1998-10-15_REVISION - M1977378 - Laserfiche WebLink

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File No. M- 77-378 Page 2 Mill area have been identified where this is occurring. Their locations are marked on the attached maps as Intercept Points A, B, and C. Outside the MLR permit area (under the voluntary Consent Decree agreement with CDPBE and permitted by the facility Stormwater Permit), interception structures will be constructed to capture and transport flowing surface water past the areas where it is currently going subsurface. The interception structures will not impede flow-thru from any storm events that may occur that are too large for the inlets to the system. These flow-through events, if they were to occur, would be controlled by the existing upland diversion system within the permit area. The transport pipes will be 24" ADS N-12 smooth interior pipe with leak-proof couplings. The 24" diameter was selected to facilitate and minimize maintenance. A 12" diameter pipe would be more than adequate to carry the anticipated and observed flows. Outfalls from the ADS pipe will be rock armored to prevent erosion. The transport pipes will enter the MLR Permit area as buried pipes. In drainages A and B, the pipes will if possible extend under the existing upland diversion ditch to discharge into the drainage path that drains runoff from the top of Tailings Pond No. 1 located at the back of the pond. This drainage is currently not large enough to carry the additional runoff water. It will be enlarged and lined with 40 mil PVC liner. The liner will be covered with rocky subsoil material. Where ditch slopes increase, the cover material will be armored with cobble/rock material to prevent erosion. If it is not possible tho carry the pipe under the existing diversion ditch, the pipe will outfall into the existing drainage ditch. The existing diversion ditch has the capacity to transport over 5 times the flow calculated from a 100 year, 1-hour storm of 2.3 inches (ie. capacity- 300 cfs, calculated flow- 55 cfs Consolidated Permit, 1987). If the outfall is into the existing diversion ditch, no changes are required since it is sized to carry the design storm from these drainages. The proposed lined ditch at the back of Tailings Pond No. 1 (to be constructed if the pipe outfall can be extended to pass under the existing drainage ditch)will have the capacity to carry greater than 50 cfs. The maximum flow anticipated would be runoff from the top of Pond No.1 (100 year, 1-hour storm of 2.3 inches) at 5.5 cfs and the capacity of the 2 - 24" pipes to be placed in drainages A and B (inlet conditions will limit the flow to less than 10 cfs for drainage A and 2 cfs for drainage B). Required capacity of the ditch to carry design flows would be 17.5 cfs which is less than the minimum flow capacity of 50 cfs for the proposed ditch. Observed flows have been less than 1 cfs. Any flows in excess of the inlet capacity will go into the existing diversion system. The drainage pipe installation is oversized to provide for ease of maintenance although the system is being constructed to minimize the potential for maintenance requirements. In drainage C, the transport pipe will also be a buried 24" ADS N-12 pipe where it enters the permit area. This line is also oversized and inlet conditions will limit flows to less than 10 cfs.