5-4 Hydrodynamic and Sediment Transport Impacts of a Historic Inlet Opening under Storm and Sea-Level Rise Scenarios: Delft3D Simulations of Nantucket Harbor

Session: Nearshore and Estuarine Research: Dynamics and Future Resiliency in the Coastal Zone (Posters)

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Abstract:

Tidal inlets exert a fundamental control on hydrodynamics, sediment exchange, and long-term morphological evolution of barrier-harbor systems. Along the Nantucket coast of MA, Haulover Inlet opened during an extreme storm in 1896 and persisted for over a decade before closing by spit accretion. Renewed interest in the potential reopening of Haulover Inlet raises important questions regarding its influence on present-day and future harbor dynamics, particularly under accelerating sea-level rise (SLR) and increasing storm intensity.

In this study, we apply a coupled Delft3D-FLOW and Delft3D-WAVE modeling to evaluate hydrodynamics and sediment transport within Nantucket Harbor under varying inlet configurations (Haulover opened and closed), SLR scenarios (0.53 m and 1.01 m), and storm forcings (November 2022 extratropical storm conditions). Model performance is validated against field observations from HOBO water-level loggers, Acoustic Doppler Current Profilers (ADCPs), and an RBR wave sensor, yielding strong agreement for water levels (skill ≈0.96), currents (0.73-0.79), and wave heights (≈0.64).

Results demonstrate that reopening Haulover Inlet fundamentally alters tidal exchange pathways within the harbor. Under present sea level and normal tidal conditions, an open Haulover Inlet reduces tidal range and increases current velocities in the northeastern part of the harbor. At the same time, the reduced tidal prism exchanged through the jetties decreases peak velocities in the channel by up to ~25%. During storm conditions, flood dominance intensifies near the reopened inlet, enhancing landward transport, while peak velocities at the jettied inlet are reduced. Under existing conditions, sediment flux calculations across multiple transects reveal the harbor is flood-dominant, explaining ongoing dredging requirements. Moreover, when Haulover Inlet is opened, sediment is exported detailing why the historic inlet remained opened for a decade.

High-SLR simulations reveal an emergent tendency for inlet formation near Coskata Pond (vestige of former Haulover Inlet) driven by elevated water levels, overwash, and redirected tidal fluxes, which highlights additional geomorphic vulnerabilities under future elevated tidal conditions. Sediment flux calculations indicate that the harbor is strongly flood-dominant and imports sediment. In contrast, modeling shows that reopening Haulover Inlet results in net sediment export, providing an explanation for the persistence of the historic inlet.

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