Hydro-Québec is testing a new method for assessing the condition of a spillway. The method uses an indexing system to objectively determine the condition of spillway components. Information from the assessment helps prioritize maintenance activities.
By Denis R. Caron and Luc E. Chouinard
Overtopping is the most frequent cause of failure of embankment dams worldwide.1 Failure to properly operate the spillway structure, resulting in overtopping, is due to equipment or operational deficiencies. These deficiencies can be associated with poor original design or gradual deterioration of the spillway.
Assessments of the condition of spillway components and their relative importance are necessary to effectively allocate maintenance and rehabilitation budgets for spillways. A method is presented to evaluate spillway condition relative to dam safety functions and to assist in prioritizing maintenance activities.
Developing the assessment method
Spillways are structures over or through which flood flows are discharged. The primary focus of the assessment method presented here is to identify and rank major deficiencies in the operation of spillways. The final selection of remedial actions and maintenance activities should include this ranking within a comprehensive asset management program. The ranking method also can be used to obtain an overall condition rating of the spillway.
This method was developed for spillways with vertical lift gates, stoplogs, or tainter (radial) gates because these are the most prevalent gate types for participants in development of the method. Participants were: Hydro-Québec, Manitoba Hydro, Ontario Power Generation, the U.S. Army Corps of Engineers, and the U.S. Department of the Interior’s Bureau of Reclamation.
Hydro-Québec’s hydro facilities have widely varying configurations. They are equipped with spillways that serve multiple purposes. For run-of-river facilities and those in a cascade configuration, the main functions of spillways are water flow regulation, flood regulation, and water level management for dam safety purposes. For facilities with large reservoirs, the main function of spillways is management of water levels in emergency situations. Since 2001, provincial law has required Hydro-Québec to keep track of the condition of structural, mechanical, and electrical equipment, as well as operational systems, for its spillways.
With increasing demand for electricity production and unpredictable floods associated with climate change, it has become even more critical to optimize water management strategies. One aspect of this is to ensure proper spillway performance. Significant increases in repair and rehabilitation costs over the past two decades require that budgets be allocated on the basis of minimizing risk.
Ranking of maintenance budgets at Hydro-Québec is based on the current condition of equipment, as well as its relative importance for the dominant dam safety concerns of each particular spillway (condition indexing). Ranking of maintenance activities is determined first for each facility, followed by ranking across all spillways once evaluations have been performed everywhere.
The utility considered risk analysis as a complement to condition indexing. However, given the amount of effort required, risk analysis is performed only in special cases where major investments are required and alternative remediation schemes are evaluated. Condition indexing is meant to be used on a more regular basis at all facilities to identify the nature and extent of deficiencies, monitor their evolution, and select those where a detailed risk analysis may be required.
Most of the work to develop this method was performed during week-long meetings of an expert panel comprised of representatives from Hydro-Québec, Manitoba Hydro, Reclamation, Ontario Power Generation, and the Corps. During each meeting, a spillway was inspected to test and validate the procedure. These meetings were supplemented with several interim meetings of smaller groups for the development of condition tables. These tables are used to rate the performance of each component, based on inspections and test results.
The method is based on the condition indexing system the Corps developed in 1984 for pavements and used in the REMR (repair, evaluation, maintenance, and rehabilitation) research program for Corps civil works infrastructure. In 1995, Hydro-Québec and the Corps developed a condition indexing method for embankment dams.2,3,4 In 2003, Hydro-Québec, Electricité de France, and Manitoba Hydro developed a condition indexing method for concrete gravity dams.
Priority rankings are established as a function of the relative importance and current condition of spillway components. Importance factors are obtained by identifying the main dam safety concerns relative to operation of a given spillway and the criticality of each component in preventing failure.
In theory, redundant components increase system reliability and should be properly identified. For example, a facility with an emergency power supply is inherently more reliable than one without an alternate power source. Similarly, components that can be the source of a common failure mode for several gates (e.g., gates operated with a non-dedicated hoist) should be properly identified and weighted. Note that common components – such as roads, monitoring systems, and telecommunication systems – that are shared by several facilities on a given river basin also can be potential common failure modes.
The condition of spillway components is inferred from qualitative or quantitative indicators (condition tables) that have meaningful diagnostic value relative to the level of performance of the component. For example, the indicator for cracks on a gate structure goes from zero to 100, depending on the members where the crack is located. If there are no cracks, the condition is 100. If there are cracks in the skin plate due to impact, condition is between 40 and 69, depending on the number and dimensions of the cracks. If there is a fracture in a critical member, condition is between zero and nine. Condition is determined using detailed periodic inspections or up-to-date evaluation reports.
The spillway condition indexing method is based on a hierarchical representation of the spillway. At each level of the hierarchy, subordinate nodes are connected to a common parent node. Importance factors are assigned to the subordinate nodes as a function of the relative effect of the deterioration in performance of the subordinate node on performance of the parent node. At each level, the sum of importance factors assigned to subordinate nodes must equal 1.
The components at the lowest level of the hierarchy correspond to the smallest units that are inspected and evaluated during a routine inspection. The rating of systems at higher levels in the hierarchy can be obtained by a weighted summation of the condition of subordinate elements at the immediately lower hierarchical level.
Elements of the method
The spillway classification method considers six levels – importance of the spillway, dam safety, gate type, types of problems, systems, and components. (See Figure 1.)
Level 1: Importance of spillway
A classification system is used to rank the spillways relative to each other. The review or development of classification systems for spillways was not within the scope of this project. Many dam owners already have such a classification system that can be incorporated in the procedure. It should be noted that these classifications might not be designed to prioritize spillways specifically.
Level 2: Dam safety
Evaluation of the importance of deficiencies for a spillway is performed relative to its dam safety functions. The five dam safety functions for spillways are: prevent overtopping during a design flood; prevent overtopping during load rejection; prevent unintentional gate opening; prevent failure to close a gate; and draw down the reservoir.
In most applications, a spillway’s main dam safety function is to prevent overtopping. The inflows that are considered for the purpose of evaluating the spillway are design flood and load rejection. The manner in which the spillway is operated – from identification of the initiating event up to the start of the opening sequence for the gates – is defined as the reaction time for gate operation. The time from the start of the opening sequence to the total opening of a gate is defined as the opening time. The summation of the reaction and opening time is defined as the total operation time. The various spillway components should be designed such that total operation time for the gates is adequate for the response times of all possible initiating events.
Level 3: Gate type
Factors that should be considered are the capacity and respective attributes of the gates and the ability to operate in the required time. For example, when load rejection requires a short response time, remotely operated heated gates with dedicated hoists typically will be the most important. In the case of the design flood, if the response time is long, the reaction time for operation of the gates may not be relevant. If so, only the relative capacity of the gates can be considered.
A spillway at a 250-MW facility was used to test the condition indexing method. Results showed that the emergency generator was the most important element in continued reliable operation of the spillway.
Gates are treated by type and attributes and do not need to be considered on an individual basis. The types of gates that have been considered in the project are vertical lift gates, tainter gates, and stoplogs. Note that flows through the power plant are not considered in the current evaluation method.
Level 4: Types of problems
Operational systems are grouped into three main categories based on their purpose: to gather information, to aid in the decision process, and to access and operate gate controls. Spillway equipment includes: the power supply and electrical equipment, mechanical systems for positioning and lifting the gates, and gate structure and its supports. The relative importance of operational systems versus spillway equipment may be difficult to determine. Recognizing this difficulty, the preferred option is to rate operational and equipment deficiencies separately. This is often desirable because evaluations for operations and equipment are usually performed by different groups of specialists and require specific remedial measures.
Level 5: Systems
The objective is to identify the relative importance of operational systems and spillway equipment for a given dam safety function. Again, this assessment is spillway-specific and should be conducted in consultation with personnel familiar with the facility. For example, gathering information is most critical for spillways located in regions of flash floods, while access and operation are most critical for remote areas, and decision processes are most critical on watersheds with shared jurisdictions.
Level 6: Components
The relative importance of components within each system has not been considered in the project. For ranking purposes, the importance factor for a type of operation or equipment is assigned to all the components listed under it. Components that are considered secondary or irrelevant to a particular dam safety function are assigned a null importance.
How the method works
Importance factors at each level are determined during discussions of relevant spillway characteristics with dam personnel. An importance factor between 0 percent and 100 percent is assigned to each item within a level. The sum of the importance factors at a level must be 100 percent, and a precision of 5 percent is considered adequate. Factors to be considered are the likelihood that an event may occur, as well as the consequences associated with failure.
Determining component condition
Component condition is rated on a scale developed by the Corps under the REMR program. The component condition tables define the function of a component and its excellent (100 percent) and failed (0 percent) conditions. Intermediate conditions are based on quantitative data or qualitative observations. These may be obtained from an on-site inspection or the examination of existing records for the spillway. Observations are noted for each indicator, and condition is rated.
The panel of experts developed about 70 condition rating tables for components associated with spillways and their operation. The condition tables for components are grouped into four categories: civil/structural, mechanical, electrical, and operational.
Determining priority ranking
The priority ranking of a component or system is obtained as the complement of the condition index multiplied by its importance factor. This priority ranking is used to develop a prioritized list of maintenance activities on the spillway, with the most important component in the worst condition ranked first. Note that the importance factor used in the calculation is a function of the level at which the deficiency is considered. If the deficiency is evaluated at the hierarchical level of the component, it is directly multiplied by its importance factor.
- PR = (100 - CI) x I
– PR is the priority ranking of a component or system;
– CI is the condition index of a component or system; and
– I is the importance factor of a component or system.
Testing the method
The method is illustrated for two spillways. In both cases, the conditions and priority rankings reflect the state of the spillways at the time of the inspections and do not reflect remedial actions that have been taken subsequently.
This spillway is located downstream from a large reservoir located at the top of a watershed. The spillway controls water levels for a 250-MW run-of-river facility, which can fluctuate very rapidly. The spillway is comprised of six gates operated by two non-dedicated hoists. Only one gate is necessary to pass all the powerhouse flow.
Overtopping during a design flood event is not perceived as a major dam safety concern because response times of river levels are about two weeks. For the design flood event, gates are fully opened and have equal importance. Overtopping during a load rejection is considered the most severe dam safety concern because: these events are relatively frequent, response times of water levels are a few hours, and two of the three sources of power to the spillway are likely to be unavailable during load rejection. Only Gates 4 and 5 are considered critical for load rejection because they are the only heated gates and are usually attached to the mobile hoists for quick response. Equipment failure is the main concern during load rejection because the spillway configuration is based on an older design with many components and few spare parts. Operational aspects are not a major concern because operators are well-trained for load rejection events. Among equipment concerns, power supply is most critical because it is a common mode of failure for Gates 4 and 5. Failure of hoisting devices is not critical because they are not maneuvered during load rejection. Redundancy is provided because a single gate is sufficient to evacuate the powerhouse flow.
When considering all dam safety functions of the spillway, the emergency generator has been ranked the highest of all the components (with a priority ranking of 21) because it is in poor condition and is crucial for load rejection. Next in importance are components of the hoists that are in poor condition, followed by components of Gate 5 that are of prime importance during load rejection. Remaining dam safety concerns are less important given a lower probability of occurrence and less severe effects.
The second spillway is operated in conjunction with a 61-MW run-of-river facility. It is a secondary spillway, with the main spillway next to the powerhouse. The low-head spillway controls water levels during spring floods and regulates water levels to optimize electricity production. The spillway is comprised of two gates with dedicated hoists and 15 openings with stoplogs that are serviced by two mobile hoists. The two gates with dedicated hoists are used primarily to regulate water levels for the powerhouse, mainly because of an upstream powerhouse that is operated by another utility for peak production. The spillway has a single access road and two sources of power (local distribution line and emergency generator). The stoplogs are typically removed in late spring during snowmelt and put back in place during the summer and winter months. In exceptional cases, stoplogs can be removed in winter months with the assistance of a steam generator (24 hours per sluiceway).
When considering all dam safety functions of the spillway, the emergency generator and transfer switch have been ranked the highest of all components because they are in poor condition and are crucial for flood control and regulation. Next in importance are components from the old mobile hoist that are in poor condition, followed by the medium voltage line, which is prone to frequent service interruptions. Remaining dam safety concerns are less important, given a lower probability of occurrence and less severe effects.
Ranking across spillways
Ranking of maintenance activities across several spillways can be accomplished by using a relative importance factor for spillways. Several classification schemes are possible. In its simplest form, the classification can be based on categories (low-, medium-, and high-hazard spillways) and rankings established within each category. Hydro-Québec uses rankings from dam classification. These rankings account for the type of dam, foundation characteristics, dam height, size of the reservoir, years in service, local seismicity, and population at risk. The factors considered for ranking reflect regulatory requirements from Québec legislation on dam safety. The highest score for Hydro-Québec spillways is 188, while the respective scores for Spillway A and Spillway B are 143 and 22 (or relative importance of 0.76 and 0.12). When this importance factor is considered, most of the maintenance activities on Spillway A are ranked higher than those at Spillway B.
Current work on the method
After testing the method on several types of spillways, the main difficulties encountered in applying the method were related to the sharing of administrative responsibilities between several units for various types of spillway equipment. Main structural elements of the spillway were traditionally under the responsibility of dam safety engineers, while electrical and mechanical equipment were under the responsibility of facilities engineers who also oversee all electric production equipment. In consequence, a new sharing of administrative responsibilities was established to provide a clearer understanding of the roles of each administrative unit.
Under this new plan, dam safety engineers are responsible for the overall performance of spillways and determine the ranking of maintenance activities to facilitate budget allocations. They oversee application of the method and are responsible for the inspection and condition assessment of support structures for lifting devices, gate structures, stoplogs, embedded parts, rolling tracks, and concrete structures. Dam safety engineers also are responsible for evaluating operational systems in collaboration with field operations personnel.
Facilities engineers are responsible for the condition assessment of electrical and mobile mechanical equipment on the spillway. Facilities engineers are responsible for the condition assessment of mechanical and electrical equipment for dam safety needs.
The application of this method has had two additional benefits. First, it provided a forum for discussion on spillway dam safety issues across groups that otherwise would have had few opportunities to interact. Second, it put in perspective the various deficiencies of the spillway.
The project is still in the development stages, and inspection and condition assessment procedures are being validated by a group of experts that provides input for developing a user’s guide in 2009. The user’s guide will provide recommendations in the evaluation of importance factors, as well as examples for the most prevalent types of spillways.
Future enhancement plans
At this stage of the project, the optimum frequency of component condition assessments has not been determined. A trial period will be required to determine inspection and evaluation policies.
The frequency of the procedure application will depend on the facility type and its criticality to a river system. The existing guidelines and framework rate the relative importance of spillways using three categories: significant-risk facilities with gated spillways, low-risk facilities with gated spillways, and facilities with stoplogs. During the development stages of the method, field exercises were performed for each type of facility. Those that were selected had known deficiencies. The objective of the field test was to validate condition tables and resulting maintenance priorities. The results of the field tests were conclusive, and the project is ongoing with the activities outlined previously.
Mr. Caron may be reached at Hydro-Québec, 75 Rene Levesque Boulevard, West, Montreal, Québec H2Z 1A4 Canada; (1) 514-289-4576; E-mail: caron. email@example.com. Mr. Chouinard may be reached at McGill University, Civil Engineering and Applied Mechanics, 817 Sherbrooke Street West, Montreal, Québec H3A 2K6 Canada; (1) 514-398-6446; E-mail: firstname.lastname@example.org. mcgill.ca.
- Dam Failures - Statistical Analysis, Bulletin 99, International Commission on Large Dams, Paris, France, 1995.
- Robichaud, J.G., Luc E. Chouinard, G. Andersen, and V. Torrey III, “Using Indexing Tools to Prioritize Work on Dams,” Hydro Review, Volume 19, No. 4, July 2000, pages 40-47.
- Chouinard, Luc E., J.G. Robichaud, G. Blanchette, and R. Gervais, “Priority Ranking for Maintenance Activities on Embankment Dams,” Proceedings of the Canadian Dam Association 1998 Annual Conference, Canadian Dam Association, Edmonton, Alberta, Canada, pages 252-265.
- Andersen, G.R., and V.H. Torrey III, “Function-Based Condition Indexing for Embankment Dams,” Journal of Geotechnical and Geoenvironmental Engineering, Volume 121, No. 8, August 1995, pages 579-588.
Denis Caron is dam safety engineer with Hydro-Québec, responsible for coordinating projects on condition assessment of embankment and concrete dams and of spillways. Luc Chouinard is professor of civil engineering at McGill University, specializing in risk and reliability analysis in civil engineering applications.
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