Unabsorbed rain causes flood cycle

Adianto P. Simamora, The Jakarta Post, Jakarta

Despite reforestation success in Jakarta’s upstream areas, flooding will continue to be an annual occurrence in the capital, a study say.

A study of the subsurface conditions of Jakarta has concluded that underground channels from Parung in Bogor to Serpong in Tangerang are blocked, preventing rainwater infiltration and causing large amounts of runoff.

Robert Delimon, a member of the geotechnology team at the Indonesian Institute of Sciences (LIPI), said massive limestone deposits below the ground — known as the Bojong Manuk formation — prevented water absorption into the ground.

“This causes a noticeable increase in the volume of surface flow to Jakarta, which is the main source of flooding in the capital.”

Robert said flooding had occurred in Jakarta over the last century, even when much of Bogor was forest and there were not many buildings in Jakarta.

The team found the volume of water in the Ciliwung River suddenly increased after passing Parung from Jakarta.

“This is strong evidence that groundwater contributes to the increased volume of water in the Ciliwung. It also proves that Bogor does not supply groundwater to Jakarta,” he said.

He said the supply of groundwater in the capital was purely from rainfall in the city.

Over the course of the study, the team monitored a number of groundwater wells citywide and found that only the water in a few wells in Kramat Jati, East Jakarta, had a similar composition to the groundwater in Bogor.

The survey funded by the Research Institute for Humanity and Nature (RIHN) of Japan will be completed in 2010.

“We will further study the subsurface conditions of Jakarta to determine which areas are suitable for water catchments,” he said.

A city regulation requires the owners of buildings with roof widths of over 50 meters to construct tanks measuring 2 cubic meters, while developers using a 5,000 square meter area are required to set aside one percent of the total area for a water catchment facility.

Robert said the regulation on water catchment wells would not be effective if developers did not have a good understanding of subsurface conditions.

“Jakarta is a former delta region, wedged between depositions of sand and clay. The depth of water catchment wells must be in line with this composition,” he said.

He said catchment wells must be located no more than 40 meters below the ground to function effectively.

The administration has said there is about two billion cubic meters of rainfall in the city every year but only 170 million cubic meters is absorbed, causing a water deficit of 36 million cubic meters per year.

sumber: http://www.thejakartapost.com/detailcity.asp?fileid=20070210.C02&irec=1