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Bigger Drain - Good Against Flood?

posted Sep 6, 2021, 8:11 AM by jeffery jim
Many simpletons have this idea of mitigating flood but flood was never in the plan when it comes to best management practices or BMP in stormwater management. The key issue and derivation of design is based on practices to avoid flash flood from occurring which may cause property and life loss.

2. In order to understand how drains and other hydraulic structures are designed to accomodate the Stormwater Management Manual or Manual Saliran Mesra Alam or MSMA, one have to understand the main objective of stormwater management itself. The main objective is to mitigate sudden torrential runoff and rise of water level within 3 hours of heavy precipitation and expecting the runoff to recede within 6 hours.

3. The hydrograph concept will guide the water retention and dispersal by comparing flow value during pre-development and post development. The concept is similar to fighting COVID-19 where we try to flatten the curve from sudden surge which will incapacitate the hospital with influx of patients in short span of time when it is possible to stretch it through engineering means by manipulating hydraulic structures or other non-structural approaches. Non-structural approaches usually are in administration and by mean of enforcement, where as structural approaches involve civil engineering via hydraulic manipulation concept.

4. Stormwater management is not similar to flood estimation. These are two different concepts and referred to two different Hydrological Procedures and Manual produced by the Drainage and Irrigation Department. Both will use hydrograph but the storm duration time for observation and modelling is different.

5. Over-engineered structural methods is costly and may not be economical apart from taking too much space and be hindrance to local development. This approach may solve issues when it comes to abnormal rainfall intensity which sees triple or quadruple in volume. With BMP, the mitigation will be balanced in sense of economics as well as risk and vulnerability.
6. Although MSMA is a manual which provides suggestions to remedy issues related to flash flood, more solutions can be developed from creativity of the hydraulic engineer.

7. I was challenged to provide hydraulic retention for a high rise condominium in a small plot of land. The manual only speaks about concept and somehow, I came up with the best solution for both developer and DID Sabah. Instead of building weir and swale and typical pipes and culverts as a mean to retain water during storm event, I did modified my basement parking as water storage area without endangering residents and their vehicles. Although the typical intensity of a 50 Years ARI would be around 124mm at Kota Kinabalu, my design have no problem catering up to 250mm of rain.

8. In order to move forward, proponents of stormwater management should be more creative and learn the fundamental ideas before even trying to use all means and procedures highlighted by the DID. This is the only way forward. As for the countrymen who are generally simpletons, I hope such explanation will educate them on why certain designs are not up to the standard as they initially thought. It is hard to fight flood but we are ready to tackle flash flood.

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