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0 <br /> • • South Ditch - A south interceptor ditch is proposed at the toe of the slope <br /> below the No. 1 Shaft yard. Section C illustrates the design. This ditch would <br /> intercept runoff from sub-basin Al only. It was designed with 2:1 and 5:1 <br /> side slopes and a nominal invert grade of 0.4 percent. Maximum flow depth <br /> will be about 0.66 foot and 0.85 foot for 10-year and 100-year rainfall events, <br /> respectively. The flow velocity will be between 1.2 and 1.5 feet per second. <br /> • West Ditch - The west ditch has been designed as shown on Detail Section <br /> A on the drawing, with 5:1 and 2:1 slopes. The invert slope was set at 1.5%. <br /> The flow depths may vary from 0.78 foot to 1.00 foot and the flow velocity will <br /> be between 2.7 and 3.2 feet per second for the 10-year and 100-year events <br /> respectively. A new culvert should be installed at the southwest corner of the <br /> impoundment to allow access to the south ditch. Safety is also a <br /> consideration due to the close proximity of the sewage treatment plant pond. <br /> During the 10-year event the flow depth will be about 0.7 foot through the <br /> culvert and the velocity should be 3.4 feet per second. <br /> • North Ditch - The north ditch has been designed as shown on Detail Section <br /> B on the drawing, with 5:1 side slopes. The flow depths may vary from 0.7 <br /> foot to 0.9 foot depending on the storm event (10-year or 100-year). The <br /> channel invert slope of 1.1% should limit the velocities to 2.2 to 2.6 feet per <br /> second, respectively. A 24" corrugated metal pipe (CMP) was used to <br /> transport flows across the paved access road. During a 10-year storm event, <br /> • the flow depth in the culvert should be 1.00 foot with a velocity of 3.8 feet per <br /> second. During a 100-year event, the flow depth and velocity in the culvert <br /> will rise to 1.50 feet and 4.4 feet per second. <br /> • East Ditch - The east ditch consists of a shallow swale leading from the <br /> flume house to an existing 24" corrugated plastic culvert. The culvert leads to <br /> the inlet ditch for the sediment pond. A special ditch for Pond 1.2 is not <br /> needed here since the Pond 1.2 containment berm is approximately 3 feet <br /> higher than the parking area. There is no physical way that the Pond 1.2 <br /> impoundment can be inundated by storm water from the east side. An <br /> analysis was performed assuming that the swale has 10:1 side slopes and a <br /> channel slope of 1.16%. The flow depth will vary from 0.30 foot to 0.38 foot <br /> and the velocity will vary from 1.3 to 1.5 feet per second (for 10-year and 100- <br /> year events). <br /> 2.3.3.2 Sediment Pond (Trap) Effectiveness <br /> A textural analysis of sediment collected from low spots in the lower yard was <br /> performed in 1994 to determine the required size of the existing sediment pond <br /> north of the impoundment (Figure 4). These data were used in a trap efficiency <br /> . model to determine the expected amount of sediment removal within the pond. <br /> A <br /> an 12 <br /> Jum 29.1BB9 TRA <br /> TR.Eoc <br />