Infiltration Trench

[cfj104]

I am trying to model an infiltration trench with a perforated pipe embedded in the stone backfill. I am able to enter the below grade stone bed with the pipe embedded but this is where I am running into problems. Above grade, there is an embankment to allow the water to pool and then feed into the embedded pipe via inlet pipes. How do I model the holding capacity on the surface with the inflow (outlet) to the trench? Do I need to set-up two ponds or can everything be inputted into the same pond node? When I tried to set-up the model with two ponds, I get an error message saying "unable to extend routing above elevation" for the 'trench pond'.

Thank you for any help or direction.

 

[ctbailey]

You need two ponds in HydroCad.

Model the first as a standard hole in the ground, but the only outlet will be exfiltration.

The calculated infiltration rate that you might come up with is what you will want to use. But the infiltration rate that your review agency will allow you to use might be very different.

Once you get past that ensueing fight about infiltration rates, then you'll need to route your hole-pond to a second prismatic pond with 30-40% voids, representing the stone.

That pond's outlet will be the perforated pipe, but the full 6 inch diameter culvert.

Remember - HydroCad is an OUTLET CONTROLLED MODEL. This means you'll have difficulty "truely" modeling the perforations, and will probably end up discarding the whole idea.

I've had a reviewer cause me to delete one of these trenches from one of my designs, because they were concerned of a static hydraulic head causing my underdrain to "blow out" downstream of my infiltration trench.



___
Craig T. Bailey, PE

http://www.bailey-associates.com

 

[psmart]

The situation you described is very similar to a raingarden, as discussed at

http://www.hydrocad.net/raingarden.htm

For the "lower" pond, the pipe diameter is likely to be the control, rather then the perforations. If in doubt, you can model both as a compound outlet, with the orifice flow going to a culvert. However, unless the exfiltration rate is high in comparison to the orifice/culvert outlet, you won't get much ponding in the stone, so you may be able to simplify the pond to the open trench structure with exfiltration.

Your error message occurs because the software is unable to extrapolate from the storage or discharge curves, so it cannot complete the routing. You can fix the problem by defining the overflow storage and/or outlet. Please click the help button for details.

If you have further questions please post your HydroCAD project file or email it to HydroCAD support.


Peter Smart
HydroCAD Software

http://www.hydrocad.net