tailieunhanh - Sediment and Contaminant Transport in Surface Waters - Chapter 3

Tỷ lệ xói mòn trầm tích được định nghĩa là tổng thông lượng, q (g/cm2/s), trầm tích từ đáy trầm tích vào nước nằm phía trên trong trường hợp không lắng đọng. Dòng này thường do áp lực cắt gây ra bởi dòng điện và hành động sóng. Bởi vì hoạt động của họ, các sinh vật đáy và cá cũng có thể đóng góp vào dòng này, nhưng tác động của chúng thường là nhỏ. Propwash và sóng từ tàu lớn cũng như các du thuyền nhỏ hơn có thể gây ra xói mòn địa phương. Sau khi bị. | 3 Sediment Erosion The erosion rate of a sediment is defined as the total flux q g cm2 s of sediment from the sediment bed into the overlying water in the absence of deposition. This flux is generally due to shear stresses caused by currents and wave action. Because of their activity benthic organisms and fish also can contribute to this flux but their effect is usually small. Propwash and waves from large ships as well as smaller recreational boats can cause localized erosion. Once eroded sediments can go into and be transported as suspended load or bedload. The resuspension rate of a sediment is defined as the flux of sediment into suspended load again in the absence of deposition. Particles in suspension in a horizontally uniform flow move horizontally with the average fluid velocity whereas their vertical motion is governed by gravitational and turbulent forces collisions between particles are usually negligible in modifying the transport. As a result of these forces the concentration of the suspended particles typically varies in the vertical direction with the concentration being largest near the sediment-water interface and decreasing approximately exponentially in an upward direction from there. The length scale of this exponential decay depends on the settling speed ws and the eddy diffusivity due to turbulence Dv. From a steadystate balance of the fluxes due to gravitational settling and turbulent diffusion and as a first approximation this can be shown to be given by Dv ws. As the settling speed increases and turbulence decreases this length scale decreases. When it is on the order of a few particle diameters collisions between suspended particles and between suspended particles and particles in the sediment bed become significant. In this limit the particle transport is known as bedload. Bedload generally occurs in a thin layer near the sediment bed with a thickness of only a few particle diameters. In bedload particle concentrations are relatively high