1 Highway Drainage: Determining Flow Rates Ch. 8

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

3 Design Flow Methods Rational method Rational method TR-55 TR-55 Regression Equations Regression Equations Historical Data Historical Data

4 Methods-Smaller drainage areas Rational Method Rational Method Drainage areas<200 acres Drainage areas<200 acres TR-55 (Technical Release-55) TR-55 (Technical Release-55) Drainage areas < 2,000 acres Drainage areas < 2,000 acres

5 Methods-Larger drainage areas TR-20 TR-20 Large drainage areas broken into subareas Large drainage areas broken into subareas HEC-RAS (Hydraulic Engineering Circular- River Analysis System) HEC-RAS (Hydraulic Engineering Circular- River Analysis System)

6 Rational Method (metric) Q=kCIA Q – peak discharge (m 3 /sec) Q – peak discharge (m 3 /sec) K- conversion factor= (cms-hr/ha-mm) K- conversion factor= (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)

7 Rational Method (english) Q=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)

8 Rational Method Drainage area Drainage area Time of concentration (need for I) Time of concentration (need for I) Rainfall intensity Rainfall intensity

9 Basic Steps to Estimate Peak Flow Rational Method 1. Estimate time of concentration Overland (sheet) flow Overland (sheet) flow Shallow concentrated flow Shallow concentrated flow Open channel flow Open channel flow 2. Choose a value of C 3. Select a frequency (return period) 4. Determine the average storm intensity from IDF Curve 5. Use rational method equation to estimate peak flow

10 Runoff Coefficients-Rural Concrete or HMA Gravel.4-.6 Steep Grass (1:2).6-.7 Turf.1-.4 Forested.1-.3 Cultivated.2-.4

11 Roff Coeff-Sub/urban Flat Residential (30% impervious).4 Flat Residential (60% impervious).55 Mod Steep Res (50% imp).65 Mod Steep Built-up (70% imp).8 Flat comm (90% imp).8

12 Rational Coefficient C Must be weighted if you have different area types within the drainage area Drainage area = 8 acres: 2 acres; C=0.35 (residential suburban) 6 acres; C=0.2 (undeveloped-unimproved) Weighted C=[(2)(.35)+(6)(.2)]/8 = 0.24

13 Example: Determine the peak flow for a 100-acre watershed in central NY (mostly forested). Assume the intensity is 3” per hour. What is the peak flow if the watershed is developed into light industrial usage? Assume the intensity is 3” per hour.

14 Example-undeveloped: C=0.13 ( )/2 I=3” per hour (given) A=100 acres Q=CIA = 39 cfs

15 Example-developed: C=0.65 ( )/2 I=3” per hour (given) A=100 acres Q=CIA = 195 cfs In reality, the peak could be higher, since the time of concentration would likely decrease (which would increase the intensity)

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

17 Detention Structures Basically a tank or pond Basically a tank or pond Complex because water coming in (inflow hydrograph) is not constant Complex because water coming in (inflow hydrograph) is not constant Water going out is not constant (single or multistaged outlet structure where Qout varies based on water elevation in the structure Water going out is not constant (single or multistaged outlet structure where Qout varies based on water elevation in the structure Water elevation in the structure is not constant Water elevation in the structure is not constant