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Several cross sections are also shown on Map 1. The profile shows the maximum tailwater slope <br />as 9.2%. <br />Runoff from the northwest portion of the watershed reports to the pond via a postmine channel <br />along the western boundary. It is modeled as an erodible channel with a 3 -foot bottom, 3H:1 V <br />side slopes, and a gradient of 2.2%. <br />Runoff from the southeast portion of the watershed reports via C-5 Ditch, an existing roadside <br />ditch that will be redirected near the western end into Pond 016. Upon completion of the pond, <br />the lowermost reach of the C-5 Ditch will be directed into the pond. This is modeled as a <br />vegetative lined ditch with a 4 -foot bottom and 3H:1V side slopes. The gradient varies. The <br />existing eastern portion along the road is at a gradient of 2.1%, but the critical section is the <br />steeper western terminus section that drops into Pond 016. This segment has gradient of 5.0 %. <br />As can be observed on Map 1, the central and western portions of the watershed flow directly <br />into the pond. <br />Pond Construction <br />Pond 016 was designed and will be constructed as a largely subgrade impoundment with a low <br />(approximately 6 feet high) embankment along the northwestern end. The details can be <br />observed on Map 1, Pond 16 Area and Pond 016, Plan View and Design Details, Map 2. <br />The emergency spillway is a 20 -foot wide channel through the southwest corner of the <br />impoundment with a crest elevation of 5618.0 which overflows to Enstrom Draw. <br />Runoff Calculation Results <br />The results of the runoff calculations are presented in the attached SEDCAD model printout. The <br />10 year, 24 hour storm was modeled not as a pond performance model but rather to estimate the <br />rate of sediment accumulation and to assess the performance of the above described ditches in <br />the 10 year design event. The system was modeled using SEDCAD for its response to a 100 <br />year, 24 hour storm of 3.0 inches as an evaluation of the system's ability to completely contain <br />the 100 year storm. For continuity, the results of the are presented below along with the <br />discussion of the physical dimensions and properties of the pond. <br />The volume of the below grade pond was input into the SEDCAD model, along with its spillway <br />details. The model watersheds were input as a series of sub -watersheds, each with its own <br />acreage, its own flow response parameters (slope, distance, time of concentration), and the <br />specific runoff Curve Numbers from the above mentioned Table 2.05.3(3) -IA. <br />The 100 year, 24 hour storm of 3.0 inches was modeled to evaluate Pond 016's ability to <br />completely contain the 100 year event. It was assumed the starting pool would be maintained by <br />pumping if necessary at an elevation 5614.0. With the watershed in its current condition, with <br />89% reclaimed from 1 to more than 3 years, but 12% still at "topsoiled and seeded" condition, <br />and the emergency spillway at elevation 5618.0, it can be observed in the SEDCAD printout that <br />the pond does not discharge. <br />Attachment 2.05.3(3)-29 Revision Date: 1/08/2016 <br />Page 3 Revision No.: TR -70 <br />