Water Quality Storm

[mherr78]

Delaware's Department of Natural Resources & Environmental Control (DNREC) require the design of water quality structures to be able to manage the rate and volume of a 2" NRCS Type II rainfall event, up to a maximum of 1" of runoff.

Is it possible for HydroCAD to stop the routing procedure at a certain runoff depth? In my project, I have subcatchments that generate runoff depths that range from 0.5" to 1.6", for the 2" storm.

 

[psmart]

You could use the SCS runoff equation to determine the rainfall depth that will produce one inch of runoff for a certain CN value. Of course, that depth will be different for different CN values. I presume that's the crux of your question.

If you apply 2" of rainfall to all areas, what does DNREC want you to do when you reach the 1" runoff depth? Truncate the hydrograph at that time? And how is the peak flow determined under these circumstances?

Although HydroCAD can simulate any rainfall depth you select, there is now way to "stop the routing procedure at a certain depth". This is not a physically realistic situation, so it's hard to simulate.

Will the regulations allow you to design for 1" average runoff over the entire site?

For a discusion of first-inch calculations please see

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

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[TerryScan]

mherr78-

You may just need to do things the old fashion way. Delaware's regulation originally was simply the 1" of runoff. One has to determine the rainfall depth that would produce this amount of runoff.

It is rather untidy, but to optimize your design, one may need to create multiple models: one for subcatchments with 2" rainfall, and other models for those subcatchments that exceed the 1" runoff. In the latter models, one would adjust the rainfall to produce the desired volume.

Perhaps a wish list item for HydroCAD would be the ability to overide the rainfall depth on individual subcatchments. I do know of other hydrology packages with this ability.

 

[psmart]

If you want to use a different rainfall depth for a specific subcatchment, you can put it in a separate file and use a link to import the hydrograph.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[mherr78]

Thanks for all of the responses. I think I knew the answer before I posted, since I use HydroCAD almost daily, but I wanted to see if I had missed something. I figured on having to improvise, I was just hoping to keep everything in the same format for simplicity.

It looks like breaking the model up for water quality analysis will be my best bet. Fortunately, it's rather easy.