# How is ponded depth calculated when an inlet has a local depression

How is ponded depth calculated when an inlet has a local depression

• Hello Christian,

It seems you want to model ponding depth at an inlet that has a local depression.

FlowMaster models free outfall at inlets using a peak flow/steady state analysis. If the HGL is high enough to cause flooding, then the contributing runoff would be accumulating/ponding. This would be better modeled using an extended period simulation to consider time and storm data. I recommend running a dynamic analysis using Bentley CivilStorm or Bentley SewerGEMS.

The following article may help:

Interpreting results when using manhole or catchbasin Surface Storage

• Hello Christian,

I assume your situation is governed by surface flow to the inlet, not HGL surcharging. Also, since you are asking about ponding, I assume you are asking about an inlet at a low point (sag) location.

Ponding depth for sag inlets is calculated using weir equation initially, and the orifice equation if depth is high enough to control. This is explained in detail in the product documentation. The calculated depth is added to the top elevation of the inlet grate or curb opening edge to determine spread.

Local depression lowers the elevation of the inlet grate or curb opening, reducing spread on the roadway surface. It does not change how ponding depth is basically calculated.

Hopefully, Jesse will provide a better answer, but I think that’s the basic answer to your question.

Karl Dauber, PE
Laurens County, SC

• Christian, based on your recent product usage I assume you are asking about OpenRoads Designer. When you say "ponded depth" are you referring to the use of the surface storage option with the Implicit or Explicit (SWMM) dynamic solver when using a CivilStorm or SewerGEMS license, or are you referring to the gutter depth result when using inlets with the GVF-Rational (StormCAD) solver?

For the former (surface storage), see the following articles which explain that the flooding depth result is measured from the catchbasin rim elevation:

For the latter (StormCAD/gutter depth), the following article explains that the gutter depth result is measured from the bottom of the local depression (but not for FlowMaster): Where is the Gutter Depth measured from?

As Karl mentioned, the way that the depth is calculated is documented in the HEC-22 equations and in the help documentation. We also have several wiki articles for certain situations (combinations of inlet type and other factor) that explain more about the calculations.

Regards,

Jesse Dringoli
Technical Support Manager, OpenFlows