Impacts of hydraulic structures on lake water quality deterioration and eutrophication in Malaysia

Reservoirs serve many essential purposes, including water resource, flood mitigation, recreation, and hydropower generation. However, these reservoirs created by constructing hydraulic structures across the waterways encounter substantial challenges in water quality. One of the issues is eutrophication, which demands attention in reservoir management. This study employs a two-dimensional (2D) depth-averaged hydrodynamic model, utilising Delft3D, to analyse the spatial distributions and hydro-environmental processes occurring in Putrajaya Lake, Malaysia. The model effectively simulates various scenarios for both dry and wet seasons. Calibration and validation were performed using measured data from 72 points within the lake. The water quality modelling focuses on key parameters (i.e. phosphate, nitrate, and chlorophyll a). The results indicate higher phosphate concentrations during dry seasons, ranging from 0.1 to 0.4 mg∙dm−3, compared to wet seasons, suggesting a higher risk of eutrophication during dry seasons. In assessing lake eutrophication, a modified Lamparelli index was developed and adopted to evaluate the lake’s eutrophication status. The findings indicate that increasing phosphorus concentration to 0.35 mg∙dm−3 at the upstream inflow will likely trigger eutrophication in Putrajaya Lake. We recommend this phosphorus concentration as a critical threshold value at all drainage inlets as the best management practice to prevent eutrophication. A detailed assessment of water quality can be established from this finding as one of the design criteria for any hydraulic structures that could jeopardise water quality, rather than an afterthought. Addressing water quality problems through rehabilitation after they arise can be costly and often irreversible.