Modeling Custom Inlets Using Inlet Capture Curves

  Product(s): SewerGEMS, CivilStorm, StormCAD
  Version(s): V8i, CONNECT Edition
  Area: Modeling



Bentley SewerGEMS, CivilStorm, and StormCAD applications provide engineering libraries containing many typical inlets, following the HEC-22 standard inlet capacity calculations. There may be instances where you need to model an inlet that either does not conform to the HEC-22 standards, or otherwise has some special, known performance curve. For example, grate manufacturer's may provide a table and/or graph containing gutter depths at corresponding captured flow, or captured flow vs approach flow.

The method used depends on the information you have and whether the inlet is in-sag or on-grade.

Solution - in-sag

For in-sag inlets, use the "gutter depth vs. captured flow" catalog inlet type. The program takes the total flow approaching the inlet and looks it up on the table to determine the depth necessary to "push" that flow into the in-sag inlet.

1. Open the Components dialog, and select Inlet Catalog.

2. Click the New icon and set the Inlet Type to "Gutter Depth vs. Captured Flow".

3. Enter captured flow (approach-flow) values and corresponding in-sag gutter depths.

The inlet is now set up and ready to use in the current model. To make it available for use in other models, click on the Synchronization Options button and select Export to Library. This article explains how to add entries to Engineering Libraries. 

4. For each catch basin you want to use this inlet, set the property Inlet Type to Catalog Inlet and set the property Inlet to the one you created. Choose "in-sag" as the "Inlet location".

Solution - on-grade

For on-grade inlets, you can use the "Flow to Inlet vs. Flow Captured" inlet type, to model a custom inlet where you know the percentage of flow that it captures for a range of approach gutter flows.

  1. In the catchbasin properties, select "on-grade" for the "inlet location" and enter the Manning's n and longitudinal slope
  2. Choose "Inflow-capture curve" as the "inlet type"
  3. Click the "..." field next to the "inflow-capture curve" field and enter the table of Total inlet flow (approach flow) vs. "inlet capture" (the amount that gets captured - the rest will bypass into the downstream gutter)

Note that you would have a separate curve for every longitudinal slope as this would effect inlet capture. So, you could consider using the inlet catalog version ("Flow to inlet vs. flow captured" type - located in the same place as the "gutter depth vs. captured flow" type mentioned above for in-sag inlets) to create these and re-use them as needed, for a given combination of size/slope/etc. Contact the inlet manufacturer for help getting this information.

Alternatively, you could try using HEC-22 standard inlet sizes that are close to your particular inlet. Try a range of values around your size and test the sensitivity - if the overall results do not vary significantly, this may give you confidence that you could approximate with the HEC-22 inlet.


  • Gutter depth and spread results are not calculated in this case, as HEC-22 calculations are required (by using "catalog inlet" as the inlet type).
  • If you are planning to use this rating curve option to model an on-grade inlet for a standard HEC-22 with the "EPA" version of the SWMM solver (instead of the "Bentley" version), you can potentially obtain the HEC-22-based rating curve in Bentley FlowMaster by using the Rating Curve option, plotting Intercepted flow vs. "Discharge" (where "discharge" means the total approach gutter flow).

See Also

Modeling Neenah Grate Inlets

Modifying and adding entries to Engineering Libraries

  Original Author: Craig Calvin