CN Design Method

Curve Number Design Curve Number Design MethodolgyMethodolgy

Jay Dorsey, P.E., Jay Dorsey, P.E., Ph.DPh.DODNRODNR--DSWCDSWCFebruary 2009February 2009

Goals for PresentationGoals for Presentation

Review Review StormwaterStormwater Modeling ToolboxModeling ToolboxEvaluation of the NRCS CN Methodology Evaluation of the NRCS CN Methodology to Generate Runoff Hydrographsto Generate Runoff HydrographsPractical ConsiderationsPractical Considerations

StormwaterStormwater Modeling ToolboxModeling Toolbox

SpreadsheetsSpreadsheetsUSGS Regression EquationsUSGS Regression EquationsRational Method/Modified Rational MethodRational Method/Modified Rational MethodSCS Curve Number Method SCS Curve Number Method

TRTR--5555TRTR--2020

HECHEC--HMS (Hydrologic Modeling System)HMS (Hydrologic Modeling System)Storm Water Management Model (SWMM)Storm Water Management Model (SWMM)

Stark County RequirementsStark County Requirements

QpostQpost < < QpreQpre for 2 for 2 –– 100 yr events100 yr events

SCS CN Method SCS CN Method vsvs Modified RationalModified RationalMinimum 2% bottom slope Minimum 2% bottom slope –– basins & basins & ditchesditchesMinimum 0.3 in/hr native soil infiltration Minimum 0.3 in/hr native soil infiltration rate for infiltration type rate for infiltration type BMPsBMPs

SCS/NRCS MethodologySCS/NRCS Methodology

StrengthsStrengthsEmpirical, lumped modelEmpirical, lumped modelWidespread use/acceptanceWidespread use/acceptanceSimplicity/ease of useSimplicity/ease of useSeems to work well for design events (P>2Seems to work well for design events (P>2””))

WeaknessesWeaknessesEmpirical, lumped modelEmpirical, lumped modelPoorly documented model development historyPoorly documented model development historyLack of locallyLack of locally--derived dataderived dataPoor performance for smaller events (P<2Poor performance for smaller events (P<2””))


StrengthsStrengthsEmpirical, lumped modelEmpirical, lumped modelWidespread use/acceptanceWidespread use/acceptanceSimplicity/ease of useSimplicity/ease of useSeems to work well for design events (P>2Seems to work well for design events (P>2””))

WeaknessesWeaknessesEmpirical, lumped modelEmpirical, lumped modelPoorly documented model development historyPoorly documented model development historyLack of locallyLack of locally--derived dataderived dataPoor performance for smaller events (P<2Poor performance for smaller events (P<2””))

The right model is “the simplest model that answers the question in a manner that is readily understood by the reviewer.“

(W. Huber)

Overview of Overview of StormwaterStormwater Design ProcessDesign Process

Simulate rainfallSimulate rainfall--runoff for specified design runoff for specified design events events -- generate inflow hydrographgenerate inflow hydrographDesign conveyance system and pond Design conveyance system and pond (volume, geometry, outlet configuration)(volume, geometry, outlet configuration)Test design by routing all design events Test design by routing all design events through pondthrough pondEnsure outflow hydrograph meets Ensure outflow hydrograph meets requirementsrequirementsCheck compatibility with drainage network Check compatibility with drainage network (i.e., (i.e., tailwatertailwater analysis)analysis)“Block” outlets and check flood routing“Block” outlets and check flood routing

Proprietary Proprietary StormwaterStormwater ModelsModels

Pond Pack Pond Pack HydraflowHydraflowStormNetStormNetHydrocadHydrocadEtc.Etc.


Identify rainfall depth, distribution for each Identify rainfall depth, distribution for each design event design event –– SCS Type II Storm, NOAA SCS Type II Storm, NOAA Atlas 14 dataAtlas 14 dataConvert rainfall depth to runoff depth using Convert rainfall depth to runoff depth using Curve Number method (TRCurve Number method (TR--20)20)Calculate the Calculate the TcTc for each contributing drainage for each contributing drainage area for existing and proposed conditions area for existing and proposed conditions ––use velocity method (TRuse velocity method (TR--55 manual) and 55 manual) and appropriate equation for unpaved appropriate equation for unpaved concconc flowflowGenerate inflow hydrographs for each design Generate inflow hydrographs for each design event for existing and proposed conditions event for existing and proposed conditions ––use SCS unit hydrograph method (i.e., TRuse SCS unit hydrograph method (i.e., TR--20 20 based model)based model)

RainfallRainfallDesign Storms Design Storms Rainfall DistributionRainfall Distribution

Rainfall DataRainfall DataNumerous sources of rainfall data (e.g., depth, intensity) are aNumerous sources of rainfall data (e.g., depth, intensity) are available vailable

including:including:

NOAA Atlas 14 NOAA Atlas 14 -- NWSNWS--NOAA. 2004. PrecipitationNOAA. 2004. Precipitation--Frequency Frequency Atlas of the United States, NOAA Atlas 14, Atlas of the United States, NOAA Atlas 14, VolVol 2, Version 3, 2, Version 3, NOAA, National Weather Service, Silver Spring, MD. NOAA, National Weather Service, Silver Spring, MD. This data can be accessed through the internet Precipitation FreThis data can be accessed through the internet Precipitation Frequency quency Data Server (PFDS): http://Data Server (PFDS): http://hdsc.nws.noaa.gov/hdsc/pfdshdsc.nws.noaa.gov/hdsc/pfds//

Huff and Angel Huff and Angel -- Huff, F.A. and J.R. Angel. 1992. Rainfall Huff, F.A. and J.R. Angel. 1992. Rainfall Frequency Atlas of the Midwest. Illinois State Water Survey, Frequency Atlas of the Midwest. Illinois State Water Survey, Bulletin 71, Champaign.Bulletin 71, Champaign.

Technical Paper 40 (TPTechnical Paper 40 (TP--40) 40) -- HershfieldHershfield, D.M. 1961. Rainfall , D.M. 1961. Rainfall Frequency Atlas of the United States, Technical Paper No. 40, Frequency Atlas of the United States, Technical Paper No. 40, U.S. Weather Bureau, Washington, DC.U.S. Weather Bureau, Washington, DC.

Selection of Rainfall Data SourceSelection of Rainfall Data SourceIt is important to use accurate precipitation data for the It is important to use accurate precipitation data for the location of the development site. location of the development site. The precipitation values from these different references are The precipitation values from these different references are typically within 10typically within 10--20% for a given design storm (e.g., 220% for a given design storm (e.g., 2--year, year, 24 hr storm). As an example,24 hr storm). As an example,

Lat: 40.530 N; Long: 82.818 W Mt. Gilead, Morrow County

5.435.436.066.064.904.90100100--yr, 24yr, 24--hrhr

4.894.895.335.334.654.655050--yr, 24yr, 24--hrhr

4.364.364.644.644.104.102525--yr, 24yr, 24--hrhr

3.703.703.863.863.703.701010--yr, 24yr, 24--hrhr

3.223.223.353.353.253.2555--yr, 24yr, 24--hrhr

2.622.622.702.702.502.5022--yr, 24yr, 24--hrhr

2.192.192.172.172.252.2511--yr, 24yr, 24--hrhr

NOAA Atlas 14NOAA Atlas 14Huff & AngelHuff & AngelTPTP--4040EventEvent

Selection of Rainfall Data SourceSelection of Rainfall Data SourceThe difference in design storm value between data sources (less The difference in design storm value between data sources (less than 10% for smaller storms and less than 20% for extreme than 10% for smaller storms and less than 20% for extreme events) is small relative to all the other assumptions and events) is small relative to all the other assumptions and inaccuracies inherent in inaccuracies inherent in stormwaterstormwater design. design. However, the National Weather Service site offers the most upHowever, the National Weather Service site offers the most up--toto--date, locationdate, location--specific information based on rainfall data from specific information based on rainfall data from over 200 individual sites within Ohio whereas Huff and Angel datover 200 individual sites within Ohio whereas Huff and Angel data a is reported regionally for ten (10) sections within the state, ais reported regionally for ten (10) sections within the state, and nd the TPthe TP--40 data is now 40+ years out40 data is now 40+ years out--ofof--date. date. For the most accurate, upFor the most accurate, up--toto--date, locationdate, location--specific rainfall data specific rainfall data for for stormwaterstormwater design, design, use the Precipitationuse the Precipitation--Frequency Atlas Frequency Atlas of the United States, NOAA Atlas 14, of the United States, NOAA Atlas 14, VolVol 2(3)2(3)-- available at the NWS Precipitation Frequency Data Server available at the NWS Precipitation Frequency Data Server (PFDS): http://(PFDS): http://hdsc.nws.noaa.gov/hdsc/pfdshdsc.nws.noaa.gov/hdsc/pfds/)./).

NWS Precipitation Frequency Data Server (PFDS) - NOAA Atlas 14:

http://hdsc.nws.noaa.gov/hdsc/pfds/

NOAA Atlas 14NOAA Atlas 14

Rainfall Depths, NOAA Atlas 14Rainfall Depths, NOAA Atlas 14

Rainfall Distribution - 1-yr, 24-hr StormColumbus Airport NOAA Atlas 14

0

0.5

1

1.5

2

2.5

0 4 8 12 16 20 24

Time (hr)

Cum

ulat

ive

Rai

nfal

l Dep

th (i

n)

NOAA Atlas 14

SCS-Type II

Type II Rainfall Distribution DevelopmentType II Rainfall Distribution Development

Design Storm Rainfall Temporal Distribution - 24-Hour

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 6 12 18 24

Dimensionless Time

Dim

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onal

ess

Cum

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ive

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l

SCS Type II

Constant Intensity

Huff 1st Q

Huff 2nd Q

Huff 3rd Q

Huff 4th Q

Triangular

Commonly Used Rainfall DistributionsCommonly Used Rainfall Distributions

Design Storm Hyetograph Comparison

0

0.05

0.1

0.15

0.2

0.25

0.3

0 6 12 18 24

Time (Hr)

Rai

nfal

l int

ensi

ty -

dim

ensi

onle

ss

SCS Type II

Constant IntensityHuff Q1

Huff Q2Huff Q3

Huff Q4Triangular

Commonly Used Rainfall HyetographsCommonly Used Rainfall Hyetographs

Source: Phil DeGroot,Hydrosphere Engineering

3Q Huff Curve 3Q Huff Curve vsvs SCS Type IISCS Type IICoshocton NAEW 172Coshocton NAEW 172

Runoff Hydrograph ComparisonPre-Development, 1-yr, 24-hr

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2

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14

0 200 400 600 800 1000 1200 1400 1600

Time (min)

Dis

char

ge (c

fs)

SCS Type IIHuff Q3

Huff Curves Huff Curves vsvs SCS Type IISCS Type II

RecommendationsRecommendationsRainfall Depth Rainfall Depth -- For the most accurate, upFor the most accurate, up--toto--date, date, locationlocation--specific rainfall data for specific rainfall data for stormwaterstormwater design, design, use use the Precipitationthe Precipitation--Frequency Atlas of the United Frequency Atlas of the United States, NOAA Atlas 14, States, NOAA Atlas 14, VolVol 2(3)2(3)-- available at the NWS Precipitation Frequency Data available at the NWS Precipitation Frequency Data Server (PFDS): Server (PFDS): http://http://hdsc.nws.noaa.gov/hdsc/pfdshdsc.nws.noaa.gov/hdsc/pfds/./.

Temporal Distribution Temporal Distribution -- For For now, we are recommending now, we are recommending the use of the SCS Type II the use of the SCS Type II rainfall distribution for all rainfall distribution for all design events with a design events with a recurrence interval greater recurrence interval greater than 1 year. We are than 1 year. We are reviewing this guidance.reviewing this guidance.

CN SelectionCN Selection

Appropriate selection of CN to Appropriate selection of CN to represent prerepresent pre--development development conditions.conditions.

PrePre--development Land Usedevelopment Land Use

PrePre--development Coverdevelopment Cover

PrePre--development Soilsdevelopment Soils

PrePre--development development

CNCN

PrePre--development development

CNCNWhat is real?What is real?

How do we know?How do we know?

What value is most What value is most supportive of achieving our supportive of achieving our stormwaterstormwater management management

goals?goals?

What is fair?What is fair?

Agricultural DrainageAgricultural DrainageMany agricultural fields have received targeted or Many agricultural fields have received targeted or systematic drainage using subsurface “tiles” (corrugated systematic drainage using subsurface “tiles” (corrugated plastic drain pipe since late 1960s) that significantly lower plastic drain pipe since late 1960s) that significantly lower the amount of surface runoff. This includes most lowthe amount of surface runoff. This includes most low--gradient HSGgradient HSG--C and HSGC and HSG--D soils in western and northern D soils in western and northern Ohio.Ohio.Though many tillThough many till--derived soils of any HSG benefit from derived soils of any HSG benefit from subsurface tiling, HSGsubsurface tiling, HSG--D soils are most affected. This is D soils are most affected. This is reflected in the multiple HSG listing for some HSGreflected in the multiple HSG listing for some HSG--D soils D soils ––A/D, B/D, C/D.A/D, B/D, C/D.Drainage should be assumed (i.e., Drainage should be assumed (i.e., use the first HSG listeduse the first HSG listed) ) unless it is verified the field is not drained.unless it is verified the field is not drained.

Updated PreUpdated Pre--development HSGdevelopment HSGNRCS has provided updated NRCS has provided updated guidance for determining the guidance for determining the hydrologic soil group (HSG) hydrologic soil group (HSG) based on a soil’s saturated based on a soil’s saturated hydraulic conductivity, depth hydraulic conductivity, depth to impermeable layer and to impermeable layer and depth to high water table.depth to high water table.

Reference: NRCS. 2007. Reference: NRCS. 2007. Hydrologic Soil Groups. Hydrologic Soil Groups. Chapter 7 in Part 630 Chapter 7 in Part 630 Hydrology, National Hydrology, National Engineering Handbook. Engineering Handbook. USDAUSDA--NRCS, Washington, NRCS, Washington, DC.DC.

Updated PreUpdated Pre--development HSGdevelopment HSG

Ohio NRCS staff applied Ohio NRCS staff applied criteria to each Ohio soil criteria to each Ohio soil series and confirmed or series and confirmed or corrected the HSG. corrected the HSG.

Upon final QA/QC, updated preUpon final QA/QC, updated pre--development development HSGsHSGs will will be published in the be published in the Rainwater ManualRainwater Manual and available by and available by county at the NRCS Soil Data Mart Server (est. by 6county at the NRCS Soil Data Mart Server (est. by 6--09): 09): http://soildatamart.nrcs.usda.gov/..

Original HSGs Updated HSGsHSG Soils* Area (%) Soils* %A 26 1.2 51 3B 217 18 104 12C 224 61.2 121 27D 85 15.8 80 11A/D 13 0.1 35 1B/D 40 2.5 93 6C/D 22 1.1 143 27Null 13Total 627 99.9 627 100

Source: Ohio NRCS

Updated Ohio Hydrologic Soil GroupsUpdated Ohio Hydrologic Soil Groups-- preliminary estimates preliminary estimates --

Soils* = number of soil series (~500), plus variants & phases with distinct HSG

Appropriate selection of CN to represent Appropriate selection of CN to represent prepre--development conditions.development conditions.Appropriate selection of CN to Appropriate selection of CN to represent postrepresent post--development development conditions.conditions.

CN SelectionCN Selection

Disturbed Soil ProfilesDisturbed Soil ProfilesAs a result of urbanization, the soil profile As a result of urbanization, the soil profile may be considerably altered and the listed may be considerably altered and the listed hydrologic group classification may no longer hydrologic group classification may no longer apply.apply.In these circumstances, select the HSG In these circumstances, select the HSG according to the texture of the new surface according to the texture of the new surface soil, provided that significant compaction has soil, provided that significant compaction has notnot occurred.occurred.

New NRCS guidance on New NRCS guidance on designating disturbed designating disturbed soil soil HSGsHSGs coming sooncoming soon

Soil CompactionSoil Compaction

Urban Urban CNsCNs

Urban CN values listed Urban CN values listed in Table 2in Table 2--2a were 2a were developed for typical developed for typical land use relationships land use relationships based on specific based on specific assumed percentages assumed percentages of impervious area.of impervious area.Table 2Table 2--2a 2a AssumptionsAssumptions

a.a. Pervious urban areas are Pervious urban areas are equivalent to open space equivalent to open space in good condition, andin good condition, and

b.b. Impervious areas have a Impervious areas have a CN = 98 and are directly CN = 98 and are directly connected to the connected to the drainage system.drainage system.

Urban Urban CNsCNs

Urban CN values listed Urban CN values listed in Table 2in Table 2--2a were 2a were developed for typical developed for typical land use relationships land use relationships based on specific based on specific assumed percentages assumed percentages of impervious area.of impervious area.Table 2Table 2--2a 2a AssumptionsAssumptions

a.a. Pervious urban areas are Pervious urban areas are equivalent to open space equivalent to open space in good condition, andin good condition, and

b.b. Impervious areas have a Impervious areas have a CN = 98 and are directly CN = 98 and are directly connected to the connected to the drainage system.drainage system.

Is this a valid assumption?

PostPost--development development

CNCN

Does this = this?


CNCN

What is real?What is real?



goals?goals?



CNCN




goals?goals?


Newly Graded Areas?


CNCN




goals?goals?


Open Space in Fair Condition?


CNCN




goals?goals?

What is fair?What is fair?Moving HSG one group to right?

HSG-C Urban CNs

70

75

80

85

90

95

100

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Impervious Area (%)

SCS

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r

CDC - FairC - GradedC - Move HSG to Right

RecommendationsRecommendationsPrePre--development Curve Numbers development Curve Numbers –– For wooded or brushy For wooded or brushy areas, use listed values in good hydrologic condition. For areas, use listed values in good hydrologic condition. For meadows, use listed values. For all other areas (including meadows, use listed values. For all other areas (including all types of agriculture), use pasture, grassland, or range in all types of agriculture), use pasture, grassland, or range in good hydrologic condition.good hydrologic condition.PostPost--development Curve Numbers development Curve Numbers –– Either: (1) require that Either: (1) require that the developer renovate the soil (subsoil, incorporate the developer renovate the soil (subsoil, incorporate compost and/or sand through top 12 inches, replace topsoil compost and/or sand through top 12 inches, replace topsoil to a minimum depth of 4”); or (2) adjust HSG one group to to a minimum depth of 4”); or (2) adjust HSG one group to right to account for topsoil removal, grading, and right to account for topsoil removal, grading, and compaction. Undisturbed areas can be treated as “open compaction. Undisturbed areas can be treated as “open space in good condition.”space in good condition.”

Coming soon Coming soon -- Look for postLook for post--development development HSGsHSGs to be to be published in Rainwater and Land Development Manualpublished in Rainwater and Land Development Manual

Example Example RegRegLicking CountyLicking County

Highest permissible curve number (CN) for cropland Highest permissible curve number (CN) for cropland shall be 82.shall be 82.Unless thoroughly documented, all “woods” shall be Unless thoroughly documented, all “woods” shall be characterized as “good”.characterized as “good”.Where development cuts and fills are anticipated to be Where development cuts and fills are anticipated to be in excess of six (6) inches, the hydrologic soil group in excess of six (6) inches, the hydrologic soil group shall be increased one category for post development shall be increased one category for post development calculations. For example: C to D.calculations. For example: C to D.

PostPost--development HSGdevelopment HSGNRCS has provided updated guidance for determining the NRCS has provided updated guidance for determining the hydrologic soil group (HSG) based on a soil’s saturated hydrologic soil group (HSG) based on a soil’s saturated hydraulic conductivity, depth to impermeable layer and hydraulic conductivity, depth to impermeable layer and depth to high water table.depth to high water table.

Reference: NRCS. 2007. Hydrologic Soil Groups. Chapter 7 in PaReference: NRCS. 2007. Hydrologic Soil Groups. Chapter 7 in Part rt 630 Hydrology, National Engineering Handbook. USDA630 Hydrology, National Engineering Handbook. USDA--NRCS, NRCS, Washington, DC.Washington, DC.

ODNRODNR--DSWC soil science staff are applying criteria to each DSWC soil science staff are applying criteria to each Ohio soil series under a cut/fill scenario. Ohio soil series under a cut/fill scenario. Upon final QA/QC, postUpon final QA/QC, post--development development HSGsHSGs will be will be published in the published in the Rainwater ManualRainwater Manual (est. by 6(est. by 6--09).09).

Tt and TcTt and Tc

Runoff HydrographSCS CN Method, D = 30 min, Q = 0.27 in

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Time (hr)

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char

ge, q

(cfs

)

Tc = 30 minTc = 20 min

qp = 7.4

qp = 9.1

~23% increase

TcTc Impact on Peak DischargeImpact on Peak Discharge

Methods for estimating TMethods for estimating Tcc

Two ways of classifying methods of Two ways of classifying methods of computing Time of Concentration:computing Time of Concentration:-- Regression based equationsRegression based equations-- Velocity based methodsVelocity based methods

Velocity Based MethodsVelocity Based Methods

NRCS Velocity NRCS Velocity Based MethodBased Method

-- Divide flow path into Divide flow path into segmentssegments

-- overland flowoverland flow-- shallow concentrated shallow concentrated

flowflow-- channel flowchannel flow

-- Estimate velocity for each Estimate velocity for each segmentsegment

-- Compute travel time for Compute travel time for each segmenteach segment

-- Sum of the travel times is Sum of the travel times is the time of concentrationthe time of concentration

Travel Time Travel Time -- Sheet FlowSheet Flow

NRCS Velocity Based MethodNRCS Velocity Based Method

Overland Flow (a.k.a. Sheet Flow)Overland Flow (a.k.a. Sheet Flow)

where:where:TTtt = Travel time, hours= Travel time, hoursn = Manning’s roughness coefficient for sheet flown = Manning’s roughness coefficient for sheet flowL = flow length, feetL = flow length, feetPP22 = 2= 2--year, 24year, 24--hour rainfall, inhour rainfall, ins = slope of hydraulic grade line (land slope), ft/fts = slope of hydraulic grade line (land slope), ft/ft

40502

800070..

.

t s)(P(nL).T =


Manning’s n values for various surface covers to be used in the Sheet flow Travel Time equation.


Shallow concentrated flowShallow concentrated flow-- after a max of 100 feet sheet flow usually begins after a max of 100 feet sheet flow usually begins

to concentrate into small rillsto concentrate into small rills-- average velocity is a function of watercourse slope average velocity is a function of watercourse slope

and type of channeland type of channel-- average depths generally less than 0.5 feetaverage depths generally less than 0.5 feet-- equations are derivatives of Manning’s equation equations are derivatives of Manning’s equation

for open channel flowfor open channel flow

Travel Time Travel Time –– Shallow Concentrated Shallow Concentrated FlowFlow


Shallow concentrated Shallow concentrated flowflow

Paved conditions : V = 20.32 s Paved conditions : V = 20.32 s 0.50.5

where: V = velocity, fpswhere: V = velocity, fpss = slope of hydraulic grade s = slope of hydraulic grade

line (slope of land) ft/ftline (slope of land) ft/ft

NRCS Velocity Based MethodNRCS Velocity Based MethodShallow concentrated Shallow concentrated flowflow

Unpaved conditions: Unpaved conditions: Grass WaterwayGrass Waterway V=16.1 sV=16.1 s0.50.5


line (slope of land) ft/ftline (slope of land) ft/ftTR-55 guidance

NRCS Velocity Based MethodNRCS Velocity Based MethodShallow concentrated Shallow concentrated flowflow

Unpaved conditions: Unpaved conditions: Grass WaterwayGrass Waterway V=16.1 sV=16.1 s0.50.5

Bare Soil Bare Soil V=10.3 sV=10.3 s0.50.5

Cult. Straight Row Cult. Straight Row V=9.0 sV=9.0 s0.50.5

Short Grass PastureShort Grass Pasture V=7.0 sV=7.0 s0.50.5

Trash Fallow, MinimumTrash Fallow, MinimumTillage, WoodlandTillage, Woodland V=5.1 sV=5.1 s0.50.5

Forest w/Heavy Litter V=2.5 sForest w/Heavy Litter V=2.5 s0.50.5


line (slope of land) ft/ftline (slope of land) ft/ftNEH-4 Guidance


Shallow concentrated flowShallow concentrated flow

Travel time for shallow concentrated flow portion:Travel time for shallow concentrated flow portion:

where:where: L = flow length, ftL = flow length, ftV = average velocity, fpsV = average velocity, fps3600 = conversion factor from seconds to hours3600 = conversion factor from seconds to hours

VLTt 3600

=


Open Channel FlowOpen Channel Flow-- open channels assumed to begin where surveyed open channels assumed to begin where surveyed

crosscross--section information has been obtained, where section information has been obtained, where blue lines (indicating streams) appear on USGS blue lines (indicating streams) appear on USGS quadrangle mapsquadrangle maps

-- Manning’s equation is used to estimate average flow Manning’s equation is used to estimate average flow velocityvelocity

-- average flow velocity determined for bankaverage flow velocity determined for bank--full full elevationelevation


Open Channel FlowOpen Channel Flow

where:where:V = average velocity, fpsV = average velocity, fpsr = hydraulic radius, ft = a/pr = hydraulic radius, ft = a/pww

a = crossa = cross--sectional flow area, sq.ft.sectional flow area, sq.ft.ppww = wetted perimeter, ft= wetted perimeter, ft

s = slope of the hydraulic grade line (channel slope), ft/fts = slope of the hydraulic grade line (channel slope), ft/ftn = Manning’s roughness coefficient for open channel flown = Manning’s roughness coefficient for open channel flow

nsrV

21

32

49.1=


Open Channel FlowOpen Channel Flow

Travel time for open channel flow portion:Travel time for open channel flow portion:

where:where:L = flow length, ftL = flow length, ftV = average velocity, fpsV = average velocity, fps3600 = conversion factor from seconds to hours3600 = conversion factor from seconds to hours

VLTt 3600

=


Time of Concentration for the watershed Time of Concentration for the watershed area:area:

TTcc = T= Tt(sheet flow)t(sheet flow) + T+ Tt(shallow concentrated flow)t(shallow concentrated flow) + T+ Tt(open channel flow)t(open channel flow)

Worksheet 3: Tc or TtWorksheet 3: Tc or Tt

Sheet Flow

Shallow Concentrated Flow

Channel Flow

Worksheet 3 can be used to calculate Tc and Tt.

RecommendationsRecommendationsUse velocity based methods to estimate travel Use velocity based methods to estimate travel time (time (TTtt) for overland (sheet) flow, shallow ) for overland (sheet) flow, shallow concentrated flow and channel flow. concentrated flow and channel flow. Be sure Be sure to use the appropriate “unpaved” velocity to use the appropriate “unpaved” velocity equation for shallow concentrated flow equation for shallow concentrated flow from NEHfrom NEH--4.4. Sum all methods for time of Sum all methods for time of concentration (concentration (TTcc). Use TR). Use TR--55 Manual and 55 Manual and NEHNEH--4 as references.4 as references.

Options for Generating a Runoff Options for Generating a Runoff HydrographHydrograph

TRTR--55/Win55/Win--TR55TR55TRTR--20/Win20/Win--TR20TR20HECHEC--HMSHMSProprietary SoftwareProprietary Software

Options for Generating a Runoff Options for Generating a Runoff Hydrograph Hydrograph –– TRTR--5555

TRTR--55 includes a method (graphical method) 55 includes a method (graphical method) for simply estimating peak discharge, but for for simply estimating peak discharge, but for detention basin design, we need to generate detention basin design, we need to generate postpost--development hydrographs to route development hydrographs to route through the detention pond. The through the detention pond. The TRTR--55 55 Tabular MethodTabular Method generates a complete generates a complete hydrograph, but is limited to the NRCS rainfall hydrograph, but is limited to the NRCS rainfall distributions (e.g., SCS Type II), may produce distributions (e.g., SCS Type II), may produce an incomplete hydrograph (i.e., no tail), and an incomplete hydrograph (i.e., no tail), and requires much data handling to accomplish requires much data handling to accomplish pond routing. pond routing.

Options for Generating a Runoff Options for Generating a Runoff Hydrograph Hydrograph –– WinTRWinTR--5555

WinTRWinTR--55 uses a TR55 uses a TR--20 “engine” to generate 20 “engine” to generate a runoff hydrograph equivalent to the TRa runoff hydrograph equivalent to the TR--55 55 Tabular Method (if SCS Type II distribution is Tabular Method (if SCS Type II distribution is used). The structure (pond) routing used). The structure (pond) routing component of TRcomponent of TR--55 is adequate for routing 55 is adequate for routing runoff through a farm pond, but is too basic to runoff through a farm pond, but is too basic to be of much use for most be of much use for most stormwaterstormwater BMP BMP design applications.design applications.

Options for Generating a Runoff Options for Generating a Runoff Hydrograph Hydrograph –– WinTRWinTR--2020

WinTRWinTR--20 generates a runoff hydrograph 20 generates a runoff hydrograph equivalent to WinTRequivalent to WinTR--55. The structure (pond) 55. The structure (pond) and stream reach routing components of and stream reach routing components of WinTRWinTR--20 are more advanced than WinTR20 are more advanced than WinTR--55, allowing adequate modeling of most 55, allowing adequate modeling of most development/drainage/detention scenarios. development/drainage/detention scenarios. The main drawback to the WinTRThe main drawback to the WinTR--20 model is 20 model is how userhow user--unfriendly it is. Life is too short unfriendly it is. Life is too short ––choose a different model.choose a different model.

Options for Generating a Runoff Options for Generating a Runoff Hydrograph Hydrograph –– HECHEC--HMSHMS

The Hydrologic Modeling System (HECThe Hydrologic Modeling System (HEC--HMS) HMS) model of the Army Corps of Engineers allows model of the Army Corps of Engineers allows the use of SCS CN methodology (as well as the use of SCS CN methodology (as well as several other options) to generate a runoff several other options) to generate a runoff hydrograph. The model has a steep learning hydrograph. The model has a steep learning curve, but is free and relatively flexible and curve, but is free and relatively flexible and powerful. If you have time to invest in getting powerful. If you have time to invest in getting up to speed, this could be a useful tool. Not up to speed, this could be a useful tool. Not for the casual user.for the casual user.

Options for Generating a Runoff Options for Generating a Runoff Hydrograph Hydrograph –– Proprietary SoftwareProprietary Software

The most common commercial The most common commercial stormwaterstormwatermodeling software packages include the SCS modeling software packages include the SCS CN methodology. They also typically have CN methodology. They also typically have several choices for rainfall distributions and unit several choices for rainfall distributions and unit hydrographs. These programs are excellent at hydrographs. These programs are excellent at channel reach and pond routing, allow multichannel reach and pond routing, allow multi--stage outlets, and are relatively user friendly. stage outlets, and are relatively user friendly. The reports (especially graphics) are a huge The reports (especially graphics) are a huge step up from step up from WinTRWinTR software. If you plan to do software. If you plan to do much design or review of much design or review of stormwaterstormwater detention detention practices, this is the way to go. practices, this is the way to go.

Recommendations Recommendations -- HydrographsHydrographs

Find a proprietary hydrologic modeling Find a proprietary hydrologic modeling software package that fits your needs.software package that fits your needs.Focus on subdividing the BMP drainage area Focus on subdividing the BMP drainage area appropriately.appropriately.Use the published SCS Dimensionless Unit Use the published SCS Dimensionless Unit Hydrograph (with 484 peak rate constant) to Hydrograph (with 484 peak rate constant) to convert rainfall excess to a runoff hydrograph. convert rainfall excess to a runoff hydrograph. Look for local/regional research or studies that Look for local/regional research or studies that would suggest modifying the peak rate would suggest modifying the peak rate constant for local conditions.constant for local conditions.

Recommendations Recommendations -- RoutingRoutingWork to conceptually understand the pond Work to conceptually understand the pond (hydrograph) routing process.(hydrograph) routing process.Find a proprietary hydrologic modeling Find a proprietary hydrologic modeling software package that fits your needs.software package that fits your needs.Practice pond routing for different development Practice pond routing for different development scenarios (with and without scenarios (with and without WQvWQv, range of pre, range of pre--and postand post--development flow peaks) and outlet development flow peaks) and outlet types (orifices, weirs, multitypes (orifices, weirs, multi--stage outlets).stage outlets).

Practical ConsiderationsPractical ConsiderationsSoftwareSoftwareData Sheets/SpreadsheetsData Sheets/Spreadsheets

ChecklistChecklistCover Sheet w/Summary DataCover Sheet w/Summary Data

By subwatershed By subwatershed –– area, land use, % impervious, area, land use, % impervious, WQvWQv, CN, , CN, TcTc

Provide Maps for both Existing and Proposed Provide Maps for both Existing and Proposed Conditions with:Conditions with:

Delineated watersheds (incl. upDelineated watersheds (incl. up--gradient areas)gradient areas)Flow path segmentsFlow path segmentsLand use/CNLand use/CN

CN Design Method

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