| 11 Feb 2026
Channel Dynamics in an Experimental Alluvial Fan Under Constant Boundary Conditions: A Classification of Avulsion and Lateral Migration Events
Abstract. Alluvial fans exhibit complex channel dynamics, spanning gradual lateral migration to sudden avulsions. Although allogenic processes are recognized as key drivers of these behaviors, the autogenic mechanisms regulating channel change remain poorly understood. In this study, we present a quantitative analysis of the main channel kinematics on a widely graded, non-cohesive experimental alluvial fan, utilizing high-temporal-resolution RGB imagery and main channel centerline tracking. By employing two key metrics – displacement magnitude (normalized by channel width) and flow continuity, defined as the degree of overlap in the active channel footprint from one image to the next – we move beyond qualitative assessments, which are often subject to researcher bias, and establish a clear framework for distinguishing between migratory (continuous) and avulsive (discrete) channel behaviours. Our findings reveal that the fan alternates between supply-limited phases, when a small number of efficient channels route sediment to the toe with only localized reworking, and transport-limited phases, when a more complex, inefficient channel network traps sediment mid-fan, favoring abrupt reorganizations (i.e., avulsions). Contrary to the conventional assumption that systematic aggradation from toe to apex triggers large-scale channel abandonment, we demonstrate that lateral sediment redistribution often prevents fan-wide sediment buildup, thereby delaying or even preventing major avulsions. These results highlight the critical role of self-regulating autogenic processes, particularly the lateral reworking of coarse sediment, in controlling both the timing and scale of channel adjustments, emphasizing the need to incorporate localized feedback mechanisms into predictive models to improve our understanding of alluvial fan dynamics.
Review of “Channel Dynamics in an Experimental Alluvial Fan Under Constant Boundary Conditions: A Classification of Avulsion and Lateral Migration Events”
This manuscript presents an experimental investigation of channel dynamics on a widely graded alluvial fan. The active flow area is identified using high-frequency imagery and optical flow analysis. Active channel centerlines were extracted from these areas, and a quantitative framework was developed to classify channel displacement into continuous lateral migration and discrete avulsion based on channel displacement magnitude and flow overlapping. The study demonstrates that cumulative lateral migration can produce net channel displacement comparable to avulsions and discusses the alternation of the fan between supply-limited and transport-limited states under constant boundary conditions. Overall, the manuscript is well written and addresses an important problem in fluvial geomorphology: how to distinguish between gradual channel migration and discrete avulsion. The use of high temporal resolution imagery and a classification scheme combining channel displacement magnitude and flow-path overlap is a valuable contribution. The results highlight the role of repeated lateral migration in redistributing sediment in alluvial fans.
However, the methodology and interpretation require further improvement and clarification before the manuscript can be considered for publication. In particular, the novelty of the proposed method relative to existing techniques needs to be discussed, the classification thresholds needs to be improved, and the implications and broader applicability of the method to natural systems require further discussion. Some figures need further refinement. Therefore, I recommend major revisions to the manuscript.
Major comments:
Minor comments:
Line 109: Suggest adding the spatial resolution of the imagery
Figure 3: It would be better if adding a colorbar and a description of “flow motion values” regarding whether it represents the magnitude or direction of the motion.
Figure 4: Panels e and f don’t have captions. Also, there seems to be no citation of Fig. 4d and e in the main text.
Line 155: “position of a point at two time steps” – How were points between the two time steps matched? For example, in Fig. 5a, there are several points at the end of the down fan in blue that do not have a corresponding point on the down fan that is colored in orange. Is there a reason why these points are removed from the angular displacement analysis?
Figure 5: There is no citation of Fig. 5 in the main text.
Line 212: “between consecutive images” – suggest reinforce the temporal resolution here
Figure 8: Section 3.4 is describing Fig. 8 but there is no citation of it in the main text.
Line 318-319: “Beyond distinguishing lateral migrations from avulsions, our data illustrate how sediment connectivity exerts a first-order control on fan dynamics” - Suggest rewrite this sentence as more data might be needed to support this statement.
Line 319-322: References might be needed for these sentences.
Line 329: “which, in turn, determine the preferred locations of avulsion nodes and ultimately drive shifts in alluvial style.” – The data here did not show anything about the location of avulsion nodes.