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I I I I I I I I I I I I I I I I I I I INTRODUCTION North of Denver, in Adams County and the City of Thornton, lies a small un-named drainageway which flows directly into the South Platte River, identified by the Denver Regional Council of Governments as Direct Flow Area 0054. Historically, the land within this drainage basin has been used for agricultural purposes and drainage was not a concern for the local residents and farmers. In a relatively short period of time, mostly within the last decade the City of Thornton has experienced rapid growth and the trend is toward fully developed conditions in the uplands of the drainage basin within a few years. The increased residential development has caused an increase in the drainage flows from the upland areas which has been the cause of numerous problems in the lower areas of the basin still involved in agricultural activities. As a result of the conflicts which have been growing over the increased runoff in Direct Flow Area 0054, the City of Thornton and Adams County together with the Urban Drainage and Flood Control District contracted with Merrick and Company to prepare this Flood Hazard Area Delineation report as part of a Major Drainageway Planning Study of the area. This report has been prepared to guide Adams County and Thornton officials in planning the use and regulation of the floodplain areas, so that appropriate consideration before planning is given to the 100-year flood and that flood hazards and future flood damages are minimized. Contained herein are floodplain and floodway data for 6.3 miles of channels for the probable 100-year future flood and the hydrological characteristics of the study area. The information in this report does not imply any state action to zone or regulate use of floodplains. The District has the authority to regulate floodplains, but to date has chosen to leave floodplain regulation responsibility with local governments. The report provides a suitable basis for the adoption of land use controls to guide floodplain management, thereby preventing the intensification of flood losses. Flood water surface elevations and floodplain boundaries are often revised by road and bridge construction, floodplain development, flood control improvem ents, or natural processes. Possible future improvements to control floods are not a consideration of this report. Prior to utilization of this report for planning or design purposes, the user is advised to contact the Urban Drainage and Flood Control District or the Colorado Water Conservation Board to determine if the information in this report has been amended. -3- The 100-year floodplain, which delineates the areas that would be inundated by such an event, is outlined on the Drawings, Sheets I through 21, at the back of this report. Flood profiles, also shown on the Drawings, show the water depths relative to the stream bed for both the 100 and 10-year events. Generally speaking, major floods which occur on Direct Flow Area 0054 are the result of intense and/or high volume rainfall events. Hazards attributable to these floods are principally interruptions to traffic and endangerment of roadway embankments, health hazards caused by flooded leach fields and sanitary sewers, and erosion and sedimentation problems a$ociated with both agriculture and land development activities. The specific problems are described in Section VI of this report. Acknowledg-ements The a$istance and cooperation of the participating agencies, firms, organizations, and private citizens in supplying information and data are appreciated and gratefully acknowledged. Mapping- Aerial photography was performed by Merrick and Company and Scharf and As- sociates. The contour mapping was prepared by Merrick and Company and represent conditions on the ground as of December 21, 1978 and February 2, 1979, the dates of the aerial photography.