STUDY ON SEDIMENT CONCENTRATION DISTRIBUTION UNDER BREAKING WAVES OF MUDDY COAST

GUIDELINES Muddy coast is an important part of the mainland coast in china. Its length is about 4000 km, and accounts for around 22% of the mainland coast (Xia Dongxing et al,1993). Muddy coast is mainly constituted by very fine sediment particles with medium diameter of less than 0.05mm. This type of coast has fairly straight coastline and broad beaches. Its slope is generally 1/500~1/2,000. The tidal flat between high and low tide is very broad(Gong,1990). When harbors are constructed on muddy coast, both deep-water channel and berth will face large scope of shoal. The main sediment movement of muddy coast is suspended load transport. Tide and wave are the main forces driving. Under normal weather condition, the sediment concentration is relatively small, but during surge storms, breaking wave produces intensive disturbance on sediment. It will cause the greater sediment concentration, and result in great increase of sedimentation in channel and berth zone. The previous experimental research on the sediment concentration under breaking wave is mainly on sand and silt(Yao,1991;Ogston et al 2002; Shibayama T et al,1993;Nielsen p,1986;kos’yan R D,1985;Smith,G.G et al,1985;Han Hongsheng et al,2006; Bian shuhua et al,2007;Xua Hua et al,2012). the research of muddy is relatively few. Mud is different from sand and silt. The result of sand and silt cannot be used in muddy research appropriately. Therefore, this experimental research has significance on design, construction and maintenance of port channel in the muddy coast.


GUIDELINES
Muddy coast is an important part of the mainland coast in china.Its length is about 4000 km, and accounts for around 22% of the mainland coast (Xia Dongxing et al，1993).Muddy coast is mainly constituted by very fine sediment particles with medium diameter of less than 0.05mm.This type of coast has fairly straight coastline and broad beaches.Its slope is generally 1/500～1/2,000.The tidal flat between high and low tide is very broad (Gong,1990).When harbors are constructed on muddy coast, both deep-water channel and berth will face large scope of shoal.The main sediment movement of muddy coast is suspended load transport.Tide and wave are the main forces driving.Under normal weather condition, the sediment concentration is relatively small, but during surge storms, breaking wave produces intensive disturbance on sediment.It will cause the greater sediment concentration, and result in great increase of sedimentation in channel and berth zone.The previous experimental research on the sediment concentration under breaking wave is mainly on sand and silt (Yao,1991;Ogston et al 2002;Shibayama T et al,1993;Nielsen p,1986;kos'yan R D,1985;Smith,G.G et al,1985;Han Hongsheng et al,2006;Bian shuhua et al,2007;Xua Hua et al,2012).the research of muddy is relatively few.Mud is different from sand and silt.The result of sand and silt cannot be used in muddy research appropriately.
Therefore, this experimental research has significance on design, construction and maintenance of port channel in the muddy coast.

Experiment Method
The experiments use the regular wave.The depths of water are 45cm, 50cm and 55cm.Type of breaking wave is spilling breaker.It is determined according to non-dimension parameter introduced by Battjes .The sea side parameter of this experiment ranges from 0.021 to 0.031( ), which type is spilling breaker (Jiang et al ,2005).
The sediment sources come from the trial channel of Xuwei harbor.Sediment medium diameter is 0.007mm.The density of original mud is 1.65-1.82kg/l.Considering the density of surface mud in Tianjin Newport and Lianyungang sea area are both 1.35～1.40kg/L，density of experimental mud are 1.35kg/l，1.40kg/land 1.45kg/l respectively, gained by deploying of fresh water and original mud.As original mud has high cohesion, high-power motor blender is adopted to blend, deploy repeatedly and mix up, then put it in mud bucket, blend mud again unit density meet experimental requirement, next pave it in preformed groove in slope and even it manually.The water must be drawing off slowly to ensure mud in channel will not be affected by flow.
Water samples are the synchronous acquisition.The specific collection time is 1min, 3min and 6min since breaking wave, then the collection would be conducted every 3 min.Each collection needs 10 seconds.Then the sandy water should be filtered, dried and weighed to get the sediment concentration.16 groups of experiments have been conducted (Table 1) .Under the breaking wave action, with the reciprocating action of the fluctuating water particles, the mud surface displayed the "smoking" state everywhere, making the entire bed surface seem to be covered by clouds and mists, which diffused upward, and formed a layer of turbid water over the bed surface soon.The sediment concentration gradually increases from the surface layer to the bottom layer, and is fairly high within the scope of 4cm above the bed surface.Since high-density sandy water exists at the bottom and increases quite rapidly, while the increase of the sediment concentration of the upper water is relatively slow, after the breaking wave action, the mud surface would not form a sand wave form.If the time of the breaking wave action is fairly short, and the density of mud is 1.35kg/L, the turbidity of the water would be quite high, large amount of mud near the wave breaking point would move towards the wave propagation direction, the wave height attenuation would be very quick, and the wave breaking point would move forward.If the density of mud is 1.40kg/L, the turbidity and the wave height attenuation would both be lower than that obtained when the mud weight is 1.35kg/L.
There is a scour pit formed at the wave breaking point position of the bottom bed.When the density of mud is 1.45kg/L, the turbidity of the water would be the lowest, the wave height attenuation would be unobvious, and the mud on the bed surface would be smooth.During the wave action, the turbidity of the water would firstly increase and then decrease.
Under the same water depth, when input the same wave period, the higher wave height would cause higher sediment concentration.The sediment concentration distribution along the distance is related to the wave breaking position.Through experimental observation, in front of the wave breaking point, high sediment concentration area is formed at the bottom, and the sediment concentration would be highest near the wave breaking point.At certain distance behind the wave breaking point, with the decrease of the wave height, the stratification of mud would appear.

Experimental Results and Analysis
Figure 3, 4 and 5 provides the sediment concentration distribution diagram of the experiment results of Group 2, 6 and 16 respectively (Gao et al, 2012).In front of and behind the breaking of the wave, the bottom shows high sediment concentration.The change of gradient of sediment concentration is fairly small over 5cm above the bed.The sediment concentration of the upper and lower water would be basically uniform when muddy density is 1.45kg/L.The breaking wave would cause the highest sediment concentration with a muddy density of 1.35kg/L, the next is 1.40kg/L, and it would be difficult to cause the change of sediment concentration with a muddy density of 1.45kg/L.
Under the breaking wave action, all mud with different density would reach certain maximum sediment concentration.Under the condition of same water depth and same wave condition, when the muddy density is small, the sediment concentration at the bottom would be high.Under the condition of same muddy density, same water depth and same period, the sediment concentration with a higher wave breaking strength would be higher.On the same vertical line, when the muddy density is 1.35kg/L and 1.40kg/L, the sediment concentration near the bottom layer would be several times that of the surface layer and the intermediate layer, and the stratification exists.The sediment concentration near the bottom layer increase slightly when the muddy density is 1.45kg/L.Several minutes later, the sediment concentration on the same vertical line would basically be consistent.
When the experimental section is muddy density 1.35kg/l , the sediment concentration of over 30kg/m 3 may be formed 1cm from the bed , and a sediment concentration of 10 kg/m 3 approximately may be formed 5cm from the bed; When the muddy density is 1.40kg/l, a sediment concentration of 10kg/m 3 approximately may be formed 1cm from the bed, and a sediment concentration of 5kg/m 3 approximately may be formed 5cm from the bed; but the muddy density 1.45kg/L, only a sediment concentration of 2kg/m 3 approximately can be formed 1cm and 5cm from the bed.The experiment on sediment concentration distribution along the vertical line shows that: ① sediment moves from bed to water; ②water turbulence and movement of water particle makes the

Figure 2 .
Figure 2. the diagram of experimental slope and measuring points.