Marshes Are the New Beaches: Integrating Sediment Transport into Restoration Planning

Abstract

Recent coastal storms and associated recovery efforts have led to increased investment in nature-based coastal protection, including restoration of salt marshes and construction of living shorelines. In particular, many of these efforts focus on increasing vertical elevation through sediment nourishment, where sediment is removed from the tidal channel and placed on the marsh plain, or preventing lateral erosion through shoreline protection. In the USA alone, millions of dollars have been allocated or spent on these coastal protection solutions over the last few decades because of their perceived sustainability and ecologically positive co-benefits including habitat provision and carbon sequestration. These projects would benefit from integration of sediment transport pathways, budgets, and metrics during planning and modeling of restoration outcomes, in order to evaluate sustainability before investment. This is analogous to the decades of experience with coastal management and engineering on the open coast. Salt marshes are geomorphic features that rely partially on external sediment supply to maintain their network of tidal channels, intertidal flats, and marsh plain. Removing sediment from one component of the overall system to nourish another component may be counterproductive, given that the net sediment budget is unchanged. For example, dredging a tidal channel beyond its equilibrium condition will cause it to fill with sediment from the tidal flat or elsewhere in the system. This may cause slumping of the marsh edge, or over-deepening of other sections of the channel to compensate. Similarly, shoreline protection that prevents edge erosion hampers the marsh plain's ability to accrete on the levee and naturally transgress landward or it starves other components of the system of regularly supplied sediment. A limited vertical or lateral-only perspective, instead of a threedimensional perspective, during project planning and evaluation may lead to suboptimal decision-making regarding restoration priorities, approaches, and outcomes. I contend that before significant investments are made in marsh restoration through sediment nourishment or shoreline protection, sediment transport measurements and models that consider sediment dynamics should be integrated into the early phases of restoration planning. This will help identify where and under what conditions marsh restoration will most likely be successful and economically justified. Triaging and prioritizing is then possible, which is a sustainable approach for restoration, given the persistent vulnerability of marshes to sea-level rise, storms, and sediment deficits.

Understanding tidal marsh trajectories: evaluation of multiple indicators of marsh persistence

“…While an ample sediment supply and substantial elevation gain can clearly enhance marsh persistence in the long-term, the majority of degrading marshes in our study had higher accretion and elevation gain rates on a decadal scale than did their paired higher persistence counterparts. This finding reinforces the notion that caution must be applied to the spatial interpretation and extrapolation of data from SETs, underscoring the need for a holistic marsh assessment (Ganju 2019). Marsh accretion data must be interpreted in the context of spatial location, elevation and vegetation.…”

Salt Marsh Dynamics in a Period of Accelerated Sea Level Rise

“…Detailed sediment budgets can be used by coastal managers to assess the vulnerability of salt marshes (Ganju, 2019). Proxies for sediment fluxes, as for instance the ratio between vegetated and unvegetated area (Ganju et al, 2017), can be easily applied to map marsh vulnerability in the absence of costly field measurements.…”

Watershed Suspended Sediment Supply and Potential Impacts of Dam Removals for an Estuary

“…High deposition rates near the mouth of the Elwha were linked to reductions in macroalgae and changes in the abundance of invertebrate and fish taxa (Rubin et al 2017), and were associated with decreased light availability (Glover et al 2019). However, increased sediment inputs can also be beneficial to a coastline, particularly in regions with limited sediment supply, where sediment from dam removals can help mitigate shoreline erosion and promote marsh resilience (Ganju 2019;Ganju et al 2013). For the Elwha, the shoreline near the river mouth was erosional prior to dam removal, but increased sediment supply reversed this trend and led to coastal accretion (Warrick et al 2019).…”

The Sensitivity of a Dike-Marsh System to Sea-Level Rise—A Model-Based Exploration

“…On the flipside, adding sediment through suppletions can also be a viable addition to flood adaptation strategies [40,41]. The suppleted sediment has to accumulate on the marsh for longer time-scales and not degrade other areas of the system [40,78]. Doing so effectively requires extensive knowledge of the local hydrodynamics and morphology.…”

Carbon Sources in the Sediments of a Restoring vs. Historically Unaltered Salt Marsh

“…This study suggests that the range of projects could be expanded to less desirable, subsided coastal areas if a longer unvegetated stage than is typical of salt marsh restorations were acceptable and expected in the restoration trajectory. Such projects would clearly benefit from an assessment of sediment dynamics during restoration planning to assure that local conditions could support marsh building (Ganju 2019). Although these types of projects would likely take longer than usual to reach parity with reference salt marshes, the tradeoff may be worthwhile if society could start benefiting from a reduction in carbon pollution early in the restoration timeline.…”