Detecting hydrodynamic changes after living shoreline restoration and through an extreme event using a Before-After-Control-Impact experiment
Introduction
Wetland and estuarine habitats such as marshes, coral and shellfish reefs, and mangrove forests provide a multitude of critical ecosystem services (Mcleod et al., 2011; Barbier et al., 2011; Grabowski et al., 2012) which, if incorporated into coastal planning, provide numerous benefits (Arkema et al., 2015). Given the increasing frequency and severity of hydrologic events and projected acceleration of sea level rise (Nicholls et al., 1999; Nicholls and Cazenave, 2010; Nicholls and Cazenave, 2010; Tebaldi et al., 2012; Stocker et al., 2013) and that over 10% of the world's population lives within 10 m of mean sea level (McGranahan et al., 2007), protection of vulnerable coastal regions is vital. In addition to climatic variability, aquatic recreational activities may exacerbate shoreline erosion, as wakes generated by boating activity may introduce high-magnitude forces to shorelines (McConchie and Toleman, 2003; Herbert et al., 2018), leading to ecosystem degradation (Wall et al., 2005; Campbell, 2015). Reinforcement of shorelines is therefore often necessary to limit erosion and preserve ecosystems.
Traditional strategies for shoreline protection often involve hardened structures (e.g. bulkheads, seawalls, revetments) which introduce above-equilibrium slopes, change local hydrodynamics (Miles et al., 2001), and modify sediment transport and nearshore morphology (Plant and Griggs, 1992; Griggs, 2005). Additionally, seawalls and bulkheads effectively eliminate the intertidal ecotone, reducing shoreline biodiversity and wetland habitat (Bozek and Burdick, 2005; Gittman et al., 2016a) and lowering potential for biogeochemical transformations in the shoreline ecotone (Vidon and Hill, 2004; Dosskey et al., 2010). As wetlands are recognized as a crucial component of the biological carbon pool (Chmura et al., 2003), ecosystem services related to sequestration of carbon (Chmura, 2009; Mcleod et al., 2011; Mitra and Zaman, 2015; Ridge et al., 2017) are largely eliminated along hardened shorelines.
Trends in shoreline stabilization have shifted toward nature-based approaches utilizing native vegetation and materials produced by living organisms (e.g. biogenic breakwaters) (Bilkovic et al., 2017). Living shoreline methods aim to promote natural ecotone processes while stabilizing eroding shorelines (Coen et al., 1999; Gittman et al., 2016b; Polk and Eulie, 2018; Morris et al., 2019) through reduction of hydrodynamic energy and trapping of sediment (Piazza et al., 2005; (Gedan et al. 2011; Horstman et al., 2014). Use of native wetland vegetation to stabilize eroding shorelines is supported by scientific literature regarding flow-vegetation-sediment interaction (Bouma et al., 2005; Temmerman et al., 2005; Larsen and Harvey, 2010). Emergent vegetation increases drag on flow, reducing mean velocities and wave heights (Nepf, 1999; Horstman et al., 2012, Horstman et al., 2013), while eddies generated downstream of individual stems increase turbulence (Nepf, 2012). However, the net effect to sediment transport from the combination of reduced flow velocity and increased instantaneous shear stresses due to turbulent bursts depends on vegetation characteristics, such as density, size, configuration and rigidity of plant elements (Yager and Schmeeckle, 2013; Yang and Nepf, 2018, Yagci et al., 2017). Neumeier and Amos (2006) observed that bed turbulence was reduced within emergent vegetation (Spartina alterniflora) due to lower flow velocities, suggesting that this interaction might enhance sediment deposition. However, Tinoco and Coco (2018) observed that turbulence generated from the interaction of emergent stems with oscillatory flow led to sediment suspension. Norris et al. (2017) found that turbulence intensity produced by interactions with mangrove pneumatophores was greatest at the fringe and that dissipation scaled correspondingly with increased vegetation density. Similarly, Kibler et al. (2019) observed that turbulence dissipation rates were greater in dense mature mangrove in a natural shoreline fringe as compared to within a living shoreline planted with marsh grasses and young mangroves. However, more than 6 years after living shoreline stabilization, flow patterns within the Restored shoreline were more similar to the natural mangrove shoreline than to a bare shoreline with a seawall, highlighting the potential impact of successful shoreline restoration on local hydrodynamics.
While the aims of living shoreline stabilization are multi-faceted, much available data focus on ecological impacts (Scyphers et al., 2011; La Peyre et al., 2014; Davis et al., 2006). Because overall effects to hydrodynamics and sediment transport by vegetation are highly context-specific and field observation of nearshore hydrodynamics in living shorelines are rare, many process-level questions remain unanswered. For instance, understanding the precise mechanisms by which living shorelines affect hydrodynamics and sediment transport, particularly at early stages when new plantings may be sparse and most susceptible to failure, are critical to design of more robust green infrastructure. The objective of this study is to observe changes in shoreline hydrodynamics during early stages of living shoreline implementation. A Before-After-Control-Impact (BACI) experimental design is applied over 16 months, which includes the implementation of a living shoreline and landfall of a major hurricane. Hydrodynamic changes at the living shoreline site are compared to those occurring simultaneously in controlled reference-condition and degraded shorelines. We address the hypothesis that hydrodynamic and related benthic sediment changes within the living shoreline exceed natural variability observed within nearby bare shoreline that is not restored. This is the first such experimental assessment of hydrodynamic effects related to living shoreline stabilization.
Section snippets
Study location and experimental design
Three shoreline study sites were selected from the east bank of Mosquito Lagoon, a shallow (mean water depth < 3 m), microtidal estuary located on the Atlantic coast of Florida, USA (Fig. 1a), where rates of relative sea level rise are around 2.4 mm/yr (Maul, 2015). Berms created in conjunction with historical mosquito impoundments have artificially steepened shoreline slopes in Mosquito Lagoon (Brockmeyer et al., 1996) and loss of wetland vegetation and erosion is widespread. The sites were
Shoreline velocities
Mean depth-integrated channel velocities ranged from 2 to 9 cm/s in summer (low water levels) and from 8 to 16 cm/s in fall (high water levels) (Table 2, Fig. 3). Prior to restoration, mean near-bed horizontal velocities observed at the degraded Restored shoreline (0.5 ± 0.4 cm/s) were similar to those observed at the neighboring Control site (0.3 ± 0.3 cm/s), and velocity attenuation rates were slightly higher from the channel to the Control shoreline (Fig. A3a). Shoreline velocities in the
Effects of breakwater structure and vegetation on shoreline hydrodynamics
The aim of this study was to detect early hydrodynamic changes following implementation of a living shoreline. The sites were not replicates and were selected to assess impacts of the restoration. Immediately following restoration, shoreline velocities in the Restored site decreased considerably (mean 62% decrease, Fig. A3b). As vegetation densities immediately after restoration were low (three plants per m length of breakwater structure), vegetation was unlikely to have significantly altered
Conclusion
This is the first before-after controlled study of early hydrodynamic impacts attributed to living shoreline stabilization. Changes to shoreline vegetation, hydrodynamic environment, and benthic sediments were observed over a 16-month period that included implementation of a living shoreline (comprised of an oyster shell breakwater structure and planted marsh grasses and mangroves) and its performance in the wake of an extreme event weeks after implementation. Immediately after stabilization,
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge Iris Peterson and Samantha Maldonado for sediment data analysis, Spencer Shannon for his review, and Barbara Nogueira Tirado, Arash Aliabadi Farahani, and Samantha Maldonado for field assistance. Funding for this research was provided by the US National Science Foundation, grant number 1617374.
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