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Tract D <br />There are no gas wells locate within Tract D that could be exposed to head cutting, therefore protection <br />of Tract D is not a priority. Using the USACE hydraulic analyses, the site is likely to overtop along the <br />Tract D berm during a 100 year event at cross section 22471, with a maximum flow depth over the berm <br />of approximately 0.4 ft and a maximum flow velocity of approximately 2.3 ft/sec. In this area the <br />proposed berm elevation for Tract C is approximately 4625, a foot higher than the proposed berm for <br />Tract D, and 0.7 ft higher than the expected WSEL, so no flow is expected to enter Tract C. In addition, <br />the two pits are separated by a drainage channel, so overtopping of Tract D is unlikely to adversely <br />affect Tract C. <br />Permissible velocities for the berm if it is topped by grass range from 2 -4 ft/sec depending on the <br />species of grass and time of year (Schwab, 1993, USDOT 2001, pg 6.51). Maximum permissible velocities <br />for the in situ soil (partially cohesive silty clay with sand) range from 1.5 ft/sec to 3 ft/sec (Julien, 2002). <br />Based on an expected overtopping velocity of 2.3 ft /sec, the berm should be allowed to remain <br />vegetated to prevent erosion. <br />Recommended Action <br />Based on the analysis above, the following measures are recommended. <br />Tract C- There are two gas wells of concern in Tract C. <br />• Provide an armored weir with 1 ft of flow depth and 330 ft width at elevation 4629 NGVD <br />upstream of the drop structure to convey and guide possible overtopping flows away from <br />critical infrastructure. <br />• Allow erosion of the interior pit walls during an overtopping event, but repair damaged areas <br />after the event, particularly in the area of the weir. <br />• After pit excavation is complete, provide rip -rap or other protection at the slope transition <br />within the pit if the water level is not at least 2 feet above this point (Approximately 28 feet <br />from the existing ground elevation) <br />Tract D- There are no gas well of concern in Tract D. <br />• Allow Tract D to overtop during the 100 year event since there is no threat to critical <br />infrastructure. <br />• Maintain existing vegetation or hydroseed to encourage vegetation establishment after <br />excavation to help control erosion on the top of the berm in an overtopping event. <br />• No armoring is necessary on the interior of the pit. <br />71Page <br />