Morphodynamic signatures derived from daily surface elevation dynamics can explain the morphodynamic development of tidal flats

Understanding the sensitivity of tidal flats to environmental changes is challenging.Currently,most studies rely on process-based models to systematically explain the morphodynamic evolution of tidal flats.In this study,we proposed an alternative empirical approach to explore tidal flat dynamics using statistical indices based on long-term time series of daily surface elevation development.Surface elevation dynamic(SED)indices focus on the magnitude and period of surface elevation changes,while morphodynamic signature(MDS)indices relate sediment dynamics to environmental drivers.The statistical analyses were applied to an intervention site in the Netherlands to determine the effect of recently constructed groynes on the tidal flat.Using these analyses,we were able to(1)detect a reduction in the daily SED and(2)determine that the changes in the daily SED were predominantly caused by the reduction in wave impact between the groynes rather than the reduction in tidal currents.Overall,the presented results showed that the combination of novel statistical indices provides new insights into the trajectories of tidal flats,ecosystem functioning,and sensitivity to physical drivers(wind and tides).Finally,we suggested how the SED and MDS indices may help to explore the future trajectories and climate resilience of intertidal habitats.

Morphodynamic response of an embayed beach to different typhoon events with varying intensities

Beach erosion has occurred globally in recent decades due to frequent and severe storms.Dongsha beach,located in Zhujiajian Island,Zhejiang Province,China,is a typical embayed sandy beach.This study focused on the morphodynamic response of Dongsha beach to typhoon events,based on beach topographies and surficial sediment characteristics acquired before and after four typhoon events with varying intensities.The four typhoons had different effects on the topography and sediment characteristics of Dongsha beach.Typhoons Ampil and Danas caused the largest(-51.72 m3/m)and the smallest erosion(-8.01 m3/m),respectively.Remarkable alongshore patterns of beach profile volumetric changes were found after the four typhoon events,with more erosion in the southern and central parts of the beach and few changes in the northern part.Grain size coarsening and poor sorting were the main sediment patterns on the beach influenced by different typhoons.Typhoons that occurred in the same year after another typhoon enhanced the effect of the previous typhoon on sediment coarsening and sorting variability,but this cumulative effect was not found between typhoons that occurred during different years.A comparison of the collected data revealed that the topographic state of the beach before the typhoon,typhoon characteristics,and tidal conditions were possible reasons for the difference in the responses of Dongsha beach to typhoon events.More severe beach erosion was caused by typhoons with higher intensity levels and longer durations,and high tide levels during typhoons can determine the upper limit of the beach profile erosion site.Taken together,these results can be used to improve beach management for storm prevention.

An investigation of the morphodynamic change of reef islands under monochromatic waves

The persistence and habitability of coral reef islands in future extreme oceanographic conditions has received increasing attention in the recent decade,concerning that the sea level rise(SLR)and more frequent and intense storms in the context of global climate change are expected to destabilize those islands.Here,we conduct a set of wave-flume laboratory experiments focusing on the morphodynamic change of reef islands to varying ocean forcing conditions(wave height and SLR).Subsequently,a phase-resolving XBeach numerical model is adopted to simulate the monochromatic wave process and its associated sediment dynamics.The adopted model is also firstly validated by laboratory experimental results as reported in this study.It is then used to examine the impacts of island morphological factors(island width,island height,island location and island side slope)on the island migration.The combined laboratory/physical and numerical experiment outputs suggest that reef islands can accrete vertically in response to the sea level rise and the increased storminess.

Processes,feedbacks,and morphodynamic evolution of tidal flat-marsh systems:Progress and challenges

Tidal flats and saltmarshes have been a long-standing research focus because of their high socio-economic and ecological values.The evolution of tidal flat-marsh systems is highly complex due to the intertwined processes operating over a variety of spatial and temporal scales.As a traditional research highlight,the role of regular hydrodynamic processes such as tides,waves,and river flows have been explored comprehensively with fruitful outcomes.Over past decades,the changing environment(e.g.,sea level rise,increasing anthropogenic activities,and extreme weather conditions)has attracted more attention with many reported insightful results.More recent advances indicate that biological activities play a critical role in tidal flat-marsh morphodynamics but are still poorly understood.The field of research that connects the bio-logical and physical processes is commonly described as"biogeomorphology"and requires the joint efforts by scientists from multiple dis-ciplines ranging from hydraulics,ecology,and geography to sociology.This review aims to provide a synthesis of the current research status of tidal flat-marsh morphodynamics,with a particular emphasis on the understanding of various processes and feedbacks underlying the devel-opment of morphodynamic models.Some future research needs and challenges are identified to facilitate a more sustainable management strategy for tidal flats and saltmarshes under climate change.

Aggregated morphodynamic modelling of tidal inlets and estuaries

Aggregation is used to represent the real world in a model at an appropriate level of abstraction.We used the convection-diffusion equation to examine the implications of aggregation progressing from a three-dimensional(3D)spatial description to a model representing a system as a single box that exchanges sediment with the adjacent environment.We highlight how all models depend on some forms of parametric closure,which need to be chosen to suit the scale of aggregation adopted in the model.All such models are therefore aggregated and make use of some empirical relationships to deal with sub-scale processes.One such appropriately aggregated model,the model for the aggregated scale morphological interaction between tidal basin and adjacent coast(ASMITA),is examined in more detail and used to illustrate the insight that this level of aggregation can bring to a problem by considering how tidal inlets and estuaries are impacted by sea level rise.

Morphodynamics of sandy beaches under the influence of storm sequences:Current research status and future needs

This paper reviews and discusses the current research status, trends, and future needs in the field of beach morphodynamics under the influence of storm sequences.The paper reviews how the three main research methods, field investigations, numerical modelling, and physical modelling, have been used to study beach morphodynamics during storm sequences.Available quantitative definitions of storm sequences at different sites are presented and discussed.It is shown that the definition of storm sequences is site-specific and requires knowledge of the storm climate, beach characteristics, and the temporal scale of beach recovery.Subsequently, the paper brings together currently available approaches aimed at describing the effect of storm sequences on beach erosion in a general way.The importance of storm chronology and the effects of an extreme storm within a sequence of storms are highlighted.Following that, the more poorly studied aspect of beach recovery in between storms within a sequence is discussed.Three indicators for defining beach recovery, namely the shoreline location, sediment volumes, and the beach state, are identified and compared.Finally, important research needs, including the need for detailed physical modelling, are identified.

Wave-dominated,mesotidal headland-bay beach morphodynamic classsfications of the Shuidong Bay in South China

Beach morphodynamic classifications have achieved extensive acceptance in foreign coastal geomorphological studies. Three beaches located in different zones of a headland-bay coast are classified according to a dimensionless fall parameter, a relative tide range parameter and a dimensionless embayment scaling parameter. Synchronous data, including wave, tide, sediment and beach morphology, are respectively collected from the tangential beach, the transitional beach and the shadow beach of the Shuidong Bay during each spring tide for 16 successive months. The research results indicate that （1） the beach in the tangential zone falls between two major categories which are low tide terrace beaches with rips and barred beaches; the beach in the transitional zone exhibits two main types which are low tide bar/rip beaches and barred dissipative beaches; and the beach in the shadow zone mainly mirrors dissipative states with presence or absence of bars; and （2） the sequential changes and differences of beach states in different coastal zones reflect spatial and temporal variabilities of the headland- bay coast, totally meeting the actual measured beach morphology changes, showing that studies on wave- dominated, meso-macrotidal beaches need to consider the influences of the tides. Meanwhile, the research mainly provides a framework about beach state studies, due to different beach states with different erosion patterns, which requires the need to strengthen the researches in this respect, in order to further enrich theoretical basis for a beach topography evolution, beach morphodynamic processes and beach erosion orevention in China.

The Morphodynamics of Wulan Delta and Its Impacts on the Coastal Community in Wedung Subdistrict, Demak Regency, Indonesia

Wulan Delta in Central Java has been experiencd coastal morphodynamics due to interface between Wulan River’s sedimentation and Java sea’s oceanographic process. This paper aims to analyze Wulan Delta’s morphodynamics as well as its impacts on the socio-economic condition of the community. The morphodynamics is interpreted visually and digitally using a maximum livelihood analysis of multiscale and multi-temporal images. The socio-economic impact is defined by field observations and in-depth interviews to the community and stakeholders. Analysis during the period of 1931-2010 showed that Wulan Delta had been changed morphologically from straight with small arc-like (arcuate) to bird-foot (digitate) with a plausible change into rounded (lobate) in the future, which represented dominant sedimentation by oceanographic rather than fluvial processes. Land expansion in Wulan Delta led to landuse change in Wedung, i.e. expansion of settlement area to 167 ha and fish pond area to 1014 ha in 2000-2010. Most settlements in the area, which are built linearly to river levees, own a high vulnerability to both river flood and tidal inundation. Therefore, dissemination about knowledge of the disaster and coastal zone management, as a means of increasing awareness, becomes necessary in integrating the coastal management in Wulan Delta together with improvement in infrastructure and proper regional development. The proper development strategy for the coastal village is aggressive maintenance strategy, including public infrastructure revitalization, quality improvement for human resources, and silvofishery system. Law enforcement on coastal zoning regulations is necessary to achieve an integrated and sustainable coastal zone management.

Investigation and Discussion on the Beach Morphodynamic Response Under Storm Events Based on A Three-Dimensional Numerical Model

A well-established 3D phase-averaged beach morphodynamic model was applied to investigate the morphodynamics of a typical artificial beach,and a series of discussions were made on the surfzone hydro-sedimentological processes under calm and storm events.Model results revealed that the nearshore wave-induced current presents a significant 3D structure under stormy waves,where the undertow and longshore currents exist simultaneously,forming a spirallike circulation system in the surfzone.Continuous longshore sediment transport would shorten the sediment supply in the cross-shore direction,subsequently suppress the formation of sandbars,showing that a typical recovery profile under calm waves does not necessarily develop,but with a competing process of onshore drift,undertow and longshore currents.Sediment transport rate during storms reaches several hundreds of times as those under calm waves,and two storm events contribute approximately 60%to the beach erosion.Sediment transport pattern under calm waves is mainly bed load,but as the fine sands underneath begin to expose,the contribution of suspended load becomes significant.

The Long-Term Morphodynamic of Barito Delta, Southern Kalimantan, Indonesia

Barito Delta morphodynamic had contradictive role with its potency and problem. Potency in Barito Delta may support the development of Banjarmasin City, but development of Barito Delta may decrease the capacity of transportation in Barito River. Multitemporal topographic map and Landsat satellite image during the period 1862-2008 were used to analyze the long-term delta morphodynamic. The analysis consisted of delta growth, yearly growth, growth orientation, delta shape, and shoreline changes. The research showed that the Barito Delta had developed during the period 1862-2008. Barito Delta had developed to south orientation. The growth of Barito Delta during the period 1862-1946 was 27.82 km^2 or 0.33 km^2/year. However, during the period 1946-1997, Barito Delta growth was 175.82 km^2 or 3.45 km^2/year. Area of Barito Delta decreased during the period 1997-2004. The reduction of Barito Delta area was 4.73 km^2 or 0.67 km^2/year. Area of Barito Delta in years 2004-2008 increased about 3.38 km^2 or 0.84 km^2/year Shoreline during the period 1862-2008 had changed. Accretion occurred in Kuala Lupak River during the period 1862-1997, but erosion occurred during the period 1997-2004. The delta morphodynamics were influenced by human activities in watershed and delta such as landuse change and land degradation.

Immediately downstream effects of Three Gorges Dam on channel sandbars morphodynamics between Yichang-Chenglingji Reach of the Changjiang River, China

Sandbars are of vital ecological and environmental significance, which however, have been intensively influenced by human activities. Morphodynamic processes of sandbars along the Yichang-Chenglingji Reach of the Changjiang River, the channel immediately downstream of the Three Gorges Dam(TGD), are assessed based on remote sensing images between 2000 and 2016. It can be found that the entire area of sandbars reduces drastically by 19.23% from 149.04 km~2 in 2003 to 120.38 km~2 in 2016, accompanied with an increase in water surface width. Owing to differences in sediment grain size and anti-erosion capacity, sandbar area in the upstream sandy gravel reach(Yichang-Dabujie) and downstream sandy reach(Dabujie-Chenglingji) respectively decreases by 45.94%(from 20.79 km~2 to 11.24 km~2) and 14.93%(from 128.30 km~2 to 109.14 km~2). Furtherly, morphological evolutions of sandbars are affected by channel type: in straight-microbend channel, mid-channel sandbars exhibit downstream moving while maintaining the basic profile; in meandering channel, point sandbars show erosion and deposition in convex and concave bank respectively, with mid-channel sandbars distributing sporadically; in bending-branching channel, point sandbars experience erosion and move downstream while mid-channel sandbars show erosion in the head part along with retreating outline. We document that the primary mechanism of sandbars shrinkages along the Yichang-Chenglingji Reach can be attributed to TGD induced suspended sediment concentration decreasing and increasing in unsaturation of sediment carrying capacity. Additionally, channel type can affect the morphological evolution of sandbars. Along the Yichang-Chenglingji Reach, sandbars in straight-microbend channel are more affected by water flow than that in bending-branching channel.

A COUPLED MORPHODYNAMIC MODEL FOR APPLICATIONS INVOLVING WETTING AND DRYING

This work presents a new finite volume Godunov-type model for predicting morphological changes under the rapidly varying flood conditions with wetting and drying. The model solves the coupled shallow water and Exner equations, with the interface fluxes evaluated by an Harten-Lax-van Leer-Contact （HLLC） approximate Riemann solver. Well-balanced solution is achieved using the surface gradient method and wetting and drying are handled by a non-negative reconstruction approach. The new model is validated against several theoretical benchmark tests and promising results are obtained.

Morphodynamic study of reticulate dunes at southeastern fringe of the Tengger Desert

Reticulate dunes are one of the commonest dune types, and yet the least understood. Reticulate dunes at southeastern Tengger Desert are constituted by NE-SW trending primary ridges and nearly vertical secondary ridges. The result of field work studying the morphodynamics and formation mechanism for reticulate dunes in this area shows that the primary ridges were formed by dominant northwest wind and the secondary ridges developed and maintained by alternating dominant wind and subdominant northeast wind on the basis of the primary ridges. Viewed from morphodynamics, reticulate dunes correspond to the complex transverse dune on which the longitudinal element superimposed.

Bank and point bar morphodynamics in the Lower Jingjiang Reach of the Yangtze River in response to the Three Gorges Project

Geomorphic dynamics of alluvial rivers in response to upstream damming have substantial impacts on navigation, habitat protection, and channel stability. The purpose of this study was to determine how flow and sediment regimes, and meander characteristics affect the morphological adjustment of bends in the Lower Jingjiang Reach(LJR) before and after the Three Gorges Project(TGP). Based on detailed field measurements and hydrological and topographic datasets from 1991 to 2016, banks and point bars morphodynamics of 12continuous bends in the LJR were comprehensively analyzed. Point bars in the LJR mainly experienced a net deposition before the TGP operation, but substantially deteriorated with a net erosion rate of 4.6 million m^(3) yr^(-1) in the post-TGP periods(2003–2016), and erosion on heads and upstream margins of point bars was a general adjustment pattern in the 12 bends.The most significant morphological changes of point bars and banks occurred in 2006–2011,indicating a delayed response of the channel evolution of the LJR to damming. Detailed observations suggested that the medium discharges(16,000–18,000 m^(3) s^(-1)) were the most contributive discharges in shaping the morphology of point bars and banks in the LJR after damming. In addition, we revealed the importance of sediment supply on meander deformation of the LJR, driven by sediment exchange over point bars, and more upstream planform deformation tended to occur in bends with high sinuosity(>2.0) in the LJR after damming.The relationship between meander deformation and sinuosity was manifested through the geometric adjustment range of point bars. The morphological adjustments of point bars in the highly curved or compound bends of the LJR were more conducive to cause flow deflections,leading to form concave-bank bars after the TGP operation.

Beach morphodynamic characteristics and classifications on the straight coastal sectors in the west Guangdong

Currently beach morphodynamic classification is the most important foundation to conduct associated coastal geomorphological studies.This paper carried out beach morphodynamic classifications for 12 straight beaches on headland-bay coasts based on field survey and evaluated the applicability of the most widely used dimensional fall velocity parameter(Ω)and relative tidal range parameter(RTR).One reflective,five intermediate and six non-barred dissipative beaches were visually classified and sand size seemed to be a key factor to differentiate these beaches.The studied beaches were in relatively low wave energy environments(Hs<1 m)and the absolute deep-water wave energy level of P0=3 KWm-1 was supposed to a critical threshold to characterize the applicability of theΩand RTR parameters.These two morphodynamic parameters were applicable for the beaches with P0>3 KWm-1 and MSR<2 m.It was found that the model of the traditional winter-and-summer profiles was not applicable in the study area in despite of distinct wave seasonality.The studied beaches were more possible to hover around a limited range due to relatively low background wave environments and variability without considering typhoon impacts,which needs further research on actual breaker wave conditions and beach morphodynamic type responses to typhoon events.

Spatiotemporal variability of channel and bar morphodynamics in the Gorai-Madhumati River,Bangladesh using remote sensing and GIS techniques

Bangladesh is a riparian country that is criss-crossed by the many tributaries and distributaries of the mighty Ganges,Brahmaputra,and Meghna river systems.Gorai-Madhumati,a distributary of the Ganges River is an example where morphological development of the river is associated with frequent channel shifting within the catchment area.The main objective of this research is to quantify the extent of channel migration,erosion-accretion,river width,sinuosity,and charland morpho-dynamics from 1972 to 2018 using geospatial techniques combined with satellite images and hydrological data.The study also addressed the impacts of Farakka Barrage construction in India on the shifting,flow behavior,and siltation of Gorai-Madhumati River.The study shows that bar surface areas have abnormally increased in both segments after 1975 due to the construction of Farakka Barrage.Water flow in the Gorai-Madhumati has dropped remarkably in the downstream and instigated huge sedimentation in this region.Analysis of the time series satellite images revealed that the morphology of the river channel experienced huge changes simultaneously with the changes in the seasonal flow and sedimentation all over the study period.Migration trend has frequently shifted and taken place in the NW and NE direction in the observed sections of the river.Throughout the study period,total amount of accretion was greater than the net percentage of erosion on both banks of the river.River discharge,bar accretion,and erosion history show that the Gorai-Madhumati River will no longer exist with the present flowing condition without attention and proper river management.

Mathematical Development and Verification of a Finite Volume Model for Morphodynamic Flow Applications

The accuracy and efficiency of a class of finite volume methods are investigated for numerical solution of morphodynamic problems in one space dimension.The governing equations consist of two components,namely a hydraulic part described by the shallow water equations and a sediment part described by the Exner equation.Based on different formulations of the morphodynamic equations,we propose a family of three finite volume methods.The numerical fluxes are reconstructed using a modified Roe’s scheme that incorporates,in its reconstruction,the sign of the Jacobian matrix in the morphodynamic system.A well-balanced discretization is used for the treatment of the source terms.The method is wellbalanced,non-oscillatory and suitable for both slow and rapid interactions between hydraulic flow and sediment transport.The obtained results for several morphodynamic problems are considered to be representative,and might be helpful for a fair rating of finite volume solution schemes,particularly in long time computations.

Erosion hotspot identified along the sandy coast of Shanwei: characteristics and origin

Based on the measured beach profile data of Sanzhou Bay from 2015 to 2019,an erosion hotspot was identified along the Shanwei coastline of eastern Guangdong,where the maximum retreat distance of the shoreline exceeded 80 m and the erosion rate was more than 20 m/a.To determine the time at which the erosion hotspot started and the potential causes of its formation,this study used 63 Landsat satellite images from 1986 to 2019 to construct a time series of shoreline positions over the past 30 years by extracting their high-tide shorelines.Next,the M-K trend test method was introduced to evaluate the non-linear shoreline behavior based on the single-transect method.The results showed that the time of approximately 2013 marked the start of the erosion hotspot,the erosion hotspot was characterized by erosion rates of more than 2 m/a(a maximum rate of 31.6 m/a),and the affected shoreline more than 4.3 km from 2013 to 2019.Furthermore,this erosion hotspot was proved to be caused by artificial sand mining in the nearshore zone,which destroyed the original beach’s morphodynamic equilibrium.With the aid of storm events,soil cliffs composed of loose sediment on the backshore were sacrificed to achieve a new equilibrium,resulting in an extremely significant retreat parallel to the coast on the west side of the study area,which reflects the combined effect of human and natural processes.This study provides a concrete example of the rapid response of shorelines to artificial sand mining activities,and the associated finding is a stark warning about the cautious development and utilization of coastal zones and the strict regulation of human activities.

Numerical Investigations on the Transient Behavior of Sand Waves in Beibu Gulf Under Normal and Extreme Sea Conditions

In this study, a morphodynamic numerical model is established with the Regional Ocean Modeling System(ROMS)to investigate the transient behavior of sand waves under realistic sea conditions. The simulation of sand wave evolution comprises two steps: 1) a regional-scale model is configured first to simulate the ocean hydrodynamics, i.e., tides and tidal currents, and 2) the transient behavior of sand waves is simulated in a small computational domain under the time-variant currents extracted from the large model. The evolution of sand waves on the continental shelf in the Beibu Gulf is specifically investigated. The numerical results of the two-year evolution of sand waves under normal sea conditions compare well with the field survey data. The transient behavior of sand waves in individual months shows that the sand waves are more stable in April and October than that in other months, which can be selected as the windows for seabed operations. The effects of sediment properties, including settling velocity, critical shear stress and surface erosion rate, on sand wave evolution are also analyzed. Then, the typhoon-induced currents are further superimposed on the tidal currents as the extreme weather conditions. Sand waves with the average wavelength generally have more active behavior than smaller or larger sand waves. The characteristics of the evolution of sand waves in an individual typhoon process are quite different for different hydrodynamic combinations. For the storm conditions, i.e., the real combination and maximum combination cases, the sand waves experience a significant migration together with a damping in height due to the dominant suspended sediment transport. For the mild conditions, i.e., the pure tidal current and minimum combination cases, the sand waves migrate less, but the heights continue growing due to the dominant bedload transport.

Routing the asteroid surface vehicle with detailed mechanics

The motion of a surface vehicle on/above an irregular object is inv.estigated for a potential interest in the insitu explorations to asteroids of the solar system. A global valid numeric method, including detailed gravity and geo- morphology, is developed to mimic the behaviors of the test particles governed by the orbital equations and surface cou- pling effects. A general discussion on the surface mechanical environment of a specified asteroid, 1620 Geographos, is presented to make a global evaluation of the surface vehicle＇s working conditions. We show the connections between the natural trajectories near the ground and differential features of the asteroid surface, which describes both the good and bad of typical terrains from the viewpoint of vehicles＇ dynamic performances. Monte Carlo simulations are performed to take a further look at the trajectories of particles initializing near the surface. The simulations reveal consistent conclusions with the analysis, i.e., the open- field flat ground and slightly concave basins/valleys are the best choices for the vehicles＇ dynamical security. The dependence of decending trajectories on the releasing height is studied as an application; the results show that the pole direction （where the centrifugal force is zero） is the most stable direction in which the shift of a natural trajectory will be well limited after landing. We present this work as an example for pre-analysis that provides guidance to engineering design of the exploration site and routing the surface vehicles.