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I I I I I I I I I I I I I I I I I I I Request for Letter of Map Revision - Skunk Creek University of Colorado at Boulder Research Park Page 4 prepared by the MSM/SP Group (now Greiner Engineering Sciences, Inc.) under contract with FEMA (Ref. 6). This hydrology has also been accepted by the UD&FCD for their Flood Hazard Area Delineation (FHAD) Program (Ref. 3). A complete summary of hydrological analyses is given in that document. Table 1 summarizes the discharge values for the study reach. TABLE 1 SKUNK CREEK DISCHARGE VALUES THROUGH STUDY AREA Floorling Source Peak Di>!charge Icf>!) lO-yr 50-yr lOO-yr 500-yr Skunk Creek 1,060 1,830 2,230 3,500 Note: These values represent basin runoff assuming future development conditions. 3.2 Hydraulic Analysis An analysis has been prepared for Skunk Creek in order to determine the extent of protection provided by the channel construction. The flood profile elevations were computed using the U. S. Army Corps of Engineers HEC-2 Backwater Computer Program (Ref. 7). Original data (Ref. 4) was input in order to recreate all existing detailed studies and is included in Appendix 1. The original data was modified to reflect existing conditions between Colorado Avenue and the Wellman Canal. An anticipated vertical walled channel was originally modeled in this reach as a part of the MSM/SP Greiner study. This vertical wall channel was never constructed. The analysis for Skunk Creek was based on the as-built drawings for the channel improvement project. The Skunk Creek channel is designed with an 83 cfs trickle channel. A 3 cfs irrigation waterway is incorporated into a 600 foot reach of the channel design. Upstream, a 450 foot long spill collection channel runs east-west parallel to and north of Colorado Avenue. This spill collection channel collects the entire 1,034 cfs of the Skunk Creek 100-year flows that spill across Colorado Avenue. The collected flows are then re- directed back to the main channel. Armor protection has been provided in high velocity areas including those areas in the vicinity of drop structures to minimize erosion. The maximum 100-year water depth in the design channel is 5.7 feet (measured from the water surface to the bottom of the trickle channel). The UCBRP will provide perpetual maintenance for Love & Associates. Inc.