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Perimeter Drainage Ditches <br />The two temporary perimeter drainage ditches wrap around the down - gradient portion of the shaft pad <br />and convey site runoff to the sediment pond (Exhibit 2). The perimeter drainage ditches were constructed <br />in cut material with a V- shaped cross section. The channels typically have 3:1 side slopes (or flatter), <br />with a centerline depth of approximately 2 feet. The drainage ditches are bare earthen channels that have <br />developed some armoring consisting of cobbles and shingles. Since vegetation may naturally encroach <br />on the perimeter drainage ditches, or the ditches may be revegetated by TCC at some point in the future, <br />the ditches were modeled as both erodible and vegetated channels for the peak runoff resulting from a 10- <br />year, 24 -hour storm event. Both scenarios were modeled to ensure that the ditches were stable at the <br />higher flow velocities experienced with erodible earthen channels, and that the ditches had enough <br />freeboard capacity at the greater water depths experienced with vegetated channels (as a result of <br />increased channel roughness caused by the vegetation). Flow velocities in both drainage ditches are less <br />than the maximum permissible velocity of 5 feet per second (fps) for the peak flow resulting from the 10- <br />year, 24 -hour storm, although the scenario for erodible Perimeter DrainageDitch 1 produces a velocity of <br />4.71 fps that approaches the 5 fps velocity limit. When the perimeter drainage ditches are modeled as <br />vegetated channels, the maximum velocity of 1.34 fps is significantly below the limiting velocity of 5 fps. <br />SEDCAD analyzes vegetated channel hydraulic characteristics for two conditions, short grass and long <br />grass. The short grass condition is used to analyze the channel for stability (relative to limiting flow <br />velocities) while the long grass condition is used to analyze the channel for capacity (relative to the <br />maximum runoff depth that will occur in the channel). Therefore, evaluation of the freeboard adequacy <br />of the as -built ditches with vegetation is based on modeling results for the long grass condition. The <br />maximum water depth obtained from the modeling is 1.07 feet, well below the as -built 2 -foot channel <br />depth providing significantly more than the required 0.3 feet of freeboard. To establish a greater factor of <br />safety regarding channel stability, it is recommended that TCC consider adding more rock protection to <br />• the channels or establishing vegetation. <br />Sediment Pond with Erodible Perimeter Drainage Ditches <br />The sediment pond has been constructed as an incised impoundment with a maximum water surface area <br />of approximately 0.566 acres (0.99 acres if the embankment strip around the pond is included), measured <br />at the top of the pond embankment. The maximum depth of the pond is 5.17 feet without sediment, and <br />5.11 feet with 3 years of sediment to the highest point on the pond embankment. The empty pond with 3 <br />years of sediment storage (0.003 acre -feet) will completely contain the 10 -year, 24 -hour runoff volume <br />with 1.0 feet of remaining freeboard to the spillway invert. The pond will completely contain three years <br />of sediment (0.003 acre -feet) and dead storage of 0.4638 acre -feet plus the runoff volume from the 10- <br />year, 24 -hour storm event without discharging. Additionally, the as -built pond will adequately control <br />sediment and retain, treat, and discharge the runoff even if the pond is full to the spillway invert at the <br />start of the 10 -year, 24 -hour runoff event. <br />Sediment Pond Spillway with Erodible Perimeter Drainage Ditches <br />The pond will discharge through a single combined principal and emergency spillway. The spillway is <br />trapezoidal shaped with a 12.0 -foot bottom width, 2:1 sideslopes, and a crest length parallel to the <br />direction of flow of 3.0 feet. If the total runoff volume from the 10 -year, 24 -hour event was retained in <br />the pond, there would be 1.0 feet of freeboard between the maximum water level surface and the invert of <br />the emergency spillway. The maximum 25 -year, 24 -hour peak spillway discharge will occur when the <br />pond is full to the spillway invert prior to the start of the runoff event, and the system is modeled with <br />erodible perimeter drainage ditches. If the pond is full at the start of a 25 -year, 24 -hour runoff event, the <br />single open - channel spillway will pass the 25 -year, 24 -hour peak flow of 6.99cubic feet per second (cfs) <br />with a maximum flow depth of 0.41 feet and freeboard of 1.08 feet between the maximum flow depth in <br />the spillway and the top of the embankment. <br />TCC 6N4N Intake Shaft Facility 2 Water & Earth Technologies, Inc. <br />Drainage and Sediment Control System Design April 24, 2009 <br />