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

Abstract

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.

Seasonality of Retreat Rate of a Wave‐Exposed Marsh Edge

“…(2022) found that inclusion of mass‐failure events (rather than only gradual erosion processes) lowered R 2 values of wave power‐edge erosion regressions at the monthly scale but increased R 2 values at the annual scale, speaking to how sufficiently long time periods between retreat measurement can effectively aggregate seemingly stochastic mass‐failure processes or time‐lagged cumulative wave power effects on edge failure (as found on the weekly scale by Cadigan et al. (2023)). Our findings support that seasons can be effective sub‐annual timescales for marsh failure observation aggregation, in contexts where seasons drive shifts in regional wind and wave climate, although large uncertainties in change metrics over short timescales may occlude clear results.…”

“…This may be because this is a system with high enough wave power to effectively erode protrusions from the marsh edge; put another way, a strong uniform wave field effectively overpowers heterogeneity in marsh-edge erodibility to produce a uniformly eroding marsh boundary on a sufficiently long timescale (Leonardi & Fagherazzi, 2014;Priestas et al, 2015). Mel et al (2022) found that inclusion of mass-failure events (rather than only gradual erosion processes) lowered R 2 values of wave poweredge erosion regressions at the monthly scale but increased R 2 values at the annual scale, speaking to how sufficiently long time periods between retreat measurement can effectively aggregate seemingly stochastic massfailure processes or time-lagged cumulative wave power effects on edge failure (as found on the weekly scale by Cadigan et al (2023)). Our findings support that seasons can be effective sub-annual timescales for marsh failure observation aggregation, in contexts where seasons drive shifts in regional wind and wave climate, although large uncertainties in change metrics over short timescales may occlude clear results.…”

“…Dimensional analysis (Marani et al, 2011) and empirical studies (Bendoni et al, 2016;Leonardi, Ganju, & Fagherazzi, 2016;McLoughlin et al, 2015;Mel et al, 2022) suggest a linear relationship between incident surface wave power and annualized volumetric erosion rates at the marsh edge. However, nonlinear relationships have been observed as well (Cadigan et al, 2023;Houttuijn Bloemendaal et al, 2023), and Houttuijn Bloemendaal et al (2023) revisited the assumption of linearity in this relationship, noting that once marsh-edge retreat and wave power distributions are log-transformed into normal distributions, some marsh systems exhibited power-law relationships. Marshedge retreat may happen in sudden moments of mass such as slump-block or cantilever failure, delayed after wave impact (Bendoni et al, 2016;Cadigan et al, 2023).…”

Interplay of Hydroperiod on Root Shear Strength for Coastal Wetlands

“…Existing numerical models of wetland evolution, of which there are many, simulate the spatial distribution of sediment fluxes and vegetation characteristics from simple empirical models that predict sedimentation patterns as a function of topographic variables (Temmerman et al., 2003) to physics‐based models that simulate water and sediment flow paths on the basis of simplified hydrodynamic schemes (D'Alpaos et al., 2007; Rinaldo et al., 1999) or on the basis of a full hydrodynamic description of the feedbacks between tidal flow and vegetation (Temmerman et al., 2005). Connecting the various geomorphological and ecological factors in numerical models is particurlay critical in changing climate conditions in order to understand the physics behind marsh evolution not currently accurately encapsulated by numerical models (Cadigan, Jafari, Wang, et al., 2023; A. J. Payne et al., 2021; A. R. Payne et al., 2019; Saintilan et al., 2023). Indeed previous discussion around the interplay between vegetation, sediment composition in terms of percentage of fine‐grained material, and marsh edge geometry notes that the lack of geotechnical knowledge on how vegetation stabilizes salt marshes severely limits the advancement of numerical models of these systems (Bendoni, 2015).…”

“…The root shear strength of wetland vegetation directly controls the ability of wetlands to resist edge erosion from wind‐generated waves (Cadigan, Jafari, Wang, et al., 2023; Chen et al., 2013; Marani et al., 2011; Reed, 2001), uprooting from tropical cyclone storm surge and waves (Howes et al., 2010; Morton & Barras, 2011), and collapse from excessive inundation (Cadigan, Jafari, et al., 2022; Chambers et al., 2019; DeLaune et al., 1994). It can also serve as a proxy of belowground biomass productivity to project restoration trajectories, from thin layer placement to freshwater diversions.…”