Swamps and lakes hold answers to flood control issues

IT HAS been reported that the cabinet is looking for a comprehensive solution to flood issues. We would like to share some points.

When rain falls, some portion of it is absorbed by the forest and soil. The balance will flow on the surface as surface run-off. As we reduce the amount of absorption capacity, naturally we will increase the amount of surface run-off.

How is the absorption capacity reduced? It is reduced by deforestation, change in land use, increase in impermeable (surface that does not absorb water) surface and development.

Flash floods recede faster than normal floods. We hear about how rubbish chokes the drains and causes flash floods. However, such floods cannot solely be blamed on the rubbish alone. Although clogged drains are one cause for flash floods, rivers and monsoon drains overflow because of poor design and lack of maintenance.

Design of drainage systems is not static. They have to be improved with time.

With the increase in impermeable surface areas caused by development and roads, a lot of rainwater will flow as surface run-off. If the drainage system is inadequate, flash floods occur.

The flood affected areas in the east coast are facing normal floods which recede slowly. Flowing water fills space, especially when it reaches a dead end or a flat surface. Lakes and fresh water swamps are good examples how nature contains flood waters. But unfortunately, many lakes and swamps have either been filled up for development or made shallower by sedimentation.

Deepening a river means that the volume of water carried at a specific time is increased. However, this must be carried out uniformly.

How do we measure the effectiveness of a flood-mitigation project? If floods occur frequently within a short time after the implementation of the flood-mitigation project, the project has failed.

Every solution must "mimic" nature. Water flows from higher to lower elevation. It forms fresh water swamps and lakes. Similarly, it needs to maintain a good infiltration of water into the soil to maintain the level of ground water.

Flood mitigation projects must be based on this principles. Regular maintenance work must be carried out on retention ponds. The ground water level also affects the functioning of such retention ponds and other such facilities. Careful study is needed before any method is used.

Some methods that can be used are:

Hollow tubes – this are holes that go vertically into the soil and are lined with hollow stones. The stones absorb the water during rainfall and slowly release it into the ground.

Recreational areas with wavy surface covered with grass (swale) – it's a temporary rainwater retention area and suitable for use in housing estates.

Hollow pipes – a piping system similar to hollow tubes. But they are laid horizontally in the soil. It flows to a collection tank that allows rainwater to slowly seep into the soil.

Man-made lakes – these retain rainwater and release the water at a predetermined rate. The lakes can also be used for recreational purposes.

Man-made freshwater swamps – swamps can hold more water compared to lakes and have a variety of plants that can improve the water quality. Putrajaya uses this method to manage surface water flow.

Engineering methods – these are very expensive and are more effective when used with the other methods mentioned above.

It is evident that delaying surface water flow is an important part of any solution.

Solving floods involves delaying surface run-off entering the flood prone areas and quickly draining large amounts of water from flood prone areas.

Prevention is better than cure, and forest cover must be preserved. Deforestation and logging must be controlled. The nature of our landscape allows a good gravity flow of water. When forest cover is lost, rainwater from higher ground will rush down as the delay mechanism is removed. When we lose our forests, we lose a natural flood retention system.

We can use the best systems available but if we fail to maintain them it is akin to planting a time bomb. There should be scheduled maintenance and regular inspections carried out.

It is also important that local communities relying on such flood control projects be given a basic understanding of how the system works. This will allow quick action when there is failure in the system.

The Drainage and Irrigation Department does not have enforcement powers. When solutions are proposed, it is up to the local councils to implement the recommendations.

We hope that the Ministry of Natural Resources and Environment as well as DID will put up a long-term flood control framework with the involvement of local governments. If the need arises, enforcement powers must be given to the DID to ensure flood control allocations are not wasted due to a lack of enforcement.

Piarapakaran S.
Association of Water and Energy Research Malaysia