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On West Fork Kenneys Run, downstream of the Welch Ditch crossing, a shallow flooding area, designated as Zone B, was shown in the right overbank. Flooding in this area is hydraulically disconnected from the main channel. The approximate studies were developed using cross sections from a U.S. Geological Survey topographic quadrangle map (Reference 19) and a computer program that generates a top width for each section based on cross section geometry, discharge value, energy gradeline slope, and roughness factor. The computed top width corresponds to flow at normal depth in the cross section. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the maps. 4.0 FLOOD PLAIN MANAGEMENT APPLICATIONS The National Flood Insurance Program encourages State and local govern- ments to adopt sound flood plain management programs. Therefore, each Flood Insurance Study includes a flood boundary map designed to assist communities in developing sound flood plain management measures. 4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the lOa-year flood has been adopted by the Federal Emergency Management Agency as the base flood for purposes of flood plain management measures. The 500-year flood is employed to indicate additional areas of flood risk in the community. For Clear Creek and Apex, Jackson, and Lena Gulches, the boundaries of the 100- and 50 a-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated using topographic maps at scales of 1:1,200 and 1:2,400, with contour intervals of 2 and 5 feet (References 20 and 21). The lOa-year flood boundaries for Cressmans Gulch, Cressmans Gulch Old Channel, Tucker Gulch, Kenneys Run, East Fork Kenneys Run, West Fork Kenneys Run, West Fork Kenneys Run Tributary No.1, North Fork of West Fork Kenneys Run Tributary No.1, West Fork Kenneys Run Tributary No.2, and Deadman Gulch were taken from Flood Hazard Area Delineation - Tucker Gulch/Kenneys Run (Reference 1). A 50 a-year flood boundary was not computed for that report. For all streams studied by approximate methods, the boundaries of the lOa-year flood have been delineated using the top widths calculated at each cross section; between cross sections, the boundaries were interpolated using topographic maps at a scale of 1:24,000, with a contour interval of 40 feet (Reference 19). 17