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SEHIES 2-YEAR 5- YEAR 10-YEAR fACTQR 0.88 0.96 0.99 No adjustment is needed for rarer return intervals since the two curves coincide after tile 1 a-year event. At this point. all adjustments have been made. Ttle next step is to proceed through further breakdown and arrangement of the Ilypothetical storm. 3.6. DIVISION INTO INCREMENTAL VAlcUES. One now takes the acljusted rainfall values for a particular storm (there are usually six values) and further sub(jivides these to arrive at a rainfall depth value for eadl time increment (for example, there will be twenty-four values for the 6-hour-duration 15-minute-interval case). This division into increments is usually performed by plotting trle values of rainfall depth (in inches) versus duration (in minutes) on logarithmic paper, fitting a curve tllroUgh these points, and then reading off accumulated depth values for each increment from the curve. Averaging the incremental change between the original point,; is usually a satisfactory alternative, since the depth-duration plot normally approximates a straight line after the first several values. Once an accumulated deplh for each interval has been determined, the depths are incremented to compute that portion of the dHpth tt-,at occurred in each period. 3.7. STORM ARBANGEMENT. The final step in tile storm definition is arrangement of the storm r2.infall into a specific pattHm. 3.7.1 Alternating Block Method. The pattern used most often by the Corps of Engineers is a "triangular" 8.rrangement, with the peak period in tt-Ie center of the storm. For out example, for a 24-hour duration storm with a computation interval of 15 minutes, the peak 15-minute depth would be placed in (assigned to) the tllirteenth hour or forly. ninth period of the twenty-four period storm sequence. ThH next-highest depth is placed just ahead of the peak (Period 48), the next highest depth just behind (Period 50), and so on until all 96 values are systematically arranged about the peak period. If a storm with a duration longer than ;~4 hours is to be arranged, all 24-llOur periods outside of the peak ;~4 hours Can be represented by an average value for each 24.. hour period. The rainfall increments cannot be moved outside the 24-hour period from which the increment was developed, however. Colorado Flood Hydrology Manual DRAFT 7.6