River Channel Change Can Affect Flood Hazard and Risk as Much as Climate

Over a billion people globally are already exposed to the risk of flooding, but by 2050 this number is expected to double due to human-induced climate change, population growth, and encroachment into at-risk areas. Global Flood Models (GFMs) are vital tools for producing flood hazard maps supporting impact estimates and policy interventions. These GFMs represent river channels by typically assuming that the bankfull flow-carrying capacity equates to a river flow with a specified return period (RP) that is spatially and temporally invariant. However, bankfull capacity is determined by channel size, shape and roughness and so varies in response to erosion and sedimentation. To quantify the extent to which channel variability biases GFM predictions here we employ a typical GFM, the Fathom model, to a 135,000 km2 region of the Mississippi floodplain in a sensitivity analysis that evaluates how inundated areas and associated population exposures respond when forced with empirically-derived bankfull capacities. Our results show that since the typical RPs (< 1 year) of these present-day bankfull flows differ from the 2-year value normally assumed in GFMs, substantial underestimates of flood extent (9 to 59%, depending on flood magnitude) and populations exposed (15 to 118%) result. We also show that, over multi-decadal timescales, changes in past channel morphology are, depending on emissions scenario, of equal or greater importance in driving changes in simulated flood hazard and risk than changes in future climate. The evolution of bankfull capacity through space and time is therefore a first order control on flood hazard and risk, meaning it is vital that river channel variability and change is represented accurately in GFMs.