Integrating anthropogenic factors, geomorphological indicators and local knowledge in the analysis of coastal flooding and erosion hazards

The settlement and development of Québec’s maritime coastline has generally taken place without consideration for coastal dynamics and coastal hazards. Consequently, fighting coastal erosion has become a necessity. Until now, the construction of rigid walls and encroachments has been the preferred approach to the problem. In the Chaleur Bay region, coastal communities are particularly vulnerable to erosion and flooding because a number of residential, commercial and transportation infrastructures have been installed on beach terraces and sand spits. Recent storms, such as the storm of December 2, 2005, have made possible a better understanding of how these rigid defence structures amplify the effects of storm surges and waves. These structures drive the sand away from the coast, lowering beach levels and even causing beaches to vanish entirely from the areas situated in front of the defence structures. The end result is a weakening of the natural capacity of these coastal systems to absorb the energy of waves and a greater risk of coastal flooding. An integrated approach using local knowledge on the one hand, along with LIDAR surveys and a DGPS system on the other hand, have made possible to map the levels reached by flooding at the time of the storm. The results indicate that such levels vary greatly in spatial terms and that the difference between the levels actually reached and the water level measured by tide gauge can be as much as 2 m; a difference that is due to anthropogenic perturbations. This raises questions concerning the safety and the reference levels used for mapping coastal flooding risk. Taking into account knowledge of local communities, analysis of water levels, geomorphological indicators and aggravating anthropogenic factors, an approach and basic criteria are put forward with a view of establishing a reference level for the mapping of flood risk that can be used by community land-use planners.