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Y <br />fxNlg/~r t-/7 <br />2308-30 <br />n <br />U <br />NORTHERN COAL COMPANY <br />HYDROLOGIC CALCULATION <br />• <br />• <br />The following calculations are provided to demonstrate a basis for <br />sizing culverts located on Northern Coa] Company's "Northern No. L (FF) <br />Mine Haul Road". <br />The main criteria for design of these culverts is based on the OSM <br />requirements of a 10-year/24-hour storm which should pass "free flowing" <br />through the design culvert. <br />The choice of the hydrologic methods used in quantifying the runoff <br />characteristics of the areas influencing drainage on the "FF Road" was <br />based in part of the size of the drainage areas. Previously planned or <br />existing construction and design was also taken into account. <br />The "FF" road, location and station of the culverts under consideration, <br />and the nearby planned facilities are shown on the accompanying map, <br />"Exhibit B - Culvert Drainage Basin - Northern No. 1. Mine". The drainage <br />areas influencing each culvert location are delineated on this map, <br />corresponding to the areas used in the design calculations. <br />The basic rainfall-runoff characteristics of this area are taken from <br />previous data submitted by Northern Coal: <br />Design Event Precipitation Runoff <br />Recurrence/Interval (In.) In. <br />10 year/24 hour 1.9 .46 <br />-1- <br />