Wave runup estimations on platform-beaches for coastal flood hazard assessment

Wave runup in a shore platform environment has been acquired by in situ measurements at high tide and along field debris following the December 6, 2010, flood near Rimouski (Quebec, Canada), south coast of the St. Lawrence estuary. Using offshore wave data and beach slopes, a linear empirical runup relationship has been adjusted to the study site and showed good predictive results. Two types of beach slopes, the upper foreshore and foreshore slopes, have been assessed in order to calculate the surf similarity parameter. It appears that the foreshore slope, located between the coastline and the end of the foreshore at the seaward edge of the platform, shows best results. This slope was thus used to calculate the wave runup on the shore platform. A static flood map, adding wave runup to observed peak tidal level including the storm surge during the December 6, 2010, flood, has been realized on a LiDAR base layer for a specific site in Sainte-Luce. A second flood map has been applied to validate the approach on an external site in Sainte-Flavie, outside the study area. In both cases, predicted flood extents highly correspond to observed flooded area. High underpredictions occur when using the observed storm tide level only (a 182-year event) without runup, where only 5 and 4 % of the studied area are considered as flooded, respectively. It appears that in a shore platform environment, a wider foreshore reduces wave runup and flood levels.