Section 9.0 Conclusions and Recommendations 1960050-03
9.0 CONCLUSIONS AND RECOMMENDATIONS
Conclusions
The Portuguese Bend Landslide is a re-activated segment of the ancient, Pleistocene-age Abalone
Cove Landslide complex and has been continuously moving since 1956. The currently active
landslide is divided into four distinct subslides. The west-central and headward subslides are
failing along one or more tuff beds above the Portuguese Tuff that have been altered to bentonite
clay. The east-central subslide and western half of the seaward subslide, however, are failing
within the ancient landslide debris.
The landslide has generally been moving. at nearly the same rate for the past 41 years. The initial
rate of movement of the re-activated landslide was up to 4 inches per day but, it soon slowed to
an average of 0.5 inches per day. This movement rate is fairly consistent throughout the current
landslide's history, except when heavy rainfall increased the rate of movement to 1.5 inches per
day. Following grading and dewatering of the landslide, which was completed in 1987, the rate
of movement further decreased to an average of 0.25 inches per day. Current movement of the
landslide averages approximately 0.25 inches per day.
• The continuing movement of the Portuguese Bend Landslide is the result of three main factors.
The high groundwater table, found within the active landslide, the weakness of the bentonite
clay, and the wave erosion of the toe are factors influencing movement of the Portuguese Bend
Landslide. The wave erosion of the landslide toe is continuously removing material which might
otherwise buttress the seaward subslide, allowing the landslide mass to continue moving
seaward.
Sediments derived from the active Portuguese Bend Landslide have been mapped within 1,000
feet of the shoreline. These sediments cover bedrock outcrops.on the seafloor with a layer up to 5
feet thick. •
The three shoreline borings drilled in 1996 encountered Portuguese Tuff at shallow depths. No
conclusive evidence was found to show that the basalt sill that crops out at Inspiration Point
extends eastward across the study area. However, the basalt sill is stratigraphically higher than
the borings. The borings did verify the continuation of shallowly dipping bedrock offshore, as
well as demonstrate that the active landslide toe is located near the present shoreline.
A volumetric study by Charles Abbott Associates calculated that approximately 5,834,000 cubic
yards of Portuguese Bend Landslide debris has been eroded into the ocean since the landslide's
re-activation in 1956. Without shoreline protection, erosion will continue.
eA-29 -
,eeurnuauneccnrrerrc toil'
1960050-03
40.
Recommendations
Based on our interpretation of available data and observations of the study area, shore protection
will be needed to prevent erosion of a gravity buttress placed at the shoreline to protect the toe of
the Portuguese Bend Landslide. Further investigation is needed for specific design criteria, and
to further refine the location for the revetment and the gravity buttress. We recommend drilling
borings offshore to evaluate the foundation materials and to determine the subsurface structure
beneath the proposed revetment.
•
•
•
•
szi
LEIGHTON AND ASSOCIATES,INC.