Evolution of Coastal Defence Structures and Consequences for Beach Width Trends, Quebec, Canada

Bernatchez, P., and Fraser, C., 2012. Evolution of coastal defence structures and consequences for beach width trends, Quebec, Canada. Journal of Coastal Research, 28(6), 1550-1566. Coconut Creek (Florida), ISSN 0749-0208. Over recent decades, amplification of coastal erosion, coupled with the expansion of the built environment in Quebec's maritime region, has led to a significant increase in defence structures. On unconsolidated coasts, severe beach width reduction was noted following the construction of hard engineered structures when compared to beaches with no such constructions. Beaches in the sectors of Sept-Iles and Perce, where shorelines have been modified by rigid defence structures parallel to the coast, have had a reduction in width of 85% and 44%, respectively. In some places, beaches have almost completely disappeared. On low-lying sandy coasts, although an immediate width reduction of 19% to 27% is attributable to the encroachment of the structures on the upper beach, most of the width reduction (74% to 81%) is due to erosion caused by the reflective nature of the defence structures. The effects of the defence structures on the sediment deficit of the beaches are much worse for coasts with a high level of sediment transit. Such consequences significantly diminish coastal resilience to climate change and storm events because, under such circumstances, beaches cannot absorb nearly as effectively the energy of storm waves. A look at the historical record of coastal construction shows that rigid defence structures parallel to the coast have not only led to beach erosion, but they have also limited the shoreline progradation process. The sediment budget variations of beaches on natural coasts correlate well with relative sea-level trends. An average decrease in beach width of 43% due to sea-level rise was measured on natural coasts. However, the results show that the reaction of a beach to an increase in sea level is much more complex than a simple translation of its equilibrium profile. Beach width is a very good geomorphological indicator for evaluating long-term trends with respect to the sediment budget of coastal systems, as well as evaluating the relative importance of natural and anthropogenic causes of the sediment deficit of beaches.