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# HEC22 vs StormCAD Inlet Capacity Comparison

## HEC22 vs StormCAD Inlet Capacity Comparison

(OP)
A coworker and I have been working on a storm drainage project using Haestad Method's StormCAD V.5.0 software. The software is supposed to be based on HEC 22 inlet design principals.  However, I have not been able to get the program to produce results which match the procedures in HEC-22.  I used to use an Excel spreadsheet I developed based on HEC-22.

I have an inlet in a sump condition.  The inlet has an inclined throat.  The following Inlet and street cross slopes were used for the HEC-22 and StormCAD runs:

All units are in feet unless otherwise noted -

Sx=0.017
Sw=0.073
a=0.112
Kc=0.56
h=0.46
kt=0.46
d=0.667
Cw=2.3
Co=0.67
Tmax=13.0ft
W=2
Theta=33.69 degrees

For the sump conditions HEC22 Equations 4-25, 4-28, and 4-31 were used along with Figure 4-17.

My spreadsheet produces answers which match the County's inlet capacity nomographs pretty well.  But, StormCAD provides capacities which are 20 to 30% less.  Which is no big deal on a small site, but huge when you have a larger development with lots of area, streets and houses.

My sump has 12.56 cfs coming to it with double 7' wide inlets.  My spreadsheet and the County's nomograph says it will accept the flow.  StormCAD says the road is overtopped.

What gives?  Any ideas?

### RE: HEC22 vs StormCAD Inlet Capacity Comparison

I don't mean to cause trouble by mentioning a competing list, but you may want to post this on Haestad's Stormtalk list.  There is usually feedback from Haestad personel in addition to peer feedback.

### RE: HEC22 vs StormCAD Inlet Capacity Comparison

I use a spreadsheet to find d and T for design.  This kind of "backs into" HEC-22, but allows maximum economy (least inlets).  Here is what I found with your data:

Sw=a/W = .112/7 = .016.  Checking with a in inches = 1.34, then Sw = 1.34/84 =.016, so a and W look OK.  Since you gave Sw as .073, your Sw is greater than your given a/W, which makes your gutter pitch exceeds your inlet pitch relative to the gutter, which will inhibit drainage.  Was this an oversight, or am I making an error?

Is the roadside curbed? This affects Perimeter, and weir flow is usually the case.
Curbed: I got depth of 4.5"", 6" curb is OK for depth.
Uncurbed: depth = 4.0", 6" curb is OK for depth.

In both cases, though Tmax is exceeded at the sump, which means efficiency is not sufficient.  Hope this doesn't add too much confusion.

### RE: HEC22 vs StormCAD Inlet Capacity Comparison

(OP)
The road is curbed.  Your numbers seem to match fairly closely what I got.  The a value of 0.112 is the additional drop at the inlet over what the normal curb elevation would be at the inlet face.  So the inlet pitch is steeper than the gutter.

I was getting depth values of .6 or greater.  The Tmax is for on grade inlets.  The County allows the spread to be exceeded in sumps.

Thanks

### RE: HEC22 vs StormCAD Inlet Capacity Comparison

It may have to do with the parameters you set for grate efficency, and the storm parameter that are somtimes automaticly set by StormCad, try setting the storm to 0 and put an artifical value in the blank labled added catchment and see what happens.

The Dot spread sheet is a very conservative method, if it works on the spreadsheet the design probably works, unless you've got something funky going on, like gutter sections changeing from 2.5' to 1.5' and stuff like that

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