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Page 2 <br />August 20, 1998 <br />Mr. Michael B. Long <br />carry overland flow, are the issue for [his NOV. The layout of the East Portal Area and <br />associated sediment and drainage control is best shown on Map No. 139 from the mine permit <br />application. <br />The natural slope above the East Portal Area is very steep and highly erosive. <br />For the purposes of the following discussion and to remain consistent with Map 139 the plugged <br />culvert at the East Portal Area Access Road is referred to as CMP #2 and the plugged culvert at <br />the East Portal Area is referred to as CMP #4. CMP #3 is located at the East Portal Area <br />between CMP #2 and CMP #4. <br />CMP #2 is a 40 foot long 24 inch culvert near the center of the access road to the East Portal <br />Area. It not only carries runoff from the majority of the road itself but from undisturbed slopes <br />above the road. This culvert became plugged during the storm of July 27 by a straw bale that <br />dislodged due to the excessively high peak runoff If plugged, runoff crosses the road and exits <br />via a riprapped ditch outlet located adjacent to the culvert. Mr. Mathews requested BME to <br />install this outlet earlier this year to carry flow should CMP #2 become plugged. The ditch outlet <br />functioned properly with only minimal erosion due to the high peak flows. Ponding due to the <br />clogging of CMP #2 allowed additional settling of sediment originating from the undisturbed <br />ground. <br />CMP #4 is a 120 foot long 18 inch culvert located under the east storage area of the East Portal <br />Area. As with CMP #2 it carries runoff from undisturbed slopes. This culvert became plugged <br />during the storm of July 27 by rock fragments and sediment carried by the excessive high peak <br />runoff. If plugged, runoff which would normally pass through CMP #4 ponds on the road and <br />passes on to CMP #3. Should the capacity of CMP #3 be exceeded the excess runoff ponds on <br />the road until it can pass through CMP #3. Should this storage capacity be exceeded, flow would <br />eventually continue on to CMP #2 and the riprapped ditch outlet. Ponding due to the clogging of <br />CMP #4 would allow additional settling of sediment originating from the undisturbed ground. <br />The applicable design specifications for the culverts require the safe passage of peak runoff from <br />a 10-year, 24-hour precipitation event (Rule 4, Section 4.03.2(4)). <br />According to the most recent isopluvials of l0-year 24-hour precipitation from NOAA Atlas 2, <br />Volume III acquired from the Colorado State Office of the Natural Resource Conservation <br />Service (p.k.a. Soil Conservation Service (SCS)) on August 14, 1998 the 10-year 24-hour design <br />storm is 1.6 inches in 24-hours (copy attached). Applying a SCS Type II synthetic rainfall <br />distribution to the l0-year 24-hour event yields a peak ]0-year 1-hour precipitation intensity of <br />0.71 inches. <br />The Type II synthetic rainfall distribution is approximated by the following equation: <br />