Hydrodynamic Zone of Influence Due to a Floating Structure in a Fjordal Estuary—Hood Canal Bridge Impact Assessment
2. Materials and Methods
2.1. Oceanographic Data Collection
2.2. The Salish Sea Model Set-Up
2.3. Hood Canal Bridge Module Implementation
- Implementation of a velocity block: This approach is identical to that described by Khangaonkar and Wang (2013)  where the impermeable surface block was incorporated into FVCOM with modification of both external and internal modes of the solver. For the baroclinic internal mode, the horizontal velocities at the selected cells and surface layers were always specified as zero such that no horizontal flow was allowed to pass through. During the barotropic external mode calculations, the cross-sectional water column depth at selected cells occupied by the block was adjusted to a new reduced value by subtracting the blocked layer thickness from the total water depth. This modification accommodates the presence of the rigid structure but is an approximation, as non-hydrostatic components of the pressure term, which are likely to be strong in the near field, are neglected. Effects of the bridge on momentum terms are addressed, but are done so as an indirect effect of setting the surface boundary to zero velocity without affecting the pressure term.
- Implementation of momentum sink at the bridge using form drag: In this approach, the cells occupying the bridge are populated with hypothetical cylinders similar to a densely packed kelp farm. The drag from the cylinders set to sufficiently high value results in blockage of nearly ~95% of surface currents. Although this represents a leaking bridge, the implementation allows effects on continuity, as well as momentum, terms of the governing equations. The implementation of form drag from suspended cylinders in the water column was described by Wang et al. (2013) . This method also requires local modification of the bathymetry to a representative average depth under the bridge for representation of the rectangular shape of the bridge pontoons.
- Free surface pressure modification with a bottom drag: This method relies on modification of the free surface pressure boundary condition; an increase in pressure equivalent of 4.57 m of head results in a model response of 4.57 m depression of the free surface. This method is an improvement over (a) and (b) in that bathymetry is unaltered. In addition to modifying the free surface, the method also employs drag formulation for the layer immediately under the bridge. This results in flow passing the bridge under modified pressure with suitable reduction in velocity induced by the form drag.
2.4. Near-Field Model Validation
3.1. Zone of Impact—Surface Layer
3.2. Zone of Impact—Vertical Transect
Zone of Influence—Quantitative Assessment
Conflicts of Interest
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|Mean Error (Bias)||RMSE||WSS|
|Variable||Max. (Δ)||South HCB||North HCB|
|ZOI (km)||ZOI (km)|
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Khangaonkar, T.; Nugraha, A.; Wang, T. Hydrodynamic Zone of Influence Due to a Floating Structure in a Fjordal Estuary—Hood Canal Bridge Impact Assessment. J. Mar. Sci. Eng. 2018, 6, 119. https://doi.org/10.3390/jmse6040119
Khangaonkar T, Nugraha A, Wang T. Hydrodynamic Zone of Influence Due to a Floating Structure in a Fjordal Estuary—Hood Canal Bridge Impact Assessment. Journal of Marine Science and Engineering. 2018; 6(4):119. https://doi.org/10.3390/jmse6040119Chicago/Turabian Style
Khangaonkar, Tarang, Adi Nugraha, and Taiping Wang. 2018. "Hydrodynamic Zone of Influence Due to a Floating Structure in a Fjordal Estuary—Hood Canal Bridge Impact Assessment" Journal of Marine Science and Engineering 6, no. 4: 119. https://doi.org/10.3390/jmse6040119