Sediment Movement Research Articles

ЗАХИСТ МОРСЬКИХ КАНАЛІВ ВІД НАНОСІВ

Seaports of the world, which built in estuaries and bays, are connected to the seas by approach channels. When they crossing shallow water depths (milky water and surf zones), they are subjected to intensive drift by bottom sediments. It is main reason of decreasing of the depths in the channels over time and to reduce the terms between repair dredging works. The wave velocities are the main reason force of movement of bottom sediments. Projections of the velocities of water particles in waves on shallow water are described by various wave theories. Intensity of drifting to deposit due to a number of natural factors: topography of the sea beds; duration and the severity of storms; speeds of currents; types of bottom soils and their physical and mechanical characteristics. In addition, this process is influenced by the design profile of the channels and their planned position to the seaport gates. In the process of engineering studies and scientific support for the design of sea channels, the total volume of sediment flows and their general direction of transit are established, options for the planned location of channels routes, the angles of their slopes as well as options for protecting channels from drifting sediment are considered, taking into account the factors listed above. The analysis of all factors, as well as, the results physical and mathematical modeling allows take optimal design decisions on the location of the routes of sea channels, their dimensions taking into account the specifics of regions. The dimensions of estimated vessels, types of channels and cargo turnover of sea ports in the near future are also taken into account. The main requirements that apply to sea channels in their design are safe navigation of ships, the minimum amount of dredging works, as in the construction process, as well as exploitation. For this reason, the protection of sea channels from drifting by sediments is a very urgent technical problem. This problem was solved on the basis of analysis of existing methods for protection of sea channels in the shallow-water zones of the seas from the bottom sediments by developing a simple and inexpensive method. The mechanism of movement of bottom sediments under the action of wind waves and current was taking into account, when developing this method. The method proposed in this article is based on the principal of operation of a flexible floating breakwater, which patented in the state Ukraine.

Seed‐ versus transplant‐based eelgrass (Zostera marina L.) restoration success in a temperate marine lake

Despite active seagrass restoration gaining traction as a tool to halt and reverse worldwide seagrass losses, overall success remains limited. Restoration strategies, through seeding or transplantation, face different environmental bottlenecks that limit success. Choosing the most appropriate strategy of the two for a specific location, however, is hampered by lack of direct practical comparisons between strategies within a single system. To investigate potential life stage dependent bottlenecks, we compared seed‐based and transplant‐based restoration of Zostera marina in the subtidal saltwater Lake Grevelingen. Our results demonstrate that seedling recruitment was negatively impacted by bioturbation from the lugworm Arenicola marina and sediment movement due to hydrodynamic exposure. Transplant‐based restoration was clearly more successful but surprisingly best predicted by leaf gluing by the ragworm Platynereis dumerilii. This previously undescribed interaction caused seagrass leaves to clump and reduce effective photosynthetic surface and leaf movement. We suggest that the observed behavior of these worms may result from a lack of foodweb interactions, illustrating the importance of trophic control for seagrass restoration. Thus, in addition to recognizing life stage dependent environmental bottlenecks for restoration strategy selection, seagrass restoration may also require the active recovery of their associated food webs.

Application of artificial step-pools in natural hazard mitigation

The step-pool is a representative riverbed structure that is highly effective in increasing flow resistance and dissipating flow energy. Well-designed artificial step-pools can control channel incision, improve riverbed stability, and further mitigate natural hazards. In this study, step-pools following the characteristics of natural step-pools in terms of their geomorphology and energy dissipation are designed and applied to mitigate channel incision and debris flow, in a small watershed where riverbed structures are poorly developed and landslides and massive debris flow have occurred frequently as a result of channel incision. The jamming ratio of the step-pools is mostly less than 6.0, and the SP values, i.e., the development degree of riverbed are over 0.1, and these values are increased significantly compared with the original values, guaranteeing the stability of the constructed steps. Field investigation indicated that the artificial step-pools experienced little adjustment and generally remained stable during the flood season. The constructed steps caused a decreasing trend in deposition downstream, and achieved the expected effects in natural hazard mitigation effects by reducing and transferring the debris flows to hyper-concentrated flows and eventually to normal flood flows. Therefore, the stable step-pools effectively suppressed massive sediment movements, with 69.3% of the loose sediment eroded from the water erosion zone being trapped in the step-pool reach, which promoted uplift of the channel bed. Numerical simulation of debris flows reveals that the maximum kinetic energy of debris flow was reduced by about 27 % with the artificial step-pools compared to the pre-construction condition. The scientific findings and practical applications concerning the use of artificial step-pools could provide guidance for natural hazard mitigation in mountainous areas at the small watershed scale.

CHARACTERISTICS OF BREAKING WAVES USING THE HINDCASTING METHOD ON MAMBORO BEACH, PALU CITY

Introduction: Mamboro Beach is one of the beaches in the northern partof Palu City, Central Sulawesi Province, approximately 14 km from PaluCity. Wave activity, currents, tides, and winds are some of the triggers forsedimentation. Mamboro Beach is prone to sediment movement which canresult in shoreline changes such as erosion which results in the retreat ofthe shoreline (abrasion), or causes siltation which results in theadvancement of the shoreline (accretion). As a result of changes in theshape of the coast, it will affect the hydrodynamic processes of the waters,such as changes in the characteristics of ocean waves. This study aims todetermine the characteristics of the breaking waves, the height and depthof the breaking waves that occur on the Mamboro beach, Palu City usingthe Hindcasting method, the input required is bathymetry data, tides for 15days, wind data for 10 years (past), fetch data. From the results ofHindcasting calculations and analysis of wave transformation using winddata for 10 years (past), it can be concluded that the characteristics of thetype of Breaking Wave that occurred on the Mamboro beach, Palu City, isa surging type where the waves break right on the shore. The calculatedbreaking wave height is 2.0911 meters and the breaking wave depth is1.882 meters.

PARAMETERS OF SEDIMENTS TRANSPORT IN RIVERS WITH A SIDE TYPE OF THE CHANNEL PROCESS

At present, in practice, to model channel reformations in rivers with a deformable bottom, a large number of different formulas are used to calculate the flow of bottom sediments. However, the reliability of channel forecasts performed using the apparatus of mathematical modeling, in most cases, still remains quite low. The quality of channel forecasts performed when designing engineering measures to ensure navigation on rivers is determined, on the one hand, by the correct assessment of the physical nature of sediment transport in natural channel flows, and, on the other hand, by the quality of the applied numerical models. This work is devoted to the verification of the sediment transport model based on studies in hydraulic flumes, in which microforms and mesoforms of a river channel with a secondary type of channel process were simultaneously reproduced. Such problems have not been considered yet. The previous experiments were carried out with a uniform steady-state mode of water movement at one structural level of bottom sediment movement - with the movement of bottom sediments in the form of microforms (dunes) - i. e. on a flat day. The results obtained in the course of the research make it possible to obtain new data, using which it seems possible to improve the quality of modeling of sediment transport in natural channel flows. This line of research, based on the use of the methodology for calculating the discharge of channel sediments using the parameters of dunes, can objectively lead to an increase in the reliability of forecasts of channel reformations when keeping navigation on free and regulated rivers with a moving bottom.

Experimental study and a physical model on the geomorphic response mechanisms of meandering rivers under progressive sediment reduction

As influenced by human activities, including high dam construction, soil and water conservation, and sand mining in the upper reaches of rivers, the amount of sediment entering the lower reaches of the river decreases significantly. Human activities disturb the natural balance between river evolution and water as well as sediment movement and induce new characteristics of river evolution. After the construction and operation of the Danjiangkou Reservoir and the Three Gorges Reservoir of the Yangtze River (China), different degrees of bend bypassing and shoal cutting in the bends downstream of the dams are observed. The reason for this interrelation has not been explained sufficiently. Based on the analysis of prototype observation data and physical model experiments, we have investigated the evolution mechanism of different topographic types of meandering rivers under the condition of decreasing sediment inflow. The data generated by our model experiment document that the sediment concentration of the convex bank flow with a large degree of curvature of the bend is lower than that of the concave bank flow for a reduction of the sediment concentration from 100% to 0%. Moreover, the convex bank of the bend gradually collapses and retreats, whereas the main channel of the river gradually swings, with a swing range of 0–800 m, toward the convex bank. This phenomenon becomes more significant with progressive reduction of the sediment inflow. Combined with theoretical analysis, we interpret that the drastic reduction in the amount of sand that has destroyed the relative natural balance of sediment transport in the riverbanks and river channels in the meandering river section is the main reason for the occurrence of bypassing and shoal cutting in the meandering river section of plain alluvial rivers. Especially during the mid-to high-water period, the capacity of sediment transport in the mainstream belt exceeds the sediment concentration in the water body significantly and is coupled with the change of the runoff process caused by the regulation of the reservoir operation. As a consequence, the river beach is constantly scouring and developing. The evolution is generally characterized by bend bypassing and shoal cutting. The results of our study can be transposed to other alluvial plain rivers and may constitute valuable reference data.

Sediment Yield Modeling and Mapping of the Spatial Distribution of Soil Erosion-Prone Areas

Sediment movement is the most critical problem in Ethiopia, notably in the Ketar River watershed, which is located in the Rift Valley Basin. The Ketar River flows through rugged terrain with steep slopes and high sediment movement. The purpose of the study is to evaluate the Soil and Water Assessment Tool's (SWAT) ability to simulate sediment and to identify areas that are vulnerable to soil erosion. This will aid water resource planners in determining the appropriate corrective action. In SWAT sediment sensitivity analysis, the USLE soil erodibility factor (USLE-K) is found to be the most sensitive sediment parameter. The SWAT model sediment simulation performance is evaluated using the coefficient of determination (R2) and Nash-Sutcliffe Efficiency (NSE). The model performance results in R2 and NSE values of 0.69 and 0.55 for calibration and 0.73 and 0.51 for validation, respectively, using the SWAT Calibration and Uncertainty Program (SWAT-CUP). The sediment-prone area subbasins have steep slopes and were mostly covered by cultivated land. Annual sediment yield from cultivated land was approximately 1872.12 t/y, while yield for moderately cultivated grassland was 171.45 t/y. Woodland and forest land have less soil erosion rate. The subbasin highly covered by Eutric Nitisols is found in very high soil erosion-prone areas. Sediment yield from a slight slope is almost zero, whereas sediment yield from steep and very steep slopes is very high. The slope of the subbasin is an important factor in determining sediment yield, followed by land cover and soil types. The very high sediment yield rate area accounted for 39.64% of the total subbasin and it needs soil conservation planning.

Field testing a High-Frequency acoustic attenuation system for measuring fine suspended sediments and algal movements

Acoustic measurements of suspended sediment have the potential to allow remote, autonomous monitoring of sediment movements at much higher temporal resolution than traditional manual sampling methods. Although suspended sands present a challenging measurement problem due to their logarithmic distribution with depth, fine clay sediments are distributed evenly throughout a stream cross section, making them amenable to point measurements. In order to improve measurement capabilities for fine sediments in stream channels, The National Center for Physical Acoustics at The University of Mississippi has developed a remote, autonomous acoustic system to monitor fine sediments transported in streams. The system was tested on the Middle Rio Grande near San Acacia, New Mexico, and in Goodwin Creek in Panola County, Mississippi. The acoustic instruments were compared to sediment concentrations from physical samples in both deployments. Diurnal patterns were found in the acoustic signals from the Middle Rio Grande, and a follow up experiment at The University of Mississippi Biological Field Station was used to investigate the potential effects of algal biomass on acoustic attenuation measurements. The results showed diurnal patterns in attenuation were associated with patterns in light, temperature, and dissolved oxygen. These results combined with information from the literature indicate diel movement of algal colonies in the water column of some water bodies may interfere with high-frequency acoustic measurements in natural environments and that acoustic methods have the potential to allow ecological researchers to evaluate mass algal movements in the field. Results from Goodwin Creek demonstrate that the acoustic system is able to provide measurements of sediment concentration with high temporal resolution that track well with expected sediment transport patterns in response to discharge hydrographs.

Dredging on water bodies and margin interventions: effects on fish

The urbanization process deeply affects rivers and streams, with numerous impacts, such as the discharge of sewers, dams, and pipework, causing profound changes in the water bodies characteristics and in their biota. In this scenario, the silting of rivers suffers one of the most impactful changes, as it undergoes a reduction in the depth and width of the rivers, triggering physical and chemical changes in the water, as well as in the structure of fish population, its feeding and reproduction habitats. As a palliative measure, it is normal to carry out the desilting (dredging) of rivers, an activity that is also very impacting. Floodings are one of the main factors that demand dredging to be carried out. This review was made to analyze desilting activities, their effects on biota and migratory fish, as well as to evaluate the best management strategies and mitigation of impacts on fish population. The shifting and removal of sediment from the riverbed can cause burial and massive death of eggs and larvae, in addition to interfering in the upward and downward migration of eggs, larvae, and adults of migratory fish. In addition, breeding and feeding sites can be impacted by sediment movement, dredging, and deposition. Some actions minimize the impacts of the silting activity recovering riparian forests, inspect the use of soil on the banks, move urban settlements away, assess the dredging site, consider the spawning sites and reduce the suspension of bottom sediments, as well as choose the best equipment and time for the performance of activities. Therefore, the development of research on the effect of dredging of water bodies on fish would contribute to a better management of the activity.

Three-dimensional simulation of silted-up dam-break flow striking a rigid structure

Different from the clear water dam-break flow, flow patterns and multiphase interactions of the silted-up dam-break flow are always more complicated, hence it may lead to more potential hazards. In this paper, three-dimensional numerical simulations of the silted-up dam-break flow striking a rigid structure are carried out by using a proposed Eulerian-Eulerian multiphase model, coupling kinetic particle theory (KPT) and computational fluid dynamics (CFD). The coupled level set and volume of fluid (CLSVOF) method and an advection-diffusion equation are employed to capture the air-water interface and sediment transport, respectively. Modified momentum equations suitable to the two-fluid method are solved by OpenFOAM, an open-source package based on fixed unstructured mesh. Furthermore, rheology-based constitutive equations of sediment are also taken into account for the simulation of scouring and deposition. The results of the benchmark case agree well with those in published papers, which demonstrates that the proposed model can effectively simulate gravity-dominated collapse flow while considering the multi-interface capturing problems. Subsequently, the Eulerian-Eulerian multiphase model is used to reproduce the problems of three-dimensional silted-up dam-break flow striking a rigid structure, mainly focusing on the aspects of front propagation, violent free surface deformation, sediment movement, dynamic pressure loads and sediment deposition. Four scenarios in the downstream are considered: dry bed, a tailwater sill, a short obstacle, and a vertical cylinder. In addition, the effects of the height of the silted-up sediment on dam-break flow are investigated numerically.

Numerical investigation of sediment transport mechanism under breaking waves by DEM-MPS coupling scheme

Plunging waves generate vortices that suspend significant amounts of sediment in the key sediment transport mechanism in the surf and swash zones of beaches, causing major changes in the beach morphology over time. The intermittency and locality of vortices and sediment transport make it difficult to measure the flow fields and sediment concentrations in laboratory experiments or field investigations, making such studies of limited value for studying the sediment transport at beaches or as a basis for predicting changes in beach morphology. An attempt is made in this study to overcome these shortcomings through a particle-scale numerical investigation of sediment transport mechanisms under breaking waves. The numerical simulation was performed using three-dimensional coupling of the discrete element method (DEM) and moving particle semi-implicit (MPS) method, with the validity of the numerical model confirmed by comparison with experimental results obtained in a small wave flume. The numerical results revealed the intra-wave sediment motion and the contribution of the wave-generated pressure gradient to the sediment transport near the ripple crest. Sediment transport mechanisms was investigated at the spatial scale of the individual sediment grains by associating it with the vortices, turbulence, and exerted forces near the ripple trough. The results revealed that vortices generated at the water surface by plunging waves affected the bed material motions mainly through the action of a large pressure gradient force that arises near the bed surface owing to a local minimum of pressure, which can be associated with a vortex region.

A process-based mesh-distributed watershed model for water runoff and soil erosion simulation

ABSTRACT Runoff and sediment movement induced by storms have serious economic, environmental, and social impacts around the world. Hydrological models have become useful decision support tools for watershed management. The development of process-based distributed models is complex as it involves a range of disciplines and spans multiple spatial and temporal scales. In this study, a process-based and mesh-distributed model, suitable for both event-based and continuous simulations, is developed. The watershed is conceptualized in three distinct zones: a surface overland region, an unsaturated subsurface zone, and a groundwater zone. Overland flow is governed by the 2D diffusive wave equation with an optional 1D channel network solver; water in the unsaturated zone is modelled through mass conservation and vertical dominance of the processes; and saturated groundwater flow is governed by the 2D Dupuit equation. Soil erosion and transport are governed by the multi-size, non-equilibrium equations incorporating soil entrainment, transport, and deposition. The model is driven by meteorological input, land use and soil type properties. The numerical approach is a generalization of existing models to overcome some current modelling shortcomings. New contributions of the model are highlighted and model verification and validation are reported against benchmark cases. The model is then applied to the Cache Creek watershed in California, USA. Satisfactory agreement with the measured data is obtained with both the runoff flow rate and sediment load.

ANALYSIS OF LOW-COST UAV PHOTOGRAMMETRY SOLUTIONS FOR BEACH MODELLING AND MONITORING USING THE OPENSOURCE QUANTUM GIS

Abstract. Coastlines are fundamentally unique features. Their behavioural patterns are predominantly subjects of numerous environmental and engineering studies. With the magnitude of the effects of coastal flooding and erosion, there is a need for accurate techniques for data capture and data processing. With an emphasis on the zero-cost open source GIS software, there is no existing evaluative procedure for demonstrating the analytical capabilities of large-scale UAV-based outputs for microscale analysis for small changes on the beach such as sediment movement, erosion/accretion of individual features. There were four different drone surveys in the study area to determine microscale change over time. A three-stage analysis procedure helps in determining the overview of the coastline and highlights the region(s) of optimum change requiring further spatial analysis with micro-scale change detection. Results obtained show the analytical capabilities of large-scale UAV-based outputs for relatively small but detailed analysis using the open-source QGIS. Results obtained show that spatial analyses of the zoomed areas at different viewpoints and scales improve the confidence level of the hillshading and contour of that particular section on the coastline. The UAV photogrammetry and the three-stage analysis procedure can detect a 1cm change on the beach using the free and open-source QGIS software. It shows the profile modelling of the coastal inundations for both pre and post-flooding events at sub-centimetre intervals can be obtained from QGIS modelling, data computation, analysis, and visualization.

Recent and Historical Background and Current Challenges for Sediment Disaster Measures against Climate Change in Japan

Japan is a country with one of the highest incidences of sediment disasters, which will become more severe and more frequent as a result of climate change. This paper reviews the recent occurrence of sediment disasters caused by heavy rainfall affected by climate change in recent years, challenges in adaptation measures, and recent policies targeting such sediment disasters in Japan. The Ministry of Land, Infrastructure, Transport and Tourism (MLIT) has been conducting non-structural and structural measures based on legislation. Recently, climate change has resulted in more severe and frequent disasters along with damage caused by sediment movement phenomena that are not covered by the present system for warning and evacuation. Efforts to establish assessment methods concerning the risk of these phenomena are shown as examples of current challenges of climate change.

Numerical Simulation of the Locality of Erosional Damages by Storm Waves in Searching for Measures to Conserve Bonggil Beach, Korea

Coastal erosion caused by extreme storms can reduce the value of beaches. Under the scenario of climate change, the storm intensity may increase and the resulting severe erosion can lead to disastrous damages on the beaches. Therefore, it is crucial to find appropriate measures and adaptation plans to conserve the beach from storm attacks. In this study, numerical models were applied to analyze the dune erosion in Boggil Beach, Korea, occurred by Typhoon Tapah in September 2019. Two models were used as Telemac-2D was run in larger domains for producing forcing conditions. XBeach was then applied to simulate the 2019 dune erosion after validation using observational data from a post-event field experiment performed in 2020. The model results showed reasonable agreement with the observational data except for the overestimation of erosion that was likely caused by characteristic pattern of sediment that was a mixture of sand and gravel and the accuracy of model results decreased due to the existence of gravel. The results also confirmed the locality of erosional damage by which the dune erosion was severest in the southern part of the beach. This locality was caused because the water depth was steeper in this area, which kept the wave energy in this area higher than that in the northern part. The uneven distribution of depth was induced by natural and anthropogenic causes. Three cases of model tests were performed to determine an appropriate measure to preserve the beach from future storm attacks – two were to place a submerged breakwater (SB), and one to place a submerged groin(SG). Although the SBs could directly protect the shore from erosion in the lee of the SBs, they could cause additional erosions at unexpected seabed areas. Although the SG was not the best in protecting the beach from the dune erosion, it could minimize the side effect. This measure was also environmentally friendly by keeping the sediments within the coastal cell around the SG so that the beach maintenance could be feasible through replenishment. In addition, the SG could also save the initial cost by reducing its size, and would be more effective, if the recovery process considered, because the SBs would disturb the onshore sediment motions under milder wave conditions. The results of this study can be applied for decision-making to establish future adaptation plans from storm impacts in Bonggil Beach.

Plugging efficiency of pouring aggregates through multiple boreholes into an inundated tunnel to prevent groundwater inrush disasters

This paper presents an experimental investigation of the plugging efficiency of aggregate pouring through multiple boreholes under flowing water conditions into an inundated mine tunnel. Aggregate pouring into an inundated mine tunnel has been widely used and constitutes the premise for the salvage of flooded underground mines through further grouting. However, corresponding in-depth research is relatively limited due to the concealment of underground engineering. A visual experimental setup for aggregate pouring into a tunnel replica was built based on the theory of similarity between sediment movement and slurry pipeline transportation. Four factors, each with four levels, including the aggregate particle size (0.25–0.5, 0.5–1, 1–2 and 2–5 mm), distance between boreholes (0.5, 0.75, 1 and 1.5 m), initial water flow rate (0, 1.5 × 10−2 m/s, 2.4 × 10−2 m/s and 3 × 10−2 m/s) and tunnel inclination (0°, 3°, 5° and 8°) were selected in orthogonal experiments to investigate the plugging efficiency. Range and variance analysis of the four-level orthogonal array experimental results indicated that the factors influencing the plugging efficiency, varying between 83.96 and 98.15%, could be ranked in descending order as the initial water flow rate, aggregate particle size, distance between boreholes and tunnel inclination. The former two factors yielded a more significant influence than that of the latter two factors. The measured water pressure difference ranging from 16 to 32% between the front and back ends of the formed aggregate mass in the pouring process indicated that there remained a high resistance to water flow, even if the aggregate mass was not capped but reached a certain length. Plugging criteria for aggregate pouring into horizontal and inclined tunnels were then proposed. Moreover, the optimal distance between boreholes to form an effective bulkhead was determined, which could be defined as the distance between boreholes when the aggregate mass exhibits the fastest build-up and the plugging capacity is reached.

A new criterion for critical suspension of nonuniform sediment

ABSTRACT As the lower boundary of suspended particles, the critical condition of suspension is an important component in the theory of nonuniform sediment transport and plays a significant role in the numerical computations in the suspended sediment movement. Previous studies on critical suspension are mostly based on Rouse number and Shields number. From the perspective of maximum particle size, this paper conducted research on the critical suspension of nonuniform sediment by flume experiment. A new criterion for critical suspension of nonuniform sediment is proposed according to the concept of maximum particle Reynolds number and analysis of experimental data. Sediments will not be suspended unless the particle Reynolds number is less than or equal to the maximum one related to flow and boundary conditions. A formula, established from experimental data, indicates that the maximum particle Reynolds number is proportional to 3 power of Froude number and 1/5 power of relative roughness. As a new criterion for critical suspension, this formula offers a reasonable description of the physical mechanism for the initial movement of the suspended load. Not only does the proposed formula agree well with laboratory and field data, but it also has its advantages compared with traditional criteria.

Advances in biophysical feedbacks and the resulting stable states in tidal flat systems

<p indent="0mm">As an integral part of the coastal wetland system, tidal flats play an important role in maintaining the health of coastal ecosystems, resisting natural coastal disasters, and blue carbon sequestration. Tidal flats can provide suitable habitats for a variety of organisms; simultaneously, such organisms mediate hydrodynamic and sediment processes, hence affecting the geomorphic evolution and stability of tidal flat systems. In recent years, tidal flat systems have faced the comprehensive impacts of rapid human activities, frequent storm surges, and reduced sediment availabilities, which have aggravated tidal flat erosion. Severe erosion is accompanied by a critical biomass loss, especially under complex biological-physical interactions. Tidal flat degradation is often amplified, threatening the stability of tidal flat systems. The ultimate goal of ecological protection and restoration of tidal flat systems is to establish a self-sustaining method with less artificial assistance. To date, only a few ecological restoration projects in China have considered the integration of biological and physical processes in tidal flats. The most important reason is the lack of understanding of the basic laws of natural construction and operation of the systems. Therefore, understanding and giving way to the self-organization process driven by the biophysical interactions in tidal flat systems have become an important scientific and technical necessity in the ecological protection and restoration of tidal flats. First, we summarized the biophysical effects of salt marsh vegetation and benthic microalgae on the hydrodynamic and sediment movement process of tidal flats. Salt marsh plants and benthic microalgae change the hydrodynamic and sediment processes through wave dissipation and sediment stabilization. Conversely, the changing physical conditions affect the growth and development of salt marsh and benthic microalgae. Second, we qualitatively analyzed the bistability and catastrophic shift of a tidal flat system driven by the salt marsh-sediment and benthic microalgae-sediment feedback, both containing some similarities and differences. Specifically, driven by biophysical feedbacks, both the salt marsh- and benthic microalgae-sediment systems have alternative stable states; with the increased bottom shear stress, both systems shift from a state characterized by high biomass and sediment deposition to that with low biomass and sediment deposition; the bistable range of the salt marsh-sediment system is wider, and the system has a greater hysteresis, indicating that once degraded, it is more difficult for the salt marsh system to be restored to its original state. Third, we discussed the self-organized bio-geomorphic characteristics. The scale-dependent feedback between organisms and physical environments results in spatial patterns. The self-organized patterns have been widely found in a tidal flat ecosystem, for example, circular salt marsh clusters, salt marsh cliffs, and patterned mudflats characterized by the microalgae-covered hummocks alternating with water-filled hollows. Self-organized patterns can improve the primary productivity and sediment deposition of tidal flats, thereby enhancing the stability of ecosystems, which is of great significance for the ecosystem function of tidal flats. Finally, this paper proposed the critical scientific problems that still need to be solved in the future, such as the continuous field observation at the system level, the regulation of biological interspecific interactions under physical conditions, and the quantitative simulation of catastrophic shift thresholds in the tidal flat bio-geomorphic system.

Roughness Effects of Subaquaeous Ripples and Dunes

Numerous questions and problems on Earth and questions with respect to other planets arise from morphodynamic processes caused by sediment movements driven by flows of fluids, such as water, air and other gases. A sediment surface opposes the current with a resistance that is determined by its skin or grain roughness. As soon as sand waves, such as ripples and/or dunes, are formed, these bedforms cause a further resistance to the flow, the so-called form roughness. Dependent on the dimensions of the ripples and dunes, the form roughness can be much more pronounced than the skin roughness. The relevant literature provides a large number of solution approaches based on different basic ideas and different result quality. The aim of this paper is a comparative analysis of solution approaches from the literature. For this purpose, 14 approaches to bedform-related friction in the subaqueous case are evaluated using 637 measurements from laboratory and natural settings. We found that all approaches were significantly more accurate for ripples than for dunes. Since this was equally the case for all approaches tested, it is reasonable to assume that this is caused by measurement inaccuracies for dunes in the natural case rather than due to the approaches themselves. The approach of Engelund 1977 proved to be most accurate among all approaches investigated here. It is based on the Borda–Carnot formulation and an additional empirical term. An analytical derivation and justification is provided for this additional term.

Numerical Simulation on the Evolution of Tailings Pond Dam Failure Based on GDEM Method

Because of the continuous exploitation of metal minerals and the limited availability of land resources, many tailings ponds have been expanded. It is of great importance to dynamically calculate the critical parameters of flooding and sand velocity after a collapse, and quickly determine the dangerous area downstream on reducing the hidden dangers for an accident. In view of the limitations of the single finite element and discrete element methods for the simulation of tailings pond instability, the authors use the finite element and discrete element coupling method (GDEM) for the first time to carry out a dynamical process for numerical simulation of the evolution of a tailings pond collapse. The sediment movement and submerged range were compared with the results of the Volume of Fluid (VOF) method. The comparison shows that the change in drainage flow at the dam foundation position and the depth change in the downstream sensitive point after the dam break are consistent with the VOF calculation results, which indicates that the GDEM can be used as an effective means for the analysis of tailings dam failure. The calculation results of this method can provide technical support for the determination of dangerous areas downstream of tailings ponds and the safety management of key areas.