Defusing the situation4 October 2002
Rubber dam failure at Lake Lyell in Australia prompted the search for a safe and reliable method of reinstating reservoir storage capacity.
IN A first for Australia, innovative Fusegate technology pioneered in France has been used to upgrade Lyell dam, on the Cox river in New South Wales. Standing 50m high, this concrete faced rockfill dam was constructed in 1983 to supply cooling water for Delta Electricity at Mt Piper and Wallerawang power stations. In 1994 the original spillway was doubled in length and two 3.5m high by 40m long inflatable rubber dams were installed on the two spillways to increase storage capacity by 7500M litres. Two uncontrolled deflations occurred, however, which caused Delta Electricity to review the suitability of the rubber dams in this particular application.
The first incident in February 1997 released 1600M litres over five hours. The river level 2km downstream of the dam rose by 2.3m, with a peak discharge of 120m3/sec.
An exhaustive investigation revealed the failure was most probably caused by a software malfunction in the programmable logic controller.
The second incident (to the other rubber dam) occurred on Sunday 11 April 1999, when it ruptured without warning, deflating instantaneously and releasing almost 6000M litres. Approximately 2km downstream of the dam the rise in water level measured 2.5m and the peak discharge was 150m3/sec. The ensuing investigation indicated that the most likely cause of the rupture was a manufacturing defect in the rubber. Fortunately neither incident resulted in any loss of life and only minor damage to property downstream.
Following the second incident it was decided not to reinflate the rubber dams. Apart for strategic reasons, there was considerable community interest in restoring reservoir water levels since the dam is a popular recreational facility. Delta Electricity was determined to investigate other methods of restoring the storage capacity to its former level.
Following the receipt of expressions of interest and a comprehensive evaluation process during which an independent international consultant was utilised, the hydroplus Fusegate System was selected.
According to Peter Siers, Delta Electricity's asset manager, a deciding factor in selecting the Fusegate technology was its inherent reliability and minimal maintenance requirements. The system comprises a number of individual units which rest on the spillway forming a watertight barrier. When flood water level reaches a predetermined level, it enters an inlet well and fills a chamber in the base of the first Fusegate, providing hydraulic uplift that triggers the Fusegate to tip off the spillway sill.
Generally a number of Fusegates are installed side-by-side on the spillway sill, each with ever increasing inlet well levels to provide a carefully controlled breaching of the raised spillway until the last tipping event which normally occurs just before the maximum flood event.
Two basic types of Fusegate are available: a straight crested unit (where the design depth of overflow can be up to three and a half times the height of the Fusegate) and labyrinth units where the maximum overflow depth is restricted to about 140% of the Fusegate height.
The technology has been used on 37 dams across Europe, the US, Africa, India and parts of Southeast Asia.
Safety is a prime consideration for Delta Electricity. It was an essential requirement of any replacement that it could be relied upon to operate without human intervention or auxiliary power when required.
The Hydroplus System offers a unique safety feature (the minimum tipping level or MTL). When flood water is below the MTL it is not possible to generate sufficient pressure in the base chamber from the applied uplift and hydrostatic loads to destabilise the unit. Since the MTL is well above the crest of the Fusegate a significant flood has to be under way before the Fusegate could tip thus effectively eliminating the possibility of 'sunny day' failure.
To pursue opportunities in Australasia and to manage the Lyell project, Hydroplus International relocated engineer Bill Hakin to Australia. Leading global consultants Sinclair Knight Merz (SKM) undertook the engineering design, environmental studies, statutory approvals process and construction inspection for the project.
Detailed design work
Before detailed design work commenced a Leica Cyrax 3D laser scanner was used to survey the existing dam structure, providing confirmation that the existing structure was built within its design tolerances. The scanning method offered speed and accuracy, as well as greater safety for the survey team as the structure could be surveyed remotely.
Following a six-month design period that included an environmental review and obtaining required statutory approvals, Hydroplus awarded locally based firm Eodo Pty a contract for construction of the Fusegates and associated civil works in November 2001.
The structure for Lyell dam comprises six straight crested concrete Fusegates supported on the modified spillway sill. Each Fusegate is 3.2m high and 9.7m wide, consists of around 115m3 of concrete and weighs around 270t. For Lyell the units were cleverly designed to be essentially a mass concrete structure with reinforcement located only where required structurally (for example the upstream face of the well shaft to resist impact from floating debris).
To control cracking in the water retaining concrete it was specified that there should be no more than a 15ºC variation between the inner and outer core temperature with an overall maximum temperature limitation of 80ºC during curing. A number of methods were employed by the contractor to meet these requirements. Early morning pours using chilled water in the mix and a Portland cement blend with high flyash content was used to meet the requirement for low heat of hydration. A Daracem plasticiser was used to increase workability in the low water:cement ratio concrete and aggregate size was increased to 40mm for the Fusegate body while 10mm aggregate was used for the slender well shaft structure. During curing the Fusegates were wrapped in a jacket comprising heavy duty bubble wrap surrounded by insulating wool, with a tarpaulin cover around the outside.
Wooden forms were placed over a steel structure bolted to the sill to form the void for the base chamber. Once sufficient concrete strength had been attained the Fusegates were jacked up 300mm and a Tirfor winch was used to pull the formwork out after the bolts were loosened. The jacking also confirmed that no bond had been developed between the Fusegate base chamber and the sill and allowed the weight of the units to be checked.
The inlet wells could be set high above the Fusegate crest level ensuring that the first Fusegate tips off the spillway only for a very rare event (a 1 in 20,000 year flood). Because the shafts are over 6m above the Fusegate crest, concrete was lifted by kibble to the inlet shaft forms, and poured into the forms via a tremie. For the main body of the Fusegates the concrete was pumped in.
The client's requirements for a low maintenance solution to replace the rubber dams was met by utilising only stainless steel components in conjunction with the concrete Fusegate superstructure. Thus the possibility for development of corrosion is almost non-existent. Stainless steel inlet wells were bolted to the concrete well shafts and carefully levelled after placement. Vertical and horizontal rubber seals were fixed to the concrete units using stainless steel flanges and bolts.
Because the Hydroplus System is a non-mechanical one having no moving parts, it does not require operational checks to prove the satisfactory working of components. The guidelines of the Australian National Committee on Large Dams (and indeed the guidelines of several other National Committees on Large Dams) stipulate that flood routings should take into account a degree of restriction of discharge capacity consistent with at least one inoperable gate. This was not a requirement for the Fusegate System at Lyell dam (and has not been a requirement for Hydroplus projects anywhere else in the world) because the System is not categorised as being a mechanically gated system.
Hydroplus Australia has recently been awarded its second contract in Australia. The construction work on the raising of Dartmouth Regulatory dam for Southern Hydro will be quite a different project, however, involving labyrinth Fusegates and an entirely new approach to the installation methodology.
Apart from inflatable dams, alternatives to the Fusegate include mechanical gates of various configurations and fuse plugs constructed of graded rock, clay, sand and gravel combinations. Compared to the alternatives, the Fusegate has design simplicity that minimises the potential for mechanical failure, and reliability in the accuracy of flood management.
Fusegates can be used on both existing and new dams. The System can be used on dams with ungated spillways to recover up to two-thirds of the distance between the spillway sill and crest of the dam without raising the maximum water level. An increasing amount of interest is being shown in using the system to improve dam safety where dams suffer from inadequate spillway capacity. In these cases it can be fairly straightforward to lower the existing sill to where it is required and then regain the lost storage by installing Fusegates to the original full supply level or even higher to provide an increase in storage capacity as well.
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