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<br />DITCH HYDRAULICS AND DESIGN <br />Diversion ditches have been designed and constructed to modify natural <br />drainage boundaries for various ponds in order to make use of favorable <br />pond locations. The ditches are located in plan view on the Mine Area <br />Surface Water Hydrology Map, (Map 16a). The ditches were designed as <br />trapezoidal, flat-bottom ditches. Either the 10-year or 100-year design <br />frequency was used depending on whether ditch overflow would cause <br />significant problems for downstream ponds. The ditches were sized to <br />handle the peak runoff from the design storm. Peak flow rates were <br />determined in accordance with the publication titled "Procedures for <br />Determining Peak Flow in Colorado", (USDA-SCS, 1980). The ditches were <br />designed in accordance with a document titled Design Charts of Open <br />Channel Flow, (u.5. Bureau of Public Roads, 1961), utilizing Manning's <br />Equation for open channel flow as follows: <br />• Q = 1.486 ARziaS~iz <br />n <br />Where Q = peak discharge for desired storm event (cfs) <br />n = Manning's roughness coefficient <br />A cross sectional area of channel flow (ftz) <br />R = Hydraulic radius (ft) <br />S - Channel slope (ft/ft) <br />also <br />~-4 <br />A <br />Where V -flow velocity (fps) <br />The ditches were assumed to be stabilized with grass lining and design <br />velocities were kept under five (5) feet per second to prevent erosion and <br />additional contributions of sediment to runoff where possible. The steep <br />reach at the end of Ditch 82 requires riprap lining due to unacceptable <br />flow velocities. <br />23-45R Revised - August, 1990 <br />