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Cross-section data for Coal Creek in the area north of the UPRR were obtained from topographic maps at a scale of 1:4,800, with contour intervals of 2 feet and 4 feet (Reference 7). Field measurements were taken by the SCS to obtain elevation data and structural geometry of all bridges and culverts. Roughness coefficients (Manning's Un") for Coal Creek, north of the UPRR, were determined by the SCS through field inspections. The "n" values range from O. 085 to 0.120 for the existing channel and overbank areas. Roughness coefficients for Coal determined by WRC (Reference 8). approved by the cwes. The \'n" channel to 0.055 in the overbank. Creek, south of the UPRR, were These values were reviewed and values range from 0.075 in the Water-surface elevations (WSELs) for floods of the selected recurrence intervals on Coal Creek and the Coal Creek West Line Overflow were computed through use of the USACE HEC-2 step-backwater computer program (Reference 9). Locations of selected cross sections used in the hydraulic analyses are shown on the Flood Profiles (Exhibi t 1). For stream segments for which a floodway was computed (see Section 4.2), selected cross-section locations are also shown on the FIRM. The hydraulic analyses for this study were based on unobstructed flow. The flood elevations shown on the profiles are thus considered valid only if hydraulic structures remain unobstructed, operate properly, and do not fail. All elevations are referenced to the North American Vertical Datum of 1988 (NAVD). 3.3 Vertical Datum All FIS reports and FIRMs are referenced to a specific vertical datum. The vertical datum provides a starting point against which flood, ground, and structure elevations can be referenced and compared. Until recently, the standard vertical datum in use for newly created or revised FIS reports and FIRMs was the National Geodetic Vertical Datum of 1929 (NGVD). With the finalization of the NAVD, many FIS reports and FIRMs are being prepared using NAVD as the referenced vertical datum. As a part of this revision for Coal Creek, an elevation datum conversion was performed, and all elevations are now referenced to the NAVD using a datum shift value of 3.0 feet (NAVD = NGVD + 3.0). All elevations are referenced to the NAVD. To obtain up-to-date elevation information on National Geodetic Survey (NGS) Elevation Reference Marks (ERMs) shown on this map, please contact the Information Services Branch of the NGS at (301) 713-3242, or visit their websi te at www. nqs . noaa. gov. Map users should seek verification of non-NGS ERM monument elevations when using these elevations for construction or floodplain management purposes. It is important to note that adjacent communities may be referenced to NGVD. This may resul t in differences in Base Flood Elevat ions (BFEs) across the corporate limits between the communities. 6