Catchment and fan geometry controls on grain size fining in northwestern Namibia

Abstract

Located at the entrance to the sedimentary basin, alluvial fans are key depositional systems in which fluvial responses to external forcing are commonly preserved in the gravel and sand fraction of the stratigraphic record. It has long been observed that upstream catchment area has an impact on fan extent, incoming sediment flux, and subsequent deposition rates (Bull, 1977). Numerical modelling results suggest that catchment size can affect the autogenic dynamics of channel incision and mobility inducing high topographic variation across the fan, transient deposition, and more rapid grain size fining in systems where the downstream drainage area is greater than the upstream source catchments (Wild et al. 2025). However, the extent to which these internal dynamics are expressed in real-world fan grain size records and any geometric thresholds within the landscape remains poorly constrained. In an area with comparable external forcing (e.g. limited tectonic activity, comparable lithology and base-level, and comparable mean annual precipitation) producing fans in a state of sediment bypass near Kowarib, Namibia, we tested additional internal dynamics and geometric (e.g. catchment area and topographic) correlations with grain size fining.The goal of our approach is to quantify internal controls observed within the landscape evolution model, GravelScape (Wild et al. 2025), on the gravel grain size fining record of real-world unconstrained alluvial fans. We implemented a geospatial analysis of high resolution (1 m) DEM, multispectral remote sensing imagery, and analysis of field captured channel bed images and bank measurements. To measure gravel grain sizes, we implemented the machine learning ImageGrains algorithm (Mair et al, 2024) on field sampled imagery from the main channel of three fans in the northern interior of Namibia. We expanded the geospatial analysis to nine Kowarib fans draining the same mountain range as the field sampled fans to provide greater geometric context on the area. Preliminary results indicate a difference in fan profile shape (convex vs concave) draining larger catchments (> 1 km2) with less grain size fining in these systems even after normalization by depositional length. The concave fans draining the smaller catchments (1 km2), displayed a more consistent sedimentation layer throughout the main channel, less topographic variation (down and across) the fan and less grain size fining. A landscape evolution model is then used to compare downstream responses in fan development and grain size fining across different catchment–basin geometries, isolating the effects of variations in sediment supply and fan evolution time or internal, autogenic, depositional dynamics within the fan.Bull (1977). Progress in Physical Geography: 1(2). doi.org/10.1177/030913337700100202Mair et al. (2024) ESPL :49(3). doi.org/10.1002/esp.5755Wild et al. (2025) ESurf: 13(5). doi.org/10.5194/esurf-13-889-2025