Securing a major role for Bengoh dam

8 July 2010

Water demand is increasing year on year in the Malaysian city of Kuching. The fear is that this will soon outstrip natural supplies during dry periods. In order to prevent this from happening, Bengoh Dam is set to play a critical role in a master plan to help secure future water supplies for Malaysia. With dam construction well underway, Tony Morison, Roland K W Ling and Danien Ak Rangu give an insight into this important project

Construction of the Bengoh Dam is progressing to schedule. With the objective of securing sufficient water supply for Kuching city and its surrounding areas, the project comes at a time when there is a real risk that present water resources are unable to meet the demands of a growing population, tourism and economic and agricultural development. When completed, the 63.2m high structure will be the second RCC dam in Malaysia after the Kinta dam near Ipoh, Perak.

Kuching is the state capital of Sarawak, Malaysia, located on the north coast of western Borneo near the mouth of the Sarawak river, where it forms a delta with the South China Sea.

In 2003, UK consultant Halcrow and Malaysia’s KTA (Sarawak) Sdn Bhd were appointed by Malaysia’s Public Works Department (JKR) to progress the development of Bengoh Dam. Initially they were to undertake an assessment study, and subsequently develop the outline and detailed designs of the structure – with the scope to meet water demand requirements until 2030.

Tony Morison, an associate director with Halcrow, explains the urgent need: “Kuching relies heavily on the Sarawak river. It’s an integral part and key feature of the city; provides water to a population of over half a million people as well as to industry and agriculture; and enables the movement of people and goods throughout the region.”

He continues: “But its natural resource is gradually being depleted and without proper management, there may come a day when scarcity of water will pose a problem for the state capital. Water supply in Sarawak is a government function, managed by the Kuching Water Board (KWB) and JKR. Bengoh Dam is a critical element of their master plan that aims to help secure water supplies for the next 20 years. The population of Kuching has grown by leaps and bounds, and it will not be long before the demand for water will outstretch the stable natural supply during dry periods.”

Greater Kuching had a population of over 500,000 in 2003, when water resource studies for the Bengoh Dam project were carried out, and this is expected to grow to over one million by 2030. In addition, increased tourism and planned industrial developments in Kuching are expected to create additional requirements for treated water. The water demand is growing fast, increasing at over 8% year on year, as individual water consumption increases and more outlying areas are connected to the treated water system.

Bengoh Dam will ensure that adequate raw water is supplied to the Kuching Water Board’s Batu Kitang water treatment plant in times of low river flows, to meet the water requirements of Kuching and surrounding areas such as Samarahan, Asajaya, Sadong Jaya, Tasik Biru-Lundu-Sematan, Tapah-Beratok-Siburan and Serian.

Construction of the dam started in 2007, at a site 40km south of Kuching and about 2km from Kampung Bengoh accessible from Kuching by bitumen road. The terrain provides a significant reservoir area with sufficient volume upstream for a relatively small dam.

Project status

Tony Morison provides a project update: “At the present time the RCC placing is complete to the underside of the spillway crest, with the left abutment block still to complete. Work on the spillway crest, crest bridge and equipment room, all of which will be conventional mass or reinforced concrete, is in hand, for completion later this year. Current works in progress include the concrete spillway side collector channels and training walls; with the grout curtain being drilled from the dam upstream heel.”

Morison goes on to add that drilling of the dam and foundation drainage (from the dam galleries) has not yet started; whilst the upstream intake shaft reinforced concrete is about 80% complete. Valves and pipe work will be installed when the river diversion is closed and the reservoir starts to impound.

“The critical path for filling the reservoir will be resettlement of the upstream villages,” Morison says, “plus clearing of vegetation from the reservoir area as required by the environmental mitigation plan.”

Kuching master plan

Between 1983 and 2000, a number of studies were carried out to develop a master plan to ensure future water supplies for Kuching and the surrounding area, with the aim of securing them beyond 2005.

The main water source is located at Batu Kitang. Abstractions here have progressively increased and the capacity of the water treatment works extended to meet increased demand, based on the natural yield of the river. However, recognition was given to the fact that this is a limited source. Alternatives considered to supplement the yield included: additional supply from other river catchments; limited local potential for groundwater development; provision of regulation storage behind a weir in the Sg Sarawak Kiri river channel; development of an emergency pumped intake on the Sg Sarawak Kanan to supply the Batu Kitang WTW; and construction of a regulating reservoir in the Sarawak Kiri catchment. (The study into this considered a number of different sites and recommended the Sg Bengoh as a suitable site for a river regulation storage reservoir.)

The findings of studies into the above alternatives were incorporated into the Kuching Water Supply Master Plan Study. As a result, the recommended plan to supplement water resources included the development of a storage weir on the river at the Batu Kitang intake by 2005, and construction of Bengoh dam by 2010.

Dam type and location options

Options for RCC gravity, earthfill, clay core rockfill and concrete face rockfill dams were considered on four different dam sites within the Bengoh gorge. Two of these sites were located where spillways, outlets and diversions could be constructed at bends in the river, more suited to embankment dams. The cost of the access road to the dam was taken into account in assessing the options. The furthest upstream site offered a lower and slightly cheaper dam. However, this would incur significantly higher access road costs plus some delay while this was constructed.

The most cost-effective option at the site is an RCC gravity dam – with a lowest foundation level 23m asl, the spillway crest level 80m asl and dam crest level of 86.2m - giving a maximum dam height of 63.2m and a dam 267m long.

Suitable geology

The gorge section of the Bengoh valley cuts through the lower Kayan formation, consisting of interbedded and cross-bedded sedimentary sandstones, pebble conglomerates and mudstones. Around the dam sites these dip in an upstream direction at angles of between about 45 and 650 skewed about 450 from the selected dam axis. The thickness of individual horizons varies from a few millimetres to tens of metres. The sandstone is the hardest of the layers and forms the steep sections of the river. The mudstone or soft shale layers tend to weather quickly and control the sliding properties of the rock mass. The valley sides are weathered to depths of typically 20m. Where unweathered, the predominantly sandstone areas form a suitable foundation for most types of dam.

Geological investigations consisted of surface mapping, borehole investigations and laboratory testing at two sites, including some 12 boreholes to a depth of up to 80m at the selected dam site. Geological assessments confirmed the suitability of the selected site for a concrete gravity dam and provide data for the dam foundation design.

Design considerations

The Bengoh dam is designed as a gravity section to be stable under normal, extreme floods and seismic loads. Seismic records show the area to be of very low seismicity. A nominal 0.1g peak ground acceleration was adopted for the dam design but proved not to be the critical design criterion.

The dam body has a vertical upstream face with the upper part at a slope of 0.78:1, and the lower at 0.9:1 to reduce tensile loads at the foundation contact.

The non-spillway section was made 8m wide for ease of RCC construction. Stresses and deflection of the section were checked by finite element analysis using ANSYS. Consolidation grouting is specified at the upstream heel and an inclined grout curtain is included to about 40m below foundation level beneath the upstream face of the dam. A drainage curtain in the foundation and dam body is connected to a system of galleries in the dam section.

The total volume of the dam and associated structures is about 160,000m3 of mass concrete and RCC, with an additional 15,000m3 of reinforced concrete in the diversion, outlet structures and spillway.

The selected aggregate for the RCC is Bau limestone. A new quarry and aggregate plant were set up within 3km of the dam site to provide and process the aggregate for dam construction. Trial mixes for the RCC using cement and flyash from sources in Kuching were used to develop an RCC design mix containing 60kg of cement, 120kg of flyash, 0.6% of water reducing admixture and a water cementitious ration of around 0.5. This has a modified VeBe time of between 8 and 15 seconds, little segregation and an average compressive strength of about 35 MPa (N/mm2) at the specified 91 days. This is significantly more than the specified 15 MPa characteristic strength but has not been changed. Reducing the cement content and increasing the flyash content to maintain the cementitious content provides little cost saving and thermal aspects are within specification.

Initial set time is between 10 to 14 hours, and final set between 24 and 36 hours, allowing most layers to be placed with hot or warm joints. The maximum recorded temperature rise from the 30°C specified maximum placing temperature is about 14°C. The average RCC density is around 2450kg/m3.

Topographic survey

Detailed topographic surveys were carried out for the dam and access road sites. In addition, contour surveys at full supply level (FSL) and maximum design flood level for the reservoir were carried out to define the reservoir area, together with a series of selected cross-sections within the reservoir area. This was done as the detailed surveys of the dam site showed that the contours on the 1:10,000 maps tended to be above actual ground level by over 10m in places. It is thought that the published mapping was based on aerial photography, and the difference reflects the density of the vegetation cover. The dam site and reservoir area surveys were done by ground survey techniques and required cutting sight lines through the dense secondary vegetation.

The survey showed the FSL surface area of the reservoir at 80m asl to be 10.2km2, an increase of about 15% on the surface area from the map contours. The storage volume is thought to be increased by more than this, although insufficient survey work was carried out to confirm the exact volume. This will be done after clearance of vegetation from the reservoir area. It was decided not to reduce the FSL of the reservoir, but to leave the expected additional storage volume as a margin for environmental releases and transmission losses in the river between the dam site and Batu Kitang. FSL was therefore maintained at 80m asl.

Environmental requirements

An EIA was carried out by the University of Malaysia Sarawak Reporting in March 2007. Environmental approval for the project was given in February 2008, subject to conditions on:

• Dam design and safety.

• Soil erosion and sedimentation.

• Control of air and water pollution.

• Waste management.

• Protection of biological resources.

• Reservoir preparation.

• Socio-economic, health and safety aspects.

• Environmental monitoring.

• Audit and site clearance.

Biomass clearance and disposal and erosion control before reservoir filling were particular requirements, as were the preparation of a dambreak impact assessment and emergency response plan for the area downstream of the dam.

River diversion

The river diversion as originally designed takes the form of twin box culverts each 4m wide by 4.5m high, located on a rock shelf on the dam left abutment. These can pass the 1:10 year flood of 365m3/sec with an upstream cofferdam level of 42m asl, about 15m high. The outlet pipework is to be installed in the diversion culverts when the river diversion is closed.

During construction, a third similar opening was added in the dam body to prevent backwater flooding of the existing villages (kampongs) upstream of the dam, up to the 1:100 year flood once the dam body construction rises above cofferdam level.

Spillway and outlets

An overflow spillway on the dam crest with a stepped downstream face was adopted, designed to pass the probable maximum flood (PMF) without overtopping the dam crest. The PMF peak inflow was determined as 2420m3/sec. Routing this over the 62m long crest, the routed outflow was 1800m3/sec with a flood rise of 5.8m. The stepped spillway is expected to dissipate around 75% of the head at the 1:10 year flow, reducing to about 45% at PMF.

The draw-off system is designed to discharge up to 21m3/sec down to a minimum operating level of 55m asl, from a multi-level intake to the river downstream of the dam. The intake takes the form of a dry well with inlets at five levels feeding a single 1.6m diameter pipe stack above one of the diversion culverts. The pipe discharges to the downstream culvert through a hooded fixed-jet cone (Howell-Bunger) valve. Each inlet is controlled by a gate valve with provision for an upstream maintenance gate. A lifting D-shaped trash screen is placed in the gate track of the operating intakes.

In addition to the draw-off, a 1.6m bottom outlet is included in the second diversion culvert, discharging with a similar arrangement to the drawoff.

When acting together, the draw-off and bottom outlet are capable of drawing down the reservoir by about 0.4m/day from FSL, and draining 90% of the reservoir volume to minimum operating level in 45 days against the highest (January) average monthly inflow.

Significant issues

Resettlement was recognised as a significant issue for the project and is likely to be the controlling factor in the programme for reservoir filling. Four villages (kampongs) were within the catchment area. However, three of these would all lose low-level agricultural land to the reservoir. The fourth kampong is remote from the reservoir and largely unaffected.

Over 1100 people need to be resettled, which is the responsibility of the Sarawak government. Extensive discussions and negotiations on resettlement and compensation have been going on and a resettlement site has been agreed on land in an adjacent catchment. Work is in progress on construction of access roads and housing.

The authors are: Tony Morison, Associate Director, Halcrow Group, UK; Roland K W Ling, Director, KTA (Sarawak) Sdn Bhd, Sarawak, Malaysia; and Danien Ak Rangu, Assistant Director, Jabatan Kerja Raya, Sarawak, Malaysia.

Progress updates

In August 2007 a turnkey contract for project implementation at Bengoh dam was awarded by JKR to Naim Cendera Sdn Bhd (NCSB). KTA (Sarawak) are providing engineering and construction supervision with assistance from Halcrow.

Work started on site on the bridge and access road in September 2007. NCSB awarded the dam construction contract to Sinohydro, and clearing of the dam site started in March 2008. Construction of the river diversion was delayed by unseasonal floods in August and September 2008, and development of the quarry was delayed by a legal challenge over access routes. However these problems have been successfully overcome and RCC placement started in June 2009.

The dam construction was originally scheduled and more or less on track to complete so as to start impounding by December 2010. This would have required casting bottom outlet and draw-off pipework and valves into the river diversion culverts (a dry-season activity).

The dam body RCC is already complete and final concreting of the spillway crest and bridge (which provides access for installation of the draw-off valves in the intake) is in progress for completion by about August 2010. Valve and pipework installation, except in the diversion, should be completed in the autumn.

Resettlement has now been agreed, but the delay in resettlement of the upstream villages means that it is no longer possible to close the diversion as originally scheduled. This means that there will be at least a one year, and possibly longer, delay in closing the diversion to start impounding. This is beyond the control of the dam construction team and no clear date has been confirmed.

Unfortunately it means that there will be an enforced indeterminate break in completion of the dam and testing of the operating equipment. The clearing of vegetation from the reservoir area has likewise been delayed, as there is no point doing this more than one year before impounding.

Bengoh dam Bengoh dam

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