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I I I I I I I I I I I I I I I I I I I SECTION 1 - INTRODUCTION The following is a brief summary of the scope of work for this proj ecL 1.1 Authorization 1. Calculation of water surface profiles for the 10-, 50-, 100-, and 500-year flood events; 2. Delineation of the 100-year floodplain boundary; 3. Development of discharge profiles indicating the relative magnitudes of the 10-, 50-, 100-, and 500-year flood events; 4. Determination of f100dways based on encroachment resulting in a maximum increase in the energy grade line of 1.0 feet; 5. Preparation of a report describing characteristics of the study area, flood history of the North St. Vrain and St. Vrain Creeks, various hazards associated with areas in or near the 100-year floodplain, tabulations indicating pertinent floodplain and floodway information, and reduced copies of floodplain maps and flood profiles. This report was authorized by Boulder County in joint sponsorship with the Colorado Water Conservation Board. The Board's power and duty is to devise and formulate methods, means and plans for bringing about the greater utilization of the waters of the state and the prevention of flood damages therefrom and to designate and approve storm or floodwater 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 Section 37-60-105 (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 1 imi t such uses on or along any stream or floodwater runoff channel or basin, as such storm or floodwater runoff channel 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 flood waters, as stated in Sections 30- 28-111 and 31-23-201 of the Colorado Revised Statutes, 1975. Upon official approval of this report by the Colorado Water Conservation Board, the areas described as being inundated by the 100-year flood may be designated as flood hazard areas and their use regulated accordingly by local governmental entities. Flood elevations and the corresponding floodplain are often altered 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 Boulder County to determine if the information in this report has been amended. A supplement to this report in the form of a technical addendum incl udes the supporting hydraul ic and hydrologic data and calculations which may further be used in engineering studies in the location and design of roads, bridges, and channel modifications. 1.3 Acknowledgements 1.2 Purpose and Scope The assistance and cooperation of the below listed agencies and organizations in addition to numerous private citizens in supplying information and data are appreCiated and gratefully acknowledged. This report was prepared to guide local officials in planning the use and regulation of the floodplain areas such that hazards and future damages are eliminated or minimized. This report includes information on historical floods, existing factors which influence the flood hazard, and the nature and extent of probable future floods. U.S. Department of the Army, Omaha District Corps of Engineers Colorado Water Conservation Board Colorado Division of Water Resources Boulder County Public Works Department Aerial photography and contour mapping was prepared by Northern Colorado Photogrammetrics of Greeley, Colorado, December, 1981, -1-