BOTTOM GRAIN SIZE DISTRIBUTIONS IN PLUM ISLAND SOUND ESTUARY: IMPLICATIONS FOR SUSPENDED SEDIMENT FOR GIVEN SHEAR STRESSES

The resiliency of New England saltmarshes is being questioned due to accelerating sea level rise and limited fine-grained sediment for vertical accretion. Plum Island Sound (PIS) saltmarsh builds vertically through production of belowground biomass and deposition of suspended sediment. In this study, we document grain size distributions throughout PIS using 148 samples from channel thalwegs combined with 225 bottom samples collected previously by Daboll (1969) in the main estuary channel, large and small tidal creeks, point bars, and the flood tidal delta. Samples were analyzed for grain size, sorting, and skewness. We compare existing grain size data from 20, 2-meter-deep sediment cores throughout PIS to our bottom samples to explore marsh sedimentation using hydrodynamic and sediment transport theory.

Stratigraphic studies (McIntire & Morgan 1963; McCormick 1968) indicate that sediment that filled PIS and facilitated initial marsh development consisted primarily of sand and silt that entered the estuary from a seaward source. This model is consistent with present sediment trends within PIS consisting of medium to coarse sand at the mouth fining to muddy silt in the landward small tidal creeks. Grain size trends of the sub-environments from highest to lowest wave and tidal energy include: Tidal Delta Mz = 2.11 Ø, range = 1.28-2.89 Ø; Point Bar Mz = 2.31 Ø, range = 1.28- 3.41 Ø; Main Channel Mz = 2.26 Ø, a range = 1.69-3.41 Ø; Large Tidal Creeks Mz = 2.71 Ø, range = 0.81-6.2 Ø; Small Tidal Creeks Mz = 4.04 Ø, range = 0.76-7.84 Ø.

Theoretical analysis of governing hydrodynamics for PIS shows that sediment entrainment and flux is proportional to the local energy as well as the advection length scale of each particle. As expected, areas with the greatest exposure to coastal storms, in terms of wave energy and surge-enhanced tidal currents, contain the largest grain sizes. Moreover, while abundant mud exists in small creeks flanking the high marsh, energy is usually too low to facilitate its delivery. In contrast, the high energy characterizing the largest channels, while conducive to high sediment entrainment and transport, are devoid of grain sizes capable of traveling long distances and contributing to marsh sedimentation. This conundrum creates challenges for future marsh sustainability and sedimentation models.