Sat, Sep 07, 2013 - Page 8 News List

The need for flood water prevention strategies

By Lee Chyi-ti 李錫堤

Taiwan is located in the world’s most geologically active region. This, together with the impact of typhoons, exposes the region to at least three kinds of natural disaster, making it one of the regions with the highest risk of experiencing a natural disaster.

Advanced nations are trying to come up with strategies for responding to the impact of extreme weather by studying climate adaptation strategies. This is something to which Taiwan must pay attention.

In geologically fragile regions, natural disasters are often compounded, as the different kinds of disaster affect each other and sometimes cause other natural disasters.

When discussing flood prevention, it is necessary to understand that natural conditions have become both extreme and complex, so that any response must consider the interaction between different kinds of disaster and the irreversible changes that occur as time passes.

For example, in terms of water reservoirs, which are hubs for water control, extreme rainfall increases the landslide area in reservoir water catchment areas and the mud from these landslides is deposited in rivers and in reservoirs. This affects the reservoir’s supply capacity and ability to collect flood water.

Insufficient supply from reservoirs results in excessive pumping of groundwater, which creates or aggravates existing land subsidence, which in turn increases the risk of floods.

When the ability of reservoirs to collect flood water is diminished, downstream peak flows are increased, which increases the risk of flooding as silt is deposited and diminishes a river’s ability to drain flood water.

Reservoir sedimentation is next to irreversible, and as sediment continues to accumulate, it will eventually fill reservoirs, which then have to be abandoned. Even without considering the changes that extreme weather will bring, southern Taiwan is already beginning to experience water shortages.

If new water resources cannot be developed within the next 20 years, water shortages will become increasingly severe. If the trend toward more extreme weather continues, floods will become increasingly severe.

The increase in land slide area, faster sedimentation and faster deterioration of reservoir functionality could soon have an unacceptable and critical impact on floods and droughts.

If that happens, it may be necessary to spend even more than the current NT$600 billion (US$20.2 billion) on flood prevention. It is necessary to take preventative measures before that happens.

The construction of a large reservoir will take well over 10 years from the initial planning stages, so it is not a solution that can be applied to an urgent problem.

This is why flood control strategies must be considered in tandem with water resource planning, why it is necessary to take a conservative approach when considering its effect on the impact of extreme weather and why we must make comprehensive overall plans to create a sustainable and durable implementation strategy.

At a more detailed level, we must also consider that mud and silt will be held up in reservoirs, which leads to erosion of the coast line. The way to solve this problem is to build off-channel reservoirs — a reservoir built next to or near a river, diverting water from a primary stream during high flows, thus avoiding the need to place a large dam directly on the main river — and to add channels for releasing sedimentation to minimize sedimentation buildup.

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