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Memo to Kate Pickford 2 January 25, 2006 <br />Detention Pond Desien File No. M-2005-045 <br />4. The following design and construction criteria are applicable to the proposed pond described in the <br />permit application: <br />4.1. Provide adequate capacity to contain or treat the inflow entering the pond as a result of 10-year, 24- <br />hourprecipitation event and the inflow from any water to be pumped from the pit into the pond. In <br />determining the runoff volume, the characteristics of the mine site shall be considered. Runoff <br />diverted from the undisturbed drainage areas and not passed through the pond need not be <br />considered in sedimentation pond design; <br />4.2. Provide for an adequate sediment storage volume and periodic sediment removal; and, <br />4.3. Provide a nonclogging dewatering device or conduit spillway to remove water storage resulting <br />from inflow. The dewatering device shall not be located at a lower elevation than the maximum <br />sediment storage volume. Dewatering shall be achieved within 72 hours of detainment of water <br />unless an augmentation plan approved by the water court is secured. <br />4.4. Diversion ditches shall be constructed, as a minimum, to pass safely the peak runoff from a <br />precipitation event with a 10-year recurrence interval and with gently sloping banks that aze <br />stabilized by vegetation. <br />4.5. The detention pond and diversion ditches shall be constructed before any disturbance of the pit azea <br />to be drained into the pond. <br />4.6. The detention pond must be designed with spillway capacity to safely pass inflows as a result of <br />100-year, 24-hour precipitation event without damage to the pond or its appurtenances. Unless <br />upland ditches are designed to bypass runoff from the 100-yeaz, 24-hour event, the spillway <br />capacity calculations must consider inflows to the detention pond that would occur following <br />overtopping of the diversion ditches. <br />4.7. The DMG understands that the embankment dam of less than five feet in height is planned. A <br />minimum embankment crest width of six feet is suitable for dams up to 10 feet in height, and shall <br />be provided. <br />4.8. Minimum freeboard of one foot, during peak flows from the design storm, is suitable for dams less <br />than 660 foot long, and shall be provided. <br />4.9. Given the nature of the soil at this site that will likely be used for construction of the embankment, <br />the dam should be constructed with slopes no steeper than 3H:1 V. <br />4.10. If earthen channel spillways are to be employed, the velocity of the peak flows through the <br />spillways must be calculated and a demonstration made the spillways will not erode. <br />The Applicant should be aware that, depending on the chazacter and adequacy of the plans, once submitted, <br />the Division may call for further engineering characterization or changes. <br />cc: Matt Carnahan, Agent for Gravel Inc. (via fax and email) <br />Harry Posey, DMG (via email) <br />Bruce Humphries, DMG (via email) <br />