Climate change, sediment delivery and flood risk explored using two-dimensional inundation modelling

摘要

Much concern has been expressed about the potential impacts of changing climate upon future flood risk. However, there has been much less attention given to the ways in which changes in catchment function in general, and sediment delivery in particular, might themselves impact upon flood risk. This includes the effects of climate change upon sediment delivery and hence upon changes in river channel conveyance and hence flood risk. In this paper, we present the results of two types of modeling. The first presents predictions of future sediment delivery rates under future climate changes (estimated for the 2050s and 2080s) in order to provide first order estimates of rates of channel aggradation in an upland river with a gravelly bed. The second couples measured channel geometry changes, over short timescales, to flood events with 1 in 0.5 year and 1 in 2 year return periods, for both the present, and for future climates (the 2050s and 2080s). This coupling is achieved through a coupled 1D-2D model, with a full solution of the 1D St. Venant equations for the channel and a 2D diffusion wave treatment on the floodplain. Our results show that, for the system and flood return periods studied, sedimentation rates are the primary control upon the frequency of overbank flows and the magnitude of flood inundation extent. However, these interact non-linearly with future climate changes to increase future flood risk dramatically. This leads us to make three important observations: (1) we must consider what happens when historical flood events are modelled contemporary geometrical surveys in aggrading systems, as this is likely to yield unrealistically low friction parameters during the parameterization process; (2) we must factor the indirect impacts of future climate change upon sediment delivery into estimates of future flood risk; and (3) we must think about how river management activities impact upon the sedimentation process and make sure that the mechanisms by which a channel maintains conveyance when subject to sediment delivery are not undermined by river management activities.