Article Title

Mass Movement and Seacliff Retreat along the Southern California Coast





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