ASSESSING THE EXTENT AND IMPORTANCE OF SEDIMENTATION FROM A 100 YEAR WINTER STORM ON SALT MARSH RESILIENCE (Invited Presentation)

An unprecedented sedimentation event took place in early January 2018 on New England marshes resulting from the January 4^th 2018 “Bomb Cyclone”. On the Great Marsh, northern MA, thick deposits were left by rafted ice, covering football field-size areas of the marsh platform. Ice rafting is an important secondary sedimentation process that redistributes sediment from tidal flats, channel beds, and ponds to the vegetated marsh surface in northern temperate climates. A period of 13 consecutive days with temperatures between 0 and -19 C led to thick ice accumulation in channels and bays, followed by highly unusual meteorologic and astronomic conditions over the New Year 2018. A powerful extratropical cyclone underwent a rapid drop in pressure referred to as bombgenesis . The associated storm surge was 0.95 m, which occurred during high tide on one of the highest tides of the year. Water levels at the Boston tide gauge reached 3.036 m above MSL, exceeding the 1% annual exceedance probability (3.02 m to MSL). Ice that had incorporated sediment from tidal flats and channels was lifted onto the marsh platform, and stranded when the water dropped, often in sites that normally receive minimal inorganic sedimentation. When the ice melted widespread layers of silty sand, mud, and peat were left across the marsh. Orthoimages obtained in late February have allowed digital mapping of storm deposits, and assessments using automated image analyses provide data on the extent of the deposits. Multiple samples were taken from 25 sites throughout the marsh system and the samples processed for grain size, bulk density and organic content. Vegetation regrowth was monitored at several sites through the spring to determine the maximum deposit thickness that did not result in smothering and plant mortality. The distribution of the deposits suggests that wind and ice-push contributed to ice rafting. Deposits were found throughout the entire marsh, including extensive deposits on the sheltered west-facing shore of the barrier. Average thicknesses varied from 0.5 to 6 cm (N=90) and bulk density ranged from 0.72 -2.43 g/cm³. Many samples contained coarse sands and gravels, or shells, in line with their provenance. The total volume of sediment deposited well exceeds the “normal” ice rafting annual deposit (~5% of annual accretion).