| Tidal flat is a significant part of the world’s coast, and the interaction between land and sea in these areas is strong. As an important geographic unit in coastal areas, tidal flat has many service functions, such as ecological function, environment function, revetment function (in engineering sense) and so on. Suspended sediment concentration is one of the important environmental indicators of estuarine and coastal water, and the variations of suspended sediment concentration and suspended sediment transport have important significance in ecology, geomorphology and sedimentary engineering. Due to various hydrodynamic factors (current, waves, water depth, submerged time, etc.) and different combinations of them, changes of suspended sediment concentration and suspended sediment transport processes and their mechanisms in tidal flat are very complex, further research is necessary. Field high-resolution observations under advanced equipment, combined with advanced numerical model of the dynamic mechanism analysis, is not only an important way to deepen the tidal flats and suspended sediment concentration and suspended sediment transport studies, but also the frontier of estuarine and coastal sedimentary dynamics.The study area is an open macro-tidal flat at the front of the Yangtze River delta and the north shore of Hangzhou Bay, with advanced high-resolution instrument wave tide gauges-(SBE-26 plus), coherent pulsed Doppler velocity meter (HR-profiler), optical backscatter turbidity meter-(OBS-3A), optical turbidity lever-(ASM-IV) during September 2014 on-site, water depth, wave height and wave period, the flow rate the flow cross-section, observing suspended sediment concentration profile and so on. Besides, we use Soulsby (2005) Model calculates wave-flow shear stress and sediment transport rate, and then to carry out research in the tidal cycle, spring-neap tide, the concentration of suspended sediment and suspended sediment transport processes and dynamic mechanism of storm events. The main results and conclusions are as follows:1) Suspended sediment concentration and suspended sediment transport during tide cycleIn calm weathers, Vertical average suspened sediment concentration of near-bottom 0.5 m water column generally has three peaks during a tide period, they are the low-level of shallow water at the early stage of flood tide, the high-level between the last stage of flood tide and the early stage of the ebb tide, and the low-level of shallow water at the last stage of ebb tide. Near-bottom high suspended sediment concentration layer appears in the high-level stage.Suspended sediment concentration peak at low-level of shallow water is mainly due to the increase of τcw which caused the resuspension of fine particles on the flat. Suspended sediment concentration peak and near-bottom high suspended sediment concentration layer at high-water period mainly due to the decrease of τcw which cause settlement of upper suspended sediment and collected near the bottom.The range of the components parallel to the shore of the vertical average of instantaneous suspended sediment transport rate is 0～4.80 kg·m-1·s-1, suspended sediment transport direction is into the Bay at flood tide, and outside the bay at ebb tide; the net transport is mainly into the Bay in a whole tide cycle. Components of instantaneous suspended sediment transport rate perpendicular to the shore is 0-0.34 kg·m-1·s-1, indicating suspended sediment transport is on-shore at flood tide, and off-shore at ebb tide; the net transport is mainly on-shore in a whole tide cycle.2) Suspended sediment concentration and suspended sediment transport and their mechanism during spring tide and neap tide cycleUnder calm conditions, the vertical average suspended sediment concentration of near-bottom 0.5 m water layer in spring tide is 2.19 g/1, and 1.67 g/1 in neap tide, the spring tide is 1.3 times higher than the neap tide.The suspended sediment concentration peak at early stage of flood tide and last stage of ebb tide during spring tide is more significantly than that during neap tide. The difference of suspended sediment concentration between flood tide and ebb tide in neap tide is more obviously than that in spring tide. The thickness of near-bottom high suspended sediment concentration is thicker than that of neap tide, ranging from 0.1 m to 0.22 m, lasting longer time than neap tide.The average suspended sediment transport rate of near-bottom 0.5 m water layer during spring tidal is 0.45 kg·m-1·s-1, and during neap tide is 0.2 kg·m-1·s-1, spring tide is 2.2 times faster than that of neap tide. The cumulative amount of suspended sediment transport during spring tide is about 1.8 times more than that of the neap tide. The difference of sediment transport between spring tide and neap tide above is the mainly due to the differences of current velocity (eg. The average current speed at near-bottom 0.5 m from the ground during spring tide (0.25 m/s) is 1.4 times higher than that of neap tide (0.18 m/s)). The difference of sediment transport between spring tide and neap tide is the comprehensive reflection of the different suspended sediment concentration and current velocity, and the difference of flux of suspended sediment transport between spring and neap tide is smaller than that of average suspended sediment transport rate, which is because that the site exposed longer time than neap tide (low tide level of spring tide is lower than low tide level of neap tide).3) The vertical differences of suspended sediment concentrationIn general, instantaneous suspended sediment concentration presents increases downwards; average suspended sediment concentration at near-bottom layer (distance to bottom is 0.09 m) is 5 times higher than that of 1.0 m level. The average suspended sediment concentration of tidal cycle generally approximate "L" shape, which means that the variability of suspended sediment concentration at near-bottom part in vertical is significantly greater than that of upper body of water. Obvious changes different stages of the shape of vertical suspended sediment concentration in a tidal cycle. For example, the vertical difference of suspended sediment concentration at high tide with low current speed is significantly bigger than that during flood-ebb tide with high current speed. The mechanism is that during high tide, upper suspended sediment collected near the bottom, and then re-suspension with the increases of current speed during ebb tide, making the suspended sediment concentration of upper body of water increase.4) The influence of storm to water dynamic、suspended sediment concentration and suspended sediment transportThe tidal average suspended sediment concentration (0.44 m/s) under the strongest impact of "Fung-wong " storm is 2 times higher than that (0.22 m/s) of the similar astronomical tide under calm weather; The tidal average significant wave height (0.8 m) under the strongest impact of "Fung-wong" storm is 3.5 times higher than that (0.23 m) of the similar astronomical tide under calm weather; the maximum wave height (2.06 m) under the strongest impact of" Fung-wong " storm is 4.5 times higher than that (0.46 m) of the similar astronomical tide under calm weather; The tidal average bed shear stress combined with wave and current τcw(0.79 Pa) under the strongest impact of "Fung-wong" storm is 7.9 times higher than that (0.1 Pa) of the similar astronomical tide under calm weather; The tidal maximum τcw (1-98 Pa) under the strongest impact of "Fung-wong" storm is 4.3 times higher than that (0.46 Pa)of the similar astronomical tide under calm weather; The tidal average suspended sediment concentration (3.89 g/1) of near-bottom 1 m water layer under the strongest impact of" Fung-wong " storm is 2.8 times higher than that (1.38 g/1) of the similar astronomical tide under calm weather; The tidal average suspended sediment transport rate (1.64 kg·m-1·s-1) of near-bottom 0.5 m water layer under the strongest impact of" Fung-wong " storm is 3.7 times higher than that (0.44 kg·m-1·s-1) of the similar astronomical tide under calm weather;The shape of averaged suspended sediment concentration profile during the storm surge tide is similar to averaged suspended sediment concentration profile during calm weather, but the distance to the bed at the inflection point of which the suspended sediment concentration rate changes obviously is higher than that of calm weather. The fundamental dynamic mechanism of the dramatic changes in storm events of suspended sediment concentration and suspended sediment transport in costal areas is that, storm transfer enormous energy to coastal waters, thereby significantly increasing the bed shear stress combined with wave and current, resulting in re-suspension of fine sediments. |
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