C153623 Extension Letter - Laserfiche WebLink

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Sanchez Ditch and Reservoir September 18, 2012 (Updated October 4, 2012) Page 5 of 8 Agenda Item 23c Issues /Additional Needs: The Colorado Division of Water Resources (Division 2 office) reviewed the feasibility study and grant application and provided the following comments: Please note we have not performed a technical review of the proposed alternatives due to their preliminary nature. (1) While we support the Sanchez Dam outlet rehabilitation proposed in the May 2012 feasibility report and believe it is a necessary project, we have also made the dam owner aware of our concerns with the right abutment seepage. We have advised them of the un- protected (i.e. no filter protection) nature of the seepage flow path in this area and the fact that this is a fairly serious potential failure mode that we will be tracking as reservoir levels rise. Due to this as an identified failure mode there is a potential for a State Engineer's storage restriction if seepage and piezometric levels continue to rise relative to historic levels when water becomes available to store. We don't want anyone to be surprised if after spending significant time and resources on the outlet works rehabilitation, additional work related to seepage protection is mandated by this office. (2) Past borings and geologic conditions indicate a potential flaw associated with embankment soils in contact with fractured basalt at the right abutment contact. Limited monitoring data indicates increased piezometric pressure and seepage rates in the right abutment since 2004. Due to the limited historic monitoring data and the current low stage of the reservoir, we cannot say conclusively that there is an immediate problem and additional data collection would be very helpful for making that determination. We have therefore required the dam owner to improve their seepage and piezometer monitoring program. The owner agrees that accurate data is the key to assessing the urgency of the seepage problem and to possibly preventing a serious safety problem. (3) We recommend that the proposed outlet rehabilitation project include, at a minimum, a new seepage collection system and measuring device(s) so that the right abutment seepage can be monitored more accurately. (4) Although we don't want to push the scope of the outlet works rehabilitation beyond available funding or delay the project, we recommend that some consideration to be given to a comprehensive evaluation of the failure modes and weaknesses at this historic dam as an aid to potentially phasing project(s) for the most beneficial and timely use of those available funds, and the most efficient rehabilitation of this important structure. We understand that the owner wishes to pursue another Roundtable grant to help address the seepage problems -- comprehensive planning may help to prioritize and phase work associated with multiple projects. For example, it could be beneficial to grout the right abutment at the same time if the reservoir is drained for outlet rehabilitation, in order to achieve a more successful grout curtain when there is no seepage. To address these concerns, the scope of work does include the installation of a 12 -inch slotted drain pipe along the right side of the outlet structure to help control seepage. A weir will also be installed to measure discharge so more data can be collected to better analyze the seepage problem. Reporting and Deliverables: All products, data and information developed as a result of this grant must be provided to CWCB in hard copy and electronic format as part of the project documentation. This information will in turn be made widely available to Basin Roundtables and the general public and will help promote the development of a common technical platform.