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<br />. . <br /> <br />Proposed Scope of Additional Work to be performed by HDR <br /> <br />I. Depth.Area-Duration Radar Analysis of Recent Historical Storms - $22,250 <br /> <br />Importance factor: Refines the depth-area-duration issues between HMR 51/52 and 55A <br />and extreme local storms in Colorado in a quantitative manner. <br /> <br />The development of a oasin-specific PMP will be enhanced by the radar evaluation and <br />morphoiogy of several recent extreme precipitation events that have direct application to the <br />Cherry Creek basin. The storms are listed below: <br /> <br />1. Fort Collins CO Flash Flood of Julv 1997: This storm is a transposition candidate and <br />dropped 10-14" of rain over a 24-30 hour period along the Larimer County foothills. <br /> <br />2. Julv 29.1997 west-central Arapahoe Countv flash flood: This lesser-known event <br />produced significant erosion and flooding of small stream basins in west-central <br />Arapahoe. northeastern Douglas and northwestern Elbert Counties the day after the Fort <br />Collins flash flood. Radar estimates of 5-8 inches of rain fell in less than 3 hours with this <br />storm that was overshadowed by the Fort Collins tragedy. <br /> <br />3. Parker/DouQlas Countv.Julv 1998: This storm dropped a measured 0.96"/5min 25 sec <br />and 3.12"/45 min of verified rainfall in the lower Cherry Creek basin.. This extreme event' <br />is the most reliable measurement of extreme short-duration rainfall in the county. . <br /> <br />4. Juiv 17. 2000. Northeastern Dou!:lIas/Northwestern ElbertCountv: This storm was. <br />partially reviewed by CWCB and dropped 4-6" of rain in an hour near Elizabeth while <br />producing significant flooding. <br /> <br />5.. Julv.B. 2001. Cherrv Creek Arts Festival storm: This storm produced 3-4 inches of <br />rain in less than an hour as a Denver Cyclone event maximized the storm system across <br />southern Denver and western Arapahoe Counties. <br /> <br />HDR proposes to complete a radar re-construction of the rainfall of these events on a 5/6-min, 0.5 <br />mile by 0.5 mile grid. This rainfall will be transferred to a GIS grid and used to determine the <br />historical storm Depth-Area-Duration (D-A-D) values to assist in the development of the site- <br />. specific PMP consistent with the historic HMR PMP procedures. <br /> <br />Unlike the 53 storms systems evaluated by the National Weather Service in the development of <br />HMR 51 and 52. few Colorado extreme storms have been evaluated for the D-A-D relationship. <br />By completing the radar-reconstruction and D-A-D anaiyses of these storms a significant step <br />forward will be accomplished at defining the regional differences of the storm sets used in the <br />PMP calculation. <br /> <br />HDR will use NWS WSR-88D radar data archived during the UDFCD Flash Flood Prediction <br />Program to perform the rainfall re-constructions that can be used to enhance the existing set of <br />extreme storms that the Colorado Climate Center currently maintains. Partial rainfall re- <br />constructions have been performed On the Parker/Douglas storm of 1998 and the Cherry Creek <br />storm of July 8. 2001 for UDFCD. Expanded analyses of these storms will be performed. In the <br />case of the Fort Collins storm existing storm radar re-constructions will be reviewed and <br />compared to the proposed evaluation. <br /> <br />HDR can develop the rainfall re-constructions, storm descriptions and D-A-D calculations <br />for a lump sum amount of $22,250 in 45 days from notice to proceed. This product will be <br />added to the existing Colorado Climate Center's Extreme Precipitation Event list upon <br />completion. <br />