Spotlight on...Spillway Gates14 December 2000
Spillway gate installations are a critical part of a reservoir and dam system. Their reliability is important for the safety of dams. Jack Lewin and Jonathan Hinks explain why spillway gate design, maintenance and operation require close attention
IN a number of exceptional cases, particularly in the former Soviet Union, bottom outlets are made large enough to discharge flood flows. However, in the vast majority of cases, a spillway is required to deal with flood inflows to a reservoir. Where an uncontrolled weir would be excessively long, or the requirement for freeboard too great, a gated spillway may be the most economic solution.
Particularly in the developing world, there is a preference for uncontrolled weirs because the risk of electrical or mechanical malfunction, or human error in operation is eliminated.
For large dams the spillway will usually be designed to pass a flood no smaller than that with a return period of 10,000 years. For the bigger dams and for all major dams in the US, UK, Australia and other countries, design will be for the probable maximum flood (PMF). There is no fixed relationship between the PMF and the flood with a return period of 10,000 years but the PMF peak outflow will often be about twice that in the 10,000 year flood.
The UK guide Floods and Reservoir Safety, published by the Institution of Civil Engineers, suggests that gated spillways should have a minimum of two gates. Many engineers require at least three gates. These should be sized so that any two can pass 70% of the spillway design flood in the event of one gate being serviced behind stop logs or non-operational for other reasons. The assessment of the need for redundancy will usually take into account whether standby facilities for gate operation are provided and whether repairs to potentially vulnerable elements can be carried out between receipt of flood warning, or onset of the flood and the advanced stage when all gates are required to discharge the flood.
Radial gates are preferred for spillways. Their advantages are:
• The absence of gate slots.
• The gate thrust is transmitted to only two bearings which can be located out of the water.
• Less hoisting capacity is needed than for vertical lift gates.
• Mechanical simplicity.
• Absence of a high superstructure.
The disadvantages are:
• The flume walls have to extend downstream at a significant height to provide attachment for the trunnion bearings.
• The load is taken by the piers as concentrated loads at the gate anchorages.
Most radial gates are operated by electric motors driving cable hoists through multi-stage reduction gears. At large gates hoisting chains are often used, although the use of hydraulic cylinders is becoming increasingly common.
There are some spillway gate installations - such as the Victoria dam in Sri Lanka - where the gate is counterbalanced, opens under gravity and relies on power closure, and others which are water operated. The latter are either of the radial automatic type or when the gate is connected by cables to counterweights in a chamber which can be flooded by the reservoir water to reduce the balancing weight. Both types of gate require wide piers to accommodate the gate operating displacers, floats or counterweights. The counterweighted gates which open under gravity and close under power, mostly by oil hydraulic cylinders, do not require wide piers but there are few examples of this construction.
There are still a number of old dams, such as the Sennar dam on the Nile, where vertical lift gates control the spillway flood discharge. Because of the advantages of radial gates there are few, if any, installations more recent than 35 years.
Limited examples exist of spillways controlled by bottom hinged overflow flap gates; the Legadadi dam in Ethiopia is one. Flap gates require an accurate, smooth flume wall for the side seals to be effective. If this is provided by embedded, machined panels, the contact face must be of stainless steel to prevent corrosion. If the concrete is ground to a smooth, accurate sealing surface, it still causes rapid wear of the seals. This factor, combined with the difficulty of effecting a good sill seal and access to the hinges, accounts for the few installations of this type.
There are two design standards for spillways. The one by the US Army Corp of Engineers is specific for the design of spillway tainter gates. The term tainter gate is used in the US for radial gates. The German standard DIN 19704: Part 1: 1998 deals with the design and calculation of all hydraulic gates and Din Handbook 179 Water Control Structures 1 extends the scope to other design and operational factors and includes basic requirements for gate hoisting machinery.
The German specifications require that the design be based on limit state. In the case of the Corps of Engineers' specifications, the design is based on load and resistance factors.
Skin plate assemblies are either stiffened by horizontal beams or by curved vertical ribs. Large gates usually combine vertical and horizontal stiffening beams. All forms of construction are arranged to transfer the load on the gate to the gate arms, which form a splayed portal with the beams tying the gate arms in the horizontal plane. The gate arms converge on trunnions.
Usually two gate arms per side are used, three per side at larger gates or even four at gates which have a projected area in the order of 300m2. Exceptionally, a single tapered box section arm per side is used. The gate arms are braced in the vertical plane. A practice at gates in the US is to cross-brace the gate arms close to the junction with the skin plate assembly. This resists the torsion when a gate jams either due to failure of suspension on one side or an obstruction.
The trunnion bearings are anchored to the piers and at the abutments by trunnion beams. Typically, the foundations for the trunnion beams are prestressed on all but small gates (below 20-30m2 of projected area). In some cases, reinforced concrete has, however, been used.
The design of trunnion bearings has been reviewed worldwide since the collapse of spillway gate No 3 of the Folsom dam in California, US. Corrosion on the loaded side of the steel trunnion pins had increased trunnion friction over time, resulting in a shear failure of a strut brace in one of the radial arms. Current design practice favours the use of stainless steel trunnion pins and bronze alloy bearings with inserts of lubricant. Different designs are discussed in Lewin (2001).
Operating machinery is either by electromechanical drive raising the gates by cables or chains or by hydraulic machinery. Cables and chains are frequently anchored at the upstream face of the gate skin plate because it simplifies the layout of electro-mechanical hoists. This results in cables or chains being immersed in reservoir water for the majority of their lives. Failures due to corrosion have occurred. Also, debris can become wedged between the skin plate and the lifting cables and the gate face has to be locally protected from chafing by the cable.
Oil hydraulic operating cylinders permit the direct application of large forces moving slowly, eliminating electric motors, brakes, large multi-stage reduction gear boxes and hoisting drums.
While spillway gate installations have a good operational record overall, there have been cases of failure, some actually or potentially catastrophic, as well as areas where persistent problems occur, such as:
• Ice problems in cold climates and failures of heating systems to prevent freezing of gates.
• Seal leakage, which can cause gate vibration and, in winter, freezing of gates.
• Hoist failures and breakdown of mains supply, as well as failure of standby generators to start and run, can result in a common cause fault.
• Gate vibration problems.
• Trunnion bearing problems, limit switch function, cable breakages, failure of chains to articulate around chain sprockets due to corrosion.
• Damage to gate arms due to late opening of gates and overflowing debris.
• Corrosion due to stagnant water on end arms and main horizontal beams (i.e. lack of adequately sized drain holes).
• Control system malfunction.
Standby generators, in particular, have a rather poor record of instant availability and need to be tested frequently (every two weeks is the usual interval) to ensure that they are available when needed.
Less well publicised than the above problems are cases where gates have been operated incorrectly as a result of procedural problems or human error. The opening of spillway gates will often involve flooding, and possibly even loss of life downstream. In some cases, operators have to seek higher authority before opening them. If senior personnel are not available, for example because the critical time is at night or over a weekend, or because the communications system is down, the operator is placed in an unenviable position whereby he may be criticised even if he takes the right decision.
To prevent such problems, it is important that operators should be issued with complete and unambiguous instructions as to how the gates should be operated under all possible conditions. In the simplest cases, such instructions will relate gate settings to water levels in the reservoir but, where there are sophisticated catchment monitoring systems in place, decision-making will be more complicated. In all cases, the necessary decisions should be taken by fully trained personnel who are adequately informed as to the procedures, and instructed as to how they should act in all circumstances.
Pyke and Grant described simplified operating rules for dams with flood control gates at the icold congress in China in 2000. The authors suggested these would be useful as backup in the event of failure of more sophisticated methods reliant on real time upstream data.
Control of spillway gates is usually from a central operations building. Often additional local control panels for one or two gates are mounted on the piers. If the two systems provide independent controls it results in a robust installation. Automatic gate operation by programmable logic controllers (PLCs) is becoming more frequent and is sometimes used in conjunction with a telemetry system for remote supervision. The transmission of telemetry can be the weakest link.
Flood routing by a PLC system offers many advantages because manual control of gates when flood routing is carried out is counter intuitive. It is likely to be infrequent and requires training. Many dam operators provide supervision by a responsible engineer.
PLC operation of spillway gate operation is not suitable in developing countries because technical back-up is usually not available. At all installations operating instructions, whether for flood release or flood routing, must be simple, consistent and unambiguous.
Standby facilities are the general rule. The back-up for the mains supply is usually by a diesel engine generator set. The typical failure on demand for a diesel engine driven generator set to start and run for one hour is 1 in 25 demands. Therefore, two generator sets provide greater reliability. Some dam operators consider that gas engines start and run more consistently than diesel engines. Portable diesel engine driven standby sets which can be connected to a shaft on the gate hoist should be available at all spillway gate installations or mobile oil hydraulic pump sets at cylinder operated gates.
Standard practice is to provide manual winding as a final standby stage or manually operated oil hydraulic pumps. Their effectiveness in an emergency can be doubtful because of the length of time required to raise a gate manually. (To raise a large spillway gate about 0.5m by hand winding takes between 1.5-2 hours if winding is carried out in relays). In spite of this, it is considered an essential provision.
Reliability assessments of spillway gate installations have become more important as reservoir safety investigations have been extended to include appurtenant structures.
The integrity of spillway gate installations under earthquake conditions is being investigated alongside the performance and safety of dams. Following an earthquake, the release of reservoir water can be a critical control function if the dam has been damaged by the seismic motion. The amplification of ground acceleration which can occur at the crest of a dam magnifies the forces experienced by the gates. The movement of gates in response to the earthquake causes reservoir water upstream of the gates to act with the gates as added mass. The computation of loads and stresses due to a seismic event can be an equivalent static or a dynamic analysis.
The former is a two-dimensional model which is used for a preliminary assessment and a dynamic finite element three-dimensional analysis is performed when it is considered necessary to account for the important three-dimensional effects of radial gates. The procedures for assessing the structural consequences of an earthquake on spillway gates have so far not been formalised into guidance rules and investigations have been based on different analytical approaches.
It is recognised that seismic assessments must include all the equipment which comprises a spillway gate installation, such as overhead electricity supply lines, usually the first to fail in an earthquake, transformer mountings, switchgear, operating panels and hoisting machinery.
After an earthquake, access to operate spillway gates and communications can be interrupted and can impede or prevent timely action. Maintenance and inspection of gates can be variable and recent risk assessments of spillway gate installations have drawn attention to this.