Proper drainage will minimise flood damage

By Sharika Tasnim

Fast industrialisation and economic expansion are accompanied by dynamic urban growth in Malaysia. Subsequently, the hydrological cycle of a city has been drastically altered according to a 2019 book chapter by Sheldon et al. entitled “Urbanization: Hydrology , Water Quality, and Influences on Ecosystem Health” (see https://www.sciencedirect.com/science/article/pii/B9780128128435000113).

Increased construction has expanded 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 mention Lusk & Toor in their 2017 book chapter “Optimizing the Hydrologic Properties of Urban Soils” (https://doi.org/10.1201/9781315154251-9).

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 Urban Stormwater Management Manual (MASMA) implementation, mentions Samsuri et al. in their 2018 journal article “Flash Flood Impact in Kuala Lumpur – Approach Review and Way Forward”.

MSMA, 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.

Managing disastrous floods and ensuring the safe passage of less frequent and more severe flood occurrences are vital.

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 Plan (ESCP) in the construction sector. A comprehensive temporary measure will be adopted during the construction period, according to the ESCP.

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 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, according to MSMA 2nd Edition – Application of Green Infrastructures for Solving Sustainable Urban Stormwater Management Challenges published in 2014.

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 percent, resulting in flood disasters.

In this case, people must exercise precautions to avoid clogging the drains as a result of indiscriminate dumping of rubbish mentions Tukiman in her 2008 PhD thesis.

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, writes Siudak in a 2001 book chapter entitled “Role of Education in Reducing Flash Flood Effects” (see https://link.springer.com/chapter/10.1007/978-94-010-0918-8_3).

Research suggests that 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, examines Sohn et al. in their 2020 journal article “How effective are drainage systems in mitigating flood losses?” (see https://doi.org/10.1016/J.CITIES.2020.102917).

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 responsible authorities.

This is because well-designed drainage systems make effective stormwater discharge possible, which reduces property losses and recovery costs states Faram et al. in their 2005 conference paper “Innovative approaches to urban stormwater management” (see https://www.researchgate.net/publication/270685476_Innovative_approaches_to_urban_stormwater_management)

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 MSMA 2nd Edition – Application of Green Infrastructures for Solving Sustainable Urban Stormwater Management Challenges.

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 for Tasnim et al. in their 2022 journal article “Understanding Flood Vulnerability Issues in Hulu Langat Residential Zone: A Study of Taman Sri Nanding, Hulu Langat, Malaysia” (https://publisher.uthm.edu.my/ojs/index.php/jaesrr/article/view/12617)

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. ***

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(Sharika Tasnim is a postgraduate student in the Kulliyyah of Architecture and Environmental Design and has published this article in New Straits Times as a columnist.)