Understanding Morphologic Flood Risk Relevant to River Management in Western Washington State

Abstract:

The frequency and intensity of high flows are typically assumed to dominate flood hazard in most basins. Changes in a river’s conveyance capacity may also contribute substantially to flood hazard, yet morphological change is often neglected in flood planning and the relative importance of morphological versus hydrological controls on flooding is not well understood. We investigate the relative influence of morphological versus hydrological change on flood hazard for 48 river-gauges in 25 river basins in Western Washington State by comparing changes in channel conveyance to changes in the flow volume of moderate floods. We find that the morphological and hydrological influences are comparable in magnitude but not regionally consistent. The magnitude of fractional channel conveyance changes at a site are not directly explainable through basin characteristics such as slope, contributing area, or elevation. Instead, we identify five categories of morphologic changes relevant for flood-risk management. The largest morphologic effects on flooding have identifiable forcing mechanisms such as extreme events and anthropogenic disturbances that may cause a channel to change by over half of its maximum capacity. Temporal patterns of channel change are most commonly non-linear and often depict multi-decadal oscillations in capacity. These findings suggest changes in flood-risk can occur both gradually (decadal timescales) and rapidly (annual timescales) due to channel morphology.