Diane P. Horn

Beach groundwater dynamics

Abstract An understanding of the interaction between surface and groundwater flows in the swash zone is necessary to understand beach profile evolution. Coastal researchers have recognized the importance of beach watertable and swash interaction to accretion and erosion above the still water level (SWL), but the exact nature of the relationship between swash flows, beach watertable flow and cross-shore sediment transport is not fully understood. This paper reviews research on beach groundwater dynamics and identifies research questions which will need to be answered before swash zone sediment transport can be successfully modelled. After defining the principal terms relating to beach groundwater, the behavior, measurement and modelling of beach groundwater dynamics is described. Research questions related to the mechanisms of surface–subsurface flow interaction are reviewed, particularly infiltration, exfiltration and fluidisation. The implications of these mechanisms for sediment transport are discussed.

Swash zone sediment transport modes

Abstract The result of field experiments, designed to investigate the relative proportions of bedload and suspended load, are described. The ratio of bedload to suspended sediment load in the swash zone is examined in both swash and backwash on four beaches by measuring the amounts collected in a sediment trap. Bedload transport is found to dominate the backwash. The relative proportions of bedload and suspended load change over the tidal cycle, with increasing bedload dominance at low tide. The total amount of sediment transported as swash and backwash is noticeably greater at high tide than at low tide. More sediment is transported on the flood tide than on the ebb.

Validation of a Boussinesq model of beach ground water behaviour

Abstract A numerical solution to the 1-D Boussinesq ground water equation is used to predict water table response to tidal and wave forcing. We argue that the solution presented here offers considerable advantages over other solutions to the 1-D Boussinesq ground water equation and is simpler to apply than 2-D ground water models. The main assumption in using the Boussinesq equation is that ground water flow in a shallow aquifer can be described using the Dupuit-Forchheimer approximation, in which it is assumed that changes in hydraulic head with depth below the water table are negligible. The predictive ability of the solution was tested against field data from ten wells and ten piezometers on a microtidal sandy beach. Two model runs were carried out: in the first the effect of tides only was considered; in the second setup due to waves was also considered. The arithmetic mean of independently measured values of hydraulic conductivity was used to describe sediment characteristics. The piezometer data show that changes in hydraulic head with depth are negligible for much of the beach, suggesting that the use of the Boussinesq equation is appropriate. The correspondence between model predictions of water table elevations and field observations is good for both the tide-only and the tide plus setup model runs, with the latter giving the better predictions.

Low frequency swash motion induced by wave grouping

Abstract This paper concerns the low frequency motion of swash directly induced by wave grouping on a steep beach. A new experimental investigation is presented which considers the hydrodynamics of the inner surf zone and swash zone using vertical wave gauges and a run-up wire. Results for regular waves, wave groups and random waves are discussed, with particular reference to low frequency motions. The inner surf zone and swash zone are found to be unsaturated at incident short wave frequencies and, as a result, significant wave grouping is apparent at the shoreline. The low frequency motion in the surf zone is found to be in phase with the incident wave grouping and may therefore be regarded as a time varying set-up (Watson and Peregrine, 1992). The low frequency motion of the swash is shown to be an order of magnitude greater than that in the inner surf zone, inconsistent with cross-shore standing long waves, for which no evidence is found. We demonstrate that the low frequency motion of the shoreline provides an excellent approximation to the run-up of individual bores and therefore describes the run-up envelope. Spectral analysis shows that the low frequency motion of the swash is directly linked to the modulations in offshore wave height i.e. the low frequency energy in the incident wave envelope. In addition, the random wave run-up spectra show an f−4 high frequency roll-off, as found by Huntley et al. (1977). The accumulated data show that, unless the surf zone is totally saturated, a significant proportion of the low frequency swash motion may be directly due to incident wave grouping and not standing long waves.

Measurements and Modeling of Swash Induced Pressure Gradients in the Surface Layers of a Sand Beach

Field measurements of swash-induced hydraulic (pressure) gradients in the surface layers of a sand beach are presented and compared to a one-dimensional diffusion model. The model provides a description of flow through a porous, quasi-saturated sediment and is driven by the time-varying swash depth on the beach surface. The field measurements broadly show four different types of pressure propagation behavior, which appear dependent on cross-shore position in the swash zone. The hydraulic gradients within the surface layers of the beach are typically found to be much greater than those likely to be generated by tidally-induced groundwater flow. In these instances the diffusion model generally provides a good description of the data. However, very large hydraulic gradients are frequently observed just below the beach surface, and these cannot be adequately described by the diffusion model. A mechanism is proposed through which these large hydraulic gradients could be generated.

Measurement and Modelling of Gravel Beach Groundwater Response to Wave Run-up: Effects on Beach Profile Changes

Abstract Beach profile change in the swash zone on gravel beaches is characterised by enhanced onshore sediment transport and berm formation, and infiltration loss in the swash zone is often given as the reason why gravel beaches are steeper than sand beaches. In this paper, we report on field measurements conducted on a gravel beach, Slapton Sands, Devon, UK, in April 2001. Collected data included surface pressures under uprush and backwash, subsurface pore water pressures, uprush and backwash velocities, and bed elevations. The field data were compared with predictions of the BeachWin model, which simulates interacting wave run-up/run-down, beach groundwater flow, swash sediment transport, and resulting beach profile changes. With a relatively large value of hydraulic conductivity, the model was able to predict the observed berm formation at the upper part of the beach. In contrast, the berm feature was absent in the simulation without swash infiltration effects included. The predicted erosion at the middle section of the beach, however, took place at a location landward of that of the measurements. Simulations were carried out to investigate the sensitivity of the model to changes in the hydraulic conductivity, friction factor, and coefficient ratio of uprush/backwash sediment transport. All parameters were shown to affect the simulation results—in particular, the formation and extent of the berm.

Video measurement of swash zone hydrodynamics

A new technique for recording two-dimensional uprush and backwash depth profiles using standard analogue camcorder technology is introduced. This technique has the potential to provide low cost, high frequency, high resolution measurements of individual swash events. Analogue to digital data conversion using multimedia software is discussed, as is the derivation of metric data via image processing and geographical information systems techniques. A series of field and laboratory experiments are described which provided an opportunity to refine and test the technique. Data from the laboratory video record are compared against measurements from standard surface piercing resistance type wave gauges, and an assessment made of the next steps required for the enhancement of the technique. The comparison between video and probe data indicates that the technique shows promise as a means of deriving two-dimensional profiles of swash lens; however, further development and testing are necessary.

MECHANISMS OF BEACH GROUND WATER AND SWASH INTERACTION

A study of alternatives including a shoreline evolution numerical modelization has been carried out in order to both diagnose the erosion problem at the beaches located between Cambrils Harbour and Pixerota delta (Tarragona, Spain) and select nourishment alternatives.

The Influence of Groundwater on Profile Evolution of Fine and Coarse Sand Beaches

This paper reports on laboratory experiments carried out on fine and coarse sand beaches under raised and lowered back beach groundwater levels. Onshore sediment transport was enhanced when the groundwater level was lowered under both accretionary and erosive conditions, whereas higher groundwater level promoted offshore sediment transport. Under swell conditions, the coarse beach showed accretion at all groundwater levels. In contrast, on the fine sand beach, lowering the groundwater level was able to induce accretion on a previously erosive profile. The groundwater level had less effect on beach profile evolution under storm conditions, suggesting that artificially lowering groundwater levels would not help much in the control of storm erosion, but could promote post-storm accretion on permeable beaches. Agreement between measured and modelled head levels was better on the coarse beach than on the fine beach, and better in the swash zone and within the beach than in the surf zone.

FIELD MEASUREMENTS OF SWASH INDUCED PRESSURES WITHIN A SANDY BEACH

Detailed studies have been undertaken to assist in the design of major extensions to the port of Haifa. Both numerical and physical model studies were done to optimise the mooring conditions vis a vis the harbour approach and entrance layout. The adopted layout deviates from the normal straight approach to the harbour entrance. This layout, together with suitable aids to navigation, was found to be nautically acceptable, and generally better with regard to mooring conditions, on the basis of extensive nautical design studies.Hwa-Lian Harbour is located at the north-eastern coast of Taiwan, where is relatively exposed to the threat of typhoon waves from the Pacific Ocean. In the summer season, harbour resonance caused by typhoon waves which generated at the eastern ocean of the Philippine. In order to obtain a better understanding of the existing problem and find out a feasible solution to improve harbour instability. Typhoon waves measurement, wave characteristics analysis, down-time evaluation for harbour operation, hydraulic model tests are carried out in this program. Under the action of typhoon waves, the wave spectra show that inside the harbors short period energy component has been damped by breakwater, but the long period energy increased by resonance hundred times. The hydraulic model test can reproduce the prototype phenomena successfully. The result of model tests indicate that by constructing a jetty at the harbour entrance or building a short groin at the corner of terminal #25, the long period wave height amplification agitated by typhoon waves can be eliminated about 50%. The width of harbour basin 800m is about one half of wave length in the basin for period 140sec which occurs the maximum wave amplification.Two-stage methodology of shoreline prediction for long coastal segments is presented in the study. About 30-km stretch of seaward coast of the Hel Peninsula was selected for the analysis. In 1st stage the shoreline evolution was assessed ignoring local effects of man-made structures. Those calculations allowed the identification of potentially eroding spots and the explanation of causes of erosion. In 2nd stage a 2-km eroding sub-segment of the Peninsula in the vicinity of existing harbour was thoroughly examined including local man-induced effects. The computations properly reproduced the shoreline evolution along this sub-segment over a long period between 1934 and 1997.In connection with the dredging and reclamation works at the Oresund Link Project between Denmark and Sweden carried out by the Contractor, Oresund Marine Joint Venture (OMJV), an intensive spill monitoring campaign has been performed in order to fulfil the environmental requirements set by the Danish and Swedish Authorities. Spill in this context is defined as the overall amount of suspended sediment originating from dredging and reclamation activities leaving the working zone. The maximum spill limit is set to 5% of the dredged material, which has to be monitored, analysed and calculated within 25% accuracy. Velocity data are measured by means of a broad band ADCP and turbidity data by four OBS probes (output in FTU). The FTUs are converted into sediment content in mg/1 by water samples. The analyses carried out, results in high acceptance levels for the conversion to be implemented as a linear relation which can be forced through the origin. Furthermore analyses verifies that the applied setup with a 4-point turbidity profile is a reasonable approximation to the true turbidity profile. Finally the maximum turbidity is on average located at a distance 30-40% from the seabed.