Search

A laboratory experiment was carried out to study sediment transport dynamics occurring in the swash zone of a coarse-sandy beach built in a large-scale wave flume. Hydro- and morpho-dynamic as well as sediment transport data were collected using sensors mounted on a scaffold rig deployed in the lower swash zone close to the moving bed. The high resolution of near-bed data permitted quantitative evaluation of suspended and sheet flow contributions to the total sediment transport. Although sheet flow sediment fluxes were higher than suspended fluxes, the vertically integrated suspended sediment load overcame the sheet flow load during uprush and it was on the same order of magnitude during backwash. The observed cumulative sediment transport was generally larger than the morphological changes occurring shoreward of the rig location implying either an underestimation of the offshore sediment transport or an overestimation of the onshore fluxes obtained from concentration and velocity profile data. Low correlations were found between net swash profile changes and runup parameters suggesting that local hydrodynamic parameters provide little or no predictability of accretion and erosion of an upper beach which is near equilibrium. The balance between erosion and deposition induced by individual swash events brought a dynamic equilibrium with small differences between the profiles measured at the start and at the end of the run. [ABSTRACT FROM AUTHOR]

Abstract: This work presents a detailed analysis of suspended sediment concentration measured in the inner surf and swash zone under large-scale wave flume conditions. Controlled hydrodynamic conditions tested in the laboratory corresponded to highly energetic (erosive) conditions. Hydrodynamic, bottom evolution and suspended sediment concentration (SSC) data were acquired at several locations within the inner surf and the swash zone. A limited number of hydrodynamic type of events have been identified to induce important suspended sediment concentrations. These hydrodynamic conditions include incident broken wave, wave capture, weak and strong wave backwash interactions or pure backwash events. The acquired data showed the variability of the suspended sediment concentration pattern depending on the cross-shore location from which measurements were taken. This suspension mechanism also varied depending on the time series characteristic. There was a clear predominance of incident wave/bore as a forcing term in the surf zone, whilst the SSC measured in the swash zone showed that wave-swash interaction controls the most significant events. Finally, the importance of a wave-by-wave analysis is highlighted in this study. It was observed that despite the fact that most of the time series induce profile accretion in the mid and upper swash area, only one or two swash events are able to erode extensive parts of the beach and change the final output of the entire time record from accretion to erosion. [Copyright &y& Elsevier]

Abstract: This paper reviews research on beach groundwater dynamics and identifies research questions which will need to be answered before swash zone sediment transport and beach profile evolution can be successfully modelled. Beach groundwater hydrodynamics are a result of combined forcing from the tide and waves at a range of frequencies, and a large number of observations exist which describe the shape and elevation of the beach watertable in response to tidal forcing at diurnal, semi-diurnal and spring-neap tidal frequencies. Models of beach watertable response to tidal forcing have been successfully validated; however, models of watertable response to wave forcing are less well developed and require verification. Improved predictions of swash zone sediment transport and beach profile evolution cannot be achieved unless the complex fluid and sediment interactions between the surface flow and the beach groundwater are better understood, particularly the sensitivity of sediment transport processes to flow perpendicular to the permeable bed. The presence of a capillary fringe, particularly when it lies just below the sand surface, has influences on beach groundwater dynamics. The presence of a capillary fringe can have a significant effect on the exchange of water between the ocean and the coastal aquifer, particularly in terms of the storage capacity of the aquifer. Field and laboratory observations have also shown that natural groundwater waves usually propagate faster and decay more slowly in aquifers with a capillary fringe, and observations which suggest that horizontal flows may also occur in the capillary zone have been reported. The effects of infiltration and exfiltration are generally invoked to explain why beaches with a low watertable tend to accrete and beaches with a high watertable tend to erode. However, the relative importance of processes such as infiltration losses in the swash, changes in the effective weight of the sediment, and modified shear stress due to boundary layer thinning, are not yet clear. Experimental work on the influence of seepage flows within sediment beds provides conflicting results concerning the effect on bed stability. Both modelling and experimental work indicates that the hydraulic conductivity of the beach is a critical parameter. However, hydraulic conductivity varies both spatially and temporally on beaches, particularly on gravel and mixed sand and gravel beaches. Another important, but poorly understood, consideration in beach groundwater studies is the role of air encapsulation during the wetting of beach sand. [Copyright &y& Elsevier]

The purpose of this study is to analyse the variations of the beach morphology surveyed in Teignmouth, United Kingdom, while the swash processes were shaping the beach, in parallel with the hydrodynamic conditions and watertable fluctuations. These data and field observations lead to a conceptual model which shows a three-phase sequence of evolution. The first phase is characterised by a quick erosion, which is linked with the sediment characteristics of the beachface. The second phase consists of a period of accretion caused by the influx of the sediment mobilised by the plunging breakers at the toe of the beachface, and finally, the seepage of the ground water induces the last phase of erosion mainly due to the enhanced backwash erosion. This model enlightens the importance of the grain characteristics for the beachface evolution and complements the model of Duncan, in the case of high-permeability sediment. [Copyright &y& Elsevier]