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RESERVOIR PROBLEMS 272 9 Reservoir Problems INTRODUCTION The need to extend dam safety investigation to include the reservoir rim was dramatically illustrated by the 1963 landslide on the valley wall of the Vaiont Reservoir in Italy (Kiersch 1964). In that case the wave caused by the slide of rock (some 200 million cubic meters) into the reservoir overtopped the dam by about 125 meters. Although the dam withstood the impact with only minor damage, the wave continued downstream into the town of Longatone killing an estimated 2,000 people. This chapter discusses the reservoir problems of slope instability, induced earthquake, excessive seepage, backwater flooding, and ice. SLOPE INSTABILITY Slides Of concern to dam safety is the possible movement of large masses of rock or soil into the reservoir. The Vaiont reservoir slide is a spectacular and catastrophic example (see Figure 9-1). These movements can be initiated by changes in the piezometric conditions within the mass as a result of reservoir loading, saturation of soil and weak rock materials with a resulting lowering of the internal coefficient of friction, erosion and subsequent undercutting of large earth masses by wave action and reservoir operations (raising and lowering), earthquake stresses on potentially unstable masses of earth, or increased erosion as a result of the removal of protective vegetation.
RESERVOIR PROBLEMS 273 Figure 9-1 Vaiont reservoir slide (aerial view). Source: Courtesy, L. Mueller. Major concerns from reservoir bank instabilities are (1) the sudden release of large masses of material may generate reservoir waves that overtop the dam; (2) large masses dropping or slowly sliding into a reservoir severely reduce reservoir capacity; and (3) highways, railroads, or developed land adjacent to the reservoir may be undercut or displaced by such movements. Sedimentation Slides can increase sedimentation in the reservoir. Another consideration is that the reservoir water may cause adverse chemical or mechanical alteration of the materials composing the reservoir banks. This would result in increased erosion and possible landslides. Increases in the siltation of a reservoir, whether from slide materials or normal sedimentation processes, can reduce the reservoir capacity to store floods. As a result overtopping can occur if such reduced storage capacity has not been considered in the dam and spillway design. Sedimentation also can block or inhibit flow through low-level gates and emergency outlets. In stability analyses of concrete dams, consideration must be given to the expected silt load on the structure. An approximation of this load can be
RESERVOIR PROBLEMS 274 obtained by including a fluid pressure of 85 pcf for the horizontal pressure from the silt and 120 pcf for the vertical pressure. Bank Storage Another possible undesirable situation could result from high-volume bank storage. The latter occurs when the rock or soil in the reservoir is cavernous or highly permeable. If such storage occurs and a reservoir is lowered rapidly, the drainage to the reservoir from the reservoir bank may be too slow to allow rapid dissipation of pore pressure within the rock or soil mass. This can produce instability in these masses. INDUCED EARTHQUAKES There is a question of whether a reservoir may induce earthquakes. To date, there is no universally accepted proof that this can occur, but it is a possibility that should be given consideration. The causative factors still are uncertain, e.g., the weight of the reservoir may increase stresses at epicentral depths sufficiently to trigger fault activity (there is little to support this thesis), the downward movement of reservoir water may increase hydrostatic pressures in the rock masses (this increase presumably would increase pressure on a fault plane that may be under a critical state of strain), or the addition of water to a critically stressed fault plane may decrease the shear friction values of the fault sufficiently to trigger an earthquake. A recent study (Meade 1982) indicates that the possibility of reservoir-induced earthquakes is very limited and probably should not be considered except for extremely large and deep reservoirs. EXCESSIVE SEEPAGE Excessive seepage through sinkholes, rock formation, or any pervious soil formation is a principal type of defect in reservoirs. The effect of this defect is normally obvious in that the reservoir will lose water. Reservoir leakage of this nature would not be expected to cause any loss of basin integrity or catastrophic release of storage, except where it might occur in the immediate proximity of the dam or thin natural barriers around the periphery of the reservoir. Seepage rates can be estimated by keeping a log of water levels, rainfall, and spillway discharges and then calculating total inflow and total water removal, including evaporation and transpiration. The difference should be seepage. Alter it is determined that there is excess seepage, the problem is to find where and how the water is escaping. By keeping a log of water levels and
RESERVOIR PROBLEMS 275 rainfall, one may sometimes obtain a good indication of the elevation at which the excessive seepage is occurring. For example, if a reservoir rises rapidly to a certain elevation but rises slowly after reaching that elevation, this may be a good indicator of excessive seepage at that elevation. Excessive seepage locations can sometimes be detected by visual inspection. It is better to make a visual inspection on a clear, calm day by walking around the rim looking for a vortex, any movement in the water surface, or differences in the turbidity. These visual inspections should especially be made during the initial filling of any new reservoir. If visual inspection fails to identify the problem, a qualified geotechnical firm probably should be employed to investigate and recommend a solution. A reservoir that leaks over a large area probably cannot be sealed economically. Excessive seepage may be corrected by using one or more of the following corrective measures: ⢠Grouting. ⢠Installation of a layer of clay or impervious soil over the problem area. ⢠Use of bentonite. ⢠Mixing soda ash with the pervious soil in the problem area. ⢠Use of liquid chemical soil sealants. ⢠Installation of a polyvinylchloride (PVC) liner. Which corrective measure should be used can only be decided after one has located and quantified the seepage loss and learned all that practicably can be learned about the surrounding soil and rock formations. Core drilling may be used in the localized problem area to learn more about the way the water is leaving the reservoir and to help in planning the corrective program. BACKWATER FLOODING Any dam and reservoir owner should own all of the land that would be flooded in the event of a maximum flood or should have the permission of the other land owner(s) involved to flood their land. ICE The development of an ice layer on a reservoir surface can cause structural damage and produce maintenance difficulties. The main damage is commonly from the impact of ice against thin-walled structures such as parapets on masonry dams. Vertical and horizontal motion of ice against a structure (because of reservoir operations or deep waves) can induce high thrusts and cause considerable damage to concrete. Freezing and thawing
RESERVOIR PROBLEMS 276 of the ice also can damage structures, particularly if poor drainage allows such action underneath or behind concrete slabs and walls. Ice can adhere to structures, and, when reservoirs are raised or lowered, there can be a concurrent drag of the ice upon the structure. This drag force can induce uplift or increase compression loads on such structures as intake gates and trash racks. There are many cases of such damage. Ice around intake gates and/or towers can impede the passage of water into the intake and interfere with the operation of gates and valves. Ice problems adjacent to intake gates and valves have been mitigated by the continuous introduction of compressed air bubbles along the water side of the structure. This inhibits formation of ice directly on metal or concrete structures. The establishment of minimum operating levels above such facilities also is a possible solution. During a thaw the floating ice can block intake areas or be driven by wind against man-made structures with resultant damage. In general, thermal expansion and wind loads are the major cause of structural damage from ice. Ice can also block spillway control structures, particularly gated ones, thereby reducing the spillway capacity, raising the water level, or resulting in a sudden water surge when the ice finally breaks loose. A number of empirical formulas have been proposed for calculating ice loads on a dam. Parameters to consider are the slope of the upstream face and the slope and roughness of the valley walls. In addition, wind blowing Figure 9-2 Ice pressure versus ice thickness. Source: Thomas (1976).
RESERVOIR PROBLEMS 277 down a reservoir can increase ice loading by 4 to 5 tons per meter on an exposed face. One study showed that, theoretically, thrusts on the order of 7.5 to 30 tons per meter were possible under North American climatic conditions, and in Canada a figure of 15 tons per meter is commonly used. At one time Norway used 45 tons per meter but now has decreased this to 5 tons per meter for dams with sloping upstream faces. Some tests by the U.S. Bureau of Reclamation near Denver, Colorado, indicated the highest thrusts were in the range of 20 to 30 tons per meter, depending on ice temperature. The latter is important because the rate of temperature rise and its duration can determine the pressure likely to be exerted. Figure 9-2 compares ice pressure with ice thickness and considers whether the reservoir banks act as a restraint. It was developed in Japan in 1970 (Thomas 1976). REFERENCES Kiersch, G. A. (1964) "Vaiont Reservoir Disaster," Civil Engineering , March. Meade, R. B. (1982) State of the Art for Assessing Earthquake Hazards in the U.S.âThe Evidence for Reservoir Induced Macro-Earthquakes, U.S. Army Corps of Engineers, Waterways Experiment Station, Miscellaneous Paper S-73-1. Thomas, H. H. (1976) The Engineering of Large Dams, Vol. 1, John Wiley & Sons, New York, p. 57. Recommended Reading Daly, W., Judd, W., and Meade, R. (1977) Evaluation of Seismicity at U.S. Reservoirs, USCOLD Committee on Earthquakes, Panel on Evaluation of Seismicity. Monfore, G. E., and Taylor, F. W. (1948) The Problem of an Expanding Ice Sheet, U.S. Bureau of Reclamation Memorandum, March 18. Muller, L. (1968) "New Considerations on the Vaiont Slide," Rock Mechanics and Engineering Geology, Vol. 6/1-2. Rothe, J. P. (1969) Earthquake and Reservoir Loadings, Proceedings of Fourth World Conference on Earthquake Engineering, International Association for Earthquake Engineering.
