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' 4 ' The 24 hour storm_dlstribution has been used to model storms with ]0 year, 25 year, and 100 year recurrence lnterv~ls. These storms have been labeled as Alternates 1, 2 and 3 in the model. An antecedent moisture ' condition of 2 was used, again in accordance with accepted criteria. Detaliled hydrology input and output for this TR-20 model is Included in the following section. G b ' The following is a summary of the hydrologic computations ~~u~~ssed ~xh~ ~~' ~ u~l t design oY the sediment basins. Discharge is from Subbasins Aland B sh~~wn ~ .Y ' on Exhibit A. 24 HOUR M ~ ~ DESIGN STORM PRECIPITATION PEAK DISCHARGE U ' ' W ' 10 Year 3.0 In. 13.3 CFS //,^, • S-I p( ~~ 25 Year 3.4 In. 16.5 CFS '7~ ' 100 Year 4,2 In. 23.3, CFS ~ ~ ~ ' III. HYDRAULICS The hydraulics of the sediment basin system consist of estimating the anticipated sediment load by use of the Universal Soil Loss Equation (USI,E); sizing of the sediment basin for the Q,o flow, design of an outlet structure which allows sediment removal Por the design storm and discharge through the ' ~ outlet of the v storm, and design of an emergency overflow spillway for the Qioo storm. Each oP these hydraulic elements are further discussed in detail. ' 3, I The sediment load was estimated by considering runoff from both the 2 -lfd'~~+isual rm-in_lrs reclaimed condition-and the Subbasins A and B considered to be in an Active mining condition. The malorlty oP the soli loss occurs from the latter area with a total soil loss estimated to be 92 tons per year. 'fhe 2~ ~~~~ USLE calculations are contained in the Following section. '-- ~ ' ~ The sediment basin system has been designed as two cells labeled upper' ~ O zj 4,t ` and lower pond on Exhibit B. This double pond configuration was made ' necessary by the topography at the site. The Qio storm was used to compute w~fY a surface area and de th The settlin article size of 0 'crons iva ~4-PS p . g p selected based on limestone fines as being the smallest particles, however muc ~ 4,K of the sediment is expected to be larger. A surface area at Qio of 6600 :..F. J ' was computed together with a settling depth of slightly less than 4 feet. , cleaning interval of every 2 years was considered for design, however actual ~~406 ~~ ' cleaning will be on an "as required" basis Prom visual inspection. A s• afzty factor of 2 was also applied to the storage zone depth. ~ o Q~. ~ ~i ---- ~ IZ 7 3 _ , - 39: ~ _ The outlet system consists of a vertical standpipe with perforati~>ns ' 17 oC-F-H ' together with weir overflow. The @io storm was used for size and denSity_of perforations to dewater the sediment basin. A series of 3" holes at 11" on 1