The City of Alpharetta retained Manhard Consulting to develop regulatory floodplain maps that reflect year 2020 “built‐out” conditions of the City. The Metro North Georgia Water Planning District is now requiring municipalities within the 16‐County district, in which the City of Alpharetta is a participating municipality, to complete floodplain mapping built‐out condition floodplains for all streams with a drainage area greater than 100 acres.
The City has approximately 39 miles of stream with a drainage area of 100 acres or greater. These basins comprise a total of over 96 square miles. All of the City’s streams and tribu taries discha rge into Big Creek, which ultimately discharges into the Chattahoochee River a few miles south of the City limits.
Manhard developed a HEC ‐ HMS flood hydrograph/hydrologic model with a HEC ‐ GeoHMS ArcView interface to simulate 100 ‐ year peak flood discharges along the project’s 39 miles of stream. Of the 39 miles of stream within the study area, approximately 24 miles of streams are currently included within the 1998 effective FEMA models and FIRM mapping.
The hydraulic analysis on these streams incorporated the peak flood discharges into the effective FEMA hydraulic models for the calculation of both the 100 ‐ year floodplain and floodway boundaries. Finally, floodplain and floodway delineations using HEC ‐ RAS and HEC ‐ GeoRAS were completed, and a comprehen ‐ sive report on the floodplain modeling and mapping effort performed was provided to the City.
The result of Manhard’s study established regulatory 100 ‐ year floodplain limits that are more stringent than the FEMA requirements and provide communities with flood hazard boundaries. These flood hazard boundaries, based on built ‐ out watershed conditions, allow reviewing authorities to asse ss the effects of a proposed development in a “worse case” scenario.