La Conchita landslide verdict

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).