Complete Tidal Cycle Research Articles

Are mangroves drivers or buffers of coastal acidification? Insights from alkalinity and dissolved inorganic carbon export estimates across a latitudinal transect

AbstractMangrove forests are hot spots in the global carbon cycle, yet the fate for a majority of mangrove net primary production remains unaccounted for. The relative proportions of alkalinity and dissolved CO2 [CO2*] within the dissolved inorganic carbon (DIC) exported from mangroves is unknown, and therefore, the effect of mangrove DIC exports on coastal acidification remains unconstrained. Here we measured dissolved inorganic carbon parameters over complete tidal and diel cycles in six pristine mangrove tidal creeks covering a 26° latitudinal gradient in Australia and calculated the exchange of DIC, alkalinity, and [CO2*] between mangroves and the coastal ocean. We found a mean DIC export of 59 mmol m−2 d−1 across the six systems, ranging from import of 97 mmol m−2 d−1 to an export of 85 mmol m−2 d−1. If the Australian transect is representative of global mangroves, upscaling our estimates would result in global DIC exports of 3.6 ± 1.1 Tmol C yr−1, which accounts for approximately one third of the previously unaccounted for mangrove carbon sink. Alkalinity exchange ranged between an import of 1.2 mmol m−2 d−1 and an export of 117 mmol m−2 d−1 with an estimated global export of 4.2 ± 1.3 Tmol yr−1. A net import of free CO2 was estimated (−11.4 ± 14.8 mmol m−2 d−1) and was equivalent to approximately one third of the air‐water CO2 flux (33.1 ± 6.3 mmol m−2 d−1). Overall, the effect of DIC and alkalinity exports created a measurable localized increase in coastal ocean pH. Therefore, mangroves may partially counteract coastal acidification in adjacent tropical waters.

Seasonal Hydrodynamics and Salt Exchange of a Shallow Estuary in Northern China

ABSTRACT Zou, T.; Zhang, H.; Meng, Q. J., and Li, J., 2016. Seasonal hydrodynamics and salt exchange of a shallow estuary in Northern China. Estuaries are important components of coastal ecosystem and function as dominant pathways of material exchange at the land-sea interface. The transport of terrestrial input through river inflow is controlled by physical process including tides, waves, and fresh water discharge. This study investigates hydrodynamic characteristic and salt flux within Xiaoqinghe River (XQR) estuary, which is a shallow estuarine system (water depth < 8 m) and exports substantial amount of nutrients and pollutants to the adjacent Laizhou Bay. Profile velocity and salinity are measured using ADCP and CTD through complete tidal cycles (25 hours) in April, July and September 2013. The instantaneous velocity and salinity data are decomposed into time-averaged means and time-varying components based on the improved Kjerfve (1986) method to quantify the contributions of various physical proces...

Spatial and temporal dynamics of nano‐ and pico‐size particulate organic matter (POM) in a coastal megatidal marine system

AbstractSurface water samples of size‐selected seston (0.7–20 μm) were collected from April 2013 to September 2013 at three similar coarse‐sand benthic habitats. Additionally, seston sampling was performed at a fixed location throughout a complete tidal cycle (2014). A combination of fatty acid (FA), isotope, and flow cytometry analyses were used to determine the quality and quantity of nano‐ and pico‐sized particulate organic matter (POM). High variability was found between fatty acid replicate samples. Similar temporal patterns were observed at two sheltered sites, while the exposed site displayed less pronounced seasonal changes. Lower concentrations of 16C and 18C polyunsaturated fatty acids were found during low tide sampling. Globally, POM was dominated by picoeukaryotes, with concentrations exceeding 50,000 cells mL−1, and (16:4ω3 + 18:3ω3)/Σω3 is proposed as novel biomarker of picoeukaryotes in this region.

Field and theoretical investigation of sediment mass fluxes on an accretional coastal mudflat

Variations in suspended sediment concentrations (SSCs) in tidal mudflats are an important influence on the ecological environment, morphological evolution, and pollutant transport. To better understand how the behavior of suspended sediment influences small-scale variations in SSC in the water column, we took simultaneous measurements of water depth, wave height, current velocity, SSC profiles, and intratidal bed-level changes during a series of continuous tidal cycles on a highly turbid macrotidal mudflat, part of a larger accretional coastal mudflat on the Jiangsu Coast, China. We estimated the relative contributions of erosion, deposition, and advection processes to variations in SSC from the field data. We used an empirical orthogonal function (EOF) analysis to examine the influence of hydrodynamic factors (water depth, wind, wave height, and current velocity) and environmental factors (salinity and temperature) on SSC variability, to determine why the contributions of the three processes (erosion, deposition, and advection) to the variability in SSC differed. Our results showed that on average, advection flux was about an order of magnitude higher than erosion–deposition flux of corresponding tide, and that advection, driven by the tidal current velocity, wind, and associated alongshore transport, accounted for most of the variability in SSC at the study site over a complete tidal cycle. An abundant sediment supply and limited resuspension of the bed sediments meant that advection was the main transport process. Our results also demonstrate that detailed analyses of transport processes provide useful information on the sources and fates of suspended sediments, and support the interpretation of morphological changes in accretional intertidal mudflats.

Riders on the Storm: Selective Tidal Movements Facilitate the Spawning Migration of Threatened Delta Smelt in the San Francisco Estuary

Migration strategies in estuarine fishes typically include behavioral adaptations for reducing energetic costs and mortality during travel to optimize reproductive success. The influence of tidal currents and water turbidity on individual movement behavior were investigated during the spawning migration of the threatened delta smelt, Hypomesus transpacificus, in the northern San Francisco Estuary, California, USA. Water current velocities and turbidity levels were measured concurrently with delta smelt occurrence at sites in the lower Sacramento River and San Joaquin River as turbidity increased due to first-flush winter rainstorms in January and December 2010. The presence/absence of fish at the shoal-channel interface and near the shoreline was quantified hourly over complete tidal cycles. Delta smelt were caught consistently at the shoal-channel interface during flood tides and near the shoreline during ebb tides in the turbid Sacramento River, but were rare in the clearer San Joaquin River. The apparent selective tidal movements by delta smelt would facilitate either maintaining position or moving upriver on flood tides, and minimizing advection down-estuary on ebb tides. These movements also may reflect responses to lateral gradients in water turbidity created by temporal lags in tidal velocities between the near-shore and mid-channel habitats. This migration strategy can minimize the energy spent swimming against strong river and tidal currents, as well as predation risks by remaining in turbid water. Selection pressure on individuals to remain in turbid water may underlie population-level observations suggesting that turbidity is a key habitat feature and cue initiating the delta smelt spawning migration.

Pollutant Flux Estimation in an Estuary Comparison between Model and Field Measurements

This study proposes a framework for estimating pollutant flux in an estuary. An efficient method is applied to estimate the flux of pollutants in an estuary. A gauging station network in the Danshui River estuary is established to measure the data of water quality and discharge based on the efficient method. A boat mounted with an acoustic Doppler profiler (ADP) traverses the river along a preselected path that is normal to the streamflow to measure the velocities, water depths and water quality for calculating pollutant flux. To know the characteristics of the estuary and to provide the basis for the pollutant flux estimation model, data of complete tidal cycles is collected. The discharge estimation model applies the maximum velocity and water level to estimate mean velocity and cross-sectional area, respectively. Thus, the pollutant flux of the estuary can be easily computed as the product of the mean velocity, cross-sectional area and pollutant concentration. The good agreement between the observed and estimated pollutant flux of the Danshui River estuary shows that the pollutant measured by the conventional and the efficient methods are not fundamentally different. The proposed method is cost-effective and reliable. It can be used to estimate pollutant flux in an estuary accurately and efficiently.

Water-level controls on macro-tidal rip currents

Field measurements and numerical modelling have been used to investigate the water-level control of rip current dynamics on a macro-tidal beach. Field data collected over 32 complete tidal cycles, spanning a range of wave and tide conditions, demonstrate that rip current strength and behaviour is modulated at the semi-diurnal frequency by tide-induced changes in the water-level over bar/rip morphology. Peak flow speeds in the rip neck 〈uv〉 correspond to the time of maximum wave breaking 1.5h before and after low water.Alongshore-directed water surface gradients ∂η/∂y were measured along the feeder channel and around the ends of the inter-tidal bar, with head differences O(0.1m). The numerical model reproduced ∂η/∂y with a good level of skill and showed that ∂η/∂y and 〈uv〉 increase with the proportion of breaking waves Qb over the inter-tidal bar; but 〈uv〉 was maximised during peak Qb, maximum ∂η/∂y occurred when wave breaking moved offshore to the sub-tidal bar and Qb was reduced. Around low water, the forcing of the rip current by the alongshore pressure-driven feeder current was reduced by the decrease in Qb over the bar and feeder regions, but an offshore flow through the rip channel was maintained by a localised intensification of ∂η/∂y around the ends of the inter-tidal bar.

Relationship between nitrogen concentration, light, and Zostera marina habitat quality and survival in southeastern Massachusetts estuaries

The relationship of eelgrass survival and habitat quality to water column nitrogen level, phytoplankton biomass, particulate matter, bottom light intensity, and light attenuation was quantified at 70 sites within 19 Massachusetts estuaries through 4 growing seasons (2007–2009, 2011). Sites included a range of eelgrass habitat quality, from stable productive eelgrass beds, to degraded beds, to areas that have lost all eelgrass coverage. Survival of transplanted eelgrass culms was used as a bio-indicator of habitat quality. Habitat quality based upon both changes in stability of eelgrass coverage and transplant survival was positively related to light intensity and percent transmittance. Transplant survival was consistent with habitat designations based upon long-term changes in eelgrass coverage, with lowest light coinciding with areas that lost eelgrass in earlier decades. Bottom light declined in proportion to increases in total nitrogen levels, phytoplankton biomass, and water column particulates determined from long-term water quality data. Field surveys indicated that eelgrass survival required bottom light ≥100 μE/m2/s and healthy eelgrass existed where tidally-averaged total nitrogen was less than 0.34 mg/L, equivalent to a mid-ebb tide water-column total nitrogen of <0.37 mg/L. Traditional sampling of water column nitrogen at mid-ebb tide was found to slightly overestimate the average nitrogen level over a complete tidal cycle. However, since long-term, ebb-tide and tidally-averaged total nitrogen are correlated, it is possible to use the monitoring average to guide management until tidally-averaged TN becomes available. Nitrogen thresholds that support eelgrass communities provide a fundamental tool for managing this habitat and for selection of transplant sites aimed at accelerating restoration of this resource under increasing nitrogen loading of the coastal zone.

Radium Sampling Methods and Residence Times in St. Andrew Bay, Florida

Activity ratios (AR) of radium isotopes have been used with success to constrain estimates of water ages and to approximate residence times in coastal waters. We compared two common radium sampling methods (grab sampling and stationary moorings) to estimate water ages and the residence time of St. Andrew Bay waters in northwest Florida, USA. Both sampling methods utilize manganese dioxide fibers (“Mn fibers”) to adsorb dissolved radium from the water column. Grab samples capture radium activities at a discrete time while moorings integrate radium activities over longer deployments. The two methods yielded similar results in this study and thus both approaches are useful for water age comparisons and residence time approximations. However, since radium often varies as a function of tidal stage, deploying moorings over a complete tidal cycle is the preferred approach. An estimated residence time for North Bay and West Bay of 8–11 days was approximated using ARs for both ex224Ra/223Ra and ex224Ra/228Ra. Some complications were introduced as St. Andrew Bay is a tidally dominated, rather than a river-dominated bay system where this method has previously been applied. The largest freshwater source to this bay system is from a man-made reservoir, with an average freshwater flow of only 20 m3 s−1. The activity concentrations and ARs measured by both sampling methods suggest that while the reservoir is the prominent radium source, it is not the only radium source. Nonetheless, a tidal mixing model applied to the western half of the system yielded an approximate flushing time of 10–12 days, similar to that derived from our radium-based water age approach.

Characteristics of the turbulence in the flow at a tidal stream power site

This paper analyses a set of velocity time histories which were obtained at a fixed point in the bottom boundary layer of a tidal stream, 5 m from the seabed, and where the mean flow reached 2.5 m s(-1). Considering two complete tidal cycles near spring tide, the streamwise turbulence intensity during non-slack flow was found to be approximately 12-13%, varying slightly between flood and ebb tides. The ratio of the streamwise turbulence intensity to that of the transverse and vertical intensities is typically 1 : 0.75 : 0.56, respectively. Velocity autospectra computed near maximum flood tidal flow conditions exhibit an f(-2/3) inertial subrange and conform reasonably well to atmospheric turbulence spectral models. Local isotropy is observed between the streamwise and transverse spectra at reduced frequencies of f>0.5. The streamwise integral time scales and length scales of turbulence at maximum flow are approximately 6 s and 11-14 m, respectively, and exhibit a relatively large degree of scatter. They are also typically much greater in magnitude than the transverse and vertical components. The findings are intended to increase the levels of confidence within the tidal energy industry of the characteristics of the higher frequency components of the onset flow, and subsequently lead to more realistic performance and loading predictions.

Salt Fluxes in a Complex River Mouth System of Portugal

Measurements of velocity and salinity near the mouth and head of the Espinheiro channel (Ria de Aveiro lagoon, Portugal) are used to study the local variation of physical water properties and to assess the balance, under steady conditions, between the seaward salt transport induced by river discharge and the landward dispersion induced by various mixing mechanisms. This assessment is made using data sampled during complete tidal cycles. Under the assumption that the estuarine tidal channel is laterally homogeneous and during moderate tidal periods (except for one survey), currents and salinity data were decomposed into various spatial and temporal means and their deviations. Near the channel's mouth, the main contributions to the salt transport are the terms due to freshwater discharge and the tidal correlation. Near the channel's head, this last term is less important than the density driven circulation, which is enhanced by the increase in freshwater discharge. The remaining terms, which are dependent on the deviations from the mean depth have a smaller role in the results of salt transport. The computed salt transport per unit width of a section perpendicular to the mean flow is in close agreement to the sum of the advective and dispersive terms (within or very close to 12%). An imbalance of the salt budget across the sections is observed for all the surveys. Considerations are made on how this approach can inform the management of hazardous contamination and how to use these results to best time the release of environmental flows during dry months.

Role of Transparent Exopolymer Particles on Phytoplankton Dynamics in a Subtropical Estuary, Canan&amp;#233;ia-Iguape (Sp, Brazil)

Transparent Exopolymer Particles (TEP) has a known crucial role in vertical fluxes of carbon in the ocean and has been intensively studied in the last decade. Nevertheless, few studies have considered horizontal fluxes of TEP. These fluxes arise prominently between estuaries and the coast, where its importance is associated to the predominant horizontal transport. This is due both to the low density of the TEP agglomerate generated in low density waters of continental origin, and to the fact that the main component of the advective transport at estuarine regions is the horizontal one. In this study, the significance of TEP in the particulate matter exchange between estuary and coast was analyzed in the estuarine lagoon system of Cananéia-Iguape (southeast coast of Brazil). TEP, total seston (TS), organic seston (OS) and chlorophyll a (Chl.a) were analyzed over complete tidal cycles, during the dry and rainy seasons both at spring and neap tides. Horizontal fluxes and net transport rates of these variables were also calculated. TEP concentrations (max. 4991 μg Xeq/L) were almost one order of magnitude higher during the rainy season. TEP horizontal transport rates as high as 1.8 g Xeq. m/s were observed at the northern inlet of the system. In terms of particulate carbon, it represents 32.7 g TEP-C m/s or 4% of the OS transport rates. Our study quantifies the advective transport of TEP and their importance in particulate matter exchange between a subtropical estuarine system and the adjacent coastal region. Our results contribute to the knowledge of exopolimer particles dynamics in subtropical estuarine systems, and their relationship to phyto-plankton biomass and particulate matter.

Morphodynamic characteristics in a tidal inlet: San Blas, Argentina

A tidal inlet on the Argentinean coast is studied. An integrated approach of combining detailed bathymetry survey, seabed sampling and tidal current measurements was used to characterize the tidal inlet morphodynamics. A Phase Measuring Bathymetric System (PMBS) “GeoSwath Plus” from GeoAcoustics Ltd. with centimetric precision and a mean swath coverage of 160 m, was used to recognize bedform morphologies and to determine bedform dynamics by comparison of successive surveys . Tidal velocities were recorded during a complete tidal cycle using an ADCP mounted on a ship operating at a frequency of 650 kHz. Maximum tidal currents, of the order of 2 m s − 1 , exhibit different patterns across the channel. While maximum ebb currents dominate in the central and southern area of the channel, flood maximum currents dominate on the northern flank. The complex morphology of a dune field in an inlet and the resulting 3-D dune movement mechanism are also presented. In the middle of San Blas channel, a field with symmetrical large dunes (10–100 m in spacing) and very large dunes (> 100 m in spacing) 4.5 to 5 m in height was found. Dune migration rates were measured by comparing two successive morphological maps of the field, reaching a rate up to 21 m year − 1 toward the ocean. In contrast, the smaller dunes present on the northern border of the field migrate in an inward direction at a rate of 15.7 m year − 1 .

Interannual Variability in Tropical Estuarine Copepod Assemblages off Northeastern Brazil

We studied copepod assemblage variability among years, seasons, and tidal states in the Mucuri River estuary (Bahia State, Brazil). Zooplankton samples were collected seasonally through five years (2002-2006) at three sampling stations, one of which was sampled over a complete tidal cycle (two ebb and two flood tides). Temperature, salinity, river flux, and rainfall data were collected. Winter and summer represented dry and wet seasons, respectively. Copepod abundances ranged from 40 to 63% of the total zooplankton assemblage and comprised 46 taxa, among which, common estuarine species such as Temora turbinata (first record for the studied area), Parvocalanus crassirostris, Acartia lilljeborgi, Oithona hebes were the most abundant (euryhaline species). Interannual and seasonal variations were most marked in stenohaline species, e.g., Notodiaptomus sp. and Thermocyclops minutus; density variations of euryhaline species, which made up the majority of the abundant taxa, were most closely related to tides. Diversity and richness also followed an intertidal pattern of variation.

Variability of nutrients and chlorophyll a in Ancão Inlet during a spring tidal cycle in April 2009

São Luis or Ancão Inlet is the most western of the six inlets of the Ria Formosa lagoon, in southern Portugal. This inlet contributes relatively little to the water volume exchange over a tidal cycle (~10%). Temperature, salinity, pH, dissolved oxygen, and chlorophyll a and nutrient concentrations were measured hourly at the surface, at an intermediate level, and near the bottom over a complete spring tidal cycle, at a sampling point in the middle of the inlet. Current velocity was also measured hourly over the entire cross-section of the inlet using an acoustic Doppler current meter. From these parameters, the flow rate and the transport of nutrients and chlorophyll a were calculated and integrated over the complete tidal cycle providing the net transport. The results show that this is a well-mixed dynamic inlet where, during spring tide, the tidal signal is evident particularly for the nutrient and chlorophyll a concentrations. The nutrient concentrations were low (almost depleted), while the chlorophyll a concentrations were high (4–6 mg m–3). The estimated transports through Ancão Inlet revealed that there was a net import of chlorophyll a, nitrate, and phosphate to the Ria Formosa lagoon, while silicate was exported to the adjacent coastal area.

Estuary-ria exchange of cadmium, lead and zinc in the coastal system of the Ria of Vigo (NW Iberian Peninsula)

Little research has been done on the land-sea exchange of trace elements, which is particularly applicable to ria coasts. In particular, trace metal enrichment in the inner part of the Ria of Vigo (the San Simon Inlet) has been observed from sediment studies but there is no information about Cd, Pb and Zn fluxes through the Rande Strait, which is the natural boundary of the estuary-ria water exchange. In order to assess metal exchanges in a ria-type system, six sampling cruises on board the R/V Mytilus (IIM-CSIC) were carried out. Water column profiles of salinity, temperature and tidal currents were obtained every 30 min. The water column for dissolved and particulate metals was sampled every two hours over a complete tidal cycle. Dissolved metal concentrations were 0.01–0.18 nM for Cd, 0.5-1.9 nM for Pb and 4-44 nM for Zn. Compared with Zn (16±12%) and especially with Cd (5.4±5.0%), particulate metal represented a significant fraction of the total concentration for Pb (41±21%). Net fluxes of dissolved Cd and Zn are higher than in the particulate phase, whereas for Pb an inverse situation was observed. The net metal exchange in the Vigo estuary-ria environment was not seasonally controlled. Dissolved Cd and Pb were driven by tidal ranges and particulate Pb by the Oitaven River flow. On the other hand, Zn did not show a defined trend. The budgets obtained for the Ria of Vigo, with the exception of Pb, were one or two orders of magnitude lower than those measured in other large European estuaries.

CARACTERIZAÇÃO OCEANOGRÁFICA E BIOGEOQUÍMICA DOS ESTUÁRIOS DOS RIOS TAVARES E DEFUNTOS, RESERVA EXTRATIVISTA DE PIRAJUBAÉ, FLORIANÓPOLIS, SC

This paper presents a preliminary assessment on the oceanographic and biogeochemical characteristics of the Tavares River (ERT) and Defuntos River (ERD) estuaries, located in the Limões Bight, Pirajubaé Extractivist Reserve, Florianópolis, SC. Field surveys were carried out in the estuaries at June 30th on July 1st, 2000, along complete tidal cycles, 13 hours, under spring tide condition. At every survey the follow parameters were monitored: current speed and direction, salinity, temperature, particulate suspended matter (PSM), pH, dissolved oxygen, and inorganic dissolved nutrients concentration (ammonium, nitrite, nitrate, phosphate and silicon). In spite of being close one another less than 1 km, the estuaries showed considerably differences in the intra-tidal variation of the monitored parameters. The parameters in the ERT showed good coherence with flood and ebb tidal phases, meanwhile in the ERD they showed more complex pattern. In terms of net transport, the ERT exports PSM and nutrients towards Limões bight, and the ERD imports PSM though exports nutrients. Beside the major influence of the astronomical semi-diurnal tide, seiches are also important to these estuaries. The main seiches periods ranged between 1 and 2 hours.

CARACTERIZAÇÃO HIDRODINÂMICA DO ESTUÁRIO DO RIO CUBATÃO, JOINVILLE.

Este trabalho apresenta uma caracterização preliminar da hidrodinâmica do estuário do Rio Cubatão baseada em dados hidrográficos obtidos durante uma campanha de treze horas (um ciclo completo de maré) em condições de maré de sizígia. Dados de velocidade e direção de corrente, salinidade, temperatura e turbidez foram obtidos à 1,5 m abaixo da superfície, próximo da desembocadura do estuário com o fundeio de um correntógrafo acústico. Perfis verticais de salinidade e temperatura foram realizados próximos ao fundeio a cada hora com um CTD para fornecer a estrutura vertical. Perfis de CTD também foram realizados ao longo do estuário durante a preamar. Os resultados mostram que o estuário do Rio Cubatão apresenta-se verticalmente homogêneo com períodos menores de estratificação durante a vazante. O regime de maré mostra uma assimetria acentuada, com correntes de vazante muito mais intensas do que as correntes de enchente.

Benthic macroinvertebrates' vertical distribution in the Tagus estuary (Portugal): The influence of tidal cycle

In this work we evaluated the vertical distribution pattern of benthic infauna during the tidal cycle at one of the most important mudflats of the Tagus estuary. Samples were collected hourly during 24 h periods at four complete tidal cycles, using a corer specifically designed for the study purpose that allowed easy and effective separation of 15 different sediment layers. A particular case of general linear models, the hurdle model, was used to analyse data sets. We found that different species have different distribution and abundance according to sediment layers. Results showed that individuals tend to go deeper into sediment with a lower water column height and that these migrations are more visible during spring tides.

Net sediment transport and morphological change in the swash zone of a high-energy sandy beach from swash event to tidal cycle time scales

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.