Swash Zone Research Articles

Habitat Preference and Abundance of Mole Crab Based on Sea Tides in Coastal Area of Kebumen Regency

Mole crab is an important resource in the food chain cycle and an initial level consumers in sandy areas. Mole crab lives in the swash zone that is affected by sea tides. Comparison of habitat and abundance of each station based on tides in terms of the type of utilization of the study location. The purpose of this study was to determine the abundance at high tide and low tide, species, and habitat preferences of mole crab, also to determine the relationship between their habitat preferences and their abundance. The method used in this study was descriptive exploratory and uses purposive random sampling technique. Abundance data collection was done by using a 10 x 10meter transect quadrant at two spring tides and two neap tides in the swash zone area. The results showed that the abundance of mole crab at high tide was 214 - 239 ind/100m2 and at low tide was 378 - 509 ind/100m2. There are three types of mole crab that has been found, Emerita emeritus, Hippa adactyla and Albunea symmysta. Favored habitat of mole crab in coast area of Kebumen Regency has a flat to sloping beach, wide swash zone at high tide 14.5 - 24.5m, and at low tide is 20 - 32.5m, medium sized beach sand (0.25-0.5mm), and has low organic material values. The results of the PCA (Principal Component Analysis) analysis that the sediment grain size and the width of the swash zone have a positive correlation with the abundance of mole crab.

Groundwater Flow and Moisture Dynamics in the Swash Zone: Effects of Heterogeneous Hydraulic Conductivity and Capillarity

AbstractA density‐dependent, variably saturated groundwater flow and solute transport model was used to investigate the influence of swash motions on subsurface flow and moisture dynamics in beach aquifers with heterogeneous distributions of hydraulic conductivity (K) and capillarity. The numerical simulations were performed within a Monte Carlo framework using field measurements conducted in the swash zone of a sandy beach. Our results show that heterogeneous capillarity causes spatially variable capillary rise above the groundwater table. In response to swash motions, heterogeneity creates capillary barriers that result in pockets of elevated moisture content beneath the swash zone. These moisture hotspots persist within the unsaturated zone even at ebb tide when the swash motions recede seaward. Heterogeneous capillarity also results in highly tortuous preferential flow paths and alters the flow rates from the sand surface to the water table. Heterogeneous K greatly enhances the seawater infiltration into the swash zone and modulates its spatial distribution along the beach surface. Due to heterogeneous K and capillarity, complex mixing patterns emerge. Both strain‐dominated and vorticity‐dominated flow regions develop and dissipate as tides and waves move across the beach surface. Complex mixing patterns of seawater percolating from the swash zone surface to the water table, with localized areas of high and low mixing intensities, are further demonstrated by analysis of dilution index. Our findings reveal the influence of geologic heterogeneity on swash zone moisture and flow dynamics, which may have important implications for sediment transport and chemical processing in beach aquifers.

Development of the Physics–Based Morphology Model as the Platform for the Optimal Design of Beach Nourishment Project: A Numerical Study

In this study, a physics-based morphology model is developed and to test the feasibility of the morphology model proposed in this study as the platform for the optimal design of the beach nourishment project, the beach restoration process by the infra-gravity waves underlying the swells in a mild sea is numerically simulated. As a hydrodynamic module, the IHFOAM wave toolbox having its roots in the OpenFoam is used. Speaking of the morphology model, a transport equation for suspended load and the Exner type equation constitute the morphology model. In doing so, the probability theory first introduced by Einstein and the physical model test by Bagnold are used as the constituent sub-model of the morphology model. Numerical results show that among many flow features that are indispensable in forming sand bars over the flat bottom and swash zone, the partially skewed and asymmetric bottom shearing stresses, a shoreward Stokes drift near the free surface, boundary layer streaming near the seabed, and undertow toward the offshore were successfully simulated using the morphology model proposed in this study. It was also shown that plunging type breaker occurring at the final stage of the shoaling process, and its accompanying second breaker, sediment entrainment at the seabed, and the redistribution of suspended load by the down rush of preceding waves were successfully reproduced in the numerical simulation, and agreements with our experience in the field were very encouraging. In particular, the sand bar formation process over the flat bottom and backshore were successfully reproduced in the numerical simulation, which has been regarded as a challenging task.

Experimental Study on the Influence of an Artificial Reef on Cross-Shore Morphodynamic Processes of a Wave-Dominated Beach

Artificial reefs are being implemented around the world for their multi-functions including coastal protection and environmental improvement. To better understand the hydrodynamic and morphodynamic roles of an artificial reef (AR) in beach protection, a series of experiments were conducted in a 50 m-long wave flume configured with a 1:10 sloping beach and a model AR (1.8 m long × 0.3 m high) with 0.2 m submergence depth. Five regular and five irregular wave conditions were generated on two types of beach profiles (with/without model AR) to study the cross-shore hydrodynamic and morphological evolution process. The influences of AR on the processes are concluded as follows: (1) AR significantly decreases the incident wave energy, and its dissipation effect differs for higher and lower harmonics under irregular wave climates; (2) AR changes the cross-shore patterns of hydrodynamic factors (significant wave height, wave skewness and asymmetry, and undertow), leading to the movement of shoaling and breaking zones; (3) the beach evolution is characterized by a sandbar and a scarp which respectively sit at a higher and lower location on the profile with AR than natural beach without AR; (4) the cross-shore morphological features indicate that AR can lead to beach state transformation toward reflective state; (5) the scarp retreat process can be described by a model where the scarp location depends linearly on the natural exponential of time with the fitting parameters determined by wave run-up reduced by AR. This study demonstrates cross-shore effects of AR as a beach protection structure that changes wave dynamics in surf and swash zone, reduces offshore sediment transport, and induces different morphological features.

Simulating wave runup on an intermediate–reflective beach using a wave-resolving and a wave-averaged version of XBeach

The prediction of wave runup, as well as its components, time-averaged setup and the time-varying swash, is a key element of coastal storm hazard assessments, as wave runup controls the transitions between morphodynamic response types such as dune erosion and overwash, and the potential for flooding by wave overtopping. While theoretically able to simulate the dominant low-frequency swash, previous studies using the infragravity-wave–resolving model XBeach (XBSB) have shown an underestimation of the observed swash variance and wave runup, which was in part related to the absence of incident-band swash motions in the model. Here, we use an incident-band wave-resolving, non-hydrostatic version of the XBeach model (XBNH) to simulate wave runup observed during the SandyDuck '97 experiment on an intermediate–reflective sandy beach. The results show that the XBNH model describes wave runup and the individual setup and swash components well. We subsequently examine differences in wave runup prediction between the XBSB and XBNH models and find that the XBNH model is a better predictor of wave runup than XBSB for this beach, which is due to better predictions of both the incident-band and infragravity-band swash. For a range of beach states from reflective to dissipative it is shown that incident-band swash is underestimated by XBSB relative to XBNH, in particular for reflective conditions. Infragravity-band swash is shown to be lower in XBSB than XBNH for most conditions, including dissipative conditions for which the mean difference is 16% of the deep water wave height. The difference in infragravity-band swash in XBNH relative to XBSB is shown to mainly be the result of processes occurring outside the swash zone, but approximately 15% of the difference is caused by explicitly resolving incident-band wave motions within the swash zone, such as swash-swash interactions, which inherently cannot be simulated by wave-averaged models.

Swash zone morphodynamic modelling including sediment entrained by bore-generated turbulence

In this paper we introduce a mathematical model of sediment entrainment due to bore-generated turbulence in a shallow water context. In this model, the entrainment is assumed to be proportional to the energy decay rate across a bore on a mobile bed. The energy decay rate across a bore on a mobile bed is derived analytically. This model is incorporated into the one dimensional morphodynamic model developed by Zhu and Dodd (2015), which includes bed- and suspended load transport. This results in new shock conditions, which allow for sediment entrainment at a shock. With it we investigate the effects of sediment entrainment due to bore-generated turbulence on beachface evolution under a single swash event driven by a solitary wave. The simulation results imply that sediment entrainment by bore turbulence at the incoming bore dominates over sediment mobilisation by bed shear stress. In contrast, the backwash bore is dominated by bed shear stress related processes. The morphodynamic impact of bore turbulence on this swash event is primarily erosion of the sea bed seaward of the initial shoreline. Any sediment remaining in the water column seaward of this point is then available to be transported by subsequent events. It is shown that the bed step is primarily a bed load related feature, with sediment entrained as suspended load counteracting the bed step growth.

Vertical sorting process in oscillating water tank using DEM-MPS coupling model

In natural beaches, sea-beds consist of graded sediments. In swash and surf zones, wave actions lead to selective transportation and segregation of sediments. Thus, size segregation needs to be considered while determining the sediment transport rate and morphological dynamics on beaches. However, because of the strong non-linearity and the large deformed free surface, previous experimental or observational investigations failed to obtain enough knowledge about the sorting process in the swash zone. In this study, as the first investigation step of the size sorting process in the swash zone, an experiment was conducted with an oscillatory water tank, while the vertical sorting process was observed. Moreover, to further investigate the structure of the flow and movable bed, a 3D numerical simulation was performed. A coupling scheme of the discrete element method (DEM) and the moving particles semi-implicit (MPS) method was applied to reproduce the vertical sorting progress. The result revealed that the development of the vertical sorting brought about bed surface armoring and a horizontal velocity transition. Additionally, an acting force investigation disclosed that the inertia of the water surface gradient induced oscillations of the sediments, which consisted of elements, in the water tank while leading to frequent element collisions and contacts, which caused the elements to be pushed upward.

Foamed Polystyrene in the Marine Environment: Sources, Additives, Transport, Behavior, and Impacts

Foamed polystyrene (PS) that may be either expanded (EPS) or extruded (XPS) is a rigid, lightweight insulating thermoplastic that has a variety of uses in the consumer, packaging, construction, and marine sectors. The properties of the material also result in waste that is readily generated, dispersed, and fragmented in the environment. This review focuses on foamed PS in the marine setting, including its sources, transport, degradation, acquisition of contaminants, ingestion by animals, and biological impacts arising from the mobilization of chemical additives. In the ocean, foamed PS is subject to wind-assisted transport and fracturing via photolytic degradation. The material may also act as a substrate for rafting organisms while being exposed to elevated concentrations of natural and anthropogenic surface-active chemicals in the sea surface microlayer. In the littoral setting, fragmentation is accentuated by milling in the swash zone and abrasion when beached, with wind transport leading to the temporary burial of significant quantities of material. Ingestion of EPS and XPS has been documented for a variety of marine animals, but principally those that feed at the sea surface or use the material as a habitat. As well as risking injuries due to gastro-intestinal blockage, ingestion of foamed PS exposes animals to harmful chemicals, and of greatest concern in this respect is the presence of the historical, but still recycled, flame-retardant, hexabromocyclododecane. Because foamed PS is particularly difficult to retrieve as a constituent of marine litter, means of reducing its presence and impacts will rely on the elimination of processes that generate foamed waste, modification of current storage and disposal practices, and the development of more durable and sustainable alternatives.

Numerical investigation of ripple in oscillating water tank by DEM-MPS coupled solid–liquid two-phase flow model

The height of ripples, representing roughness in the shallow-water area, is an important factor in the prediction of coastal processes. However, the characterization of ripples under violent flow conditions has proven challenging. For instance, in shallow water in the swash zone, significant water fluctuation occurs owing to interactions between the up-rush and down-rush of waves. Because such phenomena shows strong nonlinearity, sufficient knowledge of swash-ripple formation remains lacking. In this study, experiments generating ripples in an oscillating water tank are conducted. Furthermore, numerical simulation using the three-dimensional DEM-MPS coupled method is performed. By comparing numerical and experimental results, validation of the present numerical simulation is demonstrated, followed by an investigation of the mechanics of ripple formation. Numerical results show that not only the drag force but pressure gradient force resulting from the water surface gradient considerably affect sand-particle motion, revealing the need for accurate evaluation of the free surface flow. Moreover, pressure gradient force plays an important role in the ripple formation process, so it should be reflected in the numerical model for ripple simulations.

Morphodynamic response to extreme wave events of microtidal dissipative and reflective beaches

In this work, the morphodynamic response of microtidal dissipative and reflective beaches to extreme wave events generated by hurricanes and cold fronts in the Colombian Caribbean was established. Two sandy beaches were used as case studies, Bocagrande (Cartagena, Bolívar Department) (dissipative) and Costa Verde (Ciénaga, Magdalena Department) (intermediate–reflective). The numerical models SWAN and XBeach were used to investigate wave propagation during storm conditions for selected cases and to determine morphological changes on the two beaches. Both models were calibrated using hydrodynamic and bathymetric data obtained in field campaigns pre and post-cold fronts. The results of the numerical models satisfactorily represented the real conditions on both beaches. It was shown that regardless of beach configuration, whether dissipative or intermediate–reflective, erosive processes had a greater magnitude during cold fronts (March 2009 and December 2017) than under hurricanes conditions. Bocagrande beach is more exposed to extreme wave events, mainly due to its characteristic dissipative profile. Infragravity wave energy, fundamental in sediment transport dynamics, is not reflected on the beach front, but is dissipated along the swash zone and drags sediment to the sea. By contrast, Costa Verde shows conservative coastline retreats: its intermediate–reflective morphologic configuration played a transcendental role. Infragravity wave reflection on the beach front does not allow a longer water extension over dry areas in the swash zone under storms.

Wave-resolving shoreline boundary conditions for wave-averaged coastal models

Downscaling broadscale ocean model information to resolve the fine-scale swash-zone dynamics has a number of applications, such as improved resolution of coastal flood hazard drivers, modeling of sediment transport and seabed morphological evolution. A new method is presented, which enables wave-averaged models for the nearshore circulation to include short-wave induced swash zone dynamics that evolve at the wave group scale (i.e. averaged over the short waves). Such dynamics, which cannot be described, by construction through wave-averaged models, play a fundamental role in nearshore hydrodynamics and morphodynamics. The method is based on the implementation of a set of Shoreline Boundary Conditions (SBCs) in wave-averaged models. The chosen set of SBCs allows for proper computation of the short-wave properties at a mean shoreline (xl) taken as the envelope of the actual shoreline. The suitability of the approach is assessed through implementation of the SBCs into the Regional Ocean Modeling System (ROMS) coupled to a spectral wave model (InWave for IG waves and SWAN for wind waves). As the aim is to assess the viability of the approach, the SBCs are implemented only through a one-way coupling to ROMS (i.e. ROMS forcing the SBCs). Four different test cases – with constant, periodic and bichromatic offshore forcing – are run to assess the model performances. The main results of the analysis are: (a) the proposed SBCs can well reproduce the shoreline motion and swash zone dynamics in there for all chosen tests (RMSE and BIAS less than 20 % up to a cross-shore resolution of 4.0m (L0∕3 or L0∕5)) and (b) implementation of the SBCs allows ROMS to accurately simulate the swash zone flows even at a resolution 40 times coarser than that needed by ROMS with its own wet–dry routine to properly describe the same flows. The latter result clearly demonstrates the major computational advantage of using the proposed SBCs. We also show that most of the swash zone dynamics is due to the mean flow (i.e. incoming Riemann variable) and the local (at xl) wave height. However, especially in the case of bichromatic waves, the swash zone water volume content also seems to play a crucial role.

Surficial sediment data from the Shoalhaven River delta: Bed channel and adjacent beach.

Estuaries on wave-dominated coasts generally comprise three sedimentary environments: fluvial sands and gravels derived from the catchment; marine sands characteristic of the beaches and nearshore; and silts and clays that accumulate in the sheltered central basin. Estuarine transition to deltaic form occurs when geomorphological maturity is achieved during coastal evolution. Sedimentary plains become infilled and a narrow channel connects the catchment and facilitates the transport of fluvial sediments to the coast. Here, we present modern sedimentary data that supports the idea that the wave-dominated Shoalhaven system in southeastern Australia has transitioned from an estuary to delta, transporting fluvial sediments to the modern adjacent beach and contributing to coastal progradation. A total of 141 bed channel and swash zone samples were collected from the estuarine channel of the Shoalhaven River and the adjacent Comerong Island and Seven Mile Beach, respectively. Surficial sediments were subject to grain size analysis, whereas random quartz grains from selected samples were used to indicate a qualitative degree of weathering using a scan electron microscopy (SEM). Additionally, selected samples were examined for mineralogical composition using x-ray diffraction (XRD) to provide understanding of sediment transport and provenance. The dataset, one of the most comprehensive modern sedimentary coastal records in Australia, can be used to understand the sediment dynamics and support a diverse range of coastal management decisions. The experiment design and analyses also serve as a model that can be replicated elsewhere to better understand fluvial delivery of sediments to the coast. The dataset and analyses presented here support the research article entitled “Evolution from estuary to delta: alluvial plain morphology and sedimentary characteristics of the Shoalhaven River mouth, southeastern Australia” [1], to which readers should refer to for interpretation.

Quantifying Bore‐Bore Capture on Natural Beaches

AbstractBore‐bore capture occurs when a faster moving bore captures a slower moving bore or the shoreline while both are propagating shoreward in the surf or swash zones and is quantified here for the first time using wave tracking methods applied to data collected at seven sandy Australian beaches. The results showed that, for the locations where bore‐bore capture occurred, there was a high probability (≈40%) of bore‐bore capture occurring in the surf or swash zones. Of these ≈45% consisted of a bore capturing the instantaneous shoreline (i.e., they occurred in the swash zone). The landward‐most 10% of the nearshore region (i.e., the time‐varying surf‐swash extent) was found to be the most likely location for a capture event. Infragravity wave energy (Eig) at the shoreline, number of bore‐bore capture events, and beach morphodynamic state are shown to be correlated: more dissipative beaches had higher levels of Eig at the shoreline which directly correlated with higher rates of bore‐bore capture. Approximately 20% of the capture events led to extreme horizontal shoreline maxima; however, >97% of extreme shoreline maxima were directly driven by bore‐bore capture events. There was a clear relation between the probability of bore‐bore capture driving an extreme shoreline maxima and beach morphodynamic state: The more dissipative the beach, the lower the probability of a bore‐bore capture event causing an extreme shoreline maxima event. Such correlation has direct importance for the future development of predictive runup models, which do not currently account for this phenomenon.

Wave-driven Groundwater Flows in Sandy Beaches: A Richards Equation-based Model

Clément, J.-B.; Sous, D.; Golay, F. and Ersoy M., 2020. Wave-driven groundwater flows in sandy beaches: A Richards equation-based model. In: Malvárez, G. and Navas, F. (eds.), Global Coastal Issues of 2020. Journal of Coastal Research, Special Issue No. 95, pp. 1047–1051. Coconut Creek (Florida), ISSN0749-0208.A groundwater model is developed to simulate flows under the swash zone of sandy beach. Variably saturated beach is described by Richards equation, which is solved by the Rivage code thanks to a discontinuous Galerkin method. The SWASH code is used to simulate wave propagation in nearshore waters and compute suitable boundary conditions at the beachface to force Richards equation. An idealized case is considered to investigate the beach groundwater response to the action of swash cycles associated with long IG-like waves: infiltration/ exfiltration, water table, hydraulic head and pore velocities are outlined. A good qualitative agreement is found with observations from experimental studies in the literature.

A RANS numerical study of experimental swash flows and its bed shear stress estimation

Quantification of the bed shear stress in the swash zone is subject to much uncertainty due to the very shallow water, complex flow structure and the strong unsteadiness in this zone. This study presents a RANS modeling of experimental dam-break flow generated swash processes, giving numerical reproduction of vertical flow structure of swash flows and thus an opportunity to study the magnitude and features of the bed shear stress. Firstly, numerical solutions (i.e., temporal or spatial variation of the depth-averaged velocity and water surface elevation, the time variations of the swash front, the vorticity structure and the boundary layer thickness) are compared with existing laboratory data and the previous knowledges on swash hydrodynamics. Secondly, using the computed vertical flow structure as input data, performances of four methods (the Reynolds stress method, and the three variants of the log-law method: the depth-averaged variant, the one-position variant, the two-position variant) for estimating the bed shear stress are evaluated. It is shown that the Reynolds stress method produces non-zero bed shear stress during the reversal time, during which the swash flows may be partly stagnant. Among the three variants of the log-law method, the depth-averaged method appears to agree with the measured data much better than the other two variants: those by the two-point log-law method is very sensitive to the selected near-bed positions, and those by the empirical value variant may underestimate (overestimate) the bed shear stress in the early uprush stage (in the initial stage of the backwash).

The effect of air bubbles on optical backscatter sensor measurements under plunging breaking waves

Accurate suspended sediment concentration measurements are key to understand and quantify the sediment transport patterns in the surf and swash zones. One of the most widely used instruments to collect suspended sediment concentrations is the Optical Backscatter Sensor (OBS). However, the OBS is known to give erroneous readings when deployed in bubbly environments like the surf zone. The present study aims to quantify the influence of an aerated wave breaking environment on the OBS sediment concentration measurements. Experiments are performed in a large wave flume, which ensures full air entrapment under plunging breaking waves, and avoids scale effects that could affect the volume of entrapped air, the air bubble penetration depth and the residence time of air bubbles in the post-breaking turbulent eddies. OBS measurements are obtained at 66 locations along a fixed bed profile for 14 regular breaking wave conditions. In the absence of suspended sediment particles, OBS voltage measurements are used as a proxy for air bubble content. The presented OBS results show peaks up to 1.49 V (31% of the OBS measurement range, corresponding to 16 g/l for sediment with d50 = 0.25 mm) produced by air bubbles in the most energetic tested wave breaking conditions, while the maximum time-averaged value obtained is 0.48 V (10% of the OBS measurement range, corresponding to 5 g/l). The results highlight the importance of considering the presence of air bubbles where OBS are deployed to measure suspended sediment concentrations. A good correlation is found between the breaker depth index and the air bubble distribution and two predictive formulas are derived to forecast the area of air bubble influence in the surf zone.

The role of alongshore flows on inner surf and swash zone hydrodynamics on a dissipative beach

Cross-shore and alongshore velocities were measured over five high tide cycles in the swash and inner surf zones of a dissipative beach (significant wave height between 0.5 and 1.65 m; water levels between 0 and 0.87 m) using acoustic and electromagnetic current meters. Measurements are used to determine the importance of alongshore motions relative to cross-shore motions for bed shear stress; a parameter required for many sediment transport formulations. Velocities and water depths are infragravity dominated with running average (5 min) cross-shore and alongshore velocities being of similar magnitude near 0.25 m/s. Significant coherence squared between cross-shore velocity and water depth is found in an infragravity frequency range (0.0078 Hz–0.024 Hz). A narrower infragravity frequency range (0.012 Hz–0.022 Hz) of significant coherence squared is found between alongshore velocity and water depth. Near bed velocity profiles at high spatial resolution indicate the mean profile of cross-shore or alongshore velocity for different relative cross-shore positions are nearly depth uniform, suggesting, on average, a well-mixed water column. Time-averaged onshore-directed depth-averaged velocities are nearly constant as a function of mean water depth, whereas time-averaged offshore-directed depth-averaged velocity magnitudes decrease with an increase in mean depth. In contrast, mean alongshore depth-averaged velocity magnitudes increase with depth regardless of the corresponding cross-shore motion and were always southerly-directed. These variations result from changes in breaking wave forcing location (i.e. relative cross-shore position). Near instantaneous velocity profile data are used to estimate friction coefficients and the bed shear stress through the Law of the Wall assumption. Mean friction coefficients for alongshore flows are similar to those obtained for cross-shore flows and values of ~0.02 are appropriate for all phases of flow. Alongshore bed shear stresses are the dominant bed shear stress component for over 27% of the samples implying an enhancement of total bed shear stress through incorporation of alongshore motions. Neglecting these alongshore processes (even if alongshore uniform) in cross-shore sediment transport models will lead to errors in predicted sediment transport rates.

Lidar Observations of the Swash Zone of a Low-Tide Terraced Tropical Beach under Variable Wave Conditions: The Nha Trang (Vietnam) COASTVAR Experiment

A field experiment was conducted at a tropical microtidal intermediate sandy beach with a low tide terrace (Nha Trang, Vietnam) to investigate the short-term swash-zone hydrodynamics and morphodynamics under variable wave conditions. Continuous 2D Lidar scanner observations of wave height at the lower foreshore, subsequent run-up and swash-induced topographic changes were obtained. These data were complemented by detailed real-time kinematic GPS topographic surveys. Variable wave and tide conditions were experienced during the field experiment with relatively large swell waves (offshore significant wave height, Hs = 0.9 m to 1.3 m; peak wave period, Tp = 8 to 12 s) concomitant with spring tides at the beginning of the period, followed by mild wind waves (offshore Hs under 0.5 m and Tp 5 s) and neap tides. This resulted in the following morphological sequence: berm erosion followed by rapid neap berm reformation and beach recovery within a few days. New insights into the link between intra-tidal swash dynamics and daily beach profile evolution were found using the Lidar dataset. While waves directly cause morphology changes on a wave-by-wave basis, tidal levels were found to be a key factor in determining the morphological wave-effect (accretive or erosive) due to modulated interaction between surf and swash hydro-morphodynamics.

Variability in Onshore Sediment Transport on a Natural Beach during a Central American Cold Surge Event

Briggs, T.R.; Figlus, J.; Torres-Freyermuth, A.; Puleo, J.A.; Warren, W., and Alrushaid, T., 2020. Variability in onshore sediment transport on a natural beach during a Central American cold surge event. Journal of Coastal Research, 36(3), 487–497. Coconut Creek (Florida), ISSN 0749-0208.Beaches facing the Gulf of Mexico along the northern part of the Yucatan Peninsula experience frequent Central American cold surge events, or northerly storms locally called “Nortes,” which interrupt the usual daily patterns of sea- and land-breeze–dominated nearshore dynamics. These perturbations of prevailing hydrodynamic forcing conditions also affect sediment distribution and transport parameters in the surf and swash zone, which are critical elements in understanding and modeling complex morphodynamic processes during storm events. However, detailed measurements of sediment characteristics throughout the water column during a storm event are scarce. This study presents field measurements of bottom and suspended sediment transported in the onshore direction during a 24-hour storm event impacting the Yucatan Peninsula. Sediment load data within the water column collected in 3 hour intervals via vertical arrays of streamer traps, deployed at two cross-shore locations in the inner surf zone, are compared with sediment data from collocated bottom grab samples. Time series of grain size distributions, sediment statistics, and respective vertical variations throughout the water column are related to measured bed-level changes to verify applicability of simple erosion and accretion predictors. Results indicate that bottom sediment and sediment distributions throughout the water column deviate substantially due to storms, highlighting the complexity and difficulty of assessing sediment transport and morphodynamic processes with a single-parameter sediment characteristic. Calculated total onshore transport rates during the Norte were on average one order of magnitude higher than during nonstorm conditions.

Observation of munitions migration and burial in the swash and breaker zones

Behavior of underwater unexploded ordnance located on the sea bed is poorly understood. Different types of munitions were tested in the swash and breaker zones of a large-scale flume containing medium sand on two initial foreshore slopes (1:16 and 1:10) with regular and irregular wave forcing (Hs = 0.3–1.1 m; Tp = 2.6–9 s). Migration and burial were measured via pre- and post-run surveys and internal munition sensors. Water depths, flow velocities, wave runup and beach morphology changes were also measured. Munition sensors consist of an inertial motion unit for roll and heading and optical photocells for burial. Munition migration distances in the swash zone were greatest for the steeper slope and lower density munitions. Munition migration mimicked wave runup with net offshore transport. Breaker zone munitions experienced episodes of onshore- and offshore-directed motion without preferred direction. Denser munitions were observed to bury deeper, up to 2 munition diameters. Moderate correlation was observed between burial (r2 = 0.48) or migration (r2 = 0.37) with dimensionless parameters (such as Shields parameter, Keulegan-Carpenter number, object mobility number, munition density and Iribarren number), attributed to the high variability in munition response even under similar offshore forcing conditions.