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Impacts of 150 Years of Shoreline and Bathymetric Change in the Coos Estuary, Oregon, USA

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The diverse users of Oregon ’s Coos estuary share a need to better understand water circulation and sediment transport in the estuary under present and future conditions. A collaborative research project led by the University of Oregon and South Slough National Estuarine Research Reserve filled data gaps and modeled how estuarine circulation and sediment respond to perturbations due to both natural and human-induced causes—such as dredging or inundation caused by sea level rise.

About this article

This journal article discusses changes to the Coos estuary from 1865 to the present, including the deepening of its primary navigation channel, a decrease in estuary area, and altered tidal and salt dynamics with implications for estuarine habitats. The authors use a hydrodynamic model and detailed bathymetric dataset compiled from multiple sources to evaluate present and future impacts.

Abstract

Estuaries worldwide have evolved over the past few centuries under development activities like dredging and shoreline reclamation, which commonly lead to increased channel depths and reduced intertidal areas. The Coos Estuary offers a useful example of how these changes, common to diverse global estuaries, have altered tidal and salt dynamics, with implications for estuarine habitats. In the past 150 years, the primary navigation channel has been deepened from ~ 6.7 to 11 m, generating a 12% decrease in estuary area and 21% increase in volume. To evaluate the present and future impacts on the Coos and similar estuaries, a hydrodynamic model was implemented using a detailed bathymetric dataset compiled from multiple data sources including agency charts, water-penetrating lidar, and single-beam-sonar small-vessel surveys. The model was then re-run using grids constructed from 1865 survey data and a future proposed dredging plan. Changes in the hypsometry from 1865 to present have driven a 33% increase in tidal amplitude, an 18% increase in salinity intrusion length, a doubling of the subtidal salt flux, and an increase in ebb dominance of currents. A proposed channel-depth increase from 11 to 14 m is predicted to generate a negligible change in tidal range and a small increase in the salinity intrusion length. These results highlight the utility of curating high-resolution bathymetric datasets for coastal management applications through modeling. The historical and modern models quantify how local bathymetric modifications can significantly alter tidal and salinity regimes and provide context for estuarine response to global climate-change drivers.

Citation

Eidam, E.F., D.A. Sutherland, D.K. Ralston, B. Dye, T. Conroy, J. Schmitt, P. Ruggiero, and J. Wood, 2020: Impacts of 150 years of shoreline and bathymetric change in the Coos Bay Estuary. Estuar. Coasts, doi:10.1007/s12237-020-00732-1.