
Applies To 



Product(s): 
StormCAD, FlowMaster 


Version(s): 
08.11.XX.XX 


Area: 
Calculations 


Original Author: 
Mark Pachlhofer, Bentley Technical Support Group 

Background
This article focuses on how to do the step  by  step HEC22 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 HEC22 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 HEC22 manual:
 Example 42 on pages 413 and 414
 Equation 42 on page 49
 Equation 44, 45 and 46
T = 0.9749 m
2) Calculate the E0 ratio from equation 44
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 424
Se = 0.09069
5) Calculate Lt from equation 425, 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 423 to get the E
For example using a length of 0.94 m, the efficiency is 99.2%