Steep Beach Research Articles

Bedform contributions to cross-shore sediment transport on a dissipative beach

Field measurements of hydrodynamics, suspended sediment transport rates and bedform sediment transport rates were made in the intertidal section of a dissipative sandy beach (D50=0.26mm, slope=1/80) at Perranporth (UK). Pressure transducers, acoustic Doppler velocimeters, optical backscatter sensors and an acoustic sand ripple profiler were deployed for 12 tides, measuring in a range of wave heights from 0.5 to 2.2m, water depths from 1 to 6m, and in current strengths up to 0.4m/s. Data were analysed in terms of the distance to shore (x) normalised by the surf zone width (xs), and spanned the region 0.4<x/xs<3. Bedform heights up to 30cm and wavelengths 0.5 to 2.7m were recorded. Maximum wavelengths were observed just shoreward of the breakpoint. Bedforms were classified as sub-orbital, vortex ripples. Bedform migration was mostly onshore directed, and correlated with positive (onshore) wave skewness. Migration rates increased through the shoaling zone to a maximum of 1.5cm/min just shoreward of the breakpoint (x/xs=0.8). The bedform component of sediment transport was generally onshore directed, and maximum just shoreward of the breakpoint (0.021kg/m/s). Point measurements showed that the cross-shore suspended sediment transport 25cm above the bed was dominated by the mean component, with an offshore directed maximum at x/xs=0.5. Contributions to onshore transport were only made by the incident wave (gravity band) component. The total depth integrated suspended sediment transport was offshore directed and maximum in the mid surf zone (−0.16kg/m/s). The depth integrated suspended sediment transport dominated over the bedform sediment transport in the inner to mid surf zone (x/xs<0.5) and in the outer shoaling zone (x/xs>1.5). The fractional contribution of the shoreward directed bedform transport to the total absolute transport was up to 100%, and occurred broadly in the region of the breakpoint (0.5<x/xs<1.5). However, spatial averaging in the cross-shore indicated that a more realistic bedform contribution was up to 15% of the transport, with a maximum at x/xs=0.9. Results from this dissipative beach experiment generally agree with previous findings on intermediate beaches, steep beaches, and offshore sandbars.

Observations of wave shear stress on a steep beach

Observations are presented of the wave shear stress 〈u∼w∼〉 on a steeply sloping beach. Above the wave boundary layer (WBL), positive values of 〈u∼w∼〉 were observed and are attributed to a combination of both wave shoaling due to the large-scale bed slope, and dissipation due to wave breaking, in agreement with the wave theory of Zou et al. (2003). Within the WBL, observed vertical profiles of 〈u∼w∼〉 were also in good agreement with theory, in cases where the wave height was small. As wave heights increased, however, the WBL profile of 〈u∼w∼〉 generally did not agree with theory. Near-simultaneous rotary sonar observations of the bed suggest the disagreement with theory was due to the presence of orbital-scale ripples, which the present theory does not accommodate.

The surf zone heat budget: The effect of wave heating

AbstractSurf zone incident wave energy flux is dissipated by wave breaking which through viscosity generates heat. This effect is not present in shelf heat budgets and has not previously been considered. Pier‐based observations of water temperature in 1–4 m depth, meteorology, and waves are used to test a surf zone heat budget, which closes on diurnal and longer time scales. Wave energy flux is the second most variable term with mean contribution one fourth of the mean short‐wave radiation. The heat budget residual has semidiurnal and higher‐frequency variability and net cooling. Cross‐shore advective heat flux driven by internal wave events, rip currents, and undertow contribute to this residual variability and net cooling. In locations with large waves, steeper beaches, or less solar radiation, the ratio of wave energy flux to short‐wave radiation may be &gt;1.

SEA WALL and BEACH PROFILE INTERACTION IN RUN-UP REGION

Seawalls are the most common form of coastal defence, they are the physical barriers between land and sea. They are parallel to the beach used to prevent overtopping and flooding due to storm waves. They can be constructed in the front or back of the shoreline. However, the front face of a seawall is subjected to high energy wave action, which can induce the problem of toe scour. Scours in front of the breakwaters and the seawalls have been investigated heavily in the past, but it still remains a concern for coastal engineers. In those previous studies, the sea bottom was considered as flat or having a very mild slope. These may not be valid for steep slope beach in front of a structure. The seawall stands in run-up region in this study. The seawall and the beach profile interaction was investigated by experimentally. The beach slopes were 1/5 and 1/10 in the experiments.

Laboratory experiments of tsunami run-up and withdrawal in patchy coastal forest on a steep beach

Laboratory experiments were carried out to study tsunami flow dynamics in the presence of patchy macro-roughness, representing coastal forest, on a 1:10 steep beach. The experimental setup included four cross-shore rows of roughness patches affixed to the dry beach in a staggered array, such that 12 % of the staggered array region had higher roughness. The flow field during run-up and withdrawal was quantified using point measurements of velocity and flow depth at 20 locations, while high-resolution video was used to track bore position during run-up. Data analysis revealed that while inundated area was marginally impacted when patchy roughness was present, flow depths and flow force were, respectively, increased by more than 40 and 30 % in some areas within the patch array; a decrease in flow force was also observed in some areas. Alongshore variation in flow depth, induced by the roughness patches, was most pronounced during withdrawal. These findings suggest that patchy macro-roughness, like that created by coastal forest, will simultaneously lead to increased protection in some areas and decreased protection in others.

Intertidal habitats and decapod (Crustacea) diversity of Qeshm Island, a biodiversity hotspot within the Persian Gulf

Qualitative rapid assessments and taxonomic surveys of decapod crustaceans were carried out along the entire coastline of Qeshm Island in the Persian Gulf. Shore morphology and habitat distribution were examined. Simultaneously, decapod samples were collected from 40 selected sites. In total, 131 species from five different infraorders were identified, of which 61 were recorded for the first time from Qeshm Island and 18 species were new records for the Persian Gulf. The Brachyura possessed the highest species richness (73 species, 56 %) within the different infraorders. Among the surveyed habitats, the combined habitat “rocky/cobble”, occurring mainly along the south coast, had the highest species diversity. Rocky shores were dominated by Grapsus albolineatus, Metopograpsus messor, and Eriphia smithii. Cobble beaches were dominated by Leptodius exaratus, Epixanthus frontalis, Clibanarius signatus, Nanosesarma sarii, Petrolisthes spp. and Alpheus lobidens. Mudflats and mangrove forests, typical habitats of the north coast of Qeshm Island, were bordered along their landward fringe with Nasima dotilliformis and Uca sindensis, which were accompanied by Uca iranica in some places with coarser sediments. On muddy substrate and among mangroves, Metopograpsus messor, Parasesarma persicum, Eurycarcinus orientalis, Macrophthalmus depressus, Metaplax indica, Ilyoplax stevensi, Manningis arabicum, Opusia indica and Alpheus lobidens were the most common species. Exposed sandy beaches, mainly found on the south coast, were inhabited by Ocypode rotundata and Coenobita scaevola in high-intertidal and supralittoral zones, respectively. Emerita holthuisi occurred slightly lower in the mid-intertidal zone of relatively steep beaches where Diogenes avarus and Ryphila cancellus were found whenever the sandy beach was relatively flat. Intertidal habitats of the island are generally in relatively good condition compared with the other Persian Gulf states.

Morphological modeling of a nourished bayside beach with a low tide terrace

We present a simple one-dimensional process-based morphological model for predicting medium–long term beach profile evolution of a bayside beach with a low tide terrace. Bayside beaches are characterized by low wave energy. In regions with a large tidal range, a beach face is slowly eroded due to erosional waves inside the surf zone which shifts along the beach profile over a wide range during a tidal cycle. The model is developed using an existing model framework proposed by Mariotti and Fagherazzi (2010), who formulated a model for sediment suspension and transport due to waves and tidal currents in a tidal flat regime. The new developments in this study include an implementation of the bedload sediment transport and modification of the suspended load transport for noncohesive particles. The model parameters were calibrated using beach profile data collected over a one-year span at Xiamen Beach. Numerical experiments indicate that waves suspend sediment in the narrow surf zone, inducing pulse-like high sediment concentration in shallow water. The suspended sediment is advected by the tidal current and wave-induced undertow, causing erosion of the steep beach face at the upper portion and deposition of the flat beach face at the lower portion of a beach profile. The erosion on the upper beach face was not predicted by a bedload-only transport model, suggesting that the sediment suspension associated wave breaking may be the major mechanism for the erosion. Tide-only induced sediment transport is insignificant, but tides play a role in distributing the wave-induced erosion over a wide region on a beach profile.

Observations and modeling of steep‐beach grain‐size variability

Novel observations of surface grain‐size distributions are used in combination with intra‐wave modeling to examine the processes responsible for the sorting of sediment grains on a relatively steep beach (slope = 1:7.5). The field observations of the mean grain size collected with a digital camera system at consecutive low and high tides for a 2 week period show significant temporal and spatial variation. This variation is reproduced by the modeling approach when the surf zone flow‐circulation is relatively weak, showing coarse grain sizes at the location of the shore break and finer sediment onshore and offshore of the shore break. The model results suggest that grain size sorting is dominated by the wave‐breaking‐related suspended sediment transport which removes finer sediment from the shore break and transports it both on‐shore and offshore. The transport capacity of wave‐breaking‐related suspended sediment is controlled by the sediment response time scale in the advection‐diffusion equation, where small (large) values promote onshore (offshore) transport. Comparisons with the observed beach profile evolution suggest a relatively short time scale for the suspended sediment response which could be explained by the vigorous breaking of the waves at the shore break.

Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France

Birrien, F., Castelle, B., Dailloux, D., Marieu, V., Rihouey, D. and Price, T.D., 2013. Video observation of megacusp evolution along a high-energy engineered sandy beach: Anglet, SW France.We present an 18-month period of video monitoring of both the nearshore sandbars and the megacusps at Anglet Beach, SW France. The study site covers a 2-km long stretch of beach that is constrained to the North by a groin that extends 400 m seaward. The beach is high-energy intermediate, mostly double-barred, with a steep beach face (~1/10) favouring the formation of cusps at a large range of lengthscales, from beach cusps (O(10 m)) to megacusps (O(100 m)). A megacusp is systematically observed against the groin as a result of the persistent presence of a topographic rip. Further away from the groin, about 4–5 megacusps are typically observed. The shoreline dynamics are strongly controlled by the geometry of the surfzone sandbar. In-phase shoreline-sandbar coupling was commonly observed, that is, megacusps in the alignment of the rip channels and seaward bulges in the shoreline facing the surfzone sandbar horns. Megacusps at Anglet Beach have a typical cross-shore amplitude of O(10 m). During a severe storm with significant wave heights over 7 m, a very erosive megacusp (hotspot) formed in the alignment of the rip channel. Prior to this storm, a seaward bulge in the shoreline was observed in this location. Accordingly, 40 m cross-shore variation change in shoreline position at this location was the signature of megacusp dynamics only. This hotspot was systematically observed at the same location during 7–8 months after the storm event, before the fair weather conditions in summer resulted in an overall accretion of the beach. Megacusps were smoothed out during rapid alongshore migration of the sandbar or for high-energy wave conditions. In addition, the evolution of the alongshore-averaged shoreline position shows the exact opposite trend as that of the alongshore-averaged sandbar position, yet with less pronounced cross-shore variations. Overall, our results highlight once again the strong links between shoreline and surfzone sandbar dynamics.

Numerical modelling of pronounced sloping beach profile evolution: comparison with the large-scale BARDEX II experiment

ABSTRACT Dubarbier, B., Castelle, B, Marieu, V and Ruessink, B.G., 2013. Numerical modelling of pronounced sloping beach profile evolution: comparaison with the large-scale BARDEX II experiment .In: Conley, D.C., Masselink, G., Russell, P.E. and O'Hare, T.J. (eds.) Barriers and sandbars are ubiquitous natural coastal features, whose variability often determines nearshore morphological evolution. Wave-dominated beach profile evolution results from the interaction between wave non-linearities, wave-breaking induced turbulence, undertow, infragravity motions and swash processes. To explore each of these contributions to the sediment transport, the full-scale Barrier Dynamics Experiment (BARDEX II), performed in the Delta Flume in June 2012, provides a new dataset for the rigorous testing of the performance of beach profile evolution models in the case of steep beaches. This new experiment will improve our knowledge on (1) swash zone processes, including infiltration and exfiltration of water into the sand an...

PERFORMANCE OF PERCHED BEACH NOURISHMENTS

Perched beach nourishments are popular measures to contrast beach erosion in the presence of steep beaches. Inspired by a real case-study, in the present work the performance of two configurations of the submerged barrier used to protect the toe of the beach are experimentally investigated. Measurements have been gathered concerning wave reflection and transmission, stability and scours around the submerged sill, and beach profile evolution. Both accretive and erosive wave conditions have been considered here. A comparative analysis on the evolution of the two configurations of the sill, shows that the armoring of the filter behind the rubble mound structure is a very effective measure to increase the level of beach protection (e.g. the shoreline retreat under erosive wave attack is reduced to a third). The comparison of the experimental results with an analytical model to predict the equilibrium profile of perched beaches shows that the key phenomena controlling the process are related not only to wave reflection but also to wave dissipation mechanisms at the sill. Moreover, the comparison of the experimental results with field data proves that notwithstanding the several simplifying assumptions considered in the physical modeling, several phenomena are reasonably predicted by the model (e.g. beach evolution, stability and scours).

One-year along-beach variation in the maximum depth of erosion resulting from irregular shoreline morphology

Beach erosion and accretion occur across multiple time scales. Over long time scales (decades to millennia) the shoreface ravinement surface, which is recognized as a coarse lag deposit, forms at the shoreface toe as a result of wave- and current-induced erosion during shoreline transgression. Over short time scales (hours to days) the depth of sediment disturbance, which is recognized as coarse lamina and measured at the foreshore by devices and monitoring tracer beds, forms as a result of wave- and current-induced reworking during a tidal cycle. The maximum depth of erosion (MDOE), quantified here over 1year, is modulated by processes that operate over a time scale that is between the drivers of short-term (e.g. tides and waves) and long-term (e.g. sea-level rise) beach erosion. The MDOE integrates the erosion that occurs over a discrete time interval and records the maximum depth of erosion that is likely principally induced by storms, which is difficult to quantify by other methods that rely on discrete observations (e.g. changes in elevation or movement of the mean high-water line). A novel technique for quantifying the MDOE, based on comparing the bedding and stratigraphy between cores collected at the same locations over a discrete time interval, is presented here and applied at Onslow Beach, NC, USA. This 12km-long barrier island has irregular shoreline morphology, characterized by two embayments separated by a central headland. This shape is largely the result of variations in the depth of underlying rock strata and produces a steeper beachface at the headland than at the embayments. At each of the six sites examined along the barrier, the MDOE is found to increase from the backshore to the middle intertidal zone and is higher at the sites closer to the headland. These variations in the MDOE are likely due to the increase in average wave energy impacting the beachface from an offshore direction and steeper beaches (intermediate beach state) at the headland. Where the MDOE is within the beach facies, it is not associated with a coarsening, which is due to the heterolithic nature of the Onslow-beach strata. Where the MDOE is the contact between back-barrier and beach facies it is always associated with a coarsening and a gravel-rich lag deposit because in this case, the MDOE is an amalgamation of multiple erosional events, which is similar to the shoreface ravinement surface. Along-beach variation in the MDOE does not correspond with discrete observations of beach change over the same period and is likely a better indicator of erosion potential than long-term discrete observations, such as changes in surface elevation or the position of the mean high-water line.

Chronic offshore loss of nourishment on Nice beach, French Riviera: A case of over-nourishment of a steep beach?

The 4.5 km-long gravel beach fronting the exclusive resort of the city of Nice, on the French Riviera, in southeastern France, was artificially nourished from 1976 to 2005 to the tune of 558,000 m 3, making this long-term operation one of the most significant for gravel beaches in the world. Nourishment has ranged from nil in certain years (1979, 1980, 1983–85, and 2001–2002) to a peak of over 97,000 m 3 in 2000. Analyses of 50 transects covering the beach highlight no significant change in net beach width over this 30-year period of massive gravel nourishment. A Principal Components Analysis and a Cluster Analysis used to detect patterns in the 87 beach-width measurement dataset show no clear spatial trends in transect groups that can be interpreted in terms of the morphology of the beach and the steep inner shoreface. Significant wave height off Nice shows no change over the period 1979–2005. Since there is no possibility for alongshore gravel leakage on the strongly embayed Nice beach, the relative stability in beach width clearly implies loss of recharged gravel offshore. Gravel loss following nourishment is favoured by: (1) the steep inner shoreface inherited from the geological context of Nice beach at the flanks of the southern Alps, and (2) the practise of artificial beach widening through flattening, in summer, of a narrow (5–15 m-wide) mobile zone of the profile in order to enhance the ‘carrying’ capacity of this highly touristic beach. Beach widening and flattening following nourishment bring close to the very steep inner shoreface zone several cubic metres of gravel for each metre of beach that may be permanently lost downslope during autumn and winter storms. Recharged gravel is redistributed alongshore and offshore leakage is probably enhanced where small narrow submarine canyon heads impinge on the beach, resulting in a very narrow shoreface. Mean beach width shows an oscillating alongshore pattern that may be due to the influence of these canyons as pathways of gravel loss offshore. However, there is no correlation between mean beach width and distance to the 10-m isobath, used as a surrogate for inner shoreface width. Storms are associated with plunging waves that are particularly effective and concentrated, on this almost tideless shore, over the narrow mobile zone of the beach profile where a series of steep reflective berms are built during storms. The high dynamic pressures associated with this narrow zone of concentrated wave breaking, and energy reflection from the steepened profile, are deemed to contribute to the permanent downslope loss of gravel. This situation of long-term gravel loss is probably accepted by the beach management authority because of the low cost of obtaining nourishment material and the advantages derived from a temporarily wider beach in terms of recreational space.

Physical determinants of intertidal communities on dissipative beaches: Implications of sea-level rise

Sea-level rise is likely to cause significant changes in the morphodynamic state of beaches in the higher latitudes, resulting in steeper beaches with larger particle sizes. These physical changes have implications for beach invertebrate communities, which are determined largely by sediment particle size, and hence for ecosystem function. Previous studies have explored the relationships between invertebrate communities and environmental variables such as particle size, beach slope and exposure to wave action, and often these physical variables can be integrated in various indices of morphodynamic state. Most of these studies incorporated a full range of beach types that included wave-dominated surf beaches, where the wave action is harsh enough to enable reliable estimates of breaker height, a parameter included in several of the indices, and concluded that more dissipative beaches with gentler slopes and finer particle sizes often support a higher number of species and greater abundance than more reflective beaches. Whether these predictions remain valid for less wave-dominated beaches, where breaker height is more difficult to determine, is uncertain. In the present study, the abundance of meio- and macrofauna was quantified across a range of beaches in the UK, which are generally towards the lower energy end of the morphodynamic gradient, and their relationships with beach physical properties explored. No significant relationships were found between abundance and the standard morphodynamic indices, but significant relationships were found for both macro- and meiofaunal abundance when these indices were combined with an exposure index (derived from velocity, direction, duration and the effective fetch). All the relationships identified between abundance and combined morphodynamic indices indicated a higher abundance of both macro- and meiofauna on the more dissipative beaches. The reverse was however found for species richness. If predictions that accelerated sea-level rise will move beaches towards a more reflective morphodynamic state are correct, this could lead to declines in the abundance of meio- and macrofauna, with potential adverse consequences for ecosystem functioning.

Laboratory and numerical study of dambreak-generated swash on impermeable slopes

New laboratory experiments have produced detailed measurements of hydrodynamics within swash generated by bore collapse on a steep beach. The experiments are based on a dambreak rig producing a highly repeatable, large-scale swash event, enabling detailed measurements of depths and velocities at a number of locations across the swash zone. Experiments were conducted on two beaches, differentiated by roughness. Results are presented for uprush shoreline motion, flow depths, depth-averaged velocity, velocity profiles and turbulence intensity. Estimates of the time- and spatially-varying bed shear stress are obtained via log-law fitting to the velocity profiles and are compared with the shear plate measurements of Barnes et al. (2009) for similar experimental conditions. Experimental results are compared with model predictions based on a NLSWE model with momentum loss parameterised using the simple quadratic stress law in terms of the depth-averaged velocity. Predicted and measured flow depths and depth-averaged velocities agree reasonably well for much of the swash period, but agreement is not good at the time of bore arrival and towards the end of the backwash. The parameterisation of total momentum loss via the quadratic stress law cannot adequately model the swash bed shear stress at these critical times.

Comparison Between Moving Paddle and Mass Source Methods for Solitary Wave Generation and Propagation Over a Steep Sloping Beach

Two methods for generating nonlinear solitary waves on constant water depth in order to study their further runup over a steep sloping beach are presented. The first method is based on a prescribed wave paddle motion inducing a desired free surface profile. The second generation procedure is based on an appropriate mass source term added to the mass conservation equation in an internal source line. These two wave generation methods are implemented in the industrial Computational Fluid Dynamics code (CFD): PHOENICS (Parabolic Hyperbolic Or Elliptic Numerical Integration Code Series). The generated solitary wave’s profile and their non-breaking runup over an impermeable steep beach are in good agreement with experimental data of Lin, Chang and Liu (1999). No appreciable discrepancies are observed between experimental and simulated velocities during the runup and rundown processes except for the horizontal velocity component at the final stage of the second runup. The moving paddle and mass source methods performed equally well on reproducing non-breaking runup despite the difference in their wave generation procedures.

Validity of the equilibrium beach profiles: Nile Delta Coastal Zone, Egypt

Beach profile equilibrium is the principal concept assumed by most numerical modelling. The response of beach profile configuration to natural and anthropogenic changes could be predicted to help in selecting the most appropriate engineering design required to mitigate coastal erosion and accretion alongshore. Thus, in order to apply coastal engineering projects, the predicted profile of equilibrium should be close enough to the measured profile. Therefore, before application of numerical modelling techniques, equilibrium expressions have to be validated at the study site. This research aims to assess the validity of the equilibrium profile concept on the Nile Delta coast based on beach profiles surveyed in 1990 from the main promontories; Abu-Quir Bay, the Rosetta promontory and the El-Burullus promontory. The results indicate that the equilibrium beach profile is consistent with the measured profiles at the study sites beyond 200 m distance offshore at − 4 m depth. In contrast, the equilibrium status is not valid along the beach face at − 1 m depth. Accreted beaches at Abu-Quir bay and Burullus promontory are characterized by wide berms and gentle beach face whereas eroded stretches at Rosetta promontory have a narrow berm and steep beach faces. The measured profiles are also compared with the exponential beach profile concept. An exponential hypothesis is not valid along the Nile Delta coast. Profiles measured at Abu-Quir, Rosetta and El-Burullus depart significantly from the exponential equation. Despite the fact that equilibrium expression can describe beach profiles along the Nile Delta, one equilibrium profile equation is not sufficient to assess all beach profiles. This can be explained as the morphology of beach profiles is subjected to some factors including; sediment characteristics, wave parameters and closure depth, which vary alongshore. Analysis of the validity of the equilibrium beach profile is recommended to get accurate results in modelling simulations and design the most appropriate engineering projects required for shore protection.

Spatial distribution of the Ocypode quadrata (Crustacea: Ocypodidae) along estuarine environments in the Paranaguá Bay Complex, southern Brazil

This study examines the spatial distribution of the ghost crabs, Ocypode quadrata Fabricius, 1787, in thirteen estuarine sandy beaches located along two main axes of the Paranaguá Bay Estuarine Complex, southern Brazil. Burrow densities of ghost crabs were measured at three beach levels established around the high tide mark during the summer and winter of 2005. All beaches showed a steep beach face slope (2.6 to 8.3º) with sediment composition varying from well sorted fine sand to very poorly sorted coarse sand towards the upper estuary. Water salinity ranged from around 31 at those beaches near the bay inlet, to 14 at beaches in the inner estuary. The burrow densities of O. quadrata in the estuarine beaches were similar to those observed in the oceanic beaches. However, the absence of burrows at the four innermost beaches suggests that low salinity and sediment penetrability may prevent ghost crabs from occurring in this region of the estuary. Burrow densities showed strong seasonal variability. The low densities observed during the winter are probably related to a delay crab activities due to low temperatures in the early morning during this season. The absence of a clear zonation pattern was related to estuarine beach morphology.

Is the burrowing performance of a sandy beach surfing gastropod limiting for its macroscale distribution?

The size-specific burrowing capacity of Olivella semistriata, an extremely abundant surfing gastropod of exposed sandy beaches in the tropical East Pacific, was investigated in terms of sediment grain size. For all investigated sand classes, there was a significant increase in burial time with size. Burrowing was fastest in sand with grain size between 150 and 355 μm, as well as in native sediment (median grain size 209 μm) and in field conditions (median grain size 223 μm). Values of the burrowing rate index (BRI) were found to be between 3 and 7, rating the burrowing capacity of O. semistriata to be fast to very fast. Data from previous qualitative and quantitative sampling campaigns were used to identify the macroscale (i.e., between beaches) distribution of O. semistriata in terms of sediment grain size and swash conditions. Swash period was shorter than burial time, excluding this as a factor limiting the distribution. Swash standstill time (the time between uprush and backwash), however, was just long enough on the beaches, where O. semistriata was present to allow for securing firm anchorage. On reflective beaches, the swash standstill time is as short as 1 or 2 s, denying O. semistriata the time to burry itself before being swept away by the backwash. As such, swash standstill time is advocated as a valuable part of the swash exclusion hypothesis. A survey of the available literature on the burrowing of surfers shows that mole crabs are by far the fastest burrowers and the only surfers that burrow sufficiently fast to withstand the extremely short swash standstill time on reflective beaches. Burrowing ability of surfing gastropods is found in the same range as surfing bivalves, both being insufficient to cope with coarse sediment on steep beaches. Finally, we suggest that neither burial time nor BRI, yet rather the minimal burial time—the time needed to anchor securely in a certain sand at a given swash velocity—should be used to judge the limitations of burial in terms of sediment and swash conditions.

Field investigation on the results of non-linear shallow water equations in the Swash Zone

For the study of the cross-shore wave-induced hydrodynamics in the swash zone, a numerical model is developed based on the one-dimensional non-linear shallow water (NSW) equations for prediction of hydrodynamic parameters in the swash zone. In order to evaluate the accuracy of the outputs of the numerical model, the model's predictions in terms of water surface elevations and cross-shore velocities, are compared to field data from full-scale experiments conducted on three sites with different beach slope; mild and steep, several bed particle sizes and under various incident wave conditions. The quantitative and qualitative comparison of the results of the numerical model and the full-scale data reveals that the model can generally predict many aspects of the flow in the surf and swash zone on both types of beach. The accuracy is adequate for application in a sediment transport study. Considering the time-history and probability distribution of water surface elevation, the model is generally more accurate on steep beaches than on the mild beach. The model can adequately simulate the dominant frequency across the beach and saturation of higher frequencies on both mild and steep beaches for various incident wave energy characteristics. With regard to the horizontal (cross-shore) velocity, the sawtooth shape of time-history and negative acceleration of water are well predicted by the model for both mild and steep beaches. Due to the uncertainties in maximum and minimum values of velocity data, clear judgement about the accuracy of the numerical model in this matter was not possible. However, the comparison of the minimum velocities (offshore direction) revealed that the application of friction factors below the range which is suggested by literature best match the data.