TECHNICAL MEMORANDUM No. 1 TO: Michael Bowen California Coastal Conservancy Geotechnical & Earthquake Engineering Consultants CC: Eric Ginney Philip Williams & Associates PREPARED BY: Paul Grant SUBJECT: RESERVOIR DRAWDOWN RATES/RESERVOIR DRAWDOWN TEST Iron Gate, Copco (I & II), and JC Boyle Dams PROJECT NO.: 07-153 DATE: This memo briefly confirms our discussions regarding reservoir drawdown rates for the Iron Gate, Copco (I and II), and JC Boyle reservoirs on the Klamath River located in northern California and southern Oregon. The results of our study will be presented in detail in a separate geotechnical report that discusses the overall geology of the area and our assessment of the safety of the embankment of the Iron Gate and JC Boyle Dams and all reservoir side slopes with various rates of reservoir drawdown. The rate of reservoir drawdown is a minor concern for the Copco dams because both are concrete structures that are relatively insensitive to the drawdown rate. In general, conclusions on the stability of the embankment of the 100 foot high Iron Gate Dam would also apply to the much smaller (i.e. 30 feet high) JC Boyle Dam. RESERVOIR DRAWDOWN RATES In summary, we evaluated the stability of the Iron Gate embankment dam using various analytical procedures to determine if a rapid drawdown of the reservoir would induce a failure of the upstream slope of the dam. First, the stability of the embankment was analyzed using conventional limit equilibrium procedures which conservatively assumed the hydrostatic surface at the base of the rip rap on the upstream face of the embankment during all stages of drawdown. The stability analyses also assumed that the gravelly sand specified in the construction documents for the shell of the dam was sufficiently pervious to allow the dissipation of pore water pressures without developing significant seepage pressures. We also performed other analyses to directly assess the stability of the embankment to resist a seepage induced failure. The results of both analyses show that the upstream slope of the Iron Gate embankment dam is stable and may accommodate reservoir drawdown rates in excess of 10 feet per day. 2021A Minor Avenue East Seattle, WA 98102-3513 TEL (206) 262-0370 FAX (206) 262-0374
Page 2 of 6 Therefore, for modeling, assessment, and planning of the dam removals, we suggest using drawdown rates of 3, 6 and 10 feet per day in sensitivity analyses to determine if these drawdown rates can accommodate the anticipated sediment removal in the assumed time allotted for the dam removals. Specifically, geotechnical considerations for using different drawdown rates are as follows: 3 feet per day This represents a safe drawdown rate that would most likely be readily accepted by governmental review agencies 6 feet per day This represents a safe intermediate drawdown rate that, while technically sound, would likely require significant discussions with governmental review agencies because few dam removals have been contemplated at this drawdown rate and this drawdown rate is greater than the normal hydroelectric operational demands on these reservoirs. 10 feet per day This is also a safe drawdown rate, but one which would likely require significant discussions with governmental review agencies to achieve project approval because it is higher than normal practice. Use of this drawdown rate would likely benefit from a partial drawdown of the reservoir (i.e. about 30 feet of drawdown) at a rate 10 feet per day to demonstrate the stability of the embankment and reservoir slopes. While the Iron Gate embankment dam would remain stable during any of the above drawdown rates, incrementally higher drawdown rates would likely correspond to an increase in the number of skin or shallow surface slides of the reservoir slopes. Our full report will contain a qualitative assessment of the amount of material that could be dislodged from the reservoir slopes in each of the above drawdown rate scenarios. Based on our geological reconnaissance of the reservoir slopes, we did not identify any areas that would be particularly at risk of slope movement from the reservoir lowering. Additionally, we do not anticipate that the reservoir lowering will significantly affect any of the existing structures built around the reservoirs. In summary, from a geotechnical perspective, reservoir drawdown rates of up to 10 feet per day may be used without compromising the stability of the embankment or reservoir slopes. However, selection of the drawdown rates for the dam removals will likely be controlled by the hydraulics of the sediment removal, time constraints of construction, and potential considerations of environmental impacts. Any proposal to use drawdown rates in excess of about 3 feet per day will likely generate many questions from geotechnical reviewers and may require significant time to respond to review comments.
Page 3 of 6 RESERVOIR DRAWDOWN TEST Because the dam removals may require use of reservoir drawdown rates that exceed values typically used in normal operations, it would be prudent to conduct a reservoir drawdown test in advance of the dam removals to demonstrate the safety of a rapid drawdown at a rate equal to or greater than that which would be used in the actual dam removals. Use of a test drawdown rate that is higher than that which would be used during the actual dam removal would demonstrate a relative factor of safety for the proposed reservoir lowering. Ideally, the testing should be conducted at a drawdown rate of about 6 to10 feet per day, which would correspond to a rate that equals or exceeds the drawdown rate that may be used for the actual reservoir lowering. The results of the test would help validate a factor of safety for the draw down used for the actual dam removal. To provide sufficiently meaningful results, the depth of reservoir drawdown during the test should be on the order of 20 to 30 feet. If the test is conducted at Iron Gate, the embankment slopes on the dam should be monitored with slope stakes for a line-of-sight confirmation of stability. Slope stakes combined with visual observation of the crest and both the upstream and downstream slopes will provide an early indication of potential movement. Should movement be observed, the test would be halted to evaluate areas of suspected instability. Similar monitoring and evaluation techniques should be implemented at selected reservoir shoreline slopes to evaluate the response of those slopes to the test. In addition to providing information on slope stability, a drawdown test may provide useful data to validate and/or calibrate existing modeling of the downstream reaches of the river in terms of suspended sediment timing, concentration, etc. While the test may be conducted at either the Iron Gate or Copco Reservoirs, there are particular advantages and limitations in limiting testing to either location:
Page 4 of 6 IRON GATE RESERVOIR Advantages Test would validate stability of both embankment and reservoir slopes Direct demonstration of embankment stability could be correlated to inferred performance of JC Boyle Dam Direct demonstration of embankment stability should expedite agency review Higher concentration of sediments in tributary channel may allow direct observation of sediment stability under drawdown to refine or confirm ongoing planning and assessment of sediment transport Potentially low community concerns because of limited shoreline development Limitations Draw down rate may be limited to about 2 /day by the capacity of existing project outlet structures Higher drawdown rates would require opening the gate to the diversion tunnel with the attendant risk that mechanical problems may arise in the opening of the gate or, upon opening, the gate may become jammed or the operating mechanism may break and not allow closure. Potential reluctance of the dam operator to allow use of the diversion tunnel based on concerns for the reliability of the gate operations and potential reluctance to install a downstream gate to provide redundancy in controlling the reservoir flow Possible release of suspended sediments downstream of dam
Page 5 of 6 COPCO RESERVOIR Advantages High drawdown rates - Spillway gates may potentially support drawdown rates of up to about 10 /day High drawdown reach spillway gate elevations may support a drawdown of at least 15 feet Low Cost and Low Risk - High drawdown rates would be achieved with existing, functional dam gates that require no upgrades and the gates may be readily closed at any apparent condition of instability. Test would validate the stability of reservoir slopes in developed areas along the reservoir, thereby expediting agency review and reducing community concerns Limitations Test would only validate stability of reservoir slopes Stability of embankment slopes at Iron Gate and JC Boyle can only be inferred from performance of reservoir slopes Potentially greater community concerns because of shoreline development Potentially less information on the performance (i.e. stability) of accumulated sediments on the reservoir slopes because of the lack of large tributaries and the relative lack of sediments at higher elevations at which the test would be performed Low impact on river ecosystem downstream of Iron Gate Reservoir because release could be mitigated by storage and settlement in that reservoir Some information on sediment stability and erosion potential will be provided to validate ongoing planning and assessment; however, the amount of information be less than that of a test at Iron Gate
Page 6 of 6 Benefits to be gained from the test would include: An expedited agency review of the dam removal because of a) the demonstrated stability of the upstream face of the embankment (and relative stability of the reservoir slopes) at a drawdown rate exceeding the planned rate during the dam removals; and b) empirical data that could be collected and used to support environmental assessment activities. Data to quantify the amount of shallow skin or surficial slides on the reservoir slopes that may occur as a result of reservoir lowering. The locations and geometry of the failure areas may then be used to quantify the amount of slide material that may enter the reservoirs during drawdown. This data could be used to estimate additional sediment loading in the reservoirs and could be incorporated into planning and assessment activities. In the event that the pool lowering exposes accumulated sediment on the reservoir slopes, the drawdown test may provide a basis for evaluating the insitu strength and stability of the sediments and provide data on an anticipated angle of repose for the materials that may be incorporated into ongoing planning and assessment activities. Empirical data collected within the reservoirs and re-emerging reaches of the Klamath River would focus on assessing the stability of any sediment stranded on the emerging floodplain and reservoir side slopes, characterizing and quantifying the mobilization of impounded sediment in the main-stem river and its tributaries, and the evolution of the river channel morphology as it erodes into the aggraded sediment. This information and could be incorporated into planning and assessment activities. Finally, the drawdown test may provide useful data to validate and/or calibrate existing modeling of the downstream reaches of the river in terms of suspended sediment timing, concentration, etc. Any potential drawdown test will need to consider many factors, including the start date and duration of the test, to avoid unnecessary releases of suspended sediment and to accommodate seasonal fluctuations in reservoir level. The details of such a test will certainly need to be further refined and reviewed by review agencies for their concurrence. If you have any questions, please do not hesitate to contact our office.