Sediment discharged into the portion of the Southern California Bight extending from Santa Barbara to Dana Point enters a complex system of semi-isolated coastal cells, narrow continental shelves, submarine canyons, and offshore basins. On both the Santa Monica and San Pedro margins, 210 Pb accumulation rates decrease in an offshore direction (from ~0.5 g cm −2 yr −1 to 0.02 g cm −2 yr −1 ), in concert with a fining in sediment grain size (from 4.5φ to 8.5φ), suggesting that offshore transport of wave-resuspended material occurs as relatively dilute nepheloid layers and that hemiplegic sedimentation dominates the supply of sediment to the outer shelf, slope, and basins. Together, these areas are effectively sequestering up to 100% of the annual fluvial input. In contrast to the Santa Monica margin, which does not display evidence of mass wasting as an important process of sediment delivery and redistribution, the San Pedro margin does provide numerous examples of failures and mass wasting, suggesting that intraslope sediment redistribution may play a more important role there. Basin deposits in both areas exhibit evidence of turbidites tentatively associated with both major floods and earthquakes, sourced from either the Redondo Canyon (San Pedro Basin) or Dume Canyon (Santa Monica Basin). On the Palos Verdes shelf, sediment-accumulation rates decrease along and across the shelf away from the White's Point outfall, which has been a major source of contaminants to the shelf deposits. Accumulation rates prior to the construction of the outfall were ~0.2 g cm −2 yr −1 and increased 1.5–3.7 times during peak discharges from the outfall in 1971. The distal rate of accumulation has decreased by ~50%, from 0.63 g cm −2 yr −1 during the period 1971–1992 to 0.29 g cm −2 yr −1 during the period 1992–2003. The proximal rate of accumulation, however, has only decreased ~10%, from 0.83 g cm −2 yr −1 during the period 1971–1992 to 0.73 g cm −2 yr −1 during the period 1992–2003. Effluent-affected sediment layers on the Palos Verdes shelf can be identified in seabed profiles of naturally occurring 238 U, which is sequestered in reducing sediments. The Santa Clara River shelf, just north and west of the Santa Monica and San Pedro margins, is fine-grained and flood-dominated. Core profiles of excess 210 Pb from sites covering the extent of documented major flood deposition exhibit evidence of rapidly deposited sediment up to 25 cm thick. These beds are developing in an active depocenter in water depths of 30–50 m at an average rate of 0.72 g cm −2 yr −1 . Budget calculations for annual and 50-yr timescale sediment storage on this shelf shows that 20%–30% of the sediment discharge is retained on the shelf, leaving 70%–80% to be redistributed to the outer shelf, slope, Santa Barbara Basin, and Santa Monica Basin.

In the past decade, several large programs that monitor currents and transport patterns for periods from a few months to a few years were conducted by a consortium of university, federal, state, and municipal agencies in the central Southern California Bight, a heavily urbanized section of the coastal ocean off the west coast of the United States encompassing Santa Monica Bay, San Pedro Bay, and the Palos Verdes shelf. These programs were designed in part to determine how alongshelf and cross-shelf currents move sediments, pollutants, and suspended material through the region. Analysis of the data sets showed that the current patterns in this portion of the Bight have distinct changes in frequency and amplitude with location, in part because the topography of the shelf and upper slope varies rapidly over small spatial scales. However, because the mean, subtidal, and tidal-current patterns in any particular location were reasonably stable with time, one could determine a regional pattern for these current fields in the central Southern California Bight even though measurements at the various locations were obtained at different times. In particular, because the mean near-surface flows over the San Pedro and Palos Verdes shelves are divergent, near-surface waters from the upper slope tend to carry suspended material onto the shelf in the northwestern portion of San Pedro Bay. Water and suspended material are also carried off the shelf by the mean and subtidal flow fields in places where the orientation of the shelf break changes abruptly. The barotropic tidal currents in the central Southern California Bight flow primarily alongshore, but they have pronounced amplitude variations over relatively small changes in alongshelf location that are not totally predicted by numerical tidal models. Nonlinear internal tides and internal bores at tidal frequencies are oriented more across the shelf. They do not have a uniform transport direction, since they move fine sediment from the shelf to the slope in Santa Monica Bay, but carry suspended material from the mid-shelf to the beach in San Pedro Bay. It is clear that there are a large variety of processes that transport sediments and contaminants along and across the shelf in the central Southern California Bight. However, because these processes have a variety of frequencies and relatively small spatial scales, the dominant transport processes tend to be localized and have dissimilar characteristics even in adjacent regions of this small part of the coastal ocean.

Conventional bathymetry, sidescan-sonar and seismic-reflection data, and recent, multibeam surveys of large parts of the Southern California Borderland disclose the presence of numerous submarine landslides. Most of these features are fairly small, with lateral dimensions less than ~2 km. In areas where multibeam surveys are available, only two large landslide complexes were identified on the mainland slope— Goleta slide in Santa Barbara Channel and Palos Verdes debris avalanche on the San Pedro Escarpment south of Palos Verdes Peninsula. Both of these complexes indicate repeated recurrences of catastrophic slope failure. Recurrence intervals are not well constrained but appear to be in the range of 7500 years for the Goleta slide. The most recent major activity of the Palos Verdes debris avalanche occurred roughly 7500 years ago. A small failure deposit in Santa Barbara Channel, the Gaviota mudflow, was perhaps caused by an 1812 earthquake. Most landslides in this region are probably triggered by earthquakes, although the larger failures were likely conditioned by other factors, such as oversteepening, development of shelf-edge deltas, and high fluid pressures. If a subsequent future landslide were to occur in the area of these large landslide complexes, a tsunami would probably result. Runup distances of 10 m over a 30-km-long stretch of the Santa Barbara coastline are predicted for a recurrence of the Goleta slide, and a runup of 3 m over a comparable stretch of the Los Angeles coastline is modeled for the Palos Verdes debris avalanche.

Abstract Palos Verdes Peninsula contains large landslides in areas that are otherwise desirable for residential development. The landslides can be developed providing the factor of safety is at least 1.5 or can be raised to 1.5 during development. The greatest uncertainty in the calculated factor of safety is the residual shear strength of bentonite that forms the bases of slides. The use of too high a shear strength inflates the calculated factor of safety and can result in landslide failure after development. Tests by various investigators yield a wide range of residual shear strengths for bentonite samples from Palos Verdes Peninsula. Most residual friction angles (4φr) range between 6° and 14° with one lower value (3.5°) and several higher values reported. Residual cohesions (C r ) range mostly between 0 and 36 kPa (0 to 750 lb/ft2). Data for various bentonite samples from Palos Verdes Peninsula indicate they have similar compositions and Atterberg limits. Calcium montmorillonite is the principal clay mineral; the liquid limit is generally between 80% and 110% with the plasticity index between 40% and 70%. The data suggest that much of the reported variation in residual shear strength is the result of differences in sample preparation, testing methods, and interpretation of results rather than true differences in strength. Bulk samples of bentonite from the base of the Portuguese Bend landslide were remolded and tested by conventional direct shear, long sample direct shear, and ring shear devices to determine the effect of testing method on residual shear strength measurements. The three devices gave similar results—for conventional direct shear, φ r = 6.9° and Cr = 33.0 kPa (690 lb/ft2); for long sample shear, tyr = 6.8° and Cr = 23.8 kPa (497 lb/ft2); for ring shear, tyr = 6.7° and Cr = 7.2 kPa (150 lb/ft2). Back calculations indicate the ring shear results most closely simulate the residual strength along the base of the Portuguese Bend landslide. For all those devices, the residual shear envelope is a curve whose slope reaches an asymptote near a confining pressure of 200 kPa (approximately 4,000 lb/ft2). Reported φ r angles are commonly too high because samples were not tested at sufficient confining pressure to define the asymptote and a best fit straight line was drawn through the data points.

Abstract The Abalone Cove landslide, in southern California, is an 80 acre (32 ha) landslide within an ~870 acre (348 ha) ancient landslide complex. The landslide has developed in seaward-dipping marine strata of the middle Miocene Monterey Formation. The lower part of the landslide began moving by February 1974; the upper part did not appear to start moving until the spring of 1978. Since 1980, landslide movement has been con-trolled by the removal of ground water from the landslide mass. During years of nearly normal rainfall, subsurface inflow was the major source of ground water, contributing 55%. Percolation of rainfall and of delivered water were second and third, contributing 22% and 19%, respectively. During years of nearly twice normal rainfall, percolation of rainfall was the major source of ground water, contributing 56%. Subsurface inflow, percolation of delivered water, and surface inflow contributed 27%, 9%, and 8%, respectively. Prior to the installation of seven dewatering wells, the major loss of ground water was discharge to the surface by seeps at the toe of the landslide. The seeps accounted for 81% of the ground-water disposal. During the last two years of the study when the dewatering system was fully operational, surface seeps accounted for 34% of the ground-water disposal and pumping accounted for 54%. Other sources of ground-water disposal were evapotranspiration and subsurface outflow.