Sediment Dynamics in Coastal and Marine Environments: Scientific Advances

1. Introduction

Sediment dynamics describe the processes of the formation, distribution, and movement of sediments. Furthermore, sediment dynamics in coastal and marine environments involve sediment erosion, transport, and accumulation in marine systems and their impacts on the natural environment under certain ocean dynamics [1,2,3]. Coastal and marine geohazards are important components of sediment dynamics research. The forms of these geologic disasters are diverse, including certain geological bodies (liquefiable sand layers, underwater deltas, ancient river channels), geological effects (coastal erosion, turbidity currents, earthquakes, sea level rise), etc. [4,5]. A comprehensive understanding of sediment dynamics requires integrated analysis of the joint contributions of oceanographical, geological, hydrological, biological, and chemical processes and their coupling.

The dynamics of sediment in coastal and marine environments are complex and not fully understood, as they are influenced by various environmental factors and human activities occurring at different temporal and spatial scales [6,7]. With the use of innovative methods and techniques, some traditional issues are resolved, and new discoveries are being made. The selected articles in this special issue cover a broad range of topics in sediment dynamics in coastal and marine environments, including studies of erosion-sedimentation, ground subsidence, and buried paleochannels, which are of particular interest in marine geohazards, as well as studies of transport and dispersion of suspended sediments that focus more on interdisciplinary and integrated approaches. We hope that these findings will encourage researchers from different disciplines to collaborate closely to address practical problems related to sediment dynamics in coastal and marine environments.

2. Overview of This Special Issue

This Special Issue consists of 10 papers covering diverse marine sediment dynamics aspects. We summarize these articles using three main themes: (i) rivers and estuaries; (ii) coastal zones and islands; (iii) shallow and deep seas.

2.1. Rivers and Estuaries

Estuaries are regions where there is a high exchange of material and energy between land and sea, typically located at the confluence of rivers and oceans. The complexity of physical and chemical processes in estuaries is largely due to the size of the rivers, tidal action, mixing of salt and fresh water, flocculation and sedimentation of sediments, and frequent action of wind and waves. Li et al. [8,9] used Delft 3D numerical simulation to compare the hydrodynamic effects and sediment dispersal paths in the Yellow River delta (river-controlled type) and Yangtze River delta (tidal type) of the East Asian monsoon region, as well as the Mekong River delta (tidal type). The findings of this study can be used to analyze river sediment distribution and to assess the impact of human activities on changes in river sediments and delta types.

One of the major challenges in studying ancient meanders are the lack of measured data and the incomplete nature of their structures. Yan et al. [10] developed a linked model of meander channel planes and profile structures by examining the structural properties of channel configuration element distribution under different migration patterns and using the 3D channel configuration model system to build planes and profiles. As an example, the Shan 11 sublayer in the Su-x block of the Surig gas field was used to profile the reservoir structure of deep ancient meander channels using tight well data.

The Okavango Delta is a typical distributed river system composed of a network of sand islands, rivers, and swamps. Yan et al. [11] used satellite imagery from Google Earth and the Alaska Satellite Facility to analyze river migration in the Okavango Delta (ASF). A sedimentary phase model of the meandering river single-source distribution river system was proposed based on the features of the distinct zones. This research provides a foundation for future river projections and critical theoretical guidance for studying sedimentation patterns in groundwater reservoirs.

2.2. Coastal Zones and Islands

An artificial structure or natural rocky island placed close to a sandy beach can create a beautiful sandy plane between two headlands or a headland and a control point (such as a grave). However, due to the dynamic nature of sediment movement caused by natural forces, the shoreline shape and its possible dynamic movements can be unpredictable, leading to potential erosion or deposition of the shoreline. Therefore, it is important to consider methods that can simulate the shoreline shape and its possible dynamic movements for practical engineering design. Tao et al. [12] developed a straightforward and generic formula to describe the temporal variation of the coastline in the hooked zone shielded by the headland and in the unhooked zone directly affected by the incident wave, taking into account the balance of nearshore sediment transport. To prove the validity and effectiveness of the suggested evolutionary formula, the authors provided numerical findings and comparisons of shoreline time change between the two headlands.

Ma et al. [13] proposed a large-area, full-coverage deformation monitoring method for assessing island subsidence using InSAR technology. The working principle and distinctive advantages of InSAR data were introduced, and the SBAS InSAR key interpretation processing technique was described in depth. To increase processing efficiency, the GPU-assisted InSAR processing approach was used. InSAR data can be utilized to correctly identify millimeter-scale micro deformation experienced by island groups, confirming the data’s high-precision deformation monitoring capacity.

Jan et al. [14] show the seasonal dynamics of the Trebrevatnet lake complex in Svalbard and the evolution of the suspended sediment in the lake and the sediment plume adjacent to Ekmanfjorden. They used multi-temporal Sentinel-2 images for the period 2016–2021 to derive a normalized difference suspended sediment index (NDSSI) that combines the area extent of the sediment plume with the NDSSI to quantify the sediment influx into the marine environment.

2.3. Shallow and Deep Seas

The existence of nepheloid layers in the world’s oceans plays an important role in the oceanic carbon cycle and the source-sink system of continental margins [15]. Chen et al. [16] collected articles on nepheloid layers published in the last 30 years using the Web of Science Core Collection and analyzed some evolutionary trends related to research areas and publishers. The findings show that Germany works closely with the United Kingdom and the United States. The nepheloid layer research area is primarily centered on the multidisciplinary disciplines of oceanography, earth sciences, and marine biology, with sediment resuspension and the nepheloid layer becoming the primary focus of future research.

Tian et al. [17] explored the characteristics of oceanic nepheloid layers and the mechanism of internal solitary waves that form the middle and bottom layers. Internal solitary waves can cause powerful currents at the bottom, leading to the resuspension of sediments and the formation of the bottom nepheloid layer. The superposition of multiple bottom nepheloid layers and their propagation to the deep ocean form the middle nepheloid layer. The formation process and transport contribution of the nepheloid layer in the world ocean remain unclear and require further investigation.

The development of deep-sea mineral resources is of great importance for the energy needs of countries around the world; however, it can have a significant impact on the marine environment. Fan et al. [18] focused on the effects of deep-sea polymetallic nodule mining on the ecological environment, discussing the distribution characteristics, geoengineering characteristics, and mining methods of deep-sea polymetallic nodules. They also examined the environmental impacts of deep-sea polymetallic nodule mining from the perspectives of sediment resuspension, resuspended sediment transport dispersion, and sediment redeposition. Finally, they summarized the problems and solutions to the environmental impact of deep-sea mining.

3. Outlook

In this Special Issue, the study of sediment dynamics in coastal and marine environments has been explored from various perspectives, such as island ground subsidence, delta erosion and siltation, paleochannel migration characteristics, dynamic shoreline changes, sediment suspension and transport, etc. New methods and techniques have been applied and developed to promote the integration of multiple disciplines; however, there are still many limitations. In the future, we believe that two more directions should be pursued in research.

Firstly, a comprehensive understanding of sediment dynamics requires an integrated analysis of the combined contributions of oceanographic, geological, hydrological, biological, and chemical processes and their interactions. Researchers should strive to enhance interdisciplinary collaboration and use integrated approaches and techniques to address practical problems.

Secondly, there is a lack of research on sediment dynamics in the ocean, especially in the deep sea. We hope that more in-situ observations and investigations on sediment dynamics in the deep sea can be conducted, and at the same time, mature techniques and methods can be applied to sediment dynamics in the coastal and marine environment.

We hope that scientists, engineers, policymakers, business leaders, and private citizens can be engaged in this discussion on sediment dynamics, and the theoretical, and practical research presented in this Special Issue will further the understanding of sediment dynamics.