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All times of concentration and peak flows were determined from the SCS method using SurvCadd. <br />The basic design for all ditches assumes trapezoidal cross - section with 2.OH:1.OV sideslopes, <br />constructed of earth with some weeds, resulting in a manning's n of 0.0275. If portions of ditches <br />along hillsides encounter durable rock, the widths may be reduced since no erosion will occur on <br />the bottom. <br />The table below shows the rainfall for important frequency events at the mine site. <br />Design 24 Hour Event <br />Rainfall (inches) <br />10 <br />2.0 <br />25 <br />2.4 <br />100 <br />3.0 <br />A number of ditches and culverts in the permit area will be permanent. These structures are <br />primarily along the re- established portions of 2700 Road and BB Road. Since the County requires <br />that the culverts be designed for the 25 year 24 hour event and DMG requires that permanent <br />structures be designed for the 100 year 24 hour event, the 100 year event will take precedence and <br />all permanent structures will be designed for this event. <br />All watershed areas are determined from the Cad map and all watershed land slopes are <br />determined from a grid made from either the original or final land surface, whichever is appropriate <br />in the design. A maximum flow length was also determined for each watershed, which was then <br />used to determine the Time of Concentration, using the SCS method. This was then used to <br />determine the peak flow using the graphical method in Survcadd. <br />The ditch slope was then determined using Survcadd and the channel was designed using <br />Manning's equation. All ditches will have 0.3 feet of freeboard above the design depth. <br />Revised June 2011 (TR -60) Attachment 2.05.3(3) -1 -5 <br />