Study On Morphodynamic Processes And The Influence Of Vegetation In A High-turbidity Estuarine Tidal Flat

Estuarine tidal flats in the interface zone between land and sea are important nesting areas for migratory birds and benthic animals,and important land resources for coastal cities.Tidal flat is one of the most economically important ecosystems.In recent years,accretion rates of worldwide tidal flats have strongly decreased due to sea-level rise and reduction in sediment supply.A further understanding of tidal flat morphodynamics and its controlling factors is needed for coastal management.The accretion rates of tidal flats are decreasing and a change to erosion in the Yangtze Estuary which is well-known for receiving rich riverine sediment supplies and suffering strong human impacts.Tidal flats are potential for land reclamations and coastal protection in such an economically rapidly developing region(including Shanghai).So there is an urgent need to study the behavior and controls on tidal flat morphodynamics,such as the impacts of sediment fluxes,storms,and human modifications.Moreover,since there are significant spatiotemporal variations in morphological changes in different regions of tidal flats,understanding the morphological changes is essential to the study of tidal flat morphodynamics.The Eastern Chongming tidal flat in the Yangtze Estuary was chosen as the study area.Based on several terrestrial laser scanning(TLS)surveys,we investigated morphological changes in tidal flats at multiple timescales.Hydrodynamics and sediment transport were also studied based on field measurements using tripod observation systems.The main findings and conlusions were summarized as follows:1.We developed a method to use TLS to accurately obtain large-scale morphological data in tidal flats,including effective vegetation filtering.TLS allowed rapid acquisition of high-resolution and large-scale morphological data.Firstly,TLS data could reveal tidal flats well.Even small ripples with a height of1-2 cm and a length of 8-10 cm could also be scanned.Secondly,a combination of several single scanning at different locations made it possible for the coverage of large area in tidal flats.The TLS data,combined with external cylinder targets coordinates from the RTK-GPS surveys,was georeferenced into the China National reference frame.Thirdly,a novel vegetation filtering method was developed and applied to TLS-derived salt marsh morphology.The method can validly filter vegetation by increasing window sizes of morphological filters and thresholds progressively.With its high-resolution morphological data,relatively good portability and different spatial-temporal scales coverage,TLS technology would definitely provide us a good tool to quantify morphological changes of tidal flats over much longer time and larger space scales.2.We quantified spatiotemporal variations of hydrodynamics and sediment transport in the“mudflat-creek-marsh”system in a high-turbidity estuary and explained the mechanism of tidal flat morphodynamics.This study covered the entire“mudflat-creek-marsh”system in the Eastern Chongming tidal flats in the Yangtze Estuary which was famous for its high-turbidity environment.There were significant spatiotemporal variations in hydrodynamics and sediment transport.The results indicated that richer sediment supply in wet seasons played an important role in tidal flat evolution and explained why seasonal variations of morphodynamics occurred.This study clearly revealed that spatiotemporal variations of hydrodynamics and sediment transport in the“mudflat-creek-marsh”system finally led to different feedbacks of morphodynamics in tidal flats.Both surficial and suspended sediments were composed of mud and silt with median diameters smaller than 62?m.In wet seasons,sediments were finer than dry seasons.In dry seasons,the particle size of surficial sediment(21.9?m)differed from suspended sediment(17.7?m)in the tidal creek,indicating suspended sediments were not from resuspension.While in the mudflat,suspended sediments were likely from resuspension of surficial sediments due to the similarity in particle size.The differences of particle size between surficial and suspended sediments were significant,indicating that suspended sediment were likely from adjacent estuary or subtidal zone rather than resuspension in wet seasons.Tidal currents varied in tidal creeks,marsh edges,and mudflats.The inundation time in wet seasons was longer than dry seasons.In tidal creeks and mudflats,flood durations were always longer than ebb durations,while the situation in marsh edges reverses.In dry seasons,the average velocity in marsh edges was 0.51 m/s and larger than tidal creeks and mudflats(both 0.33 m/s).In wet seasons,the average velocity in marsh egdes,mudflats,and creeks was 0.63 m/s,0.57 m/s,and0.22 m/s,respectively.The average suspended sediment concentration(SSC)in wet seasons was larger than dry seasons,which was 1.99 kg/m~3 and 1.54 kg/m~3,respectively.The cross-shore sediment flux in wet seasons was 2.6 times more than dry seasons.3.We found that sediment availability controlled tidal flat morphodynamics.Human activities played a key role in short-term tidal flat morphodynamics.Natural variations and human modifications(e.g.,seasonal riverine fluxes,plant growth,and estuarine environments)could influence morphological changes in estuarine tidal flats.The results in this study suggested that local sediment availability would play a dominant role in the morphodynamic processes in tidal flats.When local sediment availaibility was lower,the entire tidal flat tend to erode.The seasonal variation in riverine input and plant growth led to less erosion in wet seasons compared to dry seasons.While when local sediment availability was higher,the entire tidal flat tended to accret.The seasonal variation led to higher accretion in wet seasons compared to dry seasons.The study area experienced erosion in 2012-2013 and 2013-2014,with a rate of-9.2 cm/yr and-29.4 cm/yr.After a nearby embankment was completed,tidal flats in the study area became depositional,with a rate of 11.6 cm/yr and 22.9 cm/yr in 2014-2015and 2015-2016.During the construction,large amounts of sediment were extracted from adjacent tidal flats and creeks into geotextile tubes,significantly reducing the sediment availability to the intertidal zone and aggravating the erosion in the study area.After the construction,SSC increased remarkably,leading to an increase in sediment availability and sedimentation in the study area.4.We quantified significant short-term morphological change in the study area during the typhoon and the rapid depositional recovery occurred in the tidal flat due to the high-turbidity environment.Vegetation could contribute to the stability of salt marshes during the storms.The results suggested that the typhoon led to significant erosion and short-term(day/month)change in the study area.While rapid depositional recovery occurred within several weeks.During the storm,the study area were eroded and the net change volume and rate were-752 m~3/day and-0.28 cm/day.After the storm,the depositional recovery was rapid in tidal flats with a net change volume 1011 m~3/day.Mudflats experienced severe erosion with a net change volume and rate of-899.61m~3/day and-0.53 cm/day.By contrast,marshes were slightly deposited with a net change volume and rate of 147 m~3/day and-0.14 cm/day.The results indicated that vegetation could contribute to the stability of salt marshes during the storms.After the storm and recovery,most of marshes(80%)and mudflats(98%)accreted.However,the depositional rate of vegetated marshes(0.32 cm/day)was 2.5 times larger than unvegetated mudflats(0.13 cm/day).In summary,this study creatively utilizes TLS and field measurements,to investigate hydrodynamics,sediment transport and morphological changes in tidal flats in a high-turbidity estuary.The results accurately quantify large-scale tidal flat morphological changes and explore spatiotemporal variations of hydrodynamics and sediment transport in the entire“mudflat-creek-marsh”system.This study suggests that local sediment availability plays a dominant role in tidal flat morphodynamics and evaluates the influences that human modifications and extreme weather events have on the tidal flat morphodynamics.This study gives some references for effectively monitoring tidal flat morphological changes.Our results also contribute to a good knowledge of tidal flat morphodynamics in high-turbidity estuaries and offer the complementary information for a better understanding of global coastal morphodynamics.