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HYDROLOGY ORIGINAL STUDY The hydrology and subsequent flow rates for this basin have been obtained from a 1977 study by Wright-McLaughlin Engineers titled "Ralston/Leyden Creek Major Drainageway Planning". The purpose of their study was to create a master plan for flood hazard management for Urban Drainage and Flood Control District. Due to the complex nature of the drainage basin tributary to Ralston/Leyden Creeks and the extensive hydrological evaluations necessary, the method used to compute runoff was a mathematical model known as the Massachusetts Institute of Technology Catchment Model or MITCAT. At the time the MITCAT model was known to be more .sensitive to urbanization, channelization and reservoir variables than other commonly used methods. Flow estimates for the 100 year event under the 1977 conditions are presented in Table 111-1 for all of the Ralston Creek Basin. The location of the site under study is Ralston Creek above Leyden Creek. Flow estimates for the 100 year event under future conditions are presented in Table 111-2. These values are based upon future development with the existing channel and include routing the flood through Ralston Reservoir and Leyden Lake. The 100 year floodplain was determined assuming that Blunn Reservoir was not in place. However, Blunn Reservoir is in place and is a integral part of the flood management program. It was found through hydraulic analysis and routing that Blunn Reservoir has a negligible effect on the peak flows of Ralston Creek. It was also assumed that all reservoirs are full at the time of the design storm, resulting in only that storage which would take place above the spillway crests. Land use patterns were based upon the latest plans approved by the City of Arvada and Jefferson County. These land use plans forecasted medium to low development densities in the study area. Since the publishing of the 1977 study, very little development has actually occured and today the area is still agricultural land. ~ Flow estimates were never developed for the 500 year event. The latest Flood Insurance Rate Map of the area does not include a Zone B for the 500 year flood boundary. The last flood hazard boundary map revision was published on July 5, 1977. Once the flood magnitudes were determined for the future conditions, the floodplain was developed. The procedure for determining the floodplain was the HEC-2 backwater program developed by the U. S. Army Corps of Engineers. The majority of the information used to determine the channel cross sections was obtained by a 100 scale aerial topographic map of the area and surveyed culvert information. .