SITE CONDITIONS
Topography and Bathymetry
The project site features distinct topographic and bathymetric characteristics that heavily influence the proposed design alternatives from a coastal engineering perspective. The site is situated on the eastern edge of the Great Bahama Bank and fronts the Atlantic Ocean. On the Atlantic Ocean side, water depths along the ‘iron shore’ are approximately 30 feet but increase rapidly. On the bank side, water depths are typically 5 to 10 feet and only very gradually increase to the west. Figure 17 displays a three-dimensional representation of the topography and bathymetry at the project site. The site is characterized by limestone rock cliffs fronting the Atlantic Ocean with varying widths along the centreline of the existing road. The narrowest portion of the island is located at the existing bridge.
Figure 17 - Digital Elevation Model (DEM) of site
The existing bridge and road presently experiences hazardous conditions when the site is exposed to large wave events. These large wave events may originate from offshore weather patterns, ultimately reaching the site during fair weather conditions locally. The large incoming waves result in overtopping at different sections of the island and existing bridge. An example of the overtopping conditions is shown in Figures 18 to 20 below. An analysis of the existing topography and observed overtopping from various sources was conducted to identify potential areas of concern as it relates to hazardous conditions, namely locations with relatively narrow width and/or lower elevations.
Figure 18 - Overtopping at project site
Figure 19 - Overtopping areas
Figure 20 - Cross-sectional view of overtopping areas
COASTAL EVALUATION OF BRIDGE
The selected alignment is located approximately 60 ft west of the existing bridge and at an elevation of approximately 53 feet above sea level. This elevation results in reduced wave loading on the bridge deck. Wave spray will be further limited due to the western offset of approximately 60 ft, which results in an increase between the wave breaking location and new structure, thus limiting wave spray potential. Depending on the wind conditions, spray is still anticipated to occur under extreme conditions, but the probability of occurrence is lower. The numerical modeling of overtopping for the new structure is represented in Figure 25 to Figure 28 below.
Figure 25 - 3D representation of structure in numerical model looking west
Figure 26 - 3D representation of structure in numerical model looking east
Figure 27 - Overtopping condition of island looking west
Figure 28 - Overtopping condition of island looking east
COASTAL LOADING
The coastal loading on the different components of the proposed bridge configuration is calculated in the following sub-sections.
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3.4.1 Proposed Structures
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The locations of each cross-section configuration are presented in Figure 32. The details of each configuration are presented in the following.
Figure 32 - Locations of the different pier configurations
Section 1
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The foundation of the proposed structure is proposed to be a shallow foundation with a maximum height of 1.5m (5ft).
Figure 33 - Section 1 pier configuration
Section 2
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The foundation of the proposed pier structure is proposed as circular columns, each with a diameter of D = 1.2m (4ft) and maximum height of H = 2.1m (7ft). Each column will be supported on a single drilled shaft with a diameter of 1.2m (4 ft) and a length of at least 25 ft below the top of rock. While each column is subjected to different flow velocities and water depths, the proposed design loading will be determined based on the column with the highest exposure.
Figure 34 - Section 2 pier configuration
Section 3
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The foundation of the proposed pier structure is determined to be a 4.2m (14ft) long and 2.1m (7ft) wide single elliptical column supported on drilled shafts.
Figure 35 - Section 3 pier configuration