Three-dimensional modelling of sediment transport and the effects of dredging in the Haihe Estuary

Abstract

The Haihe Tide Lock was constructed on the Haihe River in 1958 to stop salty and muddy water intrusion. Nevertheless, tidal currents carry sediment, which is eroded from the surrounding silty coast, into the river mouth and, thus siltation of the channel downstream of the tide lock becomes a major problem. Employed are trailer dredges, which stir up the silt and subsequently moves it out of the mouth with ebb tidal currents. While the application of this method is encouraging there are still problems to be studied: how high is the dredging efficiency, how far can the resuspended sediment be transported by the ebb currents, and is the sediment carried back by the next flood tide? This paper develops a 3-D model to answer these questions. The model employs a special element-interpolating-function with the σ-coordinate system, triangle elements in the horizontal directions and the up-wind finite element-lumping-coefficient matrix. The results illustrate that the efficiency of dredging is high. Sediment concentration is 4–20 times higher than the flow without dredging. About 40–60% of the resuspended sediment by the dredges is transported towards the sea 3.2 km off the river mouth and 10–30% is transported 5 km away from the mouth. Calculations also indicate that the rate of siltation of the river mouth is about 0.6 Mm 3 per year. If the average discharge of the river runoff is 0, 200 or 400 m 3 s −1 the mouth has to be dredged for 190, 99 or 75 days every year so to maintain it in equilibrium. The dredging efficiency per day is 0.53–1.31%.