flowing longshore currents accelerated through the narrow gap in a manner similar to
a jet. This resulted in high velocities that created a large scour hole over 9 m deep.
After an emergency repair to the detached breakwater and filling of the scour hole
with rock, there was concern that scour would continue to occur downstream of the
repaired scour hole. This concern was strengthened by a simple estimate of the flow
rate that caused the scour and by evaluation of the consequences of the same flow rate
passing over the rock sill.
A 1:25-scale laboratory movable-bed model of the Ventura Harbor scour region was
built to determine the probable extent of scour that could be expected, and to test and
refine the 1996 scour protection design to assure adequate protection from future scour.
The model was calibrated by adjusting the total flow discharge to achieve equilibrium
scour development in the model that reasonably matched the scour hole measured
at Ventura. Although the model similitude was attained via calibration, it appears
that the similarity criterion developed from the equation for maximum equilibrium
discharge would be appropriate in cases where validation data are not available. The
Ventura Harbor model study was completed successfully, and the following benefits
were realized as a direct result of the study:
The model study confirmed that scour would indeed undermine the detached
breakwater toe. Thus, the proposed toe reinforcement was shown to be neces-
sary, and the toe protection needed to extend over the entire toe.
The physical model was used to optimize the toe apron cross-section design and
to demonstrate the optimized design was stable with sufficient residual protec-
tion. The final design cross-section used 25 percent less stone than the 1996
design.
While working in the physical model it was realized that the original 1996 toe
protection design and the Plan 2 design were to be constructed at an elevation
that would hinder passage of the dredge through the gap between the detached
breakwater and the north jetty spur. The final design corrects this error, thus
avoiding potential costs related to grounding of the dredge and subsequent re-
building of the toe protection to correct the problem.
The 1996 toe protection design called for excavation that involved a risk of
leeside armor layer instability, whereas the final design is easier to construct
with less risk to the existing structure.
Shortly after completion of the toe protection in 1997, three large storms impacted
the project area. Visual field observations during these conditions support the exis-
tence of strong southerly flow through the gap. Estimates of surface current velocity
were made by timing floating objects as they passed between two points of known
separation distance. Flow velocity along the Ventura north jetty was estimated to be
1.5 m/s. Through the gap the current accelerated to values exceeding 2 m/s, which
corresponds well to the estimate made based on the equilibrium discharge formulation.
It was also noted that the currents "pulsated" in tune with the surf beat.
14
Hughes/Schwichtenberg
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