Andrea;

We also recognized that inlet capacity can be an important parameter. We found that old curb inlets, without grates in the gutter, would not pass 5 year return level peak flows and round inlets were similar. All curb and gutter combination grates had ample capacity.

Even where inlet capacity was ample it is still useful to include in the models if you include street storage. It is necessary to build up some amount of depth on the street gutter before inlets are flowing at significant rates and this street storage has a dampening effect on peak flows - not much for high capacity inlets because not much storage depth is required, more so for lower capacity inlets where deeper depths are required to pass peak flows.

Since you don't state it, I wonder if you are referring to coarse modeling or fine modeling. In either case you could accumulate inlet rating curves and street storage area/depth curves for coarse catchments.

In our fine modeling we are accumulating a rating curve to cover form one to 6 inlets at a model inlet location and applying a single area/depth curve. This smoothes out the inlet hydrographs and makes it a simple matter to model inlet restriction during relief design.

If your drainage basin has mostly large capacity inlets then you will see minimal effects. It is a large amount of work to add inlets and street storage to models so you should investigate the probability of seeing worthwhile benefits. I would do a detailed examination of at least two catchments to see the benefits of such detailed modeling and then develop a method of doing the modeling.

We have added what we call an Intermediate Extran model to our work process. The surface catchment Runoff interface file is directed to this intermediate model which contains only street storage curves and inlet rating curves. Each 'inlet' is actually an outfall from the intermediate model for this purpose as it's computed flows are routed to the pipe Extran model via an interface file. Thus it is necessary to have larger array sizes than standard as many outfalls are required. We are working with the regular EPA SWMM engine and using PCSWMM.

If you need more help, let me know.

Reinhard Sprenger
rsprenger@UMAGROUP.COM