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The spillway consists of a shallow 4 -foot deep vegetated channel with a 20 foot wide bottom and <br />3H:1 V side slopes. It traverses the through the crest of the embankment, into the C4 Ditch, and <br />towards the channel of Calamity Draw. <br />Other System Components <br />Two separate ditches will be constructed to route affected area runoff to the pond. As shown on <br />Map 1, 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 forming the west and east banks. The ditch delivers its 10 year design flow of <br />7.21 cfs at a velocity of less than 2 fps, which is well below the maximum allowable velocity for <br />a vegetative (grass) lining. Several ditch cross section templates with 8 foot bottom width were <br />evaluated and all delivered the 10 year flow at a velocity less than 2 fps. However, for simplicity <br />the template with 3H:1 V side slopes on both sides is the one presented in the attached SEDCAD <br />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 (3.16 cfs) with velocity below 2 fps. As can be observed on Map 1, the central portion of <br />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. <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 />Attachment 2.05.3(3)-28 Revision Date: 04/21/16 <br />Page 4 Revision No.: TR -72 <br />