spillway gate relibility

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Spotlight on...Spillway Gates

14 December 2000

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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.


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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 byoil 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.

Design standards

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.


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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 offailure, 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).


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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.

Central control

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.


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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.

Earthquake conditions

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.

spillway gate relibility

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