1.1 Geography 486/586 Splansky and Laris 2005

PALOS VERDES PENINSULA

I. HISTORY: DATES AND PLACE NAMES

In 1602 Sebastian Vizcaino, sailing northward from Acapulco, re-named the bay east of the peninsula San Pedro Bay (viewed on the Feast Day of Saint Peter). Earlier, it had been named the "Bay of Smokes" by Juan Cabrillo. Vizcaino also referred to and charted the existence of the Palos Verdes (green stalks) observed to the west of the bay.

In 1784, was granted to Juan Jose Dominguez. This change in land ownership status represented the first private land grant in Alta and it included the . Subsequent carving up of the Rancho San Pedro created one portion named Rancho de Los Palos Verdes which was acquired by the son of Juan Jose Sepulveda.

In 1792, George Vancouver, on his return voyage from the Canadian Pacific coast, met and liked Father Fermin Francisco de Lasuen while visiting the Carmel Mission. Vancouver also met and liked Friar Vicente Santa Maria while visiting the San Buenaventura Mission. In 1793, while rounding the Palos Verdes peninsula, he named and charted Points Vicente and Fermin after the two clerics.

II. PALOS VERDES: GENERAL TERMINOLOGY AND INFORMATION

The Palos Verdes Peninsula is a northwest trending dome-like ridge, 9 miles long and up to 5 miles wide. Its crest has gentling rolling topography at elevations ranging from 1480 to 1100 feet above sea level. Below this upland, remnants of a flight of marine terraces ring the peninsula and demonstrate it was an island during most of its geomorphic evolution.

A Simple Stratigraphy

1. Catalina Schist is metamorphic and the oldest rock on the peninsula (K/A dated to 120,000,000 BP). This rock was formed in a subduction zone and probably was not exposed to erosion until 15-16 M.Y.B.P. when it was unroofed in a submerged basin and range topographic province in the “ Basin Inner California Borderland.” Some of the Catalina Schist material was subsequently carried northward along fault lines that mostly parallel the San Andreas Fault Zone.

From late Miocene times (14.5 M.Y.B.P.), the schist formed an irregular, southwest sloping sea floor upon which sedimentary deposits were laid down and consolidated. Concurrently, igneous material was ejected to flow and harden atop the accumulating sedimentary deposits and intruded as sills into the consolidating sedimentary strata then forming atop the Catalina Schist. These sedimentary and both extrusive and intrusive igneous materials were to form the Monterey Formation.

The Catalina Schist thus forms the basic core bedrock and core of the peninsula site upon which the current widely seen sedimentary and igneous materials are deposited. On the Palos Verdes Peninsula today, the Catalina Schist bedrock remains buried beneath the Monterey Formation except for one site where erosion on the northeast slope of the peninsula has stripped away overlying sedimentary materials and exposed a small section of the surface of this core bedrock.

2. The Monterey Formation is the oldest and most common sedimentary unit overlying the Catalina Schist and ranges in age from 14.5 to 3.6 M.Y.B.P. The Monterey Formation was formed by sedimentary deposition into an offshore basin floored by the Catalina Schist, accompanied by the addition of extrusive and intrusive igneous material. 1.2 Subduction Diagram

Diagrams of cross sections of crustal extension

1.3

Fault and Geographic Map of onshore and offshore

1.4 Large Scale map of Lithotectonic Belts

1.5

Major Faults Geomorphic Provinces 1.6 Map of LA Basin Area displaying general fault distribution, place names, etc.

Strata of Palos Verdes Peninsula Diagram 1.7 Geologic Time Chart 1.8 During middle Miocene times (10-19 M.Y.B.P.), the area was an underwater basin that was filling with laminated diatomaceous (formed from diatoms*) mud and distal turbidites (formerly fine sediments in suspension not deposited by stream deposition) forming the base of the Monterey Formation. Subsidence of the basin kept pace with deposition and 1,000-4,900 feet of diatomaceous mud collected, while the water depth remained constant at 4,900-6,500 feet.

* A diatom is any of numerous microscopic, unicellular, marine or fresh-water algae having siliceous cell walls. Diatomaceous earth contains diatoms or their fossil remains.

The beginning of Pliocene time (5 M.Y.B.P.) was marked by the influx of increasing amounts of terrigenous (originating from land surface) sediment and increased rates of deposition. Through middle and late Pliocene times (3-2 M.Y.B.P.) the Palos Verdes Hills area evolved toward a shallower (3,900-4,900 feet) depositional environment. The shallowing is consistent with uplift of the peninsula which was probably underway by 3.0 .Y.B.P. The proto-Palos Verdes Hills already formed a submarine sill at least 600 feet above the surrounding sea floor. By early Pleistocene time (2 M.Y.B.P.), the sediments being deposited in the adjacent basins had coarsened and the rate of accumulation had increased to 1-2 mm per year, enough to compensate for subsidence and to produce complete filling of the by 1 M.Y.B.P.

The Monterey Formation may be divided into three members; 1) The Middle Miocene Altamira Shales, 2) the late middle to early upper Miocene Volmonte Diatomite, and the 3) late upper Miocene to early Pliocene Malaga Mudstone.

a) The Altamira Shales is the dominant exposed rock on the Peninsula, has an exposed thickness of 1,000 feet and contains another 1,000' concealed under the west end of the peninsula. The Altamira Shales were deposited on top of the Catalina Schist from 14.5 to 12.5 M.Y.B.P.

All but the upper 150' of Altamira Shales was deposited while local submarine vulcanism occurred. The igneous activity created sills, basalt flows and numerous areas of accumulated tuffaceous material (volcanically derived ash sometimes known as tuff). The basalt sills intruded into the Altamira Shales and are irregularly exposed (they can be seen at Royal Palms County Park Beach).

The Tuff has been weathered and altered and is often known as Bentonitic Tuff or Ash or Clay, or simply Bentonite. It is a distinctive unit as much as 75' thick in the lower portion of the exposed Altamira Shales (it could have been deposited from a single eruptive cycle from a submarine volcano or vent). A younger thinner bed of tuff (Miraleste Tuff) is at higher elevation.

Bentonite, like all clays, organizes its molecules into flat sheets. Water molecules can slip in between these sheets and act as ball bearings. The clay can absorb a vast quantity of water thus making the clay more slippery and allowing the clay to expand in volume as much as fifteen times. The potential for mass wasting is thus much enhanced as the Bentonite beds have a shearing strength of only one-fifth to one-tenth that of other materials within the bedrock.

b) The Volmonte Diatomite formation was deposited from 12.5 to 7.0 M.Y.B.P.

c) The Malaga Mudstone was deposited from 7.0 to 3.5 M.Y.B.P.

3. The Monterey Formation is partly overlain by the middle to upper Pliocene Repetto Formation dominated by the Lomita Marls and Sands. These are the youngest sedimentary materials formed on the peninsula, mostly along the north and east facing lowest elevations and were deposited between about 3.0 to 2.0 M.Y.B.P.

Sedimentation on this growing formation appears to have slowed or stopped between 1 and 2 million M.Y.B.P. as suggested by the absence of the Upper Pliocene and lower Pleistocene Pico Formation, which is rather thick (900- to 3,000 feet) and widespread elsewhere in the Los Angeles basin. Sedimentation resumed when the San Pedro Formation, an almost 600 feet thick sequence of fossiliferous silts, sands and gravels was deposited on the Miocene-Pliocene bedrock. This formation is younger than 700,000 Y.B.P. 1.9

III. UPLIFT AND FORMATION OF MARINE TERRACES

Faulting and Uplift

It has been over a century since A. C. Lawson in 1893 first recognized the importance of vertical tectonic movements in the Los Angeles Basin and the Southern California borderland. The Palos Verdes Peninsula appears to be a compressional structure resulting from vertical tectonics (uplift) along a reverse fault caught between two regional strike- slip faults. The anticlinal growth of the peninsula has long been associated with a northwest trending, southwest dipping, reverse fault. The Palos Verdes Hills are the expression of a doubly plunging anticlinorium. The emergent portion of the anticlinorium is somewhat like an iceberg as the visible part of the structure is only a fraction of its total size (9 x 5 miles and 1480’ high). Offshore stratigraphy suggests that the complete anticlinorium is at least twice as large and three times as high. Sustained anticlinal growth of the peninsula is reflected by the presence of the only exposure of basement rocks (Catalina Schist) throughout the western Los Angeles Basin, the progressive warp of originally flat-lying deposits, a sedimentary suite indicative of progressive shallowing, and the existence of a flight of emergent marine terraces..

Uplift of the Palos Verdes anticline began about 3 M.Y.B.P and has continued to the present. The end of the Pliocene has long been recognized on stratigraphic grounds to coincide with a strong compressional episode along coastal California. Recent plate tectonic reconstructions have related the onset of this compression to two distinct changes in Pacific plate motion at 2.48 and 3.4 M.Y.B.P. The 3. 0 M.Y.B.P. date is consistent with these reorientation episodes and is compatible with paleobathymetric (ancient sea floor measurement) evidence from mid and upper Pliocene sedimentary deposits, which confirm that uplift of the Palos Verdes anticline was underway by 2.5 M.Y.B.P.

At initiation of uplift (about 3 M.Y.B.P.), the ocean depth in the Palos Verdes Hills area was about 2,760 feet. By 2 M.Y.B.P. the steepening sides of the rising bank (the proto-uplifted Palos Verdes Hills) and the deepening Los Angeles Basin intercepted terrigenous sediment coming from the east, preventing its deposition on top of the protopeninsula. During 1-2 M.Y.B.P. the mainland shoreline migrated westward, and sediment deposition increased to 1-2 mm per year, more than compensating for fault-induced uplift or subsidence. By 1 M.Y.B.P. the Los Angeles basin was filled and the bank/basin topography had leveled enough for upper Pleistocene deposits to reach and lap onto the anticline flanks and later with more uplift, connect the island as a peninsula to the mainland.

The uplift of the sea floor to form the Palos Verdes Peninsula has occurred along the Palos Verdes Fault. The uplift has been accompanied by considerable local folding, faulting, warping and near-overturning of Monterey Formation materials. Results of a 1996 study that employed use of radio carbon dating of samples from borings of the ocean floor along the fault, indicate that the 80 mile long Palos Verdes Fault, comes on shore in Redondo Beach, cuts southeast across the northern foot of the Peninsula, and extends past Wilmington off shore into the harbor and ocean beyond. It remains one of the most active faults in the Los Angeles Basin.

The Peninsula side of the fault is moving up-slope (as a reverse fault movement), along a southwestward dipping fault plane, and northwest as a right lateral strike slip fault into the at a rate of 3.0 to 3.7 millimeters per year (movement along the nearby Newport-Inglewood Fault is about one to two tenths of a millimeter per year). Since initiation of uplift in the form of a reverse fault, the topography at the crest of the anticline has been raised at least 200 feet at an average rate of 0.43 mm per year.

Researchers have concluded that the Palos Verdes Fault on land dips at a steep angle of 67o to the southwest and that the fault surface is 19 km in length and extends from 6.0 to 12.4 km in depth. The proportion of slip is 56% right lateral and 44% reverse. Seismologists estimate that the Palos Verdes Fault could be the site of a 7.0 to 7.2 magnitude earthquake sometime in the next 400 to 900 years.

1.10 Reconstruction of anticlinal growth and marine terrace formation Uplift Diagram and Map of predicted uplift rates. 1.11 MARINE TERRACES

Waves as Erosive Agents in the First Stage of Marine Terrace Formation

Waves operate as agents of erosion in several ways. The actual hydraulic (water or other liquids in motion) action of a mass of water can have a direct shattering effect as it pounds the rock with physical force and breaks apart constituent pieces of the rock face. The erosive force increases as the velocity of the moving water and the total weight (mass) of the water increases. Air contained in joints and fissures in the exposed rock is compressed by the forcefully intruding water until its pressure is equivalent to that exerted by the wave so that the resultant expansion, when the wave retreats, may have virtually an explosive effect. When this process is repeated constantly, the joints and fissures are enlarged as pieces of rock are broken off and removed.

Even more potent is the abrasive action of the mass of fragments (sediment load of the waves), ranging from sand to boulders, which are carried in suspension and are pounded by the waves against the rock at the foot of the coastal bluff or cliff. This wave borne sediment acts as cutting and chiseling tools to abrade and chip away at the rock surface; the abrasion process dislodges rock particles while the sediment grinds and smoothes the rock surface of the bluff as well as talus and other fragments along the shore. Erosive dynamics are at their peak during stormy weather, rough seas and high tides.

Wave-Cut Notches, Wave-Cut Platforms and Offshore Terraces

The forces of hydraulic action, air pressure expansion and abrasion at the foot of the cliff has the effect of undercutting the face of the bluff and creating a wave-cut notch, the recessed, eroded base of the cliff. As more rock material is eroded away and the notch is enlarged, insufficient rock material may remain to support the overlying rock mass. The force of gravity, perhaps aided by rain wash, may cause the overhanging rock material to collapse and fall to the shore in broken fragments called talus.

The shoreline thus retreats along the plane of cliff collapse and wave cut notch, and the talus material becomes subject to movement by tide, eddy motion, and the in-and-out swash and backwash of wave movement. The talus fragments are now rolled or carried in and out by the wave swash and backwash between high and low tide levels, and are themselves subjected to abrasion. The pieces become smaller, smoother and more rounded. Some of the much abraded talus accumulates at the ocean-edge of the wave-cut platform and as veneer atop the wave-cut platform to form an off- shore wave-built terrace, a depositional feature. Some of the talus pieces become added to the sediment load of waves and add to their abrasive force.

Forming a Wave-Cut Platform (also known as an Abrasion Platform or Wave-Cut Bench or Wave-Cut Terrace)

Wave-cut platforms are erosional features created along some coasts at sea level and their formation can be understood in terms of stage, structure, and process. At the initial stage of their development, a smoothly sloping land surface is exposed to wave erosion at the shoreline. Wave action begins to cut a notch in the slope at the shoreline and as overhanging rock material collapses and falls onto the base of the notch, the plane of the shoreline has receded and talus covers the base of the notch. As the talus is reduced in size and moved away by wave and tidal action, the wave-cut platform below is exposed. Wave action is renewed at the shoreline where a new wave-cut notch is carved which will ultimately undercut the new rock face of the bluff above. When the bluff collapses, the plane of the shoreline recedes again while the surface width of the wave-cut platform at sea level is enlarged (though partially covered with a newly created veneer of talus from the cliff fall). As time passes, gradually a wave-cut platform grows while the cliff (shoreline) recedes.

Offshore Terrace (Wave-Built Terrace)

Although some of the talus on the beach is reduced to tiny fragments and moved away by the long-shore current, much of the talus is deposited at the ocean-edge of the wave-cut platform and as a veneer on portions of the wave-cut platform itself. As this material accumulates it may form an off-shore, wave-built terrace. Thus the surface of the wave- 1.12 cut platform may also display an oceanward extension composed of accumulated talus deposits that have been well- worked, reduced in size and smoothed by the abrasion caused by wave action. 1.13 4 Stage diagram of Stages in the Development of a Shore-Profile

Rate of Erosion and Formation of Wave Cut Platforms

The rates at which the wave-cut notch is etched inland, the overhanging cliff responds to gravity and collapses and the recession of the plane of the shoreline are dependent on multiple factors including the intensity of wave erosion, and the nature of the coastal rocks, their homogeneity, their stratification, the dip angle of the strata, the extent of jointing and fissuring, and the rock’s resistance to erosion. As long as the interface between land and sea remains stable without uplift or lowering of sea level, the marine erosion processes can continue to shape the already evolving wave-cut and the coastal bluff and shoreline will continue to recede while the wave-cut platform continues to increase in width.

Emergent Marine Terraces

Tectonic uplift or eustatic change that results in a lowering of sea level may lead to a new location for the interface of land and sea that is below the position of the former wave-cut platform. If a new shoreline level is established and the new position of sea level remains stable/unchanged for a sufficient period of geologic time, a new wave-cut notch and 1.14 platform may be carved and a new offshore terrace formed. Concurrently, the former wave-cut platform and offshore terrace, now elevated to a position above sea level, appears as an emergent marine terrace. The toe or seaward edge of the raised marine terrace marks the newly created plane of the shoreline and overlooks the newly emerged bluff or slope that is subject to new undercutting from new wave-cut notches being created at the newly established sea level.

If stability of sea level is extended for a sufficient period of geologic time, a newly created wave-cut platform and offshore terrace is fashioned at the newly established sea level. As the bluff recedes through steady erosion at the base of the bluff and collapse of the overhanging rock of the cliff, marine terrace #2 (the former wave-cut platform and adjoining offshore terrace now raised to a level some distance above current sea level), is reduced in width while marine terrace #1 is being fashioned at sea level in the form of the wave-cut platform and adjacent offshore terrace which is steadily being increased in width.

The emergent, upraised, marine terrace is likely to steadily lose its flattish profile as terrestrial erosion and deposition processes occurring at higher elevations lead to the accumulation of talus and sediment on the new marine terrace #2 (the former, now emergent, wave-cut platform/offshore terrace). These deposits mask the formerly flat wave-cut platform profile and may create a more steeply sloping gradient on all or portions of the emergent, upraised, and formerly flat wave cut and wave built marine terrace.

The tectonic and/or eustatic processes may continue once again and if a new tectonic or eustatic stability occurs, still another wave-cut platform/offshore terrace may form at the new sea level to become a new marine terrace #1. What had been the prior terrace #1 in formation is now elevated to become emergent marine terrace #2 while the former marine terrace #2 becomes emergent marine terrace #3 at a still higher elevation. The Palos Verdes Peninsula displays at least thirteen recognizable marine terraces. The highest and oldest, #12, is near the summit while #1 is currently being formed at sea level.

Palos Verdes Peninsula’s Marine Terraces

The marine terraces of the Palos Verdes Hills comprise the most extensive sequence of elevated former shorelines in California. The combination of tectonic and eustatic forces (changing sea levels as continental glaciers expanded and receded) facilitated the formation of wave-cut marine platforms around the emergent anticline that was first an island and subsequently would become the Palos Verdes Peninsula when it was later connected to the Los Angeles lowlands. Thirteen terraces have been identified between present sea level where terrace number one is now forming and almost 1480 feet above sea level. Additional terraces are thought to have been carved during Pleistocene periods of lower sea level, corresponding to times of major glacial advance, and their remnants now exist below sea level. The oldest (highest) terrace was formed around 1.0 M.Y.B.P. Terraces above and positioned inland toward the peninsula center from the 5th terrace, appear to extend around the peninsula in a bathtub ring configuration, whereas the younger, lower terraces exist only on the ocean-facing sides of the hills. This arrangement suggests that a few hundred thousand years ago, the Palos Verdes Peninsula was an island and that alluvium deposited from the Los Angeles and San Gabriel Rivers has filled the channel between the island and the mainland in recent geological time.

Researchers disagree on the dating for the formation of the different marine terraces on the Palos Verdes Peninsula. When attempting to reconstruct the history of terrace formation, multiple factors must be taken into account including the style of faulting, rate of uplift, former positions of sea level and location, dating and rate of folding and warping of strata. Suggested dates for the formation of some of the terraces are identified as follows:

12th Terrace (highest) 668,000 to 927,000 B.P. 10th Terrace 658,000 to 796,000 B.P. 9th Terrace 603,000 to 706,000 B.P. 7th Terrace 493,000 to 544,000 B.P. 6th Terrace 430,000 to 466,000 B.P. 5th Terrace 330,000 B.P. 4th Terrace 237,000 to 255,000 B.P. 1st Terrace 94,000 B.P.

1.15 Five stage diagram of Post Capistrano Time - Development of terraces and sea gullies

1.16 2 Diagrams: Beach drifting and features of an emergent marine terrace. of Terrace Development 1.17 2 maps of terraces 1.18

PALOS VERDES PENINSULA - MASS WASTING

I. TERMINOLOGY AND INFORMATION

A. Mass Wasting consists of downslope movements occurring under the pull of gravity. The movement can be slow or rapid, falls or plastic flows, and involve loose small particles or large blocks of earth and rock.

1. Although the term landslide is often used somewhat loosely to mean any fairly rapid movement of rocks and sediment down slope, it is more accurate to use the term mass wasting to refer to the wide variety of mass movement processes that wear away at the Earth’s surface.

2.A Slump Block may be defined as a large mass of bedrock or overburden that slides downward at the same time rotating backward on a horizontal axis.

3. A Rock Slide occurs when bedrock mass slips on a relatively flat inclined plane such as a bedding plane.

4. A Slide (Slip) Plane is the plane along which movement occurs.

5. Slide Plane Overload is more weight than can be supported at the point of contact with the slide plane.

6. Shearing Strength is the pressure in pounds-per-square-foot to be supported. Shearing strength is often reduced by an increase in the volume of water in the bedrock or unconsolidated material.

7. Factor of Safety or FOS (against sliding) is an attempt to quantify the potential for movement of a slope. In a

landslide zone, it is the ratio of forces tending to resist sliding at the lower end of a slide zone divided by the forces tending to cause the sliding (the driving forces along the up slope edge of the slide).

An FOS of 1.00 indicates that the two forces are balanced and the material is stable. A moving landslide has a factor of safety of less than 1.00 and a stable landslide has a factor of safety of more than 1.00 (an FOS of 1.5 indicates that the material has 50% more resistive force than driving force). The actual value of the factor of safety varies seasonally depending upon soil moisture and ground water conditions. The FOS is computed using variable factors such as shear values, slope angles, specific gravities, pore pressure, degree of compaction and water content of the materials.

B. Toe Erosion is the undercutting and removal of material from the base of a steep slope by wave action. Toe erosion results in reduction of the base of support for overlying and upslope material and contributes to the mass wasting process that is active at and Point Fermin. 1.19 GEOGRAPHY 486 SPLANSKY

II. POINT FERMIN SLIDE ZONE

A. A classic slump block slide occurred at Point Fermin in 1929 with activation of the easterly portion again in 1940- 1941, 1950 and with continuous movement since 1978.

B. The slide started as a simple block slide when a large mass of Altamira Shale (a member of the Monterey Formation) moved rapidly downward. The eastern portion was broken into numerous sub-blocks, some of which rotated backward on a horizontal axis as they moved seaward. Some of the sub-blocks have experienced as much as 45 degrees of backward rotation. The slide mass covers an area 1000 feet long along the cliff, reaches as much as 400 feet inland and extends over approximately 10.5 acres.

1. The sea cliff face exposes various components of the Altamira Shale member of the Monterey Formation. Siltstones, sandstones, and bentonite dominate the 200 feet of exposed strata of the Altamira Shale at Point Fermin. The 200 feet of strata at Point Fermin represent about 5 million years of sedimentation.

2. The slide movements were first noticed in January 1929. Eighteen months later it was found that the middle portion of the sliding block had moved seaward 7.6 feet, the western portion had moved seaward 8.1 feet and the eastern portion had moved seaward 7.5 feet. Although the original outward (seaward) movement was substantial, the extent of the downward movement ranged only from less than a foot to about 2.5 feet.

3. The slide movement caused water and gas lines to break, street improvements to crack and/or break apart, and residences and garages to become dislocated from their original placements.

4. A major cause of the slide was wave erosion (toe erosion) which undercut a thin seaward-dipping bentonite bed in the Altamira Shale. The bentonite bed dips 12 to 14 degrees southwest at an oblique angle to the coast. When wave erosion has removed debris, the basal slip surface of the bentonite bed can be seen along the easterly edge of the slide where it cuts diagonally across the face of the cliff. Ground water from natural and artificial sources seems to have contributed to a reduction of the shearing strength of the bentonite bed.

5. The western part of the slide has experienced little activity because the bentonite bed at the western base of the slide is many feet below the base of the sea cliff and is overlain by sandstone beds. The sandstone beds are resistant to development of shear surfaces across them and therefore have buckled upward along the beach at the southwest corner of the slide in order to permit the western part of the slide to move.

6. Although bedding planes (the strata) have an average seaward dip of about 15 degrees in the Point Fermin area, the Altamira shale and sandstone beds have sufficient shearing strength to support the nearly vertical sea cliff.

7. From 1929 onward, investigators have identified the presence of moisture in the rocks, as shown by a belt of dampness and water seepage along the lower portion of the sea cliff in front of the disturbed area. Since about 1950, the eastern half of the slide has experienced frequent reactivation as wave erosion (toe erosion) continues to remove the toe of the slide/cliff. Since the heavy rains of 1978, the eastern portion of the slide appears to have undergone nearly continuous movement.

C. It has been inferred that the faulting and broken character of the ocean bluff in front of the Point Fermin slide suggests that some movement took place before the area was inhabited and consequently that the movement recorded in cracked pavements and displaced foundations is merely a resumption of an older movement.

1.20 Map Point Fermin Slide Zone Transect Lines 1.21 Diagram of Geologic Cross Section a - A’ Point Fermin

Point Fermin Boring Data B-1

1.22 III. RANCHO PALOS VERDES’ MAJOR RECENT SLIDE ZONES

THE PORTUGUESE BEND SLIDE ZONE: ACTIVATED IN 1956 THE OCEAN TRAILS SLIDE ZONE: ACTIVATED IN JUNE 1999

THE PORTUGUESE BEND SLIDE ZONE

A. In the Portuguese Bend region of the City of Rancho Palos Verdes is the site of ancient (Pleistocene) landslide activity (125,000-100,000 BP). The main body of the landslide probably remained inactive during the period between 100,000 and 10,000 years ago. When sea level rose to nearly its present level about 10,000 years ago, wave erosion began to remove support from the toe (downhill edge) of the landslide. The toe erosion caused prehistoric reactivation of the seaward part of the Pleistocene landslide over a two square mile area. Prior to historic time, the Abalone Cove, Portuguese Bend and Klondike Canyon landslides experienced one or more episodes of major movement within the last 10,000 years. The currently active Portuguese Bend slide zone extends over about one square mile, became activated in 1956 and currently is thought to be the largest active slide zone in a developed urban area within the U.S.A.

B. Early real estate development and home construction generated increased ground water volumes. Few homes were connected to a sewerage system and septic tanks were the primary receptacles for domestic waste water. By 1954, 250 gallons of water per home per day were being added to the ground water volume from septic tank overflows, lawn watering, car and driveway washing and other sources generated by local residents. Approximately 32,000 gallons of water per day in total, the equivalent of 11 inches of precipitation (about double the normal (natural) recharge per year), was being added by 1954.

C. Crenshaw Boulevard construction led to the addition of 132,000 cubic yards of earth fill (weighing about 200,000 tons) to an upper portion of the slide zone.

D. In 1956, landslide activity was re-activated. Winter rains, combined with buildup of artificially deposited water affected the Bentonitic Clay material. Toe erosion, a slide plane overload and a less stable and water swollen Bentonitic Clay substrate seemed to have generated a renewed mass wasting process.

1. The slide plane follows a complex curvilinear scheme and is encountered at different depths in different portions of the slide zone. In most of the uphill part of the slide complex, the slide plane (base) is directly above the Portuguese Tuff (about sixty feet thick consisting of compacted volcanic ash altered to Bentonite). In the downhill part of the slide complex, the depth of the slide plane varies within the Portuguese Tuff strata and is as much as 70 feet below the tuff in the Klondike Canyon slide zone. Although the basal slip surface shifts from higher to lower bentonite beds from the landward to the seaward part of the landslide complex, the slide base is a continuous surface beneath the entire slide zone. The slide plane also extends beyond the shoreline beneath the sea floor.

2. Monitoring data prove that rainfall is the greatest source of ground water recharge and that it takes about one year for groundwater to flow from the crest of the peninsula to the Abalone Cove landslide and much less time to reach the upper portion of the Portuguese Bend slide. Slide problems become acute when the region receives substantial precipitation in two consecutive years.

3. The Portuguese Bend slide zone may be divided into five subslides, each with its own rate of movement and dynamics. Maximum overall lateral displacement to date has been over 800 feet.

4. From 1960 to 1978 the rate of movement was relatively steady and averaged 6 to 8 inches per month in most areas. The rate accelerated to approximately 2 inches per day following the rainy winters of 1977-80 and 1982-83. The current rate of movement varies in each of the subslide areas.

1.23 Splansky Map Portuguese Bend Slide Area

Portuguese Bend Cross section diagrams of slide

1.24 Map of Port. Bend subslides

5. The instability in the area north of Palos Verdes Drive South (PVDS) was primarily derived from the large driving force of the land mass moving seaward along the slide plane, the presence of a high water table, and continuing erosion at the toe of the landslide.

6. In addition to the infiltration from the heavy precipitation, additional water was added to the ground water via over-flowing septic tanks from homes in the slide area. No sewer system yet serves the Abalone Cove community in the slide zone and approximately 250 gallons per day, per home was and continues to be added to the ground water charge from local septic tanks.

7. Landsliding had re-directed surface drainage so that runoff, particularly from PVDS, flowed into fissures within the active landslide rather than directly to the ocean. This, in turn, elevated the water table, provided buoyancy of the landslide mass and lubricated the slide plane, thereby contributing to the continuous movement.

8. Additional slide activity occurred at Klondike Canyon along the eastern edge of the older Portuguese Bend slide zone. Coastal areas of the canyon began moving in 1979. Subsequent study suggested that the Abalone Cove and Klondike Canyon slides were both related to the earlier movement of the more central portion of the Portuguese Bend slide zone. Both the Abalone Cove and Klondike Canyon slides rotated outward away from the central and less supportive slide mass of the Portuguese Bend.

1.25 9. In 1980, an inland area above Klondike Canyon in the city of Rolling Hills, known as the Flying Triangle, began to slide. Although movement was slight at first, three homes were destroyed in late 1984 and others were threatened. All existing evidence indicates there is no connection between the Flying Triangle slide and the Klondike Canyon slide as there is a sub-surface geologic barrier that separates the two slides.

10. Landslide movement coastward of PVDS increased following the winter of 1991/1992. Until the winter storms of 1993/1994, the accelerated movement occurred primarily because of the increased toe erosion that resulted from the loss of protective gabions (destroyed by storms) that had been installed at the toe of the slide in 1988-91. By the summer of 1993, the east central portion of the Portuguese Bend slide was moving at a rate of approximately 2.5 feet per year (as much as 6 to 7 feet per year in certain months). The most rapid movement occurred immediately after the 1992/1993 winter storms.

The winter storms of 1993/1994 and the very heavy winter storms of 1994/1995 (almost 30 inches of rain) and the winter storms of 1995/1996 through 1999/2000 have continued to foster accelerated movement in the central and upper portion of the slide zone. From August 1994 through August 1996, horizontal displacement was as great as 16.9 feet in one portion of the slide zone. The average horizontal displacement, as measured at 27 measurement sites in the slide zone, was 2.5 feet during the August 1994-August 1996 period.

11. Portions of the Portuguese Bend landslide zone have moved well over 800 feet from their 1956 locations and about one square mile (approximately 1/2 of the ancient landslide area) has been affected.

E. Landsliding action has caused extensive damage to developed property and local terrestrial and marine ecosystems in the slide zone.

1. Between 1956 and 1958, 150 homes were damaged or destroyed. Following ensuing lawsuits filed by local property owners, Los Angeles County was ordered in 1965 to pay $5,450,000 in damages.

2. The Abalone Cove section of the greater Portuguese Bend zone of mass movement was discovered to be active in the summer of 1978. Extending over approximately 80 acres, the slide movement was exacerbated and accelerated during the wet winter of 1978/79 and was reported to be moving at an average rate of five feet per year at Palos Verdes Drive South. Twenty-five homes were damaged with property losses estimated to be about $1,000,000.

3. At the seaward foot of the Portuguese Bend and Abalone Cove slides, wave cut platforms and long existing tidepool settings became covered with slide material. The local sea water turbidity increased dramatically and there were notable changes in the local marine .

F. Efforts to restrain, slow, and halt the mass wasting process in the Portuguese Bend Slide Zone.

1. From 1957-59, twenty-three 4' diameter concrete and steel caissons, (each 20 feet long), were vertically lowered into holes drilled through the slide base in order to "pin" the slide in an attempt to arrest movement. The effort was useless as they were sheared off or plucked out in a matter of months and now appear at the surf 400 feet from their original placement.

2. Starting in the late 1950s, water, sewerage, and natural gas pipelines were re-located on the surface and designed for expansion as the mass movement occurred. Palos Verdes Drive South has been continuously resurfaced and repositioned. Older segments of this road can be seen coastward of its present location.

3. Engineers and geologists, in the late 1970s and early 1980s, proposed a series of actions to stabilize the Portuguese Bend slide zone. In 1984, these ideas were consolidated with the development of a stabilization plan that included the following major elements:

a. Minimize or eliminate the toe erosion process occurring at the base of the cliffs (by employing some form of sea wall or other artificial barrier to reduce the wave impact).

1.26 b. Install de-watering wells that would pump up the ground water from the slide mass and thus reduce the water volume in the substrate. Experimental dewatering wells were installed and activated at Abalone Cove in 1980- 1984. An ideal, but impractical, solution would be to lower the water table to a level below the slide plane and thus lower the water content of the Bentonitic Clay (although it is extremely difficult to remove water from Bentonite).

c. Construct surface drainage channels (culverts and pipes) that would intercept the rainfall runoff before it is lost through fissures and percolation into the ground water.

d. Grade various portions of the slide area to reduce the angle of slope, fill in fissures and depressions, and direct the runoff where intended.

e. Reposition the location of PVDS (site and elevation) and capture all of the highway's runoff to be directed by pipe directly to the ocean.

f. Relocate masses of slide material from the upper and steeper slopes (thus reducing the driving forces) and reposition the removed material to downslope locations where the added weight would increase stability and strengthen the resisting forces to movement from upslope locations.

g. Construct a sewer system to service the residences in the slide area. By thus eliminating septic tank use, less water will enter the slide mass as ground water. Septic tank use by residents in the Abalone Cove slide zone (1991) yields 10 million gallons of water per year into the slide material. The city commissioned studies and began in summer 1993 to consider various sewer designs that would serve the Abalone Cove and Portuguese Bend areas of the slide zone. A design was chosen and an Environmental Impact Report was scheduled for certification by the city in August 1996.

The process to obtain easements from property owners was underway in early 1996 and although most of the necessary easements were obtained by mid-1997, the sewer system project remained on hold in mid 1999 pending negotiations with Los Angeles County regarding the plans for repayment of the bonds that must be sold to pay for the construction of the proposed sewer system. The sewer system plan will target most of the slide area but only the Abalone Cove region will actually receive an installed sewer system at this time. If and when a sewer system is installed and operational, it is likely that the formation of a sewer maintenance assessment district will be required to fund the cost of maintenance of the installed sewerage system; as of mid 1999, there was uncertainty and even serious doubt if such an assessment district is likely a to be approved.

G. Since 1978, various landslide mitigation measures have been implemented.

1. In 1978 the city of Rancho Palos Verdes passed the Landslide Moratorium Ordinance (nothing new could be built in the Landslide Ordinance area). The Landslide Ordinance Area was to become the basis for the future creation of a Redevelopment Agency area. By 1991, the Landslide Moratorium Ordinance had become a major and controversial planning issue because of proposals to ease the rules for development within the landslide area.

The city council established the Rancho Palos Verdes Redevelopment Agency Board (RDA) which divided the moratorium area into eight sub-zones. The zone boundaries and rules for development, are in principle, based on the varying degrees of geologic stability within the landslide area. A proposal for the RDA to pursue possible development in only one of the moratorium zones (Zone #2 consisting of 130 acres located north of the existing Abalone Cove settlement) is in the consideration stage. As of August 1997, an EIR for this project was being prepared and a peer review panel was waiting to review and debate the recommendation of an outside expert regarding the geological feasibility of the proposal

2. The city of Rancho Palos Verdes successfully persuaded the California legislature to approve the "Geologic Hazard and Abatement District" enabling legislation in 1980.

1.27 3. The Abalone Cove Landslide Abatement District (ACLAD) and the Klondike Canyon Geologic Hazard Abatement District (KCGAD) were formed in 1981 and 1982 respectively. These districts were formed by the affected property owners to undertake mitigating measures designed to stabilize the slides.

Over $1,500,000 has been spent to abate the two slides (mostly de-watering well projects). By 1988 these efforts had substantially reduced the two slides' rate of movement and by 1991, the slides seemed to have been stabilized and movement arrested. By late 2002, a total of seventeen active de-watering wells and added drainage facilities in the ACLAD had lowered the water table by 25 feet in the Abalone Cove slide zone. The seventeen de- watering wells produce about 220,000 gallons per day; 85% of the water is obtained from the upper part of the slide zone.

4. In 1984 the city of Rancho Palos Verdes formed a Community Redevelopment Agency (CRA) to provide the political and tax control powers to finance the proposed solution and mitigation measures and eliminate the "blight" associated with the slide. The city's Department of Public Works is the lead agency in the planning and implementation of proposed solutions. The city's Planning Department is regularly consulted and included in the planning and environmental considerations process. The CRA established as its goal to reduce slide movement to less than three feet per year (from the then 30 - 40 feet per year).

CRA's were originally conceived of as strategies for eliminating blight and revitalizing inner city neighborhoods. The landslide area, however, has been identified as meeting the criteria for being in an existing deteriorated condition because: a. a substantial number of land parcels exhibited a lack of productive use. b. a significant number of residences (40) were actively threatened by the slide. c. public access to recreation areas was hindered. d. marine life was destroyed.

5. Elsewhere in the Portuguese Bend Slide Zone, de-watering wells were introduced in the early 1980s and by 1982 had removed 75,000,000 gallons of water. Additional de-watering wells were installed in the mid-1980s and by 1991 movement in most of the subslide sections had been slowed to less than one foot per year. Eleven additional dewatering or monitoring wells were added in the 1990s. As of late 2002, a total of twenty-two dewatering wells were operational in the entire slide zone.

6. In 1987-88, major earth moving projects were undertaken in the slide zone. In addition to grading the surface, the most dramatic project involved the movement of 600,000 cubic yards of earth from the top end of the landslide to the bottom (creating a toe berm). This helps to slow the slide by taking weight off the top where the driving forces are most concentrated and putting it on the bottom where the added weight increases the resistance forces. Although additional significant earth moving projects were proposed in the early 1990’s, they were not approved or funded and no new projects are currently contemplated.

7. Surface drainage has been improved by filling fissures, adding surface runoff channels, and completely re- positioning PVDS and ensuring capture of runoff from its surface for direct transport to the ocean.

8. Gabions. An innovative temporary solution to the problem of toe erosion, employing gabions, was attempted in 1988-89 and again in the spring of 1991. Gabions were constructed above the high tide line along portions of the base of the slide in order to reduce the erosion effects of wave action. Gabions are large wire cages filled with rocks. In the first phase, the gabions were installed so as to form a stepped barrier along the western half of the Portuguese Bend shoreline area.

a. The first phase of gabion development and the protection against toe erosion of the slide base that the gabions were intended to provide was perceived to be quite effective. Relatively little slide material was eroded away and there was a dramatic reduction in the offshore water turbidity and a notable improvement in the off-shore habitat for marine life.

1.28 b. The gabions experiment was considered to be so successful that additional funds were allocated in 1990-91 to extend the length of the gabion wall eastward, and thus increase the extent of protection. This project was completed in the late spring of 1991.

c. As the eastern gabion extension was being constructed, serious storm\wave damage in March/April 1991 destroyed a portion of the original gabion wall. Toe erosion on a large scale was renewed at the site of the destroyed gabions and the face of the slide was eroded headward by more than 40 feet by the middle of August 1991.

d. The gabions took a heavy beating in the winter of 1991/1992 and were almost completely destroyed by storms in the winter of 1992/1993. Following the damaging winter storms of 1991/1992, no funds were appropriated for the replacement or repair of the lost or battered gabions. The Coastal Commission permit to construct the gabions, issued in 1987 to the city of Rancho Palos Verdes, expired in Spring 1992. Although in October 1992, the city re-submitted an application to reconstruct the gabions, funding for this proposal never materialized and the Coastal Commission declined to approve a permit. Current thinking suggests that the gabions idea was ill founded. Storms in subsequent winters have completely destroyed remnant gabions.

9. Sea Wall/Jetty. Efforts were underway in 1989 to find the funding for construction of a sea wall, toe berm or jetty that would stop the toe erosion at the base of the slide. A sea wall or jetty would have the added advantage of facilitating cleaning of the ocean water (reducing turbidity), thus allowing native marine plants and animals to re- establish themselves off-shore. The U.S. Army Corps of Engineers had been asked to investigate the feasibility of such a project in that it might contribute to the reduction of the siltation rate at Los Angeles Harbor. In July 1993, Congress allocated $80,000 for the Army Corps of Engineers to study the slide. Geologic testing by the Army Corps of Engineers along the Portuguese Bend shoreline was ongoing from Fall of 1996 and a proposed sea wall design rendering was produced in 1999. In 1996, the City also began geologic testing of the shoreline area to obtain more data on the geologic substructure of the shore-line and the near off-shore zone. In 2000, Rancho Palos Verdes officials rejected the proposed sea wall design produced by the Army Corps of Engineers as aesthetically unacceptable. By 2001, unacceptable design concerns, higher than expected cost estimates and rising public opposition combined to bring a halt to further proposals and pursuit of a Corps built sea wall or jetty,

H. Results of Mitigation Measures: Successes and Failures. Geologists have divided the Portuguese Bend slide into five specific subslide components, and each can be examined and evaluated with regard to its own dynamics and rate of movement. The Abalone Cove and Klondike Canyon slide zone dynamics are considered as distinct units although much of the groundwater that enters the Portuguese Bend slide zone finds its way to the Abalone Cove slide material.

Measurement precision relates to the ability to repeatedly measure the same value between two fixed points. In the GPS (Global Positioning System) method used in the Palos Verdes slide zone, a stable point in the parking lot for Abalone Cove beach is used as fixed point from which the coordinates of all other points are determined.

1. Subslide A. This is the very top of the slide, an area of about 50 acres. In late 1989 it was moving about .03" per day, compared to a 1984 rate of 1.25" per day. By 1991, movement had stopped only to be renewed following the heavy winter storms of 1991/1992 and 1992/1993. The subsequent rate of movement was very slight or non existent until early 1995 when movement began again following heavy winter rains.

2. Subslide B. This is the eastern half of the area just north of PVDS (about 60 acres). Removal of water from seventeen de-watering wells, some grading, movement of one million cubic yards of earth to reduce driving forces in the northern and eastern parts of the slide and drainage improvements in the subslide B zone had slowed movement in this area from 1.5" to .3" per day by the Fall of 1991. By 1989 it was moving about 1/10 as fast as it had been in 1984. By 1991 it had been slowed still more only to experience an increase following the heavy winter storms of 1991/1992 and 1992/1993. As of May 1993, movement in this zone ranged from one to three feet per year but has accelerated since the heavy winter rains of 1994/1995. Between August 1994 and August 1996, horizontal displacement has been as much as sixteen feet.

1.29 3. Subslide C. This is the western half of the slide area just north of PVDS (includes about 40 acres of developed land). In 1989 it was moving at a rate of 0.1 inch per day to 0.4 inch per day (about 2/3 as fast as it was in 1985). In 1991, its rate of movement had been slowed almost to the point of stabilization. Following the winter storms of 1991/1992 and 1992/1993, however, movement of this subslide area had increased to almost one foot per year and the rate of movement has accelerated still more following the winter storms of 1994/95.

4. Subslide D. This is the area between PVDS and the ocean east of Inspiration Point (about 75 acres). Work completed in the neighboring subslides along with some grading and the relocation of almost one million cubic yards of earth from uphill locations to this portion of the slide zone to augment stabilizing and resistant forces in the subslide D zone had slowed movement in this area from 1.5" to .3" per day by the Fall of 1991. The winter storms of 1991/1992 and 1992/1993, however, destroyed the stabilizing gabions at the toe of the slide zone and facilitated an increase in the rate of slide movement from 2.5 to as much as 6 or 7 feet per year as of summer 1993. The heavy winter rains of 1994/1995 and storms of 1995-1996 have brought about additional substantial slide movement with 3-7 feet of horizontal displacement averaging between October 1994 and August 1996.

5. Subslide E. This is the westernmost section of the slide, between the Abalone Cove landslide and Inspiration Point (about 60 acres). De-watering wells in the Abalone Cove landslide have made the greatest contribution toward the present stabilization of this slide zone.

6. Abalone Cove Slide. Since the early 1980’s, installation of de-watering wells, new surface drainage facilities and repositioning and grading of the slide material have been effective in halting slide movement. Stabilization was achieved by 1991. In the August 1994 to May 1996 period, almost no horizontal or vertical displacement was detected in this slide zone.

7. Klondike Canyon Slide. In the August 1994 to May 1996 period, almost no horizontal or vertical displacement was detected in this slide zone.

1.30 Map Port. Bend landslide showing displacement 21 months after Phase 1 grading 1.31 Map Port. Bend landslide showing displacement after second year after Phase II grading 1.32 1.33 Proposed Development in the Landslide Moratorium Ordinance Area and the Factor of Safety (FOS).

Divided into eight zones, the moratorium landslide area zone boundaries are based on the varying degrees of geologic stability within the landslide area. Zone #2 consisting of 130 acres located north of the existing Abalone Cove settlement) is the only zone under consideration for development at this time. Of considerable concern is the Factor of Safety (FOS) level that exists for this zone. Geologists and engineers are often requested to calculate the FOS of a location before development plans proceed very far. When these professionals calculate the Factor of Safety or FOS (against sliding), they are attempting to quantify the potential for movement of a slope. In a landslide zone, it is the ratio of forces tending to resist sliding at the lower end of a slide zone divided by the forces tending to cause the sliding (the driving forces along the up slope edge of the slide). An FOS of 1.00 indicates that the two forces are balanced and the material is stable. A moving landslide has a factor of safety of less than 1.00 and a stable landslide has a factor of safety of more than 1.00 (an FOS of 1.5 indicates that the material has 50% more resistive force than driving force).

Of concern here is the gross stability of the ancient inactive landslide within which Zone 2 is located. Although it is commonly accepted that the gross factor of safety exceeds 1.0 and has not fallen below 1.0 for the last 100,000 years, determining its true value is difficult or impossible because of the size and complexity of the area and uncertainties regarding the strength parameters along its ancient failure surface. Still, expert opinion suggests that the FOS is most likely between 1.1 and 1.4 and although there is uncertainty regarding potential modes of failure, various geomorphic and dynamic factors suggest that a positive FOS is a realistic expectation.

Factors that Contribute to a FOS value exceeding 1.00 for Zone #2

During the prehistoric sliding, the base of the ancient landslide rose to the ground surface at an elevation of about 275 feet above sea level and then moved downhill across the ground surface over stable ground west of the current site of Wayfarer’s Chapel until it built a buttress that stopped this southwest movement and redirected the slide eastward. The part of the slide that is in the area west of Ginger Root Lane then moved southeast toward the head of the present Abalone Cove landslide. This movement reduced the driving force acting against the debris buttress west of the Abalone Cove landslide; consequently, its Factor Of Safety is likely to be well above 1.0 for failure in a south direction.

At the same time, large blocks of bedrock buttressed themselves in a northerly direction extending inland from Portuguese Point. Thus Portuguese Point and the blocks of bedrock buttressed against it formed a barrier that blocked slide movement along its alignment. As a result any massive landslide in the uphill area must constrict as it moves toward Abalone Cove and would have to follow behind the Abalone Cove landslide. These structural dynamics increase frictional resistance to sliding and for these reasons most geologists would not expect a massive slide to develop.

Since sliding occurred about 125,000 years ago, there has been extensive removal of driving force by erosional down cutting and widening of canyons in the uphill part of the landslide. The erosional removal of material has increased the factor of safety.

The fracturing of rocks during sliding would have allowed groundwater to drain more effectively from the landslide area. The resulting improved drainage would lower the water table and decrease the buoyancy effect of groundwater and increase the factor of safety.

Leaching of ground water through the uphill area tends to remove sodium and replace it with calcium. Added calcium increases the shear strength of clay along the slide plane because calcium-bearing clay is much stronger than sodium-bearing clay. Under the natural conditions in the slide area, the only significant source of sodium is sea spray whereas calcium is easily dissolved from natural carbonate (dolomite) present. In the rocks. This ion exchange causes the FOS to be greater than 1.5 in sampled locations.

Existing and proposed additional de-watering wells and the installation of sewers and good surface drainage will keep the water table below the level it had reached prior to the start of mitigation of the Abalone Cove landslide. These artificial projects will improve the FOS.

1.34 Assuming such stability, the most likely mode of failure would be to have secondary landslides develop along the uphill edge of the present landslide. If. however, the present landslide is kept in stable condition, secondary landslides can not develop along its uphill edge. The real issue, therefore is whether the existing Abalone Cove landslide can be prevented from moving. If it can be prevented from moving, the uphill part of the ancient landslide will be kept from moving.

Unknown Parameters Relevant to Calculating the FOS for Zone #2

Although, the above described factors suggest that a positive FOS for the Zone #2 area is likely, identifying a completely reliable and more precise FOS would require a more detailed and complete examination of currently uncertain parameters that must be included in the calculations. Currently unknown parameters that require more precise measurement include:

1. The configuration of the base of the ancient landslide and subslides within it must be known. At least a dozen deep borings (core holes) along with conventional and dip meter logging are needed to define the base of the slide in sufficient detail to precisely calculate the FOS. Most borings are needed in the area uphill from Zone #2.

2. The shear strength of clay along the slide base and whether or not there is an effective method of increasing its strength must be determined. The shear strength of the slide base is the most important and controversial factor when calculating the FOS. Sodium-rich montmorillonite (clay) which occurs along the bases of the active and recently active coastal landslides is exceptionally weak and exhibits viscous behavior. Shear strength is expressed in terms of a cohesion value and the angle of internal friction. Low cohesion values are not uncommon and can be fairly easily determined.

Angles of internal friction below about 12 degrees, however, are uncommon and require special testing methods. Sodium montmorillonite can have an angle of internal friction as low as 5 degrees at very low strain rates (on the order of an inch per year). Most tests are done at a rate of many inches per day and part of the resisting force is cased by viscous resistance to movement. Consequently, the tests may yield a significantly higher angle of internal friction than actually exists at very slow rates of movement. For example, if the true FOS is 1.00 when the angle of internal friction is 7.0 degrees, a determination of 10.5 degrees will yield a FOS of 1.5l. However, if sodium has been extensively replaced by calcium, the angle of internal friction might be as high as 18 degrees. There is also the possibility that the FOS can be increased by substituting calcium for sodium in clay along the slide base. Such a substitution might raise the FOS above 1.50.

3. The level to which the water table can be lowered and held in various sectors of the slide mass must be determined. Data regarding the elevation to which the water table can be lowered is best obtained by actual experience. Monitoring wells for this purpose would be needed and there are none in Zone #2 at present. Six to eight monitoring wells and two or three more production wells would be required in Zone #2 and additional wells would be needed uphill from Zone #2.

4. Possible future modes of landslide failure must be explored and taken into account as well as knowing to what extent the Abalone Cove landslide will be kept in a stable condition.. The most likely mode of failure would be for secondary landslides to fail along the uphill edge of the existing Abalone Cove landslide. Such failures could work their way uphill through Zone #2 and lead to massive failures in the area uphill from Zone #2. This mode of failure can only occurif ACLAD does not do its job of maintaining the Abalone Cove landslide in a stable condition. As of March 1997, a consulting geologist concluded that it was uncertain if ACLAD was performing adequately. Although ACLAD had maintained wells, it had not funded any of the other recommended remedial measures since a drainage culvert had been installed in 1983.

In summary, if the appropriate actions are taken to improve the FOS and nothing is permitted that will decrease the FOS, the landslide will not move no matter what the calculated FOS may be. In regard to the long-term need for abatement, no matter what the calculated FOS may be, the area served by ACLAD will always require ground water removal to prevent a recurrence of sliding. Well production data supports the view that much of the ground water enters the slide area from uphill beyond the boundaries of ACLAD and that the amount of water entering ACLAD has increased through time (because of uphill development). Monitoring wells are needed in Zone #2 to keep track of ground water conditions and two or three additional de-watering wells are likely to be needed to better control the inflow of water from 1.35 uphill. Most information needed to calculate the FOS would have to come from Zone #1, located uphill and to the west of Zone #2.

THE OCEAN TRAILS SLIDE

On June 2, 1999 a 16.4 acres block of Altamira Shale, containing the bluff top and seaside half of the 18th hole of the Ocean Trails Golf Course, was displaced 50 feet seaward while dropping approximately 50 feet. Geologists believe that the mass wasting event occurred on a portion of an ancient dormant slide zone.

A. Experts have calculated that about 2 million gallons of wastewater, equivalent to adding some 16 million pounds of driving force to the soil, triggered a movement that caused the earth to ram into the ocean floor. 1. The driving force lifted formerly submerged rock up about 20 feet and created a series of snaking fissures up to 90- feet deep. 2. Although the cause of the 1999 slide has yet to be legally determined, some experts argue that a break in a sewerage pipe installed in 1955, added increased weight, reduced the resistance forces of the rock mass and provided additional lubrication to the slide planes/bedding planes of the Altamira Shale. 3. Other experts suggest that the slide could have been triggered by the construction activity and relocation of earth associated with the creation of the golf course or as a result of pipe damage stemming from natural corrosion and old age of the pipe itself. 4. Still other experts contend that the slide was a natural event that began as a slow paced movement that induced a brake in the sewerage pipe. Initially the water leakage may have been minor but over time the saturation of the Altamira Shale with ground water from the broken pipe triggered the rapid slide of 1999.

UPDATE 2005 ECOLOGICAL RESERVE: A NEW FUTURE FOR PORTUGEUSE BEND?

In a strange twist of fate the long history of geological instability and the inability of engineers and geologists to halt the landslide have opened the door to the future establishment of an open space and large environmental reserve at Portuguese Bend. As of the present, a massive fundraising campaign is underway. The Annenberg Foundation has agreed to match all contributions (over $1, 000,000) between now and the deadline of September 14th .

The opportunity is now. The community must raise $4 million by Sept.15th.

Message from Joel Rojas, Head Planner for the City of Rancho Palos Verdes regarding Portuguese Bend.

The emphasis in past years has been on all the landslide stabilization efforts that have been attempted by the County and then the City. For years, one of the City's unwritten mandates was to try to stop and stabilize the landslide. That no longer is the case for a number of reasons. One, the primary persons that were driving the stabilization efforts are no longer part of the picture. Perry Ehlig, the City Gelogist who was the expert on the slide and who greatly influenced the 1.2 stabilization efforts drove passed away as did Charles Abbott, the consultant that contracted with the City to do all the stabilization work. Two, there is not enough money to carry out all of the stabilization efforts called out in the "Horan settlement". That's the settlement that resulted from the lawsuit against the County which set up a pot of money to stabilize the landslide by implementing a list of projects in a priority order. The installation of a sewer system was one of those projects, and now that it's been completed, we're realizing that there isn't much money left for anything else. Three, unlike previous city councils who felt that stabilization of the landslide was a top priority, the current city council's top priority is purchasing the open space that includes the active slide area. Although slide related activities such as the installation of dewatering wells, drainage improvements and fissure filling will likely continue if the property is purchased by the city, the reality is that once purchased for conservation there will not be the financial backing (from a private property owner who wishes to develop the property) or the political backing to stabilize the slide.

1.3

As you can see from the following news story, efforts to provide habitat for endangered species as part of the mitigation to establish Ocean Trails have had some success:

1.4 Golf Course News Magazine » News » Threatened bird thrives at Trump National 8/23/2005 Rancho Palos Verdes, Calif. – When women professional golfers tee off Sept. 30 in the LPGA tournament at the Trump National, the new course on Los Angeles’ Palos Verdes Peninsula will have recorded one major achievement already.

Although cut from one of the region’s most environmentally sensitive areas, Trump National includes 125-acres of restored coastal sage scrub in which the population of the federally listed threatened Coastal California gnatcatcher is actually growing. Breeding pairs of gnatcatchers have increased from four to 15 and more than 205 fledglings have been raised since the restoration began in the late 1990s.

“The course now exports young gnatcatchers to habitat in other areas on the peninsula,” says Mike Sweesy, project manager with Dudek and Associates, an Encinitas, Calif., environmental services firm that developed and manages the course’s habitat restoration program. “Even during the 2003 drought when other gnatcatcher populations were decreasing dramatically, the population at Trump National held steady.”

Trump National succeeded at blending land development with environmental protection. Trump National took great care of the coastal habitat of the endangered gnatcatcher when renovating the course. “It shows that economically viable development can co-exist with environmentally sensitive resources through appropriate design,” Sweesy said. “The gnatcatcher’s success at Trump National can probably be extrapolated to similar species in similar environmental conditions.” The gnatcatcher has been called the canary in the coalmine of Southern California because it is an indicator of how healthy the California coastal ecosystem is. The gnatcatcher is at risk of extinction due to a decline in natural coastal sage scrub habitat. Of the 2.5 million acres of coastal sage scrub that once stretched from Ventura to the Mexican border, only 10 percent remains. Trump National sits in the middle of one of the last habitats for the bird in the Los Angeles basin. The plan to protect the gnatcatcher began in the mid 1990s. As a precondition to construction of what was then known as Ocean Trails, regulators told the course’s original owners they would have to show they could successful restore coastal sage scrub. The owners brought in Dudek’s biologists and landscape architects who successfully demonstrated a restoration program. Hopes for the golf course opening were dashed in 1999 when a landslide claimed the 18th hole. Dudek conducted additional assessments of the landslide’s environmental impacts for the reconstruction effort (which included building an enormous retaining wall). The resulting plans to mitigate the landslide helped the course to be competed. After Ocean Trails slipped into bankruptcy, Donald Trump bought the course in 2002, renamed it Trump National Golf Club and closed it in August 2004 for reconstruction and renovation. “Mr. Trump came in and said ‘We only do things one way, the right way,’” Sweesy says. “He has been very supportive of the habitat restoration effort because it is integral to his development.” During course construction, Sweesy spent months walking the rolling hills to map out what to plant in areas sometimes as small as 20-square feet to accommodate micro-topographical features. In addition to the restoration work, Dudek also prepared the master landscape plan for the clubhouse, golf course and residential and designed the Ocean Trails public park. When completed, the restoration project installed more than 100,000 native, drought-tolerant plants. “The most challenging part of the design was the permit requirement for 20-acres of coastal sage scrub restoration inside the course,” Sweesy says. “As a result, restored slopes and out of play areas passed daily by golfers function as habitat where the gnatcatchers forage and nest. That’s significant because conventional wisdom before this project held that these restoration sites would have been too compromised for the gnatcatcher to thrive.”

Sweesy said the repopulation has been an unexpected bonus.

“The regulatory permits required only restoration of coastal sage scrub and not actual occupation by the birds,” Sweesy said. “The gnatcatchers native to the peninsula have been finding their way to the course and staying. These birds are opportunistic and we have created a place where people can enjoy the California coastline and gnatcatchers can thrive.”