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I I I I I I I I I I I I I I I I I I I V. FLOOD HAZARD AREA DELINEATION The 100- year fl ood d i sch arges shown in the fi gures ment i oned above are conservative in that the highest discharge rate calculated for a subreach is used for the entire subreach length, not just for the point for which it was calculated. In general, discharge values were selected such that they increased in step fashion as the flood progressed downstream. One exception to this is near the downstream end of the study area where a slight reduction in the peak discharge occurs. This exception is attrib- uted to the routing procedure used and reflects a more rapid increase in flood plain storage capacity than in the inflow from the tributary drainage area; i.e., if no further inflow occurred, the peak discharge would get smaller as the flood proceeded dO\-Instream since the constant flood volume would be spread out over a longer length of the channel. A. FLOOD FREQUENCY AND DISCHARGE Floods have occurred in the past and will continue to occur in the future along Dad Clark Gulch with magnitudes equal to and greater than those that have already occurred. As development progresses within the drainage basin the flooding potential will increase and actual flooding will occur more frequently. The Dad Clark Gulch drainage basin has been analyzed to determine the discharge at various key locations along the flood plain for the 100-year flood event. The 100-year flood event is defined as that which has a one percent chance of being equalled or exceeded in anyone year. The 100-year flood is considered by the Urban Drainage and Flood Control District, the Colorado Water Conservation Board, and the Federal Insurance Administra- tion, as the flood magnitude for which flood plains should be designated for regulatory and improvement purposes. In Colorado, the 100-year flood plain is an area of state interest as defined in H.B. 1041. Lesser floods with higher probabil ities of occurrence have, therefore, not been con- sidered here. Flood magnitudes greater than the 100-year flood magnitude can and wi 11 occur and plans for 1 and improvement adj acent to the 100- year flood plain should consider the probability of flood damage. B. FLOOD PLAIN AND FLOODWAY DATA The Dad Clark Gulch flood plain has been divided into six reaches in order to more readily identify the various tributary branches of the waterway. Refer to Basin Boundary Map in the Appendix of this report for exact locations of each of the six reaches. The peak flow profiles given in Figures 6 through 11 in the Appendix show the discharges developed in the hydrologic portion of this study for the 100-year flood event and are the discharges used in the hydraulic analysis portion of the flood hazard area delineation. The discharge values shown are those which could occur under fully developed conditions without ~ mitigating measures being taken to control or retard the increased runoff. A subsequent report wi 11 address reduced di sch arge rates wh ich are more representative of final design conditions and will describe the mitigating measures which will be taken to achieve a discharge rate which should not exceed pre-development conditions at the downstream edge of the Highlands Ranch development, i.e. County Line Road. The lOO-year flood plain limits and water surface profiles for each of these reaches of the Dad Clark Gulch flood plain are shown on the Flood Hazard Area Del ineat ion drawi ngs (see Appendix), and the computed 100-year flood elevations for the various cross section locations are tabulated in Tables 3 and 4 as discussed earlier. A brief description of the flooding for each of these reaches follows: 9