MLLW. The groin was intended to impound northward moving sediment before
it reached the navigation channel.
3. Construction of a 91-m extension to the south end of the detached breakwater
(labeled "BWS" on Figure 1) to provide improved wave protection for vessels
and dredge equipment in the navigation channel.
4. Deepening of portions of the navigation channel from a depth of -6 m to a new
depth of -12 m MLLW to provide sand storage volume for advanced maintenance.
Figure 1 shows the location and orientation of the improvements. The design of
the new structures were optimized using a 1:75-scale fixed-bed physical model (Bottin
1991). The model indicated wave heights would be reduced in the entrance chan-
nel by the detached breakwater extension. The effectiveness of the north jetty spur
and the south groin were examined using crushed coal tracer. Construction of the
improvements began in 1993 and was completed in August, 1994. Total cost of the
improvements was US.46 million.
Scour Concerns at Ventura Harbor
Scour Hole Formation
A few months after completion of the 1994 Ventura Harbor improvements, harsh
winter storms out of the west impacted the project. In January, 1995, on-site in-
spection revealed damage to the detached breakwater leeside armor layer originating
immediately across from the tip of the north jetty spur and extending southward. Ap-
proximately 46 lineal meters of the two-stone-thick armor layer experienced slumping
above and below the water line, and the breakwater crest in this region was signif-
icantly lowered. A hydrographic survey indicated a large scour hole had formed in
the gap between the north jetty spur and detached breakwater (see lower sketch of
Figure 4). The portion of the scour hole adjacent to the detached breakwater roughly
corresponded to the region of leeside armor damage. The scour hole had a maximum
depth of -9.5 m MLLW, whereas before the storm depths were close to the MLLW
elevation.
The scour was caused by the "jetting" action as water passed through the flow
constriction formed by the gap between the detached breakwater and north jetty
spur. Figure 2 shows the potential flow solution for an inviscid jet having the same
structure geometry as the gap region at Ventura Harbor.
The contours running across the streamlines represent lines of constant discharge,
and the contour labels correspond to values of dimensionless discharge per unit width
given by
qe
= Constant
(1)
(Q/a)
where qe is the discharge per unit width, Q is the total discharge, and a is the distance
between the breakwater and the north jetty spur. The fraction Q/a can be interpreted
as the "average discharge per unit width."
4
Hughes/Schwichtenberg
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