Sediment dynamics and hydrographic conditions during storm passage, Waquoit Bay, Massachusetts

- Storm events have impacted Waquoit Bay for 4000 years leaving a geologic signature of their passage.
- There are several natural and anthropogenic factors that have impacted sedimentation patterns within Waquoit Bay.
- There are multiple sediment sources and transport pathways that operate during storm passage within shallow water lagoons.
- Waquoit Bay is sensitive to storm passage recording both strong and week hurricanes as well as extratropical storm events.
- Age model uncertainties and site oversensitivity prevents the determination of long-term hurricane frequencies.

The impact of storm events on the sediment dynamics of the shallow groundwater fed estuaries of Cape Cod, Massachusetts, USA is little understood. To address this, the objectives of this study are to assess sediment dynamics during storm passage and determine whether shallow back-barrier lagoons like Waquoit Bay have the preservation potential for a sedimentary archive of hurricanes. When setting out in this study, it was unclear whether paleotempestological methods could be applied successfully to cores collected from the landward reaches of shallow estuaries of southern New England. Water level and bottom current data using Arm-and-Float tide gauges and SeaHorse Tilt Current Meters was collected during Tropical Storm Irene (2011) and coupled with storm surge modeling projections to better elucidate storm-induced sediment transport mechanisms. Three sediment cores were collected at the head of Waquoit Bay, located 2.8 km from the barrier beach. Grain size analysis of sediment cores was conducted with a laser particle size analyzer at 1 cm increments in order to identify coarse grain anomalies, which can act as a storm event proxy. Bayesian statistics were applied to develop age models of two of the cores based on three Pb pollution chronomarkers and 21 continuous flow 14C AMS ages. The results yield variable sediment accumulation rates between 2 mm/yr to 10 mm/yr, with significantly higher rates occurring in the upper 1 m of sediments. Grain size results are highly variable, and contain numerous large amplitude, short duration fluctuations suggesting that during storm passage coarse sand is deposited in the coring site. The sensitivity of the site to both tropical and extratropical storm events, uncertainties in the age model, and the multiple sediment sources and transport pathways limits the utility of using the Waquoit sediments to determine long-term hurricane frequencies. Results nonetheless provide insights into how extreme storm events impact coastal lagoons.