Supplementary material to "The effects of storms and a transient sandy veneer on the interannual planform evolution a low-relief coastal cliff and wave-cut platform at Sargent Beach, Texas, USA"

Abstract. Coastal cliff erosion is alongshore-variable and episodic, with retreat rates that depend upon sediment as either tools of abrasion or protective cover. However, the feedbacks between coastal cliff planform morphology, retreat rate, and sediment cover are poorly quantified. This study investigates Sargent Beach, Texas, USA at the annual to interannual scale to explore (1) the relationship between temporal and spatial variability in both cliff retreat rate and roughness and (2) the response of retreat rate and roughness to changes in sediment cover of the underlying mud substrate and the impact of major storms, using the low-lying mudstone cliff as a rapidly evolving model of a larger cliff system. A storm event in 2009 increased the planform roughness and sinuosity of the coastal cliff at Sargent Beach, TX. Following the storm, satellite image-derived shorelines with annual resolution show a decrease in average alongshore erosion rates from 4 to 12 m yr−1, coincident with a decrease in shoreline roughness and sinuosity (smoothing). A storm event in 2017 again increased the planform roughness and sinuosity of the cliff. The occurrence of storms and the presence of sediment to laterally erode the cliff influence the planform morphology and subsequent retreat. Over shorter timescales, monthly retreat of the sea cliff occurred only when the platform was sparsely covered with sediment cover on the wave cut platform, indicating that the tools and cover effects can significantly affect short-term erosion rates. The timescale to return to a smooth shoreline with a long-term steady-state erosion rate following a storm or roughening event is approximately five years, with the long-term rate suggesting a minimum of ~38 years until Sargent Beach breaches, compromising the Gulf Intracoastal Waterway (GIWW) under current conditions and assuming no future storms or intervention. The observed retreat rate varies, both spatially and temporally, with cliff face morphology, demonstrating the importance of multi-scale measurements and analysis for interpretation of coastal processes and patterns of cliff retreat.