Levee and dike breaching as a restoration tool in coastal wetlands for long-term resiliency to sea level rise

As sea levels rise, a “resilient” coastal wetland can respond in two ways; it can migrate upslope to escape rising water levels (the horizontal solution) or it can trap and accrete sediments to keep pace with the rate of sea level rise (the vertical solution). The two solutions are not necessarily mutually exclusive. The current practice of removing or breaching dikes and levees to restore historic coastal wetlands allows for both solutions; creating a pathway for landward escape and providing for the reintroduction of sediment laden waters, be they tidal, riverine or both, to the restored wetlands. Over the past two decades, using marker horizons, surface elevation tables and Pb210 dating, we’ve measured rates of accretion and elevation change in numerous coastal wetlands of the Salish Sea. From this, we present here two lines of evidence that point towards the potential and efficacy of dike removal as a restoration tool in the face of rising seas. First, in relatively unmodified, un-leveed natural coastal wetlands, open to the subsidizing energies of tides and sediment-rich river water, we consistently measure rates of sediment accretion equal to or in excess of the current rate of local sea level rise, indicating an adequate sediment supply for marsh maintenance. Second, our measurements in coastal wetland sites restored by levee breaching reveal high rates of accretion and elevation gain, far exceeding current and predicted rates of sea level rise. For example, in a recent restoration site in the Stillaguamish River estuary, we measured a mean rate of elevation gain of +3.1 cm yr-1 since levee removal in 2012. In summary, the many active deltaic distributaries of the Salish Sea provide a source of sediments that coastal wetlands, unencumbered by levees and dikes, can and do use to maintain a dynamic equilibrium with sea level rise