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The ecological dynamics of tidal flats are subject to continuous transformations attributed to the interplay of the tidal erosion and sedimentation processes. Accordingly, the realization of a dynamic monitoring system for tidal flats holds significant importance in facilitating the judicious utilization and safeguarding of tidal flats resources. In this study, an integrated methodology was implemented to monitor and assess the tidal flats in Jiangsu province. Specifically, a collection of five multispectral images from the Sentinel-2 satellite was acquired, coupled with data derived from the ICESat-2 satellite. Through the application of advanced techniques, including convolutional neural networks and machine learning algorithms, a comprehensive combined model for inverting tidal flats topography was developed. The accuracy disparity between various model combinations was meticulously evaluated, thereby enabling the acquisition of detailed topography information encompassing the entirety of Jiangsu's tidal flats for the year 2021. Comparative analysis against measured topographic data demonstrated that the overall accuracy of the combined model inversion surpassed 80%. Additionally, this study conducted an extensive investigation into the temporal evolution of tidal flats in Jiangsu by integrating data from 2008 to 2021, focusing particularly on the measured topography from 2008. Consequently, the study successfully delineated the various types of tidal flats present in Jiangsu and elucidated their corresponding evolutionary trends. [ABSTRACT FROM AUTHOR]

Ripples on tidal flats significantly influence bedform roughness and near-bed turbulence, yet their dynamics in sandy and muddy environments remain incompletely understood. While laboratory studies have elucidated the effects of mud content and extracellular polymeric substances (EPSs) on ripple formation and stability, the interactions between ripple characteristics, EPS, and mud content in natural settings are more complex and not thoroughly explored. To address this gap, we conducted field studies on sand ripples at the central Jiangsu coast, China, using high-resolution drone photogrammetry to accurately measure ripple dimensions. We performed bedload sampling at both ripple crests and troughs to analyze median grain size, mud content, and EPS concentrations. Our results show that the investigated ripple wavelengths range from 34 to 46 mm, and heights vary between 2.7 and 5.3 mm. Ripples developed near tidal creeks show pronounced asymmetry. Significantly, EPS concentrations are markedly higher at the ripple crests than at the troughs, and it follows a power-law relationship with the median grain size. These results highlight the complex intricate relationship between the ripple morphology, environmental forcing conditions, and EPS content. Our findings enrich the understanding of ripple development and the factors influencing current-dominated ripples in natural environments. This research also adds to the better prediction of the bedform roughness and quantification of the near bed sediment transport. • EPS content at the ripple crest is higher than the trough. • Ripples developed closer to the tidal creek are affected by the flow toward the tidal creek, displaying high asymmetrical pattern. • Ripple dimensions are lower than existing empirical predictions. [ABSTRACT FROM AUTHOR]

Wetland vegetation plays a critical role in the health of coastal ecosystems, serving pivotal roles in disaster mitigation and blue carbon sequestration. While extensive research on the effects of sea-level rise on the aboveground biomass of salt marsh vegetation, the spatial patterns of biomass distribution related to surface elevation remain less explored. This study investigates the growth patterns and spatial distribution of Spartina alterniflora in central coast of China through field surveys and the analysis of remote sensing data via Google Earth Engine. We have found a significant parabolic relationship between vegetation biomass and distance from the sea, with the optimal biomass influenced by the location of the marsh front edge. Further, we observe that biomass distribution varies with surface elevation and is influenced by geomorphological features notably creeks, which introduce significant elevation differences and local biomass fluctuations. These findings offer new insights into the spatial distribution of tidal flat vegetation, which are vital for effective coastal management and for modeling the evolution of tidal flats. • Discovered parabolic spatial distribution of biomass in China's tidal flat. • Unveiled positive biomass-elevation relationship within salt marshes in central China. • Used remote sensing to track Spartina alterniflora's spatial growth patterns. [ABSTRACT FROM AUTHOR]

Understanding sediment sorting and bedding dynamics has high value to unravelling the mechanisms underlying geomorphological, geological, ecological and environmental imprints of tidal wetlands and hence to predicting their future changes. Using the Nanhui tidal flat on the Changjiang (Yangtze) Delta, China, as a reference site, this study establishes a schematized morphodynamic model coupling flow, sediment dynamics and bed level change to explore the processes that govern sediment sorting and bedding phenomena. Model results indicate an overall agreement with field data in terms of tidal current velocities, suspended sediment concentrations (SSCs), deposition thicknesses and sedimentary structures. Depending on the variation of tidal current strength, sand‐dominated layers (SDLs) and mud‐dominated layers (MDLs) tend to form during spring and neap tides, respectively. Thinner tidal couplets are developed during daily scale flood–ebb variations. A larger tidal level variation during a spring–neap tidal cycle, associated with a stronger tidal current variation, favours the formation of SDLs and tidal couplets. A larger boundary sediment supply generally promotes the formation of tidal bedding, though the bedding detail is partially dependent on the SSC composition of different sediment types. Sediment properties, including for example grain size and settling velocity, are also found to influence sediment sorting and bedding characteristics. In particular, finer and coarser sediment respond differently to spring and neap tides. During neap tides, relatively small flow velocities favour the deposition of finer sediment, with limited coarser sediment being transported to the upper tidal flat because of the larger settling velocity. During spring tides, larger flow velocities transport more coarser sediment to the upper tidal flat, accounting for distinct lamination formation. Model results are qualitatively consistent with field observations, but the role of waves, biological processes and alongshore currents needs to be included in further studies to establish a more complete understanding. [ABSTRACT FROM AUTHOR]

Bank retreat in coastal tidal flats plays a primary role on the planimetric shape of tidal creeks and is commonly driven by both flow-induced bank erosion and gravity-induced bank collapse. However, existing modelling studies largely focus on bank erosion and overlook bank collapse. We build a bank retreat model coupling hydrodynamics, bank erosion and bank collapse. To simulate the process of bank collapse, a stress-deformation model is utilized to calculate the stress variation of bank soil after bank erosion, and the Mohr-Coulomb failure criterion is then applied to evaluate the stability of the tidal creek bank. Results show that the bank failure process can be categorized into three stages, i.e., shear failure at the bank toe (stage I), tensile failure on the bank top (stage II), and sectional cracking from the bank top to the toe (stage III). With only bank erosion, the planimetric shapes of tidal creeks are funneled due to the gradually seaward increasing discharge. In contrast to bank erosion, bank collapse is discontinuous, and the contribution of bank collapse to bank retreat can reach 85%, highlighting that the expansion of tidal creeks can be dominated by bank collapse process. The planimetric shapes of tidal creeks are funneled with a much faster expansion rate when bank collapse is considered. Overall, this study makes a further step toward more physical and realistic simulation of bank retreat in estuarine and coastal settings and the developed bank collapse module can be readily included in other morphodynamic models. [ABSTRACT FROM AUTHOR]

ABSTRACT Jin, C.; Gong, Z.; Geng, L.; Zhao, K.; Xu, B.B., and Coco. G., 2018. Subsurface processes in salt marsh of central Jiangsu coast(China). In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 296–300. Coconut Creek (Florida), ISSN 0749-0208. Studies of tidal flats morphodynamics tend to ignore subsurface processes like bioturbation or underground water dynamics so that changes in the surface elevation are always considered to be the result of sedimentation/erosion on the soil surface. However, at times, subsurface variations can be as large as elevation changes driven by sedimentation and so control the evolution trend of surface elevation. In this study, four observation sites were set up on the salt marsh in the central Jiangsu coast (China). A three-year field observation campaign has been carried out studying seabed elevation changes. The Surface Elevation Table-Marker Horizon (SET-MH) technique was applied to measure surface elevation and net sedimentation. The subsurface variation was derived from the difference between the surface elevation and the sedimentation. Results show that the changes in surface elevation are not equal to the sedimentation and that subsurface process, responsible for such deviation, are strongly affected by storm surge. Storm surges increase the underground water content leading to the observed expansion of the subsurface soil. The expansion results in the rapid increase of surface elevation and it takes almost one year to recover to the surface elevation before the storm season. [ABSTRACT FROM AUTHOR]

ABSTRACT Zhang, Q.; Gong, Z.; Zhang, C.K.; Lacy, J.R.; Jaffe, B.E., and Xu, B.B., 2018. Bed Shear Stress Estimation Under Wavy Condition Using near-bottom Measurements: Comparison of Methods. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 241–245. Coconut Creek (Florida), ISSN 0749-0208. Understanding the influence of waves on bed shear stress is critical for predicting morphodynamical behaviours in coastal areas. Near-bed flow was measured on the middle and lower intertidal mudflats along the Jiangsu coast, China, using a three-dimensional acoustic velocimeter that collected a 3.5-cm vertical profile at 1mm resolution and sample rate of 25 Hz. On the lower and middle tidal flats, velocities from ~2.5–6 cmab (cm above bed) and ~0–3 cmab were measured, respectively. Current-induced bed shear stresses were calculated from turbulent kinetic energy (TKE) at the 11th measurement layer (i.e., 5.1 cm below the probe) using wave-turbulence decomposition and from a logarithmic fit to the horizontal mean velocity profile (LP). A wave boundary layer extended from the bed up to 3 cmab when the significant wave height was 0.23 m; when it was present the near-bed mean velocity profile was non-logarithmic. Waves suppress the development of a vertical velocity gradient and lead to an overestimation of bed shear stress when calculated using the log profile assumption. The TKE method is more accurate than the LP method when waves are present and measurements are at least partially within the wave boundary layer. Accurate calculation of current-induced bed shear stress depends on probe height and wave conditions. [ABSTRACT FROM AUTHOR]

ABSTRACT Geng, L.; Gong, Z.; Lanzoni, S., and D'Alpaos, A., 2018. A New Method for Automatic Definition of Tidal Creek Networks. In: Shim, J.-S.; Chun, I., and Lim, H.S. (eds.), Proceedings from the International Coastal Symposium (ICS) 2018 (Busan, Republic of Korea). Journal of Coastal Research, Special Issue No. 85, pp. 156–160. Coconut Creek (Florida), ISSN 0749-0208. Tidal creek delineation is the first step in the analysis of the structure and dynamics of tidal creek networks. This study proposed a new creek definition method, in which we determined the critical elevation of creek delineation by the local elevation distribution. This method can be applied to intertidal zones with inclined bed surfaces, showing a better applicability than existing methods. By comparing creek characteristics within a certain small tidal flat using different delineation methods, we evaluated the accuracy of this new method. The size measured by the new method nicely matches results obtained on the basis of the method proposed by Fagherazzi et al. (1999). [ABSTRACT FROM AUTHOR]

Fifteen monthly field surveys were conducted from September 2012 to November 2013 at ten representative stations along a cross-shore profile, covering the entire tidal flat. Results indicate that tidal currents significantly affect bed level variations over bare flats, while subsurface processes (e.g., soil subsidence and expansion) are likely to play an important role in changing the bed level of the upper intertidal flat where salt marshes are present. The cross-shore profile shows a clear double-convex shape, and different geomorphic zones display distinctive variation. Above the mean high water level (MHWL), the bed level is generally stable. The region around the MHWL, where the upper convex point is present, is a location of high sedimentation due to the weaker hydrodynamic conditions and the settling and scour lag effects, it keeps growing with the increase of inundation frequency. A concave point occurs in the middle part of the intertidal flat, showing considerable erosion. Near the mean low water level (MLWL), the lower convex point is elevated due to the long-shore tidal current and associated sediment transport (the flood dominated transport during summer exceeds the ebb dominated transport during winter, hence the net effect favors sedimentation). Further seawards, the area below the MLWL is strongly eroded. The cross-shore profile follows a “stable-accretional-erosional-accretional-erosional” sequence. Overall, the measurements indicate that the interplay among vegetation, hydrodynamics and sediment transport is critical in shaping the cross-shore morphology of the intertidal flats along the Jiangsu coast of China. [ABSTRACT FROM AUTHOR]

Zhang Q.; Gong Z.; Zhang C.K.; Zhou Z., and Townend I., 2016. Hydraulic and sediment dynamics at times of very shallow water on intertidal mudflats: the contribution of waves. In: Vila-Concejo, A.; Bruce, E.; Kennedy, D.M., and McCarroll, R.J. (eds.), Proceedings of the 14th International Coastal Symposium (Sydney, Australia). Journal of Coastal Research, Special Issue, No. 75, pp. 507-511. Coconut Creek (Florida), ISSN 0749-0208. Intertidal mudflats are often characterized by a special 'very shallow water' environment, with a water depth in the order of 10 cm. High-resolution data including water depth, wave parameters, velocity profiles (within 3-6 cm above the mudflat surface with a vertical resolution as fine as 1 mm), stratified suspended sediment concentration (SSC) and bed deformation over the intertidal mudflat during August 8-10, 2013 were measured using a self-designed measuring system. High-resolution stratified velocities near the bottom under different tide and wave conditions were compared to the Karman-Prandtl model. Results suggest that the vertical structure of velocity profiles within 55 cm above the bed maintain a logarithmic distribution even when waves were relatively large. In addition, bed shear stresses under current and wave alone and combined current-wave conditions were calculated to evaluate the contribution of waves. This suggests that waves do not induce a significant rise in the bed shear stress, but have a pronounced impact on the sediment suspension. This is particularly the case for very shallow flows during both the early-flood period and the late-ebb period. These periods were characterized by 'surges' in velocity and SSC, which are enhanced when waves become larger. Even small waves cannot be overlooked for their contribution to sediment re-suspension and vertical mixing. We conclude that waves strengthen the response of micro-topography deformation due to flow conditions. They enhance the forcing during the 'surges' at times of very shallow water, to sculpt the micro-topography of the bed. [ABSTRACT FROM AUTHOR]

The salt marsh system on the tidal flat is one of the most productively ecological wetlands with high biological productivity and blue carbon sequestration levels. The evolution of salt marshes-bare flats boundaries and the shapes of salt marsh margins are of scientific and engineering prospects due to the capability on indicating the morphodynamic patterns of tidal flats. Previous studies are almost qualitative, so far, quantitative analysis on this issue is scarce. To improve our understandings on the spatio-temporal characteristics of salt marsh margins and the mechanism underpinning, we extracted datasets on Spartina alterniflora using orthoimages derived from the quarterly-surveyed UAV photographs and satellite imagery. Results show that the patch-size distribution corresponded to the power law, indicating the marsh patches are in the state of self-organized criticality and the system is scale-independent. It was observed that the external dynamic conditions (i.e., the sediment erosion and accretion on the tidal flat) imposed negative impacts on the self-organization process. In particular, when the short-term morphological changes were negligible, R2 of the power-law relationship notably increased to nearly 1, indicating the salt marsh patches tended to adhere to a high-order self-organization structure in this process. • The orthoimage generated by UAV is used to identify the patch size of salt marsh. •Patch-size distributions in salt marsh leading edge correspond to power laws. •Patch-size distributions present the self-organizational processes. •Both erosion and deposition impose negative impacts on the self-organization process. [ABSTRACT FROM AUTHOR]

• Model captures main mechanisms underlying sediment sorting and bedding dynamics. • Storms bring coarser sediment to upper flats, forming sand-dominated layers (SDLs). • Stronger and longer storms cause thicker SDLs with more sand fraction. • Storms occurring during spring tide generate thicker SDLs than that during neap. • Impact of clustered storms is more significant than the sum of individual storms. Field studies suggest that storms can considerably affect the morphology, sedimentology and bedding structure of tidal flat wetlands because of their high-energy, while numerical modeling studies unravelling the underlying mechanisms remain rare. With a tidal flat of the Changjiang Delta, China as a reference site, this study explores the role of storms on sediment sorting and bedding behaviors using a biomorphodynamic model that couples hydrodynamics, sediment transport, marsh dynamics and morphological change. Model results indicate that storms can leave clear signatures on tidal flats in both horizontal and vertical sedimentary features, in agreement with field observations. The high bed shear stress induced by storms can initiate the motion of relatively coarse sediment which can be brought shoreward during the flood and settle on the upper tidal flats (or saltmarshes), altering the typically observed cross-shore "landward fining" phenomenon. The storm-entrained coarse sediment on the higher flat is usually hard to be brought offshore during calm weather, and hence leaving an evident sand-dominated layer (SDL) within neighboring mud-dominated layers (MDL). Storms can also destroy tidal rhythmites formed under calm weather. With the increase in storm intensity and duration, the thickness and the sand fraction of the SDL increase accordingly. The SDL thickness generated by a weaker storm during the spring tide is larger than that of a stronger storm during the neap tide. Storm chronology also plays an important role. In particular, the impact of two overlapping or temporally close storms is larger than the sum of the two individual storms. Overall, this study results in a more in-depth mechanistic understanding of storm impacts on tidal flat biomorphodynamics, providing a scientific facet to make sustainable management strategies for coastal wetlands. [ABSTRACT FROM AUTHOR]

The flocculation process of cohesive sediment impacts upon estuaries and tidal flats by affecting the sediment dynamics, modifying the biogeochemical exchanges, and playing an essential role in coastal ecosystems and geomorphologic evolution. To understand the roles of biological activity on flocculation processes in aquaculture areas, here we undertook in situ measurements over a bare tidal flat and a nearby clam aquaculture mudflat on the Jiangsu coast, China. Near-bed in situ floc size, the grain size distribution of suspended particles in seawater, suspended sediment concentration (SSC), and currents were obtained for nine consecutive semidiurnal tidal cycles simultaneously at the two sites. Correlation analysis indicated that the flocculation and its break-up process in this study appeared to be controlled by the variations in SSC and bottom shear stress due to combined wave and current. The floc sizes showed less difference between the two sites under calm conditions. However, the near-bed in situ floc size in the aquaculture mudflat was 23% larger than that in the bare tidal flat in the severe erosion events, suggesting modulation of the flocculation process due to the extracellular polymeric substances (EPS) eroded from the seabed sediments at the aquaculture site, as the hydrodynamics were very similar between the two sites. A higher EPS content was observed in the sediment layer below the surface seabed at the aquaculture site. We conclude that abundant filter feeders alter floc properties and enhance flocculation by excretion of exopolymer particles. • The higher EPS contents in sediment were observedat aquaculture site. • The near-bed in situ floc size in the aquaculture mudflat was much larger. • Negative relationships were found between SSC, turbulent and floc size. [ABSTRACT FROM AUTHOR]