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I I I I :1 I I I I I I I I I I I I I I If the spills were likely to increase due to aggradation of the channel or degradation of the banks, the effect on any new floodplains or planning downstream would need to be reviewed. 2. Floodolain and Hazard Delineation [HI The West Valley Overflow has no delineated floodplain either for the direct tributary local drainage or in combination with the South Boulder Creek overflows. Significant 100 year floodplain areas of 1.5 to 3.5 feet deep flooding (in streets) and shallow residential flooding (0.5 to 2.0 feet) is possible. The 500 year floodplains would be significantly greater than that shown on the FEMA Flood Insurance Rate Maps (FIRM's). The extent of the work possible and needed depends on the tasks authorized above, and map accuracy standards acceptable to Boulder Community, UDFCD, CWCB, and FEMA. We would suggest, at minimum, a HEC-2 or UNET delineation following the main west bank overflow route(s) with approximate delineation methods for small or shallow flow areas, particularly along interconnecting spill zones. Any decision to re-delineate of South Boulder Creek should await the results of hydrologic investigations and possibly master plan and improvement efforts. 3. Master Planning [HI A collaborative effort is needed to understand the problems and recognize a reasonable plan to address the poor local drainage and South Boulder Creek overflow hazard through the West Valley. Numerous hazard mitigation techniques and physical improvement components are feasible. A combination of techniques is likely necessary, as overly simple solutions potentially will just shift the problems, increase or not fairly address reduction of the hazard. V-7