Inlet Type Definition by Maximum Capacity

Hello Jose, 

For inlets which are not available for modeling in StormCAD, you should receive Gutter Depth vs. Captured Flow relationship from the manufacturer and enter that data in StormCAD to model the inlet. 

Here is detailed information about modeling specific inlet type. 

Modeling Neenah Grate Inlets 

With the "maximum capacity" option for an inlet, it will capture all flow up until the specified maximum capacity, and all flow above that maximum capacity will bypass and travel down the next downstream gutter to approach the next inlet. If this is how you would like to represent your inlet, you can certainly do so, but it might be better to obtain the gutter depth vs. captured flow characteristics from the manufacturer, of flow to inlet vs. flow captured. Note that these characteristics may be different for different road/gutter slopes, though.

So, another option is to use the catalog inlet option and configure a catalog inlet to match the characteristics of the inlet you want to model. For example a grate inlet with a certain grate length/width / open area (in a separate thread, my colleague Larry showed some Connecticut DOT catalog inlets available in the engineering library for a "double" grate inlet, but you can set up your own with the respective grate length, width, etc. The catalog inlets use the HEC-22 standard inlet calculations to determine not only the capture efficiency based on the entered slopes, inlet parameters, etc, but also the depth and spread.

What originates my original question is that even though I established a maximum inlet capacity for an specific grate inlet type to be 6.94 cfs, and have a catchment area with a 2.62 cfs (theoretically all flows should go into the inlet) the program results shows a spread value above my “maximum spread constraint”.  I performed a hand calculation using HEC-22 equation and get the same spread value results. 

I have tried all kinds of different “Inlet Types” and got the same spread and depth values (only the capture flow and capture efficiency changes).

Engineering judgment dictates that a bigger or additional inlet shall be provided but I don’t get the program to produce a spread value between my project constraints.

Hi Jose,

The spread and depth at a catchbasin is measured from the upstream side of the inlet, where it tends to be the largest (just before some or all of it is captured). Therefore it won't matter what size inlet you choose for that particular catchbasin, because the inlet size comes into play just downstream of where the depth and spread are measured from. You can read more about this in the following article form our Wiki: 

Why is it that no matter how large the inlet on grade is the gutter spread always remains the same?

If you want to improve the spread and depth of an on-grade inlet, you'll need to look at the next-upstream inlet (so that less flow travels down the upstream gutter). If there is no upstream gutter and the only approach flow comes from a catchment, you may need to insert an additional upstream inlet.

As for the design constraints - for an inlet on-grade, it can only design based on capture efficiency, and the notifications about not meeting the spread/depth constraints are informational. Read more about this in the section called "Designing inlets on grade" in the following article:

Order of automated design priorities for StormCAD (GVF-Rational) and SewerCAD (GVF-Convex)