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I I I I I I I I I I I I I I I I I I I loam soil. The infiltration rate of this series, is approximately 2.0 inches per hour for level ground with minimal ground cover. However, due to the steep slopes of the Green Mountain area, the effective infiltration will be consider- The Modified Pu1s method of flood routing was used for reservoirs or roadway embankment detention areas (Reference 6). The effects of detention vary from as little as 5% peak flow reduction in Drainage D at Union Boulevard to as much as 63% peak flow reduction in Drainageway H at Colfax ably less. For the lower intensity storms (i.e.: 10 year) an infiltration rate of 0,6 inches per hour was used for this analysis, based upon the soils' data and the Criteria Manual recommendations. For the higher intensity 100 year recurrence interval storm an infiltration rate of 0.4 inches per hour was utilized, due to the soil moisture considerations reCommended by the S,C.S, The rainfall-runoff model used for this analysis is the Colorado Urban Hydrograph Procedure (CUHP) presented in Reference 5, with May 15, 1975 and July, 1977 revisions to the b~sic data and procedure. A computer program was used to generate the unit hydrograph, the excess precipitation, and resulting storm runoff hydrograph for each of the sub-basins. 2. Flood Routing The next step in the hydrologic analysis consisted of routing and combining the individual sub-basin storm hydrographs to determine the peak flows and runoff volumes at the hydrologic points, The process of routing consisted of translating hydrographs along the channel and computing detention delays and storage at drainageway constrictions (ponds). The Muskingum method of channel routing (Reference 6), which was used in this analysis, computes the channel storage based on weighting of the inflow and outflow discharges. Due to the very steep channel which is narrow and incised throughout most of the study area, channel routing had very minimal effect on the peak flows. Avenue. One of the common effects of urbanization on natural drainageways is the separation of flow or flow splitting, which is caused by the inability of a drainage facility, such as a storm sewer or culvert, to pass the entire storm flows or by splitting of flows at a street intersection. The result of the flow splitting is that portions of the runoff are carried in more than one direction. Generally, flow splitting has minimal effect on the flood routing in a major drainageway analysis. However, due to the severe flow splitting that occurs in certain channel reaches of the study area, special consideration was given to the routing procedure. The following criteria was used to determine the flood routing where flow split can occur. (a) Where a significant portion of the peak flood flow (i,e, 10 year frequency or greater) follows the m~in channel, then the full hydrographs are routed and the flood plains defined accordingly, (b) Where a significant portion of the peak flood flow leaves the main channel ~lut returns to the main c:lannel within a short distance, the full hydrographs are routed and the flood olains defined accordingly. The full flood peak is subsequently used to define the flood plains within the main channel area. (c) Where a significant portion of the peak flood leaves the main chan- nel but does not return to the channel for a significant distance, the flood 7