Coast & Ocean Vol. 21, Nos. 1&2: Cliff Erosion

The County of Santa Barbara condemned several cliff-top buildings in Isla Vista, near the University of California campus, that were judged to be unsafe to occupy. This forced about 100 students to seek other accommodations in a tight housing market with rather short notice. (See Coast and Ocean, Winter 2004–05.)

It may be useful to review the recent history of the area and the causes of the erosion that has put these and other buildings at risk. Most were built in the 1950s and 1960s, before the County began to enforce strict setback rules, and probably before the rate of cliff retreat at Isla Vista was widely appreciated. Furthermore, even when the rate of erosion began to be recognized, it was assumed that the principal—perhaps the only—cause of the cliff retreat was wave erosion at the cliff base. In response to this seemingly reasonable assumption, several wood piling seawalls were constructed after approval was obtained from the County and the Coastal Commission. These seawalls have provided scarcely any protection from the erosional processes that cause cliff retreat. On the contrary, they demonstrate that wave erosion at the cliff base does little beyond clearing away debris provided by other erosional processes.

What are these processes? Perhaps the most obvious is the effect of rain and sea spray driven against the cliff face by wind. Because the rocks making up the cliff face in the Isla Vista area are soft, laminated, and fractured shales and mudstones of the Miocene Sisquoc Formation, they succumb readily to water running down the cliff face.

Two other closely related processes are also involved. Any surface runoff from the area landward of the cliff top augments this type of erosion. Although attempts have been made to direct surface runoff from cliff-top properties to the street rather than down the face of the cliff, or in some cases to drains that discharge at the cliff base, maintenance of these systems has not been very good or consistent. Poorly maintained drainage schemes have too often exacerbated rather than prevented erosion.

Another related but very inconspicuous process is salt weathering. When sea spray is blown against the cliff face, some of this saline water soaks into porous places in the rock and evaporates during dry periods, leaving tiny salt crystals. These crystals grow during evaporation, and slowly and persistently wedge apart rock grains in the cliff. This process is similar to frost-wedging seen in high mountains, where the formation of ice in tiny cracks splits even hard rocks.

Yet another group of erosional processes are those attributable to organic agents. Among these are burrows made by gophers and ground squirrels that occasionally act as drains from the ground surface to the cliff face. Others are due to the activities of burrowing owls and bees, and perhaps of other animals as well.

Student occupants of these buildings have contributed to erosion occasionally by constructing rope ladders to climb up and down the cliff face, dislodging bits of rock as they go.

Plants too, play a role. Tree or shrub roots associated with vegetation growing near the cliff edge regularly wedge rocks apart, often leaving fairly large roots dangling in the air. It is not unusual for entire trees to wind up on the beach as their supporting soil and rock is eroded away. Further, heavy masses of plants such as ice plant regularly pull away blocks or slabs of cliff-face rock, and fall with them to the beach below.

The net result of these processes is to produce a nearly continuous, generally inconspicuous, rain of fine material, punctuated from time to time by spectacular rockfalls, some of which may have no obvious, immediate cause, but are almost certainly due to the cumulative effect of several of the erosional agencies just mentioned.

On one of the writer’s photographic visits, on a fine, windless day, several hundred pounds of rock suddenly broke free 15 or 20 feet above the beach, and certainly would have killed or seriously injured anyone sitting at the cliff base. Exactly such an event occurred at More Mesa, about a mile east of Isla Vista, in the early 1980s—a man and his dog were killed.

Landsliding of large masses of rock has been quite frequent at other places in the Santa Barbara area, but not at Isla Vista. It would certainly be foolish to assume, however, that landsliding will not occur.

For the most part, landslides in Santa Barbara County have occurred in the Miocene Monterey Formation. This rock is highly fractured, and rock layers are often separated by thin seams of soft, slippery clay. Also, the cliffs at Isla Vista are usually only about 35–40 feet high, whereas cliffs cut in the Monterey Formation, which is somewhat more resistant to erosion, often exceed 100 feet. Whenever water works its way into fractures behind a nearly vertical cliff, abrupt failure of the over-steepened block may occur with little warning. Accordingly, landslide events most often occur during heavy rains, or shortly thereafter.

No Happy Solution

Is there any practical and economically justifiable way to protect these cliff-top buildings from the effects of erosion? The answer, perhaps unfortunately, is clearly no, if one considers both costs and the difficulty of getting authorization from various state and county agencies. Theoretically, a massive concrete wall keyed into bedrock at the cliff base, extending to the top of the cliff and protected from water entering behind the wall, would provide some protection, but even such a structure would be attacked from its ends.

As far as the property owners of cliff-top buildings are concerned, there is no happy solution to the problem. Sooner or later all these buildings will be endangered by erosion. They might possibly be supported on concrete piers extending into bedrock in the surf zone, but not without intruding on the public’s beach and right of access to the shore. However, because these buildings command premium rents, any County effort to condemn them as safety hazards will result in major resistance and attempts to find engineering solutions. Some building owners have opted for temporary fixes and have merely cut off the front parts of the buildings.

Slowed, Not Stopped

What can be said about rates of cliff retreat in the Isla Vista area? The first thing worth noting is that rates of erosion behind so-called protective sea walls are essentially the same as on adjacent “unprotected” areas. There is so little difference that one must conclude that the sum total of the non-marine processes (not direct wave erosion), account for at least 90 percent of cliff retreat.

The photos above show an area in which the entire cliff face, from top to bottom, was protected by a coat of “shot-crete.” Although this did indeed slow cliff erosion at that point for a time, it was ultimately defeated by wave erosion at the base and non-marine erosion on the sides. The period of protection left a slight but noticeable seaward bulge in the cliff face.

The long-term retreat rate measured from dateable structures, such as survey monuments atop the cliff, averages about six or seven inches annually. It is important to note that this rate is based on a long period, at least 40–50 years. Short-term rates may be dramatically different, especially during very stormy El Niño years marked by heavy rains. In the winter of 2004–2005, which was not an El Niño year but had periods of heavy rain, as much as eight feet of the cliff collapsed during and immediately following a single storm.

If one lived in a world where long-term public benefits and safety took clear precedence over individual property issues, and if government agencies had sufficient discretionary funds, a county or park district would condemn and purchase all properties to at least 150 feet from the cliff top and convert them to public parkland with few or no permanent buildings. The City of Santa Barbara has just such a park on the Mesa lands just west of the harbor. This is a beautiful asset not only to the immediate neighborhood but to the entire city.