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I I I I I I I I I I I I I I I I I I I 6 it is felt that peak flow rates will rise somewhat. The basin, at present, is poorly drained; there are many streets without gutters and many areas of local drainage problems which result in inadvertant detention storage. The drainage routes are fre- quently circuitous, poorly defined, and parallel to the main channels, rather than directing flow to the channels. All these factors tend to presently "flatten out" the runoff hydro graphs and keep peak flow rates low. It is anticipated that future development will result in regrading of many poorly drained streets and elimination of the local drainage problems. These changes will result in faster runoff and higher flood peaks. TABLE 1 DESIGN RAIN STORMS ON DRY GULCH AND TRIBUTARIES lN DENVER AND LAKEWOOD. COLORADO Rainfall parameters were selected from the Urban Storm Drainage Criteria Manual (Ref.l) t.o represent the basin. Design storms of 200 minute durations were adopted and are itemized in Table 1. Parameters representing each sub-basin tributary to t_he "points Time Interval Rainfall During Interval (inches) (Minutes) 5-yr. 10-yr. 50-yr. 100-yr. storm storm storm storm -- 0-10 .05 .06 .07 .lO 10-20 .18 .23 .13 .17 20-30 .69 .86 .27 .27 30-40 .22 .26 1. 00 1. 05 40-50 .os .10 .31 .34 50-60 .07 .08 .13 .17 60-70 .06 .07 .12 .10 70-80 .04 .05 .07 .09 80-90 .03 .05 .06 .08 90-100 .03 .04 .05 .06 100-110 .03 .04 .05 .06 110-120 .03 .04 .05 .06 120-130 .03 .03 .05 .05 130-140 .03 .03 .05 .05 l40-150 .02 .02 .04 .05 l50-l60 .02 .02 .04 .05 160-170 .02 .02 .04 .05 170-180 .02 .02 .04 .04 180-190 .02 .02 .04 .04 190-200 .02 .02 .04 .04 --- --- -- ----- TOTAL 1. 69 2.08 2.61 2.92 The hydrologic analysis was initiated by the delineation of the drainage basin boundaries, or divides, which separate Dry Gulch, North Dry Gulch, and Colfax Gulch from adjacent basins as well as from each other. These basins are illustrated on Sheet 2 of the FHAD Maps in Section VII. The basins were further divided into sub-basins which represent areas tributary to specific points along each channel, so that computations of peak discharges could be made at the "points of hydrologic com- putation" designated on FHAD Sheet 2.