This paper uses numerical simulation of flood inundation based on a
coupled one-dimensional-two-dimensional treatment to explore the
impacts upon flood extent of both long-term climate changes, predicted
to the 2050s and 2080s, and short-term river channel changes in
response to sediment delivery, for a temperate upland gravel-bed river.
Results show that 16 months of measured in-channel sedimentation in an
upland gravel-bed river cause about half of the increase in inundation
extent that was simulated to arise from climate change. Consideration
of the joint impacts of climate change and sedimentation emphasized the
non-linear nature of system response, and the possibly severe and
synergistic effects that come from combined direct effects of climate
change and sediment delivery. Such effects are likely to be exacerbated
further as a result of the impacts of climate change upon coarse
sediment delivery. In generic terms, these processes are commonly
overlooked in flood risk mapping exercises and are likely to be
important in any river system where there are high rates of sediment
delivery and long-term transfer of sediment to floodplain storage (i.e.
alluviation involving active channel aggradation and migration).
Similarly, attempts to reduce channel migration through river bank
stabilization are likely to exacerbate this process as without bank
erosion, channel capacity cannot be maintained. Finally, many flood
risk mapping studies rely upon calibration based upon combining
contemporary bed surveys with historical flood outlines, and this will
lead to underestimation of the magnitude and frequency of floodplain
inundation in an aggrading system for a flood of a given magnitude.