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Drainage Terraces <br />Drainage terraces are placed periodically to minimize erosion and direct the runoff to the <br />appropriate downslope structure. The drainage terraces were designed to handle the flow for <br />each subarea and are spaced 40' vertically around the perimeter of the landfill. Figure 1 shows <br />the location of each drainage terrace. Haestad's Methods (FlowmasterTM) program was used to <br />calculate the range of depth and velocity to be as follows for the following input parameters: <br />• Bed Slope: 3% <br />• Inside Sideslope: 3H:1V <br />• Outside Sideslope: 12H:1 V <br />• Mannings n: 0.035 <br />• Flow Rate Q: Varies from 2.3 cfs to 43 cfs <br />Flow <br />Rate <br />Q Cfs Depth <br />feet Velocity <br />(ft/sec) Erosion Control <br />Measures <br />Low 2.3 0.37 2.4 Grass <br />Hi h 43 1.1 4.9 Grass <br />The peak flow depth will not exceed the depth of the terrace, and the velocities will not exceed 5 <br />feet/second; therefore, the drainage terraces design is acceptable with no additional erosion <br />control measures. Erosion control matting will be used in the benches, however, for additional <br />erosion protection in the case of larger storm events. <br />T:\0742\1406Uune1lCityApp\Comment response\Response_0818ll\P,ttachment 1\Surface Water Management Memo 08101 Ldoc 2 <br />