Abstract
Floating structures such as barges and ships affect near-field hydrodynamics and create a zone of influence (ZOI). Extent of the ZOI is of particular interest due to potential obstruction to and impact on out-migrating juvenile fish. Here, we present an assessment of ZOI from Hood Canal (Floating) Bridge, located within the 110-km-long fjord-like Hood Canal sub-basin in the Salish Sea, Washington. A field data collection program allowed near-field validation of a three-dimensional hydrodynamic model of Hood Canal with the floating bridge section embedded. The results confirm that Hood Canal Bridge, with a draft of 4.6 m covering ~85% of the width of Hood Canal, obstructs the brackish outflow surface layer. This induces increased local mixing near the bridge, causes pooling of water (up-current) during ebb and flood, and results in shadow/sheltering of water (down-current). The change in ambient currents, salinity, and temperature is highest at the bridge location and reduces to background levels with distance from the bridge. The ZOI extends ~20 m below the surface and varies from 2–3 km for currents, from 2–4 km for salinity, and from 2–5 km for temperature before the deviations with the bridge drop to <10% relative to simulated background conditions without the bridge present.
Highlights
Hood Canal is a fjordal sub-basin within the Salish Sea region of Pacific Northwest
We present an assessment of the near-field impact of the floating bridge on the tidal hydrodynamics in the Hood Canal fjord environment as a component of the Hood Canal Bridge Environmental Impact Assessment study [9]
While some effect was expected due to the existence of a large floating object in the path of the tidally influenced fjord-like waterbody of Hood Canal, the extent of influence was not quantified in the past
Summary
Hood Canal is a fjordal sub-basin within the Salish Sea region of Pacific Northwest. In the spring and summer, many Salish Sea sub-basins regularly experience algae blooms, and some of the sub-basins such as Hood Canal, East Sound, and regions of South Sound show signs of hypoxia [1]. Of particular interest was the ability to simulate low dissolved oxygen (DO) events in the Salish Sea, including those responsible for fish kills and other chronic impacts in the Hood Canal region. Numerous studies were conducted in the past to determine what contributes to low DO events in Hood Canal, such as natural meteorological and oceanographic conditions, as well as anthropogenic causes, such as excessive nutrient loading [5,6]
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