20 August 2008

La Conchita landslide verdict

Posted by Dave Petley

Many readers will be familiar with the La Conchita landslide in California. For those who aren’t, La Conchita a small (population about 340 people), unincorporated seaside village located on the Pacific coast north of Los Angeles (Fig. 1) (34.4 degrees north, 119.5 degrees west). The town is located on a narrow coastal plain, with a steep slope behind. The houses are very close to the foot of the slope (Fig 2).

Fig 1: Google Earth image showing the location of La Conchita

Fig 2: Oblique Google Earth image showing the town of La Conchita with the steep slope behind. Note the very clear landslide scars

The town shot into the limelight on 4th March 1995 when a landslide slipped off the slope and buried or damaged seven houses. Fortunately no-one was killed, but it clearly caused considerable concern. The landslide is the subject of perhaps one of the most famous of all landslide pictures, taken by the USGS (Fig. 3). The landslide was large (120 m wide, 330 m long, and >30 m deep, with an estimated volume of 1.3 million cubic metres. However, the slide occurred as a coherent earthflow and was comparatively slow.

Fig 3: USGS image of the 1995 La Conchita landslide

Unsurprisingly, in the aftermath of the landslide concerns were raised about the safety of the town. In particular, it is clear from Figure 2 that the slope is mantled with old landslide scars and deposits. There was consistency in the views of the experts that further landslides on the slope were likely. Sadly, this proved to be correct because on 10th January 2005, during a period of very heavy rainfall, a further landslide occurred. On this occasion a portion of the failed mass from 1995 remobilised and hit the upper part of the town.

Fig 4: USGS image of the 2005 La Conchita landslide

Amazingly, the failure of the upper part of the landslide was caught by a TV crew (see here or below).

Fig. 5: USGS false colour image of the La Conchita landslide complex, annotated to show the key features

Yesterday, the main part of the court case brought by the property owners against the ranch owners came to an end. I have no knowledge of the US legal system, but interestingly the verdict of the jury is that the ranch company was negligent in connection with the 2005 landslide. The land owners had claimed that the ranch company did not build an adequate drainage system. This was felt to have contributed to the event. As I understand it the drainage system was not found to be the sole cause, meaning that the negligence is shared. The court case has now moved into a second phase in which the jury will decide upon damages. Because of this it would be wrong to comment on the rights and wrongs of this case at this stage (I will do so later), but it will certainly be interesting to see what the jurors award, and to whom.