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Tract C <br />Although the USACE model indicates that the Tract C berm elevation is 0.3 ft above the expected WSEL <br />at cross section 24158 in Tract C, there is the potential for flow to overtop the berm and enter the pit <br />immediately upstream of the drop structure due to backwater effects. The exact location of the <br />overtopping is unpredictable, therefore it is recommended that either the entire bank be protected <br />from erosion, or that a weir be constructed to train flow through a suitably armored location (Plate 5). <br />The weir will capture any overtopping flows, and therefore should be placed away from the critical gas <br />infrastructure. A Weir placed as shown in Plate 5 will concentrate overtopping flow and erosion to a <br />non - critical section of Tract C. Because of this, it is not necessary to armor the inside wall of Tract C, <br />provided that erosion which occurs during an overtopping event can be repaired after the event. <br />The existing ground adjacent to Tract C has an elevation of 4630 NGVD. The weir entrance should <br />therefore be no higher than 4629 ft NGVD to protect the adjacent berm from overtopping. The USACE <br />model predicts a total flow of 901 cfs in the right overbank. Based on the weir sizing guidelines for <br />gravel weirs set forth in USDOT (2001), and a 1 foot depth of flow over the weir, the equation <br />(rearranged to solve for weir length) is: <br />6 1Page <br />Where: <br />_ Qat <br />LS KukrCr(HWr) <br />• L is the calculated weir length <br />• Q is total discharge expected through the weir (901 cfs) <br />• K,„ is a unit conversion (1 for English units) <br />• k is the submergence factor (0.65 for a 5% difference between water height before and <br />after the weir) <br />• C is the overtopping discharge coefficient (2.8 for a depth of 1 ft, gravel surface) <br />• HW, is the expected depth of water entering the weir (1 ft) <br />This equation predicts a necessary weir width of 330 feet to pass the expected flow in the right <br />overbank. <br />In addition, it is recommended that the transition from 1.25:1 to 3:1 slope at a depth of 30 ft within the <br />pit be protected after active construction is finished. However, if the water surface level in the pit is <br />maintained at least 2 feet above the slope transition (28 ft from existing ground surface), this riprap will <br />not be necessary because the standing water will acts as an energy dissipater. If the pit is reclaimed by <br />contouring a uniform slope the riprap will not be necessary as well. <br />