THE EFFECTS OF EDGE EROSION ON LONG-TERM SUSTAINABILITY OF THE GREAT MARSH

In a regime of accelerating sea-level rise the sustainability of the Great Marsh in northern New England is a function of marsh edge erosion and the ability of the marsh to accrete vertically. To study these processes, geotechnical field measurements were taken to quantify the resistance of the marsh edge to wave and current-induced retreat. Peat sampling along the face of the marsh at 30 locations provided a means of measuring shear strength, bulks density, percent biomass, and percent organic content of peats. The highest organic content occurs in the middle depths of the peats, between 0.5 to 1.0 m for peat edges ranging from 0.8 to 1.2 m high. Shear strengths ranged from 4 to >40 kPa with highest shear strengths measured at the surface of the marsh where live grasses have abundant and strong roots. High shear strengths also coincide with accretionary sites where marsh grasses are colonizing tidal flats. Slumping or eroding marsh edges typically have lower shear strengths. Organic and inorganic sediment released from eroding marsh edges is a major source of suspended sediment within the estuary. This suspended sediment is then available for deposition on the marsh platform during high tide, suggesting that the vertically accreting marsh platform benefits from edge erosion.