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Gabion Lined Downslope Structures <br />Runoff from each subwatershed is routed by drainage terraces to gabion lined downslope <br />structures. All downslope structures have been designed to handle the peak flow generated by <br />the corresponding watershed. Figure 2 shows the location of each downslope structure. <br />Calculations provided below and in the model output show the range of velocity and computed <br />water depth for flows during the 25 year, 24 hour event for the following input parameters: <br />Bed Slope: 33% Bottom Width: 8 feet <br />Left & Ri t Sideslo e: 3H:1 V Mannin s n: 0.035 <br />STRUCTURE SEGMENT PEAK Q <br />(CFS FLOW DEPTH <br />FT) FLOW VELOCITY <br />FT/SEC <br />1 U er 48.0 0.53 9.3 <br />1 Lower 56.6 0.53 11.1 <br />2 U er 34.4 0.36 10.4 <br />2 Lower 42.9 0.40 11.6 <br />Based on the expected velocities, each downslope structure will be lined with rock gabions <br />mats. Energy will be dissipated upstream of access path crossings with a line of gabions <br />perpendicular to flow and at the toe of the slope. Flow will be forced to flow over the gabions <br />and dissipate on gabion splash pads prior to entering the pipes across the access road. <br />Sedimentation Basins <br />The proposed expansion design consists of two sedimentation basins. Sedimentation basin P-1 is <br />proposed on the southwest corner of the site. P-1 will accept the runoff from the western half of <br />the expansion including downslope structures (DS-1) and stormwater discharge outlet from <br />sedimentation basin P-2. <br />T:\0742\I406Uune1lCityApp\Commwt response\Responce_081811\Attachment 1\Surface Water Manageme~d Memo 08101 l.doc 3 <br />