Background

This article focuses on how to do the step - by - step HEC-22 calculations for finding the curb length needed to capture 100% of gutter flow in a curb inlet with a continuously depressed gutter with no clogging.

 The analysis is shown using FlowMaster, but this can also be applied using StormCAD.

*Note: In some situations, StormCAD and FlowMaster need to make interpretations of HEC-22 and modify/expand the equations or logic.

Steps to Accomplish

Step by step procedure for finding curb length needed to capture 100% of gutter flow in a curb inlet with a continuously depressed gutter, with no clogging.

 1) Calculate spread

 *Note the following sections of the HEC-22 manual:

- Example 4-2 on pages 4-13 and 4-14
- Equation 4-2 on page 4-9
- Equation 4-4, 4-5 and 4-6

T = 0.9749 m

2) Calculate the E0 ratio from equation 4-4

T = 0.9749 m
W = 0.38 m
Sw = 0.1 m/m

Sx = 0.0253 m/m

E0 = 0.875893955

3) Calculate S'w:   S’w =  a/ (1000W)

a= gutter depression = 28.386 mm
W = 0.38 m

S’w = 0.0747

4) Calculate Se from equation 4-24

Se = 0.09069

5) Calculate Lt from equation 4-25, which uses Se from step 4. Lt is the length required to intercept 100% of gutter flow (0.00688 cms)

n = 0.016

SL = 0.0045 m/m

Q = 0.00688 cms

Kt = 0.817 (SI)

Lt = 1.007

6) For lengths shorter than those required for 100% capture, use equation 4-23 to get the E

For example using a length of 0.94 m, the efficiency is 99.2%