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I 18 I I To obtain most of the design flow hydrographs, two or more of the "computer hydrographs" were added graph i ca 11 y. Th i s was neces sa ry, especially on Fourmile Canyon and Twomile Canyon Creeks, because the upper portions of the larger basins have different hydrologic charac- teristics than the lower portions of the basins, thus making it neces- sary to compute the different hydrologic regions separately. When adding hydrographs from different basins together, such as where Wonderland Creek enters Goose Creek, the absolute time of the hydro- graph had to be determined and the addition lagged appropriately since the peak flow from each basin would not reach the confluence at the same time. In the smaller channels the shape of the upper basin hydro- graph as it came downstream was assumed to retain its original shape, In the larger channels of Goose Creek and Fourmile Canyon Creek, some channel storage was accounted for which lowered the peak of the hydrographs and spread out the hydrograph time. (This is an example of benefits to be derived by the protection of channel storage areas from encroach- ment .) I I I I The U. S. Geological Survey Water Supply Paper 1680 was used to check the unit hydrograph computed flow from the foothill basins of Fourmile Canyon and Twomile Canyon Creeks with the procedure recommended in the Urban Storm Drainage Criteria Manual. I I I SPECIAL CONSIDERATIONS This study is concerned only with rather small tributaries to Boulder Creek; however, when completed, these major channels will be signifi- cant factors in the floods on Boulder Creek below Boulder. After urbanization the estimated combined 100-year flood flow peak from the project area of 17 square miles will be approximately 8,000 cfs. This compares with the 11,000 cfs estimate of the U.S.G,S. for all of Boulder Creek at the confluence with Goose Creek under present con- ditions. This points out how significantly urbanization increases peak runoff flow, I I I Several detention ponds are included in the proposed project. Three ponds are schematically shown at the canyon mouth of Fourmile Canyon Creek. These ponds, besides acting as sediment collectors, will store part of the peak of the flood hydrograph and reduce the design flood flow from 3,200 cfs above the ponds to 2,000 cfs below the ponds. These ponds should have a minimum active flood storage volume of about 100 acre-feet. I Another detention pond of five acre-feet at the beginning of Elmer's Twomile Creek will reduce the downstream design hydrograph from 950 cfs to 750 cfs, The effect of this pond will be reflected on the design flow down to the confluence with Goose Creek. I I I This study assumed that all water tributary to Wonderland Lake during the major flood would be detained by Wonderland Lake and would not I II