Laserfiche WebLink
Guidelines for Permanent Drainageways and Diversion Ditch Construction <br />• The locations of existing and potential permanent drainageways are <br />shown on I~ap 28-3 (TR 87-14-3). The construction of these drainageways <br />will follow the guidelines as presented belo~•+. In some instances, diversion <br />ditches will be constructed to channel runoff directly to engineered <br />permanent drainageways. <br />The design of permanent drainageways and diversion ditches will follow <br />methodology outlined in "Procedures for Determining Peak Flows in Colorado" <br />(SCS, 1977). The generalized guidelines are given below. <br />Typically, the channels will be designed to accommodate the peak runoff <br />flow from the 10-year, 24-hour precipitation event. Normal antecedent <br />moisture conditions (AMC-II) will usually be assur~ted for these <br />calculations. Curve numbers (CN) will be assigned to the drainage area <br />based on guidelines outlined in the SCS publication. If several distinct <br />surface conditions exist, then appropriate values will be assigned to each <br />• area and the total flows based on area-weighted calculations. <br />Once the design peak flows for the channel are calculated, its <br />dimensions will be designed based on I"anning's equation: <br />V = 4 = 1.486 R2/3 S~ <br />A n <br />Where: <br />U =flow velocity in feet per second. <br />Q = discharge in cubic feet per second. <br />A = cross-sectional of flow area in square feet. <br />n = h?arining's roughness coefficient. <br />R =hydraulic radius in feet <br />= area (A) / wetted perimeter (P). <br />S =slope of the energy gradient (usually equal to the slope of the <br />channel). <br />• <br />28-13 <br /> <br />