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Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

The project is a road extension that connects two neighborhoods together and crosses a stream (no FEMA floodplains). I did some preliminary hydro with GIS shown below.

DA: 450 acres
Weighted CN: 86
Tc: 180 min

There is a 100 acre subdivision within the drainage area, and the subdivision looks to have 7 large interconnected ponds. I do not have survey on the ponds or their outfall structures. I accounted for the subdivision with a curve number landuse of Residential Districts: 1/4 acre lots. No subdivision ponds were modeled.

I feel my current modeling methodology is simple and very conservative. I am not accounting for any storage in those upstream ponds, which will yield much higher peak flows versus modeling those individual ponds and outfall structures. Currently this design would lead to a very large (and costly) box culvert structure.

Is there a methodology you've used (or can think of) that accounts for upstream pond storage without actually modeling the upstream ponds to get more realistic peak flows?

RE: Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

typical procedure is to obtain the drainage report and grading plan as-builts for the subdivision. the town/city/county should provide those

RE: Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

What about this?

Estimate area of pond and swamp areas spread throughout watershed using contour map and your local knowledge.
Use Table 4-2 from TR55 June 1986 to obtain a pond and swamp adjustment factor, Fp.
Multiple your current peak flow by this Fp to obtain a smaller peak flow.

I do not believe you can use the pond and swamp adjustment factor if you are already storage-routing your watershed.
I do not believe you can take into account pond and swamp areas in the Tc flow path when you are determining the total area of pond and swamp areas in the watershed.
Perhaps soils identified as ponding by the NRCS Soil Survey would help estimate the total area of pond and swamp areas.
I do not know how this method looks at ponds that are mostly always full and thus have no storage.

RE: Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

I find all three of the methods listed here to be reasonable. Your method is overly conservative, scavey, but if it doesn't hurt your project, go with it. Your Tc may be high, check it. If you really want to do it "right," then do what cvg says.

Is all you need a peak flow? If so, you might be able to sneak by with a simple rational method analysis.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com

RE: Upstream Ponds - How Would You Account for Those (If At All) Without Modeling Them?

I would look at refining your tc path to account for the ponds upstream. Consider making an adjustment to the Tc path.

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