FAST industrialisation and economic expansion are accompanied by dynamic urban growth in Malaysia. Subsequently, the hydrological cycle of a city has been drastically altered.
More construction has increased impermeable surfaces and soil compaction has reduced the land's ability to absorb and store floodwater.
This leads to the run-off pattern being impacted by the changing hydrologic cycle. Often, flash floods caused by intense, brief rainfall occur in urbanised areas with insufficient drainage and storage systems.
Besides, flash floods also occur for a variety of other reasons, including obstruction to river flow, sediment and debris clogging the drainage system, internal drainage systems with insufficient capacity, intense and prolonged rainfall, rapid and dense urban growth, and lack of MSMA implementation.
MSMA, or Urban Stormwater Management Manual, is a guide for professionals, particularly engineers, on how to design drainage infrastructure that is ecologically beneficial using the idea of quantity and quality management at the source level.
Therefore, the aim is to manage disastrous floods and ensure the safe passage of less frequent and more severe flood occurrences.
During heavy and continuous rainfall in metropolitan areas, onsite stormwater detention (OSD) is essential to slow the rate at which run-off enters the drainage system or other properties by temporarily storing stormwater run-off.
Moreover, it helps to control water flow and minimise the possibility of floods downstream. According to the guideline, onsite detention — underground storage and rainwater harvesting — must be included in any construction built on land less than 5ha.
On the other hand, a development of 5 to 10ha is advised to incorporate a dry pond. As for development greater than 10ha, a wet pond is recommended for storage.
Therefore, the restoration of wetlands and incorporation of retention/detention basins into community development in flood-prone locations is encouraged, to avoid financial losses due to floods.
Another major issue is the preparation of the erosion and sediment control (ESCP) plan in the construction sector. A comprehensive temporary measure will be adopted during the construction period, according to an ESCP plan.
The general parameters for ESCP include reducing soil erosion, protecting topsoil and other assets, managing access routes and sites, controlling run-off, managing sediment prevention, controlling earthwork and erosion, stabilising slopes, and site maintenance.
In addition, to highlight the run-off management facilities in the ESCP plan to prevent flash flood hazards: decrease the amount of runoff, detain run-off to reduce its velocity, divert run-off from erodible areas, and dissipate the flow of run-off.
When it comes to drainage systems, historical flash flood occurrences in Malaysia show that silt, rubbish and other obstacles reduce the carrying capacity of the drainage system by up to 50 per cent, resulting in flood disasters.
In this case, people must exercise precautions to avoid clogging the drains as a result of indiscriminate dumping of rubbish.
Educational programmes and awareness campaigns can facilitate the locals greatly in comprehending the causes and effects of flash flood occurrences and strive to minimise flash flood hazards.
Research suggests the storage-based drainage system may be the most successful in preventing or minimising flooding-related property damage. There are three drainage systems - storage-, conveyance-, and infiltration-based facilities.
Generally, systems based on storage outperform conveyance-based methods. Green spaces unconnected from sewage systems are swiftly water-logged after being saturated, and excess overflow is not efficiently drained, making infiltration-based facilities less effective in mitigating property damage.
Aside from that, the smaller width of the drains is also a contributing factor to the difficulty of rainwater movement. Overall, there is an immediate obligation to have proper maintenance of the drains by the responsible authorities.
This is because well-designed drainage systems make effective stormwater discharge possible, which reduces property losses and recovery costs.
Causes of the overall flash flood events account for extreme rainfall (31 per cent) followed by ineffective drainage system (30 per cent), heavy rainfall with high tide (18 per cent), poor maintenance of the drainage system (16 per cent), and construction sites not complying with the ESCP plan (5 per cent).
According to residents of Hulu Langat who often experience flash floods, the key issues are widening the river and improving the drainage.
Furthermore, the problem of funding is another never-ending burden.
Flash floods, often the worst traumatic experiences for residents in Hulu Langat almost every year, necessitate implementing the proper flood management strategy to improve safety and living standards of the people.
* The writer is a Committee Member at the Royal Institute of British Architects and a student at the International Islamic University Malaysia (IIUM)