Non-Monotonic Floodplain Responses to Changes in Flooding Intensity

Overbank flooding is common along most rivers, and it influences the dispersal of sediment to floodplains. While variable discharge is a critical aspect of fluvial landscape evolution, it is typically modeled by simplifying the hydrograph to an equivalent steady discharge: the channel-forming discharge. However, for all formulations used to simplify hydrographs, many different inputs can produce the same channel-forming discharge. Here, we investigate how hydrographs with different flood intensities affect channel mobility, sediment accumulation patterns, and alluvial morphology using a suite of physical experiments where a fan delta grew by dispersing a cohesive sediment mixture into a basin. The experiments spanned three levels: no flooding, low-intensity flooding, and high-intensity flooding, while the time-averaged water and sediment discharge was equivalent between all flooding regimes. Across this gradient, channel mobility, alluvial morphology and sediment dispersal scaled non-monotonically with flooding intensity, and the data suggest that levee-building feedbacks are the cause. We found that flood intensity modulates the relative balance between sediment delivery to channel margins, which nourishes levee growth, and the intensity of overbank flow, which inhibits levee growth. When flooding was absent, levees experienced consistent overbank flow and sediment delivery, leading to moderate levee aggradation and sporadic levee breaches. In contrast, when low-intensity flooding was imposed, levees experienced enhanced sediment delivery by low-amplitude floods, but only intermittent scouring. A further increase in flood intensity generated intense overbank flows that inhibited levee growth altogether. These results imply the existence of an optimum levee-building condition, where flooding conditions stabilize channels through levee-building feedbacks.