A Numerical Model of Armor Development in Flash Flood-Dominated Channels: Sensitivity to Sediment Supply, Hydrograph Shape, and Base Flow

Abstract:

Most gravel-transporting rivers with perennial base flow are armored, with bed surface grain size distributions that are coarser than the subsurface. In contrast, ephemerally flowing rivers, which are commonly flash flood-dominated, are typically unarmored. Both high sediment supply and elevated shear stresses have been identified as processes that may prevent armor development. We use a one-dimensional morphodynamic model that conserves mass and momentum, and tracks grain size distributions of bedload, surface, and subsurface to explore the parameter space of the armor ratio (Surface D₅₀ / Subsurface D₅₀) varying sediment supply, flood peak magnitude, flood duration, and base flow magnitude, including ephemeral hydrographs with no base flow. Our base case in terms of sediment supply, grain size distribution, and hydrograph is based on the Nahal Yatir, an ephemerally flowing river in the Negev Desert. Sensitivity analysis was conducted to quantitatively rank the importance of each input parameter. We found that armor ratio was the most sensitive to changes in base flow magnitude. In perennially flowing rivers, base flows can preferentially transport smaller grain sizes. In ephemerally flowing rivers, the modeling suggests that a large percentage of flow occurs above the threshold of motion for all grain sizes, transporting all grain sizes nearly equally, resulting in minimal armor development. We find that sediment supply rate and hydrograph shape have measurable but smaller influences on armoring.