A Hydrology and Hydraulic Case Study on January 2015 Flash Flood in UniGarden, Kota Samarahan, Sarawak

Bong, Charles Hin Joo and Lim, Leonard Lik Pueh and Er, Hauzh (2017) A Hydrology and Hydraulic Case Study on January 2015 Flash Flood in UniGarden, Kota Samarahan, Sarawak. In: Proceedings of the 37th IAHR World Congress, 13-18 August 2017, Kuala Lumpur, Malaysia.

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Abstract

This study investigated the possible causes for occurrence of flash flood in UniGarden by looking into the hydrologic and hydraulic factors. This study consists of the assessment of water balance (precipitation, surface runoff, and infiltration) as well as the suitability of the hydraulic capacity of the existing earth drain based on MSMA guidelines. The site works involved analysis on precipitation, measurements of size of the earth drain and the corresponding flow velocity, and water quality testing. The consistency in annual rainfall and comparison between storm in year 2007 and 2015 indicated that precipitation was not the main cause of the flash flood. The infiltration of both storm events were limited due to precipitation few days prior to the extreme storm event that partially saturated the previous ground. There was a 45% increase in the average peak flow in 2015 than that in 2007. The TSS in both low flow and high flow were relatively low, providing estimated sediment loading up to 84.81 g d-1, which is not likely to have a direct effect on reducing the earth drain size. Rapid vegetation growth slow down flow, reduced free flow area of the earth drain, thus decreased its hydraulic capacity. Lowest invert level point of the drain was found to be located some distance away from the outlet, indicating potential backflow of water. These findings suggests that the increase in runoff, decrease in hydraulic capacity, and the improper invert level of the earth drain contributed to the flash flood in UniGarden on 18 January 2015. This finding is essential in providing insight to the incident as well as call for consistency review of drainage network in land development policy and decision making.

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