Is Bengoh Dam built according to maximum requirements?

VIEWPOINT: The Bengoh Dam is located on the upper reaches of Sungai Sarawak Kiri, about 60km south of Kuching city. It is a water catchment area meant to provide adequate supply of raw water for the growing population in Kuching and Samarahan areas for the next 30 years, and is the second of its kind in the country after the Kinta Dam in Perak.

Recently, many low-lying areas in the two divisions were badly flooded after continuous heavy rainfall hit the region. The rain and floods continued for three days during which some 5,000 victims were evacuated to various flood relief centres.

The flood has now receded and most victims are back in their homes to access the damage to their properties and crops.

Many are wondering why the dam, which is supposed to mitigate floods, has failed to do so. A local newspaper report on February 22 quoted Assistant Health Minister Dr Jerip Susil, who is also the state assemblyman for Bengoh, as saying that "continuous heavy rainfall had caused the dam to overflow and made it unable to take in extra water to prevent flooding downriver of the Sarawak Kiri."

Impoundment of the dam was just completed on February 7 and Dr Jerip's statement left many worried. In laymen's language, they are asking: "How could water from the dam overflow? Was it not built to maximum requirements to hold rainwater during the landas season?"

Construction on the dam began in 2007 and it was completed in 2010. Impoundment did not begin until August last year after relocation and compensation issues involving 204 affected families were resolved. Five villages – Rejoi, Pain, Sait, Semban and Bojong – were relocated to a new resettlement scheme downriver called Bengoh Resettlement Scheme.

The dam cost RM310 million to build and is 267 metres long and 63 metres high. The reservoir is in the form of a lake with a surface area of approximately 10km square and has a capacity of 144 million cubic metres or 144 million tonnes of water. Reputed to have used Roller Compacted Technology (RCC) in its construction, it offers a projected service life of 100 years.

It is supposed to act as an upstream barrage to store rainwater when the incoming tide is high and the Kuching Barrage downstream is locked, and to release excess water when the tide is low and the downstream barrage is opened. This mitigating function of the dam is supposed to prevent or minimise flooding downriver during the rainy season.

The fact that the dam had overflowed recently raised many questions. Dams should not be allowed to reach their full capacities. The Bengoh Dam should not have overflowed if the right modelling and designing was used in its construction, many argued.

Didn’t the builders and planners of the dam have input of the maximum possible rainfall in the area? Was there any attempt to save construction costs?

A dam that overflows constantly can exert pressure on the walls of the dam. Over time, the walls might deteriorate to an extent that they become a ticking time bomb (hopefully not) waiting for the right trigger like a some seismic impact.

If the walls of the dam were to collapse, it is anybody's guess to imagine 144 million tonnes of water, plus moving earth, plunging downstream.

I do not want to sound like a prophet of doom but dam failures, though rare, do happen. In 1975, the Banqiao Reservoir Dam in China burst and caused more casualties than any other dam failure in history. The disaster killed an estimated 171,000 people and 11 million people lost their homes. Only last November, a Brazilian mining dam burst and took the lives of 28 people with many still missing.

I asked an engineer friend what in his opinion could be done to strengthen the dam or to increase the capacity of the water catchment area so as to prevent the dam from overflowing in future.

This is what he told me: "There is no single solution. We must not destabilse the integrity of the existing dam. We must consider other factors like land structure, sendimendation, and so on. Detailed inputs must be done by a panel of impartial engineers and environmental impact experts."

Well, when man interferes with nature to serve his needs, there is a price to pay for nature may retaliate with a vengeance. It may not happen within our life time but some future generation may have to pay for it.-THE ANT DAILY

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