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-10- <br />Increased surface water sediment yields generated by mining would ordinarily <br />cause a notable impact on the quality of water in t4ann Draw. Increases in <br />suspended solids from disturbed runoff will be mitigated through the use of <br />sediment ponds. <br />The sediment control system which is intended to reduce affected water <br />sediment loads was designed in accordance with Rule 4.05. Runoff volumes were <br />calculated using procedures outlined in Chapter lU of the National Engineering <br />Handbook, Section 4, to ensure that the ponds could detain the runoff produced <br />from a 10-year, 24-hour precipitation event. Peak rates of runoff were <br />calculated using the method outlined in Procedures for Determining Peak Flows <br />in Colorado, SCS, 1971. Using these rates, emergency spillways were designed <br />for the 25-year, 24-hour event. Although some questions arose over the <br />steepness of the spillway channel, it was decided that the design was in <br />compliance, since the design peak flow was not routed and did not consider the <br />effects of attenuation due to pond storage. <br />Storage routing was attempted for pons H. This method vividly exemplifies the <br />amount of conservatism in spillway designs. Anon-routed 25-year peak flow <br />for pond H is 29.4 cfs. The flow routed by the applicant utilizing a <br />triangular hydrograph approximation was 9 cfs through the spillway. And <br />finally, oy using a curvilinear hydrograph routing technique, the Division <br />calculates only 3.6 cfs will exit the spillway. From this, it becomes <br />apparentr that the spillways are conservatively designed for the 25-year, <br />24-hour event. <br />During the course of the review, another problem was identified with the <br />sediment control system. This problem dealt with the method of dewatering <br />pond H. Pond H was constructed to detain and trap sediment from a small <br />portion of disturbed area and also store undisturbed drainage for augmentation <br />purposes. In order to augment water when a "call" exists on Mann Draw, a <br />large gated spillway was installed at the bottom of the pond. However, if <br />this gate is opened, it will sluice sediment stored in the pond downstream and <br />violate effluent limitations. Since the area of disturbance is far from the <br />spillway and the gradient oetween them is small, sediment should not <br />accumulate at the spillway inlet if removed periodically. Therefore, if <br />sediment removal is accomplished before it accumulates at the spillway, <br />effluent limitations should be net. The applicant has committed to removal of <br />sediment before accumulated sediment reaches the level of the gated spillway. <br />Two additional requirements of Rule 4.05 have been addressed during the course <br />of the permit review, The applicant has provided markers on sediment pond <br />riser pipes which indicate the 60% sediment storage level. Additionally, the <br />applicant has provided professional engineers certification that all ponds <br />have been constructed in accordance with the plans contained in the <br />application. <br />