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I I I I I I I I I I I I I I I I I I I ..- ., 12 HYDROLOGICAL APPENDIX DETERMINATION OF DESIGN FLOWS - CUHP RAINFAll The CUHP was used to determine the magnitudes of the 100-year flood for each subbasin. This procedure, which was developed for DRCOG is based on the synthe- tic unit hydrograph theory. Hydrographs were then routed and combined with MITCAT, which is explained in the next section. Depth-duration-frequency curves were developed for the basin to represent the various frequencies of design rainfall. These curVeS were determined from the isohyetal maps present in the Rainfall Section of the DRCOG Storm Drainage Criteria Manual and are shown in Figure HA-l. The time incremented rainfall is derived from these curves, rearranged to a design rainfall for the various frequency events and presented in Table HA-l. The unit hydrograph principle waS originally developed by Sherman in 1932. The synthetic unit hydrograph, which is used for analysis when there is no rainfall-runoff data for the basin under study, as is the case for South lakewood Gulch, was developed by Snyder in 1938. TABLE HA-l A unit hydrograph is defined as the hydrograph of 1 inch of direct runoff from the tributary area resulting from a unit storm. A unit storm is a rainfall of such duration that the period of surface runoff is not appre- ciably less for any rain of shorter duration. The unit hydrograph tht.: represents the integrated effects of factors such as tributary area, shape, street pattern, channel capacities, and stream and land slopes. INCREMENTAL RAINFAll DATA (All Values in Inches) Time Incremental Rainfall For Various Frequencies (M i nutes) 5-Year 10-Year 50-Year 100-Year 0 0 0 0.00 0.00 10 0.03 0.04 0.06 0.06 20 0.06 0.08 0.09 0.10 30 0.14 0.22 0.25 0.25 40 0.59 0.80 0.95 1.04 50 0.19 0.30 0.30 0.36 60 0.09 0.10 0.15 0.28 70 0.08 0.10 0.12 0.17 80 0.05 0.06 0.08 0.09 90 0.04 0.05 0.06 0.09 100 0.04 0.05 0.06 0.08 110 0.04. 0.05 0.05 0.08 120 0.03 0.04 0.05 0.08 130 0.03 0.04 0.05 0.07 140 0.02 0.04 0.04 0.07 150 0.02 0.02 0.04 0.04 160 0.02 0.02 0.03 0.03 170 0.02 0.02 0.02 0.02 180 0.01 0.02 0.02 0.02 To apply the unit hydrograph, the effective precipitation depths for the "unit storm" periods are multipl ied by the ordinates of the unit hydrograph and added to obtain a design storm runoff. The basic premise of the unit hydrograph is that individual hydrographs resulting from the successive increments of rainfall excess that occur throughout a storm period will be proportional in discharge throughout their length, and that when properly arranged with respect to time, t~e ordinates of the individual unit graphs can be added to give ordinates representing the total storm discharge. The hydrograph of total storm discharge is obtained by summing the ordinates of the individual hydro- graphs. The derivation and application of the unit hydrograph are based on the following assumptions: 1. The rainfall intensity is constant during the storm that produces the unit hydrograph. 2. The rainfall is uniformly distributed throughout the whole area of the drainage basin. 3. The base or time duration of the design runoff due to an effective ra infa 11 or un i t durat ion is cons tan t. 4. The ordinates of the design runoff with a common base time are directly proportional to the total amount of direct runoff repre- sented by each hydrograph. 5. The effects of al I physical characteristics of a given drainage basin, including shape, slope, detention, infi I tration, drainage pattern, channel storage, etc., are reflected in the shape of the unit hydrograph for that basin.