Sulby Reservoir
Introduction
With the increase in numbers of summer visitors to the Isle of Man in the late nineteenth century came a need for a reliable source of water for the main centre of population, Douglas.
Improvements in transport, particularly railways and ships, coupled with increased prosperity and the establishment of the seaside holiday as a regular annual event for the industrial workers if the north west of England led to the growth of the tourist industry on the Island. Douglas became and has remained the principle resort.
To meet the resulting demand for water throughout the summer season it was necessary, towards the end of the century, to construct the Clypse and Kerrowdhoo Reservoirs in the Groudle valley. Although these schemes sufficed for a number of years demand continued to rise and in 1900 Douglas Corporation started construction of the reservoir at Injebreck in the West Baldwin valley. These reservoirs have remained Douglas' principal source up to the present day. Prior to 1948, despite it comparatively small physical size and population, the Island's water provided by nine separate water undertakings. As a consequence, sources for the areas outside the main towns were developed on a somewhat piecemeal basis, few of the individual undertakings being able to afford significant investment in major works. In 1948 the water supply undertakings on the Island were rationalized, Douglas Corporation alone retaining its own water department. The remaining eight local authority and private company under-takings serving the remainder of the Island were amalgamated to form the Isle of Man Water Board. This therefore left two water undertakings on the Island.
By the early 1970's Douglas Corporation's reservoirs were being used increasingly to augment supplies to the south and west during dry weather, and 1972 an act of Tynwald was passed establishing a single all-Island Water Authority. The Isle of Man Water and Gas Authority was formed in 1974 by the amalgamation of the former water and gas undertakings but this lasted only until 1982 when the water and gas undertaking were separated.
The need for a new source.
The drought of 1975 was an exceptionally dry period on the Island, and it became clear that during that summer that the combined yield of the various sources available to the Water and Gas Authority was barely sufficient to meet current demands. For some years past the catchment of the upper Sulby River had been under consideration for development as a new source, and indeed by 1973 had already been utilised by the construction of a river intake upstream of Druidale designed to augment supplies to the Sulby filter plant. In early 1976 the Authority commissioned the consulting engineers, G.H Hill & Sons, to advise them on the new impounding reservoir on the site.
Development of the Sulby Scheme
Exploratory work was carried out to determine the suitability of the site for construction of a dam, and included the sinking of a series of trial borings along the proposed centre line. The consultant's report, confirming the site as suitable for construction of an embankment dam, was submitted to the Authority in 1977. The report provided estimates for dams of various heights ranging from 25 metres up to 70 metres and included also estimates for dams designed to be capable of development in stages. Following due consideration if the options available to them, the Authority decided that to build a dam of maximum size, fully utilizing the resources of water available in the valley, would impose an unacceptably heavy financial burden on their consumers, and that a smaller dam would be sufficient to meet the expected increase in demand over the next 25 years.
The design of the dam was therefore to be such that it could readily be built in two stages, the first about 35 metres high, the second and final stage adding a further 25 metres in height. In this way, an economically viable dam could be built immediately without sterilizing the site against further development by future generations.
The consultants were therefore instructed to prepare the necessary detailed designs and tender documents, and early in 1979 competitive tenders were invited from eight companies selected for their proven competence in dam construction. Shephards Hill Ltd., based in Uxbridge, Middlesex, were successful in winning the Contract and construction began in June 1979.
The construction of the Sulby Reservoir made the small Block Eary Reservoir, built in 1940's in a side valet downstream of the new reservoir, virtually redundant as a source of water for the Sulby filter plant, and this together with the rapid rise in the cost of fuel oil used to generate all electricity in the Island, led in 1977 to the Electricity Board promoting the construction of a small hydro-electric generating station in Sulby Glen designed to use the surplus water from Block Eary. As fuel oil prices continued to rise during the late 1970's the idea of using the Sulby Reservoir itself as a source of hydro power was proposed, and a detailed appraisal of the possibilities was initiated in late 1979.
The consultants were therefore commissioned to prepare a report and estimates, and the conclusions reached were that there were advantaged to be gained in continuing with the second stage dam immediately while making use of the surplus water which would then be available as a source of energy. The Manx Government, with the foresight reminiscent of the Douglas Corporation in promoting the West Baldwin Reservoir, decided to finance the extended scheme, and a new contract for construction of the high dam was negotiated with the contractor already on site. Approval was given to the implementation of the enlarged scheme in December 1980.
Construction Programme
The original contract required two main targets to be met, firstly completion of drawoff main by Spring 1981 so that if 1981 was a dry year, water could be pumped from the Sulby River over the watershed at Beinn-u-Phott into the West Baldwin Reservoir, and secondly completion of the first stage of the dam by the winter if 1981/82 to enable the reservoir to be fully operational by the summer of 1982. Whilst the extension of the scheme to include construction if the second stage dam inevitably meant that completion would be postponed for at least 12 months, it remained vital that the two main target dates should be met and that a reliable new source of water should be available by summer 1982. For this reason it was necessary to design the works in such a way as to permit the second stage dam to be constructed concurrently with the reservoir being partially filled and used for water supply.
Description of the scheme
The dam site
The overall Manx geological structure consists of eleven distinct formations of sedimentary strata of Canrian age, arranged in an acute symcline which has subsequently been subjected to three phases of tectonic movement.
The trial borings carried out on the site indicated beneath superficial deposits of boulder clay, strata of the Manx slate series, consisting of fractured and contorted sedimentary rocks exhibiting varying degrees of weathering. The indications were that the underlying strata would not be ideally suited to construction of a concrete dam, due to the presence of numerous crush zones and faults in several places of considerable depths of weathered and broken material. It was also apparent that no adequate source of suitable concreting aggregate existed within the reservoir area. From an early stage therefore, not only aesthetic considerations but also engineering and economic factor dictated the choice of an embankment dam design. The final design made the most economical possible use of the materials available on the site. The outer shoulders of the embankment are constructed from the rockfill obtained from a quarry within the reservoir basin whisk the core made use of the finer material from the quarry and from the excavations elsewhere. Insufficient suitable boulder clay was available in the valet to form the impermeable membrane within the dam, and asphaltic concrete was chosen as the most satisfactory alternative material. The original or first stage of the dam is provided with a 750mm thick central asphaltic concrete core which is connected to an asphalted concrete membrane constructed on the upstream face of the second stage of the dam. The initial choice of a combination of asphalt core and membrane as the main waterproof membrane was much influenced by the need to construct the dam in two stages. It also proved to be a most satisfactory arrangement during construction, enabling the deepest section of the cutoff to be located in the soundest part of the rock foundation and allowed the placement of the main fill material of the upper part of the dam to be completed without the impediment of having to keep core and dam fill in step with each other.
Loss of water from the reservoir through fissures in the rock foundation is limited by the provision of a concrete filled cutoff trench, excavated 2 to 3 metres deep into sound rock across the valley beneath the dam. The rock beneath was sealed by means of cement grout being injected into a closely spaced curtain of holes drilled to depths of up to 50 metres. The asphalt core and membrane are sealed onto the concrete cutoff.
Drawoff and overflow
The dam is provided with separate drawoff and overflow tunnels or culverts. The drawoff culvert, about 300metres in length, passes beneath the dam in the base of the valley and contains the pipes through which water is drawn from the reservoir. The culvert provides access into the inclined drawoff gallery which runs up the western side if the reservoir basin, and thence into the gallery house.
The main water supply pipe is installed in the gallery, and it has valved intakes at three levels to enable the optimum quality of water to be selected. The hydro electric supply pipe has an intake at the upstream end of the reservoir, and the two pipes pass through the culvert to the control house and pumping station downstream toe of the dam. Water is supplied by gravity to the filter plant at Sulby village and also to the hyrdo-electric generating station below Block Eary. In dry years up to 3 million gallons a day may be pumped over the watershed at Beinn-y-Phott to the south, a lift of about 300 metres, to augment flows into West Balwin Reservoir from which supplies for Douglas and the south of the Island are drawn.
The bellmouth overflow is designed to pass the probable maximum flood from the catchment area of about 200 cubic metres per second without risk of the water overtopping the dam. Water passes over the circular weir and down a vertical shaft within the reservoir and then out through the 200 metre long overflow culvert beneath the dam and down a steeply graded spillyway channel west of the pumphouse into the stilling basin. The energy of the water passing down the overflow during heavy rainfall is destroyed in the stilling basin to prevent sourcing of the riverbed downstream.
The drawoff and overflow culverts are of the same internal cross section, being about 4.5 metres high by 3.6 metres wide. The drawoff culvert was used during construction of the first stage dame to contain the river, and was sealed by means of a massive concrete plug at its upper end in December 1980. The reservoir was then allowed to fill partially, provision being made for excess storm flows to pass through a temporary opening in the base of the overflow culvert during the construction of the second stage dam and placing of the membrane. The temporary overflow was sealed on completion of the dam in August 1982 using a second mass concrete plug, and controlled filling of the reservoir proceeded during the following winter. Top water level was reached by the end of April 1983.
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