For the understanding of longshore currents along a natural beach, the effects of
bottom unevenness are considered to be important, especially for the flow in the
swash zone. Currents in the swash zone are strongly influenced by the bed slope
because the effect of gravity overwhelms the effect of the depth change. In the present
paper, we investigate these effects and focus on waves propagating from offshore over
a flat ocean basin of constant depth to a beach with a sloping wavy bottom. The waves
are incident at a small angle to the beach normal, and the bed slope in the alongshore
direction is varied slowly. To simplify the problem, only cnoidal waves and solitary
waves are considered and the bed level is varied sinusoidally in the longshore direction.
A perturbation method is applied to the two-dimensional nonlinear shallow water
equation (two-dimensional NLSWE) for the wave motion in order to generate a
more simplified model of wave dynamics consisting of a one-dimensional NLSWE
for the direction normal to the beach and an equation for the alongshore direction.
The first equation, the one-dimensional NLSWE, is solved by Carrier & Greenspan's
transformation. The solution of the second one is found by extending Brocchini
& Peregrine's solution for a flat beach. Two methods for the solution of the one-
dimensional NLSWE are introduced in order to get a solution applicable to large-amplitude swash motions, where the amplitude is comparable to the beach length.
One is the Maclaurin expansion of the solution around the moving shoreline, and
the other is Riemann's representation of the solution, which exactly satisfies the one-dimensional NLSWE and the boundary conditions. After doing a consistency check
by confirming that Riemann's method, a numerical solution, agrees with the exact
solution for an infinitely long, sloping beach, we assumed that the Maclaurin series
solution can also describe wave motion in the swash zone properly not only for this
model but also for our ‘wavy’, finite beach model.
The solution obtained from the Maclaurin series is then plugged into the equation
for the alongshore direction to calculate the shore currents induced by wave run-up
and back-wash motions, where a ‘weakly two-dimensional solution’ is derived from
geometrical considerations. The results show that since the water depth near the
shoreline is comparable to the bed level fluctuations, the flow is strongly affected
by the bed unevenness, leading to recognizable changes in shoreline movement and
the time-averaged velocity and the mass flux of the flow in the swash zone. More
specifically, the inhomogeneity of the alongshore mass flux generates offshore currents
because of the continuity condition for the fluid mass.
