Critical width of tidal flats triggers marsh collapse in the absence of sea-level rise

Abstract

High rates of wave-induced erosion along salt marsh boundaries challenge the idea that marsh survival is dictated by the competition between vertical sediment accretion and relative sea-level rise. Because waves pounding marshes are often locally generated in enclosed basins, the depth and width of surrounding tidal flats have a pivoting control on marsh erosion. Here, we show the existence of a threshold width for tidal flats bordering salt marshes. Once this threshold is exceeded, irreversible marsh erosion takes place even in the absence of sea-level rise. This catastrophic collapse occurs because of the positive feedbacks among tidal flat widening by wave-induced marsh erosion, tidal flat deepening driven by wave bed shear stress, and local wind wave generation. The threshold width is determined by analyzing the 50-y evolution of 54 marsh basins along the US Atlantic Coast. The presence of a critical basin width is predicted by a dynamic model that accounts for both horizontal marsh migration and vertical adjustment of marshes and tidal flats. Variability in sediment supply, rather than in relative sea-level rise or wind regime, explains the different critical width, and hence erosion vulnerability, found at different sites. We conclude that sediment starvation of coastlines produced by river dredging and damming is a major anthropogenic driver of marsh loss at the study sites and generates effects at least comparable to the accelerating sea-level rise due to global warming.salt marsh boundary erosion | wave erosion W ave-induced boundary erosion is a leading process threatening salt marshes (1, 2), but it is remarkably unexplored compared with the vertical dynamics of the marsh platform (3, 4). Wave-induced boundary erosion is particularly relevant along coastlines with limited subsidence such as the Mid-Atlantic coast of the United States, where large marsh areas are deteriorating (5, 6) despite marsh accretion keeping pace with contemporary rates of sea-level rise (7,8). Here, we focus on the evolution of three salt marsh sites on the US Atlantic Coast, subjected to different rates of wave-induced boundary erosion: Cape May, NJ, Virginia Coast Reserve, VA, and Charleston Sound, SC (Fig. 1). All sites are characterized by barrier islands sheltering shallow bays with extensive salt marshes and tidal flats. The bays are connected to the open sea by multiple inlets, experience limited direct riverine inputs (9, 10), and are subject to similar wind conditions (SI Text). Relative sea-level rise (RSLR) is on the order of 2 mm/y and tidal range of ∼1.4 m (SI Text). These embayments are characterized by rounded tidal flats surrounded by salt marshes, which are referred to as marsh basins (11,12).Stevenson et al. (13) reported loss of brackish marshes driven by the enlargement of marsh basins, referred to by the authors as ponds. They suggested the existence of a pond threshold width that, once exceeded, leads to ponds widening by wave-induced boundary erosion. Here, we expand this idea by (i) developing a physical...

Physical and biogeochemical controls on light attenuation in a eutrophic, back-barrier estuary

“…Conversely, site LEI sh is on the landward side of the estuary adjacent to the mainland, which is fringed by extensive marsh. These marshes represent a local source of fine sediment that can be released during marsh collapse under wave forcing (Mariotti and Fagherazzi, 2013;Ganju et al, 2013). …”

Salt-marsh evolution at Northwick and Aust warths, Severn Estuary, UK: a case of constrained autocyclicity.

“…A new marsh may be initiated on the mudflat provided that some of the preceding marsh with viable plants survives at the scarp foot. Mariotti and Fagherazzi (2013) and Fagherazzi et al (2013) studied salt marshes along the US Atlantic coast and provide evidence that sediment starvation also promotes the initiation of salt marsh erosion.…”

Marsh Collapse Does Not Require Sea Level Rise

“…However, Feagin et al (2009) In fact, high inputs of sediment can counteract very fast rates of sea level rise (Yang et al, 2001). If the rate at which waves and currents are removing sediment from the marsh boundary is higher than the rate at which sediment is provided by rivers and by the adjacent sea or continental shelf, the marsh will enter into an erosive state, and this state can be irreversible even in absence of sea level rise (Mariotti and Fagherazzi, 2013).…”