2020-03-05_PERMIT FILE - M2020018

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The drainage areas for each pit and the facilities area during mining can be seen on Map G-1. Reclamation drainage patterns are shown on Map G-2. The runoff value for each drainage area and the capacity of each area based on a two -foot berm is listed in Table G-2. There is more than enough capacity at each location for the area that drains to it. Table G-2: Stormwater Runoff and Retention Capacity Pit 1 46.2 85.6 2.97 52.4 Pit 2 16.6 78.8 1.59 24.6_ Pit 3 8.0 84.8 -- j 0.74 14.2 - - Facilities Area 99.4 77.9 6.06 17.8 The stormwater controls for the site are designed based on the maximum overall drainage area that each pit will intercept. However, mining and reclamation occur contemporaneously, and therefore the area of disturbed ground will be limited (5-10 acres) at any given time. The rest of the site will consist of either undisturbed ground to be mined, or reclaimed ground behind mining. The active mining area (5-10 acres) will always be bounded by a stormwater barrier or berm of at least two feet of height. This may be windrowed soil, stockpiles overburden, or mining highwall. Assuming a minimum of two feet of stormwater barrier surrounding the active mining area, at a full 10 acres, there is 20 acre -ft of storage available. The largest storm event that any of the pits will see is 2.97 acre -ft. Therefore, there will always be sufficient storage for the 100 -YR event during mining. The stormwater that is collected in disturbed area will infiltrate into the underlying alluvium. Alluvium (sand and gravel) of this type has a hydraulic conductivity of 10-3 to 10-1 cm/sec, which means infiltration occurs at a rate of <1 to 5 min/inch. A 2.97 ac -ft storm event over a 10 -acre ' Composited from various soils and land uses' curve numbers. See attached Stormwater control plan Rincon Materials G-4 February 2020 crcq Lewicki and Aswcia0.�x. PITC