Hydrologic and geomorphic drivers of changing flood hazard

Flooding is a major hazard to lives and infrastructure, but trends in flood hazard are poorly understood. In flood risk analysis and channel design engineering, channel capacity is generally assumed to be constant, and changes in flood frequency are assumed to be driven primarily by changes in streamflow. However, trends in channel capacity will also modify flood hazard, even if the flow frequency distribution does not change.

Using daily discharge records and manual field measurements of channel cross-sectional geometry for USGS gauging stations that have defined flood stages (water levels), we present novel methods for measuring long-term trends in channel capacity of gauged rivers, and for quantifying how they affect flood frequency. We apply these methods to 401 U.S. rivers and detect measurable trends in flood hazard linked to changes in channel capacity or the frequency of high flows.

We found increases in flood frequency at a statistically significant majority of sites. Trends in channel capacity were smaller, but more numerous, than those in streamflow, with a slight tendency to compensate for streamflow changes. Recognizing and quantifying the joint influence of trends in channel capacity and streamflow on flood frequency is necessary to determine changes in flood hazard accurately.