phenomena in the nature and their influence on the underwater slope are different.
Tsunami waves and strong storm surges are possible only in the specific coastal
regions of the World Ocean, but even here, their recurrence is very low. In the most
cases, dynamic processes in the coastal zone of the sea are determined by the
surface waves and swell whose velocity fields have a direct impact on the bottom
sediments. In addition, during dissipation of the wind waves energy and interaction
between these waves and breaking, secondary motions appear in the water: longperiod waves, alongshore currents, water circulation in the vertical and horizontal
planes whose spatial and temporal scales most likely determine the scales of the
morphodynamic elements of underwater slopes and coasts (Kosyan and Pykhov
1991).
Wind is the main energy source of waves, which always blows over the water
basins. Parameters of stationary waves are determined by the wind speed, duration
of its forcing, and fetch. Wind waves are usually three-dimensional and irregular.
After the wind calms or beyond the zone of wind forcing, the waves propagate in
the sea in the form of swell; they are characterized by approximately constant
period and form.
When waves propagate from the open sea to the coast they reach a point, in
which their length becomes shorter than the local depth. From this moment, the
waves induce oscillating motions of water near the bottom whose amplitude
increases as the waves propagate to shallower depths. Under specific conditions
the motion and transport of the bottom sediments particles starts. If the waves
propagate not normally to the coast, refraction starts. As a result, they tend to
approach the isobaths normally. Deformation of waves occurs simultaneously with
refraction: their form and height change. After further deformation the wave breaks,
and water motions of various scales are generated as the wave energy dissipates:
small-scale turbulence, large-scale eddy motions in the breaking wave, long-period
waves, and coastal currents, which determine the intensity and direction of the
hydrogenic sediment transport in the surf zone.
Sediment motion in the region offshore the wave breaking zone occurs not only
under the wave forcing, but also under the influence of currents of different origin
(flood-ebb tides, wind drift, etc.). In the regions, where the velocities of the floodebb tidal currents may reach tens of centimeters per second, they can strongly
determine the total transport of bottom sediments in the upper region of the shelf
(Soulsby 1983). Superposition of surface waves and currents increases intense
water motions at the bottom; hence it leads to the increase in the transport of
fragmentary material. Beyond the zone of wave breaking, the motion of sediments
after its initiation occurs near the bottom; hence, the needs of modeling the
hydrogenic transport requires the knowledge of the dynamics of the bottom boundary layer both for the pure wave motion and in the case of joint wave and current
forcing.
When surface waves reach shallow depths, oscillatory motions of water at the
bottom and bottom friction form the boundary layer. Transformation of sediments
motion occurs immediately in the oscillating boundary layer, which is accompanied
by the bottom erosion, formation of microforms of bottom topography that actually
124
R.D. Kosyan and B.V. Divinskiy
Tsunami waves and strong storm surges are possible only in the specific coastal
regions of the World Ocean, but even here, their recurrence is very low. In the most
cases, dynamic processes in the coastal zone of the sea are determined by the
surface waves and swell whose velocity fields have a direct impact on the bottom
sediments. In addition, during dissipation of the wind waves energy and interaction
between these waves and breaking, secondary motions appear in the water: longperiod waves, alongshore currents, water circulation in the vertical and horizontal
planes whose spatial and temporal scales most likely determine the scales of the
morphodynamic elements of underwater slopes and coasts (Kosyan and Pykhov
1991).
Wind is the main energy source of waves, which always blows over the water
basins. Parameters of stationary waves are determined by the wind speed, duration
of its forcing, and fetch. Wind waves are usually three-dimensional and irregular.
After the wind calms or beyond the zone of wind forcing, the waves propagate in
the sea in the form of swell; they are characterized by approximately constant
period and form.
When waves propagate from the open sea to the coast they reach a point, in
which their length becomes shorter than the local depth. From this moment, the
waves induce oscillating motions of water near the bottom whose amplitude
increases as the waves propagate to shallower depths. Under specific conditions
the motion and transport of the bottom sediments particles starts. If the waves
propagate not normally to the coast, refraction starts. As a result, they tend to
approach the isobaths normally. Deformation of waves occurs simultaneously with
refraction: their form and height change. After further deformation the wave breaks,
and water motions of various scales are generated as the wave energy dissipates:
small-scale turbulence, large-scale eddy motions in the breaking wave, long-period
waves, and coastal currents, which determine the intensity and direction of the
hydrogenic sediment transport in the surf zone.
Sediment motion in the region offshore the wave breaking zone occurs not only
under the wave forcing, but also under the influence of currents of different origin
(flood-ebb tides, wind drift, etc.). In the regions, where the velocities of the floodebb tidal currents may reach tens of centimeters per second, they can strongly
determine the total transport of bottom sediments in the upper region of the shelf
(Soulsby 1983). Superposition of surface waves and currents increases intense
water motions at the bottom; hence it leads to the increase in the transport of
fragmentary material. Beyond the zone of wave breaking, the motion of sediments
after its initiation occurs near the bottom; hence, the needs of modeling the
hydrogenic transport requires the knowledge of the dynamics of the bottom boundary layer both for the pure wave motion and in the case of joint wave and current
forcing.
When surface waves reach shallow depths, oscillatory motions of water at the
bottom and bottom friction form the boundary layer. Transformation of sediments
motion occurs immediately in the oscillating boundary layer, which is accompanied
by the bottom erosion, formation of microforms of bottom topography that actually
124
R.D. Kosyan and B.V. Divinskiy