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<br />II <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />4.3.4 HEC-FFA Flood Frequency Analysis <br /> <br />HEC-FFA is the Army Corps of Engineers Hydrologic Engineering Center's Flood Frequency <br />Analysis computer program, This program allows existing stream flow data to be entered from <br />gauging stations or other sources, It will then calculate the peak discharges for the 100-yr flow <br />along with other peak discharges, The only other parameter that is needed is a regional skew <br />number that can be obtained from the USGS, For more information on HEC-FF A consult the <br />HEC-FFA Flood Frequency Analysis User's Manual, <br /> <br />4,3.5 HEC-HMS RainfalllRunoffModel <br /> <br />The Hydrologic Engineering Center has developed a Hydrologic Modeling System (HEC-HMS) <br />that can be used to perform a rainfalVrunoffmodel. An ArcView extension, called HEC- <br />GeoHMS, has also been developed to ease this analysis. HEC-GeoHMS requires a grid data <br />source to be used as a terrain model for the drainage basin being studied. Therefore, the first step <br />in conducting the detailed hydrology for Cedar Run was to develop this grid data set. This was <br />accomplished by using the 20 foot contour ArcView shape files, obtained from David Barnet of <br />BristleconeGIS, of the drainage basin of Cedar Run and using them as mass points with the <br />contour assigned as the height source to create a triangulated irregular network (TIN) using the <br />3D Analyst ArcView extension, This TIN was then converted to an ArcView Grid using the <br />Spatial Analyst extension, The grid was created to have a 4,62-meter cell size, which <br />corresponds to approximately 10,000,000 cells, <br /> <br />One problem that was encountered was the HEC-GeoHMS extension found that the Cedar Run <br />channel flowed into Surface Creek in various locations, Artificial barriers were then created as <br />polygon shapes in ArcView that effectively confined Cedar Run to the appropriate location, <br />Appendix C gives the steps needed for completion of this task of creating a "Hydrologically <br />Corrected" terrain model. Once the grid data set is developed, HEC-GeoHMS creates a <br />"Depressionless" terrain model that will more accurately determine the hydrologic characteristics <br />of the basin. It then performs a number of steps that will set up the basin characteristics so that <br /> <br />18 <br />