determine the boundary conditions for modeling of the mass transport of sediments
under forcing of sea waves.
7.4 Elementary Hydrogenic Processes
One can distinguish several main elements in the group of hydrogenic displacements of the sedimentary material: initial motion of particles, bottom transport,
suspension transport to the ripple and smooth phases of the sediment motion. It was
suggested to call the mechanisms characterizing the sediment transport as elementary processes in the analysis of lithodynamic oceanic systems (Longinov and
Pykhov 1981). Elementary processes, being the first stage and the basis of the
lithological studies, can be investigated theoretically and experimentally as a
physical process of the interaction between the suspension flow and bottom.
As a rule, all types of the sediment transport can be observed simultaneously in
the natural conditions; although one can distinguish the regions on underwater
slopes, in which one or another elementary process dominates. In the outer part
of the coastal zone, the extreme offshore point of hydrogenic displacement of
sediments is the point on the underwater slope, in which the first displacements
of bottom particles occur. This point may move along the slope depending of the
size of the sediment particles and parameters of the surface waves. As the velocities
of the orbital motion near the bottom increase, more and more particles are involved
into motion. In this phase of the interaction between the flow and current, the
particles in the bottom layer are involved into the rolling motion, sliding, or
saltation (the height of the jumps usually does not exceed a few diameters of the
particles), which determine the bottom transport of sediments over a smooth
bottom. This regime is called the smooth phase of sediment motion up to the
beginning of formation of bottom microforms. The further increase in the velocities
upslope leads to the increase in the transport of displaced particles. Formation of
ripples starts under specific conditions. Ripples are observed on the bottom up to the
zone of wave breaking. Bottom transport of sand over windward slopes of microforms and in the suspension layer whose approximate thickness is of the order of the
ripple length occurs simultaneously in this phase of the interaction between the flow
and the bottom, which is called the ripple phase.
Ripples are washed out in the region of the wave breaking point where the wave
forcing on the bottom is maximal. Here, sand transport occurs in the suspension
layer over smooth bottom. We shall call this regime the upper smooth phase of
sediment motion. Suspension motion of sediments dominates immediately in the
zone of wave breaking owing to the development of eddy motions, while all
elementary processes may exist simultaneously closer to the shore in the surf
zone where the bottom sediment particles are larger and bottom forms can appear
again. The motion in the form of sand layer over smooth bottom dominates when
the wave energy finally dissipates in the runup zone; this is the upper smooth phase
of sand transport.
7 Lithodynamics of the Coastal Zone
125
under forcing of sea waves.
7.4 Elementary Hydrogenic Processes
One can distinguish several main elements in the group of hydrogenic displacements of the sedimentary material: initial motion of particles, bottom transport,
suspension transport to the ripple and smooth phases of the sediment motion. It was
suggested to call the mechanisms characterizing the sediment transport as elementary processes in the analysis of lithodynamic oceanic systems (Longinov and
Pykhov 1981). Elementary processes, being the first stage and the basis of the
lithological studies, can be investigated theoretically and experimentally as a
physical process of the interaction between the suspension flow and bottom.
As a rule, all types of the sediment transport can be observed simultaneously in
the natural conditions; although one can distinguish the regions on underwater
slopes, in which one or another elementary process dominates. In the outer part
of the coastal zone, the extreme offshore point of hydrogenic displacement of
sediments is the point on the underwater slope, in which the first displacements
of bottom particles occur. This point may move along the slope depending of the
size of the sediment particles and parameters of the surface waves. As the velocities
of the orbital motion near the bottom increase, more and more particles are involved
into motion. In this phase of the interaction between the flow and current, the
particles in the bottom layer are involved into the rolling motion, sliding, or
saltation (the height of the jumps usually does not exceed a few diameters of the
particles), which determine the bottom transport of sediments over a smooth
bottom. This regime is called the smooth phase of sediment motion up to the
beginning of formation of bottom microforms. The further increase in the velocities
upslope leads to the increase in the transport of displaced particles. Formation of
ripples starts under specific conditions. Ripples are observed on the bottom up to the
zone of wave breaking. Bottom transport of sand over windward slopes of microforms and in the suspension layer whose approximate thickness is of the order of the
ripple length occurs simultaneously in this phase of the interaction between the flow
and the bottom, which is called the ripple phase.
Ripples are washed out in the region of the wave breaking point where the wave
forcing on the bottom is maximal. Here, sand transport occurs in the suspension
layer over smooth bottom. We shall call this regime the upper smooth phase of
sediment motion. Suspension motion of sediments dominates immediately in the
zone of wave breaking owing to the development of eddy motions, while all
elementary processes may exist simultaneously closer to the shore in the surf
zone where the bottom sediment particles are larger and bottom forms can appear
again. The motion in the form of sand layer over smooth bottom dominates when
the wave energy finally dissipates in the runup zone; this is the upper smooth phase
of sand transport.
7 Lithodynamics of the Coastal Zone
125