StormCAD Modeling

[Kdetto]

Hi All,

I am trying to model a design and I am coming to the conclusion StormCAD cannot truly, accurately model it. Wanted to get other's opinions who may be more experienced with the program/design to make sure my assumption is correct and whether the modeling capabilities is enough to ensure/vet the design.

We are dealing with a ECA-type slope and want to capture the 100 year storm event and get it out of the area as soon as possible. The slope varies from 50% to 75%. We designed a bench at the toe of the slope of ~5 foot width, flat, with several area drains connected with a 6" perf pipe about 2.5 ' below grade. The slope bench is also lined with an impenetrable ditch liner as we do not want water to infiltrate. "Drain" set at 3" above grade. The goal is to have the water drain into the gravel bench into the perf pipe before it is conveyed to our SD connection. This bench is up against the Building/landslide-designed wall.

How it was modeled was with catchments going to each "drain" with the conduit connecting and conveying the flow to our SD connection. However, the model doesn't take into consideration that the conduit is perforated and is the first line of drainage before the inlet. The model shows our design gets the water out via drains and conduit, but is it acceptable given the true design intent?

Thanks!

 

[cvg]

Environmentally Critical Areas?

as far as I know, Stormcad is not designed for porous media flow through sand and gravel drains or transporting "groundwater" in a subdrain. I dont know if you can model the gravel layer as an "inlet" to a storm drain. it may be able to handle the rainfall runoff component of your system.

 

[Kdetto]

Yes, ECA = environmentally critical area. We are not allowed to touch the hill/slope at all. Hence the small bench to collect any drainage from the hill. I figured it couldn't be accurately modeled, but is it fair to assume that how it is modeled is conservative?

 

[cvg]

well, you really haven't provided any details on how it is actually modeled, so maybe it is conservative. A sketch might help. I would be concerned with the gravel getting clogged with silt, and then the water does not make it to the subdrain pipe very fast. So the question is can the water seep into the gravel drain and flow out of the pipes fast enough to handle the peak flow or does it flow over the bench and continue downstream? This question needs to be answered for both clean gravel at end of construction and also for clogged gravel in the future.

 

[Kdetto]

The bench is up against the landslide-rated wall. If the flow doesn't seep into the gravel fast enough, there are overflows 3" above each drain to pick up excess flow. That is how I modeled it, as if the flow is picked up by all the area drains and conveyed out to the SD connection. 100% captured for a 100 year storm event.

 

[cvg]

so you assume the water flows through the gravel at the peak flow rate, does not rise more than 3 inches and the subdrains handle the peak flow? So if the gravel clogs, which it probably will, than most of the flow will go out the overflows? And have you determined that there is a high enough hydraulic conductivity in the 5 foot wide gravel bench (clogged or unclogged) for the water to all flow through the gravel at the peak flow rate without exceeding 3 inch rise? I don't think Stormcad can do that calculation.

 

[Kdetto]

No, I have modeled it to where each drainage basin goes to each drain and is conveyed to our outfall point. As if the gravel bench is impervious (so worst case scenario). We are also installing a CB at each connection point (there are two) with enough sump to collect any silt and debris and later pumped out.

 

[LincolnPE]

You can model the infiltration trench/perf pipe setup in SWMM. Sounds like your design approach assumes the trench is clogged, but the surface grading design has enough capacity by itself to convey the design flows, therefore you would have a redundant and conservative design. Don't forget your freeboard though and use appropriate clogging factors for the inlet designs.

 

[Kdetto]

Thanks LincolnPE!