Environmental flow assessment using phabsim modelling for tropical mountainous river: a case study at Tekai River system, Malaysia / Maisarah Abd Ghani

Environmental flow (EF) analysis was never been made a mandatory requirement in the dam construction in Malaysia. However, Department of Environment (DOE) had made the EF study scope to be included in the Environmental Impact Assessment (EIA) approval conditions for hydroelectric projects in Malaysia. Environmental flow releases from dam impoundments will eventually give tremendous effect not only on the environment but also on aquatic organisms from their construction and operation. As such, the potential significance effects on instream flow conditions due to the proposed construction of Upper Tekai Dam (UTD) and Lower Tekai Dam (LTD) on the natural integrity of Tekai River, which is located at Jerantut, Pahang was evaluated in this study. The main aim of this study is to verify the suggested EF requirements in the DEIA report with latest modelling tools supported with field verification A Detailed Environmental Impact Assessment (DEIA) and Environmental Management Plan (EMP) outlined by Department of Environment Malaysia (DOE) as part of the approval conditions suggested that the water level at Tekai River should be maintained at a depth of 50 cm suggested whereby 5m3/s and 8m3/s should be released during wet and dry season subsequently at the impoundment stage though a detailed EF modelling and analysis was not performed to confirm the suggested EF values. The feasibility of the suggested environmental flow requirements was analyzed using PHABSIM habitat simulation model to determine the optimum environmental flows release during dry weather periods for typical fish species captured during fish sampling. The series of proposed environmental flows released was checked its compliances to sustain 50 cm depth of Tekai River evaluated using WinXSPRO analysis. Overall, it was found that the minimum environmental flows to be released for availability of fish habitat is 3m3/s up to 7.8m3/s whereby the WUA increases with the discharge up to a maximum value, then followed by a decreasing trend. Thus, the proposed environmental flow requirement of 8 m3/s during dry season can be reduced up to 3 m3/s which is adequate to sustain the downstream ecosystem and at the same time provide additional 5 m3/s to be sustained at the dam and utilized for power generation needs.