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.
Overtopping and Run-up Hazards Induced by Solitary Waves and Bores
Solitary waves are wave forms that consist of a single wave, rather than waves that form part of a series of regular waves or random waves, the latter being typical of ocean wind and swell waves. A solitary wave that propagates with constant form has a unique shape for a given water depth and height, but in general, single waves of arbitrary shape are referred to as solitary waves. Single waves that have broken can be regarded as solitary bores. While the exact solitary wave shape has long been used to represent tsunami, Madsen et al. (2008) show that this is not likely to be usually the case and more complex wave shapes result. However, tsunami waves often closely resemble a solitary wave type and such waves still form the basis of most tsunami modelling. Tsunami are generated by impulsive geophysical events on ocean bed, and take the form of long waves with small steepness in the open ocean (Synolakis and Bernard, 2006). As witnessed in the 2004 Indian Ocean Tsunami, at the shore, the leading waves of a tsunami may steepen sufficiently to break and form very long surf bores. The impact of tsunami bores may be very different to that from non-breaking or standing solitary waves, and this may require different disaster management strategies and evacuation plans, particularly when considering the initial impact and first few minutes after the wave arrival. Hall and Watts (1953) provided the first study of such waves with later laboratory studies of bore run-up providing further details on the speed of the run-up in comparison to theory (Yeh, 1991). In addition to tsunami, inundation of coastal zones by overwash is a major flooding hazard in many regions. In this instance, the overwash may be a result of run-up and overtopping on the sub-aerial beach, or as a result of overwash from waves in shallow water as the tide level inundates a barrier. In these cases, the overwash may be driven by surf zone bores or long wave surges that arise from wave groups in the shallow water or surf zone. The leading part of the long wave surge typically exhibits a solitary wave shape, as shown by Baldock (2006) and Nielsen and Baldock (2010). This overland flow in the run-up zone on beaches is frequently termed swash, and comprises an uprush (landward) and backwash (seaward) phase. This work considers these issues, and presents the results of recent laboratory experiments measuring the overtopping flows from solitary waves and long bores. The results are discussed in the context of recent run-up theory, and used to further verify that theory for application to long bores, solitary waves and potentially initial tsunami impact. In addition, using the recent solutions, some estimates of the fluid forces, maximum inundation depths
Using nonlinear forecasting to learn the magnitude and phasing of time‐varying sediment suspension in the surf zone
The time-dependent response of sediment suspension to flow velocity was explored by modeling field measurements collected in the surf zone during a large storm. Linear and nonlinear models were created and tested using flow velocity as input and suspended-sediment concentration as output. A sequence of past velocities (velocity history), as well as velocity from the same instant as the suspended-sediment concentration, was used as input ; this velocity history length was allowed to vary. The models also allowed for a lag between input (instantaneous velocity or end of velocity sequence) and output (suspended-sediment concentration). Predictions of concentration from instantaneous velocity or instantaneous velocity raised to a power (up to 8) using linear models were poor (correlation coefficients between predicted and observed concentrations were less than 0.10). Allowing a lag between velocity and concentration improved linear models (correlation coefficient of 0.30), with optimum lag time increasing with elevation above the seabed (from 1.5 s at 13 cm to 8.5 s at 60 cm). These lags are largely due to the time for an observed flow event to effect the bed and mix sediment upward. Using a velocity history further improved linear models (correlation coefficient of 0.43). The best linear model used 12.5 s of velocity history (approximately one wave period) to predict concentration. Nonlinear models gave better predictions than linear models, and, as with linear models, nonlinear models using a velocity history performed better than models using only instantaneous velocity as input. Including a lag time between the velocity and concentration also improved the predictions. The best model (correlation coefficient of 0.58) used 3 s (approximately a quarter wave period) of the cross-shore velocity squared, starting at 4.5 s before the observed concentration, to predict concentration. Using a velocity history increases the performance of the models by specifying a more complete description of the dynamical forcing of the flow (including accelerations and wave phase and shape) responsible for sediment suspension. Incorporating such a velocity history and a lag time into the formulation of the forcing for time-dependent models for sediment suspension in the surf zone will greatly increase our ability to predict suspended-sediment transport.