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i~ <br />~ .. <br />Sue Mowry - 2 - July 11, 1985 <br />HYDROLOGY <br />1. The application indicates that the Hubbard Creek culvert will pass the <br />10 year - 24 hour storm event of 838 cfs, but not the 20 year - 24 hour <br />event of 1675 cfs. The culvert is sized for 1149 cfs or 526 cfs <br />under design. The haul road Rule 4.03.1 (4) (e) requires that the <br />crossing be designed for the 20 year event. There is no variance <br />provision in this subsection. Also, in the past two years, the culvert <br />has been exceeded during at least three different precipitation events. <br />In at least one instance, the failing of the culvert resulted in <br />stream-flow being diverted and washing a portion of the coal pile <br />downstream. This would indicate that the under-design of the culvert is <br />not as environmentally sound as the required 20 year design standard. In <br />addition, Blue Ribbon has been producing at a slower production rate, has <br />recently rebuilt the mine bench, and has been negotiating for additional <br />coal reserves. All of this indicates that the mine may be in existence <br />longer than originally indicated. This makes the likelyhood of an <br />additional failure more probable. <br />Based on the above, the Division requires 81ue Ribbon to re-design the <br />crossing to safely handle a 20 year - 24 hour precipitation event. After <br />approval of the designs, the crossing is to be reconstructed and <br />re-certified by the end of the 1985 construction season. <br />2. High stream-flow in Hubbard Creek in the past two years has over-flowed <br />the sediment pond/reservoir on the north end. This may be due to high <br />flow, debris, channel realignment, and/or embankment settlement or <br />erosion. In any case, the breaching of the dam disrupts the sediment <br />collection function of the dam and poses the potential for failure since <br />the pond was not designed to handle stream-flow. <br />To protect the dam and sediment control functions, Blue Ribbon is <br />directed to increase the embankment height. The height is to be <br />increased to give at least one foot of freeboard over the 100 year event <br />or high water mark. The fill should be constructed in accordance with <br />Rule 4.05.6 and re-certified by the end of the 1985 construction season. <br />The source of the fill must be approved by the Division in advance. <br />BGL/mis <br />1931E <br />