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" t l S + . t ... + i 7'l?'~ t ~Id ~ & j ~~ j f J ~ J ~~~ ~DUYOFITIM'MU ,~--- --------- ff ~ ~i+-~ t 8CAU!.-Ft:IET s Figure 2. Precipitation pattern derived in this studY for the period 1830 MDT 31 July to 0300 MDT 1 August 1976. timing of the flash flood peaks seemed to give the best fit for the period of intense rain over most of the subbasins. The rainfall for this 4- hour period was initially estimated as Po percent of the total rainfall occurring between 1830 MDT 31 July and 0300 MDT 1 August 1976. The pattern was sUbsequently refined as analysis of the reconstructed flood peaks indicated. The adopted dimensionless depth-duration curve was applied to each subbasin rainfall to derive 10-minute incre- ments of rainfall. Guided by unitgraph ordinates, the 10-minute rainfall increments were arranged so that peak discharges from the subbasins occurred near the reported times. A double burst arrangement was devised for ,those subbains which were reported to have had double peak floods. The detailed shape of the isohyetal pattern for this 4-hour period was developed by trial on the basis of flood peak discharge reconstructions. The 2-inch iShoyet was assumed to be the limit of the contributing area for this rainfall period. The final 4-hour precipitation pattern (1830 to 2230 MDT 31 July 1976) is shown in Figure 4. The three mass curves shown in Fi9ure 5 were prepared for use in distributing subbasin rainfall. They were developed by applyin9 the adopted dimensionless depth-duration curve to subbasin average precipitation from the 4-hour pattern in Figure 4 and rearranging incremental values to provide peak discharges at appropriate times. The areas to which these three distributions apply are shown in Figure 6. Curve A in Figure 5 was used for subbasins 5, A, 137