Flood hazard mapping techniques with LiDAR in the absence of river bathymetry data

In many areas of the world, flood risk assessment is either out of date or completely lacking. In Quebec (Canada), one of the challenges to map flood risk is the very large territory combined with very few datasets on river bathymetry, which are required to run hydraulic models. The objective of this study is to assess the precision and accuracy of 2D flood hydraulic modelling exclusively based on LiDAR elevation data which do not include information on in-channel river bathymetry. Hydraulic simulations (HEC-RAS 5.0) are carried out, for discharges of 20-, 100- and 500-year recurrence intervals, using two techniques that do not require bathymetry data, either subtracting discharge of the LiDAR survey from the flood discharge or estimating flow depth from the water surface slope. These techniques are compared to a traditional approach using bed topography obtained from detailed field surveys, on two long reaches (several kilometers). Sensitivity tests were conducted to assess the impacts of the main sources of error on simulated flood levels. Results show that both techniques can be applied with limited introduction of error in the modelled flood stages, and that errors are greatly reduced if calibration data are available.