FLOOD03304 - Laserfiche WebLink

Home Browse Search

5.2 PMS FOR FRONT RANGE. Hegionalized estimates used in the derivation of the all-season PMS for Colorado between the 103rd meridian and the Continental Divide are found in HMR 55A, HMR 55A provides information needed for determination of the general PMS for durations up to 72 hours, for area sizes up to 20,000 square miles in basically nonorographic regions, iilnd areas up to 5000 square miles in the orographic influenced portions of the study area, AlsCI, local PMS crileria is provided for dlJrations up to six hours and area sizes up to 500 square miles, Because of the complexily of terrain, variety of storm types, and lacl< of available storm areal precipitation data, a modilied approach to the PMS determination to that used in the eastern nonorographic United States was required, Because of the variation in terrain features and dilficulties encountered in transposing observed areal precipilation in such complex regions, the committee decided to evaluated the maximum storm potential using a "storm separation" method, This method assumes that total precipilation from individual storms occurring in an orographic setting can be divided into two separate components for evaluation, Using storm the storm separation method, the total 24-hour storm precipilation was evaluated based on individual storm relationships of observed precipitation, isohyetal patterns, and meteorological data and analysis to develop an estimate on amount of precipilation due to free atmospheric forcing and amount due to orography, HMR 55A presents 24-hour PMP for specilic locations assumed to represent areas of 10 square miles, The calculationS were determined for a dense grid coverinll the study region, and a smooth analysis of the data was produced, After meteorological and topographical inconsistencies were addressed, a final map of a 24-hour, 10-square mile PMP was obtained, Using smoothed regional analyses of 1- to 6-hour, 6- to 24-hour, and 72-hour to 24-hour precipilation ratios derived from observed storms of record, a 10-square mile PMP was derived for 1-, 6-, and 72-hour durations, Using relationships from severe storms of record and data supplied form adjacent PMP studies, depth-area relationships were assembled for various subregions based on topographical and meteorological variations in such parameters, Nornographs were then constructed that allowed the user to determine values of PMP for various size drainages wilhin the HMR 55A study region and within the areal and durationallimilations of the report, (Bureau of Reclamation, 1989) 6. SUPPLEMENTAL INFORMATION. Addilional material to consider in deriving these storm data which are also applicable to hypothetical storm developmEmt include: 6.1. EXTRAPOLATION OF FRECIUENCY DATA. The technical Papers of the NWS indicate that a limited extrapolation (to 200-year return periods) is appropriate based on the available generalized data, However, extrapolation to a SOO-year retum period, as is often required for the hydrology necessary in flood-insurance studies, is of qUElstionable validily, Rainfall for the extrapolated 500-year event may be only 20 to 30% greater than Colorado Flood Hydrology Manual DRAFT 7,13