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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 IOO-year flood discharges shown in the figures mentioned 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 dOI,nstream end of the study area where a sl ight reduct ion in the peak discharge occurs. This except ion is attrib- uted to the rout ing 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 downstream 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 wi 11 cant inue 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 IOO-year flood event. The IOO-year flood event is defined as that which has a one percent chance of being equalled or exceeded in anyone year. The IOO-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 IOO-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 IOO-year flood magnitude can and will occur and plans for land improvement adjacent 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 100-year flood plain limits and water surface profiles for each of tbese reacbes of the Dad Clark Gulch flood plain are shown on the Flood Hazard Area Del ineation drawings (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: 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 IOO-year flood event and are the discharges used in the hydraulic analysis portion of tbe flood hazard area delineation. The discharge values shown are tbose which could occur under fully developed conditions without any mitigating measures being taken to control or retard the increased runoff. A subsequent report will address reduced discharge rates which are more representative of final design conditions and will describe the mitigating meas ures wh ich wi 11 be taken to acb i eve a d i sch arge rate wh i ch does not exceed pre-development cond it ions at tbe downstream edge of the Hi gh 1 ands Ranch development, i.e. County Line Road. 9