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<br />5.1.8. With the maximized storm pattern established, the spatial <br />variability of the precipitation is determined, again by HMR-52, through figures and <br />nomographs. Spatial variability is greatest for the maximum six-hour period, <br />diminishes for the second and third greatest six-hour periods, and has no variability <br />for the remaining six-hour periods. Precipitation profiles are given in HMR-52 to <br />develop the spatial variation in the peak 18 hours of the PMS. <br /> <br />5.1.9. Final development of the PMS calls for a temporal arrangement <br />to give the most critical hydrologic response (maximum runoff). the six-hour values <br />of the PMS are arranged such that they decrease progressively to either side of the <br />greatest six-hour value. the four greatest six-hour increments are placed at any <br />position in the sequence except during the first 24 hours. <br /> <br />Only the general outline of PMS development has been given in this <br />appendix. The engineer should refer to HMR-51 and HMR-52 for detailed guidance in <br />determination of the PMS. <br /> <br />6. SUPPLEMENTAL INFORMATION. <br /> <br />Additional material to consider in deriving these storm data which are also <br />applicable to hypothetical storm development include: <br /> <br />6.1. EXTRAPOLATION OF FREQUENCY DATA. The technical Papers of <br />the NWS indicate that a limited extrapolation (to 200-year return periods) is <br />appropriate based on the available generalized data. However. extrapolation to a 500- <br />year return period. as is often required for the hydrology necessary in flood-insurance <br />studies. is of questionable validity. Rainfall for the extrapolated 500-year event may <br />be only 20 to 30% greater than the 100-year total, but significantly less than the SPS <br />for the same storm duration. While an SPS cannot be assigned a specific frequency <br />of exceedance. it is likely that it is of the same order of magnitude as a 500-year <br />event. An evaluation of the 500-year rainfall should include an examination of the <br />SPS rainfall to assist in development of appropriate estimates of the 500-year event. <br />An adjustment of an extrapolated 500-year rainfall total may be necessary to ensure <br />reasonable compatibility yvith SPS. <br /> <br />6.2. URBANIZATION EFFECTS. Meteorological studies have shown an <br />increase in the number and intensity of thunderstorm rainfall events for watersheds <br />downwind from major urban areas (population greater than one million). Studies of <br />downwind rainfall for Chicago, St. Louis, Detroit. Washington. Houston. New Orleans. <br />and Cleveland have shown an increase in warm-season rainfall ranging from 10% at <br />New Orleans. to 25% at Chicago for areas up to 30 miles downwind from the city. <br />A study of St. Louis weather patterns found that the number of heavy rainstorms has <br />increased dramatically since 1960. with 5-minute rainfall rates increased by at least <br />50% over large downwind areas. While there is currently no direct way of <br /> <br />7-21 <br />