Francisco Sancho

Observations of velocities, sand concentrations, and fluxes under velocity‐asymmetric oscillatory flows

[1] U‐tube measurements of instantaneous velocities, concentrations, and fluxes for a well‐sorted, medium‐sized sand in oscillatory sheet flow are analyzed. The experiments involved two velocity‐asymmetric flows, the same two flows with an opposing current of 0.4 m/s, and a mixed skewed‐asymmetric flow, all with a velocity amplitude of 1.2 m/s and flow period of 7 s. We find that the net positive transport rate beneath velocity‐ asymmetric oscillatory flow results from large, but opposing sand fluxes during the positive and negative flow phase. With an increase in velocity asymmetry and, in particular, velocity skewness, the difference in the magnitude of the fluxes in the two half cycles increases, leading to larger net transport rates. This trend is consistent with the observed increase in skewness of the oscillatory bed shear stress. Phase‐lag effects, whereby sand stirred during the negative flow phase has not settled by the time of the negative‐to‐positive flow reversal and is subsequently transported during the positive flow phase, are notable but of minor importance to the net transport rate compared to earlier experiments with finer sands. In the vertical, the oscillatory flux is positive above the no‐ flow bed. Within the sheet flow pick‐up layer, the oscillatory flux is negative and similar in magnitude to the positive flux induced by the residual flow. The 0.4 m/s opposing current causes more sand to be picked up during the negative than during the positive flow phase. Above the no‐flow bed the resulting negative oscillatory flux is comparable in magnitude to the current‐related flux.

Sediment transport in nonlinear skewed oscillatory flows: Transkew experiments

New experiments under sheet flow conditions were conducted in an oscillating water tunnel to study the effects of flow acceleration on sand transport. The simulated hydrodynamic conditions considered flow patterns that drive cross-shore sediment transport in the nearshore zone: the wave nonlinearities associated with velocity and acceleration skewness and a negative mean current, the undertow. Net transport rates were evaluated from the sediment balance equation and show that (1) the acceleration skewness in an oscillatory flow produces a net sediment transport in the direction of the highest acceleration; (2) the net transport in the presence of an opposing current is negative, against the direction of the highest acceleration, and reduces with an increase in flow acceleration; and (3) velocity skewness increases the values of the net onshore transport rates.

Modelling Horizontal Velcocities within the Wave Bottom Boundary Layer

As waves travel and shoal towards a beach, their surface elevation becomes peaky (sharp crests) and asymmetric relative to the vertical, differing from the sinusoidal profile of linear waves. Below the surface, the passage of the progressive waves induces fluid velocities, showing similar (time) asymmetries. These nonlinearities are inextricably linked to sediment transport, but the processes involved are not well understood. This work analyses the data collected during a recent experimental project under skewed oscillatory flows. It validates a simple method based on the defect law to reproduce the horizontal velocities within the wave bottom boundary layer. Results indicate a good agreement between the measured and modeled velocities using this methodology.

Coastal Erosion Management in Algarve (Portugal) — A Beach Nourishment Case Study

ABSTRACT PROENÇA, B., OLIVEIRA, F.S.B.F and SANCHO, F., 2011. Coastal Erosion Management in Algarve (Portugal) — a Beach Nourishment Case Study. In: Micallef, A. (ed.), MCRR3-2010 Conference Proceedings, Journal of Coastal Research, Special Issue, No. 61, pp. 328–334. Grosseto, Tuscany, Italy, ISSN 0749-0208. In the Portuguese southern coast, national and international appeal turns it into a highly relevant area concerning the countrys economy. Vale do Lobo is a resort located in the centre of this coast, where the beach has been suffering from erosion problems for several years now. Local interventions have been executed in order to prevent the erosion of the beach, yet the efficiency of such strategy has not been verified. This studys main purpose is to enable a better understanding of the evolutionary trends of the coastline between the Quarteira and Ancño inlet and to analyse the efficiency of the beach nourishment interventions as a protection technique for a coastal environment with the wave energy exposure and geomorphologic characteristics of the present one. The study focussed on a 10-year period, during which, 700×10 and 370×103 m3 of sand with D50=0.76 mm (slightly larger than the D50 of the native beach) were extracted from offshore, and deposited in the foreshore. The wave climate in the study area was processed, analysed and used as input for modelling the nearshore processes. In addition, beach profiles measured alongshore have been analysed to obtain the representative beach profile. Coastlines of different dates have been derived from aerial photographs. The longshore transport model Litdrift and the coastline model Litline were applied to study the sediment dynamics. Litdrift was used to estimate the longshore transport capacity of the beach for the hydrodynamic series considering the representative beach profile previously obtained. These results were then used to calculate the evolution of the coastline due to the incident wave climate with Litline model. Aerial photographs were used to verify the model. The analysis of the two beach nourishments performed allowed inference on the efficiency of this coastal protection technique to delay the erosion process.

MORPHODYNAMIC INSTABILITIES IN THE SURF ZONE

The bed forms in the surf zone can grow as a result of natural morphodynamic instabilities, forced by the wave-induced flow. This work gives a contribution to the dynamics of the formation and evolution the bed forms in surf zone. We use the new morphodynamic model, M-SHORECIRC, to analyze the growth and dynamics of bar-type, rhythmic, bed forms of a barred beach. These analyses include the effects of initial wave angle, wave energy, and bar location. These are all seen to influence the dynamics of the rhythmic bed-forms regarding the longshore length-scales, and intensity and location of bed-forms.

Use of the velocity defect law to assess sand transport rates

ABSTRACT Abreu, T., Silva, P.A., Sancho, F. and Michallet, H., 2013. Use of the velocity defect law to assess sand transport rates. This work uses a simple method based on the defect law that is able to reproduce the horizontal velocities within the wave bottom boundary layer, using a limited number of parameters. The theory is checked against the measured velocity profiles gathered with a high resolution Acoustic Doppler Velocity Profiler. The velocities were collected during an experimental project performed at the Large Oscillating Water Tunnel of Deltares under flat-bed/sheet flow conditions, allowing the analysis of the effects of wave nonlinearities and of a net current on the sediment transport processes. The velocity estimates are used to infer the time-varying bed shear stress, using the momentum-integral method. Secondly, the bed shear stresses are incorporated in a quasi-steady bed load formulation and the transport rate predictions are compared with the net transport rate measurements obtained in the same facility. The results show that the methodology presented in this work provides a suitable estimation of the horizontal near-bed sediment transport rates under non-linear oscillatory flows.

Ondas assimétricas em praias com barra

Natural beaches often present a breaker bar that significantly affect physical phenomena like, for example, wave transformation, wave refle...