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COMMENTS BY COLORADO MINED LAND RECLAMATION DIUISIOP; <br />' COMMEF;T <br />' 2. The design of the recently constructed pond at Pit 4 has not yet been <br />approved under the permanent program by the Division. Water would be <br />discharged from a 30" diameter culvert primary spillway onto a 600-foot <br />' slope mapped as a landslide deposit with slope gradients approaching <br />50%, Figure 9 indicates riprap placement for a short distance below <br />both the emergency and primary spillways, but no further detail is <br />provided. As required by Rule 4.05.7, a plan for a discharge structure <br />designed according to standard engineering design procedures should be <br />submitted to the Division. Energy dissipation should be provided <br />wherever design flow velocities would result in erosion. Diversion of <br />' discharge flows along the contour to the adjacent stream channel may be <br />an acceptable approach. <br />' RESPONSE <br /> A 6-inch diameter plastic pipe will be attached to the end of the 30" <br />t diameter primary spillway discharge pipe. The plastic pipe will be <br /> attached in a manner that diverts all low flows while not diminishing the <br /> ultimate capacity of the pipe, as shown on the attached sketch. The 6" <br />' diameter plastic pipe will extend directly down the natural slope, <br /> dischargi ng into Grassy Creek. Intermittent anchors will be provided to <br /> structura lly secure the pipe. This will eliminate the e rosion potential <br />' under all conditions less severe than the 10 year-24 hour precipitation <br />event. <br /> <br /> COMMENT <br /> <br /> 3. Statements in the text and Table 4 seem to indicate that the pond at <br /> Pit 4 has been sized to store the volume of a 10 year-24 hour precipi- <br /> tation event t ogether w ith a one year accumulation of sediment without <br />t discharging. However, Figure 9, the Detail Drawing, indicates the <br /> water level in the pond resulting from a 10 year-24 hour storm would be <br />' two feet above <br />should clarify the leve <br />whether l of the primary spillway riser. The applicant <br />any water would be discharged from the primary <br /> spillway as a result of a 10 year-24 hour or smaller runoff event. <br /> PO <br />E <br /> R <br />S <br />NSE <br />1 The existing riser pipe (principal spillway) crest elevation is 7522.50. <br />The emergency spillway crest is 7524.50. The riser pipe crest will be <br />extended 1 foot to elevation 7523.5. An area/capacity curve for the <br />existing sediment pond was developed based on actual field measurements. <br />This area/ capacity curve is attached. The curve shows that with the <br />extended riser the entire volume of the 10 year-24 hour event can be <br />stored. A small amount of water would be discharged through a 5-inch <br />' diameter hole in the riser which is discussed in response to the next <br />comment. <br />