Near-continuous monitoring of a coastal salt marsh margin: Implications for predicting marsh edge erosion

Mechanisms that control marsh edge erosion include wind-generated waves, vegetation productivity, land use and land change, and geotechnical properties of sediments. However, existing models for predicting marsh edge evolution focus primarily on edge retreat rates as a function of wave energy while accounting for other controlling factors as empirical constants. This simplification arises from a lack of high-frequency monitoring of marsh evolutions. In particular, marsh erosion is timescale dependent, and conducting field observations on short temporal and spatial scales could elucidate the progression of erosion, which may improve marsh erosion predictive models. This study developed and validated a near-continuous camera and erosion pin monitoring system to document marsh edge erosion at a high frequency (i.e., daily) in Terrebonne Bay, Louisiana. This was supplemented with daily wave power to explore the relationships between daily erosion and wave power. Long-term average erosion rates derived from satellite and aerial imagery from 1989 through 2019 compare similarly to rates derived from longer-term site visits (i.e., monthly) at approximately 2.2 m/yr. High-magnitude erosion events (>20 cm/day) are driven by a buildup in wave energy over a 7-day time period coupled with a strong 1-day wave event, indicating a gradual reduction in marsh edge resistance with continued wave attack. Long-term erosion monitoring methods, including monthly field visits, provide results that align well with previously reported relationships between wave power and erosion. High-frequency measurements, however, illustrate that the previously published trends smooth over the large-magnitude short-term erosion events, potentially obscuring the physical processes of marsh edge erosion. For example, satellite and aerial imagery provide a long period of record, but they may underestimate the average annual erosion rate in the region, the effect of which may become exasperated over the varying temporal scales considered in coastal planning efforts across the USA and worldwide.