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I I I I I I I I I I I I I I I I I I I IV. FLOODPLAINS AND AREAS OF POTENTIAL LOCAL FLOODING The 100-year potential flooding areas shown in Figure 5, which are al so delineated on the Preliminary Drainageway Design Plans (Sheets 1 through 15), of the Phase B study, can be used in the land use planning and development process in Lafayette and surrounding Boulder County, until all drainageway construction is realized. Residual lOO-year flood prone areas resulting from channel or culvert designs of less than 100-year frequency are also delineated on the drainageway drawings in this report. The flooded areas identified on Figure 5 have been prepared in anticipation of problems that would result from continued land development without providing for drainageway facilities to handle increased runoff due to urbanization. Flood Frequency The State of Colorado, Urban Drainage and Flood Control District, Boulder County and the City of Lafayette, consider the 100-year frequency flood as the flood event to be used in designing and protecting structures and dwellings for human occupation. Therefore, all floodplain regulations are based upon the 100-year flood. The 100-year flood occurs, on the average, once in a 100-year period, and has a one percent chance of bei ng equalled or exceeded every year, Flood Profil es The potential flooding areas caused by a 100-year frequency flood event with full basin development were submitted to the City of Lafayette, Boulder County and Urban Drainage and Flood Control District in the form of a 100-year Flooded Area Map, contained in the Phase A Development of Alternative Plans dated March, 1980. This map is presented in Figure 5 of this report. The Lafayette basin is al so impacted by the IOO-year fl oodpl ain from Coal Creek. The floodplain information for Coal Creek is contained on the "Flood Hazard Analyses, Coal and Rock Creek, Boulder and Weld Counties, Colorado," pre- pared by the Soil Conservation Service, (Reference 2), The approximate limits a re shown on Fi gure 5, but it is recommended that Reference 2 be used for detailed information. The 100-year floodplain identified in the Coal Creek Flood Hazard Analysis report has been designated by the State of Colorado for use in floodplain regulation and management. It is recommended that the enforcement of the 100-year floodplain regulations for Coal Creek be continued by Lafayette and Boulder County. The major drainageways recommended in this study will have no impact on damage mit i gat i on from a 100-year fl ood event on Coal Creek. Peak discharges for each sub-basin delineated on Drawing B were determined for both existing and future basin conditions for the 2-, 5-, 10- and 100-year frequencies, The sub-basin peak discharges were then routed down existing drainage paths in order to identify the potential flooding areas in Lafayette and adjacent Boulder County. The water surface profiles were computed using the HEC-2 backwater program developed by the U.S, Army Corps of Engineers, Digit i zed potent i al flood area cros s- sect ions \\ere obtai ned from aeri al photos and checked against topographic mapping with 2-foot contour, I-inch to 200-foot scale. Roughness coefficients and hydraul ic structures (cul verts and bridges), were evaluated based on field investigation and survey information. Flooding Potential Review of the potential Flooded Area Map (Figure 5) reveals that the potential for future fl ood damages is greatest in the lower portions of the drainage basin. Flows from upper' sub-basins in Basin 6 result in large shallow flooding 16 17