Adapting with future increases to coastal and riverine flood risk in deltas and estuaries: a global perspective

Deltas and estuaries concurrently represent some of the most delicate natural environments and highly populated regions of the world. Due to climate change, among other global and local variables (e.g., urbanization, land subsidence, etc.), these areas face unprecedented increases to flood risk over the next several decades. A large portion of total future coastal flood risk is found in the world's deltas and estuaries, while overall global riverine flood risk exhibits more expansive spatial distribution. Still, the presence of high levels of potential human and monetary impacts due to these two types of flooding presents a unique challenge for decision makers worldwide. For these areas of the world to survive, disaster risk reduction action is needed in the form of adaptation. Our work shows the adaptations options for a selection of critical deltas and estuaries throughout the world. We model dykes and levees (structural hazard reduction), saltmarsh preservation and mangrove restoration (nature-based hazard reduction), dry-proofing of urban assets (vulnerability reduction), and zoning restrictions on future urban development (exposure reduction), and quantify the costs and benefits of these adaptation measures. Certain adaptation measures are better suited for some cases depending on the income level, environmental characteristics, and spatial population distribution of the region in question. For example, in the Ganges delta, expanding mangrove forests proves to be the most cost beneficial adaptation strategy. Meanwhile, restricting future urban development away from floodplains in the Congo estuary shows the most potential for flood risk reduction due to large amounts of projected population growth. And highly urbanized deltas and estuaries, such as the Mekong and the Hudson, demonstrate potential risk reductions via widespread dry-proofing. With this work, we are able to identify specific adaptation pathways forward into the future for these crucial areas of the world, while also opening the dialogue for related topics, such as compound flood risk and adaptation hybridization.