Flood Zone Calculation 3

[uncivilized]

I am trying to set a Base Flood Elevation for a subdivision. This is my first endeavor so please bear with me. A few questions:

1. Hydrologic method? I am using the SCS method. The peak that I am getting is a lot higher than the USGS regression equation (due to storage effects?). Should I use the more conservative?

2. Software? I have XPSWMM which I think is approved. But I also downloaded QUICK 2. The drainage area is 979 acres. At the lower end of the drainage area are (4) 36" RCPs which will cause the area to pond.

any help as far as outlining a solution would be most helpful.

 

[francesca]

Usually we construct have our DTM created by experts from aerial photographs. They're accurate to two feet and check out with field survey data. In some cases we use the local planning commission DTM for the county, which is accurate to 5'. We've had some cases where it was spot on, and others where it was out by 10'. We definitely don't design subdivisions off USGS topo maps!

I guess that should be past tense, because I went back to school in September, but they still work the same way.

 

[queque]

I just read the thread and have some small advice. I worked on developing the Regional USGS equations for 12 states in the Southwest.
The standard error is so large that they may be of limited value. I suggest you contact the USGS District Surface Water Specialist in your State and ask him to explain what is the error possible imbedded in applying the equations within your State.
I use the Tr 20 Methododolgy (HydrolCAD) and Hec 1 to get a range of flows for the same watershed. I then compare the results to the USGS regional equations. The answers can vary greatly .

 

[francesca]

In our area the errors were of the order of +/-50%. To account for this we took into consideration weather patterns (winter frontal rain more likely to produce a basin-wide flood than summer isolated thunderstorm) and the consequences of saturated ground or frozen ground in winter and modeled the winter Manning's coefficients (which are lower) with the regression equation flow + maximum error. We then modeled a second scenario with summer Manning's coefficients and the regression equation flow. The flows differed vastly, but as a result of the higher summer Manning's coefficients, the water surface elevations differed by less than a foot. We used the average of the two WSEs to plot the 100-year elevation.

The method we used is more art than science, but we used the 100-year elevation to set property boundaries (the developer sold off the land below the 100-year flood elevation to a farmer) rather than finished floor elevations, which will be on average 10' higher.

We did not plot the 100-year elevation for the opposite side of the creek, which belonged to a different developer.