Spatial and Temporal Variability in Erosion Generating a Sea Cliff and Wave-Cut Platform that make up the Holocene Transgressive Ravinement Surface at Sargent Beach, Texas, USA

Texas mainland beaches are characterized by a mudstone substrate and an overlying mobile layer of sand. Beaches exhibiting the highest rates of shoreline retreat erode into wave-cut platform or sea cliff morphologies. This... [ view full abstract ]

Texas mainland beaches are characterized by a mudstone substrate and an overlying mobile layer of sand. Beaches exhibiting the highest rates of shoreline retreat erode into wave-cut platform or sea cliff morphologies. This study examines the relationship between temporal and spatial variability in shoreline retreat rates at Sargent, TX and how they are jointly controlled by changes in sand cover and storm wave intensity. At Sargent Beach, the mudstone substrate is composed of subhorizontal beds, centimeters to decimeters in thickness with compressive strengths that vary with water saturation between 206 kPa and 412 kPa. The mobile layer of sand covering the mudstone varies from zero to over a meter in thickness. Mudstone removed from the bed efficiently disaggregated and then suspended, leaving the beach system entirely, while sand moves back and forth across the foreshore and shoreface. The amount of sand present in the system strongly influences the amount of erosion, either facilitating erosion by acting as abrasional tools or inhibiting erosion by mantling and protecting the mudstone. Retreat of the cliff face is estimated as an annual rate by calculating the difference in cliff face position from repeat aerial photographs in 2010, 2011, 2012, and 2014. These average rates are 9.39 m/yr, 4.63 m/yr, and 3.73 m/yr, respectively. Monthly retreat rates for the year 2015 are measured using erosion pins. Monthly erosion rates can be as low as 0 - 0.054 m/month and indicate that intervals of extreme erosion are necessary to produce the retreat observed over the course of a year. Monthly surveys of the wave-cut platform positioned immediately adjacent to the sea cliff record approximately 5 meters of shoreline retreat in 2015, demonstrating a strong spatial variability in shoreline retreat rate. We seek to understand how this variability in time and space controls the long-term evolution of Sargent Beach in order to shed further light on the factors governing rapid development of this modern transgressive ravinement surface.