Flood risk to human populations is defined by the impacts of frequency, magnitude, and duration of overbank streamflow on vulnerable communities, which may potentially be affected by climatic or anthropogenic alteration of water supply from upstream basins. We call these ‘flow frequency’ factors of flood risk. The flow of water transports sediment through river channels and over years to decades, it may accumulate within or be eroded from particular river reaches, yielding progressive changes to river topography and channel capacity. These ‘channel capacity’ factors thereby produce decadal trends (nonstationarity) in flood risk that are superimposed upon the flood risk trends generated by ‘flow frequency’ factors. In the statistical analysis of floods, the separate contributions of ‘flow frequency’ and ‘channel capacity’ factors have not been sufficiently identified. Therefore, insurance premiums and public sector planning are not reflective of ‘true flood risk’, which is comprised of the sum of these two potential causes of flood risk/hazard trends.

We are working to characterize ‘true flood risk’ in a manner that is consistent with recent government legislation (e.g., US National Flood Insurance Program). Specifically, we have developed a new method to separately quantify flood hazard associated with ‘flow frequency’ and ‘channel capacity’ effects. We have applied this method to sites across the continental USA to improve understanding of the relative causes of flooding and to quantify how we expect them to change in the future. This and our prior work on flood risk has enabled us to identify impairment of flood control structures, to assess the efficacy of dams in controlling floods at various locations in the river network, and to, for example, identify sites where the ‘flow frequency’ effects that decrease flood risk are offset by ‘channel capacity’ trends that increase it. We are looking at combining this analysis with STORM to understand the evolution of flood risk over broad regions under various scenarios of future climate change.

Projects on this research theme include:

Doctoral Training Grant, Funded by Natural Environment Research Council, L. Slater; Principal Supervisor: M. Singer, 2010-2014

Tracking hydraulic mining sediments from the Sierra Piedmont into flood bypasses of the Sacramento Valley, CA. Funded by National Science Foundation Geography and Spatial Sciences, M. Singer (PI), L.A. James, R. Aalto, T. Dunne, 2005-2008

Developing and validating a flow and sediment transport model for large-scale restoration on the Sacramento River. Funded by CALFED Bay-Delta Program. T. Dunne, M. Singer, 2002