100
4 TRANSPORTATION AND SEDIMENTATION
Fig. 4.12. Diagrams showing (A) the orbital motion of open waves and (B) the ellipsoidal motion of shoaling waves.
sea wind-generated currents may also make a tidal cycle asymmetric. Thus in most cases,
though sand is constantly reworked, there is a net direction of sediment transport.
Consider now the much shorter cycle of a moving wave. A particle floating on the
surface describes an orbit as the wave passes, but there is no net transport. The orbital
motions of particles in the water diminish with increasing depth to a surface below
which there is no motion. This surface is termed "effective wave base," and is normally
half the wavelength (Fig. 4.12A). Below effective wave base there are no wave-generated
currents. Sand cannot be mobilized, unless there are currents of other origins; they may
only settle out of suspension. As water depth shoals, the shoreward effective wave base
impinges on the sea bed. Orbital motion in the water becomes modified into ellipses
as the water shallows (Fig. 4.12B). The ellipsoidal motion transports sand to and fro to
form current ripple (sometimes also called vortex ripples). These are generally symmetrical and have a chevron-like internal lamination. Unlike current ripples they occasionally bifurcate when viewed from above. Moving up the beach the ripple profiles
become asymmetric with the steep face directed landward, and with landward-dipping
cross-lamination (Fig. 4.13). These are termed wave current ripples (Reineck and Singh,
1980, pp. 32-36). It has proved possible to define the relationships between current velocity, grain size, and bed form for shallow waves (Fig. 4.14).
4 TRANSPORTATION AND SEDIMENTATION
Fig. 4.12. Diagrams showing (A) the orbital motion of open waves and (B) the ellipsoidal motion of shoaling waves.
sea wind-generated currents may also make a tidal cycle asymmetric. Thus in most cases,
though sand is constantly reworked, there is a net direction of sediment transport.
Consider now the much shorter cycle of a moving wave. A particle floating on the
surface describes an orbit as the wave passes, but there is no net transport. The orbital
motions of particles in the water diminish with increasing depth to a surface below
which there is no motion. This surface is termed "effective wave base," and is normally
half the wavelength (Fig. 4.12A). Below effective wave base there are no wave-generated
currents. Sand cannot be mobilized, unless there are currents of other origins; they may
only settle out of suspension. As water depth shoals, the shoreward effective wave base
impinges on the sea bed. Orbital motion in the water becomes modified into ellipses
as the water shallows (Fig. 4.12B). The ellipsoidal motion transports sand to and fro to
form current ripple (sometimes also called vortex ripples). These are generally symmetrical and have a chevron-like internal lamination. Unlike current ripples they occasionally bifurcate when viewed from above. Moving up the beach the ripple profiles
become asymmetric with the steep face directed landward, and with landward-dipping
cross-lamination (Fig. 4.13). These are termed wave current ripples (Reineck and Singh,
1980, pp. 32-36). It has proved possible to define the relationships between current velocity, grain size, and bed form for shallow waves (Fig. 4.14).