Near-Continuous Monitoring of a Coastal Salt Marsh Margin: Implications
to Predicting Marsh Edge Erosion
Abstract
Mechanisms that control marsh edge erosion include wind-generated waves,
vegetation, mudflats, anthropogenic factors, 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 rises from a lack of high frequency
monitoring of marsh evolutions. In particular, marsh erosion is
time-scale dependent and conducting field observations on short time 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 monitoring system to document marsh
edge erosion at a high frequency in Terrebonne Bay, Louisiana. Erosion
pins were monitored with the cameras and daily erosion rates were
estimated. This was supplemented with daily wave power to explore the
relationships between daily erosion and wave power. The largest
magnitude erosion events are driven by a buildup in wave energy over a
seven-day time period coupled with a strong one-day wave event,
indicating a gradual reduction in marsh edge resistance with continued
wave attack. Long-term erosion monitoring methods, including monthly
field visits, smooths over the large magnitude short-term erosion
events. For example, satellite and aerial imagery provide a long period
of record, but they seem to underestimate the average annual erosion
rate in the region, the effect of which may become exasperated over the
varying temporal scales considered in the planning efforts of projects
meant to protect the Louisiana coastline.