18,773,620 research outputs found
Methods for field measurement and remote sensing of the swash zone
No full textSwash action is the dominant process responsible for the cross-shore exchange of sediment between the subaerial and subaqueous zones, with a significant part of the littoral drift also taking place as a result of swash motions. The swash zone is the area of the beach between the inner surfzone and backbeach that is intermittently submerged and exposed by the processes of wave uprush and backwash. Given the dominant role that swash plays in the morphological evolution of a beach, it is important to understand and quantify the main processes. The extent of swash (horizontally and vertically), current velocities and suspended sediment concentrations are all parameters of interest in the study of swash processes. In situ methods of measurements in this energetic zone were instrumental in developing early understanding of swash processes, however, the field has experienced a shift towards remote sensing methods. This article outlines the emergence of high precision technologies such as video imaging and LIDAR (light detection and ranging) for the study of swash processes. Furthermore, the applicability of these methods to large-scale datasets for quantitative analysis is demonstrated
Effects of surfzone wave transformation on swash dynamics
No full textSwash oscillations on two natural beaches were measured to show that the shape and magnitude of energy spectra can be largely dependent on processes occurring inside the surfzone. The observations took place on a steep, intermediate beach on the east coast (Tairua Beach), and a low-sloping, dissipative beach located on the west coast of New Zealand (Ngarunui Beach, Raglan), and aimed at improving the understanding of the effects of wave breaking, beach slope, and nonlinear wave interactions on the swash oscillations. These problems were addressed by analysing datasets obtained from field experiments undertaken at these two sites.
A field experiment at Tairua Beach showed that swash oscillations were critically dependent on the stage of the tide which controlled the degree of wave energy dissipation over the sandbar crest. Under mild, near-constant offshore wave conditions, the presence of a sandbar and the tidally-controlled water depth over its crest determined whether most of the incoming waves broke before reaching the shoreline. This forced a change in the pattern of wave energy dissipation across the surfzone between low and high tide, which was reflected by changes to swash elevation (runup) on time-scales of a few hours. Significant runup height Rs, defined as 4 times the standard deviation of the waterline time series, varied by a factor of 2 between low tide, when most of the waves were breaking over the sandbar and high tide, when the waves were barely breaking. The increase in wave energy dissipation during low tide was associated with changes in swash maxima distribution, decrease in mean swash period and increasing energy at infragravity frequencies (0.004–0.05 Hz). Bispectral analysis suggested this infragravity modulation might be connected with the presence of secondary waves at the shoreline.
Swash oscillations at Tairua were not homogeneous along the beach. Alongshore variability in Rs of up to 78% was observed and was mainly driven by changes in the sea-swell (0.05–0.4 Hz) band of the swash. This variability was predominantly controlled by alongshore changes in beach face slope, although alongshore patterning in wave breaking over the sandbar caused alongshore changes in wave dissipation and also resulted in alongshore swash variation in the sea-swell bandwidth. At infragravity frequencies, alongshore swash variability was not well associated either with changes in beach slope or wave breaking and was possibly linked to the presence of low-mode edge waves, observed from frequency-wavenumber spectra of the swash time series.
A final experiment was conducted to understand the surfzone control on incident and infragravity runup on a gently-sloping beach. The observations showed that runup saturation at infragravity frequencies can occur under mild offshore energy conditions if the beach slope is sufficiently gentle. Infragravity saturation was observed for higher-frequency (> 0.025–0.035 Hz) infragravity waves, where typically less than 5% of the (linear) energy flux was reflected from the beach and where, similar to the sea-swell band, the swash energy was independent of offshore wave energy. The infragravity frequency range of saturation was determined by the tide, with saturation extending to lower frequencies at low tide when the local beach face slope over the concave-shaped profile was gentler. Runup was strongly dominated by infragravity frequencies, which accounted on average for 96% of the runup variance, and its energy levels were entirely consistent with strong infragravity wave dissipation observed in the surfzone, particularly when including the nonlinear contributions to the wave energy fluxes. Our observations show evidence of nonlinear interactions involving infragravity and high-frequency, harmonic waves, and suggest that these harmonics could play a role in the wave energy balance near the shoreline on low-sloping, dissipative beaches
Hydrodynamics and morphodynamics in the swash zone: hydralab III large-scale experiments
No full textThe modelling of swash zone hydrodynamics and sediment transport and the resulting morphodynamics has been an area of very active research over the last decade. However, many details are still to be understood, whose knowledge will be greatly advanced by the collection of high quality data under controlled large-scale laboratory conditions. The advantage of using a large wave flume is that scale effects that affected previous laboratory experiments are minimized.
In this work new large-scale laboratory data from two sets of experiments are presented. Physical model tests were performed in the large-scale wave flumes at the Grosser Wellen Kanal (GWK) in Hannover and at the Catalonia University of Technology (UPC) in Barcelona, within the Hydralab III program.
The tests carried out at the GWK aimed at improving the knowledge of the hydrodynamic and morphodynamic behaviour of a beach containing a buried drainage system. Experiments were undertaken using a set of multiple drains, up to three working simultaneously, located within the beach and at variable distances from the shoreline. The experimental program was organized in series of tests with variable wave energy. While a positive effect was observed under low energy conditions, for medium and high energy conditions the benefit of having the drains operative was not always clear. In any case, it was evident that any positive effect of the drains on the beachface was confined by the position of the cone of depression in the aquifer’s surface.
The tests carried out in the large wave flume at UPC had the intent to investigate swash zone under storm conditions. The main aim was to compare beach profile response for monochromatic waves, monochromatic waves plus free long waves, bichromatic waves and random waves. Both erosive and accretive conditions were considered. The experiments suggest that the inclusion of long wave and wave group sediment transport is important for improved nearshore morphological modelling of cross-shore beach profile evolution, and provide a very comprehensive and controlled series of tests for evaluating numerical models. It is suggested that the large change in the beach response between monochromatic conditions and wave group conditions is a result of the increased significant and maximum wave heights in the wave groups, as much as the presence of the forced and free long waves induced by the groupiness. The equilibrium state model concept can provide a heuristic explanation of the influence of the wave groups on the bulk beach profile response if their effective relative fall velocity is larger than that of monochromatic waves with the same incident energy flux
An analytical model for bore-driven run-up
No full textWe use a hodograph transformation and a boundary integral method to derive a new analytical solution to the shallow-water equations describing bore-generated run-up on a plane beach. This analytical solution differs from the classical Shen-Meyer runup solution in giving significantly deeper and less asymmetric swash flows, and also by predicting the inception of a secondary bore in both the backwash and the uprush in long surf. We suggest that this solution provides a significantly improved model for flows including swash events and the run-up following breaking tsunamis
Swash
No full text‘Swash’ was Prior’s first collaboration with architect, Frances Crow, working as ‘liminal’. ‘Swash’ is a 24 channel sound installation commissioned by Paignton Zoo as an artwork for its ‘Living Coasts’ sea life visitor attraction, and was funded by RALP (Regional Arts Lottery Programme) Arts Council of England, South West.
'liminal' set out to explore possibilities for an ‘expanded’ architecture in which a loudspeaker system might provide an additional prosthetic layer to the building, through which the physical constraints of the space could be explored, augmented or subverted. We mounted independently controlled speakers in a tunnel space at equidistant intervals, sufficiently close together to enable smooth transitions to be made from one speaker to the next. Using the speaker system we devised a series of complex movement models derived from behavioural patterns found in the movement of water which could be applied to sounds which, themselves, had all derived from recordings of water in different environments. We made underwater recordings using equipment from the University of Plymouth's Ocean Science labs land deployed a wide range of location and studio recording techniques prior to the processing of material used in the installation
Observations of nonlinear run-up patterns on plane and rhythmic beach morphology
No full textApplication of non-linear forecasting and bispectral analysis to video observations of run-up over cuspate topography shows that these alongshore patterns in the morphology are accompanied by changes to the fundamental behaviour of the run-up timeseries. Nonlinear forecasting indicates that at beach cusp horns, the behaviour of swash flow is more predictable and global (meaning that characteristics of individual swash events are well represented by the behaviour of the timeseries as a whole). Conversely, at beach cusp bays, the behaviour of swash flow is less predictable and more local (meaning that the characteristics of individual swash events are best represented by the behaviour of a small fraction of the timeseries). Bispectral analysis indicates that there is a nonlinear transfer of energy from the incident wave frequency f to infragravity frequency ~f/2 which only occurs in the bay, suggesting that the local behaviour is caused by interactions between successive swash cycles which are magnified by channelling caused by the beach cusp geometry. The local behaviour and the bispectral signatures are not present in offshore measurements, and are not present in runup timeseries collected when the beach was planar. These results provide evidence that interactions between successive run-ups 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
Numerical investigation of swash–swash interaction effects on beachface evolution using Nonlinear Shallow Water Equations
No full textThis paper presents a numerical investigation of multiple identical swash events to study the swash–swash interaction processes and their impacts on beachface evolution. The numerical model, based on the Nonlinear Shallow Water Equations, is first calibrated/validated against two different single-event-based data-sets. Multiple swash events are generated by identical solitary waves separated by different time intervals, to achieve weak and strong wave-backwash interactions. After a small number of weak interaction events the main feature is erosion from lower and mid swash region and deposition seaward of the swash in a bed-step, created by a backwash bore, primarily due to bed-load. As the number of waves increases, the strength of this backwash bore reduces because of the reduced beach slope caused by the growing bed-step. This eventually leads to a net quasi-equilibrium between bed- and suspended-load per period in most of the swash and surf zones. For strong interaction, initial bed evolution per event is much slower, due to interactions, and is bed load dominated. A quasi-equilibrium is also established as the influence of suspended load grows. Overall bed change per period within the domain eventually converges in both cases. Final bed profiles (i.e. after the same elapsed time, but different numbers of waves) are fairly similar, both with an offshore swash bar. Both profiles continue to evolve on the offshore side of this bar. However, this evolution is driven by suspended load for the weak interactions and bed load for strong interactions. The implication is that similar swash morphological features can emerge from different swash processes, and also be maintained distinctly
Overtopping a truncated planar beach
No full textRun-up on a truncated impermeable beach is analysed theoretically and experimentally to find the volume of fluid, associated with a single wave event, that flows over the end of the beach. The theoretical calculations investigate the motion using the shallow-water equations and the fluid is allowed to flow freely over the end of the beach. Two models of wave events are considered: dam-break initial conditions, in which fluid collapses from rest to run-up and overtop the beach, and a waveform that models swash associated with the collapse of a long solitary bore. The calculations are made using quasi-analytical techniques, following the hodograph transformation of the governing equations. They yield predictions for the volume of fluid per unit width that overtops the beach, primarily as a function of the dimensionless length of the beach. These predictions are often far in excess of previous theoretical calculations. New experimental results are also reported in which the overtopping volumes due to flows initiated from dam-break conditions are studied for a range of reservoir lengths and heights and for a range of lengths and inclinations of the beach. Without the need for any empirically fitted parameters, good agreement is found between the experimental measurements and the theoretical predictions in regimes for which the effects of drag are negligible
The use of video imagery to analyse groundwater and shoreline dynamics on a dissipative beach
No full textGroundwater 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-minute 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 watertable was within 2 cm of the beachface surface. Timestacks, 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
Validation of Swash for wave overtopping
No full textComparison of measured overtopping data with computations with Swash. The SWASH output discharges are smaller than the 5% lower limit of Eurotop and Neural Network predictions. Nevertheless, they were close values and they have the same order of magnitude, which is very important when comparing overtopping values. No refection analysis was done. The smaller discharges are believed to be explained by the fact that SWASH does not describe splash, and therefore, only overtopping due to waves running up the slope, but not due to “spray” is taken into account. In addition to this, it is thought that SWASH does not model accurately enough wave breaking over abrupt changes in bottom geometry or steep slopes resulting in underestimation of overtopping. On the other hand, SWASH is not able to predict wave overtopping at rubble mound breakwaters. A breakwater is not well modelled in SWASH, because porous structures are dealt as a numerical dissipation box and not as a physical obstacle for incoming waves. As a result, waves are damped but not diverted upwards, so there is no overtopping. The reasons why this happens and some recommendations are given in this thesis. Besides, some ideas about how to introduce a multi-layer structure are explained.Hydraulic EngineeringCivil Engineering and Geoscience
- …
