Large Sandbars Research Articles

Photogrammetric Investigation of Storm-Induced Erosion Process on Sandy Beach Profile in Medium-Scale Flume

In this study, laboratory experiments were conducted to investigate the influence of changes in storm wave height and water level on beach response in a medium-scale wave flume. A schematic storm was simulated (rising, apex, and waning phases). A non-intrusive photogrammetric method was used to collect high-resolution and synchronous data regarding the free surface water elevation and bed level, from which shoreline location, sandbar position, cross-shore sediment transport rates, and nonlinear wave parameters were derived. The cross-shore sediment transport was in agreement with previous laboratory measurements, including the monotonous exchange from foreshore erosion to shoaling zone accretion in most stages of the storm simulation. The surf zone was the main region supplying sediment for beach morphology modification and sandbar generation. The degree of storm erosion was not completely determined by the largest wave height and water level or the cumulative wave power of the apex phase. The largest gradients of the wave parameter sequence change occurred in the rising phase, and this was the main factor generating efficient beachface erosion. It induced an increase in sandbar size, accompanied by the cross-shore motion of maximum velocity amplitude, more violent disturbances of wave nonlinearity, and increased surf zone erosion, with these factors increasing beach instability and leading to more severe storm erosion. The large wave height and water level resulted in shoreline retreat, with a more significant swash zone erosion under a higher runup. The offshore sediment transport turned toward the onshore direction as the original large sandbar deteriorated under the decreasing wave parameter sequence in the waning phase.

Livelihood Changes of Char (Dry Land) Women Due to Involvement in Activities of the Char Livelihood Program

Climatic hazards are common in Bangladesh on delta and riverine island communities, which are large sandbars formed in riverbeds due to silt and alluvium deposition. Specifically, these hazards are caused by the dynamic erosion and accretion of riverbanks. The natural disaster caused by the subsequent sediment buildup on the riverbed and river bank erosion poses a serious threat to the way of life for the residents of Char, particularly the women, by causing damage to or destruction of houses, crops, and family income. The Char Livelihood Program (CLP) attempted to strengthen women's resilience and sense of empowerment while simultaneously improving their livelihoods, nutrition, access to clean water, sanitation, and hygiene habits of char women. This study was mainly undertaken to investigate the extent of changes in the livelihood status of char women due to the involvement in Char Livelihood Program (CLP), explore the factors that may influence the livelihood changes of the char women due to involvement in the CLP. Besides, the problems faced by the char women while working in CLP were also investigated. Data were collected from four villages of Islampur Upazila (sub-district) under Jamalpur district purposively and analyzed using Multiple Linear Regression Models. The goal was to identify the variables that affect the char women's changes in their livelihood. Findings indicated that involvement in char livelihood program had significant influence on the livelihood of char women. The main factor included years of schooling, family size and knowledge of CLP. Collectively, the factors explained 32.3% variation in the livelihood changes of char women. The most influential challenges, from highest to lowest, included unavailability of transport and poor communication status, and lack of commitment of the service provider. Therefore, in order to overcome these obstacles, efforts must be made to ensure seasonal transportation system and enhance road communication infrastructure to enable char women to move around freely in order to carry agricultural products and to connect with relevant stakeholders for better farming and income operations.

Investigating mixing patterns of suspended sediment in a river confluence using high-resolution hyperspectral imagery

In a river confluence, the fluvial process is complex because of the merging of different flows from rivers with distinct morphologies. However, the mixing of suspended sediment in river confluences has been inadequately investigated owing to the low resolution of conventional techniques for measuring suspended sediment. In this study, we investigated the mixing of suspended sediment at the confluence of the Hwang and Nakdong Rivers in South Korea by analyzing the spectral characteristics of suspended sediment from high-resolution hyperspectral data. We retrieved the suspended sediment concentration (SSC) from high-resolution hyperspectral imagery using the conversion technique, which is a cluster-based machine learning regression with optical variability. Hyperspectral clustering was first applied to classify the water regions of the main river (Nakdong) and its tributary (Hwang). The clustering result at a near-field after the confluence point was strongly related to the mixing degree of flow and sediments from two different rivers. Through segmentation, regressors with hyperspectral clusters enabled accurate SSC measurement. In particular, they precisely retrieved the concentrations adjacent to the mixing layer, even when the relationship between the spectral data and suspended sediment concentrations of the main river and tributary was substantially different. Using a detailed SSC map, we compared two cases of mixing patterns by calculating the variation in sediment concentration along the downstream river confluence. We found that the mixing pattern was different because of the flow velocity and a large sandbar near the stagnant area even though river discharge was similar for both cases. In the slow mixing case, the sandbar caused a velocity-deficit with a wake that reduced the variance of SSC due to the irregular flow near the mixing layer. In this case, dual counter-rotating secondary flow cells also limited mixing across the post-confluence reach. However, when the wake and dual counter-rotating secondary flows weakened due to the erosion of the sandbar and lower flow velocity, the transverse mixing coefficient increased by approximately 40 % compared to the slow mixing case. This study demonstrated that the hyperspectral approach can be used to investigate the mixing of suspended sediment in greater detail in complex river systems, as it provides high-resolution information that conventional measurement methods cannot explore.

Morphological evolution of a large sand bar in the Qiantang River Estuary of China since the 1960s

Morphological evolution of a large sand bar in the Qiantang River Estuary of China since the 1960s

Bank erosion in a macrotidal tropical river: Exploring the relative impact of boat wash on riverbank erosion

AbstractMacrotidal tropical rivers are dynamic systems where wet‐season floods and tidal flows cause significant riverbank erosion and sediment transport. This study aimed to explore patterns of riverbank erosion and deposition in a large, tropical, macrotidal river in Northern Australia; the Daly River. In particular, we aimed to determine if recreational boat use was impacting bank erosion in this dynamic river. Erosion pins were installed at multiple levels on both banks at 10 sites along a 34 km reach of the river. Measurements were made every four to six weeks during the low water dry season, and opportunistically during the wet season (flooding period) and seasonal transition periods. A bank geotechnical assessment, riverbed cross‐sections and site bathymetry were undertaken. Whilst the wet season was a period of substantial erosion (mean rate of 0.64 mm day−1), the highest mean erosion rate (3.6 mm day−1) was observed in the early dry season (April to May), a period of stabilizing water levels but greatest boat traffic. Bank erosion at this time was measured on both sides of the river and the inside of meander bends, which is atypical of normal riverine bank erosion patterns, and indicative of erosion due to boat wash. As the dry season progressed, significant spatial differences in erosion rates were evident, where erosion was observed at sites upstream of a large shallow sand bar, while sites downstream from the sand bar gained material through the deposition of tidally transported sediment. This study highlights the importance of understanding the significance and interaction of various erosive factors in tropical tidal rivers and has demonstrated that boat wash may be an important contributor to dry season bank erosion in these systems. We encourage management agencies to consider the role of boats in any future river management program in these systems.

Modes of onshore sandbar migration at a single-barred and swell-dominated beach

Eroded sediment can return to the subaerial beach through onshore sandbar migration, thus understanding this process is key for rebuilding coastal morphology. While offshore sandbar migrations are fairly predictable based on wave conditions alone, landward migrations are less predictable and more dependent on sandbar morphometrics. In this study, monthly collected topographic and bathymetric data from August 2014 to September 2018 were analyzed and related to the incoming wave conditions to investigate the control of sandbar location and morphometry on the landward migration process. Four distinct onshore sandbar migration modes were identified, primarily governed by the sandbar crest location and size (volume and height), and the preceding cumulative wave energy. A cross-shore distance of 150 m equivalent to ~3 m depth was defined as a boundary for the fate of the sandbar during the landward migration process. Sandbar welding (SW; Mode I) occurred during low-energy conditions after a moderately energetic winter (cum Pt ≤ 50 kWm−1) when the sandbar (of up to 1 m height and ~ 100 m3m−1) was located shoreward from the cross-shore boundary. A large sandbar (> 100 m3m−1; > 1 m height) that formed offshore from the boundary during a highly-energetic winter (cum Pt >50 kWm−1) turned into a terrace-bar (STT; Mode II) in a subsequent very low-energy period. A terrace-bar of ~100 m3m−1 located near the boundary (~ 3 m depth) became a sandbar (TST; Mode III) during moderately energetic conditions at the beginning of the winter. Sandbar splitting (SS; mode IV) occurred during low-energy conditions when a sandbar of ~100 m3m−1 situated offshore from the boundary; the outer section flattened and followed a net offshore migration cycle while the inner section migrated shoreward. This study highlights the need for considering sandbar morphometric properties in addition to the preceding wave conditions to adequately predict the shoreward migration process of the sandbar.

Channel and vegetation recovery from dredging of a large river in the Gulf coastal plain, USA

AbstractAnthropogenic impacts in large rivers are widely studied, but studies of recovery once a disturbance has stopped are uncommon. This study examines the biogeomorphic recovery of a 40‐km river corridor on the mid‐Apalachicola River, Florida following the cessation of dredging, disposal, and snag removal in 2002. This failed navigation project resulted in vegetation losses (~166 ha between 1941 and 2004), river widening, and increased point bar areas. We used paired sets of imagery for a 10‐year period during the recovery process at two different flow levels to assess sand bar change, land cover change, and their spatial variations.Most large sand bars decreased significantly in area due to growth of pioneer species, typically from the bankside of the bar. Mean bar area shrank 0.17 and 0.20 ha for the 30th and 1st percentile flows, respectively. For the entire study area, both water‐level comparisons showed gains in vegetation (23.36 and 15.83 ha), compensated by losses in the extent of water (16.83 and 8.55 ha) and sand bar losses (6.53 and 7.28 ha). Overall, these gains during the 10‐year passive recovery period are equivalent to ~15% of the vegetation losses that resulted from the navigational dredging.As found in other studies, most of the pioneer vegetation grew approximately 2 m relative elevation above the low‐water surface. The initial length of the tree line and the area of herbaceous growth both had a significant and positive relationship with the area of new vegetation growth over the study interval. As parts of the river are healing, reduced channel capacity from narrowing and tree growth will benefit the floodplain. As elsewhere, understanding of a river's biogeomorphology, hydrology, and disturbance history can help in selecting appropriate recovery metrics to further advance the understanding and management of disturbed floodplains. © 2020 John Wiley & Sons, Ltd.

Deposition Potential and Flow‐Response Dynamics of Emergent Sandbars in a Braided River

AbstractSandbars are ubiquitous in sandy‐braided rivers throughout the world. In the Great Plains of the United States, recovery and expansion of emergent sandbar habitat (ESH) has been a priority in lowland rivers where the natural extent of sandbars has been degraded. Recovery efforts are aimed at protection of populations of the interior least tern (Sterna antillarum) and piping plover (Charadrius melodus). But quantitative observations of deposition and erosion dynamics of populations of sandbars across long segments of rivers are rare. We present a three‐part case study which used Bayesian regression models to examine relations between hydrology, channel morphology, and ESH responses in the Platte River, eastern Nebraska. Logistic regression indicates presence of ESH is positively related to the Parker, (1976) stability criterion and a gradient in sediment transport mode, and negatively related to presence of vegetation. Hierarchical linear regression modeling shows direct coupling between sandbar top‐surface height and formative flood magnitude, but the gap between formative flood stage and sandbar top‐surface increases with increasing discharge. Finally, linear regression modeling of sandbar erosion demonstrates rates of ESH erosion are on the order of 10−1 ha/day during high‐flow periods and 10−2 during low‐flow periods, but sandbar persistence is largely a function of sandbar starting size. The collective observations highlight the importance of large floods (>3‐year recurrence) in creating very large sandbars that persist as high‐quality ESH over periods of years whereas lower‐magnitude, more‐frequent flood events create lower‐quality ESH that typically does not persist into the following nesting season.

Quantifying effects of water and sediment regulation scheme on the sand bar in the yellow river estuary in 2014

Water and sediment regulation scheme (WSRS) is an annual water conservancy project which aims to alleviate sand depositions and reduce flood risks in the Yellow River of China since 2002. There are few researches on the sand bar within Yellow River tail channel and its estuary from each WSRS effect, though these relative researches are important to be carried out for a better understanding of WSRS. By interpreting two GF-1 satellite images in the end of 2013 and the start of 2015 after carefully controlling some potential bias factors from the change of water level, precipitation or tidal, we compared and analyzed the change of sand bars by one WSRS implemented in 2014 in this paper. Our results indicated this WSRS changed obviously the area/shape of sand bars. It made the area of the river sand bar decreased totally by 5.55 km2, while the estuarine sand bar increased by 2.08 km2. And it achieved the highest scour efficiency (90.11%) in the straight channel, but it achieved the lowest scour efficiency (44.71%) in the more bends channel. From the prospect in the change of estuarine sand bar, the west side area increased, while the east side area shrank. There were some broken small sand bars on the north side and they were forming a large sand bar.

Eolian and fluvial modification of Missouri River sandbars deposited by the 2011 flood, USA

The 2011 flood, an approximately 500-yr flood on the Missouri River, built many large sandbars that have since been modified by eolian and fluvial processes. The ‘59-mile’ segment of the Missouri National Recreational River, located between Yankton, South Dakota and Ponca, Nebraska, USA is a relatively free-flowing stretch of the river downstream of a major dam which has drastically reduced sediment loads since 1955. Critical sandbar habitat has gradually declined on this stretch of the river, presenting challenges to river management. Satellite and LiDAR imagery was used to measure changes in area, volume, and surface characteristics of sandbars over three years since the 2011 flood. The Portable in situ Wind Erosion Lab (PI-SWERL) was used to measure the potential for bare sandbars to emit dust. Our results suggest that eolian processes are important in the early evolution of sandbars, as the wind modifies the surface into dunes and gravel lags. Furthermore, sandbars are important local sources of dust until gravel lags stabilize the surface, limiting further eolian deflation. The lowering of sandbar surfaces by wind erosion makes them more susceptible to being overtopped by high water events. Fluvial erosion along the margins of the sandbars ultimately reduces their area over time. Vegetation growth further stabilizes sandbar surfaces. The rate of eolian and fluvial erosion ultimately depends on the position of the sandbars relative to the thalweg as well as wind direction. New sandbars are rarely built by fluvial processes on a regulated river like the Missouri, and are in a constant state of erosion and modification, ultimately limiting their longevity as quality habitat.

Flood Capacity of Shirakawa River at Tatsudajinnnai Area in Kumamoto Prefecture

In the Shirakawa River, remarkable deformation due to sediment deposition was observed after the river channel improvement work. The sediment deposition was observed at the bend of the river channel. In this research, a model experiment, a quasi-three-dimensional numerical simulation and planer two-dimensional bed variation analysis were conducted in order to evaluate the effect of river bend curvature variation caused by improvement works on channel response and flow capacity. In the model experiment, large sandbars were formed, and the channel become narrower. In the numerical simulation, flood flew over sandbars and gravel deposited on sandbars, which caused decreasing of flow capacity. The gravel deposition was most active when the flow charge was in its peak. There was less inundation at the areas where sediment deposition was occurred.

Sandbars in alluvial channels: their formation processes and interaction with groins

Large sandbars resulted from the instability of loose sedimentary materials are very common in lowland rivers. These, not only, interrupt the inland waterways at low flow, but also make the channels highly unstable forming anabranches, influencing bank erosion, and so on. Groins have key roles to play in such cases. Formation processes of sandbars and their interactions with groins become very urgent to learn for better management of river engineering. RIC-Nays, a two-dimensional model for flow and morphology, is utilized in this study. Computation results reveal that different initial conditions lead to different equilibrium states, and periodic boundary conditions with a small computation domain tend to stabilize multiple bars. Intrusion of groins accelerates the flow in the main channel, which triggers the sediment movement there. Thus the bars move downstream reducing their scale and finally they disappear from the main channel.

Dynamic stratigraphy of composite peripheral unconformity in a foredeep basin

AbstractThis study employs facies analysis and basic principles of sequence stratigraphy to correlate isolated outcrop sections and reveal the depositional history of the Chmielnik Formation – a prominent mid‐Serravalian clastic wedge formed on the basinward forebulge flank of the Polish Carpathian Foredeep. The coarse‐grained clastic wedge, up to 30 m thick and spanningca1·1 Ma within biozoneNN6, consists of fluvio‐deltaic, foreshore and shoreface deposits with a range of large littoral sand bars, all enveloped in muddy offshore‐transition deposits. Its dynamic stratigraphy indicates rapid shoreline shifts and environmental changes due to the interplay of forebulge tectonism, sediment supply and third‐order eustatic cycles. A similar interplay of tectonism and eustasy is recognizable in the whole middle Miocene sedimentary succession deposited on the forebulge flank, demonstrating an extreme case of an accommodation‐controlled shelf and indicating tectonic cycles of the forebulge uplift and subsidence spanningca800 to 900 ka. The episodes of forebulge uplift correlate with the main pulses of orogen thrusting. The resulting composite peripheral unconformity differs markedly from the idealized model of a ‘steady‐state’ stepwise onlap driven by forebulge continuous retreat. It is concluded that the foredeep peripheral unconformities, instead of being simplified in accordance with this idealized model, should rather be studied in detail because they bear a valuable high‐resolution record of regional events and give unique insights into the local role of tectonics, eustasy and sediment supply.

The origin and depositional architecture of Paleogene quartz-glauconite sands in the Lubartów area, eastern Poland.

The study uses quantitative methods to analyse the latest Bartonian to Early Rupelian sedimentary succession at the SE outskirts of the Polish Lowland Paleogene Basin, in the back-bulge zone of the Carpathian orogenorebulge. The vertical lithotype proportion diagrams from a large number of well logs are compiled to reveal the area’s sequence stratigraphy. Six sequences are recognized and correlated with 3rd-order eustatic sea level cycles. The basal sequence of type 1 is overlain by three sequences of type 2 and followed by a fifth sequence of type 1, whose depositional forced-regressive and lowstand systems tracts brought the main volume of quartz-glauconite sand to the study area. The study focuses further on the deposits of this fifth sequence, exposed and surveyed with GPR in the Nowodwor-Piaski sand pit. Their sedimentary facies analysis reveals the local spatial pattern of a wave-dominated and tidally-influenced sedimentation, supporting the earlier notions of a southern palaeoshoreline and a tectonically-controlled sedimentation.The analysis, aided by multidimensional GPR survey, indicates syndepositional development of a tectonic graben filled laterally by fault scarp-attached large sand bars and an axial action of tidal ebb currents. The bars were formed of shore-derived sand swept by littoral waves from the graben footwall areas. As the graben’s tectonic activity ceased, it became buried by the lowstand regressive sands overlain by gravelly foreshore deposits, most of which were later removed by the Pleistocene glacial erosion. A 3D model of the deposits in the Nowodwor-Piaski area is constructed on the basis of outcrop and GPR data with the use of multiple-point statistical methodology to depict the internal architecture, heterogeneity and spatial relationships of main sedimentary facies. The model can serve as a guide for the future exploration and exploitation of the quartz-glauconite sands in the area and as instructive example of how a petroleum reservoir model of a complex sedimentary succession can be constructed with the use of modern statistical methods.

Coastal Flooding Hazard Related to Swell Events in Cartagena de Indias, Colombia

Andrade, C.A.; Thomas, Y.F.; Nicolae Lerma, A.; Durand, P., and Anselme, B., 2013. Coastal flooding hazard related to swell events in Cartagena de Indias, Colombia.In the city of Cartagena de Indias, the urban areas of Bocagrande and Castillogrande were built over large sandbars and are extremely sensitive to the risk of coastal flooding during storm surges. However, the risk of flooding is not directly related to local meteorological conditions, but results essentially from hydrodynamic phenomena due to the arrival of heavy swell from far away. A statistical study based on hourly sea-level observations from 1950 to 2000 showed the existence of overflow levels of 0.29, 0.34, and 0.37 m corresponding to return periods of 10, 50, and 100 years. Numerical modeling of overflows was performed from a reanalysis of atmospheric pressure as well as wind and wave data sets produced by the National Oceanic and Atmospheric Administration. The models used were ADCIRC® for the simulation of tides and wind setup, SWAN® for the simulation of wave setup, and pCOULWAVE® for wave run-up estimation. Coastal flooding that affected the city in January 2010 was validated using a HORUS® video monitoring system. Simulations were conducted on expected extreme sea levels during the 21st century. Flooding simulations were performed using both a light detection and ranging high-resolution digital elevation model and a water propagation model based on a recursive algorithm. Results showed the extreme vulnerability of Bocagrande and Castillogrande areas facing the risk of submersion. They raise questions for future risks of flooding taking into account the rates of sea-level rise as well as the effects of wave erosion.La ville de Carthagène des Indes est un pôle industriel et patrimonial important du littoral caribéen de la Colombie. Les quartiers de Bocagrande et Castillogrande édifiés sur un mince cordon littoral sont extrêmement sensibles à l'aléa submersion marine lors des épisodes de Mar de leva. Toutefois, le risque d'inondation n'est pas lié aux conditions météorologiques locales, mais résulte, pour l'essentiel, de phénomènes hydrodynamiques générés par l'arrivée de houles lointaines. L'étude statistique des chroniques horaires du marégraphe entre les années 1950 et 2000 montre l'existence de niveaux de surcote de 0.29, 0.34 et 0.37 m pour des occurrences de 10, 50 et 100 ans. La simulation numérique des surcotes a été conduite par réanalyse d'un certain nombre d'évènements majeurs grâce aux données de pression, vents et houles fournies par la NOAA et l'utilisation conjointe des modèles ADCIRC, pour la simulation de la marée et du set-up lié au vent, SWAN® pour la simulation du set-up lié aux vagues et pCOULWAVE pour le run-up. Les épisodes de submersion ayant affecté la ville en janvier 2010 ont été validés grâce aux caméras vidéo du système HORUS®. Des scenarii de submersion basés sur l'utilisation d'un modèle numérique de terrain haute résolution et d'un modèle de diffusion de crue ont permis de suivre la propagation des flux d'inondation pour les niveaux marins extrêmes attendus pour le 21ème siècle. Les résultats montrent l'extrême vulnérabilité de la ville face à l'aléa submersion. Ils posent la question du risque futur compte tenu des rythmes d'élévation du niveau marin et de l'impact érosif des vagues.La ciudad de Cartagena de Indias es un polo industrial y patrimonial importante del litoral Caribe colombiano. Los barrios de Bocagrande y Castillogrande, construidos sobre un estrecho cordón litoral, son extremadamente sensibles al riesgo de sumersión durante los episodios de Mar de leva. Las inundaciones no son debidas a sobreniveles vinculados a condiciones meteorológicas locales, si no que resultan esencialmente de fenómenos hidrodinámicos generados por la llegada de olas distantes. El estudio estadístico de las crónicas por hora del mareógrafo de Cartagena entre los años 1950 y 2000 muestra la existencia de sobreniveles de 0.29, 0.34 y 0.37

Tsunami-induced coastal change: scenario studies for Painan, West Sumatra, Indonesia

There exists a high probability of a great earthquake rupture along the subduction megathrust under the Mentawai Islands of West Sumatra in the near future. Six rupture models were used to assess the tsunami inundation and the accompanying sediment movement in Painan, West Sumatra, Indonesia. According to a worst scenario, the potential tsunami might hit the coast of Painan about 26 minutes after the rupture and the entire city could be inundated with a maximum inundation depth of about 7 m. Severe erosion may also occur in the near-shore region. Two scenarios, one scenario with a positive leading wave and the other with a negative leading wave, were selected to simulate the tsunami-induced morphological changes. A positive leading wave would cause severe erosion in the shoreline area and a large sandbar in the offshore area adjacent to the shoreline; a small amount of sediment could be deposited in the city area; a negative leading wave could cause moderate erosion in the further offshore area due to the strong retreating wave front, an offshore sandbar could form in the bay area, while no noticeable large area of sand deposit could be found in the city area. The difference in the erosion and deposition patterns between these two scenarios provides very helpful information in the investigation of historical tsunamis through tsunami deposits.

Modeling the formation of a sand bar within a large funnel-shaped, tide-dominated estuary: Qiantangjiang Estuary, China

The Qiangtangjiang Estuary (the outer part being known as Hangzhou Bay) located on the east coast of China is a large funnel-shaped, tide-dominated and well-mixed estuary. The equilibrium estuarine morphology has been attained and characterized by a large sand bar having a total length of 125km and an elevation of 10m above the average adjacent seabed. In order to investigate the physical processes governing the formation of this morphological feature, two-dimensional depth-averaged process-based morphodynamic modeling (Delft3D) was carried out on a schematized funnel-shaped domain with exponentially decreasing widths based on the dimensions of the Qiangtangjiang Estuary. The model simulated a 6000-year period, the output showing the development of a sand bar that reached equilibrium within about 3000years. The general shape, size and position of the modeled sand bar are consistent with the observations. Short-term simulations of hydrodynamic and sediment transport processes at the initial stage indicate that, in response to the interactions between river discharge and tidal currents, which are strongly influenced by the funnel-shape, the sand bar developed in the transition zone between the river-dominated upper estuary and the flood-dominated lower estuary where sediment transport pathways converge. A series of sensitivity analyses suggest that the estuarine convergence rate, sediment grain size, and river discharge are the main controlling factors of sand bar formation. Similar to other large funnel-shaped, tide-dominated estuaries of the world, a sufficient supply of fine cohesionless sediment (derived from the adjacent Changjiang Estuary), a large river discharge, and a strong shoreline convergence rate have shaped the large sand bar in the Qiangtangjiang Estuary.

Morphodynamics of Sandbar in a Macro-tidal Estuary: A Model Approach

This paper studied the development and the underlying physical mechanisms of the large sandbar in Qiantang estuary, China, using an idealized morphodynamic model. The model coupled shallow water equations, cohesive sediment transport and bed level update equations. The model was given a fixed geometry and impermeable boundaries, and forced by M2 tide at mouth and river discharge at the upstream. The sandbar was reproduced on a timescale of 30 years, and the spatial scales of the sandbar were comparable to the real situation. The formation of the sandbar was controlled by the flood dominance due to tide wave deformation and sufficient sediment supply. The morphological development was driven by the nonlinear interaction between hydrodynamics, sediment transport and bed evolution. This morphodynamic model provides a new tool for the analysis of the large-scale morphological behaviour of tidal sandbar.

Bathymetric change around artificial reefs along Tottori coast

This study deals with bathymetric changes around artificial reefs (AF) which have been constructed along Tottori sand dune coast. Three types of AF groups are investigated applying ARGUS method. In one AF group with a wide alongshore interval, sand bars which are a part of the local alongshore bar system appear between the intervals. In AF group with small intervals, offshore concave sand bars appear between AF intervals inside the AF line. Rip channel appears shore side of each bar and the channel changes the direction toward the center of each AF after it meets with the neighboring channel. A large alongshore sand bar appears from one of the AF interval outside the AF line. It may show that there is a sand movement from inside the AF line.

A large mid-channel sand bar in the macrotidal seaway of outer Asan Bay, Korea: 30 years of morphologic response to anthropogenic impacts

Over the past three decades, dike constructions along the shores and dredging activities for navigation purposes have caused significant sedimentologic/morphologic modifications of a mid-channel bar situated in the tidal seaway of outer Asan Bay, Korea, as shown by a comparison of two sediment datasets collected in the channel and over the bar in 1977 and 2008. In addition, three hydrographic charts published in 1976, 1989, and 2006 were analyzed using GIS to estimate sand volume budgets and any bar migration. The bar is currently about 15 km long and 2–5 km wide, with a relief of 15 m. It is elongated in the tidal flow direction, but asymmetric in cross section. Chart analyses reveal a dramatic reduction in size and volume over the study period. An approximate 19% net loss in total sediment volume was calculated between 1976 and 1989. Since then, a net gain of about 7% has been estimated. The surface areas between selected depth contours reveal a similar pattern, i.e., substantial shrinking until 1989, followed by slight expansion up to 2006. Comparing selected textural parameters (mean grain size, sorting) of a 2008 sediment sampling campaign with those of a 1977 survey, it was found that mean grain sizes have decreased by as much as 1-phi interval, and that the sediments have become more poorly sorted in the vicinity of the bar over the 30-year study period, probably due to a sudden decrease in tidal energy flux after completion of the adjacent dikes. A particular feature is the temporary response of the bar to man-made impacts, characterized by substantial shrinking along its southwestern flank, and slight expansion along its northeastern flank. The observed changes in bar morphology can plausibly be explained by a sudden decrease in sediment supply resulting from a reduction in tidal prism in the wake of diking. The bar initially shrunk, but since 1989 has begun to gradually recover. To compensate the deficit, the bar has lowered its base level by scouring troughs along both flanks.