| Estuarine and coastal regions are characterized by dense human activities with advanced economy and highly utilized resources. The geomorphic evolution of these regions is remarkbly affected by human beings. Building tidal sluices in the vicinity of river mouth could change original riverine and marine dynamic patterns associated with sediment transport and geomorphic evolution tendency. Taking Xinyanggang River as an example, this paper focuses on the sediment dynamic processes in the estuary with tidal sluice.Xinyanggang River is located at Yan Chen City, Jiangsu Province, which is one of the main outlets of Lixia River district drainage system easterly to South Yellow Sea. Being builded in 1957, the tidal sluice was used to control runoff from upstream and prevent salt-water encroachment. At August and December of 2012,I undertook a simultaneous sediment dynamic survey during both spring and neap tides at three mooring stations in the river channel between the tidal sluice and the river mouth. I also collected sediment and water samples. Based on the data from field observation, I analyzed the pattern of water speed and direction, SSC, sanility. I also studied the characteristics of BBL and water stratification, the varying pattern of resuspension flux, the response of suspensed sediment to resuspension.The result shows that the tidal waves deforms after the building of tidal sluice. For example, the duration of ebb tide is about 1.5-3 hours longer than flood tide. The average water speed of flood tides ranges from 0.17 ms-1 to 0.62 ms-1; the average water speed of ebb tides ranges from 0.14 ms-1 to 0.38 ms-1. The average water speed of spring tides and flood tides is larger than neap tides and ebb tides, respectively. During the period of opening tidal sluice, both water speed and SSC are quite large. The fielded data of water speed ranges from 0.49 ms-1 to 1.18 ms-1 and fielded average SSC ranges from 0.21 gL-1 to 1.60 gL-1 The fielded water speed and SSC gradually decrease from the estuary to the upstream.The water sanility (psu) ranges from 12.48 to 24.32 without the affection of freshwater runoff, and there exists sanility gradient during flood and ebb tides. Fresh runoff has great impact on the water sanility in the downstream of tidal sluice, and the fielded data ranges from 0.48 to 5.25 during this period, and this influence becomes more obvious when getting closer to the tidal sluice. The water temperature remains quite stable with no obvious gradient during the observation, the range of temperature of bottom and surface water is less than 1 ℃. The water temperature of summer ranges from 22 ℃ to 32℃ and 3℃ to 9℃ in winter, respectively; and water temperature of flood tides of both summer and winter is higher than ebb tides with the impact of local weather change.By fitting logarithmic velocity profiles, the BBL parameters near the bottom (e.g. friction velocity and roughness length) were obtained.The result shows that the friction velocity rises with the increasing water velocity, and can reach as high as 0.061 ms-1. However, the roughness length decreases when water velocity rises. Vice versa and can reach as high as 120 mm. Taking the mooring station in the middle part of the river(S2) as example,the average friction velocity during flood tide in winter is 0.018 ms-1, the average roughness length is 120 mm, while the friction velocity during the period of freshwater runoff ranges from 0.019 ms-1 to 0.061 ms-1 with an average roughness length of 8.5 mm. The Caculation shows that critical bottom shear stress ranges from 0.095 Nm-2 to 0.11 Nm-2, and bottom sediment would resuspend when bottom shear stress is larger than critical shear stress during the period for drainage, ebb or flood tides.Meanwhile, The resuspension flux rises when the velocity of the runoff and the tide rise, the order of magnitude ranges from 10-4 kgm-2s-1 to 10-3 kgm-2s-1 and resuspension flux can reach as high as 0.0025 or 0.0071 kgm-2s-1. During the other time, resuspension flux is comparatively low, and the order of magnitude is only between 10-5 kgm-2s-1 and 10-4 kgm-2s-1. The resuspension flux has the tendency of increasing when it is closer to the estuary and the average resuspension flux of spring tide is larger than that of neap tide at the aspects of temporal and spatial variations.The water stratification becomes obvious, as there exists certain gradients of temperature,sanility, SSC and water velocity, and the duration of stratification of ebb tide is longer than flood tide. Moreover, Water stratification could restrain vertical exchange of suspended sediment and salt,preventing bottom sediment from resuspending and bring down the erosiveness of water near the sea floor, which could finally lower the SSC and is unfavourable for the desilting of the river channel.The physical conditions of Xinyanggang River channel are suitable for fine sediment flocculation settling, and the period of summer and ebb tide are better due to higher temperature and longer time span. Flocculation settling makes suspended sediment deposit on river floor and makes it hard to be transported into the ocean because of the effect of water stratification, actually, the mentioned factors may contribute to the lower SSC of ebb tides in comparison with flood tides.Results of sediments grainsize analysis show that sediments on the river floor belongs to clayey silt, with mean value ranging from 13.30μm to 23.46μm, proportion of silt ranging from 65.50% to 72.60%, percentage of clay ranging from 14.26% to 19.15% and sand percentage ranging from 8.26% to 19.36%. Percentage of clay declines when it getting closer to the estuary. Most suspended sediments grainsize range from 7 μm to 9μm,average percentage of sand, silt and clay is 1.49%75% and 3.51% respectively. Based on the response of sediments grainsize to sediment resuspension and the relationship between SSC and bottom velocity, grainsize of bottom and middle layers suspended sediment from station S1 and station S3 respond well with sediment resuspension action, while the middle layer suspended sediment of station S2 respond well with sediment resuspension action. This phenomenon shows that most of the suspended sediments from S1 and S3 derived from bottom resuspension, which probably come from advection transport. The suspended sediments grainsize do not getting coarser during the period of opening tidal sluice. The reason may be,the "shielding effect" may prevent deeper erosion of the river floor sediment, or, on the other hand, the sampling height may be higher than the range of altitude of sediment resuspension action, making the gathered sediment mostly comes from advection transport. The water and sediment flux can be used to evaluate the tendency of scouring and silting during different tidal cycles. Our result shows that water flux can keep balanced when the tidal sluice is closed, but the net sediment flux is landward. In general, there are more deposition in the upstream of section S1 and downstream of section S2, the total weight can be 2.34×106 kg and 2.52×106kg respectively (spring tide of winter), and there are less deposition between these two sections. The amount of freshwater runoff and the time span of opening tidal sluice could directly affect the desilting, this usually cause effective scouring in the upstream of section S2.As Xinyanggang is located at South Yellow Sea where the coast is silting each year, with the runoff varies seasonally and the marine dynamics keep relatively steady, this paper points out that desilting should relies on runoff during wet seasons and both runoff and tidal water during dry seasons, and adopts mechanical measurements at channel with serious deposition.There are two key aspects for further study in the future. Firstly, adopting observation instruments in the field to survey the suspended sediments grainsize and settling velocity, quantitative method can be used to study the flocculation and settling and its contribution rate to the variations of SSC. This is important for the time selection of opening the tidal sluice. Secondly, building three dimensional model by combining three dimensional terrain data and sediment dynamic data from field observation, the areas of erosion, deposition and equilibrium in the river channel could be determined. This is helpful for the river administrative department to start mechanical desilting with lower economic cost. |
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