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III -2 2. Field inspection and testing and analysis of data pertaining to impervious cover, slopes, infi ltration rates and other phy9icaJ phenonemon pertaining to runoff flow. The inforrration was par'- tially presented in Table 11-2, with supplemental information to be found in the Hydrological Appendix. 3. Determination of runoff flows based 011 the. data establ ished in 1 and 2 above. The primary method applicable in the Manual is the ColoradoUrban Hydrograph Procedure (CUHP) which is based on a synthetic unit hydrograph developed by Snyder in 1938. This method assumes that one inch of effective rainfall during a spec- ific unit time basis will result in a certain runoff flow pattern and that this response can be sca~ed to various amounts of rain- fall and rainfall patterns. The flow pattern is determined from similarities in other gaged basins, c.ompal-ing general basin shape, area, development and slope characteristics. Another method was also used to establish a better understanding of the effects of basin development, channelization, on-stream reservoirs and other sensitivities. This method uses a computer model call the Massa- chusetts Institute of Technology Catchment I~odel (tIITCAT). The concept is to use basic fluid mechanics to analyze separately the ovel-land flo>1 and stream flow portions of surface runoff. The design rainfall is the most basic input to the overland flow area, called a catchment, and the catchment then inputs to the stream. One can then vary individual param- eters to understand the sensitivity of stream discharges to development patterns. For instance, the amount of impervious area and the. over.land flow distances to collecting streams are easi ly control~ed variables. LENA GULCH FLOOD HYDROLOGY A more detailed explanation of the hydrology is presented in the Hydro- logical Appendix, which presents additional factual data for both methods. Each of the analytical techniques used resulted in hydrological estimations which are within reasonable limits of one another. That is, a ten to twenty percent variation between methods ;;n determining flow peaks wi 11 not flood substantially different areas because that variation wi I 1 change the flow depth by only a few inches. The variation between pl-edictions is about 11 percent for present conditions and about 17 percent for future conditions. These variations are thought to be basically due to inaccu- racies in the lagging processes used in summing the CUHP hydro\jraphs, In other words, when one uses the CUHP, there is not a practical method of routing the hydrographs downstream and summing them, other than educated estimates. The following Table 111-1 presents the estimates of both methods at different basins for prese~t and future conditions by the 100- year event. Also presented are the earlier estimates prepared by the U.S. Army Corps of Engineers (USACE).