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v e r s i o n 2 . 0

R E F E R E N C E

M A N U A L

Visual OTTHYMO™ v2.0Reference Manual

July 2002

Table Of Contents i

Table of Contents

PROGRAM REFERENCE................................................................................................................1INTRODUCTION ...................................................................................................................................1VISUAL OTTHYMO FILES...................................................................................................................1

Project Files ..................................................................................................................................1Rainfall Files..................................................................................................................................2Hydrograph Files...........................................................................................................................4

IMPERVIOUSNESS ...............................................................................................................................5COMMAND LIST...................................................................................................................................5

Watershed Commands .................................................................................................................5Routing Commands ......................................................................................................................6Operational Commands ................................................................................................................6Utility Commands..........................................................................................................................7STANDHYD ..................................................................................................................................7NASHYD .......................................................................................................................................9WILHYD ........................................................................................................................................9SCSHYD .....................................................................................................................................10ROUTE CHANNEL .....................................................................................................................10ROUTE MUSKCUNG .................................................................................................................11ROUTE PIPE ..............................................................................................................................12ROUTE RESERVOIR .................................................................................................................13ADD HYD....................................................................................................................................13SHIFT HYD .................................................................................................................................13DIVERT HYD ..............................................................................................................................14DUHYD .......................................................................................................................................15READ HYD..................................................................................................................................15STORE HYD ...............................................................................................................................16

TIPS FOR MODELLING UNGAUGED RURAL CATCHMENTS.....................................................................16Initial Abstraction Parameter, IA .................................................................................................16Modified Curve Number, CN* .....................................................................................................17Time to Peak Parameter, TP ......................................................................................................17

TIPS FOR MODELLING UNGAUGED URBAN CATCHMENTS ....................................................................19Imperviousness...........................................................................................................................19Loss Routine ...............................................................................................................................20Parameters for the Pervious Component ...................................................................................21Parameters for the Impervious Component................................................................................21

SWM POND MODELLING...................................................................................................................22How to Build a Rating Curve Using ROUTE RESERVOIR ........................................................22

THEORY REFERENCE .................................................................................................................24COMPUTATION OF RAINFALL LOSSES .................................................................................................24

Critical Review of SCS Curve Number Procedure......................................................................24Calibration of the Modified SCS CN Procedure..........................................................................29Infiltration Procedures in STANDHYD ........................................................................................34Considerations in Using the Rainfall Losses ..............................................................................35

UNIT HYDROGRAPH OPTIONS IN VISUAL OTTHYMO..........................................................................38IUH Relations..............................................................................................................................39The STANDARD IUH..................................................................................................................40The NASH IUH (NASHYD) .........................................................................................................41The SCS IUH (SCSHYD)............................................................................................................42The WILLIAMS IUH ....................................................................................................................42Use of IUH’s For I/I Simulation and Baseflow (DWF) .................................................................43Unit Hydrograph Options for Rural Areas ...................................................................................44

ROUTING OPTIONS IN VISUAL OTTHYMO .........................................................................................49Simulation Time Steps ................................................................................................................49Time Shift Routing ......................................................................................................................50

ii Visual OTTHYMO v2.0Reference Manual

Variable Storage Coefficient Routing in Visual OTTHYMO........................................................50Muskingum-Cunge Channel Routing..........................................................................................50

DESIGN STORMS FOR STORMWATER MANAGEMENT STUDIES.............................................................55Methodology Of Design Storms..................................................................................................58Methodology for Comparing Design Storms and a Historical Storm Series...............................59Results Of Peak Flows From Design Storms And Historic Storm Events ..................................64Conclusions.................................................................................................................................71

A REVIEW OF DESIGN STORM PROFILES ...........................................................................................72Intensity Duration Frequency Curves .........................................................................................72Frequency Of Real Storms And Synthetic Storms......................................................................75Uniform Design Storm.................................................................................................................76Composite Design Storm............................................................................................................76Chicago Design Storm................................................................................................................76SCS 24-Hour Design Storm........................................................................................................79SCS 6 Hour Design Storm..........................................................................................................80Illinois State Water Survey Design Storm...................................................................................81Atmospheric Environment Service Design Storm.......................................................................84Flood Studies Report Design Storm ...........................................................................................86Pilgrim and Cordery Design Storm .............................................................................................87Yen and Chow Design Storm......................................................................................................88

REFERENCES ...............................................................................................................................90

INDEX...........................................................................................................................................102

Program Reference 1

PROGRAM REFERENCE

IntroductionWe have created two separate documents for Visual OTTHYMO v2.0, a User’s Guide anda Reference Manual. The User’s Guide contains information on how to use the program,complete with a description of all the features. We have also included example modelsand a chapter on troubleshooting. The User’s Guide is for the basic to average user whois not concerned with how the model works or background theory. The Reference Manualcontains all of the hydrologic theory behind the program and gives guidance for users onhow to select or measure object parameters. The Reference Manual also gives morehistory on the model and how it has developed over the years. The Reference Manual isfor advanced users who need to know “why” and from “where”.

The VO2 Reference Manual has been assembled to provide users with more detailedinformation on the theory and history of Visual OTTHYMO and OTTHYMO. TheReference Manual is divided into 3 sections, as follows:

• Program Reference: This section contains the command list specifications andadditional details pertaining to VO2 that are not covered in the User’s Guide.

• Theory Reference: This section contains all the theory behind VO2.

• References: This section contains all the published references from where the VO2theory and practice have been developed.

Visual OTTHYMO Files

Project FilesIn order to effectively work with projects, one must understand what is meant by a Projectand how it is interpreted by VO2. To the user, a project may represent a specific type ofwork which consists of multiple hydrologic models(e.g. Subwatershed Study). To yourcomputer a project is a collection of files contained within a specific folder. These fileswould include: hydrologic scenarios, rainfall files, external hydrograph files, and outputfiles, to name a few. When the user creates a new project VO2 prompts for a projectname and that name is used to create a new folder on a fixed disk. All files used duringsimulation will be stored in that folder.

The following table lists the more common file types that are stored in a project folder:

File Extension Description*.vop Visual OTTHYMO project file. This is the main file associated with

the project. It keeps track of all the other files that are part of theproject. Users can double-click on *.vop files to immediately launchthe selected project into VO2.

*.sce Scenario File. This file contains the parameter data for each of theobjects in your model.

*.mdr Connectivity File. This file contains the GUI data necessary fordisplaying the model schematic.

vo.mdb This is the project’s database file where all output information isstored when a simulation is executed.

*.out This is the detailed output file in ASCII format. These files are namedbased on the scenario name.

2 Visual OTTHYMO v2.0Reference Manual

*.sum This is the summary output file in ASCII format. These files arenamed based on the scenario name.

*.stm Storm file. This is an ASCII file that is read into the model. The file isin a hyetograph format (refer to following section).

*.mst Mass storm file. This is an ASCII file that is read into the model. Thefile is in a mass curve format (refer to following section).

*.hyd External hydrograph file. This is an ASCII file that is read into themodel. The file is in a hydrograph format (refer to following section).

Rainfall FilesStorm data files that are required for the use of Mass Storm and Read Storm, are createdin the same format as the storm files used in OTTHYMO-89/INTERHYMO. Therefore,previously made storm files may be used for both Mass Storm and Read Storm.

READ STORM FilesThe following is an example of how to create a storm data file, for use in the read stormcommand. This file can be created in any text editor (e.g. Wordpad, Notepad).

1st line: 2 (1 indicates in/hr, 2 indicates mm/hr)

2nd line: comment line (up to 60 characters in length)

3rd line: 10 (storm time step, min)

4th line: 24 (number of rainfall increments, max.=400)

5th line: 2.071 (1st rainfall intensity)

6th line: 2.266 (2nd rainfall intensity)

7th line: 2.524 (3rd rainfall intensity)

..th line: …

xth line: 2.135 (xth and last rainfall intensity, x corresponds to the numberin the 4th line)

last line: -1 (indicates the end of the file)

Notes: The comments shown to the right of the inputs are not necessary. The file mustbe saved with a file extension of *.stm and must be in an ASCII text format.

An example of a Storm data file (25mm4hr.stm) is as follows:2TWENTY-FIVE MM FOUR HOUR CHICAGO STORM10242.0712.2662.5242.8803.3824.1755.69610.77750.21413.3668.2866.2955.1944.466

Program Reference 3

3.9493.5603.2523.0102.7992.6222.4762.3462.2332.136-1

MASS STORM FilesThe following is an example of how to make a mass curve data file, for use in the massstorm command. This file can be made in any text editor (Wordpad, Notepad).

1st line: comment line (up to 60 characters in length)

2nd line: 20 (the time increment between each ordinate, min)

3rd line: 13 (the # of ordinates used to describe the mass curve,max.=400)

4th line: 0.00 (the first ordinate of the mass curve)

5th line: 0.01 (the second ordinate of the mass curve)

6th line: 0.04 (the third ordinate of the mass curve)

..th line: …

xth line: 1.00 (xth and last ordinate of the mass curve)


Notes: The comments shown to the right of the inputs are not necessary. The file mustbe saved with a file extension of *.mst.

An example of a Storm data file (aesmass.mst) is as follows:AES Mass curve data with twenty minute time step20130.000.010.040.120.270.550.700.820.900.950.980.991.00-1


Hydrograph FilesIf the READ HYD object is used to read an external hydrograph file into the model, thenthe file must be coded in the correct format. This format is in the same format as thehydrograph files used in OTTHYMO-89/INTERHYMO. Therefore, previously madehydrograph files may be used.

The following is an example of how to create a hydrograph data file, for use in the READHYD object. Please note that this format is also used by the Save Hydrograph featurelocated in the Hydrograph Data form. Hydrograph files can be created in any text editor(e.g. Wordpad, Notepad).

1st line: 2 (1 indicates in/hr, 2 indicates mm/hr)

2nd line: comment line (up to 60 characters in length)

3rd line: 5 (hydrograph time step, min)

4th line: 50 (the catchment area from which the hydrograph wasobtained, ha / acre)

5th line: 0.000 (1st hydrograph ordinate, m3/s / cfs)

6th line: 0.100 (2nd hydrograph ordinate, m3/s / cfs)

7th line: 0.200 (3rd hydrograph ordinate, m3/s / cfs)

..th line: …

xth line: 0.100 (xth and last hydrograph ordinate)


Notes: The comments shown to the right of the inputs are not necessary. The file mustbe saved with a file extension of *.hyd and must be in an ASCII text format.

An example of a Hydrograph data file (test.hyd) is as follows:2HYDROGRAPH FROM OUR TEST CATCHMENT5500.0000.1000.2000.3001.0001.2004.5008.15010.0009.5005.4502.0001.0000.5000.3000.100-1

Program Reference 5

ImperviousnessImperviousness is the decimal fraction of the total watershed area covered byimpermeable surfaces such as: paved roadways, roofs, sidewalks, driveways, parking lots,etc. Imperviousness values range from 0 to 1.0 with the larger values representing higherurban development intensities.

Imperviousness can be directly or indirectly connected areas. Directly connectedimpervious areas are those that form a continuous pathway from the point of runoffgeneration to the outlet point. Indirectly connected areas are those which convey runoff topervious areas prior to reaching the watershed outlet. For example, in developedcatchments, roofs with roof-leaders discharging to front or rear backyard lawns areindirectly connected impervious areas. Driveways sloped towards the roadway areconsidered directly connected areas. Sidewalks may or may not be directly connected.

Command ListWatershed Commands

STANDHYD Used to simulate design hydrographs from urbanwatersheds. With this command, the model uses twoparallel standard instantaneous unit hydrographs toconvolute the effective rainfall intensity over the perviousand impervious surfaces. The losses over the perviousarea can be calculated by one of the three methods: i)Horton’s soil infiltration equation; ii) SCS modified CNprocedure; or iii) Proportional Loss Coefficient. A baseflowcan be added to the total simulated hydrograph. To obtainadequate results, the command should be applied to areaswith impervious ratios larger than 20% (for smallerimpervious ratios the watershed can be broken down intourban and rural basins).

NASHYD Used to simulate design hydrographs with the Nashinstantaneous unit hydrograph. This hydrograph is madeof a cascade of ‘N’ linear reservoirs. The command ismainly used for rural areas but can also be used for verylarge urban watersheds and to simulate the effects ofinfiltration /inflow in sanitary sewers. Rainfall losses can becomputed by a SCS modified CN procedure or ProportionalLoss Coefficient.

WILHYD Used to simulate design hydrographs from ruralwatersheds with long recession periods. The programuses the Williams and Hann’s (1972) unit hydrographdeveloped in the original HYMO program and the ModifiedSCS Curve Number procedure to calculate the rainfalllosses.

SCSHYD Essentially the same as the NASHYD command with theexception that it uses parameters for the SCS procedure(i.e. Initial abstraction is a function of the SCS CurveNumber and the number of linear reservoirs ‘N” is set to5.0). This command can be used when the SCS procedureis required by agencies or for comparison with otheroptions.


Routing CommandsROUTE CHANNEL Used to route hydrographs through typical channel cross-

sections using the variable storage coefficient (VSC)method. The open channel cross-sections are describedwith X and Y co-ordinates. Other inputs are the averagelongitudinal slope and the variation of Manning’s roughnesscoefficient across the width. The command computes arating curve and travel times prior to routing with the VSCmethod.

ROUTE MUSKCUNG Used to route hydrographs through typical channel cross-sections using the Muskingum-Cunge routing method. Thismethod is based on the continuity equation and thestorage-discharge relation. The open channel cross-sections are described with X and Y co-ordinates. Otherinputs are the average longitudinal slope, the variation ofManning’s roughness coefficient across the width, and aconstant, Beta, of the stage-discharge curve that is also afunction of the kinematic wave celerity.

ROUTE PIPE Used to route hydrographs in circular or rectangular pipes.It uses a simplified form of the ROUTE CHANNEL input.Only the pipe diameter or width and heights are requiredand only one Manning’s roughness coefficient is allowed.The command automatically resizes the pipe cross-sectionif the dimensions entered are not sufficient toaccommodate the peak flow without surcharging.

ROUTE RESERVOIR Used to route hydrographs through reservoirs using theStorage-Indication method.

Operational CommandsADD HYD Used to add two hydrographs. A total hydrograph obtained

from the addition of the two hydrographs derived atdifferent time steps will have the smaller time step.

SHIFT HYD Used as an alternate routing method when the peak flowattenuation expected is negligible. The command shifts theentire hydrograph forward to the nearest equal number oftime steps specified by user-entered time shift.

DIVERT HYD Used to simulate diversion channels and multi-outletstructures. By entering a table of inflow-outflowrelationships the command can split any hydrograph into amaximum number of five hydrographs.

DUHYD Used to separate the major (street flow) and the minor(pipe flow) hydrographs from a total hydrograph.

Program Reference 7

Utility CommandsREAD HYD Used to read a previously saved hydrograph.

STORE HYD Used to enter ordinates of a hydrograph directly into theinterface.

STANDHYDSTANDHYD is used to simulate runoff flows from urban watersheds. The program usestwo parallel standard instantaneous unit hydrographs to convolute the effective rainfallintensity over the pervious and impervious surfaces. The losses over the pervioussurfaces are calculated by one of three methods: i) Horton’s soil infiltration equation; ii)SCS modified CN procedure; or iii) Proportional Loss Coefficient (see Program Theory forfurther reference). A baseflow can also be added to the total simulated hydrograph. Toobtain adequate results, the command should be applied to areas with impervious ratiolarger than 20% (for smaller impervious ratios the watershed should be sub-divided intourban and rural basins).

ParametersNHYD Hydrograph number given by the user for reference,

display and printing purposes. The number must be aninteger number between 1 and 9999.

DT Simulation time step increment (min). It should be lessthan the storage coefficient SCP and SCI.

AREA Watershed area (ha / acre).

XIMP Ratio of total area directly connected impervious areas arethose that form a continuos pathway from the point ofrunoff generation to the outlet point. For example, the areadirectly connected to the sewer system. The value must bein the range of 0 to 1.

TIMP Ratio of total impervious area. The value must be in therange of 0 to 1 and greater than or equal to XIMP.

DWF A constant Dry Weather Flow or baseflow (m3/s / ft3/s).

LOSS Selects the rainfall loss method to be applied to thepervious area. Value must be 1, 2 or 3.

If LOSS = 1 Selects Horton’s soil infiltration equation method. Thenenter:

Fo Initial infiltration rate (mm/hr / in/hr).

Fc Final infiltration rate (mm/hr / in/hr).

DCAY Decay constant (1/hr).

F Accumulated moisture in the soil at the beginning of the


storm (mm / in).

DPSP Depression storage available over the pervious area (mm /in).

If LOSS = 2 Selects SCS modified CN procedure method. Then enter:

CN Soil’s SCS or Modified Curve Number for the perviousarea.

IA Initial abstraction (mm / in).

If LOSS = 3 Selects Proportional Loss Coefficient method. Then enter:

C Proportional loss coefficient ratio (between 0 and 1).

IA Initial abstraction (mm / in).

SLPP Average slope of the pervious area (%). Value must begreater than 0.0.

LGP Overland flow length of the pervious area (m / ft).

MNP Manning’s roughness coefficient for pervious surfaces.Note that this coefficient should be selected based onsheet flow, not channel flow.

SCP Storage coefficient for the linear reservoir of the perviousarea (hr). Enter 0 to allow the program to internally selectthe value.

DPSI Available depression storage over the impervious area(mm / in).

SLPI Average slope of impervious area (%).

LGI Selects the overland flow length of impervious area (m / ft).

If LGI = 1 LGI is manually input in a separate form.

If LGI = 2 LGI is calculated from A=1.5(LGI)2 where A is area in (m2 /ft2) and LGI is in (m / ft).

MNI Manning’s roughness coefficient for impervious surfaces.Note that this coefficient should be selected based onchannel flow (i.e. sewer and/or road flow).

SCI Storage coefficient for the linear reservoir of the imperviousareas (hr). Enter 0 to allow the program to internally selectthe value.

RAIN Optional list of rainfall intensities (mm/hr / in/hr) entered attime steps equal to DT. If the list is not given the model willuse the rainfall assigned using the rain gauge tool.

LABEL Label options.

Program Reference 9

NASHYDNASHYD is used to simulate runoff flows with the Nash instantaneous unit hydrograph.This hydrograph is made of cascade of ‘N’ linear reservoirs. The command is mainly usedfor rural areas but can also be used for very large urban watersheds and to simulate theeffects of infiltration /inflow in sanitary sewers. Rainfall losses can be computed by a SCSmodified CN procedure or Proportional Loss Coefficient (see Theory Reference for furtherinformation).



DT Simulation time step increment (min).



CN SCS Modified Curve Number or (/) Proportional LossCoefficient (if negative value between 0 and –1 entered).

IA Initial abstraction (mm / in). If IA is negative, the programuses the SCS method where IA = 0.2 x S, and S is afunction of Curve Number.

N Number of linear reservoir used for the derivation of theNash Unit Hydrograph.

TP Unit hydrograph time to peak (hr). It is approximately equalto (N-1)/N x TC where TC is the Time of Concentration.


LABEL Label Options

WILHYDUsed to simulate design hydrographs from rural watersheds with long recession periods.The program uses the Williams and Hann’s (1972) unit hydrographs developed in theoriginal HYMO program and the Modified SCS Curve Number procedure to calculate therainfall losses.





AA/DWF Printout parameter or if less than 0, to enter as a constantDry Weather Flow or baseflow (m3/s / ft3/s). If AA ispositive, the unit hydrograph will be printed. If AA is 0,neither happens.


BB Printout parameter. If BB is positive the rainfall excessordinates will be printed. If BB is 0, excess ordinates willnot be printed.

CN SCS Modified Curve Number.

IA Initial abstraction (mm / in). If IA is negative, the programuses the SCS method where IA = 0.2 x S, and S is afunction of Curve Number.

K Recession constant in the Williams and Hahn unithydrograph equation (hr).

TP Unit hydrograph time to peak (hr). For this command, thetime step DT should be smaller than TP.



SCSHYDThis is essentially the same as the NASHYD command with the exception that it usesparameters for the SCS procedure (i.e. Initial abstraction is a function of the SCS CurveNumber and the number of linear reservoirs ‘N’ is set to 5.0). This command can be usedwhen the SCS procedure is required by agencies or for comparison with other options.






CN SCS Modified Curve Number. The program uses the SCSmethod where IA = 0.2 x S, and S is a function of CurveNumber.

TP Unit hydrograph time to peak (hr). It is approximately equalto (N-1)/N x TC where TC is the Time of Concentration).For this command, the time step DT should be smaller thanTP.



ROUTE CHANNELUsed to route hydrographs through typical channel cross-sections using the variable

Program Reference 11

storage coefficient (VSC) method. The open channel cross-sections is described with Xand Y co-ordinates. Other inputs are the average longitudinal slope and the variation ofManning’s roughness coefficient across the width. The command computes a rating curveand travel times prior to routing with the VSC method.



DT Routing simulation time step (min).

CHLGTH Length of channel reach (m / ft).

CHSLOPE Average longitudinal channel slope (%).

FPSLOPE Average flood plain slope (%).

VSN Valley Section Number used for identification and printingpurposes. Values must be between 0 and 9999.

NSEG Number of segments in the channel cross-section withconstant Manning’s roughness coefficients. A maximum ofsix segments across the section are permitted. NOTE: TheManning’s roughness coefficient that describes the mainchannel, must be entered as a negative (e.g. -0.025).

ROUGH, SEGDIST Paired values describing the roughness over the segmentdistance (X co-ordinate). Each roughness value, ROUGH,is applied over the distance specified by SEGDIST whichshould also be one of the distance co-ordinates found inDIST, ELEV (below). SEGDIST has units (m / ft).

DIST, ELEV Co-ordinates describing the shape of the cross section as(X, Y). A maximum of 20 points can be entered. Units are(m / ft).


ROUTE MUSKCUNGUsed to route hydrographs through typical channel cross-sections using the Muskingum-Cunge routing method. This method is based on the continuity equation and the storage-discharge relation. The open channel cross-sections are described with X and Y co-ordinates. Other inputs are the average longitudinal, the variation of Manning’s roughnesscoefficient across the width, and a constant, Beta, of the stage-discharge curve and is alsoa function of the kinematic wave celerity.




CHLGTH Length of channel reach (m / ft).

CHSLOPE Average longitudinal channel slope (%).

FPSLOPE Average flood plain slope (%).

VSN Valley Section Number used for identification and printingpurposes. Values must be between 0 and 9999.


BETA Is a function of the kinematic wave celerity and is aconstant of the stage-discharge curve. Beta is a reflectionof the channel shape. Beta has an upper limit of 1.67 anda lower limit of 1. Beta equals 1.67 for natural and widerectangular channels, 1.5 for trapezoidal channels, 1.33 fortriangular channels, 1.5 for rectangular channels.

NSEG Number of segments in the channel cross-section withconstant Manning’s roughness coefficients. A maximum ofsix segments across the section are permitted.

ROUGH, SEGDIST Paired values describing the roughness over the segmentdistance (X co-ordinate). Each roughness value, ROUGH,is applied over the distance specified by SEGDIST whichshould also be one of the distance co-ordinates found inDIST, ELEV (below). SEGDIST has units (m / ft).

DIST, ELEV Co-ordinates describing the shape of the cross section as(X, Y). A maximum of 20 points can be entered. Units are(m / ft).


ROUTE PIPEUsed to route hydrographs in circular or rectangular pipes. It uses a simplified form of theROUTE CHANNEL input.

Only the pipe diameter or width and heights are required and only one Manning’sroughness coefficient is allowed.

The command automatically resizes the pipe cross-section if the dimensions entered arenot sufficient to accommodate the peak flow without surcharging.

ParametersITYPE Denotes the pipe section type. A 1 is entered if the pipe is

circular and a 2 if the pipe is rectangular.

If ITYPE = 1 Selects a round pipe. Then enter:

DIAM The pipe diameter (mm / in).

If ITYPE = 2 Selects a rectangular pipe. Then enter:

WIDTH, HEIGHT Are the width and height of the pipe (mm / in).

NHYD Hydrograph number given by the user for reference,display and printing purposes. The number must be aninteger number between 1 and 9999.

PIPE Pipe identifier used for identification and printing purposes.Values must be between 0 and 9999.

PLNGTH The length of the pipe (m / ft).

ROUGH The Manning’s roughness coefficient.

SLOPE The average slope of the pipe (m/m / ft/ft).




ROUTE RESERVOIRUsed to route hydrographs through reservoirs using the Storage-Indication method.




RATING CURVE Allows input of discharge-storage curve

DISCHARGE,STORAGE

Are pairs of values entered to describe the Discharge-Storage relationship of the reservoir (m3/s & ha.m. / ft3/s &ac.ft.). A maximum of 20 co-ordinates can be entered.


ADD HYDUsed to add two given hydrographs. A total hydrograph obtained from the addition of thetwo hydrographs derived at different time steps will have the smaller time step.




Note: ADDHYD must have two inputs and one output. With only a single input, theprogram will not run.

SHIFT HYDUsed as an alternate routing method when the peak flow attenuation expected is negligible.The command shifts the entire hydrograph forward to the nearest equal number of time stepsspecified by user-entered time shift.



TLAG The amount of time by which the hydrograph is to be


lagged (min). The command will shift the hydrograph bythe nearest multiple of DT to TLAG.


DIVERT HYDThis command can be used to simulate diversion channels and multi-outlet structures. Byentering a table of inflow-outflow relationships the command can split any hydrograph intoa maximum number of five hydrographs. The five hydrographs must add up to the originalinflow hydrograph.


display and printing purposes. The number must be aninteger number between 1 and 9999. All five outlethydrographs need to be given an NHYD number.

FLOW TABLE Allows input of flow splitting relationship up to 5hydrograph.

Q1(1) Is the outflow for the first output hydrograph, NHYD1, whenthe inflow is QTOTAL(1).

Q1(2) Is the outflow for the first output hydrograph, NHYD1, whenthe inflow is QTOTAL(2).

……

Q1(Total 20) Is the outflow for the first output hydrograph, NHYD1, whenthe inflow is QTOTAL(20).

Q2(1) Is the outflow for the second output hydrograph, NHYD2,when the inflow is QTOTAL(1).

Q2(2) Is the outflow for the second output hydrograph, NHYD2,when the inflow is QTOTAL(2).

……

Q2(Total 20) Is the outflow for the second output hydrograph, NHYD2,when the inflow is QTOTAL(20).

Q3(1) Is the outflow for the third output hydrograph, NHYD3,when the inflow is QTOTAL(1).

Q3(2) Is the outflow for the third output hydrograph, NHYD3,when the inflow is QTOTAL(2).

……

Q3(Total 20) Is the outflow for the third output hydrograph, NHYD3,when the inflow is QTOTAL(20).

Q4(1) Is the outflow for the fourth output hydrograph, NHYD4,when the inflow is QTOTAL(1).

Q4(2) Is the outflow for the fourth output hydrograph, NHYD4,


when the inflow is QTOTAL(2).……

Q4(Total 20) Is the outflow for the fourth output hydrograph, NHYD4,when the inflow is QTOTAL(20).

Q5(1) Is the outflow for the fifth output hydrograph, NHYD5, whenthe inflow is QTOTAL(1).

Q5(2) Is the outflow for the fifth output hydrograph, NHYD5, whenthe inflow is QTOTAL(2).

……

Q5(Total 20) Is the outflow for the fifth output hydrograph, NHYD5, whenthe inflow is QTOTAL(20).


DUHYDUsed to separate the major (street flow) and the minor (pipe flow) hydrographs from a totalhydrograph.



CINLET The peak flow capture rate per inlet (m3/s / ft3/s).

NINLET The number of inlets in the drainage system which havethe capture rate of CINLET. Note: The maximum minorsystem capture equals CINLET x NINLET.

FLOW TABLE This allows the user to flip the major and minor connectionsbetween the downstream NHYD’s.

MAJID NHYD of major system connection.

MINID NHYD of minor system connection.


Note: The major and minor flows are differentiated on the screen by different line types.The major (street) flow is shown by a solid line, while the minor (pipe) flow is shown by adashed (broken) line.

READ HYDUsed to read a previously saved hydrograph.


display and printing purposes. The number must be an


integer number between 1 and 9999.

FILEPN The filename, of the saved hydrograph, to be read. Refer tothe Visual OTTHYMO files section for the file format.


STORE HYDUsed to enter ordinates of a hydrograph directly.



DT The time step at which the hydrograph is entered (min).

AREA The watershed area from which the hydrograph wasderived (ha / acre).

HYD POINTS A list of hydrograph ordinates, entered at time steps equalto DT. Up to 2000 values can be entered (m3/s / ft3/s).


Tips for Modelling Ungauged Rural CatchmentsThis section outlines different methodologies for modelling ungauged rural catchments.While it is preferable to use a calibrated hydrologic model for water resources studies,especially for rural catchments, this is not always possible. Satisfactory results may stillbe obtained for macro level studies provided that the modeller chooses the appropriateparameters for each catchment.

The focus of this section is on the Initial Abstraction parameter, IA, and the Time to Peakparameter, TP, parameter. While the CN parameter plays a large role in determining therunoff characteristics of a particular catchment, this parameter can be readily determinedand is rarely in dispute by watershed regulating authorities. Guidance is provided in thissection on determining the Modified CN parameters, called CN*.

Initial Abstraction Parameter, IA

Modified Curve Number Method (CN*)

When using the Modified Curve Number Method the IA parameter should be set to a valuein the range of 1.0 mm and 5.0 mm, depending on the circumstances. The IA value mustthen be used to calculate CN* (see below).

SCS Method (CN)

When using the SCS Curve Number Method, IA should be set to 0.2S where S is the soilstorage (a function of CN). Bear in mind that this method may underestimate the peakflow for small storms because the initial abstraction is higher than the total rainfall, which isnot accurate. A literature review of this method has found that for lower CN values, alower IA should be used. Suggests guidelines are as follows:

CN ≤ 70 IA = 0.075S

CN > 70 ≤ 80 IA = 0.10S


CN > 80 ≤ 90 IA = 0.15S

CN > 90 IA = 0.2S

Please note that the above guidelines are for the SCS Method only, where the SCS CurveNumber is used to define the soil type.

Modified Curve Number, CN*The Modified Curve Number method was first proposed by Paul Wisner & Associates in1982, and was based on their research and monitoring of rural and urban catchments inCanada. This method has been used successfully in Canada for the past 20 years andhas correlated well with measured flows.

Rather than having a varying IA parameter, as in the SCS method, the IA is fixed, asdescribed above, and the CN is altered. The modified CN, called CN* is a function of theIA, and total rainfall. CN* is calculated as follows:

• Select an appropriate IA (see above) for catchments being modelled.

• Determine the SCS CN value from soils maps and/or calculations. Convertthe CN (AMC II conditions) to a CN (AMC III conditions).

• Determine the largest precipitation volume, P, for a rainfall event that wouldjust represent AMC III soil moisture conditions. In most cases this is the 100year storm event. For example, in Markham Ontario the 100 year stormvolume for the 3 hour storm is 80 mm.

• Calculate the soil storage S, based on the SCS Method using CN (AMC IIIconditions). The metric equation is S = (25400 / CN) –254 and the imperialequation is S = (1000 / CN) – 10. This will give you the soil storage duringyour large storm event.

• Calculate the IA based on the SCS Method, where IA = 0.2S. Note that thisrelationship is also valid for the Modified CN Method because it is assumedthat the runoff volume, Q, for large events is the same using both methods.

• Determine the runoff volume, Q, based on the familiar:Q = (P – IA)2 / (P - IA + S)

• Next calculate S* using the above equation again but this time setting IA tothe value calculated for the Modified CN method (i.e. 1.0mm to 5.0mm). ThisIA will be the value used in the model simulations.

• Once you have calculated S*, calculate CN* from the equation:S* = (25400 / CN*) –254 metricS* = (1000 / CN*) – 10 imperial

• The above calculation will give you the CN* for AMC III soil conditions. Younow finally determine the CN* for AMC II soil conditions by using publishedtables relating CN for AMC II and AMC III conditions.

The above method is easily adaptable to a spreadsheet so that for future uses, you caneasily and quickly calculate the CN* once you know the IA, P, and CN.

Time to Peak Parameter, TPUnlike the urban catchments hydrographs, rural catchment unit hydrographs do notcalculate the time to peak TP as a function of the other variables. The TP parameter musttherefore be determined by the modeller. It should be noted that most methods ofestimate TP, start by calculating the time of concentration, tc. Time of concentration is thetime at which the centroid of the flow reaches the bottom of a catchment. TP is usually afixed ratio of tc, depending on the unit hydrograph chosen.

Over the past 40 years there have been numerous studies in both the United States andCanada in which empirical, semi-empirical, and mathematical relationships for tc have


been derived. Most of the relationships state that tc is a function of catchment slope,catchment area, and ground cover. While no single method can be used for everysituation we have included the most common methods in this manual so that the modellercan choose what is appropriate for their situation.

Listed below are five methods for calculating TP. We have included both the source of themethod as well as the context in which it was derived. This way the modeller should beable to choose a method that was derived for a similar situation as their own.

Upland’s Method

Figure 1: Uplands Method of Estimating Time of Concentration (SCSNational Engineering Handbook, 1971)

With Upland’s Method the average overland flow velocity is determined for a catchmentbased on the catchment slope and ground type, as shown in Figure 1. Once the velocityhas been determined then the time of concentration is determined by dividing thecatchment length by the overland flow velocity.

Bransby - William’s Formula

In catchments where the runoff coefficient, C, is greater than 0.40, the Bransby Williamsformula is a popular choice. The method calculates time of concentration as a function ofcatchment area, length, and slope as follows:

1.02.0 **057.0ASLt

wc = (1)

where:


tc = time of concentration (min)L = catchment length, (m)Sw = catchment slope (%)A = catchment area (ha)

Airport Method

For catchments where the runoff coefficient, C, is less than 0.40, the Airport formula mayprovide a better estimate of the time of concentration. This method was developed forairfields and calculates time of concentration as a function of runoff coefficient, length, andslope as follows:

33.0

5.0*)1.1(*26.3

wc S

LCt −= (2)

where:

tc = time of concentration (min)C = runoff coefficientL = catchment length, (m)Sw = catchment slope (%)

William’s Equation (1977)

Williams, who co-developed the William’s Unit Hydrograph (WILHYD in Visual OTTHYMO)with Hann in 1973 later derived empirical relationships for both the K and TP variables inWILHYD. These relationships are:

84.024.01.16 −= SAK (3)

50.039.054.6 −= SAtp (4)

The above relationships were derived for watersheds in the southern United States. Referto the Theory Reference section of this manual for more information on the derivation ofthe WILHYD unit hydrograph.

Tips for Modelling Ungauged Urban CatchmentsThis section provides direction for modellers who are modelling ungauged urbancatchments. In most cases, urban catchments are not gauged since the response torainfall can be accurately simulated. However, like any model the user should be awarethat the inappropriate selection of parameters can lead to erroneous output. This sectionwill guide the modeller in selecting parameters that have been successfully used in thewater resources industry.

ImperviousnessThere are two impervious ratios required, the amount of directly connectedimperviousness, XIMP, and the total imperviousness, TIMP. XIMP must be less than orequal to TIMP.


TIMP is a function of the land use of the catchment. Land use is a planning term thatdescribes the approved, or proposed, use for the catchment (e.g. residential, commercial,industrial). Water resources studies are generally tied to planning applications anddepending on the level of planning application, (i.e. Secondary Plan, Official PlanAmendment, Draft Plan), the modeller will have a little or a lot of information about the landuse. Therefore it is important to select a conservative value for the imperviousness whenperforming more macro level studies so that when the subsequent more detailed studiesare completed, the more refined land use calculations will still be valid in the overall model.

The following table gives examples of suggested TIMP and XIMP values, based on landuse, for the macro-level studies. These values can be used with the information suppliedby the planner to determine area weighted values for the catchment of interest.

Land Use XIMP TIMPEstate Residential 20 40

Low Density Residential(e.g. Single Units)

25 50

Medium Density Residential(e.g. Semi-detached Units)

35 55

High Density Residential(e.g. Townhouse Units)

50 60

School 55 55

Commercial 85 85

Park 0 0

For more detailed level studies (i.e. Site Plan), there should be more information availableso that the XIMP and TIMP can be calculated.

Loss RoutineIn both the United States and Canada, either the Horton’s Method (LOSS = 1) or the CNMethod (LOSS = 2) are commonly used for urban catchments. The Proportional LossMethod (LOSS = 3) has been successfully used in France for urban catchments. Whilethe selection of Loss Routine can be somewhat arbitrary and at the discretion of the user,there are a few things to keep in mind when choosing a loss routine.

Horton’s Method is what is used in the SWMM model, therefore if the user is comparingresults with a SWMM based model, or working in a watershed where the overall modelused was SWMM, then this method may be the most appropriate. However, the usershould bear in mind that for longer duration storms (greater than or equal to 12 hours) theHorton’s Method may not accurately predict the runoff from pervious areas. We haveseen cases where the model simulates no runoff from a pervious area during a 12 hour100 year storm. This is clearly erroneous. The CN Method does not have any limitationswith respect to storm length and often yields more conservative results as compared toHorton’s Method.

If the user selects the CN Method, then the IA parameter should be set somewherebetween 1.5 mm and 5 mm. Note that this is a different value than what would be used fora rural catchment with the same CN value. An urban catchment generally has lesspervious depression storage than the same catchment in its rural state.


Parameters for the Pervious ComponentThe pervious slope, SLPP, is the average slope of the pervious areas. This is not thecatchment slope from highest point to lowest point, but an average when considering onlythe pervious areas. For example, if the catchment consists of a residential subdivision,this value would represent the average slope of the pervious lot surface. In this examplethe slope would not be less than 2% or whatever the municipal minimum is.

The overland flow length, LGP, should be set to the representative value for the perviousareas. It is not the length of the catchment from high point to low point. This valuerepresents the average length over which flows from pervious areas would travel beforebeing intercepted by channels, sewers, or roads. For example, in a residential subdivisionthis value might be the representative lot length which is typically 40 m.

The Manning’s roughness coefficient for pervious surfaces, MNP, should be selectedbased on sheet flow and not channel flow. This is a common mistake for modellers. Mostlisted values of Manning’s values are for channel flow, whereas the pervious runoffsimulated is sheet flow. Therefore if we assumed a grassed surface then the sheet flowManning’s roughness coefficient would be approximately 0.25, whereas the channelroughness coefficient for the same material might be 0.025.

For an ungauged urban catchment the pervious storage coefficient, SCP, should be set to0, which will let the program determine the storage coefficient.

Parameters for the Impervious ComponentThe impervious depression storage, DPSI, should be set to an appropriate value for therepresentative impervious surface. For roads, driveways, and roofs, this value is typicallybetween 0.8 mm and 1.5 mm.

The impervious slope, SLPI, is the average slope of impervious areas. This is not thecatchment slope from highest point to lowest point, but an average when considering onlythe impervious areas. For example, if the catchment consists of a residential subdivision,this value would represent the average slope of the impervious road surfaces. In thisexample the slope would not be less than whatever the municipal minimum is. TypicallySLPI ranges between 0.5 to 2.0.

The impervious length, LGI, is one of the most important parameters for modelling urbancatchments. A common mistake when modelling unguaged urban catchments is to setLGI equal to the measured catchment length. Previous studies by Paul Wisner AssociatesInc. have determined that LGI is related to the catchement area based on the followingequation:

25.1 LGIA = (5)

where:A = catchment area (m2)LGI = impervious length (m)

This relationship will yield runoff characteristics similar to those which would be measured.The LGI parameter should only be adjusted from this relationship if the model is beingcalibrated.

The Manning’s roughness coefficient for impervious surfaces, MNI, should be selectedbased on channel flow, not sheet flow as in MNP. For example, if the representativeimpervious surface were a road, then the MNI should be set around 0.013.

For an ungauged urban catchment the impervious storage coefficient, SCI, should be setto 0, which will let the program determine the storage coefficient.


SWM Pond ModellingProbably the single biggest use for Visual OTTHYMO is to help create water resourcesstrategies whereby stormwater management ponds are implemented to address issues ofwater quality control, erosion control, and water quantity (i.e. flooding) control. VisualOTTHYMO can be utilized to examine many scenarios that help water resources plannersand engineers determine the most effective strategy, on a watershed or sub-watershedbasis.

How to Build a Rating Curve Using ROUTE RESERVOIRA rating curve for any stormwater management pond describes how the pond operates. InVisual OTTHYMO the command ROUTE RESERVOIR is used to enter a pond ratingcurve and simulate routing. The rating curve is described by the Discharge (i.e. outflow)and Storage relationship. Note that the Stage or water depth variable is taken out of theinput, since both Discharge and Storage are a function of Stage. The Stage-Storage andStage-Discharge rating curves are essentially combined into one Discharge-Storagecurve. An example of a Discharge-Storage Curve is as follows:

Discharge (m3/s) Storage (ha-m)

0.00 0.000.06 0.340.21 0.480.37 0.600.66 0.830.94 1.00

Designing a Discharge-Storage curve, at the watershed or sub-watershed planning level,involves determining each storage ordinate for every given discharge ordinate. Dischargeordinates are usually known or can readily be determined. They may represent allowableflows or release rates that when combined with other flows are the allowable flows at keylocations. Storage ordinates are what the modeller is trying to calculate in order to meetthe discharge targets.

For single event analysis the Discharge-Storage curve is built from the smallest to largestvalues, which corresponds to the smallest to largest rainfall events. For example, theabove Discharge-Storage curve was based on the following design storm events.

Discharge (m3/s) Storage (ha-m) Design Storm

0.00 0.000.06 0.34 25 mm0.21 0.48 2 year0.37 0.60 5 year0.66 0.83 25 year0.94 1.00 100 year

When building a curve the storms must be run from smallest to largest and the storageiterated until the pond outflow matches that of the target value in the Discharge-Storagecurve. Only then can the modeller move onto the next largest storm. The proper pondsizing methodology is therefore:

1. The modeller enters the first 2 sets of points on the curve, (0,0) and the first targetflows (e.g. 0.06). The modeller guesses a storage value and then runs the model withthe storm that corresponds to the target flows.

2. The modeller checks the outflow and compares it with the target. If the outflow is too


high then the modeller must increase the storage. If the outflow is too low then themodeller must decrease the storage. Note that if the storage curve has beenexceeded then the outflow may be erroneous. It is better to iterate from a largestorage value to the correct storage than from a small storage value.

3. The modeller iterates step 2 until the calculates outflow matches (or is slightly less)than the target outflow. At this point the calculated storage should also match thestorage in the input table.

4. The modeller then enters the next discharge ordinate for the next largest storm,guesses a new storage and runs the model.

5. Steps 2 and 3 are repeated until the outflow and storage are matched.

6. Step 4 is repeated with the next largest storm until the final storm is reached.

7. Once the last storm is iterated then the Discharge-Storage curve is complete. (e.g.when the (0.94,1.00) point is determined in the above example curve).

If the modeller is designing a SWM pond based on a real storm, or is analyzing an existingpond with design storms, then the actual discharge storage curve must be used. This canbe obtained by combining the pond’s Stage-Storage curve (i.e. geometric relationship) andthe Stage-Discharge curve (i.e. hydraulic relationship).

Also, a SWM pond’s actual Discharge-Storage curve must be used when creating a detailpond design, to ensure that the outflows match the targets from the design curve that wasdetermined in the watershed or sub-watershed analysis.


THEORY REFERENCE

Computation of Rainfall LossesCritical Review of SCS Curve Number Procedure

The SCS CN procedure is based on the equation

)()( 2

SIPIPQa

a

+−−

= (6)

It is assumed in the procedure that the initial abstraction Ia = 0.2 S. This results in theequation

)8.0()2.0( 2

SPSPQ

+−= (7)

The curve numbers CN are functionally related to S by

101000

−=S

CN (8)

CN can be obtained from tables based on land use, soil type and soil moisture conditions.However the soil moisture is determined only for three antecedent moisture conditions(AMC), classified on the basis of precipitation in the previous 5 days. CN has no intrinsicmeaning but is only a non-linear transformation of S, which is a storage parameter. CNvaries from 0 (Q=0 for all P) to 100 (Q=P for all P). In Eqn. 8, the 10 and 1000 have inchdimensions. Conversion can be made to the metric system.

Background information on the derivation of the procedure can be found in a paper byRallison and Cronshey (1979). In the mid-50s when the SCS CN procedure wasdeveloped, the only data available were daily precipitation and runoff records fromagricultural watersheds and infiltration curves from infiltration studies. Rainfall versusRunoff (P vs Q) data were plotted. A grid of plotted CN for Ia = 0.2S was then overlaid andthe median CN selected. The values in the SCS NEH-4 manual (1971) represent theaverages of median site values for hydrologic soil groups, land cover and hydrologicconditions. The SCS work involved considerable interpolation and extrapolation fordifferent soil types and land cover. The rainfall versus runoff plots were also used todefine enveloping CN for each site.

The SCS CN procedure is in widespread use and there has been criticism of theprocedure (Hawkins 1978, Altman et al. 1980, Golding 1979) because it is often appliedbeyond the original conditions and intended use.

Some of the concerns about the procedure are over:

1. why the antecedent moisture range of values is a discrete rather than acontinuous relationship,

2. the lack of a clear definition of AMC II, the standard reference moisture condition,

Theory Reference 25

3. use of a 5-day time interval as a basis for classifying antecedent moistureconditions,

4. why is Ia = 0.2S,

5. what probability levels are associated with the envelopes in defining AMC I andAMC III.

The SCS CN procedure may severely underestimate the runoff volume, especially forsmall rainfalls. It was found that the runoff volumes obtained from real measurements ontwo residential watersheds were greater than those computed using CN = 90(corresponding to a high degree of imperviousness) (Figure 2).

A study in Texas (Altman et al. 1980) involving four watersheds found that the optimizedCN were greater than the weighted CN for four of the six watershed conditions studied(Table 1). For areas with low CN, the SCS procedure may give significant errors. Golding(1979) utilized the SCS CN procedure to simulate runoff from a gauged urban basin inSouth Florida (58.3 ac., Group A soil, 36% imperviousness, 18% directly connectedimperviousness). He found that the computed Ia amounted to 0.86 inches (CN=70), whichwas greater than the total recorded rainfall on the basin, which had peak flows of up to 40cfs in many cases. Reduction of the initial abstraction may give a more realistic runoffvolume. Figure 3 compares the runoff volumes obtained for different Ia and Ia = 0.2S forstorms of 3 return periods (Rowney, 1982). The SCS CN procedure is still a popular andsimple tool, which will be around for some time to come. It is felt that with someimprovements in the procedure and proper application, the method is still useful.

The methodology used in OTTHYMO involves determining the initial abstraction Ia fromthe runoff threshold curve obtained from rainfall and runoff records (Jobin, 1982). Aprogram called SECSER has been written in order to do this. The runoff volumes for thestorms are then used to calibrate the CN with this Ia. The resulting CN are called CN*.Instead of using 3 discrete AMC classes, the antecedent moisture condition is classified bythe API (antecedent precipitation index) which is calculated from the hourly rainfallrecords. The API for each storm is then plotted against the CN*. The CN* for otherstorms can then be determined from this CN*-API relationship once the API for thesestorms are determined. This relationship would be a continuous one as compared to the 3discrete classes used in SCS. It also would not require the definition of a standardreference moisture condition.

A small program (Figure 4) has been written to calculate the runoff volumes Q for differentrainfalls P using the specified Ia. The results can then also be plotted on the Q-P chart(Figure 5). These charts are useful for a quick comparison of CN and CN*. Since CN* area function of the Ia, different charts will result for different Ia.


Figure 2: Relationship between Rainfall and Runoff: CN and Real Measurements (Wisner, Gupta, Kassem, 1980)

Theory Reference 27

Figure 3: Relation between Runoff Volume: Q(Ia) (using initial abs. = Ia), Q(0.2S) (using initial abs. = 0.2S), (Rowney, 1982)


Figure 4: Program for CN Procedures

Theory Reference 29

Figure 5: Hydrology of Runoff Equation, Q = (P-0.2S)/(P+0.8S)

Calibration of the Modified SCS CN Procedure

The modified SCS CN procedure was tested first on the Seymaz watershed in a joint studyby the University of Ottawa and the Ecole Polytechnique Federale de Lausanne who hadpreviously done extensive monitoring. This watershed is composed of 30.2 km2 of ruralareas and 8 km2 of urban areas and is located in the suburbs of Geneva, Switzerland.

Using the SECSER program and the rainfall and runoff records, a runoff threshold curvecan be plotted. From the curve, the initial abstraction Ia was found to be 1.5 mm. Figure6 also shows a comparison between the simulations using both Ia = 1.5 mm and Ia = 0.2S.In the latter case, the first peak cannot be simulated accurately because of the large initialabstraction. The variation of CN* with API for some storms on the Seymaz watershed isshown in Figure 7. CN* is the calibrated CN obtained by using Ia = 1.5 mm as obtainedfrom the runoff threshold curve. (It was not possible to find a similar correlation of this typewith Ia = 0.2S). With the Ia = 0.2S assumption, peak fitting for small rainfalls is possible ifthe CN values are increased, without consideration of antecedent conditions, to unrealisticvalues, e.g., CN = 90 or higher. Two of the simulated storms obtained with the new Ia arecompared with the observed storms in Figures 8 and 9. The CN* values used in thesimulations are determined from the curve, Figure 7, once the API for the storms areobtained. A similar CN*-API relationship has been determined for the Etobicoke Creekwatershed (Figure 10) in Metro Toronto. One of the typical comparisons betweensimulated and observed storms is shown in Figure 11.


Table 1:Comparison of Calculated and Optimized CN (Altman, Espey, Felman, 1980)

Watershed Date CN(calc.)

CN(opt.)

Austin, Texas RegionWaller Creek (urban)

Wilbarger Creek

1957-19591962-19651971-1973

1964-1975

848484

83

927981

85

Dallas, Texas RegionTurtle Creek (urban)

Spanky Branch

1967-1976

1973-1975

86

84

93

96

Figure 6: Observed and Simulated Results for Event of 77/10/24

Theory Reference 31

Figure 7: Relation of CN versus API for Seymaz Watershed

Theory Reference 33

Figure 10: CN*-API Relationship for Etobicoke Creek Watershed

Figure 11: Comparison of Simulated and Observed Hydrographs for EtobicokeCreek Watershed


Infiltration Procedures in STANDHYD

Horton's Equation

For pervious areas, there are two options for calculating the infiltration losses. The firstoption is Horton's equation where the infiltration capacity rate is an exponential function oftime, which decays to a constant rate. It is written as follows:

tcoct effff α−−+= )( (9)

where ft is the infiltration capacity rate (in/hr or mm/hr) at time t;fo is the initial infiltration capacity rate (in/hr or mm/hr);fc is the final infiltration capacity rate (in/hr or mm/hr);∀ is the decay rate (1/hr).

The equation is only satisfactory for the condition that the rainfall intensity is higher thanthe infiltration capacity rate. To overcome this problem, the cumulative form of theequation can be used. It has the advantage that the infiltration rate becomes a function ofthe amount of water accumulated into the soil.

)1()(0

tcoc

t

t efffdtfF α

α−−−+== ∫ (10)

Where, F is the cumulative infiltration volume, at time t.

The average infiltration capacity rate during the next time step is

ttFttFft ∆

−∆+= )()((11)

In order to determine the actual infiltration rate f, the average infiltration capacity rate isthen compared with the average rainfall intensity i during the time period )t.

If

t

tt

fiifif

f<>

= (12)

then the calculation proceeds to the next time step with the cumulative infiltration volumeat F(t + )t). If f = i, then the actual cumulative infiltration would be

titFFact ∆+= )(. (13)

where

)(. ttFFact ∆+< (14)

Theory Reference 35

The new time t1, which would correspond to the cumulative infiltration Fact., is determinedby means of an iterative process. The calculation then continues from this point for thenext time step.

The antecedent moisture condition can be represented by the water, F, accumulated intothe soil before the start of the storm. F can be directly specified as input. The otherinfiltration parameters also need to be specified.

For a decay rate of 4.0 hr-1 the infiltration capacity rate declines 98% towards the limitingvalue fc after 1 hour (if the rainfall intensity is always higher than the infiltration capacityrate). For ∀ = 2.0 hr-1, the decline is 76% after 1 hour. This should be considered whenselecting the time increment )t for computation.

Figure 12: Cumulative form of Horton’s Infiltration Equation

Modified CN Procedure

The second option for infiltration losses in the previous area is the modified CN procedure,which is used in NASHYD.

Considerations in Using the Rainfall LossesFor flood control purposes and master drainage planning, there are both rural and urbanareas in the watershed. In Visual OTTHYMO, the rainfall losses in the rural areas arecomputed by means of the CN* procedure. The critical storms for rural conditions arelong-duration storms such as the Southern Ontario Regional Storm with a peak intensity of2.08 in/hr. The modified SCS method (CN*) is used in such conditions. The Horton modelmay result in underestimating the runoff mainly for low intensity storms, since it generatesrunoff only if the rainfall intensity is higher than the infiltration capacity rate. In such cases,the rainfall losses in the pervious portion of the urbanized areas should also be computedwith the CN* procedure. The ratio of the peak rainfall excess intensity to the peak rainfallintensity is an indicator of the effect of rainfall loss model. This ratio is called RI andFigure12 shows RI against CN* and the maximum infiltration capacity rate for (Horton) forthe Regional Storm. RI for this storm would be sensitive to the fc (minimum infiltrationcapacity rate) selected.

If the same storm is used in studying the effects of urbanization (e.g. comparing pre- andpost-development flows), the CN* procedure can continue to be used for post-development conditions with STANDHYD.

For design purposes under urban conditions, however, the critical storms are the short-duration, high intensity storms such as the Chicago-type storms. Here Horton's procedureis preferred because it is more sensitive to the storm intensity and in general results inhigher peak flows than the CN* procedure. This is shown in Figure 13 for a residentialwatershed (30% imperviousness) for three storms, the 5-year, 100-year Chicago and the


Regional storms.

A series of numerical experiments have been done to find a range of values in which theHorton and CN* procedures would give the same runoff volumes. The runoff volumetriccoefficient Cv was calculated for different combinations of fo, fc and values (Horton) andCN* values (with Ia = 0.10 in). The range of values tested were 1.0 to 5.0 in/hr for fo, 0.10to 0.50 in/hr for fc and 2.0/hr and 4.14/hr for ∀ < (decay constant). The results are shownin Figures 14 and 15. The peak flows for a 121-acre residential watershed for the valuesshown in Figure 15 are plotted in Figure 13. It is observed that equivalent Cv does notmean that the corresponding peak flows are equivalent.

It is also found that total runoff for the Regional storm is more sensitive to fc while for theChicago storms they are more sensitive to fo. There is no range of values for which the Cvare matched for all three storms. Figure 14 shows that the Cv for the Regional storm canbe matched by varying fc and Figure 15 show that the Cv for the Chicago storms can bematched by varying fo.

These results show that for consistency the selection of infiltration parameters shouldconsider the characteristics of the soil and also those of the storm. Tables given inliterature in which infiltration parameters like fo, fc and CN are given in terms of soil groupsA, B, C, D alone may not give consistent results.

If data is available and the CN*-API relationship has already been derived during theplanning stage, the CN* procedure can also be used for design purposes. The use of theCN* procedure with design storms is discussed in the section on design storms. This willresult in compatibility between the planning and the design stages for the watershed.

Figure 13: RI versus CN* and fo: Regional Storm (fc=0.30 in/hr, αααα=2.0/hr)

Theory Reference 37

Figure 14: Peak Flows for Residential Watersheds (30% imperv.) (fc=0.30 in/hr, αααα=2.0/hr)

Figure 15: Cv versus CN* and fc with fo=3 in/hr, αααα=2, Ia=0.10 in.

38

Figure 16: Cv versus CN* and fc with fo=3 in/hr, αααα=2, Ia=0.10 in.

Unit Hydrograph Options In Visual OTTHYMOIn Visual OTTHYMO, the response of a watershed to the effective rainfall is obtained byconvolution of a short duration unit hydrograph (UH) derived from the theory of conceptual“instantaneous unit hydrographs” or IUH. The characteristics of these unit hydrographsare not dependent on rainfall duration. However, depending on the size of the area beingsimulated, their use usually requires short computational time steps (1 to 15 minutes).

Visual OTTHYMO has three types of IUH's which have a common parameter, the time topeak, tp. Another parameter, K, is related to the hydrographs’s recession limb. K is alsocalled a ‘storage coefficient’ and has different values in each IUH.

tpNASH IUH

tpSTANDARD

Another option in Visual OTTHYMO is the SCS non-dimensiona

tpWILLIAMS

Visual OTTHYMO v2.0Reference Manual

l UH, which is a specific

Theory Reference 39

NASH IUH defined only by Tp (see IUH Relations).

For tp equal to the computational time step, the STANDARD IUH is identical to the singlelinear reservoir IUH from the URBHYD command in OTTHYMO 83.

IUH Relations

TYPE OF IUH RELATION REMARKS

STANDARD q/qpeak = t/Tp , for t < Tp

q/qpeak = e-( t - Tp ) / k, for t > Tp

For Tp = DT (thecomputational time step) theSTANDARD IUH becomes theURBHYD IUH fromOTTHYMO 83.

NASH q/qpeak = (t / Tp )(N-1) e(1 – N) ( t / Tp - 1) N = Tp / k + 1N is also the “number ofreservoirs”

WILLIAMS for t < to*

q/qpeak = (t / Tp )(N-1) e(1 – N) ( t / Tp - 1)

and, qpeak = [ 1/(Kn Γ(N)) ] e(1-N) (N-1)(N-1)

for to< t < t1 (where t1 = to + 2k)q/qo = e( t

0-t) / k

for to< t < to + 2kq/q1 = e( t

1-t) / 3 k

Calibration recommended.

Where t0 is the inflection pointafter the peak; Kn is thestorage coefficient of eachreservoir; N is the number ofreservoirs, and Γ (N) is thegamma function.

SCS Is the NASH IUH with N = 5


The STANDARD IUH

The standard IUH is used mainly for urban areas with pervious and imperviouscontributions calculated separately.

Net rainfall over Net rainfall

Impervious area pervious area

Convolute Standard Convolute Standard

IUH with K and Tp for IUH with K and Tp for

Impervious Area Pervious Area

Impervious Area Pervious Area

Hydrograph Hydrograph

Total Hydrograph

The standard IUH was developed and tested in Germany by Verworn and Harms in 1978.It is used in the model HYSTEM. In Visual OTTHYMO, Tp ∃ DT and therefore, for a givenstorm, Tp varies with the size of the watershed. (The URBHYD command in OTTHYMO83 is equivalent to a STANDARD IUH with the time to peak equal to the time step, DT).

A relation derived from overland routing by the kinematic wave method (Peterson andAltera) gives the storage coefficient, K. This relation is close to the relation by Neumannused in HYSTEM.

Where, L = an equivalent flow length which requires calibration. A default value forimpervious areas obtained from A = 1.5 L2.

3.04.0

6.06.0

sinLCK =

Theory Reference 41

Where A is the watershed area, was frequently tested with measurements.

For pervious areas, the default value is L = 40 m, representing an average travel length oninter-spaced green areas.

n = the roughness coefficient. Testing shows that adequate results are obtained it n =0.013 for impervious areas and n = 0.25 for pervious areas.

i = the dominant rainfall intensity (maximum average intensity during K).

S = the characteristic slope in m/m.

C = a constant (0.00775 for L in feet, i in inches/hour).

The STANDHYD command is based on analysis of comparisons with measurements andpractical applications. In STANDHYD, the dominant rainfall intensity is averaged over theduration of K. Since K varies with rainfall intensity this IUH varies from one rainfall to theother, the STANDARD IUH is a quasi-linear model.

For the impervious area, the time to peak, Tp, in the STANDARD IUH of VisualOTTHYMO is equal to the storage coefficient, K. For the pervious areas, fragmented inbackyards and connected to storm sewers, Tp is equal to K pervious + K impervious, attime of convolution, Tp is rounded to the nearest multiple of the time step, DT.

In the STANDHYD command, the pervious hydrograph and the impervious hydrographhave, in general, different Tp values. There is also a lag between the peak discharge ofthe total hydrograph and the end of the peak rainfall intensity.

For watersheds with large estate lots and semi-urban areas with relatively large perviouscomponents, it is recommended to simulate two component hydrographs:

a) The first, an equivalent smaller urban area can be simulated with STANDHYD.

b) The remaining area which is only (or mostly) pervious, can be simulated withNASHYD.

The equivalent urban area and the imperviousness of this area (e.g., say 30 %) satisfy thefollowing rule of thumb:

Equivalent Aurban * 0.30 = Atotal * (Real imperviousness)

Fore very large urban areas (> 200 hectares), STANDHYD requires calibration.

The NASH IUH (NASHYD)This linear IUH is used mainly for rural areas. With Nash, the peak discharge increaseswith N and decreases with Tp. Measurements in Ontario and in Switzerland indicate thatan average of 3 number of linear reservoirs may be appropriate.

The time to peak, Tp, is obtained from the time of concentration, Tc:

Tp = (N-1)/N Tc where, N, is the number of linear reservoirs

Tp = 0.67 Tc

In general, the time of concentration, Tc, can be determined using one of three methods:

Empirical formulas (only recommended if they are based on regional verifications).

Velocity methods, Tc = 3 (Li/Vi). The overland velocities are determined with an SCS


graph, and channel velocities can be determined from Manning’s equation.

Kinematic wave method (which accounts for the rainfall intensity).

The NASHYD command is used for non-homogeneous areas, and in the case of SCSabstraction methods, uses a weighted average of CN. Comparisons with measurementsshow a better performance if the response from the pervious and impervious areas aresimulated separately.

Fore very large urban areas (> 200 hectares), NASHYD requires calibration.

Furthermore, if the response time of an urban watershed is increased by significantchannel storage, this effect must be simulated by channel routing (unless Tp is calibrated).

The SCS IUH (SCSHYD)The shape of the SCS UH is obtained from the NASH relation, with N=5. This value isgreater than the one determined from studies in Ontario, Switzerland, and the UnitedKingdom. It is, however, conservative if the time to peak is correct.

The SCS non-dimensional unit hydrograph is used by SCS abstraction methods for bothrural and urban areas. Comparisons with measurements show that even if Tp, Ia, andCN* are calibrated, the proper shape of the hydrograph is not always generated.

The 1986 SCS TR-55 publication indicates the following limitations:

1. Hydrographs obtained by this method are not developed for comparisons withmeasurements.

2. The method should only be used in cases where runoff is greater than 12.5 mm.

3. The lag formula given in previous SCS publications is no longer recommended (itmay underestimate the peak flow).

The SCS methods apply the non-dimensioa1 UH in conjunction the SCS CN method withthe assumption that Ia = 0.2 x S. Although-this may overestimate the rainfall losses, it wasmaintained in the SCS command for special agency requests.

It is recommended the to determine Tc with the velocity method:

Tc = 3 (Li/Vi).

The WILLIAMS IUHThe method is recommended for rural watersheds where observations indicate a longhydrograph recession limbs. The Williams formula for Tp is not recommended in Ontarioas it has been shown to give significant errors.

Theory Reference 43

Figure 17: Williams IUH

As is predecessor (INTERHYMO / OTTHYMO.89), Visual OTTHYMO does notrecommend default values for K and Tp in the Williams command, since it is consideredthat this IUH requires calibration.

Figure 18: Comparison of Williams and Nash IUH

Use of IUH’s For I/I Simulation and Baseflow (DWF)Visual OTTHYMO can be used to simulate the Infiltration/Inflow into sanitary sewers orcombined sewers. The four types or rainfall-induced infiltration/inflow are:

1. Fast responses from directly connected impervious areas.

2. Rapid responses from grassed areas in combined sewers systems.

3. Semi-rapid responses from weeping tiles.

4. Slow responses from cracked pipes and leaking joints in the sewers.

Visual OTTHYMO can simulate these responses during a single event by adding individualresponse hydrographs from each type of contributions within the same area. The firstthree responses can be simulated with the quasi-linear instantaneous Unit Hydrograph(STANDHYD) while the fourth, slow response, can be simulated with the NASH unithydrograph.

Baseflow can be super-imposed to account for the domestic sewage contributions duringwet conditions.


Unit Hydrograph Options for Rural AreasFor computation of flows from rural watersheds, the subroutines NASHYD, WILHYD orSCSHYD (NASHYD with n=5) can be used. The rainfall excess distribution is obtained bymeans of a modified CN procedure, which is then convoluted with the unit hydrographobtained by means of the Nash model (NASHYD) or the Williams and Hann unithydrograph (WILHYD).

Instantaneous Unit Hydrograph

Many ways of deriving synthetic unit hydrographs or IUH have been proposed since theearly studies of Snyder in 1938. One frequently used way is by means of a conceptualmodel made up of a cascade of equal, linear reservoirs, first proposed by Nash in 1957(Figure 19). The IUH for Nash's model can be written as:

1

)(1),0(

−−

Γ

=n

n

Kt

n Kte

nKtq n (15)

Where ∋ (n) is the gamma function;n is the number of reservoirs;Kn is the storage coefficient of each reservoir.

By differentiating Equation 15 with respect to t/Kn and equating to zero, the time to peak tpin terms of n and Kn is obtained.

np Knt )1( −= (16)

The peak flow then becomes

11 )1()(

1 −− −Γ

= nn

np ne

nKq (17)

By substituting Equations 16 and 17 in Equation 15, the 2-parameter gamma equation isobtained

−−

−

= 1

)1(1

pttn

n

pp ettqq (18)

Williams and Hann (1973) use this equation from the time of rise to the inflection point forthe IUH in WILHYD. Figure 19 shows the variation of the outflow hydrograph fromNASHYD, with the number of reservoirs, n, for a fixed time to peak.

As shown in Figure 19, for the same time to peak, the peak flow is sensitive to n in therange 2 to 6. The parameter n, can be a non-integer. The calibration of watersheds withareas of less than 15 km2 on the Seymaz and Etobicoke studies presented in the previoussection has shown that a first estimate for N = 3 can be used if data is unavailable. Forconsistency, the various subwatersheds should use the same n unless data is available foreach subwatershed.

Theory Reference 45

Figure 19: Variation of Hydrograph (NASHYD) with the Number of Reservoirs (n) for Fixedtp


Estimation of Time to Peak (tp) in NASHYD

It is, of course, best to obtain tp by calibration with measurements. If data is available, thefollowing procedure may be utilized to estimate tp.

Figure 20: Definition of Time Lag

The first step involves determining the time lag tL which is defined as the time differencebetween the centroids of the rainfall excess hyetograph and the direct runoff hydrograph(after subtracting baseflow). tL is related to n and Kn in the Nash conceptual model by

nL nKt = (19)

Once tL is determined and n is estimated by 3 for example, then tp can be obtained byequation 16.

np Knt )1( −=

If n = 3, tp = 0.667 tL

Since measurements are usually available only at the outlet of a watershed, the tp valueswould still have to be determined for each subwatershed after discretization. The mainparameters that affect tp are the slope and the area. Since in small watersheds the slopedoes not vary too much, an approximate relation tp = m(area)n can be utilized. With thecalibrated tp at the outlet, constants m and n can be obtained by trial and error.

In the Seymaz and Etobicoke studies, the Williams and Hann equation for tp was foundadequate. For smaller watersheds, the tp values obtained can be checked by using thevelocity charts in the SCS TR-55 tables (1975) for overland flow and swale flow.

Several relations for tp or tL can be found in the literature such as Chow (1962), Kibler et al(1982), Boyd (1978) and Nash (1960).

William’s Unit Hydrograph

WILHYD is the subroutine that uses the unit hydrograph proposed by Williams and Hann

Theory Reference 47

(1973). The unit hydrograph is divided into three parts for computation. The first part, fromthe beginning of rise to the inflection point, to, is computed by the 2-parameter gammadistribution equation (Equation 18). The second part from the inflection point, to to tl wheretl = to + 2K, is computed by

Ktt

o

o

eqq)( −

= (20)

The third part from tl onwards is computed by

Ktt

eqq 3)(

1

1−

= (21)

n is computed as a function of K/tp and qp is a function of n and tp. Therefore only 2parameters, K and tp are necessary to compute the entire unit hydrograph. Empiricalrelations have been derived for K and tp (Williams 1977) based on Southern U.S.watersheds. These relations may not be applicable in other areas.

84.024.01.16 −= SAK (22)

50.039.054.6 −= SAtp (23)

where:

K is the recession constant (hr);tp is the time to peak (hr);A is the watershed area (sq.miles); andS is the difference in elevation in feet, divided by flood plain distance in miles, betweenwatershed outlet and most distant point on the watershed.

The unit hydrograph in WILHYD has a longer recession tail than that in NASHYD and asmaller peak. It can therefore be used in those watersheds where the recession limb islonger.

A comparison of the two unit hydrographs is shown in Figure 21.


Figure 21: Comparison of Unit Hydrograph by: (i) William’s and Hann’s Method (ii) Nash’sMethod

Theory Reference 49

Routing Options In Visual OTTHYMO

METHOD BRIEF DESCRIPTION COMPLEXITY

SHIFT HYDROGRAPH

A simple translation of thehydrograph. Does notattenuate the peak discharge.

LOW

ROUTE CHANNEL

Combines the three routingcommands of HYMO into asingle command, based on thehydrologic method VSC(Variable Storage Coefficient).

MEDIUM

ROUTE MUSKCUNGEApplies the Muskingum-Cungemethod of routing, which isbased on the continuityequation and the storage-discharge relation.

HIGH

ROUTE PIPEApplies the VSC method forconduits, and gives theminimum size to avoidsurcharge.

MEDIUM

Simulation Time StepsThe storm time step is determined by the format of meteorological data. For syntheticstorms it is usually five to ten minutes. The hydrograph computational time step, DT, isdetermined from the watershed characteristics. For example:

Convolution with NASHYD requires DT < Tp (time to peak - preferably DT about 1/5 Tp)

Visual OTTHYMO will transform automatically for each sub-watershed, new storm inputwith the time step DT.

In routing with the VSC method, it is recommended to maintain a small time step.Although this is not required for mathematical stability, Ponce and others recommendshort time steps and the use of the Courant criterion for hydrologic routing.

CELELERITYLENGTHDT =


The celerity is given by:

DepthAvggQCELERITY

.∗=

Celerity ranges from 1.1 to 1.6 times the average velocity. Using the above criterion, it isfound that, for time steps used in convolution (hydrograph commands) the length cannotbe very short. For short reaches, the hydrograph should be simply ‘shifted’ in time. Incomparison, routing with EXTRAN is usually conducted with time steps of 2 to 10 seconds,and gives an error message if the courant criterion is not met.

Time Shift RoutingFor discharges close to critical or supercritical flow, and for very short reaches (with timestep constraints), SHIFT HYD can be used. Comparisons with the kinematic wave methodshow that, for a circular conduit, the time lag can be selected with the relation

Time lag = reach length / (alpha - full pipe velocity)

Where alpha is given by the following table:

Qpeak flow/Qfull ALPHA0.40 1.100.60 1.170.80 1.191.00 1.11

Variable Storage Coefficient Routing in Visual OTTHYMOLike other hydrologic routing methods, the variable storage coefficient (VSC) is based onthe continuity relation. It does not apply empirical or calibrated parameters. It calculateschannel storage based on average channel characteristics, and travel time based onManning's relation. It can be used for artificial and natural channels with three roughnesscoefficients in the same cross-section.

In Visual OTTHYMO, the three routing commands of the original HYMO model are lumpedin a single command ‘Route Channel’. The VSC routing cannot be used when backwatereffects are significant. In such cases, a fully dynamic model (e.g. EXTRAN should beused).

For circular or rectangular pipes, ROUTE PIPE command should be used. The commandsizes the pipe to the minimum diameter necessary to avoid surcharging. For design, theuser should increase the size to the next standard diameter.

Muskingum-Cunge Channel RoutingThe Muskingum method is based on the continuity equation and the storage-dischargerelation. Cunge (1969) extended the method into a finite-difference scheme. TheMuskingum-Cunge channel routing technique is a non-linear coefficient method thataccounts for hydrograph diffusion based on physical channel properties and the inflowhydrograph. The advantages of this method over other hydrologic techniques are:

• it is very simple conceptually, and can be readily applied by desk calculation, and ismuch cheaper than the other methods when applied by computer,

Theory Reference 51

• it would be advantageous to use the Muskingum-Cunge method for rivers that havemajor tributaries and are not well gauged,

• this method can include a tributary as a discrete lateral inflow, which the othermethods cannot do in a simple way,

• the hydrologic approach greatly improves computational efficiency and speed, andreduces the amount and detail of field data traditionally needed for hydraulic routing,

• the parameters of the model are physical based, the scheme is stable with properlyselected coefficients,

• the method has been shown to compare well against the full unsteady flow equationsover a wide range of flow situations,

• it produces consistent results in that the results are reproducible with varying gridsolution,

• it is comparable to the diffusion wave routing,• it is largely independent of the time and space intervals when these are selected within

the spatial and temporal resolution criteria,

The major limitations are:

• it cannot account for backwater effects,• the method begins to diverge from the full unsteady flow solution when very rapidly

rising hydrographs are routed through flat channel sections,• a disadvantage with the Muskingum-Cunge method arises when there is a disturbance

such as a tide affecting the flow in the river upstream of the downstream boundary,• it does not accurately predict the shape of the discharge hydrograph at the

downstream boundary when there are large variations in the kinematic wave speed,such as due to the inundation of a large flood plain.

Basic Flow Equations

The outflow hydrograph at the downstream end is calculated using the following formula.

4131

2111 CQCQCQCQ n

jnj

nj

nj +++= +

+++ (24)

where,

,21 D

tKxC

∆+= (25)

,22 D

Kxt

C−∆

= (26)

,2)1(

3 D

txKC

∆−−= (27)

,4 DxtqC ∆∆= (28)

.2

)1( txKD ∆+−= (29)

where,


Q = dischargeK = travel time in secondsx = weighting factor, 0 <= x <= 0.5∆x = subreach length∆t = time intervalq = lateral flowc = wave celerity

The parameters of K and x are expressed as follows (Cunge, 1969 and Ponce, 1978):

cxK ∆= (30)

∆−

=

xcBSQ

x12

1(31)

where,

B = top widthS = the channel slope.

Solution of Flow Equations

The outflow hydrograph is iterative and is calculated based on equation 24, the routingcoefficients (Cl, C2, C3, C4) are re-calculated for every distance step ∆x and calculationtime step ∆t.

Numerical Stability∆t and ∆x are chosen internally by the model for accuracy and stability.

∆t is selected as the smallest of the following 3 rules:

1. the user defined computation interval, DT,

2. the time of rise of the hydrograph divided by 20,

3. the travel time of the channel reach.

The model checks the difference between the computational time interval (DT) and thetime increment of the inflow hydrograph (SDT). If DT is less than SDT, the inflowhydrograph will be interpolated. The calculation time step must be equal or less than theinflow hydrograph SDT.

A computational space increment ∆x can be equal to the length of the entire routing reachor to a fraction of that length. It is initially selected as the entire reach length. If the size ofthis space increment does not meet the accuracy criteria for flow routing given by Ponceand Theurer (1982), it is re-evaluated by subdividing the length of the routing reach intoeven subreaches that produce ∆x’s that satisfy the accuracy criteria.

where,

Theory Reference 53

+∆<∆

BSCQtcx

21

(32)

where,

)(5.0 BpB QQQQ −∗+= (33)

QB = baseflow from the inflow hydrographQp = peak flow from the inflow hydrograph

The Courant (C) number can be defined as:

xtcC

∆∆= (34)

Main and overbank channel portions are separated and modelled as two independentchannels. Right and left overbanks are combined into a single overbank channel.Momentum at the flow interface between the two channel portions is neglected, and thehydraulic flow characteristics are determined separately, for each channel portion. At theupstream end of a space increment, the total inflow discharge is divided into main channeland overbank flow components. Each are then routed independently, using the previouslydescribed routing scheme. The flow redistribution between the main and overbankchannels is based on Manning's equation.

Data Requirements

Data required for the Muskingum-Cunge method are as followings:• channel length• main channel bed slope• floodplain bed slope• beta parameter (a function of the storage-discharge curve)• channel cross section data• number of cross section segments• Manning roughness coefficient


Simulation Results

The channel routing in Visual OTTHYMO was tested using a natural channel, 5200 mlong, main channel bed slope is 0.001, Manning's n is 0.03, floodplain bed slope is 0.001,Manning's n is 0.05, no lateral flow, the cross section parameters are shown in Figure 22.

Figure 22: Natural Channel

The simulation results from Visual OTTHYMO-MC are compared with the completeunsteady flow equation (SWMM-EXTRAN) and Visual OTTHYMO-VSC and are shownbelow in Figure 23.

Figure 23: Comparison of Test Results

The results show that the Muskingum-Cunge (MC) routing method compares very wellwith the complete unsteady flow equations of EXTRAN. The peak discharge is attenuatedslightly more from EXTRAN than that from the MC method; however, the time to peak forboth methods is the same. The difference in peak discharges could be due to the fact thatthe inertial terms in the complete unsteady flow equations are becoming more dominantwhen rapidly rising hydrographs are routed through the flat channel, compared to the bedslope, as the channel slope is decreased. The Muskingum-Cunge routing method does

Theory Reference 55

not account for the inertial effects, and consequently the method tends to show morediffusion than what may actually occur.

Design Storms For Stormwater ManagementStudies

Flow simulation for urban drainage studies is mostly done with one-event simulationmodels. The single event models determine flows produced by a single storm event.Continuous simulation models require rainfall data over a continuous period for the desiredlength of analysis. A frequency analysis is then conducted on the peak flows so that a flowof a desired return period may be found.

The flow with a single event model may be found by using a series of selected historicalevents or by using a ‘design storm’. The historical storm series may be selected using acontinuous simulation program or by analyzing a rainfall record using a selection criteria.Each event in the selected series is then run through the event simulation model. Thegenerated peak flows are then analyzed to determine their return period.

Design storms or model storms are single event rainfalls that are assumed to produceflows of a desired return period. They are of two types; synthetic design storms andhistoric design storms. Synthetic design storms are storms developed from intensity-duration-frequency (IDF) curves. Historic design storms are large single storm events;usually containing the maximum precipitation on record. In southern Ontario, hurricaneHazel is used as an historic design storm. In this text only synthetic design storms areexamined.

Each design storm has a unique temporal variation of intensity. Two general methods areused to determine the hyetograph shape. The first method derives the storm patternbased on an IDF curve. The design storms using only an IDF curve are the Uniformdesign storm, the Composite design storm and the Chicago design storm. The secondmethod obtains the temporal structure of the design storm from an analysis of historicstorm events. These are the U.S. Soil Conservation Service (SCS) 24-hour design storm,the SCS 6-hour design storm, the Illinois State Water Survey (ISWS) design storm, theAtmospheric Environment Service (AES) design storm, the Flood Studies Report (FSR)design storm, the Pilgrim and Cordery design storm and the Yen and Chow design storm.Design storms that are not discussed are the Sifalda design storm, the Hamburg designstorm and the Desordes (French) design storm. A more detailed description of eachdesign storm is contained in the Design Storm Profiles section of this document. Table 2summarizes the main characteristics of these design storms.

Each of the design storms has a different hyetograph shape. Storm hyetographs wereconstructed and compared for some of the design storms. A five year return period wasselected and the storm volumes were obtained from the Bloor Street station (Toronto) IDFcurve. The duration of the storms are not all the same, for this reason the storm volumesare different. The storm hyetographs for the Uniform, Composite, Chicago, SCS 24-hr.,ISWS, AES, FSR, and Yen and Chow design storms are shown in Figures 24 and 25.

All of the design storms are different. The peak intensities, storm profiles, durations andvolumes vary even though they all have the same return period. The Uniform designstorm has the lowest intensity. It has a constant intensity and is not recommended for usewith an event simulation model. The Chicago design storm has a high peak intensity. Thepeak intensity of this storm depends on the time step one selects. In Figure 25 the timestep was increased from 5 to 10 minutes this reduced the peak intensity by 29% from 168mm/hr to 120 mm/hr. The FSR and Composite design storms also have high peakintensities, but their shapes are not similar. The ISWS, SCS 24-hr., AES and Yen andChow design storms have peak intensities that are in the same range. The peak rainfallfor the SCS 24-hour and the Yen and Chow storms that were computed are the same.The wide variety of hyetograph profiles is why design storms of the same return period willnot produce the same peak flows. Studies are therefore required to determine if designstorms can be used to predict flows of a desired return period.


Table 2:Summary of Design Storm Characteristics

DesignStorms

DesignReturnPeriod

StormDuration

TotalRainfallDepth

TemporalDistribution

AntecedentMoisture

Conditions

IntendedApplication

Uniform Userspec.

tc i∗tc No variationin intensity

No Sewer sizing

Composite Userspec.

td i∗td User selected No Sewer sizing

Chicago Userspec.

Usuallybetween 2-

6 hrs. T

i∗T Based on anIDF curve

No Sewer sizing

SCS 24-hr.

Userspec.

Longduration

usually 12-24 hrs. T

i∗T Tabulatedtype 1 & 2

distributions

Yes Rural watersheds

SCS 6-hr. Userspec.

6 hrs. Given inmaps

Tabulated Yes Design of smalldams

ISWS Userspec.

1 hr. I∗1 Huff 1st

quartile 50%distribution

No Sewer sizing

AES Userspec.

1 or 12 hrs.T

i∗T Regionalcharts for the1 and 12 hr.

durations

No Not specified

FSR Userspec.

12, 30, 60,120 min. td

i∗td 50% summerprofile

Yes Non urban studies

Pilgrim &Cordery

Userspec.

User spec.T

i∗T Local analysisof stormevents

Yes Urban and ruralareas

Yen &Chow

Userspec.

tc i∗tc Triangular No Drainage facilitiesin small areas

tc – time of concentrationT – user selected storm durationtd – storm duration selected using an iterative procedure. The design storm is tested using differentdurations. The one with the largest peak flow is selected.i – average intensity for the return period and selected duration.

Figure 24: Comparison of Design Storms

Theory Reference 57

Figure 25: Comparison of Design Storms

A review of previous studies showed that there is contradictory opinion regarding the useof design storms. Marsalek (1978) does not recommend the use of design storms whilethe results of Arnell (1982) and Watson (1981) suggest that design storms should be used.Other researchers have concluded that further studies are required.

Those who recommend the use of design storms consider that their advantages outweighthe shortcomings. The advantages of using design storms are that:

1. They are an inexpensive procedure for obtaining flows of a desired return period.

2. If properly selected they give conservative results for peak flows and volumes.

3. They are widely used in current engineering practice.

Some of the disadvantages of design storms are that:

1. The runoff frequency is assumed to be the same as the rainfall frequency. Thisequivalence of return period has not been shown to be true.

2. The rainfall volume is not the rainfall volume of real storm events.

3. Using IDF relationships to obtain a design storm hyetograph may be incorrect.

A study was conducted using IMPSWM procedures to test two design storms commonlyused by Canadian engineers. The uniform design storm was not tested because of itslow, unrealistic intensity. The Chicago and the SCS 24-hr design storms were selected forthe study. The AES design storm could have been used but the 30% profile with a 1-hourduration gives peak flow results close the Chicago storm and historical storm flows


(Wisner and Gupta, 1980). With a 12-hour duration the AES 50% profile gives resultssimilar to the SCS 24-hr. design storm. The third chapter contains the results thatcompare the Chicago and the SCS 24-hr design storms. The methodology can be usedfor any other design storm by a municipality.

Methodology Of Design StormsThe researchers comparing peak flows from design storms and historical storms used differentcatchments and different simulation programs. A comparison of the peak flow frequencyresults for the Chicago design storm is presented first in this chapter. The results for this stormare summarized in Table 3.

A study conducted in the IMPSWM program is also presented here. The methodology used tocompare the Chicago and SCS 24-hr design storms with the historical storms is given in thesecond section of the chapter.

Results for the Chicago Design Storm

J.F. McLarens Ltd. (1978) has conducted studies on catchments in Edmonton andWinnipeg. They found that the ratio of the Chicago storm peak flow to the flows from anhistoric storm series ranged between 1.0 and 1.2. It was recommended that the Chicagodesign storm be used for urban drainage design.

Marsalek (1979) developed a Chicago design storm for the Burlington area. He found thatthe peak flows produced from the Chicago design storm are 80% larger than thoseproduced from historical storm events. He also found that the peak flow was attenuatedas the catchment size increased. The peak flow increased as the catchmentimperviousness increased but the peak flow overestimation remains at approximately80%. These results were analyzed in the IMPSWM program by Wisner and Gupta (1980).They concluded that discrepancies can be reduced if the peak intensity of the designstorms are reduced to values in agreement with measured peak intensities.

Watson (1980) compared the peak flows obtained from the Chicago design storm andhistorical storm events. A 2-hr. duration and a non-dimensional time to peak of 0.28 isused to develop the Chicago storm. The rainfall data is discretized at 5 min. intervals.

Table 3:Summary of Results from Pervious Studies (Average percentage difference between

the Chicago Design Storm and Historical Storm Flows)

Study Catchment Chicago DesignStorm

Arnell (1980) BergsjonLinkoping 1Linkoping 2

-2.2%10.3%6.0%

Marsalek (1979) Burlington(area 26 ha, imp.30%)

80.0%

Watson (1980) PinetownKew

2.0%-5.0%

Watson found that on the Pinetown catchment the peak flows from the Chicago stormagreed closely with those from the historical storms. The agreement for the Kewcatchment was not quite as good. The peak flow is slightly underestimated. It is within95% confidence interval bands of the historical storms, though. The Kew catchment isless impervious than the Pinetown catchment; therefore, it is more sensitive to antecedent

Theory Reference 59

moisture conditions.

Arnell (1982) used a Chicago design storm with a 4-hr duration. The non-dimensionaltime to peak, r, is 0.43 if the return period is less than 1 year. If the return period is greaterthan 1 year, r is 0.35. The Chicago storm is developed with a step size of one minute.

Arnell found that the Chicago design storm overestimated the peak flow by approximately5%. On the Bergsjon catchment, the peak flow is underestimated by 2.2%. On theLinkoping 1 and Linkoping 2 catchments the flow is overestimated by 10.3% and 6%respectively. The Bergsjon catchment was the smallest of the three catchments. TheChicago storm produces peak flows almost identical to the historical storms on thiscatchment.

With the exception of Marsalek (1979), the estimation of peak flow produced by theChicago storm gave acceptable results compared with that produced by historical storms.Differences range from a 2% underestimation to an 10% overestimation of peak flow.Watson recommended that the Chicago design storm be used for peak flow design. Arnellalso found that the deviation of the Chicago Storm peak flow values from the historicalstorm peak flow values are not large. He concludes that the Chicago Storm shouldoverestimate peak flows because of the way it is developed. He does not recommend theuse of the Chicago design storm because of the large overestimation of peak flowMarsalek found.

Methodology for Comparing Design Storms and a Historical StormSeries

Rainfall Input

The rainfall inputs used with the event simulation models were a historical storm seriesand two design storms. The historical storm series was selected from the Bloor Streetstation rainfall record. A criteria was selected based on the storm volume and intensity sothat approximately one storm event for each year in the record was chosen. This resultsin some years having more than one event and other years having no events. Theselected events were then discretized to ten minute time intervals. A summary of thestorm events and their characteristics is given in Table 4.

The SCS 24-hour and Chicago design storms were compared with the historical stormseries. The design storms were developed from the Bloor Street station IDF curves forreturn periods of 5, 10 and 25 years. The Chicago storm was 4 hours in duration and wasdiscretized at 10 minute intervals. The SCS storm was 12 hours in duration and wasdiscretized at 12 min. interval. The peak intensity and antecendent moisture conditionsshould be adjusted so that the design storm resembles real storm conditions.

The adjustment is necessary on urban catchments because the peak flows are dependenton the peak intensities. The scattergram in Figure 26 shows that the correlation betweenpeak flows and peak intensity is close to 1 in urban areas.

The choice of the time step is important in obtaining a peak intensity close to the peakintensity of real storms. An analysis was conducted to demonstrate the importance of thetime step used with a design storm. The 5 and 10 min. intensities were extracted for thehighest recorded storms in Toronto (Hogg,1980). These were plotted along with the peakintensities of the Chicago design storm peak intensities discretized at 5 and 10 minuteintervals (Figure 27a, 27b). For the 5 minute intensities, the Chicago design stormintensities are higher than the real storm intensities, while for the 10 minute intensities theyare slightly lower. Wisner and Gupta (1979) show that there can be a large variation inflows depending on the step size chosen for the design storm. Time steps between 10and 20 minutes are recommended for use with the Chicago design storm. If the designstorm peak intensity is still larger than that of real storms it should be adjusted so that thetwo peak intensities are similar.


Table 4:Historical Storm Characteristics

Date Duration(hrs.)

Volume(mm.)

Time toPeak(hrs.)

PeakIntensity(in./hr)

AverageIntensity(in./hr)

API(mm)

CN*

Sept. 15/57 6.67 47.84 4.167 59.18 7.19 19.8 37.5July 9/60 5.50 62.33 5.000 82.37 11.33 12.3 24.0June 19/61 6.17 37.12 2.867 49.28 6.02 24.5 45.0Sept. 13/62 2.00 42.62 0.167 159.26 30.28 13.2 25.0Nov.9-10/62 12.00 58.03 5.867 17.83 4.88 14.0 27.0Aug. 11/64 6.50 40.61 4.167 39.62 6.25 12.7 24.5Aug. 5/68 4.67 42.38 4.167 70.64 9.09 10.5 19.0Aug.22/68 9.00 72.90 3.167 58.62 8.10 31.6 53.5Aug.29-30/70 3.50 67.60 11.867 92.25 14.15 8.1 15.0May 16/74 12.00 58.32 8.500 56.34 4.85 49.4 74.0Aug. 23/74 0.67 51.20 0.333 153.62 76.81 5.5 8.0Aug. 23/75 9.17 57.22 2.667 77.72 6.25 7.7 14.0July 6/77 7.17 51.10 7.167 50.29 7.41 28.0 49.0July 31/77 0.87 45.19 0.333 156.77 54.23 16.2 31.0Sept. 24/77 10.67 60.96 8.333 26.24 5.99 24.8 45.1

The antecedent moisture conditions are usually not considered as being important when adesign storm is used with an event simulation model. Some studies, though, have beenconducted to investigate this. Wenzel and Voorhees (1979) tested design storms usingboth wet and dry antecedent moisture conditions, but they do not recommend a procedurefor determining what conditions should be used with a design storm. The Flood StudiesReport (NERC,1975) present a procedure for determining the antecedent moistureconditions. Using a relationship between the Urban Catchment Wetness Index and theStandard Average Annual Rainfall the antecedent moisture conditions can be determinedfor the FSR design storm in any area in the U.K. In the present study the modified curvenumber is used to represent the antecedent moisture conditions. With the OTTHYMOmodel an average modified curve number, for a watershed, is used with the designstorms.

Theory Reference 61

Figure 26: Correlation between Peak Rainfall and Flows (Urban Area 50% Imp).


Figure 27: Comparison of Real Storm and Chicago Storm Intensities

Theory Reference 63

Watersheds Studied

Three different types of watersheds were examined in this study; a rural watershed, anurban watershed and a mixed land use watershed. Two rural watersheds in southernOntario were tested, one was large and had an area of 6540 ha. the other was smallhaving an area of 44 ha.

Simulation runs were also conducted on three southern Ontario urban watersheds. Thecatchment characteristics are summarized in Table 5. On two of the catchments the urbanarea routine URBHYD of OTTHYMO was used. Impervious conditions of 35% and 50%were used on urban catchment No.1 to observe the change in difference between thedesign storm peak flows. On the third urban catchment the SWMM simulation programwas used. A schematic of this catchment is shown in Figure 28.

Table 5:Urban Watershed Characteristics

Watershed Area(ha.)

Imperviousness(%)

No. 1 294.4 35 & 50No. 2 290.3 50No. 3 150.5 30

A mixed land use watershed in Metropolitan Toronto, having a rural area of 1597 ha. andan urban area of 5536 ha., as also tested. The flows on this watershed were found usingthe OTTHYMO model. The total area contained 21 urban subwatersheds and 12 ruralsubwatersheds. They ranged in size from 53 ha. to 778 ha. The urban subwatershedshad an imperviousness of 35%.

Figure 28: Schematic of Catchment no. 3


Simulation Models and their Calibration

The two, single event simulation models used to test the design storms were the SWMMand the OTTHYMO models. The SWMM model was used on urban catchment No.3. TheChicago design storm with a 10 min. time step, was compared with a series of flowsgenerated from real storms. The antecedent moisture conditions are accounted for usingvalues for the Horton equation’s initial infiltration capacity and final infiltration capacity.

The lumped OTTHYMO model was used on the rural, urban and mixed watersheds. Therural watershed in southern Ontario is used as an example to show how the model wascalibrated. The catchment data was first obtained and used to compute the time to peaktp and the storage coefficient K. The initial abstraction Ia was found by examining therainfall record and the stream flow record and distinguishing between the rainfall eventsthat produce runoff and those that do not produce runoff. The relationship between themodified curve number CN* and the antecedent precipitation index API was found usingfive storm events for which there were discretized rainfall measurements as well as streamflow records. Simulations were then conducted and the generated hydrographs werecompared with the measured hydrographs and found to be similar. The model wastherefore properly calibrated.

Results Of Peak Flows From Design Storms And Historic StormEvents

Rural Watersheds

In this study design storm and historic storm flows were generated on large ruralwatersheds. The results from the historic storm events were examined to determine if thepeak intensity or the antecedent moisture conditions influence the flows on a ruralcatchment. The peak intensity was found to be independent of the peak flows (Figure 29).On the other hand the antecedent moisture conditions, as measured by the API, arecorrelated with the peak flows. The correlation coefficient between the API and the peakflows is close to 1.

The flow frequency curves for the large rural watershed is shown in Figure 30. The SCS24-hour design storm flows are greater than those given by the real storm series. On thelarge rural watershed, in Southern Ontario, the flows are overestimated by 5% to 10%.Using the SCS 24-hour design storm with an average CN* resulted in flow frequencycurves slightly larger than-the historic storm series flows on the rural watersheds.

The Chicago design storm was also tested on these catchments. It had a shorter durationthan the SCS 24-hour storm but a higher peak intensity. The Chicago design storm gavelower flows than those from the historic storm series. The flows on the southern Ontariowatershed were from 2.9% to -27.3% different from the historical storm flows. On thesmall rural watershed the 100 yr. Chicago and SCS 24-hr. design storms gave almostidentical peak flows. The flows were 1.28 cms and 1.37 cms respectively. The Chicagodesign storm flows produced lower flow frequency curves than the historic storm seriesflows on the rural watersheds.

This comparison of the design storm flows on the rural watersheds shows that the SCS24-hour design storm gives a good prediction of the peak flow frequency curves. Theimportance of antecedent moisture conditions in determining the peak flow was alsodemonstrated. On rural catchments the OTTHYMO model uses an average CN* with thedesign storm. This was found to give good predictions of the peak flow when used inconjunction with the SCS 24-hour design storm.

Theory Reference 65

Figure 29: Rural Areas(a) Correlation between API and Peak Flows.

(b) Correlation between Peak Rainfall and Flows.


Figure 30: Flow Frequency Curves for a Rural Watershed in SouthernOntario

Urban Watersheds

The dependence of the historic storm flows to the peak intensity and antecedent moistureconditions were examined on the urban catchments. The peak intensity was found to bean important factor in determining the peak flows. The correlation between the peakintensity and peak flows was close to 1 (Figure 29). Antecedent moisture conditions donot show any correlation with the peak flows (Figure 31). It was found that runoff fromurban catchments was independent of CN*.

The SCS 24-hour design storm flows were compared with the historic storm series flowson urban watersheds No.1 and No.2. The comparison of the flows is shown on Figures 32and 33. The SCS 24-hour design storm underestimated the flow on both of thewatersheds. On watershed No.1 the flow was underestimated by 11% to 22% while onwatershed No.2 from 14% to 26%. On these urban watersheds the SCS 24-hour designstorm has underestimated the peak flow.

The Chicago design storm was tested on the three urban watersheds. The flowpredictions given by this storm are slightly below the historical storm series on urban areas

Theory Reference 67

No.1 and No.2 (Figures 32 and 33). The flows were underestimated by approximately 6%on watershed No.1 and 4% on watershed No.2. On urban watershed No.3 the SWMMsimulation program was used. The Chicago storm gave results that were similar to thereal storm flows (Figure 34). From the tests conducted on these three urban areas theChicago design storm gave peak flow predictions close to the flows from the historic stormseries.

The effect of changing the catchment imperviousness was examined on urban watershedNo.1. Flows were generated on this catchment using the design storms and the historicstorms for impervious conditions of 35% and 50%. The flow frequency curves for the twoimpervious conditions are shown in Figure 34. Increasing the catchment imperviousnessresulted in the SCS storm giving lower flows with respect to the real storm series. Therelative position of the Chicago storm flows did not change. The Chicago storm is lesssensitive to changes in the catchment imperviousness than the SCS 24-hour storm.

The comparison of the design storms on the urban catchments has shown that theChicago storm gives a good prediction of peak flow on urban catchments. Flows in theurban areas were found to be dependent on peak intensity and independent of antecedentmoisture conditions. The sensitivity of peak flows to peak intensities showed theimportance of having design storm peak intensities similar to real storm peak intensities.The Chicago storm that was used obtained the peak intensities by having a 10 minute stepsize. Increasing the imperviousness of urban catchment No.1 demonstrated that theChicago design storm continued to give a good prediction of the peak flow. For thesereasons the Chicago design storm may be used with single event simulation models onurban watersheds.

Figure 31: Urban Area (50% Imp.) Correlation between API and Flows


Figure 32: Flow Frequency Curves, Urban Watershed No. 1, 35% and 50% Imp.

Theory Reference 69

Figure 33: Flow Frequency Curves, Urban Watershed No. 2, 50% Imp.


Figure 34: Comparison of Real Storm and Chicago Storm Flows, Urban CatchmentNo. 3

Mixed Land Use Watershed

The mixed land use watershed tested combines both urban and rural areas. The previoussections have shown that the SCS storm can be used on larger rural areas and theChicago storm on urban areas. It would not be satisfactory to use the SCS storm on therural segments and the Chicago storm on the urban areas of the mixed watershed. Boththe Chicago and the SCS 24 hr. storm were tested on this watershed.

The flow frequency curves for the design storms and the historic storms are shown inFigure 35. The SCS design storm underestimated the flow by 11% for a 5 yr. return periodand overestimated the flow by 18% to 39.9% for return periods of 10 to 100 years. TheChicago storm was found to give better estimates of the peak flow; for a 5 year returnperiod the flow was, underestimated by 5% and for return periods from 10 to 100 years theflow was overestimated by 0.3% to 28.0%.

Theory Reference 71

The flow predictions made on this watershed with the design storms were not the same asthe historical storm series estimates. This shows that the peak flow cannot be obtained byusing an arbitrarily selected single design event. An analysis using a historical stormseries should be conducted before a design storm is selected. In general, it is moredesirable to use a historical storm series on this type of watershed.

Figure 35: Flow Frequency Curves, Mixed Landuse Watershed

ConclusionsSimulations with the Chicago and SCS 24-hr design storms were compared with a seriesof real storms to determine if the flow frequency results they produce are reliable. Thedesign storms and a historical storm series were tested on rural, urban and mixed landuse watersheds using the OTTHYMO model. For the historical storms antecedentmoisture conditions were accounted for using the modified curve number CN*. Accordingto the IMPSWM methodology the antecedent precipitation index, API, is determined at thebeginning of a storm event. The modified curve number is then found by using the API vs.CN* relationship for the watershed. In addition, the SWMM model was used with theChicago design storm on an urban area. The conclusions of the study are listed below.


1. In rural areas antecedent moisture conditions were shown to be critical indetermining peak flows.

2. A modified curve number was calibrated which in conjunction with the SCS 24-hr.design storm gives adequate results on rural watersheds. This "design curvenumber" is proposed to be part of the design storm concept.

3. Using the "calibrated design curve number" on large rural watersheds it was foundthat Chicago design storms underestimate the flow. The SCS 24-hr design stormgave good flow predictions on both large and small rural watersheds. For a smallrural watershed the difference between the Chicago and SC-S 24-hr peak flowwas found to minimal.

4. For urban areas the SCS 24-hr design storm underestimated the peak flow. TheChicago storm gave consistent peak flow results. It was also demonstrated thatthe peak intensity is an important factor in determining he peak flows in urbanareas. For this reason design storm peak intensities should be selected on thebasis of a study of peak intensities of critical real storm events.

5. On the large mixed land use watershed antecedent moisture conditions must beconsidered if the contribution of the rural watershed is important. For flood controlpurposes in large mixed areas it is preferable to use a series of real storms. Onecannot tell prior to analysis of the watershed which design storm will produceacceptable results.

6. Results found for the homogeneous watershed indicate that for routine SWMstudies design storms can be used. It is however desirable to compare the twotypes of design storms and eventually a critical historic storm.

A Review Of Design Storm Profiles

Intensity Duration Frequency CurvesIn Canada, the Atmospheric Environment Service (AES) of Environment Canada does thecollection of rainfall records. The rainfall amounts are collected using a tipping bucket raingauge. The rain gauge typically records every 0.01 inches of rainfall. The AES standardprocedure locates an Type B Standard Rain Gauge (AES standard) together with thetipping bucket and adjusts the tipping bucket data so that the daily totals of both gaugesagree. In the United States the U.S. Weather Service collects data which is then storedand analyzed at the National Climate Centre (NOAA, Environmental Data Service AshvilleN.C.). World wide meteorological information may be obtained from the WorldMeteorological Organization.

The AES analyzes the daily records obtained from the tipping bucket rain gauge. In thecase of strip charts, these are analyzed to determine the maximum rainfall volumeoccurring in 5, 10, 15, 30, 60, 120, .360, 720, and 1440 minute intervals during each dayof a year. The daily record of maximum volumes is scanned at the end of each year todetermine the annual maximum volumes for each of the time periods. This analysis isconducted for every year of the rainfall record to form an annual maximum series. Eachset of volumes is divided by the appropriate fixed duration to obtain the annual maximumseries for intensities.

The return period of extreme intensities is of special interest to engineers who want tocalculate the return period of the rainfall-induced peak flows. The return period can bedetermined using a plotting position formula or by using an extreme-value distribution. Forexample, the Atmospheric Environment Service uses the extreme value type 1 distribution.The statistical parameters of mean, standard deviation and skew are computed for eachset of annual maximum intensities. The intensity of a particular frequency is computed

Theory Reference 73

using the statistical parameters and the frequency factor form of the probability distribution(Chow,1954). For each duration, the intensities for the desired return periods arecomputed. The points of intensity and duration for known return periods are plotted. Linesjoining all the points having the same return period are drawn to form an IDF curve. Anexample of the intensity-duration-frequency (IDF) curves developed by the AES for theBloor Street station in Toronto are shown in Figure 36.

Engineers have also found advantageous to fit empirical equations to the statisticallydeveloped IDF curves. Equations 35 and 36 are the general forms of the empiricalequations used most frequently.

bctai

)( += (35)

ctai b +

= (36)

To describe the variation in average intensity with duration for a given frequency, theparameters a, b and c must be determined. This is done by fitting the either of the twoequations to the statistically determined values for i and t. The value of b is estimated,then the following equation can be solved for the constants c and ln(a).

)ln()ln()ln( btai +−= (37)

The values of a, b and c are unique for each IDF curve. They cannot be used to developIDF curves of different frequencies or IDF curves at different locations.


Figure 36: Bloor St. Station

Theory Reference 75

Frequency Of Real Storms And Synthetic StormsThe design storms developed from IDF curves are not representative real storm events.This is because the method of analyzing rainfall to obtain the IDF curves is independent ofthe real storm events. Each annual maximum volume for the 5, 10,, 15, 30, 60, 120, 360,720 and 1440 minute duration may come from a different storm event. This isdemonstrated in Figure 37. For the series of storm hyetographs shown, the maximum 5min. volume occurred in event 4, and the maximum 10 min. volume in event 2. For timeperiods such as 12 hrs. and 24-hr., the maximum volume may come from more than onestorm event.

Figure 37: Real Storm Hyetographs

In a real storm event, the maximum average intensities for fixed duration within the eventhave different frequencies. Real storm events do not have a single frequency. Forexample, event 4 has a return period of 2 yrs. for the 5 min. duration, a return period of 1year for the 10 minute duration, and a 2 year return period for the average intensity of thetotal event duration. The frequency of average intensity for different duration in somestorms may vary from 1 yr. to 50 years. Design storms on the other hand are developedso that the maximum average intensity for each fixed duration in the design storm has thesame frequency. Since design storm are developed from IDF curves, they contain themaximum average intensities from different storm events. Design storms are thereforemodels of real storms and their validity requires testing.


Uniform Design StormThe uniform design storm is the oldest and simplest design storm. The storm originatesfrom the use of the rational method. By selecting a duration and return period, theaverage maximum intensity is found from an intensity .duration frequency curve. Theintensity remains constant for the duration of the storm.

The uniform design storm contains only a part of the real storm volumes. The volume ofthe uniform design storm is obtained from an IDF curve it is not the volume from a realstorm event. The uniform hyetograph does not show any variation in intensity with time.Real rainfall events have intensities that are highly variable, this variability affects the peakflow.

Composite Design StormTo develop this design storm, the storm duration is first selected. Average intensities arethen found by reading an IDF curve at selected duration shorter than the storm duration.The selected duration should be separated by a constant time step. Next, theaccumulated storm volume is computed by multiplying each duration by the averageintensity. The incremental rainfall volumes and intensities are computed and thenarbitrarily rearranged to form a storm pattern. If the incremental intensities are notrearranged a front-end hyetograph, following the shape of the IDF curve, is obtained.

Chicago Design StormThe Chicago design storm is a design storm distribution widely used by practisingdrainage engineers. This representation of the temporal distribution of rainfall wasproposed by Keifer and Chu in 1957. They developed a storm pattern which wouldpreserve the maximum volume of water falling within a specified duration, the averageamount of rainfall before the peak intensity and the relative time of the peak intensity.

To determine the time distribution of rainfall and preserve the previously mentionedcharacteristics, they adopted the empirical IDF curves. By using IDF curves, they stayedwith a procedure and concepts engineers were familiar with and is simple to obtain andtherefore, it has become widely accepted for use in engineering practice.

Derivation of the Chicago Design Storm

The Chicago design storm is developed from empirical IDF relationships. Keifer and Chuobserved that if they had a continuous function representation of the instantaneousintensity and they integrated this function over a given duration, the rainfall volumeoccurring in that duration is obtained. The rainfall volume divided by the duration gives theaverage intensity. Therefore, multiplying the average intensity by the storm duration anddifferentiating with respect to the duration they obtained expressions for the instantaneousintensity:

1)())1((

+++−= cbtbtcai (38)

2)())1((

ctctbai b

b

++−= (39)

These equations for the shape of the storm hyetograph are for a storm pattern where thepeak is at the beginning of the storm event. If the peak occurs after the beginning of thestorm, the storm duration t is divided into two time periods, the time before the peak tb andthe time after the peak ta. The ratio of the time to peak to the total storm duration is givenby the equation:

Theory Reference 77

ttr b= (40)

The storm duration in terms of the time before the peak and the time after the peak isgiven by equations:

rtt b= (41a)

rtt a

−=

1(41b)

The storm hyetograph shape is given by:

( )b

a

a

b

crt

crtba

i +

+

−

+

−−

= 1

1

11

(42)

( )b

a

a

a

crt

cra

tbai +

+

−

+

−−

= 1

1

1(43)

A typical shape for the Chicago design storm hyetograph is shown in Figure 38, where theintensity before the peak is given by ib and the intensity after the peak is given by ia.


Figure 38: Chicago Design StormParameter Estimation

The procedure for determining the constants for the empirical IDF curve was discussedpreviously in the Intesity Duration Frequency Curves section. The only constant that isrequired for the Chicago design storm is the ratio of the time before the peak to the stormduration, r. Two procedures may be used to determine the value of r:

1. The ratio of time to peak intensity to the storm duration is computed for a series ofevents for various duration. For a given duration, the average time to peak isdetermined from a number of rainfall events of that duration. This is done for a setof duration. The mean value of the time to peak to the storm duration ratio iscomputed as a weighted average. The following equation (is an example of how ris computed.

321

332211 )()()(

ddd

pdpdpd

ttttttttt

r++

++= (44)

where, tdl, td2, td3 are the durations of the different rainfall occurrences, tp1, tp2, tp3 are theaverage time to. peak for the different rainfall occurrences and r is the ratio of time to peakto total storm duration.

2. The ratio can be computed by analyzing local storm distributions to determine therainfall depth before the peak intensity. The design storm duration is then chosen.

Theory Reference 79

The rainfall depth prior to the peak is determined for durations less than thedesign storm. A weighted average for r is determined based on the ratio of theantecedent rainfall depth to the total rainfall depth. This is done by using:

)( ttdda tIItrd −= (45)

whereda = depth of antecedent rainfall,td = design storm duration,t = rainfall duration,Itd = maximum average intensity for the total duration,It = maximum average intensity for duration t.

Specific applications of each procedure have been documented. For example,McPherson (1958) has criticized the second procedure while both have beendemonstrated by Bandyopadhyay (1972). The later obtained a value 0.416 for the firstprocedure and an r value of 0.37 for the second. Table 6 presents a list of r valuesobtained by different researchers.

Table 6:Values of r for the Chicago design storm

Location R Source

Baltimore 0.399 McPherson, 1958

Chicago 0.375 Keifer and Chu, 1957

Cincinnati 0.325 Preul et al., 1973

Ontario 0.488 Marsalek, 1978

Philadelphia 0.414 McPherson, 1958

Determination of the Chicago Design Storm Hyetograph

The steps to produce a Chicago design storm Hyetograph are:

1. Select a design storm duration.

2. Select a time step . The time step size should not be less than the minimumduration that was used when determining the IDF curves. For the Canadian IDFcurves, step size should be at least 5 minutes. It is recommended that the stepsize should be about 10 minutes.

If the parameters a, b, c and r for the Chicago design storm are not known they should bedetermined.

The instantaneous rainfall intensities at the different time intervals from the peak intensityare computed.

SCS 24-Hour Design StormThe U.S. Soil Conservation Service (SCS) has developed the ‘Type 1 and 2’ design storm.These have and are used both, in the U.S.A. and Canada. The SCS determined the masscurve for percent of accumulated rainfall depth over a duration of 24-hr. To obtain themass curve, rainfall was analyzed across the U.S.. SCS characterized the rainfall usingthe 2 types of storm patterns: The Type 1 rainfall distribution is applicable to Hawaii,Alaska, the Coastal Sierra Nevada, the Cascade Mountains in California, Oregon andWashington. The type 2 rainfall distribution applies to the remainder of the United States,Puerto Rico and the Virgin Islands. Figure 39 illustrates the SCS 24-hour rainfalldistribution. In Canada, the type 2 curve applies in most areas, however, there are someregions in British Columbia where the type 1 curve is used.


To create a SCS design storm, a duration and return period is first selected. Thecorresponding volume is then distributed over the steepest portion of the SCS-24 hourcurve. The incremental rainfall volumes and intensities are then obtained based on thevolume distribution over the selected duration.

It’s worth noting that the Composite Design Storm can be rearranged to give a stormpattern very similar to the SCS design storm. This occurs because the SCS type 1 andtype 2 curves have been produced so that, for a selected 24-hour rainfall depth , thedepth-duration curve derived from a SCS distribution would be very similar to the curveproduced by the U.S. Weather Bureau.

Figure 39: SCS II – 24 Design Storm

Cronshey (1980) developed regional rainfall distributions to replace the SCS type IIdistribution in the 37 eastern and central states. The distributions consider variations dueto the rainfall return period as well as regional differences. The ratio of rainfall volume in afixed duration to the 24-hour rainfall volume was determined for many stations in theeastern United States. It was found that the rainfall ratios increase with distance from thecoast and that the ratios are lower for a 100-year return period than for a 2-year period.Four regions were identified with similar rainfall ratios and a special distribution wasdeveloped for each. Map are available within each area corresponding to fixed durationsand return period. The maps are for 5, 15 and 60 minute durations and return periods of 2and 100 years. There are also locations in the eastern U.S. where 2 or 3 maps could beused depending on the duration and return period being considered. However, these newdistributions are not significantly different from the SCS Type 2 distribution and they do notapply to Canadian conditions.

SCS 6 Hour Design StormThis is a second type of design storm developed by the U.S. Soil Conservation Service.The SCS 6 hour design storm was developed for designing small dams. The duration is

SCS II - 24 Hours

25

812

18

66

8288

9295

98 100

0

20

40

60

80

100

120

2 4 6 8 10 12 14 16 18 20 22 24

Time (hours)

Percent Distribution (%)

Theory Reference 81

selected as 6 hours or the time of concentration which ever is larger. The rainfall depth isdetermined from maps of probable maximum precipitation or from the 6 hour precipitationdepth for a 100 year return period. The hyetograph profile is determined using a 6 hourdesign storm distribution, an example is shown in Figure 40.

This storm was developed for use in conjunction-with the SCS method of runoffcomputation. With the SCS method the rainfall depth and duration must be known. Thewatershed soil type and antecedent moisture conditions are also important. Thesecatchment characteristics are reflected in the SCS curve number. The-amount of directrunoff is computed using the total precipitation and the curve number. The hyetographshape is not important in determining the direct runoff with the SCS method. It isimportant, though, when the runoff is computed using a single event simulation model.

The rainfall depth for the SCS 6 hour design storm has not been determined usingCanadian meteorological data. It therefore should not be used on Canadian catchmentsunless local data is used. The return period is much larger than is typically used in urbandrainage design. The design storm was developed for rural conditions and shouldtherefore not be used in urban areas.

Figure 40: 6-Hour Design Storm Distribution

Illinois State Water Survey Design StormThe Illinois State Water Survey (ISWS) design storm is based on research conducted byHuff (1967). Huff examined storm events in central Illinois having durations between 3hours to 48 hours. He divided the storms into 4 groups depending on the time period inwhich the majority of the rain occurred. The storms with the most of the rain occurring inthe first quarter of the event duration are termed first quartile. All of the storm events thatexamined were placed in one of the four quartile groups. The rainfall mass curves in eachquartile group were determined for various probability levels (Figure 41). It was found thatshort duration storm events dominate the first and second quartile. For this reason, themedian distribution in the first quartile is commonly used for design. Terstriep and Stall


(1974) recommended that the first quartile storm with the 50% probability level be used asthe design storm with the Illinois runoff model.

The ISWS design storm is determined by selecting a design storm duration. Themaximum depth for the duration and the given frequency are derived from local data orfrom an IDF curve. The rainfall depth is then distributed according to the Huff quartilemedian distribution.

The ISWS design storm is derived in a similar way to the SCS design storm. When it isused in Illinois it has the advantage of being developed from an analysis of local data. Ananalysis of rainfall similar to Huff's analysis is required if the rainfall structure is to beknown for a region other than Illinois.

Theory Reference 83

Figure 41: Cumulative Distribution of Rainfall (Huff, 1967)


Atmospheric Environment Service Design StormThe Canadian Atmospheric Environment Service (AES) examined the temporal variationof rainfall using almost 2000 extreme events in Canada. Hogg (1980) conducted theanalysis of the time distribution of rainfall in short duration events. Rainfall duration of 1and 12 hours were selected for the analysis and to develop Time-Probability curves. Theevents were chosen to have samples from both thunderstorms and large-scale cyclonicorigin. The selected events did not have to be individual storm entities, they could also bepart of a larger storm sequence. Fixed rainfall durations were selected so that the samedefinition would be used to identify rainfall that is used in obtaining depth-duration-frequency curves.

In analyzing the rainfall, the one hour events were divided into twelve 5 min. incrementsand the 12 hour events into 1 hour increments. Rainfall for each event was expressed asa cumulative percentage of, total event rainfall for the twelve equal increments through thestorm. All the events for a particular duration were analyzed and the cumulative rainfalldistributions for different probability levels were computed (Figure 42).

The temporal distribution patterns were found to vary in the different regions of Canada.The temporal distributions for the coastal regions were quite different from the distributionsfor the continental regions. A comparison was made between Huffs 50% distribution forthe second and third quartile storms and those Hogg developed for southern Ontario. Thesecond quartile distribution was up to 25% different in the cumulative rainfall. The thirdquartile distribution does not resemble any of the distributions Hogg computed.

Theory Reference 85

Figure 42: AES Rainfall Distribution


Flood Studies Report Design StormThe Flood Studies Report (FSR) design storm was developed in the United Kingdom.Rainstorm profiles of historical events were examined, and the cumulative percentage ofrainfall for different probabilities of storm peakness was developed. The peakness of astorm was defined as the ratio of maximum to mean intensity. A study of summer andwinter storm profiles showed that summer storms were more peaked than winter storms.The curves of storm peakedness were published by the National Environment ResearchCouncil (NERC,vol.2,1975).

The FSR design storm duration is selected so that the largest peak flow calculated at eachpoint in the system is taken as the design discharge. The developers of the FSR designstorm found that the peak discharge is not very sensitive to the storm duration, a doublingof duration caused a change of less than 10% peak discharge. Therefore, a fairly coarseseries of rainfall duration may be used; values of 15, 30, 60, and 120 minutes arerecommended. The return period is selected to be the same as that of the requireddischarge. This is usually established by the local government agency.

The rainfall depth is determined using a procedure developed for all of the U.K. Usingmaps and a formula, the depth for a particular return period is computed. The depth canalso be computed from a depth-duration-frequency curve.

The antecedent moisture conditions were examined and incorporated into the modeldeveloped by the FSR. The antecedent conditions are expressed by the UrbanCatchment Wetness Index (UCWI). Runoff simulations were conducted on a catchmentusing a variety of UCWI's for return periods between 1 and 10 years. An optimum UCWIwas found for the catchment that resulted in good predictions of the peak flow. Similarstudies were conducted on other catchments in the U.K. An analysis of the optimumUCWI’s and rainfall data for the catchments led to a relationship between the UCWI andthe Standard Average Annual Rainfall (SAAR). This relationship can be used to determinethe UCWI for any location in the U.K.

The FSR design storm recommended for use in the U.K. has the following properties:

1. The storm duration should be that which gives the maximum discharge.

2. The return period of rainfall should equal that of the required discharge.

3. The storm profile should be the 50% summer profile.

4. The UCWI varies as the average annual rainfall and should be determined fromthe Figure shown below.

Some things to consider before applying this type of storm: A) the FSR design storm peakintensity always occurs at the centre of the storm. B) A local analysis of historic events isnot conducted to determine the time-to-peak. C) The FSR design storm was developedfor use in the U.K. only. A similar analysis would have to be conducted if it was to be usedoutside the U.K.

Theory Reference 87

Figure 43: Relationship between UCWI and SAAR

Pilgrim and Cordery Design StormThe Pilgrim and Cordery design storm was developed to provide an approach which wouldproduce storm patterns consistent with the storm patterns of historical events. To developthe Pilgrim and Cordery Design Storm, a duration is selected and a set of events with alarge rainfall for the specified duration are selected. The duration is divided into a numberof time periods. The rainfall volume in each of the periods is ranked and An averageranking for each period is computed. The percentage rainfall in each period is computedand is ranked from the largest to the smallest. An average percentage rainfall is thencomputed for each rank in the rain period. The average percentage rainfall is thenassigned to the average ranking in each period. The percentage rainfall in each perioddetermines the hyetograph shape. It is recommended that 50 events be used in theanalysis.

The storm hyetograph is determined from an analysis of local rainfall data. If the analysishas not been conducted, it would be inconvenient for an engineer to use this design storm.

The hyetograph shape that is obtained using this procedure depends on the number ofinternal divisions that are selected for the design storm duration. The number ofrainstorms that are used in the analysis is also important. Yen and Chow (1980) show thatreal storm events have a very uneven temporal distribution of rainfall. For this reason thevalue of the average ranking of intervals tend to a single number when a large number ofrainstorms are used.


Yen and Chow Design StormAll of the previously discussed design storms were developed from rainfall frequency-duration relationships or from observations of rain gauge records, they did not use astatistical procedure to analyze the historical rainfall record. Yen and Chow (1980) use themethod of moments to statistically determine the geometry of hyetograph. Using atriangular hyetograph representation, only the first moment is required to determine thelocation of the hyetograph peak.

Rainstorms are analyzed to determine the average depth per time interval

∑=

==n

jjavg nDd

nd

1

1(46)

and the first moment arm of the hyetograph

D

djtt

n

jj

avg

−⋅∆

=∑

=1)5.0(

(47)

where: n = number of intervals in the storm,dj= rainfall depth in interval j,D = total storm depth,davg = average storm depth,t = time step of each interval,.tavg = average storm duration.

To describe the hyetograph in more general terms, the hyetograph is non-dimensionalizedusing D, the storm depth, and td, the storm duration. The non-dimensionalized form ofthese two equations can be expressed in the following two forms:

nd oavg

1= (48)

davg t

tt =0 (49)

where: doavg = non-dimensional average storm depth,

t0avg =non-dimensional average storm duration.

The hyetograph is described with geometric variables a, b and h. In a non-dimensionalized form, the variables are expressed by:

00 3ta = (50a)otb 320 −= (50b)

20 =h (50c)

The historical rainfall record is then analyzed to determine values for a, b, h for 6 particularlocality. Yen and Chow have only done this for a rainfall record from Illinois and fromBoston. Once the design storm non-dimensionalized variables are determined, thedesign storm can be developed. After selecting the duration and return period, the rainfalldepth is found from an IDF curve. Using the non-dimensionalized variables for the area

Theory Reference 89

the design storm is constructed.

The Yen and Chow design storm requires a rainfall data analysis before it can be used ina particular locality. The non-dimensionalized variable a, can only be found by analyzing alocal rainfall record. It should, not be transported from one locality to another.


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59. *Consuegra, D., “Application du Modéle Hydrologique OTTHYMO et propositionpour son application,” M.A.Sc. thesis, University of Ottawa, Ottawa, Canada.

60. Cunge, J.A., Holly, F.M., Verwey, A. (1981), Practical Aspects of ComputationalRiver Hydraulics, Pitman, London.

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115. Marsalek, J. (1979), “Malvern Urban Test Catchment, Vol. II,” Research Report,No. 95, Canada-Ontario Agreement on Great Lakes Water Quality, Project No. 73-3-12.

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117. Marsalek, J. (1980), “Sewer Inlets Study - Laboratory Investigation of SelectedInlets,” Environmental Hydraulics Section, National Water Research Institute,Burlington, Ontario, July, 1980.

118. Marsalek, J., Watt, W.E. (1983), “Design Storms for Urban Drainage Design,”Proceedings, 6th Canadian Hydrotechnical Conference, Ottawa, June, 1983.

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177. Williams, J.R. (1972), “Concept of a Technique for an Analysis of WatershedRunoff Events,” Proceedings of the Second International Symposium inHydrology, Colorado State University, Fort Collins,.Colorado, September, 1972.


178. *Wisner, P., Gloor, R., Lam, A. (1984), “Application des microordinateurs dans lecontr�le des inondations dans les petits bassins urbains,” Proceedings, WorldMeteorological Symposium on Microcomputer Applications in OperationalHydrology, Geneva, September, 1984.

179. *Wisner, P.E., Gupta, S., Kassem, A. (1980), “Considerations Regarding theApplication of SCS TR-55 Procedures for Runoff Computations,” Proceedings,SWMM User Group Meeting, USEPA 600-9-80-064, pp. 23-44.

180. *Wisner, P.E., Kassem, A.M. (1980), “Review and Comparison of RoutingMethods in Storm Water Modelling,” IMPSWM Report No. 1, Department of CivilEngineering, University of Ottawa, Ottawa, Canada, October, 1980.

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183. *Wisner, P.E., Kassem, A.M. (1983), “OTTSWMM, A Model for the Analysis andDesign of Dual Drainage Systems,” Part IV, IMPSWM Urban Drainage Modelling,Procedures, 2nd Edition, Department of Civil Engineering, University of Ottawa,Ottawa, Canada, February, 1983.

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Index

A

ADD HYD.................................................................................................................................................... 13Airport Method ............................................................................................................................................. 19

B

Baseflow ....................................................................................................................................................... 43Bransby-William's formula........................................................................................................................... 18

C

Command List ................................................................................................................................................ 5

D

Design stormsAES design storm ..................................................................................................................................... 84Chicago design storm................................................................................................................................ 76Comparison with historical ....................................................................................................................... 59Composite design storm............................................................................................................................ 76Conclusions............................................................................................................................................... 71Flood studies report design storm............................................................................................................. 86Frequency ................................................................................................................................................. 75IDF curve .................................................................................................................................................. 72Illinois State water survey design storm ................................................................................................... 81Introduction............................................................................................................................................... 55Methodology............................................................................................................................................. 58Pilgrim and Cordery design storm ............................................................................................................ 87SCS 24hr design storm ............................................................................................................................. 79SCS 6 hr design storm .............................................................................................................................. 80Uniform design storm ............................................................................................................................... 76Yen and Chow design storm..................................................................................................................... 88

DIVERT HYD.............................................................................................................................................. 14DUHYD........................................................................................................................................................ 15

F

File extensions ................................................................................................................................................ 1

H

Hydrograph Files ............................................................................................................................................ 4

I

ImperviousnessDefined ....................................................................................................................................................... 5Suggestions for ungauged catchments ...................................................................................................... 19

Introduction..................................................................................................................................................... 1IUH equations............................................................................................................................................... 39

NASH IUH ............................................................................................................................................... 41SCS IUH ................................................................................................................................................... 42STANDARD IUH..................................................................................................................................... 40WILLIAMS IUH ...................................................................................................................................... 42

M

MASS STORM file format............................................................................................................................. 3Modified SCS CN procedure

Calibration of ............................................................................................................................................ 29Using for ungauged rural catchments ....................................................................................................... 17

Index 103

N

NASHYD........................................................................................................................................................ 9

P

Pond rating curveCommand list............................................................................................................................................ 13How to construct....................................................................................................................................... 22

ProjectsWorking with .............................................................................................................................................. 1

R

Rainfall files.................................................................................................................................................... 2READ HYD.................................................................................................................................................. 15

File format .................................................................................................................................................. 4READ STORM file format............................................................................................................................. 2References..................................................................................................................................................... 90ROUTE CHANNEL..................................................................................................................................... 10ROUTE MUSKCUNG ................................................................................................................................. 11ROUTE PIPE................................................................................................................................................ 12ROUTE RESERVOIR.................................................................................................................................. 13Routing options

Command list............................................................................................................................................ 10Discussion of ............................................................................................................................................ 49

Rural areasTime-to-peak calculations......................................................................................................................... 17Tips for modelling .................................................................................................................................... 16Unit hydrograph options ........................................................................................................................... 44

S

Save Hydrograph file format .......................................................................................................................... 4SCS curve number procedure ....................................................................................................................... 24SCSHYD ...................................................................................................................................................... 10SHIFT HYD ................................................................................................................................................. 13STANDHYD .................................................................................................................................................. 7

Considerations in using rainfall losses...................................................................................................... 35Infiltration procedures............................................................................................................................... 34

STORE HYD................................................................................................................................................ 16Storm files....................................................................................................................................................... 2SWM pond modelling................................................................................................................................... 22

T

Time-to-peak parameter................................................................................................................................ 17Tips for Modelling Ungauged Rural Catchments ......................................................................................... 16Tips for Modelling Ungauged Urban Catchments ........................................................................................ 19

U

Unit hydrograph options ............................................................................................................................... 38Upland's Method........................................................................................................................................... 18

W

WILHYD ........................................................................................................................................................ 9William's Equation........................................................................................................................................ 19

2 vi sua l o t t h ymoV E R S I O N 2 . 0

h t t p : / / w w w . g r n l a n d . c o m

T H I S S O F T W A R E P A C K A G EC O N T A I N S :

The hydrological simulation program Visual OTTHYMOTM Version 2.0

A User’s Guide that aids new and experienced users will all of the procedures and features associated with operating Visual OTTHYMO v2.0.

A Reference Manual that presents all of the background technical information on which themodel is based.

A concise on-line help system to aid users while operating the modelling environment.

D E V E L O P E D A N DD I S T R I B U T E D B Y :

Greenland International Consulting Inc.7880 Keele Street, Suite 100Concord, ON Canada L4K 4G7

Tel.: (905) 738-1818Fax: (905) 761-8880E-mail: [email protected]

S Y S T E M R E Q U I R E M E N T S :

MINIMUM REQUIREMENTS• Windows 95/98/ME/NT/2000• Pentium 233 MHz• 32 MB of RAM • 4 MB of Video Card Memory• 100 MB of hard disk space

RECOMMENDED REQUIREMENTS FOREDUCATION/PROFESSIONAL VERSIONS• Pentium-II 400MHz• 64 MB of RAM • 8 MB of Video Card Memory

RECOMMENDED REQUIREMENTS FORENTERPRISE VERSION• Pentium-III 850 MHz• 128 MB of Ram• 16 MB of Video Card Memory

I N S T A L L A T I O N :

1) Insert Visual OTTHYMO cd into CD or DVD Rom drive.

2) From the Start Menu, choose RUN, then SETUP.EXE from the cd.

3) Follow install instructions on screen.

4) After install, restart Windows.

5) Double click the Visual OTTHYMO icon and call Greenland to obtain your software pass code.

© Copyright 1996, 2001 Schaeffer & Associates Ltd. All rights reserved.

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