Figure 2: Potential flow map of jetting action through the gap.
As water flows through the gap the flow cross-section is constricted to almost
half of the gap width. This causes the flow to accelerate rapidly until the discharge
per unit width downstream of the gap reaches 1.7 times the "average discharge."
Although the jet flow map is instructive, it does not include the effects of turbulent
flow entrainment at the jet boundary that begins immediately downstream of the gap.
Flow entrainment reduces the flow velocities and spreads the discharge distribution
as the jet progresses downstream as illustrated in Figure 3. Nevertheless, estimates
from potential jet flow theory are reasonable in the region less than one or two "gap
widths" downstream of the gap.
It was concluded that the principal cause of damage to the detached breakwater
leeside armor layer was due to jet-induced scouring of the breakwater toe and subse-
quent slumping of the armor layer. The -6 m MLLW contour of the scour hole intruded
along the detached breakwater toe a distance of approximately 55 m (see Figure 4).
This undermined the breakwater toe which was constructed at -3.7 m MLLW. Wave
overtopping may have contributed to the breakwater leeside armor damage; but over-
topping can ruled out as the principal cause of damage because the actual damage
was confined to just one area, whereas overtopping damage would be expected to
occur in isolated pockets along the entire breakwater length. The north jetty spur
experienced no toe damage due to the scour hole because the toe was constructed at a
lower elevation (-4.2 to -5.2 m MLLW), and the jetty spur toe was lined with 1-tonne
stone left over from the original spur jetty construction.