Mass Movement and Seacliff Retreat along the Southern California Coast

Abstract

Seacliff retreat is a complex response between the magnitude and fre- <br /><br />quency of interactive processes, primarily mass movement and marine erosion, <br /><br />and the properties of coastal terrain. Mass movement occurs on or above seacliffs <br /><br />when resisting forces are overcome by driving forces, especially when the shear <br /><br />strength of cliff-forming materials is reduced by absorption of water or when shear <br /><br />stress is increased by loading, vibration, or removal of toe support. Many land- <br /><br />slides of the Malibu and Palos Verdes coasts are prehistoric features reactivated <br /><br />in recent time by heavy winter rains or by human impacts on slope hydrology <br /><br />and buttressing. Marine erosion removes both solid rock and mass-movement <br /><br />debris from the cliff base by hydraulic forces and abrasion, the efficiency of which <br /><br />reflects hydrodynamics and resistance properties. Seacliff retreat is episodic and <br /><br />site-specific. Harder rocks, like Cretaceous sandstone, tend to erode more slowly <br /><br />than softer Quaternary deposits. Recent retreat rates along the San Diego and <br /><br />Santa Barbara coasts vary from negligible to 0.5 m/yr. The nature and rate of <br /><br />seacliff retreat were largely ignored during the region's early development but have <br /><br />recently been investigated quantitatively and incorporated into coastal manage- <br /><br />ment plans.