LARGE SCALE BEDFORMS AND SEDIMENT TRANSPORT IN FRONT OF CLIFFS
Elena Sánchez and M.A. Losada
29th International Conference on Coastal Engineering, ICCE ( September 2004)

Internacionales

Abstract:
MOTIVATION
In dissipative uniform coasts, the alongshore current induced by the spilling or plunging wave breaking under oblique incidence used to be the main sediment transport mechanism. However, in nature it is possible to observe how these uniform coasts are interrupted by cliffy stretches of finite length. This is the case of ‘La Barrosa’ (Cádiz, Spain) and ‘El Garraf’ (Barcelona, Spain). What does it happen with the sediment in this case? In the examples mentioned above, it seems that the beaches at both sides of the cliffs are ‘full’. How does occur the sand by-passing alongshore if any?, does it changes to a predominant cross-shore? What is the effect of a ‘shallow’ and finite cliff stretch in the hydrodynamic and morphological conditions of an uniform coast?. The objective of this paper is to analyze the ‘cliff induced’
hydro and morphological effects in mild environmental conditions and to evaluate their contribution to the alongshore sediment transport. For this analysis, ‘La Barrosa’ cliff-beach system will be used as study case.

FORMULATION OF THE PROBLEM
The presence of a ‘shallow’ cliff stretch in an uniform dissipative coastline with oblique incident waves causes the scattering (radiation and reflection) of the incident waves and the change of the wave breaking conditions (from spilling to ‘clapotis’, plunging or collapsing
types). Both processes can contribute, as function of the environmental regime, to decrease the cross-shore gradient of the radiation stress and consequently, to diminish the forcing mechanism of the alongshore current. In the case of mild environmental conditions, the dominant process is the wave reflection and radiation. Its effect on the across-shore gradient of
the radiation stress and on the mean water level can be evaluated analytically for the
case of regular waves and spilling breaking in the framework of the linear wave theory (Mendez et al., 1998).


PHASE AVERAGED HYDRODYNAMIC AND SEDIMENT BALANCE IN FRONT OF CLIFFS
Neglecting the possible effect of wind induced currents, in mild environmental conditions the only possible mechanisms to enhance the sediment transport are those induced by the wave scattering in the cliff. Here, we analyze the next ‘cliff induced’ processes:
(1) The mass transport and the streaming patterns induced by the interaction of the reflected, radiated and incident wave trains. For that, a second order solution of the turbulent wave boundary layer based on a Grant Madsen eddy viscosity law will be applied.
From the hydrodynamic patterns, the large scale bedforms induced as function of the grain size and their possible contribution to the alongshore sediment transport by means of the bedforms migration in presence of groups or free long waves will be also discussed.
(2) The alongshore gradient of the mean water level induced by the wave reflection and
radiation on the finite cliff. Its influence on the generation of steady or transient currents and on propagation of phase-locked radiation waves and edge waves will be analyzed.
The contribution of the above mentioned processes on the cliff morphology and on the sediment bypassing will be discussed in terms of the cliff length and reflectivity. In the final paper, all
these mechanisms will be considered to calculate the sediment balance along ‘La Barrosa’ coastline.

REFERENCES
Darymple, R.A., M.A. Losada and P. Martin, 1991. Reflection and transmition from porous structures under oblique wave attack. J.F.M. Vol. 224, pp. 625-644.
Mendez, F.J., I.Losada, R.A. Darymple and M.A. Losada, 1998. Effects of wave reflection and dissipation on wave-induced second order magnitudes. Proc. Coastal Engineering, pp.537-550.