Variations in flood magnitude–effect relations and the implications for flood risk assessment and river management

• Flood magnitude–impact literature synthesised
• Original examples of flood impact variability at sites in two contrasting regions
• High variability of impacts at a site over time demonstrated
• Factors influencing temporal variation in flood impact analysed
• Framework for assessing these factors in river management provided
• Impact variability and feedback effects need to be incorporated in river management.

In spite of major physical impacts from large floods, present river management rarely takes into account the possible dynamics and variation in magnitude–impact relations over time in flood risk mapping and assessment nor incorporates feedback effects of changes into modelling. Using examples from the literature and from field measurements over several decades in two contrasting environments, a semi-arid region and a humid–temperate region, temporal variations in channel response to flood events are evaluated. The evidence demonstrates how flood physical impacts can vary at a location over time. The factors influencing that variation on differing timescales are examined. The analysis indicates the importance of morphological changes and trajectory of adjustment in relation to thresholds, and that trends in force or resistance can take place over various timescales, altering those thresholds. Sediment supply can also change with altered connectivity upstream and changes in state of hillslope–channel coupling. It demonstrates that seasonal timing and sequence of events can affect response, particularly deposition through sediment supply. Duration can also have a significant effect and modify the magnitude relation. Lack of response or deposits in some events can mean that flood frequency using such evidence is underestimated. A framework for assessment of both past and possible future changes is provided which emphasises the uncertainty and the inconstancy of the magnitude–impact relation and highlights the dynamic factors and nature of variability that should be considered in sustainable management of river channels.