2015-12-11_PERMIT FILE - C1981008 (2)

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maximum flow is only 11.5 acre feet for a full dam break, it is not enough to cause the water to exceed the channel and flow uphill to the Enstrom structures. Once the water goes downstream in the tributary channel, it enters Tuttle Draw a short distance downstream and there are no structures that could be affected by a small volume in Tuttle Draw since it is large enough to handle substantial peak flows, as shown on Map 2.05.3(3)-1. For these reasons, Pond 011 is given a Class A designation. Ponds 012 and 013 - As seen on Map 2.05.3(3)-1 Surface Hydrology, Ponds 012 and 013 are located immediately outside the floodplain of Tuttle Draw. Any dam break of these ponds would discharge water directly into Tuttle Draw. There is no chance the water could harm anything or anyone at other locations. There are no structures that could be affected by a small volume in Tuttle Draw since it is large enough to handle substantial peak flows, as shown on Map 2.05.3(3)-1. For these reasons, Ponds 012 and 013 are given a Class A designation. Ponds 014, 015, and 018 - These ponds are meant to handle the flow from a 100 year 24 hour event. They are also incised ponds without any real embankment structure. Also, their small volume and location would not result in any damage to property or life. For these reasons, Ponds 014, 015, and 018 are given a Class A designation. G. Pond 008 through 018 Designs As shown above for Sediment Pond 007, Tables 2.05.3(3)-1, 2.05.3(3)-lA, and 2.05.3(3)-2 list the values of CN and "k" for different soil and vegetation types at the New Horizon Mine. The tables have been revised to address new soils found in the area west of 2700 Rd The curve number (CN) and "k" factor for each of the ponds is determined as a weighted average. The undisturbed area curve number and "k" factor information is based on the soils and vegetation information found within Sections 2.04.9 and 2.04.10, respectively, of the New Horizon mine permit application. The pond volumes are calculated by first using the Curve Number and Runoff Routine within the Hydrology module of the SURVCADD software and then using the USLE sediment storage routine within the Hydrology module of the SURVCADD 2000 software. The LS factors for the USLE are determined from the 3D CAD grid model of the drainage areas for each pond. The water volume is determined from the weighted average CN for the watershed based on the worst case conditions over the life of the mine and the appropriate design storm rainfall. Curve numbers used are from 78 to 90 based on the existing conditions, level of disturbance and the hydrologic soil type of each watershed. Once the required volume of sediment and storm inflow is known, these two are combined to get the total required volume. SURVCADD was then used to design ponds in each location which had the parameters described in the previous section. All Page 2.05.3(3)-17 October 2015 (TR -69)