I am working on a flood study to evaluate the 100-yr BFE for a creek directly adjacent to a proposed landfill. (19.4 sq. mi drainage area, all agriculturual very flat, a lot of depressional storage in fields. The stream is highly channelized(4+ miles) and ungaged.
Using TR-20 flow calculated at 9,100 cfs using state regression equation (Clark Unit Hydrograph) flow is calculated at 4000+/- cfs.
I have read through FEMA's "Guidelines and Specifications for Flood Hazard Mapping Partners, Appendix C: Guidance for Riverine Flooding Analyses adn Mapping".
I Also read through the "Evaluation of Flood Frequency Estimates of Flood Frequency Estimated for Ungaged Watersheds", by the Hydrologic Frequency Analysis Workgroup(HFAWG), and the Minority Opinion.
It appears that the acceptable method is the USGS Regression Equation, unless the watershed does not fall into the specifed parameters of the USGS Regression Equations; however the Minority Report by the (HFAWG) indicates that using the Regression and standard error to determine if a Rainfall-Runfall model is acceptable, appears to rely to heavily on the Regression Equations at the Benchmark.
My concern: Due to the sensitive nature of the proposed site usage (landfill), I do not want to blindly use one method over another, however it seems to me that if one uses the regression equations, and one has to defend their position in court, it seems to me like someone could easily run the runoff-rainfall calcs and show a flow is nearly double what the regression equation shows (and subsequent increase in the BFE).
We have been collaborating with another consulting firm in town and they feel the regression equation is the way to go, based on the IDNR and FEMA's willingness to accept the regression equations, but I am not sold yet.
The acceptance of the regression equation seems counter intuitive when one can use a rainfall-runoff calculation and calculate a flow that is nearly double the regression flow.
My personal feeling is that if you are calculating the existing BFE for an unstudied stream and you know the potential use such as a school, landfill, residences, business, etc, it is easier to justify the additional earthwork, comp storage, etc. than it is to explain why the facility flooded because you used method x over method y to determine the flows...
Using TR-20 flow calculated at 9,100 cfs using state regression equation (Clark Unit Hydrograph) flow is calculated at 4000+/- cfs.
I have read through FEMA's "Guidelines and Specifications for Flood Hazard Mapping Partners, Appendix C: Guidance for Riverine Flooding Analyses adn Mapping".
I Also read through the "Evaluation of Flood Frequency Estimates of Flood Frequency Estimated for Ungaged Watersheds", by the Hydrologic Frequency Analysis Workgroup(HFAWG), and the Minority Opinion.
It appears that the acceptable method is the USGS Regression Equation, unless the watershed does not fall into the specifed parameters of the USGS Regression Equations; however the Minority Report by the (HFAWG) indicates that using the Regression and standard error to determine if a Rainfall-Runfall model is acceptable, appears to rely to heavily on the Regression Equations at the Benchmark.
My concern: Due to the sensitive nature of the proposed site usage (landfill), I do not want to blindly use one method over another, however it seems to me that if one uses the regression equations, and one has to defend their position in court, it seems to me like someone could easily run the runoff-rainfall calcs and show a flow is nearly double what the regression equation shows (and subsequent increase in the BFE).
We have been collaborating with another consulting firm in town and they feel the regression equation is the way to go, based on the IDNR and FEMA's willingness to accept the regression equations, but I am not sold yet.
The acceptance of the regression equation seems counter intuitive when one can use a rainfall-runoff calculation and calculate a flow that is nearly double the regression flow.
My personal feeling is that if you are calculating the existing BFE for an unstudied stream and you know the potential use such as a school, landfill, residences, business, etc, it is easier to justify the additional earthwork, comp storage, etc. than it is to explain why the facility flooded because you used method x over method y to determine the flows...
