[H-CG20] Scaled experiments in Earth surface dynamics

[HCG20-P01] Influence of discharge rates of experimental alluvial fans: Insights from geomorphic changes in planform shapes

*Hyojae Lee¹, Wonsuck Kim¹, Duhwan Keum¹, Haein Shin¹, Yunhyeong Lee¹ (1.Department of Earth System Sciences, Yonsei University, Seoul, Republic of Korea)

Keywords:Alluvial Fans, Planform Shape

Alluvial fans evolve into distinct planform shapes and topographic surfaces with capricious channel migration depending on their depositional environments. In particular, the channel morphology that regulates the surface of alluvial fans is mainly controlled by the upstream boundary conditions: sediment discharge, Q_s, and water discharge, Q_w. To examine the effect of each discharge condition on the surface morphology, we conducted alluvial fan experiments using a range of sediment and water discharge rates with a bimodal sediment mixture of coarse quartz sand and fine crushed walnut shell sediment. In each experiment with constant sediment and water discharge rates, a fan developed a steeper upstream sand reach, a shallower downstream walnut reach, and a grain-size transition (GST) at the boundary between these two reaches. The overall fan margin and GST progradation patterns measured using overhead time-lapse images in the experiments indicate that the spatiotemporal variations in the planform shape (characterized with eccentricity) and roughness of the fan margin and GST can be a tool for deciphering upstream boundary conditions. Time series data of the fan morphology show that 1) a fan with a higher sediment discharge tends to be more semi-circular in the planform pattern (eccentricity is close to zero) and 2) a fan with a higher water discharge organizes rougher fan margin and GST. We also discovered that, 1) difference in eccentricity between the fan margin and GST could indicate different water discharge, and 2) a wide range of the eccentricity of the fan is highly correlated with high sediment discharge as lateral migration of the channels becomes dominant comparable to nodal avulsions. Depending on the source conditions, channel dynamics leaves distinct record on the surface morphology of alluvial fan. Based on the current findings, quantification of fan morphology (planform shape and roughness) might provide a framework to disentangle the upstream boundary conditions of the fans in nature.