Abstract
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 cliff retreat rate, roughness, and sinuosity and (2) the response of retreat rate and roughness to changes in sand and shell hash cover of the underlying mud substrate as well as the impact of major storms using field measurements of sediment cover, erosion, and aerial images to measure shore platform morphology and retreat. A storm event in 2009 increased the planform roughness and sinuosity of the coastal cliff at Sargent Beach. Following the storm, aerial-image-derived shorelines with annual resolution show a decrease in average alongshore erosion rates from 12 to 4 m yr−1, coincident with a decrease in shoreline roughness and sinuosity (smoothing). Like the previous storm, a storm event in 2017 increased the planform roughness and sinuosity of the cliff. Over shorter timescales, monthly retreat of the sea cliff occurred only when the platform was sparsely covered with sediment cover on the shore platform, indicating that the tools and cover effects can significantly affect short-term erosion rates. The timescale to return to a smooth shoreline following a storm or roughening event, given a steady-state erosion rate, is approximately 24 years, with the long-term rate suggesting a maximum of ∼107 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.
Highlights
Coastal cohesive cliffs may recede at rates of meters per year or more (Sunamura, 2015) depending on the intensity of waves, sea level rise, and the tools and cover effects of sediment abrasion (Sunamura, 1992, 2015; Limber and Murray, 2011; Young et al, 2014)
Sargent Beach, Texas, USA (Fig. 1), is a consolidated Holocene mud beach composed of floodplain sediments that outcrop as a low-relief sea cliff and Type-A shore platform (Sunamura, 1992), ephemerally covered by sand and shell hash
Storm occurrence and sediment cover jointly control the relationship between cliff roughness and cliff retreat on this cohesive cliff face
Summary
Coastal cohesive cliffs may recede at rates of meters per year or more (Sunamura, 2015) depending on the intensity of waves, sea level rise, and the tools and cover effects of sediment abrasion (Sunamura, 1992, 2015; Limber and Murray, 2011; Young et al, 2014). Sargent Beach is found in a 17 km stretch of coast eroding at an average of 15 m yr−1 over the last 3 decades, which makes it one of the fastest eroding shorelines globally (Luijendijk et al, 2018) This small and dynamic system can be examined as a model for erosion of larger coastal cliff systems, allowing us to understand and explore the feedbacks between planform morphology and the evolution of cliffs over timescales of months to years. A substratum consisting of subhorizontal beds with different erodibilities leads to the production of discrete, seaward-facing steps that are centimeters to decimeters in relief (Fig. 2e) These steps, potholes, and runnels are cut into a landward-migrating and gently seawarddipping shore platform at Sargent Beach
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