AGU18 presentation: An impact-based assessment of riverine flooding from compound events at the global scale

Current global flood risk studies are based on either fluvial or coastal flood drivers; potentially underestimating total flood risk as the dependencies between these drivers are ignored. In this contribution a global impact-based assessment of riverine flooding from compound events is presented. These events occur when: 1) high sea water levels propagate up a river pushing up water levels in the river; or 2) the drainage of high river discharge is limited by elevated sea water levels.

In order to assess the riverine flood impact of compound events we use a numerical experiment with a coupled state-of-the art global fluvial flood model (CaMa-Flood) and global tide and surge model (GTSM). For all rivers with an upstream area larger than 1000 km2, the downstream sea level boundary in CaMa-Flood is set based on time series of nearshore water levels from GTSM. In the experiment we simulate fluvial flooding based on discharge and tide (case 1); discharge climatology and tide and surge (case 2), and combined discharge and tide and surge (case 3). In case 1 the surge component in the sea level boundary is ignored while in case 2 the peak discharge is ignored in the CaMa-Flood forcing. Flood impact metrics for flood extent and volume as well as people and GDP affected are calculated for low lying delta areas and compared between the individual (1 and 2) and compound flood (3) cases to determine the additional impact from compound flooding. All runs are based on 35 years of consistent forcing data from the EartH2Observe and ERA-Interim datasets.The uncertainties in the model approach are assessed through validation against observations of the timing and magnitude of discharge and surge peaks as well as the dependence between both. Furthermore, the sensitivity of the coupled model to the timing of surge and discharge peaks is analyzed by looking at an ensemble of model forcings with a spread in discharge and surge peak timing.This analysis will identify the delta regions most affected by compound flood events.