A Study On Characteristics Of Hydrodynamics And Sediment Transport, And Analysis Of Vegetation Effects In Chongming Dongtan, Yangtze Estuary

Chongming Dongtan located in the east of Chongming Island, is biggest, fully developed and typical tidal flat in estuary area. Large intertidal zone, strong sediment transportation and abundant vegetation make Chongming Dongtan a perfect example for studying tidal flat. Field investigations were carried out in Sept. 2005 and Sept. 2006 in Chongming Dongtan. Using the data of tidal current (by ADP, ADV), suspended sediment concentration (SSC by OBS), bed sediment and wave, this paper tries to discuss the sediment transport process in the following four aspects.(1) Tidal currentTidal current correlated to tidal range in the study area was strong. At the outside of bare flat, max velocity was 113 cm/s in spring tide. The average flood current was 36 cm/s and average ebb current was 62 cm/s. In neap tide the average velocity was 1/2～1/3 of that in spring tide. At the outside of bare flat current direction was nearly the same as that in nearby sea, which was reciprocating in north and rotational in south. The velocity decreased form bare flat to vegetation zone. In spring tide average velocity in Scirpus zone (the outer vegetation zone) was 10cm/s, while in Sparina zone that was less than 1 cm/s. At the beginning of flood and end of the ebb the velocity was 1～3 times larger than that in other period because of the accelerating effect in shallow water. During period of high water level in flood and period of fast tide level change rate, velocity rose slightly. When tide reached the middle level, tide level change rate was maximal. At that moment the flood-front was on bare flat with strong turbulence.The near bed velocity profile fit the logarithm law well. More than 78.6% of total profile had the correlation coefficient r>0.95. Calculated friction velocity caused by tidal current (u*_c)) is 0.39～6.8cm/s. Roughness length z₀=0.31cm.(2) Sediment transportThe average SSC was controlled by tide, which had a range of 0.7 to 3.8 kg/m³ and maximum value of 14.2 kg/m³. Spatial distribution of SSC value was complex. Commonly SSC increased form bare flat to edge of vegetation zone, and slightly decreased inside the vegetation zone.There was certain relationship between velocity and SSC. SSC often had peak value (5-14 kg/m³) at the beginning and end of a tidal circle. The flood-front peak was caused by high-speed front water and tidal surge effect. In the middle of ebb period, SSC also had peak value due to rise of velocity and advection of high SSC water from upside of tidal flat. Vegetation zone had a smaller peak (3-5 kg/m³). The bare flat had a good linear regress result while vegetation zone had a bad result. This indicates strong resuspension occurred in bare flat zone and quantity of suspended sediment in vegetation zone relied on advection of tidal current.Bed sediment size got smaller from sea-side to bank-side. D₅₀ value which in bare flat zone was 76μm, decreased rapidly at the edge of mud flat, and almost stayed at the same value in vegetation zone. (3) Effect of waveMost of time during observation the wind speed was not bigger than 10m/s. Effect of wave was below middle level. The max significant wave height at the outside of bare flat is 0.74m. Energy attenuation of wave was not significant when high tide level period. But wave was attenuated rapidly after getting close to vegetation.Due to wave effect, near bed instantaneous velocity showed periodic movement with the similar period as wave. Calculated U*_cw (friction velocity caused by wave and current) was 1.3-5.6 times as large as U*_c (friction velocity caused only by current). The near bed SSC had close relationship with U*_cw as shear stress can induce sediment incipient motion. According to analyzing the SSC trend, a result critical shear stressÏ„_cr=0.48 N/m² was obtained. Base on it, several critical velocity values on bare flat were calculated under different conditions. The incipient velocity was 6.9-8.4 cm/s with presence of normal wave, much smaller than the value 31.8 cm/s under no wave condition.Wave strengthened the sorting effect on bed sediment of tidal current. Because of the wave energy distribution, D₅₀ rapidly dropped at the edge of mud flat.(4) Effect of vegetationThe vegetation have three aspects of effect on sediment transport: slowing current, attenuating wave energy and trapping sediment.Attenuation rate with distance on bare flat was estimated as 10 times above than that in the vegetation zone. Attenuation rate in Scirpus zone was 2-3 times as that in Spartina zone. Vegetation can effectively resist wave propagation towards high flat. Wave height dropped 34% in front of vegetation zone. SSC dropped after water entering vegetation zone, where resuspension was weak and SSC trend was gentle. The decrease range was less than 1/5.Changing sediment transport mode on tidal flat is a more important effect. Vegetation zone formed a new barrier for tidal current. Amount of fine grain size sediment deposited in front of vegetation zone, which helped vegetation grow. In this way vegetation expanded toward sea with growth of tidal flat.