Combined effects of waves and plants on a mud deposition event at a mudflat-saltmarsh edge in the Bahía Blanca estuary

This study was carried out at the Bahía Blanca Estuary, Argentina, at the seaward edge of a saltmarsh. The saltmarsh-mudflat boundary in the study area shows sediment deposits at a higher elevation immediately seaward of the saltmarsh edge. We compared field determinations of water velocity, bed shear stress, wind wave conditions and variations of the bed elevation in the mudflat and within the Spartina alterniflora canopy at the seaward edge of a saltmarsh, and we evaluated the relative role of vegetation in the observed morphology. A mud deposition event that raised bed elevation in more than 5 cm occurred during the study period, with TSS concentrations > 500 mg l⁻¹, but simultaneous measurements performed on the bed levels confirmed that the sediments deposited did not originate from local resuspension within the edge of the canopy. In similar tidal cycles in terms of local wave activity and bed shear stresses at the sampling site, deposition occurred only with winds aligned with the azimuth of the Canal Principal, reaching a maximum fetch of more than 20 km in front of the sampling site.     

The effect of temporal wave averaging on the performance of an empirical shoreline evolution model

The effect of using time-averaged wave statistics in a simple empirical model for shoreline change is investigated. The model was first calibrated with a six-year time series of hourly wave conditions and weekly shoreline position at the Gold Coast, Australia. The model was then recalibrated with the hourly waves averaged over intervals up to 1 year. With wave averaging up to 2 days, model performance was approximately constant (squared correlation r² ~ 0.61–0.62), with only small changes in the values of empirical model parameters (e.g. the beach response coefficient c varied by less than 4%). With between 2 and 40 day averaging, individual storms are not resolved; model skill decreased only modestly (r² ~ 0.55), but c varied erratically by up to 40% of the original value. That is, optimal model coefficients depend on wave averaging, an undesirable result. With increased averaging (> 40 days) seasonal variability in the wave field is not resolved well and model skill declined markedly. Thus, temporal averaging of wave conditions increases numerical efficiency, but over-averaging degrades model performance and distorts best-fit values of model free parameters.     

A traversing system to measure bottom boundary layer hydraulic properties

This study describes a new convenient and robust system developed to measure benthic boundary layer properties, with emphasis placed on the determination of bed shear stress and roughness height distribution within estuarine systems by using velocity measurements. This system consisted of a remotely operated motorised traverser that allowed a single ADV to collect data between 0 and 1 m above the bed. As a case study, we applied the proposed traversing system to investigate bottom boundary layer (BBL) hydraulic properties within Coombabah Creek, Queensland, Australia. Four commonly-employed techniques: (1) Log-Profile (LP); (2) Reynolds stress (RS); (3) Turbulent Kinetic Energy (TKE); and (4) Inertial Dissipation (ID) used to estimate bed shear stresses from velocity measurements were compared. Bed shear stresses estimated with these four methods agreed reasonably well; of these, the LP method was found to be most useful and reliable. Additionally, the LP method permits the calculation of roughness height, which the other three methods do not. An average value of bed shear stress of 0.46 N/m², roughness height of 4.3 mm, and drag coefficient of 0.0054 were observed within Coombabah Creek. Results are consistent with that reported for several other silty bed estuaries.     

Effect of bed roughness on turbulent boundary layer and net sediment transport under asymmetric waves

This paper presents an investigation of the roughness effects in the turbulent boundary layer for asymmetric waves by using the baseline (BSL) k–ω model. This model is validated by a set of the experimental data with different wave non-linearity index, N_i (namely, N_i = 0.67, N_i = 0.60 and N_i = 0.58). It is further used to simulate asymmetric wave velocity flows with several values of the roughness parameter (a_m/k_s) which increase gradually, namely from a_m/k_s = 35 to a_m/k_s = 963. The effect of the roughness tends to increase the turbulent kinetic energy and to decrease the mean velocity distribution in the inner boundary layer, whereas in the outer boundary layer, the roughness alters the turbulent kinetic energy and the mean velocity distribution is relatively unaffected. A new simple calculation method of bottom shear stress based on incorporating velocity and acceleration terms is proposed and applied into the calculation of the rate of bed-load transport induced by asymmetric waves. And further, the effect of bed roughness on the bottom shear stress and bed-load sediment transport under asymmetric waves is examined with the turbulent model, the newly proposed method, and the existing calculation method. It is found that the higher roughness elements increase the magnitude of bottom shear stress along a wave cycle and consequently, the potential net sediment transport rate. Moreover, the wave non-linearity also shows a big impact on the bottom shear stress and the net sediment transport.     

A Lagrangian model for boundary layer growth and bed shear stress in the swash zone

A new model for the boundary layer development and associated skin friction coefficients and shear stress within the swash zone is presented. The model is developed within a Lagrangian reference frame, following fluid trajectories, and can be applied to both laminar flow and smooth turbulent flow. The model is based on the momentum integral approach for steady, flat-plate boundary layers, with appropriate modifications to account for the unsteady flow regime and flow history. The model results are consistent with previous measurements of bed shear stress and skin friction coefficients within the swash zone. These indicate strong temporal and spatial variation throughout the swash cycle, and a clear distinction between the uprush and backwash phase. This variation has been previously attributed the unsteady flow regime and flow history effects, both of which are accounted for in the new model. Fluid particle trajectories and velocity are computed using the non-linear shallow water wave equations and the boundary layer growth across the entire swash zone is estimated. Predictions of the bed shear stress and skin friction coefficients agree reasonably well with direct bed shear stress measurements reported by Barnes et al. (Barnes, M.P., O’Donaghue, T., Alsina, J.M., Baldock, T.E., 2009. Direct bed shear stress measurements in bore-driven swash. Coastal Engineering 56 (8), 853–867) and, for a given flow velocity, give stresses which are consistent with the bias toward uprush sediment transport which has consistently been observed in measurements. The data and modelling suggest that the backwash boundary layer is initially laminar, which results in the late development of significant bed shear during the backwash, with a transition to a turbulent boundary layer later in the backwash. A new conceptual model for the boundary layer structure at the leading edge of the swash is proposed, which accounts for both the no-slip condition at the bed and the moving wet–dry interface. However, further development of the Lagrangian Boundary Layer Model is required in order to include bore-generated turbulence and to account for variable roughness and mobile beds.     

In situ flume measurements of resuspension in the North Sea

The in situ annular flume, Voyager II, was deployed at three sites in the North Sea in order to investigate resuspension events, to determine the physical characteristics of the seabed, to determine the threshold of resuspension of the bed and to quantify erosion rates and erosion depths. These are the first controlled, in situ flume experiments to study resuspension in the North Sea, and were combined with long-term measurements of waves and currents. Resuspension experiments were undertaken at two muddy, and one sandy site: north of the Dogger Bank (DG: water depths ∼80 m, very fine, poorly sorted, very fine-skewed sediment experiencing seasonal thermal stratification of the water column along with oxygen depletion); the Oyster Grounds (OG: ∼40 m, similar bed properties, year round water column thermal stratification, Atlantic forcing); and in the Sean Gas Field (SGF: ∼20 m, moderately sorted, very coarse-skewed sand, and well mixed water column). The erosion thresholds of the bed were found to be 0.66-1.04 Pa (DG) and 0.91-1.27 Pa (OG), with corresponding erosion depths of 0.1-0.15 mm and 0.02-0.06 mm throughout the experiments.Evaluation of a year of current velocities from 2007 indicated that at OG, resuspension of the consolidated bed was limited to on average ∼8% of the time as a result of tidal forcing alone for short (<30 min) durations, but would potentially increase during the winter as a result of wave influences. At DG, under similar conditions this would increase to 13%, and in the SGF, wave-induced resuspension events occurred throughout the year, with the potential exceedance of the threshold for suspension greater than 50% in January and March.Resuspension of bed material and erosion rates were closely related to applied bed shear stresses, and eroded depths were significantly correlated with the physical properties of the bed. Therefore, while complex variations in biogeophysical factors affected the critical threshold of erosion, once exceeded, erosion rates were related to the nature of the sediment.     

Laboratory measurements of large-scale near-bed turbulent flow structures under spilling regular waves

The instantaneous turbulent velocity field created by the breaking of spilling regular waves on a plane slope was measured in a plane running parallel to the slope using particle image velocimetry. The measurement plane was located at a height of about 1 mm above the bed. The measurement area encompassed the region where the large eddies generated at incipient wave breaking impinged on the bottom inside the surf zone. A total of 30 trials were conducted under identical experimental conditions. In each trial, six consecutive wave cycles were recorded. The measured velocity fields were separated into a mean flow and a turbulence component by ensemble averaging. The instantaneous turbulent velocity fields were analyzed to determine the occurrence frequency, location, geometry and evolution of the large eddies, and their contributions to instantaneous shear stresses, turbulent kinetic energy and turbulence energy fluxes. The motion of single glass spheres along the bed was also investigated. The two-phase flow measurements showed that the velocity and displacement of large solid particles on a smooth bed were significantly affected by the magnitude and direction of turbulence velocities. Overall, this study has examined the kinematic and dynamic properties of large eddies impinging on the bed and the interaction of these large-scale turbulent flow structures with the mean flow. The study has also highlighted the important role of large eddies in sediment transport.     

Direct bed shear stress measurements in bore-driven swash

Direct measurements of bed shear in the swash zone are presented. The data were obtained using a shear plate in medium and large-scale laboratory bore-driven swash and cover a wide range of bed roughness. Data were obtained across the full width of the swash zone and are contrasted with data from the inner surf zone. Estimates of the flow velocities through the full swash cycle were obtained through numerical modelling and calibrated against measured velocity data. The measured stresses and calculated flow velocities were subsequently used to back-calculate instantaneous local skin friction coefficients using the quadratic drag law. The data show rapid temporal variation of the bed shear stress through the leading edge of the uprush, which is typically two–four times greater than the backwash shear stresses at corresponding flow velocity. The measurements indicate strong temporal variation in the skin friction coefficient, particularly in the backwash. The general behaviour of the skin friction coefficient with Reynolds number is consistent with classical theory for certain stages of the swash cycle. A spatial variation in skin friction coefficient is also identified, which is greatest across the surf-swash boundary and likely related to variations in local turbulent intensities. Skin friction coefficients during the uprush are approximately twice those in the backwash at corresponding Reynolds number and cross-shore location. It is suggested that this is a result of the no-slip condition at the tip leading to a continually developing leading edge and boundary layer, into which high velocity fluid and momentum are constantly injected from the flow behind and above the tip region. Finally, the measured stress data are used to determine the asymmetry and cross-shore variation in potential sediment transport predicted by three forms of sediment transport formulae.     

Seabed shear stress and bedload transport due to asymmetric and skewed waves

A simple conceptual formulation to compute seabed shear stress due to asymmetric and skewed waves is presented. This formulation generalizes the sinusoidal wave case and uses a variable friction factor to describe the physics of the boundary layer and to parameterize the effects of wave shape. Predictions of bed shear stresses agree with numerical computations using a standard boundary layer model with a k–ε turbulence closure. The bed shear stress formulation is combined with a Meyer-Peter and Müller-type formula to predict sheet flow bedload transport under asymmetric and skewed waves for a horizontal or sloping bed. The predictions agree with oscillatory water tunnel measurements from the literature.     

Laboratory observation of boundary layer flow under spilling breakers in surf zone using particle image velocimetry

A boundary layer flow under spilling breakers in a laboratory surf zone with a smooth bottom is investigated using a high resolution particle image velocimetry (PIV) technique. By cross-correlating the images, oscillatory velocity profiles within a viscous boundary layer of O(1) mm in thickness are resolved over ten points. Using PIV measurements taken for an earlier study and the present study, flow properties in the wave bottom boundary layer (WBBL) over the laboratory surf zone are obtained, including the mean velocities, turbulence intensity, Reynolds stresses, and intermittency of coherent events. The data are then used to estimate the boundary layer thickness, phase variation, and bottom shear stress. It is found that while the time averaged mass transport inside the WBBL is onshore in the outer surf zone, it changes to offshore in the inner surf zone. The zero Eulerian mass transport occurs at h/h_b ≈ 0.92 in the outer surf zone. The maximum overshoot of the streamwise velocity and boundary layer thickness are not constant across the surf zone. The bottom shear stress is mainly contributed by the viscous stress through mean velocity gradient while the Reynolds stress is small and negligible. The turbulence level is higher in the inner surf zone than that in the outer surf zone, although only a slight increase of turbulent intensity is observed inside the WBBL from the outer surf zone to the inner surf zone. The variation of phase inside and outside the WBBL was examined through the spatial velocity distribution. It is found the phase lead is not constant and its value is significantly smaller than previous thought. By analyzing instantaneous velocity and vorticity fields, a remarkable number of intermittent turbulent eddies are observed to penetrate into the WBBL in the inner surf zone. The size of the observed large eddies is about 0.11 to 0.16 times the local water depth. Its energy spectra follow the − 5/3 slope in the inertial subrange and decay exponentially in the dissipation subrange.     

利用坐底观测估算济州岛西南海域再悬浮临界应力

底边界层中沉积物的再悬浮和沉降是控制陆架海悬浮沉积物的输运的关键过程。沉积物输运过程的数值^*模拟也依赖于沉积物侵蚀和沉降的关键参数的研究。本文根据济州岛西南泥质区的坐底观测估算了此处临界应力。通过底边界层声学仪器ADV和PC-ADP的流速和悬浮物浓度同步观测,基于湍生成与耗散平衡假设,使用惯性耗散法计算沉降速度。这种方法得到的沉降速度ws平均值为0.91 mm s^-1,标准差为0.20 mm s^-1,此结果远大于Soulbsy（1997）和LISST-ST现场观测粒径分析仪等经验方法的结果。这主要是由于两种方法的本质不同,惯性耗散法形象的刻画了底边界层的水动力,并且更加合理的现场估计沉降速度w_s,然而Soulsby的方法通常适用于静水环境。我们提出了一种估计临界应力的新方法,根据悬浮颗粒物浓度时空变化的统计分析（深度平均的悬浮颗粒物浓度对时间求导数）和对应的底应力估算侵蚀临界应力τ_ce和沉降临界应力τ_cd。侵蚀临界应力τ_ce和沉降临界应力τ_ce的变化范围为0.11-0.25 Pa,对应的中值分别为0.20 Pa和0.16 Pa,这也证实了侵蚀临界应力略大于沉降临界应力。除此之外,我们还使用了另一种方法估算临界应力,通过沉降速度间接估算的临界应力范围为0.06-0.17 Pa。     

Circulation and transport of water masses in the Lazarev Sea, Antarctica, during summer and winter 2006

The distribution and circulation of water masses in the region between 6°W and 3°E and between the Antarctic continental shelf and 60°S are analyzed using hydrographic and shipboard acoustic Doppler current profiler (ADCP) data taken during austral summer 2005/2006 and austral winter 2006. In both seasons two gateways are apparent where Warm Deep Water (WDW) and other water masses enter the Weddell Gyre through the Lazarev Sea: (a) a probably topographically trapped westward, then southwestward circulation around the northwestern edge of Maud Rise with maximum velocities of about 20 cm s⁻¹ and (b) the Antarctic Coastal Current (AntCC), which is confined to the Antarctic continental shelf slope and is associated with maximum velocities of about 25 cm s⁻¹.Along two meridional sections that run close to the top of Maud Rise along 3°E, geostrophic velocity shears were calculated from CTD measurements and referenced to velocity profiles recorded by an ADCP in the upper 300 m. The mean accuracy of the absolute geostrophic velocity is estimated at ±2 cm s⁻¹. The net baroclinic transport across the 3°E section amounts to 20 and 17 Sv westward for the summer and winter season, respectively. The majority of the baroclinic transport, which accounts for ∼60% of the total baroclinic transport during both surveys, occurs north of Maud Rise between 65° and 60°S.However, the comparison between geostrophic estimates and direct velocity measurements shows that the circulation within the study area has a strong barotropic component, so that calculations based on the dynamic method underestimate the transport considerably. Estimation of the net absolute volume transports across 3°E suggests a westward flow of 23.9±19.9 Sv in austral summer and 93.6±20.1 Sv in austral winter. Part of this large seasonal transport variation can be explained by differences in the gyre-scale forcing through wind stress curl.     

The reduction effects of mangrove forest on a tsunami based on field surveys at Pakarang Cape,Thailand and numerical analysis

Using an integrated approach including satellite imagery analysis, field measurements, and numerical modeling, we investigated the damage to mangroves caused by the 2004 Indian Ocean tsunami at Pakarang Cape in Pang Nga Province, Thailand. Comparing pre- and post-tsunami satellite imagery of the study area, we found that approximately 70% of the mangrove forest was destroyed by the tsunami. Based on field observations, we found that the survival rate of mangroves increased with increasing stem diameter. Specifically, we found that 72% of Rhizophora trees with a 25–30 cm stem diameter survived the tsunami impact, whereas only 19% with a 15–20 cm stem diameter survived. We simulated the 2004 Indian Ocean tsunami using the nonlinear shallow-water wave theory to reproduce the tsunami inundation flow and investigated the bending moment acting on the mangrove trees. Results of the numerical model showed that the tsunami inundated areas along the mangrove creeks, and its current velocity reached 5.0 m s⁻¹. Based on the field measurements and numerical results, we proposed a fragility function for mangroves, which is the relationship between the probability of damage and the bending stress caused by the maximum bending moment. We refined the numerical model to include the damage probability of mangrove forests using the obtained fragility function to investigate the tsunami reduction effect of mangrove forest. Under simple numerical conditions related to the mangrove forest, ground level, and incident wave, the model showed that a mangrove forest of Rhizophora sp. with a density of 0.2 trees m⁻² and a stem diameter of 15 cm in a 400 m wide area can reduce the tsunami inundation depth by 30% when the incident wave is assumed to have a 3.0 m inundation depth and a wave period of 30 min at the shoreline. However, 50% of the mangrove forest is destroyed by a 4.5 m tsunami inundation depth, and most of the mangrove forest is destroyed by a tsunami inundation depth greater than 6 m. The reduction effect of tsunami inundation depth decreased when the tsunami inundation depth exceeded 3 m, and was mostly lost when the tsunami inundation depth exceeded 6 m.     

Experimental Study on the Bed Shear Stress Under Breaking Waves

The object of present study is to investigate the bed shear stress on a slope under regular breaking waves by a novel instrument named Micro-Electro-Mechanical System (MEMS) flexible hot-film shear stress sensor. The sensors were calibrated before application, and then a wave flume experiment was conducted to study the bed shear stress for the case of regular waves spilling and plunging on a 1:15 smooth PVC slope. The experiment shows that the sensor is feasible for the measurement of the bed shear stress under breaking waves. For regular incident waves, the bed shear stress is mainly periodic in both outside and inside the breaking point. The fluctuations of the bed shear stress increase significantly after waves breaking due to the turbulence and vortexes generated by breaking waves. For plunging breaker, the extreme value of the mean maximum bed shear stress appears after the plunging point, and the more violent the wave breaks, the more dramatic increase of the maximum bed shear stress will occur. For spilling breaker, the increase of the maximum bed shear stress along the slope is gradual compared with the plunging breaker. At last, an empirical equation about the relationship between the maximum bed shear stress and the surf similarity parameter is given, which can be used to estimate the maximum bed shear stress under breaking waves in practice.     

Experimental study of cohesive sediment consolidation and resuspension identifies approaches for coastal restoration: Lake Lery,Louisiana

The purpose of this study was to evaluate related processes of sediment consolidation and resuspension in a coastal basin and how these processes influence retention of fine sediment delivered by a river diversion. Sediment samples were collected from Lake Lery, a coastal receiving basin of the Caernarvon Diversion from the Mississippi River, Louisiana. Consolidation was tested for six initial sediment concentrations (14.0–105 kg m^–3) in a settling column over 15-day periods. Mud erodibility was tested at seven shear stress regimes (0.01–0.60 Pa) using a dual-core Gust erosion microcosm system, on cores containing suspensions that consolidated for 1, 2, and 4 weeks. Consolidation rates were found to be inversely and exponentially related to initial suspension concentration, over concentrations ranging from fluid mud (10–200 kg m^–3) to hydraulic dredge effluent. Consolidation is best predicted by a function consisting of two exponential terms and one asymptotic constant, describing rates of rapid initial and slower subsequent settling. Coupled resuspension and consolidation tests (concentrations of 20–21 kg m^–3) show that shear stresses generating the highest turbidity peaks increase from ≤0.30 Pa after 2 weeks of consolidation to ≥0.45 Pa after 4 weeks, and this strengthening cannot be attributed solely to increasing sediment concentration over time. Comparison of measured erosion shear stresses with bed shear stresses typical of coastal lakes and bays suggests that this degree of strengthening, if given time to occur, could increase the overall retention of fine sediments deposited on lake and bay floors.     

Characteristics of the boundary layer at the bottom of a solitary wave

The results of direct numerical simulations of the boundary layer generated at the bottom of a solitary wave are described. The numerical results, which agree with the laboratory measurements of Sumer et al. (2010) show that the flow regime in the boundary layer can be laminar, laminar with coherent vortices and turbulent. The average velocity and bottom shear stress are computed and the results obtained show that the logarithmic law can approximate the velocity profile only in a restricted range of the parameters and at particular phases of the wave cycle. Moreover, the maximum value of the bottom shear stress is found to depend on the dimensionless wave height only, while the minimum (negative) value depends also on the dimensionless boundary layer thickness. Diagrams and simple formulae are proposed to evaluate the minimum and maximum bottom shear stresses and their phase shift with respect to the wave crest.     

Turbidity and sediment transport in a muddy sub-estuary

Sub-estuaries, i.e. tidal creeks and also larger estuaries that branch off the stem of their main estuary, are commonplace in many estuarine systems. Their physical behaviour is affected not only by tributary inflows, winds and tides, but also by the properties and behaviour of their main estuary. Measurements extending over more than an annual cycle are presented for the Tavy Estuary, a sub-estuary of the Tamar Estuary, UK. Generally, waves are small in the Tavy because of the short wind fetch. A several-hour period of up-estuary winds, blowing at speeds of between 7 and 10 m s⁻¹, generates waves with significant wave heights of 0.25 m and a wave periodicity of 1.7 s that are capable of eroding the bed over the shallow, ca. 1.5 m-deep mudflats. Waves also influence sedimentation within and near salt marsh areas. An estuarine turbidity maximum (ETM) occurs in the Tavy's main channel, close to the limit of salt intrusion at HW. Suspended particulate matter (SPM) concentrations typically are less than 40 mg l⁻¹ at HW, although concentrations can exceed 80 mg l⁻¹ when tides and winds are strong. Flood-tide SPM inputs to the Tavy from the Tamar are greater during high runoff events in the River Tamar and also at spring tides, when the Tamar has a high-concentration ETM. Higher SPM concentrations are experienced on the mudflats following initial inundation. Without wave resuspension, this is followed by a rapid decrease in SPM for most of the tide, indicating that the mudflats are depositional at those times. SPM concentrations on the mudflats again increase sharply prior to uncovering. Peak ebb tidal speeds at 0.15 m above the mudflat bed can exceed 0.26 m s⁻¹ at spring tides and 0.4 m s⁻¹ following high runoff events, which are sufficient to cause resuspension. Time-series measurements of sediment bed levels show strong seasonal variability. Higher and lower freshwater flows are associated with estimated, monthly-mean sediment transport that is directed out of, or into, the upper sub-estuary, respectively. Seasonal sediment transfers between the estuary and its sub-estuary are discussed.     

Resuspension of benthic fluff by tidal currents in deep stratified waters, northern North Sea

Suspended particulate matter (SPM) concentration and properties (particle size and settling velocity), water column and boundary layer dynamics were measured during a 60-d period at a site in 110 m water depth in the northern North Sea. The site was in stratified waters and measurements were made during September–November as the seasonal thermocline was progressively weakening. SPM concentration was low, c. 1 mg dm⁻³ in the surface mixed layer and maximum values of 2 mg dm⁻³ in the bottom mixed layer. The bottom layer was characterised by larger mean particle size. SPM signals in the two layers were decoupled at the start of the period, when the thermocline was strong, but were increasingly coupled as the thermocline progressively weakened. A spring-neap cycle of resuspension and deposition of SPM was observed in the bottom mixed layer. Bed shear stresses were too small to entrain the bottom sediment (a fine sand) but were competent to resuspend benthic fluff: threshold bed shear stress and threshold current velocity at 10 mab were 0.02–0.03 Pa. and 0.18 m s⁻¹, respectively. Maximum SPM concentration in the bottom layer preceded peak spring tide currents by 3 d. Simulation of fluff resupension by the PROWQM model confirms that this was due to a finite supply of benthic fluff: the fluff layer was stripped from the seabed so that fluff supply was zero by the time of peak spring flow. SPM was redeposited over neap tides. Fluff resuspension must have been enhanced by intermittent inertial currents in the bottom layer but unequivocal evidence for this was not seen. There was some resuspension due to wave activity. Settling velocity spectra were unimodal or bimodal with modal values of 2×10⁻⁴–2×10⁻³ mm s⁻¹ (long-term suspension component) and 0.2–5.7 mm s⁻¹ (resuspension component). The slowest settling particles remained in suspension at peak spring tides after the fluff layer had been exhausted. There was evidence of particle disaggregation during springs and aggregation during neaps.     

Observations of surfzone alongshore pressure gradients onshore of an ebb-tidal delta

The relative importance of radiation stress gradients and alongshore pressure gradients to surfzone dynamics is investigated using observations of water levels, waves, and flows measured onshore of a large ebb-tidal delta. Incident wave heights measured along the ~ 11-m depth contour varied about 10% over a 1.2-km alongshore transect, resulting in alongshore wave setup differences on the order of 10 cm over the 600-m extent of the surfzone instrument array in 1.5-m depth. Despite the moderate alongshore variability in wave heights, the southerly alongshore pressure gradient, associated with the alongshore variability of wave-driven set-up, was typically twice as large as the northerly radiation stress gradient forcing, consistent with the observed southerly currents during the week-long experiment. The magnitude of the alongshore forcing and resulting alongshore velocity is reproduced by the two-dimensional depth-averaged numerical model of Shi et al. (JGR-Oceans, 2011). These observations, together with the numerical results, indicate that moderate alongshore wave height gradients (O(10^− 4)) outside the surfzone owing to alongshore variations in the offshore bathymetry can result in alongshore pressure gradients that are larger than radiation stress gradients.