PETALING JAYA: A factor contributing to the recent spate of flash floods in the Klang Valley is the loss of topsoil due to over-development in the past few decades.

Drainage and Irrigation Department deputy director-general (I) Ahmad Fuad Embi said topsoil served to retain water, acting like a sponge when it rains.

Topsoil, the topmost layer of soil and rich in humus, was especially important during periods of heavy rain, he added.

“The amount of water that could be stored by topsoil just half-a-metre deep in a one square kilometre area would be enough to fill over 130 Olympic-sized swimming pools,” he said yesterday in an interview.

However, he said, too much development had taken place and developers were not replacing the topsoil they had removed.

According to Ahmad Fuad, development over the years in areas such as Hulu Kelang had led to large areas of hillslopes being cut and tonnes of topsoil removed.

This, he said, had resulted in a domino effect as the absence of topsoil meant that there was nothing to hold the rainwater.

“The reddish-brown soil that everyone sees in project sites is laterite and this type of soil is largely impermeable,” he said.

This, he noted, had resulted in rapid run-off into drains and then rivers.

“Now, without any topsoil, the water reaches the rivers much quicker.

“Of late, this has resulted in river levels peaking too fast and the end result is flash floods when the banks overflow,” he said.

Ahmad Fuad said that according to bylaws that should be enforced by the local authorities, developers who had removed topsoil in preparing land for projects were required to replace it with at least 5cm of new topsoil.