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. .- . .- Six Streams Group I Hydrology, Second Creek & Third Creek April 1, 1975 Page 3 more severe flooding conditions. However, the probability of occurrences for such a combination of storm events such as a thunderhead moving over the basin at a constant lag time ahead of the flood peak is considered to be much less than the basin wide 100-year storm event assumed here. The basin wide storm event is a reasonable and consistent meteorlogical event upon which to base a flood plain management program. After evaluation of the soil characteristics, infiltration rate, and pervious-impervious storage, the following hydro- logical factors assumed to be typical of the basin were se- lected and used in the computation of effected rainfall: Impervious depression and detention storage Pervious depression and detention storage Impervious loss Infiltration for I-hour .05"-.1" .30"-.35" 5% .50" SECTION D - HYDROGRAPHS Design storm hydrographs were computed at each of the design points for the 100-year storm frequency. Prior to calculating the storm hydrographs at each design point, storm hydrographs were calculated for each of the sub-basins. These sub-basin hydrographs were then lagged and added to obtain flows at each of the design points. Because of the complexity of this pro- cedure, and due to the number of hydrographs which have to be considered in an analysis of this type, the total process of computing the storm hydrograph, lagging and adding to the next downstream design point, has been computerized to operate an IBM 370 computer using disk storage. ! (i.'" , /' ( In completing the hydrology, flood plain storage was not con- ')Sidered ~n ~h7 lower reaches wher7 flc;>od fringe storage ~ould ~play a s~gn~f~cant role. Channel~zat~on could result wh~ch ,._d' ) will eliminate the present storage within the flood plain, therefore, the developed sub-basin hydrographs were simply lagged and added assuming no channel storage or flood peak reduction. ( l~t. Ii r I A summary of the peak flows for each creek for the 100-year frequency storm under a future developed condition, is pro- vided on the discharge profile identified as Figure I. The Peak Flows are also tabulated in Table 3, Design Point Flows. To supplement discharge profiles, plotted hydrographs have been attached as Figures II-IX. The hydrographs are representative of projected, planned conditions in the basins above each design point.