Coastal Erosion-Induced Landslide in South Orange County

Coastal erosion and sea level rise have become common issues along the coast of Southern California not only from an environmental standpoint but also from a geotechnical standpoint. This case study presents reactivation and partial movement of an ancient landslide along the Orange County coastline due to the significant loss of sand buttressing the landslide at the beach. The landslide reactivation resulted in tension cracks that extended through a community parking lot and several residential lots above the beach. A forensic geotechnical engineering investigation was performed to characterize the subsurface soils and groundwater conditions and to monitor the landslide movements using inclinometers, piezometers, and crack monitors. This study indicated that a portion of the neighborhood was constructed over an ancient landslide which, until recently, was essentially buttressed by beach sand deposited along the toe of the landslide. Over the last several decades, the beach sand slowly eroded until the beach was almost completely gone following a high tide event in July of 2020. The removal of beach sand that buttressed the toe of the ancient landslide led to the reduction of the slope stability safety factor to less than 1.0 triggering reactivation of a portion of the landslide. The reactivation resulted in damage to the parking lot area, underground utilities, one empty residential lot, and four homes at the head scarp of the reactivated portion of the landslide as well as compromising and deforming the railroad track near the landslide toe. The beach sand depletion was confirmed through our geologic modeling and slope analyses as the triggering method for the reactivation. Temporary stabilization measures were put into place consisting of adding approximately 17,400 tons of rip-rap along the west side of the railroad that served to add weight at the toe of the active portion of landslide. The temporary stabilization measures served to temporarily halt the movement, however, high tide events triggered by Hurricane Kay resulted in additional beach sand loss and landslide movement. Several more rounds of rip-rap were added while permanent stabilization measures were being developed to satisfy the multiple stakeholders. As of July 2023, permanent stabilization measures have been completed. This case study of coastal erosion induced landsliding in southern Orange County highlights both the need for sustainable designs and future mitigation measures as well as the geotechnical challenges as the coastline erodes due to sea level rise, urban sprawl, state mandated water quality measures reducing sediment transport to the ocean, and other manmade improvements that otherwise change/impede sand deposition along the coast.