FLOOD03304 - Laserfiche WebLink

Home Browse Search

SERIES 2-YEAR 5- YEAR 10-YEAR FACTOR 0,88 0,96 0.99 No adjustment is needed for rarer retum intervals since the two curves coincide after the 10-year event, At this point, all adjustments have been made, The next step is to proceed through further breakdown and arrangement of the hypothetical storm, 3.6. DIVISION INTO INCREMEN'tAL VALUES. One now takes the adjusted rainfall values for a particular storm (there litre usually six values) and further subdivides these to arrive at a rainfall depth value for each 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 the values of rainfall depth (in inChes) versus duration (in minutes) on IogarRhmic paper, fitting a curve through these points, and then reading off accumulated depth values for each increment from the curve, Averaging the incremental change between the original pOints is usually a satisfactory altemative, since the depth-duration plot normally approximates a straight line after the first several values, Once an accumulated depth for each interval has been determined, the depths are incremented to compute that portion of the depth that occurred in each periOd, 3.7. STORM ARRANGEMENT. The final step in the storm definRion is arrangement of the storm rainfall into a specffic pattem, 3,7,1 A1temating Block Method, T'he pattern used most often by the Corps of Engineers is a "triangular" arrangement, with the peak period in the center of the storm. For out eXample, for a 24-hour duration storm wRh a computation interval of 15 minutes, the peak 15-minute depth would be placed in (assigned to) the thirteenth hour or forty- ninth period of the twenty-four period storm sequence, The 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 24 hours is to be arranged, all 24-hour periods outside of the peak 24 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 DRAFI' 7.6