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Highway Drainage: Highway Drainage: Determining Flow RatesDetermining Flow Rates

Ch. 8Ch. 8

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ObjectivesObjectives

Know how to determine peak flow using the Know how to determine peak flow using the rational method rational method

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Design Flow Methods Design Flow Methods

Rational method Rational method TR-55TR-55 Regression EquationsRegression Equations Historical DataHistorical Data

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Methods-Smaller drainage areasMethods-Smaller drainage areas

Rational MethodRational Method Drainage areas<200 acresDrainage areas<200 acres

TR-55 (Technical Release-55)TR-55 (Technical Release-55) Drainage areas < 2,000 acresDrainage areas < 2,000 acres

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Methods-Larger drainage areasMethods-Larger drainage areas

TR-20TR-20 Large drainage areas broken into subareasLarge drainage areas broken into subareas

HEC-RAS (Hydraulic Engineering Circular-HEC-RAS (Hydraulic Engineering Circular-River Analysis System)River Analysis System)

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Rational Method (metric)Rational Method (metric)Q=kCIAQ=kCIA

Q – peak discharge (mQ – peak discharge (m33/sec)/sec) K- conversion factor= .00278 (cms-hr/ha-mm)K- conversion factor= .00278 (cms-hr/ha-mm) C-runoff coefficient (unitless)C-runoff coefficient (unitless) I-rainfall intensity (mm/hr)I-rainfall intensity (mm/hr) A-drainage area (hectares--100 square meters)A-drainage area (hectares--100 square meters)

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Rational Method (english)Rational Method (english)Q=kCIAQ=kCIA

Q – peak discharge (cfs)Q – peak discharge (cfs) K (conversion factor)=1 (cfs-hr / in-acre)K (conversion factor)=1 (cfs-hr / in-acre) C-runoff coefficient (unitless)C-runoff coefficient (unitless) I-rainfall intensity (in/hr)I-rainfall intensity (in/hr) A-drainage area (acres)A-drainage area (acres)

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Rational MethodRational Method

Drainage areaDrainage area

Time of concentration (need for I)Time of concentration (need for I)

Rainfall intensityRainfall intensity

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Basic Steps to Estimate Peak Flow Basic Steps to Estimate Peak Flow Rational MethodRational Method

1.1. Estimate time of concentrationEstimate time of concentration Overland (sheet) flowOverland (sheet) flow Shallow concentrated flowShallow concentrated flow Open channel flowOpen channel flow

2.2. Choose a value of CChoose a value of C

3.3. Select a frequency (return period)Select a frequency (return period)

4.4. Determine the average storm intensity from IDF CurveDetermine the average storm intensity from IDF Curve

5.5. Use rational method equation to estimate peak flowUse rational method equation to estimate peak flow

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Runoff Coefficients-RuralRunoff Coefficients-Rural

Concrete or HMAConcrete or HMA .95-.98.95-.98

Gravel Gravel .4-.6.4-.6

Steep Grass (1:2)Steep Grass (1:2) .6-.7.6-.7

TurfTurf .1-.4.1-.4

ForestedForested .1-.3.1-.3

CultivatedCultivated .2-.4.2-.4

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Roff Coeff-Sub/urbanRoff Coeff-Sub/urban

Flat Residential (30% Flat Residential (30% impervious)impervious)

.4.4

Flat Residential (60% Flat Residential (60% impervious)impervious)

.55.55

Mod Steep Res (50% imp)Mod Steep Res (50% imp) .65.65

Mod Steep Built-up (70% imp)Mod Steep Built-up (70% imp) .8.8

Flat comm (90% imp)Flat comm (90% imp) .8.8

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Rational Coefficient CRational Coefficient C

Must be weighted if you have different area Must be weighted if you have different area types within the drainage areatypes within the drainage area

Drainage area = 8 acres:Drainage area = 8 acres:

2 acres; C=0.35 (residential suburban)2 acres; C=0.35 (residential suburban)

6 acres; C=0.2 (undeveloped-unimproved)6 acres; C=0.2 (undeveloped-unimproved)

Weighted C=[(2)(.35)+(6)(.2)]/8 = 0.24Weighted C=[(2)(.35)+(6)(.2)]/8 = 0.24

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Example:Example:

Determine the peak flow for a 100-acre Determine the peak flow for a 100-acre watershed in central NY (mostly forested). watershed in central NY (mostly forested). Assume the intensity is 3” per hour.Assume the intensity is 3” per hour.

What is the peak flow if the watershed is What is the peak flow if the watershed is developed into light industrial usage? Assume developed into light industrial usage? Assume the intensity is 3” per hour.the intensity is 3” per hour.

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Example-undeveloped:Example-undeveloped:

C=0.13 (0.06+0.2)/2 C=0.13 (0.06+0.2)/2

I=3” per hour (given)I=3” per hour (given)

A=100 acresA=100 acres

Q=CIA = 39 cfsQ=CIA = 39 cfs

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Example-developed:Example-developed:

C=0.65 (0.5+0.8)/2 C=0.65 (0.5+0.8)/2 I=3” per hour (given)I=3” per hour (given)A=100 acresA=100 acres

Q=CIA = 195 cfsQ=CIA = 195 cfs

In reality, the peak could be higher, since the In reality, the peak could be higher, since the time of concentration would likely decrease time of concentration would likely decrease (which would increase the intensity) (which would increase the intensity)

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Detention StructuresDetention Structures

Store runoff temporarily and then release it in Store runoff temporarily and then release it in a controlled manner to limit the peak flow a controlled manner to limit the peak flow leaving a siteleaving a site Mitigates destructive effects of increased runoffMitigates destructive effects of increased runoff May improve water qualityMay improve water quality May increase rechargeMay increase recharge

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Detention StructuresDetention Structures

Basically a tank or pondBasically a tank or pond Complex because water coming in (inflow Complex because water coming in (inflow

hydrograph) is not constanthydrograph) is not constant Water going out is not constant (single or Water going out is not constant (single or

multistaged outlet structure where Qout varies multistaged outlet structure where Qout varies based on water elevation in the structurebased on water elevation in the structure

Water elevation in the structure is not constantWater elevation in the structure is not constant