Hong Kong Water Supply – Lower Shing Mun Reservoir

November 13, 2022 Tymon Articles, Construction, Utilities

Tymon Mellor: The development of the Shing Mun Valley for its water resources was first proposed back in 1924, and this led to the development of the Shing Mun reservoir. The 1924 study identified the opportunity to build a smaller reservoir down-stream from the main reservoir, but this was not progressed. It took the post-war water crisis for the Government to implement the Lower Shing Mun reservoir as part of the larger Plover Cove scheme.

Plover Cove Scheme

In 1958, the Hong Kong Government first considered whether large reservoirs could be created in coastal inlets. Early in 1959, one such project at Plover Cove and another at Hebe Haven, both in the New Territories, were found to be feasible after initial investigations. With the provision of water from the new Shenzhen reservoir, an integrated scheme was developed including the provision of Lower Shing Mun reservoir under the Plover Cove Water Scheme.

Shing Mun Valley

The Lower Shing Mun dam or embankment is located within a steep sided valley 2km down-stream of the existing Shing Mun reservoir. The valley was uninhabited but the scheme impacted 27 families who were paid a cash compensation totalling $120,000 for crops and loss of land. Running across the valley were veins containing the mineral tungsten, which was commercially mined between 1917 and 1967 at the Needle Hill mine and illegally so by itinerant miners.

Despite the operating mining works, the valley was isolated with only road access to the top of the existing Shing Mun dam. Thus, with endorsement of the scheme, work commenced in early 1961 on the construction of road access to the site.

Reservoir Function

The reservoir was designed to balance flood flows from the different sources with the demands of the Shatin Treatment works. With only a limited catchment, along with overflows and discharge from the adjacent Shing Mun River, the reservoir had only around one fifth the supply needed to support the treatment works. The remaining difference was to be made up with water supply through the tunnel from the Tai Po Tau pumping station. This additional source collected water from the northern sources, streams along the tunnel alignment and in time from the future Plover Cove reservoir.

To avoid the pumps at the Tai Po Tau pumping station having to accommodate a variable reservoir head, the pumped water bypasses the reservoir and discharges directly into a supply basin at the foot of the embankment. This supply basin then feeds the draw-off tunnel to the Shatin Treatment works. In addition to the water from Tai Po Tau, the supply basin can be topped up with water from the reservoir when needed. If required, an auxiliary pumping station can pump water from the Tai Po Tau supply into the reservoir to increase storage capacity[i].

The storage capacity of the reservoir, height of the dam and size of the supply tunnels was modelled and optimised with an early computer, see Plover Cove. The arrangement was such that Lower Shing Mun could gravity feed either the Shatin Treatment works or the flows could be reversed to feed the Plover Cove reservoir. With only a limited capacity of 900 million gallons, the reservoir fills and empties rapidly but it would also provide useful storage of water at the end of the wet season.

Construction

The Main Dam is 55m high and 228m long at the crest, a composite rock-and earth-fill embankment with an impervious core and a concrete cut-off wall. The rock fill was sourced from quarry sites just upstream of the dam and from tunnel excavations, and the earth-fill was obtained by stripping soil from the surrounding hills.

Several dam sites were identified in the valley and requiring a detailed ground investigation to locate the best location for the foundation of the embankment, and competent rock was found to be nearly 40m below the ground level in some locations. Investigations for the dam site and quarries required the drilling of 66 holes totalling 930m of drilling and casing through soil and 1,920m through rock, with the holes ranging from 18m to 82m deep. Use was made of 10m diameter unlined trial pits up to 21m deep and of soft-ground boring on the dam site, and also to prove adjoining earth borrow areas[ii].

Following the investigation and the computer modelling, the dam was located at the lowest possible site. The dam was curved in plan to improve the aesthetics as well as giving the best cut-off line, along with more capacity in the supply basin and greater stability, the slight arch action also closes any cracks in the fill that might occur during construction.

The contract for the construction of the reservoir was awarded to Nishimatsu Construction Co, on the 3 May, 1962 for $37.6 million[iii]. Following mobilisation on site, the construction of roads, offices, accommodation and a 50 T/hr crushing plant, work commenced on the construction of the 320m long bypass tunnel. The tunnel passed through the northern spur of the valley and allowed the contractor to divert the river clear of the dam site allowing access to the valley floor for the construction of the dam foundations.

With the diversion in place, work commenced on the dam in September, 1962 with the clearance of boulders and riverbed deposits[iv]. Construction of the embankment started in January 1963 with the placement of the down-stream rockfill for the 314m wide foundation. The embankment consisted of a central clay core with sandy materials making up the up-stream and down-stream shoulders. To protect the up-stream face from water erosion a filter layer was added, covered with rock-fill.

Material	Quantity
Core Fill	121,000 m3
Shoulder Fill	752,000m3
Rock Fill	243,000m3
Filter	121,000m3
Total	1,237,000m3

To achieve the water cut-off below the embankment, the soft material was removed to expose the fresh granite. A 1.8m wide mass concrete wall up to 36m high was then constructed along the centre line of the embankment across the valley. Through this wall three lines of grout holes were installed for the injection of cement grout to fill any flow paths within the rock.

Working day and night on two 11-hour shifts, the scheduled was to complete the embankment by the 31^st December 1964. However, the contractor was asked in November 1963 to accelerate progress so as to finish the dam by the 10th July 1964. Despite the mobilisation of extra personnel and plant to complete the works 5.5 months ahead of the original schedule, exceptionally bad weather and unusually frequent typhoons during the summer in 1964 seriously hampered efforts to meet the revised targets. The embankment was completed in December 1964[v].

With good progress on the embankment, impounding of water commenced in the summer of 1964 allowing water to be supplied to the new Shatin Treatment works in August 1964.

Valve Tower and Spillway

To manage flooding, the reservoir includes a bell-mouth spillway designed to take a flow of 566 m3/s. This arrangement utilised the bypass tunnel and avoided the need to build a complex overflow structure within the embankment. A valve tower was located adjacent to the spillway, allowing water to be drawn off from the reservoir. Access was provided by a precast concrete bridge passing over the top of the spillway. From the valve tower, water could be drawn off at different heights then piped through a 1.2m diameter pipe within the bypass tunnel to the supply basin.

Over the years, the valve tower suffered damage from the effects of water hammer action from closing valves and in 2007, it was decided to demolish and reconstruct the valve tower. The tower damage was located at the base of the structure and it was considered to be in such a poor state that workers were not to approach the structure in case it would collapse.

Given the poor condition of the structure, it was decided to push the tower over using the connecting bridge. After some initial movement in the tower, the concrete bridge buckled and collapsed. A new plan was required. The use of explosives was considered but dismissed as it required workers to approach the structure. Thus, pushing it over was still the preferred solution. A new steel truss was erected along with pushing jacks and with sufficient safety margin to ensure that the system would work. On the day for demolition, guests were invited to witness the event. Pushing of the tower commenced in the morning and it was visibly moving but did not collapse. By now it was lunch time, so while the team reconfigured the equipment, the pushing stopped while they took a break for refreshments. With no one watching, the tower collapsed. It did not rotate and pivot about the base as expected, but dropped on the spot without a fanfare. The debris was cleared and the structure was rebuilt.

Sources

[i] Investigation and Design of the Plover Cove Water Scheme, S E H Ford and S G Elliott, ICE Nov 1965

[ii] Investigation and Design of the Plover Cove Water Scheme, S E H Ford and S G Elliott, ICE Nov 1965

[iii] Building of Lower Shing Mun Dam, SCMP 4 May 1962

[iv]Lower Shing Mun Main Dam, Chung Ho, The Engineering Society of Hong Kong, Oct 1965

[v] Lower Shing Mun Main Dam, Chung Ho, The Engineering Society of Hong Kong, Oct 1965

This article was first posted on 1st September 2021.

Hong Kong Water Supply – Lower Shing Mun Reservoir

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