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-- II I II I I I 1- - I I I I I I I I I I I Lower Big Dry Creek Hydrologic Study insufficient culvert capacity under the Union Pacific Railroad embankment (WWE 1989a). Greiner indicated that potential flood damage in its study area was due primarily to un- maintained channel ar~:as with high roughness values, un-maintained culverts full of sediment, and undersized stream crossings. Standley Lake and Great Western Reservoir were identified as providing significant flood control benefits. 5.4 FEMA Floodplain Mapping Additional sources of hydrologic information for the watershed include FEMA's Flood Insurance Rate Maps (FIRMs). The Weld County portion of the floodplain mapping shown on Figure 7 is based on these FIRMS. FIRMs identify the 100- and 500-year floodplains, base flood elevations (i.e., elevation of flood having a one-percent annual chance of being equaled or exceeded in any given year), and risk premium zones to enable insurance agents to issue accurate flood insurance policies to homeowners in communities participating in the National Flood Insurance Program (NFIP). FEMA mapping panels 08001C0018 G, 08001C0016 G, 08001C0017 G, 08001C0030 G, 08001C0035 G cover the portion of unincorporated Adams County along Big Dry Creek. All of the communities in unincorporated Adams County along Big Dry Creek were identified as flood- prone based on an initial identification date of February 1, 1979 (FEMA 1995). FEMA mapping paneLs 0802660981 C, 0902660990 C, and 0802660980 C cover Weld County along Big Dry Creek. This mapping was completed by September 28, 1982. The floodplain delineation for Big Dry Creek in Weld County is an approximate floodplain. A detailed study, including hydraulic modeling, has not been conducted. 971-179.092 June 2005 Wright Water Engineers, Inc. Page 38