Non-monotonic surface process response to increased flooding intensity

Abstract:

Flooding is a ubiquitous condition in fluvial environments that impacts morphodynamics and landscape evolution. In alluvial channels, high-magnitude, low-frequency (“high intensity”) flooding produces morphologies and kinematics that contrast strongly with landscapes that experience low-magnitude, high-frequency flooding (“low intensity”). Theory predicts that channel geometry equilibrates to increased flood intensity by adjusting width, depth and slope to convey the characteristic formative discharge. A key outcome of this framework asserts that equilibrium channel geometry and dynamics vary as a monotonic function of flooding intensity. To evaluate this prediction, we conducted a suite of physical experiments in which deltas constructed with cohesive sediment evolved in response to three discrete flooding intensity states: no flooding, low intensity flooding, and high intensity flooding. For each of these states, we monitored the surface expression and dynamics of the alluvial system to quantify the impact of the flooding regime on autogenic processes. Steep and wide channels subject to high intensity flooding built broad, shallow levees and migrated rapidly across floodplains. In contrast, channels subjected to low intensity flooding were narrower and constructed thick levees that confined flow and limited lateral migration. The case of no flooding, however, experienced intermediate channel geometries and rates of autogenic processes like channel migration and avulsion. Our findings indicate that as the flooding intensity increases, the associated change in autogenic processes is nonmonotonic; contradicting existing theory of flow intermittency. Taken together, these results suggest that in alluvial systems, the intensity of flow variability may induce a cascade of feedbacks that enhance levee growth during low intensity flooding but inhibit levee growth when flooding intensity exceeds a threshold value. In most floodplains, channel migration and floodplain aggradation represent the highest level of morphodynamic hierarchy. Based on our results, flooding intensity regulates the characteristic timescales of these processes, and therefore is expected to be a key factor influencing the fidelity of environmental signals preserved in the stratigraphic record.