Mitigating beach loss : an investigation of foreshore erosion at Royal Roads Bay, Colwood, Canada

Abstract

The City of Colwood, British Columbia, Canada, has struggled with the eroding coastline of Royal Roads Bay as a result of a sediment source deficit. The area suffers from strong longshore currents which extract the fine sediments of the sandy foreshore provided by the Metchosin Gravel Pit, which has been a source of sediment supply to the beach for over 100 years; operations ceased in 2007. The study site, a 600 m stretch of beach between the gravel pit and the sewage pump station at Lagoon Road, had not previously been studied quantitatively. This study was undertaken in order to ascertain the rate and causation of erosion, the future expectations of erosion, and provide appropriate mitigation strategies for the City of Colwood. A series of seven transect surveys were taken using a total station theodolite. The first survey was undertaken in July 2018, the second in November 2018, and the third in January of 2019. Data provided insight into the changing slope of the beach profile throughout the mild summer and fall compared to the storm intensive winter. Data indicated that erosion is more evenly distributed across the profile of the beach during the calmer summer months as a result of chronic sediment loss. During the stormy winter months, episodic erosion events occur as a result of high intensity wave impacts. A steepening of the beach is prominent. Wave run up causes the backshore sediments to become unstable and erode. Paired T-tests with a 95% confidence interval were used to determine the significance of the data and provide conclusive evidence that changes were occurring. The difference in change of volume produced a p-value of 0.0936, which is not quite statistically significant yet supports the hypothesis of the rate of erosion. Based on an average yearly erosion rate of 0.429 m3m-2, immediate action is necessary. The study produced a usable set of strategies that are appropriate based on the rate and causation of erosion to inform the City of Colwood as a decision is made to mitigate erosion.