Local Sediment Transport Research Articles

Time development of live-bed scour around an offshore-wind monopile under large current–wave ratio

Scour around a circular monopile in coastal regions has been investigated extensively over the past decades, but the time development of scour depth around an offshore-wind monopile under large current–wave ratio still lacks a predictive model. By considering the conservation of sand volume and adopting the conventional exponential law for temporal variation, a semi-empirical model, which has three parameters, i.e., an equilibrium scour depth, a shape coefficient and a scour characteristic time scale, is developed for predicting the time development of scour depth around a monopile under live-bed conditions of combined wave–current flows. A series of laboratory experiments was conducted in current-only and wave–current flows to obtain data for model calibration and validation. Experimental results indicate that adding weak waves on current accelerates scour development, which is successfully captured by the proposed model through using the far-field bottom shear stress as a key model input. The overall model inaccuracy is within 25%, and the model’s applicability is further confirmed by field measurements from an offshore wind farm in east China. This model can help to determine the timing of installing scour protection around offshore monopiles, especially for the circumstances with very strong local sediment transport (live-bed).

Morphological Modelling to Investigate the Role of External Sediment Sources and Wind and Wave-Induced Flow on Sand Bank Sustainability: An Arklow Bank Case Study

Offshore anthropogenic activities such as the installation of Offshore Renewable Energy (ORE) developments and sediment extraction for marine aggregates have been shown to disrupt current flow, wave propagation, and sediment transport pathways, leading to potential environmental instability. Due to the complexity of the interconnected sediment transport pathways in the south-western Irish Sea combined with an increase in planned anthropogenic activities, the assessment of this risk is imperative for the development of a robust marine spatial plan. Subsequently, this study uses two-dimensional morphological modelling to build upon previous studies to assess the dependency of Arklow Bank’s local sediment transport regime on external sediment sources. Additionally, scenario modelling is used to identify vulnerable areas of this offshore linear sand bank to wind and wave-forcing and to examine the nature of this impact. A sediment budget is estimated for Arklow Bank, whereby seven source and nine sink pathways are identified. New evidence to support the exchange of sediment between offshore sand banks and offshore independent sand wave fields is also provided. The areas of the bank most vulnerable to changes in external sediment sources and the addition of wind- and wave-induced flow are analogous. These high vulnerability zones (HVZs) align with regions of residual cross-flow under pure current conditions. The restriction of sediment sources off the southern extent of Arklow Bank impacts erosion and accretion patterns in the mid- and northern sections of the bank after just one lunar month of simulation. Where tidal current is the primary driver of sand bank morphodynamics, wind- and wave-induced flow is shown to temporarily alter sediment distribution patterns. Wind and wave-induced flow can both accelerate and decelerate the east-west fluctuation of the upper slopes of the bank, yet the nature of this impact is inconsistent due to the misalignment of the directionality of these two forces. The methods and new knowledge derived from this study are directly applicable to tidally-dominated environments outside the Irish Sea.

Impacts of sea-access roads on wetland landscape dynamics in the Yellow River Delta front

The coevolution between oilfield development and deltaic wetlands is critical for ensuring environmental sustainability in mega-deltas. In the Yellow River Delta (YRD), the construction of offshore artificial islands and their sea-access roads (SARs) in delta fronts, providing accommodation for oilfields development of oilfields influences the evolution of adjacent wetland landscapes. This study utilized the threshold segmentation method based on MNDWI and the object-oriented random forest algorithm to extract the shoreline and classify typical wetland vegetation in the study area. It also analysed the impact of SAR on the landscape dynamics of coastal wetlands from 2004 to 2020. The results show that the SAR significantly affected the local sediment transport and erosion and accretion patterns in its adjacent coast, which has gone through accretion-domination, erosion, and continuous accretion stages, with significant accretion and erosion on the north and south sides of the SAR, respectively. From 2004 to 2020, 13.74 km2 of new-born wetland was formed, and the vegetation dynamics in the study area transformed from primary succession to human-disturbed succession. Spartina alterniflora (S. alterniflora) grew rapidly from 4.15 km2 to 18.04 km2 during 2010–2016, showed an imbalanced distribution on both sides of the SAR. This research can provide references for the sustainable coevolution of offshore oilfields and coastal environments.

EFFECT OF LARGE-SCALE FORCING ON THE LOCAL SEDIMENT TRANSPORT POTENTIAL AT THE SCHLESWIG-HOLSTEIN BALTIC SEA COAST

The Schleswig-Holstein (SH) Baltic Sea coast consists of different morphological features (e.g., bars, beaches, dunes, cliffs), which are continuously shaped by various along- and cross-shore sediment transport patterns driven by local currents generated from tides, seiches, winds and waves (Soomere et al., 2012). Understanding local scale sediment transport is of utmost importance to manage the scarce coastal sediments and to identify suitable mitigation strategies against extreme forcing scenarios (Hofstede and Hamann, 2022). This investigation was carried out at the exemplary cliff coast at Stohl, at which the glacial till cliff (maximum height about 20 m) is the source of nearshore sediments (Averes et al., 2021). The main objective of this study is to investigate the relative importance of different forcing conditions on local sediment transport pathways.

Field-based monitoring of instream leaky barrier backwater and storage during storm events

Engineered leaky barriers are increasingly used as natural flood management methods providing ecosystem and water quality benefits in addition to flood attenuation, complementing hard engineering flood defences. Field-based monitoring of a natural flood management site, Wilde Brook in the Corvedale catchment, England (UK) studied the rainfall-runoff relationship for a 5.36 km reach with 105 leaky barriers over two years. Paired pressure transducers were placed upstream and downstream of three channel spanning leaky barriers, allowing evaluation of upstream backwater rise relative to rainfall intensity, storm magnitude, and frequency. By increasing backwater rise, the leaky barriers caused overbank flows, resulting in a reduction in the cross-sectional area velocity after the event. The incidence of overbank flow depended on the local stream cross-sectional profile, barrier properties, location in the reach, and storm magnitude. Barrier operational flow conditions were classified into five modes according to relative bank inundation and barrier submergence extent. The backwater rise magnitude depended on barrier physical properties and evolution over time through cycles of accretion and build-up of brash and leafy material in the barrier, in addition to local bedload sediment transport dynamics, where instances of scour around the barriers were observed. Backwater rise and net volume hydrographs showed rapid filling up behind the barriers on the rising limb and slower water release on the falling limb. For a ∼ 4 yr return period storm event, results indicated that one leaky barrier increased storage volume by up to 102 m3, which translates to an overall net volume increase of ∼ 10,700 m3 for the full reach. These new findings provide quantitative evidence of leaky barrier backwater and storage performance, and leaky barrier design recommendations for storms up to a 4 yr return period. This evidence can be used to develop and validate flood modelling generalised approaches for smaller, more frequent storms, and work towards the development of an approach for modelling leaky barriers for larger storm magnitudes.

Influence of boulder submergence ratio on local flow field: Implications for sediment and fish instream habitat

Boulder submergence ratio may influence surrounding flow structures and as a result, local sediment transport as well as instream habitat preference. In this work, particle image velocimetry (PIV) measurements were used to study the effects of varying submergence ratios in Regimes 2 (1.3 < submergence ratio < 4) and 3 (1 < submergence ratio < 1.3) on the local flow field downstream of a boulder within a rock-ramp boulder array. It was found that in the wake of a boulder, the mean and turbulent flow characteristics for the lowest submergence ratio (Regime 3) varied substantially from the higher ones in Regime 2. From the variations of principal Reynolds shear stress and turbulent bursting events, a higher likelihood of intense downstream erosion was predicted for the lowest submergence ratio. Increased levels of mean velocity and turbulence also indicated that the local flow in the lowest submergence ratio may provide a less preferred habitat for fish. The findings suggest that based on the most common flow stages using a boulder size that generates higher submergence ratios in Regime 2 may improve the performance of stream restoration projects.

Hydrodynamic Processes Controlling Sand Bank Mobility and Long-Term Base Stability: A Case Study of Arklow Bank

Offshore sand banks are an important resource for coastal protection, marine aggregates, and benthic habitats and are the site of many offshore wind farms. Consequently, a comprehensive understanding of the baseline processes controlling sand bank morphodynamics is imperative. This knowledge will aid the development of a long-term robust marine spatial plan and help address the environmental instability arising from anthropogenic activities. This study uses a validated, dynamically coupled, two-dimensional hydrodynamic and sediment transport model to investigate the hydrodynamic processes controlling the highly mobile upper layer of Arklow Bank, while maintaining overall long-term bank base stability. The results reveal a flood and ebb tidal current dominance on the west and east side of the bank, respectively, ultimately generating a large anticlockwise residual current eddy encompassing the entire bank. This residual current flow distributes sediment along the full length of the sand bank. The positioning of multiple off-bank anticlockwise residual current eddies on the edge of this cell is shown to influence east–west fluctuations of the upper slopes of the sand bank and act as a control on long-term stability. These off-bank eddies facilitate this type of movement when the outer flows of adjacent eddies, located on both sides of the bank, flow in a general uniform direction. Whereas they inhibit this east–west movement when the outer flows of adjacent eddies, on either side of the bank, flow in converging directions towards the bank itself. These residual eddies also facilitate sediment transport in and out of the local sediment transport system. Within Arklow Bank’s morphological cell, eight morphodynamically and hydrodynamically unique bank sections or ‘sub-cells’ are identified, whereby a complex morphodynamic–hydrodynamic feedback loop is present. The local east–west fluctuation of the upper slopes of the bank is driven by migratory on-bank stationary and transient clockwise residual eddies and the development of ‘narrow’ residual current cross-flow zones. Together, these processes drive upper slope mobility but maintain long-term bank base stability. This novel understanding of sand bank morphodynamics is applicable to bedforms in tidally dominated continental shelf seas outside the Irish Sea.

Impacts of Sea‐Level Rise on Morphodynamics and Riverine Flooding in an Idealized Estuary

AbstractThis research simulates the morphodynamic evolution of an idealize estuary under four different SLR scenarios of increasing severity to investigate how SLR will influence riverine flooding in estuaries. We find that estuarine response to SLR is influenced by both morphological changes to channel capacity and the associated changes to channel hydrodynamics. Low and moderate SLR scenarios result in an increase in flood extent throughout the estuary relative to the no SLR base case. Surprisingly, more severe SLR scenarios result in decreased flood extent in upstream reaches. This shift is due to penetration of tidal energy and erosion further upstream with greater SLR, which increases channel capacity locally. A periodic pattern of local sediment transport is additionally observed due to SLR, which we attribute to the time response lag between hydrological and morphological response. The finding that increased SLR does not result in increased flood extent everywhere emphasizes that flood mitigation measures need to carefully account for non‐linear responses in the estuarine morphodynamic systems, such as the feedbacks resulting from increased tidal erosion.

Development and Dynamics of Sediment Waves in a Complex Morphological and Tidal Dominant System: Southern Irish Sea

With the recent push for a transition towards a climate-resilient economy, the demand on marine resources is accelerating. For many economic exploits, a comprehensive understanding of environmental parameters underpinning seabed morphodynamics in tidally-dominated shelf seas, and the relationship between local and regional scale sediment transport regimes as an entire system, is imperative. In this paper, high-resolution, time-lapse bathymetry datasets, hydrodynamic numerical modelling outputs and various theoretical parameters are used to describe the morphological characteristics of sediment waves and their spatio-temporal evolution in a hydrodynamically and morphodynamically complex region of the Irish Sea. Analysis reveals sediment waves in a range of sizes (height = 0.1 to 25.7 m, and wavelength = 17 to 983 m), occurring in water depths of 8.2 to 83 mLAT, and migrating at a rate of 1.1 to 79 m/yr. Combined with numerical modelling outputs, a strong divergence of sediment transport pathways from the previously understood predominantly southward flow in the south Irish Sea is revealed, both at offshore sand banks and independent sediment wave assemblages. This evidence supports the presence of a semi-closed circulatory hydrodynamic and sediment transport system at Arklow Bank (an open-shelf linear sand bank). Contrastingly, the Lucifer–Blackwater Bank complex and associated sediment waves are heavily influenced by the interaction between a dominant southward flow and a residual headland eddy, which also exerts a strong influence on the adjacent banner bank. Furthermore, a new sediment transfer system is defined for offshore independent sediment wave assemblages, whereby each sediment wave field is supported by circulatory residual current cells originating from offshore sand banks. These new data and results improve knowledge of seabed morphodynamics in tidally-dominated shelf seas, which has direct implications for offshore renewable developments and long-term marine spatial planning.

The Hummocky Patches and Associated Sediment Dynamics Over an Accretional Intertidal Flat

Tidal flat system is composed of multiple sub-scale geomorphological units. We found a new mesoscale geomorphological unit on sand-mud mixed intertidal zone along China coast, which was defined as hummocky patches. Hummock patches are most developed in middle tidal flat, with horizontal magnitude of 10–20 m and vertical magnitude of ∼15 cm. Hummocky patches significantly influence local sediment transport, thus affect morphological evolution of tidal flats. In order to understand the formation and development mechanisms of hummocky patches, we collected hydrodynamics, topography, sediment properties, and substrate erosion thresholds data through an in-situ field observation covering a spring-neap tidal cycle over an accretional intertidal flat in Jiangsu, China. We found that sediment characteristics including sorting coefficient (σ: measuring the uniformity of sediment particles) and silt/clay/organic matter/water content are essentially different between hummocky patches and nearby seabed, which leads to spatially varied substrate erosion resistance. The measured erosion thresholds for patches are two times higher comparing to surrounding seabed sediment, which provides foundation for the formation of hummocky patches. Under the impact of periodical tidal currents and waves, surrounding seabed experiences considerably more erosion than patch area, which finally develops to hummocky patches. Therefore, hydrodynamic forces drive the formation and development of patches. The erosion resistance of patches decreases vertically from seabed surface to sublayer, causing initial erosion in the lower sublayer, followed by a mass collapse of the seabed surface layer. Hummocky patches are commonly found in middle tidal zone and their sizes decrease to the shore as tidal currents and waves attenuate with shoreward propagation. Hummocky patches have distinct seasonal variations, which only appear in summer and autumn when wind waves are comparatively lower than winter and spring. We infer the reason is that the strong hydrodynamics caused by winter storms cause larger bottom shear stress than the erosion threshold of hummocky patches, leading to destruction of hummocky patches and a smooth tidal flat surface. This study shed new insight on the knowledge of mesoscale geomorphological units and their formation and development in intertidal flats, which provides crucial information for developing more realistic tidal flat sediment transport and morphological models.

Morpho‐sedimentary evolution of a microtidal meandering channel driven by 130 years of natural and anthropogenic modifications of the Venice Lagoon (Italy)

AbstractTidal channels form the pathways for tidal currents to propagate and distribute clastic sediments and nutrients, thus providing a primary control on tidal‐landscape ecomorphodynamics. Most tidal channels in both estuarine and lagoonal environments have a tendency to meander, yet very few studies exist that investigate the full spectrum of processes controlling tidal meander morpho‐sedimentary evolution. The Venice Lagoon (Italy) offers a unique opportunity to shed light on this topic, because a long record of morphological and sedimentary data is available, which allows one to relate tidal channel evolution to the hydrodynamic and morphological changes undergone by the lagoon. In particular, during the last 130 years, feedback between rising relative sea levels and anthropogenic interventions has caused severe modifications of the Lagoon hydro‐ and morphodynamics. Here we investigate how these modifications fed back into the morpho‐sedimentary evolution of a meandering tidal channel located in the northern lagoon. Combining extensive datasets of aerial photographs, topographic and bathymetric surveys, geophysical investigations, sedimentary core analysis, and numerical modelling, we show that enhanced local tidal ranges and water discharges determined adjustments of channel cross‐sectional geometries proportional to increasing tidal prisms, while changes in local tidal asymmetries caused modifications of the local sediment transport regime, resulting in the development of bar–pool patterns according to the dominant tidal phase. Such bar–pool patterns eventually determined channel migration through a bar‐push mechanism controlled by a fluvial‐like, quasi‐linear relationship between local channel curvature and lateral migration rates. Critical differences in sediment transport regime are, however, highlighted between fluvial and tidal meanders the latter being potentially characterized by high concentrations of suspended sediment during periods of slack waters when wind‐driven sediment transport processes are not negligible. This could hamper the formation of high‐relief bedforms, with profound implications for the sedimentology of tidal point‐bar deposits.

Modelling early medieval flood-induced breaching of a coversand ridge in the IJssel valley, Rhine delta, the Netherlands

Connecting geologically mapped data to numerical modelling can help in understanding river landscape evolution. This study focuses on flood-induced breaching of the coversand ridge in the IJssel valley floodplain (Rhine delta, the Netherlands). The development of the breach would explain why this river branch came into existence in early medieval times. Prior to the breaching, the coversand ridge formed a barrier for the Rhine River to discharge towards the north. A palaeoflood model was coupled to a local sediment transport model to predict sediment transport rates in two competing coversand ridge breaches. The results show that the breach at the current IJssel River location was expected to expand earlier and faster in time for various upstream discharge waves and initial breach dimensions. This provides quantitative argumentation for the IJssel river avulsion case to have been triggered by the breaching of the coversand ridge during a large palaeoflood.

Study on Local Sediment Scour and Stress State of Submarine Cables in Offshore Wind Farms

The laying of submarine cables will destroy the local seabed topography, leading to changes in the submarine hydrodynamic boundary, which will cause the local unbalanced sediment transport phenomenon when the submarine cable is buried. When the submarine cable begins to be exposed or the topography changes greatly, the scour phenomenon will be more severe. This article starts from the safety and stability of the submarine cable during the operation and maintenance of the 200MW offshore wind farm in Xuwen Wailuo, Guangdong Province, China and refers to the partial scour of the submarine cable in the “Southern Main Grid and Hainan Power Grid Second Interconnection Project” near the site near the Qiongzhou Strait. In this case, the measured ocean hydrological data at the site is used as the input condition, and Fluent is used to establish a refined hydrodynamic model of the submarine cable scour, to simulate the development process of the submarine cable scour of the wind farm under real sea conditions, and to perform the force analysis after the submarine cable is exposed. These will provide a certain scientific basis for ensuring the safe and stable operation of submarine cables during wind farm operation and maintenance.

Contribution of continuously stable sediment input to the formation of the Pearl River delta since the middle Holocene

Pearl River sediments have been reported to have changed greatly in composition due to the impact of human activities on erosion and landscapes in southern China since 2500 years BP. Here, sediments from four cores (PRD-1, PRD-3, PRD-6 and PRD-7) from the Pearl River Delta (PRD) were collected for high-density clay mineral analyses to determine the vertical variations in clay mineral compositions since the middle Holocene. The results showed that the clay mineral compositions of these four cores remained constant. In the PRD, the clay mineral assemblage dominantly consists of illite and kaolinite (averaging more than 85%), with smaller amounts of chlorite (averaging 10%) and scarce smectite (averaging 1%). The stable clay mineral assemblages in the PRD confirm that the clay mineral compositions of the sediments continuously supplied by the Pearl River have not changed. However, the smectite percentage in core PRD-7 increased by 7% downward from a depth of 12.4 m, which was correlated with the weathering of local volcanic materials and sediment transport in the east of the Pearl River under the influence of the Guangdong coastal current in winter. The stable clay mineral compositions in the core sediments indicate that the Pearl River sediments have accumulated naturally in the PRD instead of under the influence of human activities, which would have altered the compositions of the core sediments.

Turbulent Events around an Intermediately Submerged Boulder under Wake Interference Flow Regime

In-stream boulders significantly affect local flow hydrodynamics, sediment transport, and aquatic habitats. This study experimentally investigates the Reynolds shear stress (RSS) profiles and turbulent events around an intermediately submerged boulder under a wake interference flow regime. The frequency of turbulent events and their contribution to the RSS are quantified at two specific submergence ratios of 1.56 and 1.90 around the boulder in the near-bed and top-boulder regions. Ejection and sweep events were generally dominant in the near-bed and top-boulder regions. The dominance of turbulent events around the boulder varied as the submergence level changed. Relationships are proposed to predict the contribution of the near-bed turbulent events to the RSS around the boulder.

The role of bed-penetrating Kelvin–Helmholtz vortices on local and instantaneous bedload sediment transport

We present results from open-channel, Euler–Lagrange (EL) simulations of turbulent flow over an erodible particle bed at a shear Reynolds number of$Re_{\tau }=180$. Upon space and time averaging, our simulations correctly reproduce the Wong &amp; Parker (ASCE J. Hydraul. Engng, vol. 132, issue 11, 2006, pp. 1159–1168) bedload transport relation (WP). However, local and instantaneous sediment flux shows orders of magnitude scatter around the WP prediction. Visualization of the vortical structures using swirling strength shows the existence of bed-penetrating Kelvin–Helmholtz (KH) vortex packets, which coupled with particle inertia are primarily responsible for the large scatter. The results also show that Euler–Euler (EE) simulations, where the individual sediment grains are not distinguished, are still able to reliably capture the turbulent shear stress variation, however, they do not capture the wide distribution of sediment flux indicative of saltating transport. The KH vortices induce non-zero streamwise and bed-normal velocities at the upper surface of the bed, which must be considered in EE simulations.

Experimental analysis of flow and turbulence in the wake of neighboring emergent vegetation patches with different densities

Patches of vegetation in natural water bodies grow close to each other and may affect each other’s wake pattern with significant implications on nutrient uptake and local sediment transport. This experimental study analyzed the wakes of two neighboring circular patches of emergent artificial vegetation with different densities, with the tested solid volume fractions being equal to 0.059, 0.114, and 0.188. The neighboring patches were positioned in two different configurations, namely side-by-side ( $$L/D=0$$ and $$T/D=1.5$$ ) and staggered ( $$L/D=3.5$$ and $$T/D=1.5$$ ) configurations, with D, L, and T denoting the patch diameter, the patches center-to-center longitudinal distance, and the patches center-to-center transverse distance, respectively. Results show that neighboring patches with different densities generated two distinctly different wakes at the near downstream while after 7–10D these two wakes started merging into one. The flow immediately downstream of a patch was not significantly affected by the presence of a neighboring patch and remained similar to that of an isolated patch, besides the wake of the upstream patch in staggered configuration, which was significantly affected by the downstream patch. The solid volume fraction of the neighboring patch determined the flow velocity and turbulence intensity in between the patches, which were much different compared to measurements at the side of an isolated patch.

An Image-Based Uninterrupted Lagrangian Tracking Method for Measurement of Sediment Motion at Grain Scale

The imaging technique provides an efficient non-intrusive way for studying local sediment transport with a low rate in open-channel flows. It aims to track all sediment trajectories above the background consists of similar particles (i.e., top-view images of the channel). For this area of interest, currently used imaging methods can be summarized as a two-step framework for identifying and matching active bed-load particles. While effective against a simple and clear background, the two-step approach fails to yield accurate and uninterrupted Lagrangian paths for the sediment particles against complex image background consists of similar particles. This study presents a three-step approach to improve the accuracy of the tracking method. The first two steps of this approach based on image subtraction, centroid exaction and Kalman filter entail improvements to those of the classic methods. The third step based on the nearest neighbor algorithm and spline interpolation manage to identify broken chains and connect them to reconstruct uninterrupted Lagrangian paths of the sediment particles. The verification against simulated images and experimental data shows that the improved three-step approach yields more accurate estimation of bed-load kinematics than the classic two-step method.

In-situ measurements of turbulent flow over intertidal natural and degraded oyster reefs in an estuarine lagoon

Oysters are ecosystem engineers that form reefs with rough surfaces. The complex surface of a reference-condition reef contains clusters of live oysters that protrude into and interact with flows. A degraded oyster reef contains fewer live oyster clusters and subsequently has different bed roughness. Such morphological differences may translate to varied hydrodynamic function, with influence to local and larger-scale flow fields and sediment transport, as well as larval recruitment and long-term reef sustainability. The objective of this study is to compare a degraded and a reference-condition intertidal oyster reef, and assess how differences in structure (reef morphology and roughness) influence near-bed hydrodynamic function within the roughness sublayer defined by oysters. Sediment entrainment and bed deformation were observed only on the reference oyster reef, despite that mean velocities were lower than those at the degraded reef. This was attributed to slightly finer bed material and higher turbulent kinetic energy, k, at the reference reef (k = 29.2 ± 4.5 cm2/s2 with mean velocity 11.5 ± 0.6 cm/s at 1 cm above the bed) compared to the degraded reef (k = 22.6 ± 4.2 cm2/s2 with mean velocity 16.1 ± 1.7 cm/s at 1 cm above the bed). At 1 cm above the bed, shear turbulence production at the degraded reef was significantly larger than turbulence dissipation rate, while at the reference reef this pattern was observed only for time periods with low turbulent kinetic energy. For measurements 5 cm above the bed at the reference reef, shear turbulence production and turbulence dissipation rates exhibited a relatively balanced pattern, with dissipation exceeding shear production for the time period with elevated turbulence. Four bed shear stress calculation methodologies converged over the degraded oyster reef but deviated considerably at the reference oyster reef. The drag coefficient associated with bed shear stress at the reference reef was almost two times greater than that at the degraded reef and almost an order of magnitude greater than those observed at sand and mud beds.

Seasonal Variability of Forces Controlling Sedimentation in the Sundarbans National Forest, Bangladesh

Southwest Bangladesh, located on the Ganges-Brahmaputra-Meghna delta, is experiencing the impacts of sea level rise (SLR) due to processes at both the local and global scale. In particular, regional alterations of the hydrodynamic network due to embankment construction has drastically altered effective SLR, placing millions of inhabitants at risk of prolonged inundation, and threatening the world’s largest continuous mangrove stand, the Sundarbans National Forest (SNF). In order to effectively employ landscape recovery solutions, an understanding of local sediment transport and deposition is critical. This field-based study investigates the sediment dynamics between the mangrove platform and tidal channels of the SNF using data from a variety of instruments and sediment samples collected within a forested sub-basin (~20 km2) fed by a major tidal channel. We observe profound seasonal variability within the sub-basin, with the wet season exhibiting deeper and longer inundation of the mangrove platform and greater suspended sediment concentrations (SSC). Further, there exists a trend of decreasing SSC and median grain size from the perimeter of the SNF to the interior, and decreasing SSC from the tidal channel to the platform at both locations. We project seasonal platform sedimentation rates ranging from 0.17 ±0.16 cm in the dry season to 1.8 ± 0.35 cm in the wet season. Importantly, the annual deposition rate measured at either location is sufficiently rapid to keep pace with observed rates of effective SLR published in other studies (~1.0-1.7 cm/yr). Based on our results, it appears that many controls on sedimentation are both covariant and of similar importance to land aggradation in the SNF. While inundation depth and frequency will likely increase under future SLR scenarios, sediment supply is threatened by India’s proposed River Linking Project, which could decrease the sediment loads of the Ganges and Brahmaputra Rivers by as much as 75% and 25%, respectively. These rivers provide the sediment for the entire delta, and we predict that with decreasing SSC, some regions—particularly interior sediment depleted regions—may begin to deteriorate and become submerged, including within the SNF.