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c. Such other measures to minimize erosion resultant siltation and disturbance to the <br />prevailing hydrological balance. <br />Culverts will be installed to avoid plugging or collapse and to avoid erosion at the inlets and <br />outlets. Riprap will be installed as necessary to reduce the exit velocity. The minimum width <br />will be the width of the natural downstream channel. The riprap will be sized in the field based <br />on the "as -built" slope of the culvert and final configuration of the exit channel slope area. The <br />sizing shall be based on the Federal Highway Administration's HEC No. 11 "Use of Riprap for <br />Bank Protection" or other standard methods. All pipes will have a minimum cover of 12 inches. <br />Culverts and drainage ditches will be maintained periodically to prevent accumulation of debris <br />on the culverts inlets and outlets. <br />The following is an outline of the general design procedures used in the design of culverts: <br />1. Identify the need for a structure from topographic maps, and a visit to the site; <br />2. Determine the size and hydrology of the watershed in question; <br />3. Perform SEDCAD, or other acceptable software, computer run to determine the peak <br />runoff from a 10 -year, 24-hour precipitation event. <br />4. Using SEDCAD, or other acceptable software, computer run to determine the proper size <br />of culvert to handle the peak runoff form a 10 -year, 24 hour precipitation event. Also <br />check the pipe sizing using the hydrograph of corrugated steel pipe culverts with inlet or <br />outlet control, the discharge quantity, and the amount of entrance head desired. <br />Other applicable methods of culvert design include the use of charts developed by the Federal <br />Highway Administration, published in Hydrologic Engineering Circular HEC -5 (FHA, 1980), <br />and Hydrologic Design Series HDS-3 (FHA, 1980). <br />Charts published in HEC -10 (FHA, 1978) are also used; however, exit velocities must then be <br />determined by other methods. Headwater conditions are typically examined by using HEC -5 <br />inlet control nomograph. To be conservative and to allow for adequate freeboard, New Horizon <br />usually uses "projecting" conditions. <br />As virtually all culverts have free out falls, inlet control assumptions can be verified by the "Pipe <br />Flow Charts" in HDS-3. If flow in the culvert has a free surface, entrance control exists, and exit <br />velocity can be approximated by using the greater of normal or critical velocity determined by <br />the "Pipe Flow Charts" in HDS-3. <br />Page 2.05.3(3)-23 January 2016 (TR -70) <br />