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A field study was conducted on a dissipative beach to quantify inner surf and swash-zone bed level change during periods of inundation and throughout a tidal cycle. Elevation changes were acquired at millimeter resolution with a new conductivity concentration profiler that allowed quantification of the bed level throughout the duration of the wave/swash cycle and also during periods of bed exposure. Bed level change spectra showed the highest energy at low frequencies even though event-scale net bed level changes were observed to exceed the tidal-scale net bed level change. Net bed level change for individual events was nearly normally distributed with most individual events displaying little or no net bed level change. “Large” erosion and accretion events with bed level elevation magnitudes that exceeded net tidal elevation change occurred with similar frequency. The similarity between the frequency of large erosion and large accretion events suggests that a few events may be ultimately responsible for the observed net elevation change over the tidal cycle. The large events displayed different hydrodynamic characteristics. Erosion events had longer duration onshore-directed flow and higher maximum onshore-directed velocity magnitude than offshore-directed velocity magnitude. The opposite was found for accretion events that had longer duration offshore-directed flow and a higher maximum offshore-directed velocity magnitude than onshore-directed velocity magnitude.

A field experiment was conducted on a high energy sandy beach (Truc Vert, France) to examine swash zone processes and shoreline erosion–accretion in response to changing wave and tide conditions. Beachface morphological changes were measured at the swash-by-swash timescale using an array of ultrasonic bed-level sensors and at the tidal timescale through total station surveys. Hydrodynamic and sediment dynamic data were collected using arrays of pressure transducers, electromagnetic current meters, optical backscatter sensors and total load sediment traps. Data were collected for 16 consecutive days (33 tides) over a spring–neap–spring tidal cycle with offshore significant wave heights of 1–4 m and periods of 5–12 s. The swash motion was predominantly at infragravity wave frequencies. Over the measurement period, the upper beach experienced phases of accretion, erosion and stability. Trapped sediment loads for the uprush and backwash were predicted reasonably well by cross-shore velocity skewness, but the prediction of net transport over individual swash events by velocity-based models was more problematic, particularly lower in the swash zone where swash interactions were more prevalent. Net sediment fluxes over individual swash events often exceeded 100 kg per unit meter beach width and such rates were only one order of magnitude less than the net transport typically experienced across the high tide swash zone over complete tidal cycles. Upper beach erosion–accretion at the tidal timescale was reasonably well predicted by the Dean criterion based on offshore wave steepness and dimensionless fall velocity, but the beach gradient also played a significant role in controlling the occurrence of erosion and accretion.

Repeated high-resolution multibeam bathymetric surveys from 2002 through 2006 at the head of the Monterey Submarine Canyon reveal a sandwave field along the canyon axis between 20 and 250 m water depth. These sandwaves range in wavelength from 20 to 70 m and 1 to 3 m in height. A quantitative measure was devised to determine the direction of sandwave migration based on the asymmetry of their profiles. Despite appreciable spatial variation the sandwaves were found to migrate in a predominantly down-canyon direction, regardless of season and tidal phases. A yearlong ADCP measurement at 250 m water depth showed that intermittent internal tidal oscillations dominated the high-speed canyon currents (50–80 cm/s), which are not correlated with the spring–neap tidal cycle. Observed currents of 50 cm/s or higher were predominantly down-canyon. Applying a simple empirical model, flows of such magnitudes were shown to be able to generate sandwaves of a size similar to the observed ones.

Channels are relatively common on river-mouth deltas, but the process by which they arise from river sediment discharge is unclear because they can potentially be explained either by negatively buoyant (hyperpycnal) flows produced directly from the river outflow or by flows generated by repeated failure and mobilisation of sediment rapidly deposited at the delta front. Channels eroded through a dump site of dredge spoils are described here from multibeam and older sonar data collected in Commencement Bay, at the mouth of the Puyallup River. Shallow channels on the seaward upper surface of the dump site, away from any flows that could have been produced by delta front failures, suggest that at least some hyperpycnal flows were produced directly from the positively buoyant river outflow up to 200 m from the edge of the river mouth platform. The form of channel bed erosion is revealed by the longitudinal shape of the main eroded channel compared with the adjacent dump site profile. It suggests that the channel evolved by its steep front retreating, rather than by simple vertical entrenchment or diffusive-like evolution of the profile, a geometry interpreted as evidence that repeated failure of the bed occurred in response to shear stress imposed by bottom-travelling flows. Model calculations based on shear strengths back-calculated from the geometry of channel wall failures suggest that, if the main channel were eroded solely by hyperpycnal flows, their generation was remarkably efficient in order to create flows vigorous enough to cause channel bed failure. Besides the sediment concentration and discharge characteristics that have been considered to dictate the ability of rivers to produce hyperpycnal flows, it is suggested that the timing of floods with respect to the tidal cycle should also be important because extreme low tides may be needed to ensure that coarse sediment is transferred vigorously to the edge of river mouth platforms.

The result of field experiments, designed to investigate the relative proportions of bedload and suspended load, are described. The ratio of bedload to suspended sediment load in the swash zone is examined in both swash and backwash on four beaches by measuring the amounts collected in a sediment trap. Bedload transport is found to dominate the backwash. The relative proportions of bedload and suspended load change over the tidal cycle, with increasing bedload dominance at low tide. The total amount of sediment transported as swash and backwash is noticeably greater at high tide than at low tide. More sediment is transported on the flood tide than on the ebb.

Tidal resuspension experiments were carried out on two occasions during the winter of 1988–1989 at a disposal site for hydraulically dredged sediments in northern Chesapeake Bay to determine the influence of tidal resuspension on erosion of recent deposits. The results indicate that normal tidal erosion depths were only a fraction of a millimeter per half tidal cycle and probably did not account for the majority of the apparent sediment loss. Erosion rate was found to be a linear function of the excess of estimated shear stress over a critical value, but both the constant of proportionality ( M = 0.5 mg/cm 2 /h) and the critical shear stress ( τ c = 0.16 dynes/cm 2 were much less than many previously reported results. The most likely explanation for these low values is that the thin layer of surface sediments involved in regular tidal resuspension had only a few hours at most to consolidate between resuspension events. Observed resuspended sediment concentrations (up to 35 mg/l above background levels) were much less than those reported for previous observations of tidal resuspension in the nearby channel, presumably due to greater stratification and lower tidal velocities at the disposal site. Salinity-induced stratification of the water column is estimated to have reduced shear stresses by up to 50% relative to the neutrally stratified case. Regular tidal resuspension of a thin layer of surface sediments is implicated as a potentially important aspect of the typical benthic environment of northern Chesapeake Bay, even if it is not the most important factor in massive sediment erosion and transport.

Nearshore suspended sediment concentration along the muddy Surinam coast is highly variable; maximum values are many times greater than on other muddy coasts. Water samples taken at four field stations during various stages of the tide range in concentration from 15 to 3,700 mg/l near the surface and from 100 to 30,000 mg/l near the bottom. Highest overall concentrations and greatest variability in concentration occur in water over large banks of fluid mud (thixotropic gel) that extend 2–3 km offshore and 5–10 km along-shore. On both the intertidal and subaqueous portions of these mudbanks highest concentrations are found at low tide. Results provide evidence that an exchange between fluid mud and suspended sediment takes place during each tidal cycle.

Holes, 15–30 m deep and 2–5 times deeper than adjacent tidal channels, occur commonly at the junction of marsh drainage creeks in coastal South Carolina. They extend through marsh and channel sediments and are often cut 5–15 m into the underlying consolidated Tertiary Cooper marl limestone bedrock. We hypothesize that these holes primarily scour from excessive macro-scale turbulence during the flood portion of the tidal cycle, when the flow in the main channel branches into the tributaries and is highly variable in speed and direction. The holes are then swept clean of unconsolidated sediments by the swift ebb tidal current.

Odanam Satoe, a subtidal, tide-dominated sand body in the Yellow Sea, Korea, is linear in plan and asymmetrical in cross-section. It consists of fine- to medium-grained, well-sorted subangular sand. Bedforms consist of high-amplitude (1–2 m) sandwaves on the lower flanks of the gentler-sloping bar surface, and medium-amplitude (0.5-1 m) sandwaves on the sand body trough adjoining the steeper face, the bar crest and shallower parts of the gently sloping bar surface. Bedforms are absent on the relatively steeper bar surface, which is characterized by 2° slopes. Bedform orientation on the gentler slope is oblique by 30° to the bar crest, parallel to the sand-body crest on the crest itself, and opposite to the steeper sand-body face in the trough below the steeper slope of the bar. Bottom current velocity data show that tidal currents are semi-rotary with a flood time—velocity asymmetry over the gentler slope, and ebb time—velocity asymmetry over the steeper slope during most of the tidal cycle. Tidal-current flow parallels bar elongation over the steeper slope, whereas over the gentler slope, tidal-current flow is directed at 30° to the bar crest and changes to normal to the crest one hour prior to low tide. Bedform orientation mapped with side-scan sonar shows agreement with these flow directions. Sand dispersal around the sand body is controlled by time—velocity asymmetry and partial rotary flow directions of tidal currents. This circulation causes not only a trapezoidal mode of grain dispersal, but also westerly migration of the sand body documented from comparative bathymetric surveys in 1964 and 1980.

Spectral and cross-spectral analysis have been applied to two time series of beach data, published by ESSA (Harrison et al., 1968). The first series consisted of seven variables recorded at four-hourly intervals (with hourly value interpolated) and the second of four variables, recorded at high and low tide. In the first set, peaks in the spectrum at the semidiurnal tidal period were prominent for beach slope and breaker height, and in the second set for longshore current velocity, beach slope and sand size. Cross-spectral analysis gave coherence values for all possible pairs of variables; four notably high values occurred in the first set and three in the second. The results of the analysis are summarized in Tables I, II, III and IV which demonstrate the importance of the semidiurnal and diurnal tidal cycle in both process variables and the beach slope response variable. A strong coherence between beach slope, sand size and longshore current velocity shows that they are all affected by the tidal cycle.

Detailed echo sounding along a fixed transit off Avonmouth (England) has proved that sand waves reverse their orientation during the tidal cycle.

This contribution uses a 3 dimensional coastal area numerical model, DHI's MIKE3, to simulate the impact of storm and surge events on offshore sandbanks. Three offshore sandbanks in the Bristol Channel are considered due to the region's sensitivity to anthropogenic pressures and the gradients in wave and tidal forcing in the area. Two extreme storm and surge events are simulated: one co-incident with spring tide and the other with neap tide. Reference simulations of astronomical tidal forcing only are also presented. It is shown that for the two sandbanks with greater wave exposure, storm conditions can provide a mechanism for the maintenance of crest position. For these cases, bed level changes over the crest are in the opposite direction compared to astronomically forced change. For the least wave exposed bank, both wave and tide only cases exhibit similar patterns of bed level change. Volumetric changes under astronomical forcing are shown to vary with changing maximum tidal current. Accretion occurs over a neap tidal cycle for all three sandbanks and as maximum tidal current increases the amount of accretion increases; however, over a spring tidal cycle accretion is observed for the less tidally dominated site but increasing maximum tidal current leads to reduced accretion and then erosion for the most tidally dominated bank. Volumetric changes under storm conditions are related to sandbank morphology and setting rather than relative wave exposure. The two single banks closely tied to headlands show similar magnitude of percentage volumetric change despite being at the two extremes of wave exposure and greater erosion occurs over the neap tide event. The sandbank that has associated secondary banks shows lesser percentage change and greater erosion over the spring tide event.