Geophysical investigation of coastal roads vulnerable to erosion, Bay Bulls, Newfoundland and Labrador

Arshian, Marzieh (2023) Geophysical investigation of coastal roads vulnerable to erosion, Bay Bulls, Newfoundland and Labrador. Masters thesis, Memorial University of Newfoundland.

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Abstract

In the town of Bay Bulls, on the east-facing coast of the Avalon Peninsula, Newfoundland, coastal roads run close to the steep, rocky shoreline on both sides of a deep bay. Three sections of road, each about 100 m in length, have been identified as vulnerable to erosional processes. To image the surface and subsurface structure in these three areas, two geophysical methods: ground penetrating radar (GPR), direct current resistivity/induced polarization (DCR/IP), and real-time kinetics global positioning system (RTK) were used. The primary aims of this study were to characterize the subsurface in these three vulnerable areas and to test the utility of the geophysical methods in identifying structural weaknesses. On the north side of the bay, at “Bread and Cheese”, the road dips down over a culverted creek in a highly fractured area. At “the Cliff”, the road has been widened bay-ward with the use of a wooden retaining wall. “The Quays” is on the south side of the bay, where a narrow inlet with steep cliffs comes very close to the road. At all three sites, RTK data revealed locations where the road surface sloped bay-ward, suggesting undesirable creepage in the roadbed. Also, at all three sites, DCR data from deeper levels showed broad (10s of m) variations in the steeply dipping strata of the bedrock, possibly related to their ease of erosion. At Bread and Cheese, GPR showed depth to bedrock and the horizontal extent of the weaker, fractured region, while DCR measurements suggest the most fractured location is to the west of the culvert. At the Cliff, GPR profiles and 3D imaging identified the locations and lengths of wooden beams extending from the retaining wall under the widened road. At the eastern parts of the Quays, GPR identified shallow bedrock following the along-road topography of the road. To the west, the road appears to cover sediments. Analysis of the geophysical data and geological information indicates that the hard bedrock and the shape and orientation of the bay mean that wave action is not a major factor affecting the stability of coastal roads and that the overland flow of water and groundwater is more of a concern at the three vulnerable sites.

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