ERDC/CHL CHETN-I-65
June 2002
Prior to the latest entrance channel improvements, Morro Bay Harbor was known as one of the
most dangerous in the United States with numerous injuries, deaths, and vessel damages
occurring due to steep and breaking wave conditions in the entrance. Entrance problems
experienced were due to a combination of exposure to storm wave conditions and bathymetry in
the entrance. A feasibility study (U.S. Army Engineer District, Los Angeles, 1991) considered a
wide array of navigation improvements. Since structural alternatives lacked economic
justification, channel modifications, which were expected to allow large waves to pass through
the entrance without breaking and steepening, were selected for the design (USAED, Los
Angeles, 1994). In December 1995, entrance channel improvements were completed that
consisted of construction of a deepened, expanded entrance channel. The authorized depth
increased from -4.9 m (-16 ft) to 9.1 m (-30 ft). However, the plan also provided for advanced
maintenance dredging to a depth of 12.2 m (-40 ft). In addition, a 9.1-m-deep (-30-ft-deep)
deposition basin was dredged north of the south breakwater.
PREDICTED DESIGN PERFORMANCE: Initial modeling of the Morro Bay project was
conducted using the ERDC numerical model HARBD (Harbor, Deep Water). Representative
incident wave conditions were input, and the model was used to determine wave action near the
entrance and inside the outer harbor for both existing and improved conditions (Kaihatu,
Lillycrop, and Thompson 1989). The HARBD model provided valuable information relative to
wave conditions in the Morro Bay entrance; however, it was noted the model had limitations. At
the conclusion of the numerical investigation, a physical model study was recommended to gain
an accurate prediction of absolute wave heights in the harbor entrance and broken wave
propagation through the proposed dredged channel configurations.
A 1:90-scale, three-dimensional hydraulic model of Morro Bay Harbor entrance was constructed
and tested at ERDC to investigate the design of proposed channel depth modifications to
improve navigation conditions and reduce maintenance dredging costs (Bottin 1993).
Representative wave conditions (unidirectional, spectral waves) from various directions as well
as steady-state ebb tidal currents were reproduced in the model. The impact that proposed depth
changes had on wave conditions in the entrance was addressed. Results indicated that the
initially proposed, deepened entrance channel was effective in reducing wave heights in the
entrance; however, wave heights at the head of the south breakwater were significantly
increased. The deepened entrance allowed more wave energy to reach the structure, as opposed
to breaking and losing energy as with the existing contours. After studying numerous
configurations, an optimum channel configuration was selected that resulted in improved
navigation conditions and had no negative impact on the existing structures. The configuration
recommended in the physical model investigation was the one constructed in the prototype in
December 1995.
PROTOTYPE DATA: Prototype wave gauges were deployed at Morro Bay on 11 September
1998 (Garcia 2001). They consisted of a directional gauge outside the harbor (designated
CA002), a nondirectional pressure gauge inside the harbor entrance (designated CA001), and a
nondirectional buoy in the exposed harbor entrance (Figure 2). The directional gauge, CA002,
was a short-baseline pressure gauge array deployed at a water depth of 14.3 m (47 ft). This depth
corresponded to that used for incident wave conditions in the physical and numerical models.
The gauge was bottom-mounted and data were collected for 2,048 sec every 2 hr and stored
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