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<br />potential virtually precludes additional development in the canyons <br />property values. Consequently, public agencies engaged in land-use <br />regulation seek an optimal balance between protection of life and <br />recognition of individual property rights. <br /> <br />and reduces <br />planning and <br />property and <br /> <br />Existing techniques used in deriving flood characteristics for foothill <br />streams are inadequate or unverified; thus, vastly differing results can be <br />obtained from independent analyses of the same flood record. Because some floods <br />result from snowmelt at higher elevations while others result from intense <br />rainfall at lower elevations, available flood records for foothill streams are <br />applicable only to the collection sites or to nearby sites on the same stream. As <br />a result of the above problems and the potential serious consequences, a project <br />was begun October 1, 1977, to provide hydrologic data and improved techniques for <br />flood-frequency analyses of foothill streams. <br /> <br />Purpose and Scope <br /> <br />This report documents a comprehensive plan for collection and analysis of <br />flood data for streams in selected foothill areas of Colorado. Because of the <br />anticipated length of the project, the report will provide future investigators <br />with ideas and concepts from which the overall project was developed. The report <br />also may be useful in planning similar projects in other areas. <br /> <br />Although foothill-type streams occur adjacent to all mountainous areas in <br />Colorado, severe floods produced by rainfall appear to be extremely rare for <br />streams in the North Platte River basin and the Rio Grande basin. For this <br />reason, streams in these two basins are not included in current project plans. <br /> <br />Evolution of Project Concepts <br /> <br />Two general methods have traditionally been used for deriving flood charac- <br />teristics of streams in Colorado. These methods are described below. <br /> <br />Flood-frequency analysis and regionalization of gaging-station flood data.-- <br />Where available, gaging-station records have been used to derive flood-frequency <br />characteristics for gaged sites and nearby sites on the same stream. The <br />predominent computation method requires fitting of the annual flood series for the <br />gaging station to the Log-Pearson Type-I II frequency distribution as recommended <br />by the U.S. Water Resources Council (1976). <br /> <br />Flood characteristics for ungaged sites are obtained by regionalizing gaging- <br />station data on the basis of physical or climatic basin parameters. Prior to <br />1976, a series of U.S. Geological Survey reports defined flood characteristics for <br />the four river basins in Colorado (Patterson, 1964; Patterson, 1965; Patterson and <br />Somers, 1966; and Matthai, 1968). In these reports, sites on streams in the foot- <br />hill sections of Colorado were generally classed as mountain-type sites having <br />only snowmelt floods. No provision was provided for a transition zone between <br />plains or plateau streams and mountain streams. Consequently, computed flood <br />characteristics for adjacent basins could be drastically different depending on <br />the two basins' relative locations within hydrologic regions. <br /> <br />2 <br />