Assessing the relative importance of hydrological and geomorphological controls on river flood conveyance at a global scale

Abstract

Climate variability is a significant driver of flood events. However, geomorphological changes in river channels, including variations in local and upstream sediment supply, play a crucial role in determining flood conveyance capacity and flood stage variations. The interplay between hydrology and geomorphology, and their relative impact on flood conveyance, can vary in different river systems depending on both the degree of internal channel dynamics and the nature and magnitude of external forcings. For example, rates of bank erosion, vegetation establishment on bar surfaces, and overbank sedimentation control the time required for floodplain reworking, the adjustment of channel morphology and the associated evolution of river flow conveyance capacity and stage-discharge relations.To investigate the relative significance of hydrological and geomorphological controls on flood-stage variability, we employ a new computationally-efficient model of river and floodplain morphodynamics. This model simulates the evolution of river morphology and flow conveyance capacity by representing the interaction between processes of bank erosion, floodplain construction and river bed-level change over multiple centuries. The simple nature of the model enables its application at large spatial scales – e.g., to explore global variations in the controls on flood conveyance and its sensitivity to future environmental change. Simulated changes in conveyance capacity for a range of environmental settings were evaluated against trends in observed river gauging datasets. Convergent cross-mapping analysis was then applied to investigate the cause-and-effect relationships between controlling factors, including: (i) hydrologic regime; (ii) river sediment load; (iii) floodplain composition (e.g., fine versus coarse sediment); and (iv) lateral river dynamics (e.g., rates of erosion and accretion). Our analysis quantifies the causality between these factors and the resulting variability in river morphology (width and bed level), flood stage and channel conveyance capacity. Results indicate that in dynamic river systems, while the importance of climate-driven hydrological changes in driving conveyance capacity changes are acknowledged, geomorphological changes – specifically, variations in sediment supply and lateral sediment sources – may dominate over climate-driven trends.