Hood canal bridge effect on hydrodynamics and nearfield zone of influence

Hood Canal is a deep and long estuarine sub-basin within the Salish Sea that exhibits characteristics of classic fjords. Presence of the Hood Canal Bridge (HCB), a floating barge-like block near the mouth of Hood Canal is under investigation for potential environmental impacts on water quality and pelagic ecosystem. In this study, the effect of HCB on Hood Canal stratification and transport were evaluated using the Salish Sea Model, a 3-D hydrodynamic model with Hood Canal Bridge embedded at a high local resolution. The effects of the bridge as an obstruction to tidal currents and circulation were examined near the structure with the objective of characterizing the nearfield zone of influence (ZOI). This was accomplished through a combination of field measurements, and model simulations. ZOI was defined as the 3-D space near the floating bridge where ambient water properties were noticeably affected relative to background or natural water approaching the bridge during ebb or flood conditions. Field measurements included tides, currents, salinity, and temperature profile time series data collected from locations near the bridge over a 4-week period. These included stations upstream, downstream, and directly below the structure. The model results were in good agreement with the observed data. The simulated results show significant reduction of velocities in the surface layers near the structure. The Effect of HCB on temperature and salinity was also noticeable and extended over a larger zone predominantly during peak ebb and flood periods. HCB blocks the advection of warmer temperature and brackish water from the inner part of Hood …