THE IMPACT OF FLOOD HYDROGRAPH SHAPE ON ALLUVIAL FAN MORPHODYNAMICS: AN EXPERIMENTAL STUDY

Alluvial fans are predominantly built during flood events. Despite this, most alluvial fan experiments have used constant flow to model fan and channel dynamics. To evaluate the impact of flood events on alluvial fan morphodynamics, we conducted four experiments, with different flood hydrographs repeated throughout each experiment: 1) no flood, constant flow, 2) stepped flood vs interflood 3) a low flood peak that decayed slowly, and 4) a high flood peak that decayed rapidly. Crucially, the mean flow rate and flood peak volume were constant across each experiment. Nevertheless, fan gradient was significantly altered by the addition of floods. Moreover, the peaked floods generated larger avulsions than the stepped floods, and influenced lateral channel migration rates and erosion and deposition volumes. Our results show that temporal variations in flow during a flood are a key influence on stream channel response. On natural fans, changes to hydrograph shape may be driven by land cover alteration due to climate change and/or forest fire. Our results suggest that such changes to flood hydrographs may alter the fan’s geomorphic response, as well as the associated natural hazards.