Modeling the effectiveness of detention ponds in improving the quality of stormwater

A considerable amount of sediment is transported by the flowing water in a river system, especially during the flood period. This results in adverse environmental and economic effects. High sediment concentration is considered the main source of pollution, which affects the water quality in the river system and restricts the surface water use. In some river systems in Malaysia, detention ponds are being used for temporary storage of the stormwater but these ponds also have the capacity to retain part of the sediment load being carried by the flowing stormwater. In the present study, a mathematical model was developed and employed to predict the sediment concentration in the outflow water from a detention pond. The mathematical model was based on hydrological continuity equation which was applied to the flow of water and sediment through the detention pond. The main assumption made in the model formulation is the food mixing inside the pond so that the sediment concentration in the pond water is equal to the sediment concentration in the outflow water from the pond. Hypothetical data was used to run the model and to check the effectiveness of detention pond for reducing the sediment load from the stormwater. Since the stormwater carried a wide spectrum of the settling velocity of their particles. Class 1 represents the fine clay and has a sediment particle size less than 4 micrometer, settling velocity 1.77 x 10 -6 m/s and average sediment concentration of 53 mg/1. Class II sediment type represents the clayey silt and has sediment size from 4 micrometer to 62 micrometer, settling velocity 0.00074 m/s and average sediment concentration 185 mg/I. Class III represents the medium sand and has sediment size from 62 um to 500 micrometer, settling velocity 0.007m/s and average sediment concentration of 100 mg/I. Validation for the proposed model was conducted by comparing the model output with output obtained by applying other models using same data. Physical model was also employed to study the effectiveness of using filtration layers positioned at different angles in the detention pond to enhance the stormwater quality. Data collected from the physical model showed the effectiveness of using the filtration layers. The screening action of the filtration layers could reduce the sediment load carried by the stormwater flowing in the pond. Experiments were carries to study the best position of the filtration layers inside the pond for optimum performance in removing sediment load from the stormwater. The experiments revealed that the angle of inclinations and the location of the filtration layers could influence the performance of the detention pond. The results obtained from the physical model are considered as a preliminary and more experiments will be conducted to simulate the optimum performance of the detention ponds for polishing the quality of the stormwater. The efficiency of the pond in trapping the sediment was tested in this study using the standard trap efficiency equation. The trap efficiency of the pond is dependent mainly on the sediment class and was found to be over 90% for the coarse sediment and 50% for the fine sediment.