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1.0 INTRODUCTION The US Army Corps of Engineers (USACE), Kansas City District retained URS Corporation to perform an independent review of the hydrology of Frenchmen Creek near Holoyoke, Colorado and to make recommendations for peak flows to be used in the hydraulic analysis for the creek, The USACE completed hydrologic analysis of the creek using the HEC-HMS model, which resulted in relatively high peak flows at Holoyoke, Colorado. This document describes the review process for the HEC-HMS/HEC-l modeling and other data developed by the USACE and supplementary analyses conducted to develop reasonably conservative 1 OO-year peak flows for Frenchman Creek at Holoyoke, Colorado, This submittal constitutes the completion of the first task included in the modified scope of work for the delivery order for Floodplain Management Services Special Study, City of Holoyoke, Colorado. 2.0 RAINFALL-RUNOFF MODELING 2.1 FRENCHMAN CREEK WATERSHED The USACE estimated the drainage area of Frenchman Creek at Holoyoke, Colorado to be 247,5 square miles (sq mi). The watershed is located in the northeastern comer of the state of Colorado near the state boundaries of Colorado, Nebraska, and Kansas, Frenchman creek has a length of about 35 miles upstream of Holoyoke. About 14 miles downstream (east) of Holoyoke, the creek enters the state of Nebraska and traverses a distance of about 70 miles in the state of Nebraska before joining the Republican River near Culbertson, Nebraska. To develop, route, and combine surface runoff hydro graphs of various sub-watersheds within the drainage basin of 247.5 sq mi, the USACE divided the total watershed into 19 sub-watersheds. The drainage areas, hydraulic lengths, and average slopes of these sub-watersheds are shown in Table 1. Draft Letter Report Independent Review of Hydrologic Analysis for Frenchman Creek Contract No, DACW4I-00-D-0026-0001 Flood Plain Management Services Special Study Holyoke, Colorado June 18, 200 I Page 1