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Coastal Engineering: Theory and Practice
vortex System due to combination of temporal agitation at the bed [Shen
et al. 1966].
7.2.4 Scour due to waves
In the case of scour developed by waves only, the mechanism is different
from that associated with steady flow. The steady current is thought of
having persisted over a great distance so that the boundary layer (région of
velocity gradient flow) is relatively thick. In contrast, the fluid particle travel
for an oscillatory flow is not great enough for a substantial boundary layer
height to develop. Therefore, the horseshoe vortex is smaller and weaker
than that produces by steady current flow. Sédiment scour in oscillatory
flow appears to be caused by the accélération of the primary flow past the
obstruction, as well as in the variation of the flow patterns in the wake.
Even when a wave induced scour hole fully developed the orbital current
boundary layer remains thin and does not contribute to the formation of
the horseshoe vortex.
7.2.5 Scour due to simultaneous action of waves
and current
In a wave-current boundary layer flow, as the wave action increases, the
mean current speed near the bed is progressively reduces whist the shear
stress is increased. Thus, it appears that the ratio of wave to current speed
as well as their magnitudes influences the scour development in wavecurrent flow. In the case of combined waves and current, the presence of
waves modifies the vortex structure considerably. The near bed flow consists of formation of vortex and its shedding on the lee side of the pile
(which is formed due to the interaction between the two shear layers emanating from the side edges of the pile). Each shed vortex lifts the grains
into the upper portion of the vortex by the updraft. Entrainment of bottom sédiments is proportional to the shear acting at the sédiment water
interface. If the steady flow velocity is stronger than the oscillating flow
velocities, the equilibrium hole size and shape is expected to be greater
than the one developed from steady current alone. This is because of stagnation pressure of the square of the velocities at two different heights. The
implication is that, given an increased velocity due to both waves and currents, the secondary flow is stronger. Hence, the stronger the secondary
flow, the stronger is the vortex System. Rate of scouring is expedited due
to the superposition of waves on current. However, the size and pattern of
Coastal Engineering: Theory and Practice
vortex System due to combination of temporal agitation at the bed [Shen
et al. 1966].
7.2.4 Scour due to waves
In the case of scour developed by waves only, the mechanism is different
from that associated with steady flow. The steady current is thought of
having persisted over a great distance so that the boundary layer (région of
velocity gradient flow) is relatively thick. In contrast, the fluid particle travel
for an oscillatory flow is not great enough for a substantial boundary layer
height to develop. Therefore, the horseshoe vortex is smaller and weaker
than that produces by steady current flow. Sédiment scour in oscillatory
flow appears to be caused by the accélération of the primary flow past the
obstruction, as well as in the variation of the flow patterns in the wake.
Even when a wave induced scour hole fully developed the orbital current
boundary layer remains thin and does not contribute to the formation of
the horseshoe vortex.
7.2.5 Scour due to simultaneous action of waves
and current
In a wave-current boundary layer flow, as the wave action increases, the
mean current speed near the bed is progressively reduces whist the shear
stress is increased. Thus, it appears that the ratio of wave to current speed
as well as their magnitudes influences the scour development in wavecurrent flow. In the case of combined waves and current, the presence of
waves modifies the vortex structure considerably. The near bed flow consists of formation of vortex and its shedding on the lee side of the pile
(which is formed due to the interaction between the two shear layers emanating from the side edges of the pile). Each shed vortex lifts the grains
into the upper portion of the vortex by the updraft. Entrainment of bottom sédiments is proportional to the shear acting at the sédiment water
interface. If the steady flow velocity is stronger than the oscillating flow
velocities, the equilibrium hole size and shape is expected to be greater
than the one developed from steady current alone. This is because of stagnation pressure of the square of the velocities at two different heights. The
implication is that, given an increased velocity due to both waves and currents, the secondary flow is stronger. Hence, the stronger the secondary
flow, the stronger is the vortex System. Rate of scouring is expedited due
to the superposition of waves on current. However, the size and pattern of