Swash Cycle Research Articles

Experimental study of the solitary wave induced groundwater hydrodynamics

Coastal groundwater hydrodynamics are affected by the swash uprush and backwash processes. In this study, laboratory experiments were carried out to investigate the solitary wave induced groundwater hydrodynamics over a coarse-grained sand beach. Pore water pressures at 27 cross-shore positions (covering both the subaqueous and subaerial areas) along 3 different beach depths were measured, and the synchronized shoreline position as well as the local water surface elevations (WSE) were also recorded. Temporal variation of the total water head (TWH) was found almost hydrostatically controlled by the instantaneous WSE in the subaqueous area. In the capillary truncated zone, the capillary effect contributes to the non-hysteresis phenomenon between the TWH and WSE. In other subaerial area, time-varying features of the TWH and WSE show clear differences in terms of their magnitudes and phases. Regarding the experimental observations, four representative regions could be identified along the cross-shore direction, i.e., Region I corresponding to the initially subaqueous area, Region II in between the still water shoreline and the top of capillary fringe (saturated beach), Region III in between the upper boundary of Region II and the maximum swash run-up, and Region IV landward beyond the Region III. With respect to the cross-shore distribution characteristics of the maximum TWH and the maximum WSE, as well as the time instants when the maximum TWH and the maximum WSE occur, different groundwater hydrodynamics were observed during the swash event in these four regions. In addition, spatiotemporal characteristics of the TWH were scrutinized and discussed. Both the vertical and horizontal TWH gradients in different regions were considered to reveal the sophisticated groundwater hydrodynamics. Finally, the groundwater movement pattern and its percolation circulation under the solitary wave swash were also specified. • Solitary wave induced groundwater hydrodynamics are investigated through detailed laboratory experiments. • Four regions are identified regarding the spatial characteristics of the maximum total water head and its occurrence time. • Hydrostatic effect, capillary effect and the wetting front are significant for the swash groundwater hydrodynamics. • Horizontal groundwater seepage flow is evident, even larger than the vertical one, during the entire swash process. • Two groundwater circulations are observed in the subaqueous and subaerial areas during the swash cycle.

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.

Direct Measurements of Bed Shear Stress under Swash Flows on Steep Laboratory Slopes at Medium to Prototype Scales

Robust measurements of bed shear stress under wave runup flows are necessary to inform beachface sediment transport modelling. In this study, direct measurements of swash zone bed shear stress were obtained in medium and prototype-scale laboratory experiments on steep slopes. Peak shear stresses coincided with the arrival of uprush swash fronts and high-resolution measurement of swash surface profiles indicated a consistently seaward sloping swash surface with minimal evidence of a landward sloping swash front. The quadratic stress law was applied to back-calculate time-varying friction factors, which were observed to decrease with increasing Reynolds number on smooth slopes, consistent with theory for steady flows. Additionally, friction factors remained relatively constant throughout the swash cycle (except around flow reversal), with a variation of approximately ±20% from the mean value and with only small differences between uprush and backwash. Measured friction factors were observed to be larger than expected when plotted on the Moody or wave friction diagram for a given Reynolds number and relative roughness, consistent with previous field and laboratory studies at various scales.

Sand transport processes and bed level changes induced by two alternating laboratory swash events

Sand transport processes and net transport rates are studied in a large-scale laboratory swash zone. Bichromatic waves with a phase modulation were generated, producing two continuously alternating swash events that have similar offshore wave statistics but which differ in terms of wave-swash interactions. Measured sand suspension and sheet flow dynamics show strong temporal and spatial variability, related to variations in flow velocity and locations of wave capture and wave-backwash interactions. Suspended and sheet flow layer transport rates in the lower swash zone are generally of same magnitude, but sheet flow exceeds the suspended load transport by up to a factor four during the early uprush. The bed level near the inner surf zone is relatively steady during a swash cycle, but changes of O(cm/s) are measured near the mid swash zone where wave-swash interactions lead to strongly non-uniform flows. The two alternating swash events produce a dynamic equilibrium, with bed level changes up to a few mm induced by single swash events, but with net morphodynamic change over multiple events that is two orders of magnitude lower. Most of the intra-swash and the single-event-averaged bed level changes in the swash zone are caused by a redistribution of sediment within the swash. The transport of sediment across the surf-swash boundary is minor at intra-swash time scale, but becomes increasingly significant at swash-averaged time scales or longer (i.e., averaged over multiple swash events).

Patrones en las excursiones de swash en dos playas contrastantes: Playas Hollywood y Costa Verde, Colombia

La zona de swash es la parte de la playa que esta consecutivamente seca y húmeda debido al ascenso y descenso del mar producto del oleaje y la marea. Esta zona es de gran importancia en los sistemas de playas, ya que allí se desarrollan los procesos de erosión/acreción de la línea de costa y las inundaciones; por lo tanto, el entendimiento de las dinámicas que ocurren en esta zona son vitales para predecir y mitigar los posibles impactos en playas, dunas y estructuras costeras bajo diferentes escenarios climáticos.Mediciones de variables in situ como presiones, velocidades y concentraciones de sedimento en la zona de swash, fueron llevadas a cabo en dos playas en el Caribe Colombiano: Playa Costa Verde, (Ciénaga) y Playa Hollywood, (Cartagena). Los datos fueron usados para comparar los patrones de velocidades y concentraciones de sedimentos en la zona de swash debido a la influencia de la pendiente de la playa en esta zona y la influencia de las diferentes frecuencias de las ondas (infragravitatorias (IG) o Sea-Swell (SS)). Adicionalmente, se propone una aproximación para ligar pequeñas escalas espacio-temporales (ciclos de swash) con procesos espacio-temporales mayores (erosión-acreción).

Subsurface Flow and Moisture Dynamics in Response to Swash Motions: Effects of Beach Hydraulic Conductivity and Capillarity

AbstractA combined field and numerical study was conducted to investigate dynamics of subsurface flow and moisture response to waves in the swash zone of a sandy beach located on Cape Henlopen, DE. A density‐dependent variably saturated flow model MARUN was used to simulate subsurface flow beneath the swash zone. Values of hydraulic conductivity (K) and characteristic pore size (α, a capillary fringe property) were varied to evaluate their effects on subsurface flow and moisture dynamics in response to swash motions in beach aquifers. The site‐specific modeling results were validated against spatiotemporal measurements of moisture and pore pressure in the beach. Sensitivity analyses indicated that the hydraulic conductivity and capillary fringe thickness of the beach greatly influenced groundwater flow pathways and associated transit times in the swash zone. A higher value of K enhanced swash‐induced seawater infiltration into the beach, thereby resulting in a faster expansion of a wedge of high moisture content induced by swash cycles, and a flatter water table mound beneath the swash zone. In contrast, a thicker capillary fringe retained higher moisture content near the beach surface, and thus, significantly reduced the available pore space for infiltration of seawater. This attenuated wave effects on pore water flow in the unsaturated zone of the beach. Also, a thicker capillary fringe enhanced horizontal flow driven by the larger‐scale hydraulic gradient caused by tides.

Bed level motions and sheet flow processes in the swash zone: Observations with a new conductivity-based concentration measuring technique (CCM+)

Detailed measurements of bed level motions and sheet flow processes in the lower swash are presented. The measurements are obtained during a large-scale wave flume experiment focusing on swash zone sediment transport induced by bichromatic waves. A new instrument (CCM+) provides detailed phase-averaged measurements of sheet flow concentrations, particle velocities, and bed level evolution during a complete swash cycle. The bed at the lower swash location shows a clear pattern of rapid erosion during the early uprush and progressive accretion during the middle backwash phase. Sheet flow occurs during the early uprush and mid and late backwash phases. Sheet flow sediment fluxes during these instances are highest in the pick-up layer. Sediment entrainment from the pick-up layer occurs not only during instances of high horizontal shear velocities but also in occurrence of wave–backwash interactions. As opposed to oscillatory sheet flow, the pivot point elevation of the sheet flow layer is time-varying during a swash event. Moreover, the upper sheet flow layer concentrations do not mirror the concentrations in the pick-up layer. Both differences suggest that in the lower swash zone the dynamics of the upper sheet flow layer are not only controlled by vertical sediment exchange (such as in oscillatory sheet flows) but are strongly affected by horizontal advection processes induced by the non-uniformity of the flow.

Direct bed shear measurements under loose bed swash flows

Shear plate measurements of bed shear stress under fixed and loose bed swash uprush flows are presented and contrasted. A shear plate previously developed for fixed bed flows was modified and tested for coarse and fine grain loose beds. Transient flows were generated by a dam-break flow over initially dry horizontal and sloping beds, with flows upslope analogous to swash uprush flows on beaches. Performance of the shear plate varied with grain size. For the coarse grained loose beds only the peak shear stress at the leading edge of the swash flow was consistently obtained; for fine grained loose beds the shear stress was reliably obtained over the whole uprush swash cycle. Data were obtained for varying thickness of loose bed material, commencing with a single grain layer, which correspond to partially starved bed conditions. The direct measurements clearly indicate that the bed shear stress comprises of a fluid shear stress component and a grain shear stress component. The grain shear stress increases linearly with sediment load and is significantly larger for the coarse loose grain bed tests due to the much larger normal stress compared to that for fine grain loose bed tests with the same number of grain layers. The data show a high degree of consistency with the classical theory of Bagnold (1956) and the derived coefficient of dynamic internal friction is similar to that observed by Hanes and Inman (1985).

Modeling swash‐zone hydrodynamics and shear stresses on planar slopes using Reynolds‐Averaged Navier–Stokes equations

AbstractA numerical model solving the Reynolds‐Averaged Navier–Stokes equations, with a volume of fluid‐tracking scheme and turbulence closure, is employed for estimating hydrodynamics in the swash zone. Model results for run‐up distance, water depth, and near‐bed velocity are highly correlated (r2 > 0.97) with ensemble‐averaged dam‐break‐driven swash data. Moreover, modeled bed shear stresses are within 20% of estimates derived from measured velocity profiles. Dam‐break‐driven swash simulations are conducted to determine the effect of foreshore characteristics (bed roughness and foreshore slope) on bore‐induced swash‐zone hydrodynamics and bed shear stresses. Numerical results revealed that the boundary layer vanishes during flow reversal, grows during the backwash, and becomes depth limited at the end of the swash cycle. In general, the uprush experiences larger shear stresses but for a shorter duration than the backwash. Some variability in this pattern is observed depending on the bed roughness, foreshore slope, and cross‐slope location in the swash zone, implying that large spatial gradients in shear stresses can occur on the foreshore. The mean tangential force per unit area supplied to the bed is offshore directed for the simulated cases, with the exception of the mild‐slope (1:25) cases, owing to the skewed nature of swash flows. The temporal evolution of the momentum balance inside the swash zone shows an important contribution to the total force from turbulence and advection at the early/final stage of the swash cycle, whereas the local acceleration does not appear to be a significant contribution.

FIELD MEASUREMENTS OF SHEET FLOW SEDIMENT TRANSPORT IN THE SWASH ZONE

A newly developed Conductivity Concentration Profiler (CCP), capable of rendering a 29-point sediment concentration profile at 1 mm vertical resolution, was deployed on a macrotidal beach in Perranporth, UK, as part of a comprehensive field study on swash zone hydrodynamics and sediment transport. Initial field results show the occurrence of sheet flow during both the uprush and backwash phases of the swash cycle. Concentration profiles in the sheet flow layer are approximately linear, with a power-law tail at the top of the layer. The data suggest that shear stresses are the dominant forcing for sediment mobilization, but pressure gradients provide a secondary mobilization mechanism.

Net beach change in the swash zone: A numerical investigation

A range of bed-load sediment transport formulae are used to run fully coupled, morphodynamic simulations of one [1] swash cycle on an erodible plane beach. A system comprising shallow water equations and Exner equation is solved, in which sediment transport rate q is either dependent only on depth averaged velocity (u), or on u and water depth (h). The results are in agreement with equivalent uncoupled results [2] in that all sediment transport formulae considered applied to the event of [1] yield net erosion in the whole of the swash. Consistent with [3], however, full coupling yields significantly less erosion for all the q=q(u) formulae compared to the equivalent uncoupled results. It is shown that differences between uncoupled and coupled approaches (for most formulae) accumulate over the course of the swash event. The main reason for the reduced net erosion is the smaller maximum inundation. It is also shown that including a dependence on h in the bed load sediment transport formula for fully coupled simulations can result in net deposition in the upper swash.Bed shear stress described by a Chezy law is further included in fully coupled simulations to examine net beach change. Much reduced maximum inundation and net offshore sediment transport are predicted both for q=q(u) and q=q(h,u). It is shown that although the net sediment flux at the base of the swash under one [1] swash event is still offshore, deposition in the middle or upper swash may be predicted when bed shear stress is included, particularly when the drag coefficient in the backwash is reduced compared to that in the uprush, consistent with some in-situ measurements. The implication is that bed shear stress must be included not just to obtain correct quantitative beach change, but also to obtain correct qualitative beach behaviour.

SIMULTANEOUS SURFACE AND SUBSURFACE AIR AND WATER FLOWS MODELLING IN THE SWASH ZONE

This study presents the results of the numerical simulation of a bore-driven swash flow over a permeable coarse- grained beach, carried out using the THETIS code. This code, based on a VOF-RANS approach, was previously used to simulate the swash flow over an impermeable beach (Desombre et al. 2013). For the present study, the code is extended to account for infiltration and exfiltration into a permeable immobile beach using the Volume-Averaged momentum equation that solves simultaneously the surface and subsurface flows. The results are compared with a laboratory data set from an experiment performed in the swash facility of the University of Aberdeen (Steenhauer et al. 2011). Comparisons between measurements and model results show the ability of the model to simulate the main features of subsurface flow during an entire swash cycle.

The use of video imagery to analyse groundwater and shoreline dynamics on a dissipative beach

Groundwater seepage is known to influence beach erosion and accretion processes. However, field measurements of the variation of the groundwater seepage line (GWSL) and the vertical elevation difference between the GWSL and the shoreline are limited. We developed a methodology to extract the temporal variability of the shoreline and the wet–dry boundary using video imagery, with the overarching aim to examine elevation differences between the wet–dry boundary and the shoreline position in relation to rainfall and wave characteristics, during a tidal cycle. The wet–dry boundary was detected from 10 min time-averaged images collected at Ngaranui Beach, Raglan, New Zealand. An algorithm discriminated between the dry and wet cells using a threshold related to the maximum of the red, green, and blue intensities in Hue-Saturation-Value. Field measurements showed this corresponded to the location where the water table was within 2 cm of the beachface surface. Time stacks and time series of pixels extracted from cross-shore transects in the video imagery, were used to determine the location of the shoreline by manually digitizing the maximum run-up and minimum run-down location for each swash cycle, and averaging the result. In our test data set of 14 days covering a range of wave and rainfall conditions, we found 6 days when the elevation difference between the wet–dry boundary and the shoreline remained approximately constant during the tidal cycle. For these days, the wet–dry boundary corresponded to the upper limit of the swash zone. On the other 8 days, the wet–dry boundary and the shoreline decoupled with falling tide, leading to elevation differences of up to 2.5 m at low tide. Elevation differences between the GWSL and the shoreline at low tide were particularly large when the cumulative rainfall in the preceding month was greater than 200 mm. This research shows that the wet–dry boundary (such as often used in video shoreline-finding algorithms) is related to groundwater seepage on low-sloped, medium to fine sand beaches such as Ngaranui Beach (mean grain size ∼0.27 mm, beach slope ∼1:70) and may not be a good indicator of the position of the shoreline.

Laboratory investigation of pressure gradients induced by plunging breakers

This investigation presents laboratory observations on the spatio-temporal evolution of pressure fields and gradients induced by plunging waves propagating across the nearshore zone. Experiments were carried out to obtain detailed measurements of pressure time-curves and fluid velocities across the shoaling, breaking and swash zones. Pressure envelopes showed a consistent shape in all tests, with minimum values in the swash zone and maximum values at the impinge points. In particular, the nature of the pressure gradient at different locations (shoaling, breaking and swash zones) across the impermeable bed is investigated, revealing maximum values at the impact points. The relationship of the pressure gradient to horizontal velocity and local acceleration is also examined. This investigation enabled the elucidation of an apparent contradiction identified in recent works regarding the relationship between local acceleration and pressure gradient within the surf and swash zones. Consistent with findings of prior studies, results in the shoaling and breaking zone indicated that local fluid accelerations can be used to account for the contribution of pressure gradients. In contrast, the validity of this assumption is not established within the swash zone. In this region, minimum pressure gradients are identified and local acceleration is negative (offshore) for almost the whole swash cycle, consistent with previous field observations. Thus, for plunging breakers driven-swash, results point toward the significance of the advective terms acting against local acceleration. Furthermore, measurements of the velocity field by a validated Bubble Image Velocimetry (BIV) technique provide a qualitative description of the highly aerated flow, revealing that maximum values of vertical velocities are associated with the wave-impact from a plunging breaker impinging on the bed. These results indicate the need for a detailed characterisation of the pressure gradient under different types of wave breaking.

Alongshore fluid motions in the swash zone of a sandy and gravel beach

Field experiments were conducted on a low-gradient, high-energy sandy beach (Truc Vert, France) and a steep, low-energy gravel beach (Slapton, UK) to examine alongshore-directed currents within the swash zone. At Truc Vert, data were collected over 33 tidal cycles with offshore significant wave heights of 1–4 m and periods of 5–12 s. At Slapton data were collected during 12 tides with wave heights of 0.3–1 m and periods of 4–9 s. The swash motion was predominantly at infragravity frequencies at Truc Vert and incident frequencies at Slapton. During selected tides, strong alongshore currents of the same order of magnitude as the cross-shore flows were a consistent feature of both swash zones; however, while the cross-shore flows were saw-toothed with a distinct flow reversal, the alongshore flows were unidirectional and continuous. On the sand beach, alongshore momentum was advected into the swash from the surf zone by standing long-wave motion resulting in the strongest flows in the lower swash. In contrast on the gravel beach, highly oblique uprush directly forced the longshore-directed motions and resulted in strong flow velocities in the upper swash. On the sandy beach, the cross- and longshore boundary layers were consistently different with strong vertical shear during the uprush for the cross-shore and during the backwash for the alongshore flows. A logarithmic profile better fitted the longshore flows during the backwash, compared to during the uprush for the cross-shore flows, and enabled the drag coefficient to be computed. The alongshore drag coefficient was C d, v = 10 − 3 and of the same order as for the cross-shore, but was generally larger during the backwash compared to the uprush. Sediment transport efficiency during the backwash is potentially enhanced by longshore-directed swash flows. Significant bed shear stress is maintained throughout that cross-shore flow reversal enabling down-slope transport earlier in the swash cycle.

Can standard energetics models be used to predict net cross-shore sediment flux at the beach face?

This paper makes use of comprehensive new measurements of net cross-shore sediment flux and flow velocity from hundreds of individual swash events to assess the use of velocity moments to predict cross-shore sediment transport in the swash zone. It is shown that the commonly used Bagnold energetics model and Shields parameter model make inadequate predictions of net sediment transport in the swash zone over individual swash cycles. In their basic form it is demonstrated that both these approaches are too simplistic to correctly capture aspects of the complex physical phenomena that drive sediment transport in the swash, such as sediment advection from the surfzone, localised suspension of sediment and swash-swash interactions. Due to the significant remaining uncertainties involved in incorporating these modifications into a fully deterministic modelling scheme, it is proposed that parallel development of longer-term, behavioural swash zone models also be pursued.

Observations of nonlinear runup patterns on plane and rhythmic beach morphology

Application of nonlinear forecasting and bispectral analysis to video observations of runup over cuspate topography shows that these alongshore patterns in the morphology are accompanied by changes to the fundamental behavior of the runup time series. Nonlinear forecasting indicates that at beach cusp horns, the behavior of swash flow is more predictable and global (meaning that characteristics of individual swash events are well represented by the behavior of the time series as a whole). Conversely, at beach cusp bays, the behavior of swash flow is less predictable and more local (meaning that the characteristics of individual swash events are best represented by the behavior of a small fraction of the time series). Bispectral analysis indicates that there is a nonlinear transfer of energy from the incident wave frequency f to the infragravity frequency ∼f/2 which only occurs in the bay, suggesting that the local behavior is caused by interactions between successive swash cycles which are magnified by channeling caused by the beach cusp geometry. The local behavior and the bispectral signatures are not present in offshore measurements, and they are not present in runup time series collected when the beach was planar. These results provide evidence that interactions between successive runups are a fundamental characteristic of beach cusp bays. Ultimately, these interactions could lead to the growth of an infragravity wave with an alongshore wavelength forced by the presence of beach cusps.

Validation of volume continuity method for estimation of cross-shore swash flow velocity

The use of flow volume continuity to estimate depth-averaged cross-shore swash flow velocity is validated using appropriate field data. Swash surface and beach face elevation were measured by a network of ultrasonic altimeters mounted through the swash zone of a sandy beach. Estimates of cross-shore flow velocity derived from these data compare well with concurrent measurements obtained from a vertical array of four electromagnetic current meters (EMCMs). Significantly, the continuity technique allows swash velocity to be estimated throughout the full swash cycle. This is not possible using fixed current meters and the new measurements indicate that swash velocity is more negatively skewed than previously observed.

A Conductivity Sensor for Nearbed Sediment Concentration Profiling

Abstract A new sensor that measures the vertical profile of nearbed sediment concentration is described. The conductivity-based sensor is composed of eight electrode pairs separated in the vertical by 2.5 × 10−3 m. Electrode pairs are sampled at 16 Hz, with higher rates achievable. Each electrode pair response is linear over the range of conductivity tested from 0.2 to 0.65 mS cm−1 that exceeds the range of conductivity values corresponding to sediment–water mixtures from clear water to the packed bed limit of 0.65 m3 m−3. A laboratory test over a planar sloping beach indicates the capability of the sensor to simultaneously quantify sediment concentration profiles from roughly 0.01 m below to 0.1 m above the at-rest bed. The data indicate that the upper few millimeters of the bed are highly mobile and that bed dilation and sediment mobility vary considerably over a swash cycle.

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

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