How would I route this problem in Hydrocad?

[Mack454]

I have a site that has a large drainage line passing through it. The line conveys stormwater runoff from a large residential site upstream of me (roughly 250 acres). Running a quick calc, the pipe is undersized.

What I need to prepare is an analysis demonstrating that the low point of the residential development (this low point is just upstream of my site) does not allow surface water to flood to the point that it drains on my site. Between my site and the residential development is a berm that almost would act as a dam if the drainage system failed. I thought of running the low point (which is the intersection of 2 public streets) as a typical basin, but not sure that is the correct way to do it. I thought about breaking the 250 acres up into smaller drainage areas and using reaches with some assumed pipe sizes and slopes based on existing ground elevations.

Any advice/tips would be greatly appreciated.
Thanks

 

[psmart]

It sounds like you could model the "low point" as a pond, using a culvert outlet (the large drainage line) plus an overflow weir (the berm). Of course, you would need the appropriate inflow hydrograph(s) for the pond, based on the site runoff. The resulting model would give you the peak elevation in the pond, plus the overflow rate, if any.

PS: Please use the HydroCAD forum for HydroCAD-specific questions. (Although this seems to be a general modeling issue.)

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

I'm not sure I see a good reason to complicate things by modeling the street flow in the adjacent subdivision. While this may have some minor effect on the routing of the flow, what is the point if everything ends up ponding at the low point behind the berm (dam)? Make some simplifying assumptions and move on with the analysis. However, if this berm (dam) is large enough, I would be more concerned with analyzing it and the associated pond behind it for capacity, freeboard, and protection against overtopping. Your 250 acre watershed will create quite a bit of runoff which the berm(dam) must be able to hold or pass through the pipe without failure – otherwise you could have a breach failure causing a substantial flood wave into your property. For an example, think about those recent levee breaches in Missouri.

 

[Mack454]

Thanks guys. Sorry, I did not know there was a separate HydroCAD forum.

That is exactly how I started the routing. Low point in the street was the pond, using a culvert as my outlet pipe and the berm as a weir. It was topping the weir very easily which when looking at the site with some common sense would never happen. That would mean all the homes at that intersection would have water at their upper level and I know that is not true. I think I need to re-evaluate my CN and Tc numbers for a more realistic analysis.