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Other System Components <br />Two separate ditches will be constructed to route affected area runoff to the pond. As shown on <br />Maps 1 and 2, along the western perimeter the C4A Ditch will be constructed. It is modeled as a <br />trapezoidal ditch with a 8 foot bottom width and 3H:1 V side slopes. The ditch gradient assumed <br />for design is the overall average slope for the full 900 foot length of this ditch, and is 2.9%. The <br />3H:1 V side slopes have been selected because somewhere below Station 5 + 00, this ditch is <br />expected to encounter rock at its invert. Once this condition is encountered, rather than <br />excavating in rock, the ditch will be formed by keeping the invert on existing natural grade and <br />adding a low (2 foot high) earth berm on the right (west) bank to form the channel. The berm <br />will be constructed at 3H:1V side slope, forming the west bank. The ditch delivers its 10 year <br />design flow of 7.05 cfs at a velocity of less than 2 fps, which is well below the maximum <br />allowable velocity for a vegetative (grass) lining. Several ditch cross section templates with 8 <br />foot bottom width were evaluated and all delivered the 10 year flow at a velocity less than 2 fps. <br />However, for simplicity the template with 3H:1V side slopes on both sides is the one presented <br />in the attached SEDCAD models. <br />A similar ditch along the southern perimeter, called the Pond 018 East Ditch will convey runoff <br />from the eastern portions of the watershed to the pond. That ditch is expected to be excavated in <br />soil for its entire length, so an conventional prismatic trapezoidal section of 8 foot flat bottom <br />with 3H:1 V sides slopes (both sides) has been modeled. This ditch also conveys the 10 year <br />flow (2.67 cfs) with velocity below 2 fps. As can be observed on Maps 1 and 2, the central <br />portion of the watershed flows directly into the pond. <br />C4A and Pond 018 East Ditches were designed using specific gradients and cross sections for <br />SEDCAD parameters. Field conditions, construction equipment, and revegetation will require <br />flatter side slopes and wider channel bottoms. Specifically, side slopes may be as flat as 6H:1 V <br />and the bottom width may be as wide as 20 feet. Since one or both of these parameter changes <br />will reduce flow velocities, channel stability is not an issue. Both ditches have been designed <br />with adequate freeboard as shown on Map 3. <br />There is also an existing ring ditch around the topsoil pile which collects runoff from the east <br />face of the pile and routes it to the headwaters of the planned Pond 018 East Ditch. <br />Runoff Calculation Results <br />The results of the runoff calculations are presented in the attached SEDCAD model printout. The <br />system was modeled using SEDCAD for its response to a 10 year, 24 hour storm of 2.0 inches. <br />For continuity, the results of the modeling are presented below along with the discussion of the <br />physical dimensions and properties of the pond. <br />The 100 year, 24 hour storm has also been modeled. It produces a total runoff volume of 2.11 <br />ac -ft and flows 0.7 feet deep in the emergency spillway, assuming the pond is brim full at the <br />start of that precipitation event. The brim full condition is not expected to occur in actuality as <br />the pond is expected to be nearly dry most of the year due to out -seepage and evaporation. <br />Attachment 2.05.3(3)-28 Revision Date: 11/24/15 <br />Page 2 Revision No.: TR -69 <br />