Spillway condition is an important aspect of dam safety, yet spillways often are not evaluated as thoroughly as the dams themselves. Properly assessing the conditions that could lead to failure of a spillway during a flood can help project owners find and correct deficiencies before they become major problems.
By John Trojanowski
Failure of a spillway has potentially serious consequences, which can include loss of reservoir storage, loss of life, and downstream damage. Although the safety of a dam is evaluated thoroughly, spillways often receive less attention than other features. As a result, there is growing need to raise awareness among dam owners regarding safety issues associated with developing conditions that could result in a spillway failure during a flood.
Potential failure modes for spillways include: flows exceeding spillway discharge capacity, cavitation damage, hydraulic jacking, foundation erosion, improper gate operation, and mechanical failure of gates. In addition, spillway chute or stilling basin failures can develop into a condition of head-cutting, leading to loss of the reservoir.
Although spillway design engineers generally have a good understanding of this subject, dam safety inspectors and specialists should know how to identify and evaluate potential failure modes in spillways. A thorough assessment of a spillway includes evaluation of design and construction details as compared to current standards, as well as observed field conditions. Often, examiners can overlook signs of serious problems because the observed damage appears minor. Failure to recognize developing failure modes can result in recommendations to make minor surface repairs when major modifications are needed.
Conditions that can lead to failure modes
There are five notable hydrologic conditions that can lead to a spillway failure mode during a spill event. These are:
— Existing structural damage that compromises the spillway;
— Flows that exceed spillway capacity;
— Cavitation damage;
— Significant stagnation pressures that can lead to hydraulic jacking or structural collapse; and
— Foundation erosion related to seepage or groundwater.
Improper gate operation and mechanical gate failures also are possible but are a separate issue and are not covered in this article.
Government organizations, such as the U.S. Department of the Interior’s Bureau of Reclamation, have a comprehensive facility review program for their dams. These reviews are performed on each dam every six years, on a rotating basis. The reviews include evaluation of hydrologic failure modes. Recent findings from the comprehensive facility review program show that past spillway evaluations were not always thorough. As a result, there currently is an increased focus on spillway failure modes during the facility reviews. Potential failure modes related to normal and seismic loading conditions also are evaluated by Reclamation but are not discussed here.
Existing structural damage
Throughout the history of dam construction, technology related to concrete and reinforcement design, hydraulic evaluation, hydrology, foundation treatment, and drainage has changed considerably. Therefore, it is important to gather available information related to the spillway being evaluated. When drawings and reports of dam and spillway construction are available, they can be the best source of information. If this information is not available, knowing when a spillway was constructed and the engineering practices used at that time is important. Without detailed documentation, it may be necessary to make assumptions that must be verified during the on-site evaluation.
Most structural damage is easily observed during a site inspection, unless the damaged areas are inaccessible or require special inspection (such as climbing or underwater diving). Spillway damage may have occurred as the result of a wide range of problems, including:
— Previous spillway flows that initiated one of the other four failure modes;
— Deterioration of the structure;
— Foundation problems; or
— External loading.
The inspector should seriously consider the possibility that observed structural damage can affect the performance of the spillway. If the damage significantly increases the risk of spillway failure, the dam owner and/or engineer will want to determine the cause of the damage. The next step is to develop a program to repair the damage, modify the structure, and/or impose operating restrictions to avoid uncontrolled release of the reservoir due to spillway failure.
However, not all structural damage is obvious during an inspection. Indicators of structural damage may be subtle. For example, an inspector might attribute cracking, spalling, or offsets at structural joints to temperature effects or minor foundation movements. But these same findings could be indicators of excessive structural loading, settlement, foundation erosion, frost heave, alkali-silica reaction, delamination of concrete at the reinforcement mat, or other underlying problems.
Inspectors should compare design loads to the actual or suspected loading conditions to determine if the structure was under-designed. Often, older structures lack adequate strength for loading conditions caused by earthquakes, saturated backfill, high-differential uplift pressures, dynamic pressure fluctuations, or foundation movements. The concrete and reinforcement may not have been designed to current standards for the expected loading. Depending on age of the structure, advances in concrete technology may not have been employed.
With regard to drainage, drainage elements placed behind or beneath the structure may not be adequate or may have been damaged. Drains may not be properly filtered to prevent foundation erosion. In addition, foundations may not have been adequately designed to prevent damage due to settlement, frost heave, or swelling soils.
Inspectors should always investigate the possible cause of observed structural damage. This may involve a study of available design and construction documentation, combined with field investigations.
Flows exceeding spillway capacity
Typically, spillway design capacity is known or can be determined through engineering evaluation. However, the original dam design may have been based on limited hydrologic data. Updated hydrology can result in flood discharges that exceed the design spillway capacity. Hydrologic evaluations should not focus solely on the ability of the waterways to pass flood flows without overtopping the dam. While a spillway gate or inlet structure may be capable of passing flows that exceed the design discharge, the spillway chute or stilling basin may not fare well under these conditions. Water surface profiles for various flows, including those flows exceeding the design capacity, will aid in evaluation of this failure mode. Close attention should be paid to areas of rapidly varying flow, including changes in slope or cross section.
It is important to evaluate:
— Flow depths and velocities in spillway chutes;
— Stilling basin capacity;
— Tailwater depths required to develop a hydraulic jump within the stilling basin for safe energy dissipation;
— Sweep out conditions that occur when the tailwater is not sufficient to develop a hydraulic jump (resulting in high-velocity flows in the downstream channel); and
— The range of expected downstream tailwater conditions for various spillway flows.
Flows that overtop spillway walls can lead to erosion of backfill and possible collapse of the structure. Higher flow depths also can collapse a structure designed for much lower flow. Even if the stilling basin can contain a hydraulic jump, sweep out or low flow depth conditions at the upstream end of the basin where the jump has not yet formed can result in unusual differential loading. This results when uplift pressures equal the tailwater on the underside or outside of the structure and there are low flow depths (or sweep out) on the inside. Sweep out or a hydraulic jump that develops beyond the end of the stilling basin can result in excessive erosion and undermining of the structure. If damaged, chutes or stilling basin can fail, initiating head-cutting.
Various design aids, including Reclamation’s Engineering Monograph No. 25, can be used to evaluate stilling basin capacity.1
The potential for cavitation damage can be determined using design aids, such as Reclamation’s Engineering Monograph No 42.2 To evaluate cavitation damage potential, the inspector must develop water surface profiles for various flow conditions. Unchecked cavitation damage can erode through a spillway lining, compromising the foundation below. Large holes developed in hard rock foundations at the Hoover and Glen Canyon dams in Colorado after the concrete liner at each dam was damaged by cavitation and the foundations were exposed to high-velocity flows. Lower velocity flows could be more damaging in softer foundations due to greater potential for foundation erosion.
If cavitation is a significant concern, spillways that have operated at relatively high flows for extended periods typically will have visible signs of cavitation damage. However, spillways that have operated at limited flows may not show signs of potential future damage. It is important to understand the operating history and cavitation potential before ruling out this failure mode based solely on visual observation.
Stagnation pressure occurs when offsets into the flow create a stagnation point where velocity is converted to pressure. Two potential failure mechanisms can result.3 These are structural collapse due to foundation erosion and hydraulic jacking due to excessive uplift caused by stagnation pressures. Previously published research can be useful in determining stagnation pressures at offset joints projecting into the flow for flow velocities up to 15 feet per second.4,5
Ongoing research by Reclamation (see Figure 1 on page 46 and Figures 2 and 3 on page 50) indicates that stagnation pressures increase with higher flow velocities that might be expected on steep spillway slopes.6 This research also includes measurement of flow through an offset joint or crack. Studies of Reclamation spillways indicate that flows into offset joints could easily exceed the capacity of typical spillway underdrains.
Figure 2: Stagnation pressure for a 1/8-inch-wide crack with no drainage increases with increasing velocity of flow. This shows that flow velocities are a critical component of potential hydraulic jacking failures.
Structural collapse occurs when the foundation is exposed to flowing water that causes significant erosion and a portion of the spillway liner settles or collapses into the resulting void. Stagnation pressures at offset joints or cracks can produce flows such as those shown in Figure 3. The presence of erodible foundation materials contributes to this potential failure mode. A structural collapse can expose foundation materials to high-velocity flows, initiating a potential head-cutting failure.
Significant consolidation of foundation materials beneath the concrete after construction can result in cracking or joint offsets. Lack of defensive measures to control seepage into the foundation can result in free-flowing water. Voids caused by settlement, frost heave, or other conditions can result in open flow channels in the foundation. Foundation materials can erode through these channels. Unfiltered drains, open joints, or cracks in concrete provide an exit point for flowing water. Water flowing from drains, joints, or cracks can be monitored if they are visible and accessible. A discolored discharge or deposits of sediment near drain outfalls could be signs that this failure mode is developing.
A well-designed filtered drainage system can reduce the potential for erosion. Additionally, water stops through concrete joints, keying one side of the joint into the other to prevent differential movements resulting in offsets, and adequate reinforcement can reduce the risk of this potential failure mode. The drainage systems for many older spillways consist of clay tile pipe in a gravel envelope. These drains generally do not meet modern filter criteria.
Reinforcement in some spillways, especially those constructed before the 1960s, often consisted of a single layer of light reinforcing steel near the exposed surface. This reinforcement can withstand very little bending. However, when voids develop in a foundation, there may be a period when the invert slab or chute wall footing temporarily bridges the void. Initially, minor cracking may not appear significant. Once the shear or bending capacity is exceeded, the structure can experience sudden failure. Loads on the structure and the potential for additional foundation erosion will increase during a spill, making failure more likely. Discolored discharge from the drain outfall or sediment in the outfall pipe may be signs that this failure mode is developing. Investigations that may include ground-penetrating radar and/or drilling through the spillway slab can help determine if voids are present in the foundation.
Conditions contributing to the hydraulic jacking failure mode include:
— Concrete cracking or offsets at joints due to settlement;
— Deterioration of joints (freeze-thaw or other) that produce offsets into the flow;
— Lack of intact water stops and/or keys at joints in the concrete; and
— High-velocity flows in the spillway chute.
Flows passing over offsets in spillway liners can induce either high uplift pressures that can lift concrete slabs or flows into the crack that can erode foundation materials. This occurs when offsets are projecting into the flow (the downstream side is higher than the upstream side). (See Figures 2 and 3.) Figure 2 shows pressures that can develop on the underside of a spillway chute slab when there is not adequate drainage in the foundation. Figure 3 shows flows through an offset crack or joint when adequate drainage is present. Once hydraulic jacking occurs, the increased offsets will allow high-volume flows to enter the foundation, causing erosion. Model testing has provided some guidance for evaluating this condition.4 Ongoing studies by Reclamation indicate that uplift pressures increase significantly with higher flow velocities.
Two Reclamation spillways have documented failures resulting from hydraulic jacking.5 The spillway of Dickenson Dam in North Dakota failed during a 4,000 cubic foot per second (cfs) spill in 1954 that had an average flow velocity of 21 feet per second. The failure occurred after the underdrains were plugged due to freezing and erosion occurred in the soft sandstone and shale foundation.
Observation of foundation materials in the outflow of the underdrain (inset) for the spillway chute at this dam indicates that a void may be forming in the foundation.
The spillway of Big Sandy Dam in Wyoming failed during a discharge of 400 cfs in 1983 that produced an average velocity of 31 feet per second. Flow was entering the foundation through spalled and open joints. Reclamation believes that one of the Big Sandy underdrains was plugged with debris during the spill. The uplift pressure in the undrained foundation caused failure of anchor bars grouted into a soft sandstone foundation. From Figure 2 on page 50, it is easy to see how uplift caused by hydraulic jacking could lift a thin concrete slab. At 31 feet per second of flow, an offset into the flow produces a stagnation pressure of up to 12 feet of water. The spacing of anchor bars was probably not close enough to resist the uplift that likely developed before failure.
Neither of these spillway failures resulted in a breach of the reservoir because the foundation was somewhat resistant to erosion. Consequently, head-cutting did not initiate.
Over the 70-year life of the structure, erosion occurred in the soil foundation of the spillway of Hyrum Dam in Utah during yearly runoff flows. Most flows into the foundation exited through the unfiltered clay tile underdrains, causing significant erosion. However, hydraulic jacking failure did not occur because the drain capacity was not exceeded. Additional joint offsets caused by ongoing erosion had the potential to produce several cubic feet per second of flow in the foundation (see Figure 3 on page 50).
Spalling of the joints of the spillway at Scofield Dam indicates potential offsets into the flow where stagnation pressures can develop.
At Hyrum Dam, the highest flow velocities in the chute may have exceeded 60 feet per second. This could result in a flow of up to 0.2 cubic feet per second per foot of joint length. With a 16-foot chute invert width, there could potentially be 3.2 cubic feet per second of flow at each offset joint. This would exceed the drain capacity, causing the drains to become pressurized. The foundation of the Hyrum Dam spillway is highly erodible, and a hydraulic jacking failure would have resulted in head-cutting and possible breach of the reservoir. Using the methods described in this article, Reclamation identified the developing failure mode before a spillway failure occurred and modified the spillway to prevent flow into joints and cracks in the concrete. Had the spillway failed, the uncontrolled flows could rapidly erode the foundation, causing an uncontrolled release of the entire reservoir volume.
Flowing water in the foundation does not always originate in the spillway chute during a spill event. Even if there are no open joints or cracks in the chute concrete, foundation erosion can be caused by reservoir seepage, flowing groundwater, or seepage from local precipitation. Generally these conditions will not produce problems in well-drained foundations with adequate filtering of drainage materials.
Many spillways have been designed without adequate drainage. Unfiltered drains such as a clay tile pipe in a gravel envelope provide an open path for erosion of fine-grained foundation materials. Open joints in the spillway also can provide a path for erosion into the spillway chute. Erosion of the foundation can produce structural collapse similar to the collapse that results from stagnation pressure-related erosion, except that a spillway discharge is not necessary for this condition to develop. Poorly consolidated foundations also may settle when exposed to groundwater. This condition can create seepage paths between the concrete and foundation and/or structural damage.
Signs of groundwater-related erosion are discolored discharge from drains, seepage through joints and cracks, and accumulations of foundation materials near these seepage exit points. Signs of foundation movement, settlement, or voids adjacent to the structure also may indicate erosion is occurring.
Four-step process for assessing a spillway
Identification of the potential failure modes described above can be done in four stages.
The first stage is to become familiar with the design and construction details. It is important to gather as much information as possible to make an assessment. Design drawings and specifications — along with geology, construction, and laboratory reports — can be useful if they are available. Climate and temperature data also are useful. However, some of this information may not be available for older structures. Even if available, geology reports for the dam may not include details of the spillway foundation. When there is a lack of data, evaluation will be based on inferred conditions and/or field investigations. Identifying design and construction practices from the original construction period can improve understanding of possible as-built conditions. Of particular concern are details regarding the joints, cutoffs (for erosion and seepage), drainage, filtering, reinforcement, and water stops.
The second stage is to perform analysis. The design capacity of a spillway may be given or computed. Updated hydrology should be compared to the design floods. Flood routing studies may be needed to determine the spillway discharge resulting from various floods entering the reservoir. Water surface profiles can help determine flow velocities, cavitation potential, and when spillway and stilling basin capacities are exceeded. If conditions exist that can result in the foundation being exposed to stagnation pressures, the water surface profile results can give an indirect indication of the magnitude of these pressures. Structural analysis can help determine if the structure can withstand the flood loading and potential high uplift pressures.
The third stage involves site inspection. To an untrained eye, minor offsets or damage may not appear significant. However, they can be signs that foundation problems are developing. Knowing how the spillway was constructed, where drains discharge, and other potential problem areas will help in the evaluation during visual observations. Knowing the hydraulic conditions and operating history can help when identifying cavitation damage or signs of erosion damage. When a potential problem is identified, it should be evaluated further through field investigations. An example is the voids found along the sides of the Hyrum Dam spillway, which were being used by burrowing animals. Initially, it appeared that the animals caused these voids. However, further investigation showed that they were caused by erosion. Normally inaccessible areas may require special inspections.
The fourth stage involves field exploration. Although this can be expensive, it should be done when developing failure modes are suspected. Inspectors should not recommend repair to damaged areas without fully understanding the cause of the damage. This may require further investigation because foundation damage may not always be apparent from surface observation. Some tools to consider are geologic mapping and investigation, test flows to observe the performance of drains and other features, sounding, ground-penetrating radar, drilling, and remote cameras to investigate drains or drilled holes.
Mr. Trojanowski may be reached at U.S. Department of the Interior’s Bureau of Reclamation, P.O. Box 25007, Mail Code D-8130, Denver, CO 80225; (1) 303-445-3263; E-mail: jtrojanowski@ do.usbr.gov.
- “Hydraulic Design of Stilling Basins and Energy Dissipators,” Engineering Monograph No. 25, U.S. Department of the Interior’s Bureau of Reclamation, Denver, Colo., 1984.
- “Cavitation in Chutes and Spillways,” Engineering Monograph No. 42, U.S. Department of the Interior’s Bureau of Reclamation, Denver, Colo., 1990.
- Trojanowski, John, “Assessing Failure Potential of Spillways on Soil Foundation,” Dam Safety 2004, Association of State Dam Safety Officials, Lexington, Ky., 2004.
- Johnson, P.L., “Research into Uplift on Steep Chute Lateral Linings,” U.S. Department of the Interior’s Bureau of Reclamation, Denver, Colo., 1976.
- Hepler, T.E., and P.L. Johnson, “Analysis of Spillway Failure by Uplift Pressure,” ASCE National Conference on Hydraulic Engineering, American Society of Civil Engineers, Reston, Va., 1988.
- Trojanowski, John, “Can Your Spillway Survive the Next Flood?” 26th Annual USSD Conference, United States Society on Dams, Denver, Colo., 2006.
John Trojanowski, P.E., civil engineer with the U.S. Department of the Interior’s Bureau of Reclamation, has participated in dam safety evaluations for Reclamation since 1978.
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