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I I I I I I I I I I I I I I I I I I I FLOODPLAIN INFORMATION REPORT FOR COAL CREEK CRESTED BUTTE, COLORADO I. INTRODUCTION In December 1976, Engineering Consultants, Inc. prepared a Floodplain Information Report for Coal Creek through Crested Butte, Colorado. The 100 year flow was estimated at approximately 480 cfs and the 500 year flow was estimated at approximately 600 cfs. This flow estimate was developed from a regional frequency analysis of stream flow data in the region, because limited flow records were available for Coal Creek. In 1978 and 1979, according to a letter issued by Mr. Larry Lang, Chief Flood Control and Floodplain Management Section at the Colorado Water Conservation Board, the snow melt flows measured in Coal Creek through Crested Butte were: 1. 1978 Q = 580 cfs 2. 1979 Q = 600-630 cfs. The Colorado Water Conservation Board found that the 1976 study was based on a hydrologic analysis that significantly understates potential flood flows. They concluded that the 100 year flow should have been in the range of 750-800 cfs. The purpose of this study is to redefine the potential flooding problems in the Town of Crested Butte caused by flooding of Coal Creek during a 100 year snow melt condition. The scope of the study is to: 1. Develop the proper Hydrology for the Coal Creek Basin. 2. Visit the area, layout the cross sections needed for the study, and determine what changes have been made since the 1976 study. 3. Utilize the 1976 two-foot contour base mapping from the Colorado Water Conservation Board. Obtain a field survey to generate the cross section data required for the hydraulic analysis. 4. Perform a detailed hydraulic analysis for the 100 year Peak flow from snow melt. 5. Update the base map with pertinent features. 6. Perform a detailed hydraulic analysis for a possible 100 year f100dway for the northern portion of the Town. 1