Global flood exposure simulations show that about 600 million people, of which 320 million in urban areas, are at risk to the impacts of global sea-level rise and changing storm intensity and frequency. Increasing availability of data and computational efficiency enables to assess flood hazard on a global scale. Implementation of salt marshes and mangroves as nature based flood defences has gained strong interest in the field of hydraulic engineering during the last decade, because these ecosystems can be an innovative, sustainable and cost-effective supplement to manmade traditional coastal protection measures. This thesis focuses on wave damping by salt marshes and mangroves and aims to map flood hazard reduction by foreshore vegetation on a global scale.

Wave attenuation by foreshore vegetation is accessed using transects which are aligned perpendicular to the Open Street Map (OSM) coastline. The study area contains in total 495361 transects, which have an intermediate distance of roughly 1 km and are individually assessed. The assessment of each transect involves construction of the foreshore profile, derivation of vegetation presence and the corresponding vegetation type, determination of the governing hydrodynamic conditions and computation of wave propagation over the foreshore. Wave damping by foreshore vegetation is determined by comparing the wave height at the end of the vegetated zone with a bare foreshore situation. The analysis is based on topobathymetric data from the Global Interidal Elevation map (20 – 30 m resolution, 1 m vertical accuracy) and a merged set of MERIT elevation data (3 arc-seconds resolution, 2 m vertical accuracy) and GEBCO bathymetry data (30 arc-seconds resolution, 10 m vertical accuracy). Vegetation presence is based on the VegGEE map (resolution 10 -30 m) and the cover type is based on the Salt marsh map, Mangroves, GlobCover or CLC. Wave characteristics are derived from ERA-Interim reanalysis and extreme levels from the Global Tide and Surge Model (GTSM). The nearshore wave height is based on a depth limited criterion and wave propagation over the derived foreshore is determined using a dataset of XBeach pre-runs.

The results of the global assessment show that 31% of the coastline in the study area (between the Arctic circle and -60 degrees South), is vegetated, of which more than half has a vegetation width that is larger than 100 m and 5% has a vegetation width exceeding 1000 m. Along 5% (15%) of the studied (vegetated) coastline is a significant wave height reduction observed of at least 30 centimetres. The performed global flood hazard reduction assessment is unique, as it uses global open source data to express quantitatively wave damping by salt marshes and mangroves on a global scale. The results show the effect of foreshore vegetation in terms of wave attenuation, reduced dike height and the potential social impact. Based on the performed analysis can be concluded that foreshore vegetation has a high potential to mitigate flood hazard at various areas around the globe.