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I I I I I I I I I I I I I I I I I I I 1.0 1.1 1.2 PREFACE Authorization This report was authorized by the Urban Drainage & Flood Control District. The District has the power to enact District-wide floodplain regulations. At present, the District makes floodplain information and mapping available for local agencies to adopt and administer their own floodplain regulations with the assistance of the District. The Colorado Water Conservation Board has the power and duty "...to designate and approve storm or floodway runoff channels or basins, and to make such designations available to legislative bodies of cities and incorporated towns; to county planning commissions; and to boards of adjustment of cities; incorporated towns; and counties of this state..." as stated in SectiDn 37-60-106 (1) (c) of the Colorado Revised Statutes 1973. The cities, incorporated towns and counties within the study area may provide zoning regulations... "...to establish, regulate, restrict, and limit such uses on or along any storm or floodwater runoff channel or basin, as such storm or floodwater runoff or basin has been designated and approved by the Colorado Water Conservation Board, in order to lessen or avoid the hazards to persons and damage to property resulting from the accumulation of storm or floodwaters..." 1.3 as stated in Section 30-28-111 for the county governments and Section 31-23- 301 for municipal governments of the Colorado Revised Statutes 1973. Upon acceptance of this report by the Urban Drainage and Flood Control District and the designation and approval of this report by the Colorado Water Conservation Board, the areas described as being inundated by the 1 DO-year flood (Intermediate Regional Flood) can be designated as flood hazard areas and their use regulated accordingly. It should be noted that the terms "Intermediate Regional Flood," "1 DO-year Flood," and "one percent flood" can be used interchangeably as they all define the same type of flood event. Puroose of Scooe This study consists of a floodplain analysis of Rangeview Gulch, Jackass Gulch, Lower Dad Clark Gulch, and the adjacent direct flow areas. The purpose of this report is tD provide IDcal officials with a guide for f10Ddplain management so that flood hazards and environmental problems can be controlled. The 1 DO-year event is the regulatory flood as identified by the District, the Colorado Water Conservation Board, the Federal Emergency Management Agency, the City of Littleton, and Arapahoe County. The information in this report does not imply any action by the District or the State of Colorado to regulate use Df flDodplains. The District has the authority to regulate floodplains, but currently has chosen to leave this responsibility with local governments. The 1 DO-year floodplain was delineated based Dn future developed basin conditions for each of the three drainageways analyzed in this study. Both floodplain maps and flood profiles were prepared. In addition, floodways were identified. The floodway is a high hazard area characterized by deep water and high velocity flows during flood events and should remain free of any development which would obstruct flow. As an amendment to the Scope of Work, two separate floodplain analyses were prepared for the Lower Dad Clark Gulch drainageway--one withDut floDd attenuation in McLellan ReservDir (the current situation) and one with flood attenuation in McLellan Reservoir. Other unrecognized flood control facilities or possible future imprDvements were not cDnsidered in this report. Flood water profiles and floodplain bDundaries are often changed by road and bridge construction, floodplain development, flood control improvements, or natural processes. Prior to utilization of this report for planning or design purposes, the user is advised to contact the Urban Drainage and Flood Control District to determine if the information in this report has been revised. Acknowledaements This report was prepared by Centennial Engineering, Inc., consulting engineers of Arvada, Colorado, at the request of the Urban Drainage and Flood Control District. All surveying and topographic data for this study was based on the USGS datum for mean sea level and was collected and compiled by Landmark, Ltd., Denver, Colorado, under a separate contract with the Urban Drainage and Flood Control District. Various agencies, including the City of Littleton, provided infDrmation pertaining to the analysis of these basins. AlsD, coordination was done with J.F. Sato & Associates, (subconsultant to DeLeuw Cather) on the improvements to Santa Fe Drive. Technical data developed in this study is on file with the Urban Drainage and Flood Control District. 1