Formation of repeating bar‐flat bedforms in ephemeral gravel bed channels: 2. Bridging mathematical modelling and field observations

Single‐ and multi‐thread gravel‐bed ephemeral channels in semi‐arid and arid regions have a characteristic repeating, channel‐wide pattern of low angle, fine‐grained ‘flats’ alternating with steeper, coarse‐grained ‘bars’. The genesis of these macroforms, while a topic of ongoing discussion, has not been fully elucidated. We have documented the formation of these macroforms after both artificially homogenising the bed material of a reach of the Nahal Yatir in the northern Negev, Israel, and ensuring its surface was planar. Here, we integrate the empirical data gained from these field experiments with a novel mathematical model. The model we propose, within a one‐dimensional framework, is based on the analysis of flow over an initially planar, erodible bed consisting of a bimodal grain size mixture of sediment. We apply linear stability analysis to derive a solution that provides insight into the formational dynamics of these macroforms. Our results indicate that bar‐flat patterns may arise from a free‐instability mechanism driven by sediment size heterogeneity, provided that the standard deviation of the sediment grain size distribution (GSD) is sufficiently large. Application of the model to a selection of ephemeral channels of the northern Negev suggests that repeating bar‐flat patterns are likely to develop during the recession of flash flood hydrographs, specifically when flow conditions approach the critical threshold for bedload transport. Besides providing a possible explanation for the formation of these macroforms, this study also contributes to a broader understanding of geomorphic processes in dryland river systems.