Infiltration pools can help alleviate floods, drought

David Solaiman, Lecturer, Civil Engineering, Universitas Pelita Harapan, Tangerang, Banten

There is still hope for the alleviation of the floods that usually occur at this time of year. However, despite this, people continue to be subjected to stress as they are told to prepare for another traumatic disaster.

The river desilting work currently being carried out by the government is good, but it represents a passive approach toward flood alleviation. What is really needed now is action that is aggressive in nature; as in football, attack is the best method of defense. Something must be done in the catchment areas in the upstream reaches of the 13 rivers that caused severe inundations in Jakarta early last year.

At any time, rainfall causes run-off. The Ciliwung river is the biggest and has the largest catchment area among the 13 rivers flowing through Jakarta. The West Flood Canal was not able to accommodate the excess flow from the Ciliwung and its banks overflowed at several locations.

Population growth in Jakarta, estimated at an annual 2 percent, entails the need for new residential areas. And, unfortunately, these are mostly constructed in the catchment area of the Ciliwung and the other rivers, which in turn increases the run-off of rainwater into the rivers because the residential areas are no longer available for the infiltration of rainwater.

If the same intensity of rain were to fall again this year, the discharge from the Ciliwung would be even greater than last year because of changes in land use.

In 1996, the Ciliwung produced its biggest flow so far recorded at 743 cubic meters per second, a flow that has a return period of 100 years. Between 1981 and 1985, the greatest discharge was never more than 275 cubic meters per second, but after 1985 the discharge kept on increasing in the direction of 400 cubic meters per second.

A security system is needed to deal with the issue, otherwise there will be a never-ending flood problem in Jakarta. Attention has to be paid to catchment dynamics in terms of development plans for residential and industrial areas. Human needs and desires cannot be met without manipulation of the water cycle and the landscape.

Development must be based on environmental consciousness, as solving the flood problem is no simple matter.

In the development of new commercial and residential areas, the increase in run-off must be controlled by providing infiltration pools and wells or pits.

By providing such tools, run-off will certainly be reduced to an extent that will depend on the volumetric capacity of the pools and pits.

This is what is meant by an attacking football strategy. These measures will not only alleviate flooding but will also increase the ground water available, which will be of crucial importance in alleviating drought during the dry season.

The pools and wells should be built by the householders or the real-estate developers themselves. So this idea is really like taking small steps with a view to achieving big results, and therefore needs social participation, acceptance and the understanding of political decision-makers.

It would be really worthwhile to try and implement this idea, preferably in the catchment area of the Ciliwung river throughout the entire Puncak and Bogor areas. If 40,000 house compounds were each provided with a pool with an average capacity of 10 cubic meters, then 400,000 cubic meters of rainwater would be retained.

And if the duration of rainfall averaged one hour, this would reduce the river discharge by 111 cubic meters per second, quite a significant amount in preventing rivers from overflowing their banks. The water collected in the pools and pits would infiltrate into the soil and increase groundwater capacity, which would emerge several months later in the downstream reaches of the rivers and in domestic wells.

Even in Japan, people make use of tennis courts as retention pools by enclosing them with brick walls of up to 30 centimeters in height. When the time of flooding is over, the pools are allowed to drain. Outdoor parking lots are treated in the same way. The flat concrete roofs of high-rise buildings are also used as retention pools.

Efforts are needed to bridge the gap between public knowledge and scientific understanding. Erect billboards to show the need for infiltration pools, or place advertisements in the media to urge people dig such pools!

Since infiltration pools will have to be dug independently by individual householders. This work could, however, be carried out simultaneously, so that thousands of pools would be completed in not more than two weeks.

The infiltration pools and pits would certainly truncate the peak flows of our river whenever rain falls, so let us dig them now!