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Roughness coefficients (Manningls null) used in the hydrauli-:: computa- tions were chosen by engineering judgment and based on field observa- tior.s of the channel and overbank areas. A roughness value of 0.038 was selected for the chan~el of Brush Creek~ with overbank roughness values rangin~ fr~m O.O~5 to a.CEO =or all floods. starting water-surface elevations for Brush Creek were calculated usinq cri~ical dept~ at the mouth of Brush Creek. Flooa profiles were drawTI showing computed water-surface elevations to an accuracy of 0.5 foot for floods of the selected recurrence intervals (Exhibit 1). Approximate flood elevations were determined using normal depth calculations. All elevations are referenced to the National Geodetic Vertical Datum of 1929 (NGVD). Elevation reference marks used in the study are shown on the rnaps~ The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on tile profiles are thus considered valid only if hydraulic structures remain uno~structed, operate properly, and do not fail. 4.0 FLOOD PLAIN ~~AGEMZNT APPLICATIONS A prime purpose of the National Flood Insurance Program is to encourage State ar_a local govern~ents to ado?t sour~ ~lood ?lain management pro- graus. Each flood Insuracce Study, ther€fore~ includes a flood bounda~y map designed tc assist comn~niti€s in deve~o~ing so~nd flood plain manage- ment meaS'..lr~s. 4.1 Flood Boundaries In order to provide a national standard without regional discrimina- tion, the 100-year flood has been adopted by the Federal Insurance Administration as the base flood for purposes of flood plain manage- ment measures. The Sea-year flood is employed to indicate additional areas of flood risk in the community. For each stream studied in deta~l, the bo~~daries of the 100- and SOQ-year floods have been delineated using the flood elevations determined at each cross section; between cross sections, the boundaries were interpolated u~ing topographic maps at a scale of 1:1200, with a contour interval of 2 feet (Reference 8). In cases where the 100- and SOO-year flood boundaries are close together, only the 100-year flood boundary has been shown. 6