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.
Fluid acceleration effects on suspended sediment transport in the swash zone
[1] Suspended sediment concentrations and fluid velocities measured in the swash zone of a high-energy steep beach were used to investigate the importance of fluid accelerations to suspended sediment transport. Swash flow acceleration was nearly constant at about one-half downslope gravitational acceleration with two important exceptions. We observed strong, short-lived periods of accelerating uprush at the beginning of the swash cycle and decelerating backwash at the end of the swash cycle (magnitudes of both approximately twice that of the expected downslope gravitational acceleration). Interestingly, spikes in suspended load followed the anomalies in acceleration in a way that was not apparent from the nearly symmetric (in magnitude) ensemble averaged velocity time series. Suspended load values were largest during accelerating uprush associated with the shoreward propagating turbulent bore or swash front. During backwash, suspended loads were generally not as large. Correspondingly, suspended sediment transport rates obtained from the sediment concentration and velocity measurements showed best comparisons with a modified sediment transport model that includes a physical mechanism for enhancing transport rates due to flow acceleration. The modified sediment transport model reduced the overall root-mean square prediction error by up to 35% and shifted the predicted peak in uprush sediment transport rate earlier in the swash cycle, resulting in a better fit to the observations. These findings suggest that the inclusion of the acceleration term may account for physical mechanisms that include bore turbulence and horizontal pressure gradients typically associated with the accelerating portion of uprush.
Instruments for investigating shore and nearshore processes
Abstract On shingle beaches, changes in foreshore elevation and sediment distribution landward of the break point are produced largely by variations in the uprush and backwash of waves. However, very little is known about the forces active in this zone. A field instrument system which senses and records some of the parameters thought to influence beach erosion and deposition in this zone has been constructed. The equipment is also suitable for the investigation of a number of other shore and nearshore processes including erosion on sandy and rocky shores, and flow processes affecting littoral biological communities. In the swash zone two sensing heads, a dynamometer and a depth recorder, sense variations in uprush and backwash velocities, energies, discharges, and depths of flow. Both devices are electromechanical and are coupled to a recording unit on land by PVC‐insulated cable. The dynamometer (two force plates mounted back‐to‐back on a compression spring and coupled to variable resistances) has been c...