Swash overtopping and sediment overwash on a truncated beach
New experimental laboratory data are presented on swash overtopping and sediment overwash on a truncated beach, approximating the conditions at the crest of a beach berm or inter-tidal ridge-runnel. The experiments provide a measure of the uprush sediment transport rate in the swash zone that is unaffected by the difficulties inherent in deploying instrumentation or sediment trapping techniques at laboratory scale. Overtopping flow volumes are compared with an analytical solution for swash flows as well as a simple numerical model, both of which are restricted to individual swash events. The analytical solution underestimates the overtopping volume by an order of magnitude while the model provides good overall agreement with the data and the reason for this difference is discussed. Modelled flow velocities are input to simple sediment transport formulae appropriate to the swash zone in order to predict the overwash sediment transport rates. Calculations performed with traditional expressions for the wave friction factor tend to underestimate the measured transport. Additional sediment transport calculations using standard total load equations are used to derive an optimum constant wave friction factor of fw=0.024. This is in good agreement with a broad range of published field and laboratory data. However, the influence of long waves and irregular wave run-up on the overtopping and overwash remains to be assessed. The good agreement between modelled and measured sediment transport rates suggests that the model provides accurate predictions of the uprush sediment transport rates in the swash zone, which has application in predicting the growth and height of beach berms. D 2005 Elsevier B.V. All rights reserved.
Application of a Non-Linear Shallow Water Theory to Swash Following Bore Collapse on a Sandy Beach
The non-linear shallow water theory is believed to be capable of describing many features of wave behaviour in the coastal zone. A set of solutions to the governing equations exists for swash following bore collapse on a hydraulically smooth and impermeable beach. These solutions predict that the maximum swash height is proportional to the square of the initial shoreline velocity, the locus of shoreline position through time is parabolic, the maximum swash depth at any position on the beach is a quadratic function of its distance from the initial shoreline position, the maximum swash depth at any position on the beach occurs before the time of maximum uprush, and a retrogressive bore forms in the backwash. All of these predictions have been observed in field data collected from a number of sandy beaches in southeast Australia. However, the theory consistently over-predicted the magnitude of the parameters measured. Evidence is presented to suggest that this discrepancy is due to the effects of friction and infiltration acting on the swash lens, which are not initially accounted for in the theory. If the available evidence is accepted, then the combined effects of friction and infiltration on a sandy beach serve to reduce the maximum swash height to approximately 65% of that expected from theory. Aside from the theoretical over-prediction of the magnitude of the swash parameters measured, the gross flow behaviour of the uprush on a rough and permeable beach face appears to be successfully described by the inviscid theory. There is some promise, therefore, for successfully modelling the effects of friction and infiltration within the framework provided by the theory. Unfortunately the backwash stage of the swash cycle is not so well predicted. It seems that a better understanding of both the backwash bore and the behaviour of granular-fluid flows is required before the backwash is successfully modeled.
ENERGY MEASUREMENTS IN THE SWASH—SURF ZONE1
A technique has been devised for obtaining energy and sediment data in the swash-surf zone of a beach. The instruments used are a bidirectional compression spring dynamometer and a rapid surface sand sampler that have been designed specifically for use shoreward of the breakers. On the basis of the information gathered to date, three distinct depositional environments have been defined inside the breaker zone. These are: 1) the swash zone, 2) the surf zone, and 3) a well defined transition zone separating the first two.