| Sediment erosion is the basis of sediment transport research. In this paper, the distribution of critical shear stress of sediment erosion in the Changjiang Estuary is presented based on physical mechanism and numerical experiment through the analysis of characteristic of current and sediment transport, sediment distribution. The results could provide the basis for the numerical model configuration of sediment critical erosion shear stress. It has certain significance and practical value in sediment research in the Changjiang Estuary. The main works of this paper are described as following:1) The critical shear stress of sediment erosion is divided into different zone according to the measured sediment particle size, water content, related sediment data from literatures and the regional character of bottom current and wave shear stress. Then the distribution of critical shear stress of sediment erosion calculated with the formula proposed by Guoren Dou is given out preliminarily.2) The three dimension numerical sediment model, wind model and wave model were configurated. The wind model and wave model were validated by the observed wind speed and direction, significant wave height, and results shown that the model validations are in good agreement with measured data.3) Critical shear stress of sediment erosion was regionally calibrated and validated using the three dimension numerical sediment model by comparing the model output with the measured suspended sediment on the basis of verifying the current and salinity. The value is in the range of 0.3-1.1 N/m2. Overall, in the flood season, the critical shear stress of sediment erosion τe is 0.8 N/m2 in the South Branch and South Channel, is 0.7 N/m2 at the lower reach of the South Passage and North Channel, is 0.5 N/m2 in the North Passage, has a high value of 1.0 N/m2 in the shoal outside the north dike, and is in the range of 0.3-0.5 N/m2 outside the mouth. In the dry season, the critical shear stress of sediment erosion τe is 0.8 N/m2 in the South Branch, is 0.7 N/m2 at the upper reach of the North Channel and 0.3 N/m2 at the middle reach of the North Channel, is 0.5 N/m2 in the South Channel, is 0.6 N/m2 at the upper reach of the South Passage, is 0.7 N/m2 at the lower reach of the South Passage and North Channel, is 0.8 N/m2 in the North Passage, is relatively high in the range of 0.8-1.1 N/m2 outside the North Passage and South Passage, and is in the range of 0.5-0.8 N/m2 outside the 10m depth contour off the river mouth. In the North Branch, the critical shear stress of sediment erosion τe has the same value in flood and dry season, and increases gradually from upper reaches to lower reaches, i.e., is 0.3N/m2 at the upper reach, is 0.45 N/m2 at the middle reach, is 0.5 N/m2 at the lower reaches, and is 0.5 N/m2 outside the North Branch.The value of critical erosion stress τe by model calibration and validation is different with the one calculated by Guoren Dou’s formula in some area. The formula overvalues the τe in the area where the partical size is larger than 19 μm and water cotent is lower than 40%, which may be because the formula underestimates the impact of compaction on τe. Another reason may be the collection of samples in this paper is only restricted on the surface uncompacted sediment, which leads to the water content is overvalued and lowers the impact of compaction on τe. Besides, in the area where the particle size is smaller than 8 μm, the value calculated by Dou’s formula is larger than the one calculated by the model which may be due to the reason that the formula exaggerates the impact of cohesive between particles. But for the sediment with particle size of 8-19 μm and water content of 40%-60%, the Dou’s formula is suitable.In the model calibration and validation, it is found that τe is not a constant but temporal variation due to the ben erosion and deposition. The erosion in spring tide leads to more compacted bed exposed and the newly deposited sediment in neap tide reduces the compaction of sediment. So the formula of the τe adjusted with the height of bed is given and used in our model calculation. The result indicates that our model is in better agreement with measured data.Moreover, τe has difference between flood season and dry season. In the South Channel, τe is 0.8 N/m2 in flood season and 0.5 N/m2 in dry season, is larger in flood season than in dry season. The reason may be the difference of the bottom shear stress induce by current and wave between the two seasons. The bottom shear stress in flood season is much larger than the one in dry season so that the sediment is more compacted due to more elutriation by flow. But in the North Passage and outside the mouth of the South Passage and North Passage, the τe is in the range of 0.8-1.1 N/m2 in the dry season and in the range of 0.5-0.6 N/m2 in the flood season. The τe in the dry season is larger than flood season. On the one hand, the cohesive among particles on the bed is higher in dry season for the reason that the particle size in the dry season is thinner than flood season. The cohesive decides the value of τe when particle size is smaller than 100 μm, so τe in dry season is higher where the particle size is smaller than 100μm. On the other hand, the τe is higher in the area where the bottom shear stress is 0.6 N/m2 higher in dry season than in flood season, which results in the bed undergo more elutriation by flow in dry season. |
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