Numerical Modeling of the Effect of Riverbed Dredging on Water Environment in Sujin-Shitan Section of Lianshui River in Hunan Province

It has been experimentally well documented that the polluted bed sediment will release quite a lot pollutant onto its overlying water when it is disturbed to suspension. In this study, a two-dimensional numerical model was developed to simulate the effect of river-bed dredging on river water environment and some multi-site dredging schemes were also numerically investigated. Due to the extreme lack of data in Sujin-Shitan Reach of Lianshui River, Hunan Province, China, the model was validated using measured data in other river reaches. In view of the opinions from relevant construction authorities, the pollutant-releasing processes due to disturbance of polluted bed sediment in three dredging sites with different disturbing areas were designed for numerical consideration. In the numerical study, three hydrographs of Lianshui River were considered and they are the driest monthly average flow rate of 13.4 m³/s, smaller flow rate of 20 m³/s and the average annual flow of 116 m³/s. The preliminary conclusions are as follows from the simulation study:

Generally speaking, the water depth and flow velocities in Sujin-Shitan Reach of Lianshui River increase with the flow rate.

At the same time, with the increase of the flow rate, the pollutant dilution capacity of the river reach increased significantly, and the pollutant concentration decreased significantly. Therefore, it can be considered with caution that as long as the number of dredging areas and dredging intensities are controlled and dredging seasons are not in the driest season, the effect of pollutants release due to disturbance of polluted riverbed by dredging on river water environment is limited.

It should be stressed that this numerical study is an extremely simplified one due to the extreme lack of in-situ data and any practical environmental problems caused in the river reach still requires a specialized study.