2015-01-21_REVISION - M1977526 - Laserfiche WebLink

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ENVIRONMENT, INC. PAGE 2 JANUARY 14, 201 5 3) In review of the design submitted DRMS believes that this design does not provide a long -term solution. DRMS believes that there is high potential for the open channel to: fill in with silt, become clogged with vegetation and the soil walls of the 1 H:1 V open channel to slump off and restrict flow. Please provide a groundwater mounding mitigation plan that will have a lower likelihood of failure as described above. The design must demonstrate long -term success. The design as submitted, was a draft plan that was changed when construction began. During construction the ditch section was increased in width and depth from the slough bank to the road crossing. This open channel was done to match the historic conditions on the site where an open channel existed prior to mining. The increase in bottom width and flattening the side slopes is intended to provide more carrying capacity to the river, then what existed historically, to help prevent the issues you raise. Once the water elevation in the groundwater drain system stabilizes and we can put the final touches on the new out fall ditch,'as built plans will be submitted to the Division for the file. The attached photos show construction of the ditch and pipe that were taken on 1/9/15 and the ditch with water flowing thru it on 1/14/15 ?. 4) Please demonstrate that the proposed groundwater mounding mitigation plan has the capacity to convey adequate flow to account for the loss of flow through the Worthing Pit due to the lower hydrologic conductivity of the backfilled material. We know of no way to accurately predict the amount of water that passed thru the Worthing Pit prior to backfilling. The ditch and pipe sizes have been base on the observations of the historic flow through the existing groundwater drain since it was installed in. 2013 and the flow through the pipes on the easement until they became plugged with vegetation. The flow thru the groundwater drain was unrestricted even with the smaller pipes and no direct flow to the river until this happened. We estimate with, the, proposed slope and the pipe size, that the pipe is capable of passing 3.25 cfs. The new outlet channel width increases the carrying capacity of the outlet ditch and armoring will reduce the vegetation that can get to the outlet pipe and reduce the possibility of plugging. If needed a second pipe can be added at the road to increase the discharge if water flow is restricted. Then bank was cut mid -day on Friday January 9th allowing the groundwater drain to begin to flow into the out fall ditch. By night fall the water level had stabilized in the new ditch and was flowing thru the pipe with 'room to spare. 5) It appears that the proposed groundwater mounding mitigation plan will increase the surface area of the exposed groundwater. As required by Rule 3.1.6, please demonstrate compliance with all applicable Colorado water laws and regulations governing injury to existing water rights. Thank you for the information. We will address it with the SEO if needed. We believe the surface area will actually be smaller once the drain is opened and the system equalizes.