SWMM and HEC-22 Inlets
HEC-22 inlet calculations were introduced into EPA SWMM version 5.2.0. CivilStorm now leverages the use of this feature when computing with the SWMM engine, and upon export of the CivilStorm data into the EPA SWMM file format.
Computation Differences
Although CivilStorm and EPA SWMM both implement HEC-22 inlet calculations, the description of the methodology laid out in chapter 4 of the ‘Hydraulic Engineering Circular No. 22, Third Edition: Urban Drainage Design Manual published by the Federal Highway Administration is open to interpretation. This led to slightly different implementations of the methodology, especially for inlets operating in sag.
Take the following list into account when comparing results between CivilStorm and EPA SWMM:
| Use Case | CivilStorm | EPA-SWMM |
|---|---|---|
| In sag curb inlets acting as a weir, with both a local depression and a gutter depression | Takes the maximum of the gutter depression width and the local depression width when calculating the weir flow | uses the width of the local depression as W in HEC-22 eqn 4-28 regardless of the width of the gutter depression |
| In sag curb inlets transitioning between orifice and weir flow | Assumes weir flow up 1.4 * height of the curb opening, and then switches to orifice flow |
Calculates weighted average of orifice and weir flow. The transition begins at the top of the curb opening, and becomes full orifice flow at 1.4 * height of the curb opening. |
| In sag grate inlets operating as a weir | Uses the average depth across the grate to calculate the weir flow with HEC-22 eqn 4-26 |
Divides the calculation into two parts and sums those values together for the total captured weir flow: Part1 The two sides perpendicular to the gutter flow, where depth is assumed to be average depth across the grate Part 2 The portion of the gutter flow entering through the side of the grate opposite the curb, where the depth used is the actual gutter depth at that location. |
| Grate inlets in sag with grate type 45 deg tilt-bar use different open area ratios | Open ratio: 0.34 | Open ratio: 0.17 |
| Combination inlets in sag with equal curb and grate opening lengths | Excludes the curb from intercepting flow until the gutter depth is higher than the curb's orifice threshold | Excludes the curb from intercepting flow until orifice flow begins on the grate |
Result Differences
The maximum depth and spread results in the SWMM Results Report are based on the maximum depth of flow in the incoming gutter and are not based on the depth calculated based on the HEC-22 inlet equations.
To see these calculated results, look at the Depth (Gutter) and the Spread/Top Width results on the catch basin.
Using the SWMM Style Inlets
When using the SWMM Engine, the user can choose to calculate HEC-22 style inlets using either the Bentley or EPA SWMM varieties by selecting the desired option from the Inlet Methodology property in the calculation options.