Vertical Profile Of Nonuniform Suspended Sediment Concentration In Yangtze Estuary

The Yangtze River is the largest river in China, as well as the Yangtze River channel is known as China’s "golden waterway". Yangtze Estuary is an estuary with abundant discharge and sediment load, which make serious deposition and hinder navigation at downstream. With three phases of engineering management in Yangtze River Deep-Water Channel during 1998 to 2010, the navigable depth in south channel-north passage is achieved to 12.5m. However, there was still a large amount of deposition over the past few years and which cost too much in maintenance. The purpose of article is research the vertical profile of nonuniform suspended sediment concentration in Yangtze Estuary, provide helps to analysis the reasons of normal back-silting.A new formula for a concentration profile of nonuniform sediment is derived by solving the 1-D diffusion differential equation. The Jasmund-Nikurads logarithmic velocity distribution and a modified turbulent diffusion coefficient is taken into account. The new formula introduced a correction coefficient in settling velocity equation and combined with Larsen’s hypothesis, avoids the sediment concentration is infinite at the bottom and zero at the water surface in Rouse formula with equilibrium bed-concentration. The accuracy of predicted sediment concentration profile is improved at the same time. The distribution of suspended sediment is more concentrated in the vicinity of bed and the gradient of the curve is increased during the increase of relative particle size. Also the formula is self-adaptable for uniform sediment. The profile of suspended sediment is more uniform with the decrease of suspension index. When suspension index Ai=2, the suspended sediment is more concentrated from the bed to 0.2H, and it is almost uniformly distributed along the direction of whole water depth when Ai= 0.01.Obtain the transport rate formula of suspended sediment by integrate the velocity and the vertical concentration profile. There is no need to solve this formula thorough numerical integration because of its explicit form. The influence of sediment transport rate near the bottom to the total depth is considered. The near bed transport rate divided by total transport rate also called relative sediment transport rate is proportional to the suspension index. When suspension index is equal to 2, the relative sediment transport rate is about 30%. The relative sediment transport rate is also proportional to the relative particle size (Di/ Dm).As relative particle size is greater than 3, the relative sediment transport rate is more than 25%, which will make a greater impact in overall transport rate if we ignore the sediment transport rate near the bottom. In author’s opinion, it is accepted to ignore the influence of sediment transport rate near the bottom if the relative sediment transport rate is less than 5%. After calculation, the contribution of near bed sediment transport rate to overall should be considered when suspension index is greater than 0.15 in Yangtze Estuary.Two peaks of suspended sediment concentration appear during a whole ride cycle in Yangtze River Deep-Water Channel. The profile of suspended sediment at equilibrium moment after maximum rising and ebb tide is more uniform than the minimum rising and ebb tide, and it is less uniformly at equilibrium moment after maximum rising tide than the maximum ebb tide. During spring tide, the profile of suspended sediment is more uniform than neap tide at equilibrium time. The profile of fine suspended sediment at equilibrium time in dry season is less uniform than flood season, while the coarse and total concentration is in contrast. Suspended sediment transport rate during spring tide is higher than neap tide, which in flood season is also greater than dry season and it in spring tide is more than ebb tide. From upstream to downstream, the transport rate decreases after an initial increase and the peak appears at the central flexed position. Direction of net transport rate points downstream.