Modeling the Importance of Baseflow and Sediment Supply on Armor Development: Contrasting Intermittent Dryland and Perennially-Flowing Gravel-Bedded Rivers

Abstract:

Most gravel-bedded rivers with perennial base flow are often armored, defined as having a bed surface grain size distribution that is coarser than the subsurface. In contrast, ephemerally flowing rivers, which are commonly flash flood-dominated, are typically unarmored. Although both types of channels have high peak discharges during floods, a key difference between these hydrological settings is in flow intermittency and how much flow occurs during moderate and low flow periods. High sediment supply and elevated shear stresses during floods have been identified as processes that may reduce or prevent armor development, but the preferential transport of smaller grain sizes during lower flows may also strongly affect armor development. We use numerical modeling to explore the relative importance of these armor-related processes. Calculations are based on a previous model called the Spawning Gravel Refresher, which includes mixed grain size sediment transport, morphodynamic feedbacks with channel slope, variable discharge hydrographs, and interactions with the subsurface grain sizes as the bed surface aggrades and erodes. Model runs are calibrated to natural rivers with varying degrees of bed surface armoring and flood hydrograph shape. These natural rivers include: i) the Nahal Yatir, an ephemerally flowing unarmored river in the Negev Desert, Israel, and ii) Trinity River, California, a larger perennially flowing, armored mountain river with annual snowmelt floods. We present a thorough exploration of model parameters, including base flow duration and magnitude, flood hydrograph shape and sand supply, combined with data from natural rivers. This produces quantitative bounds on how armor development varies in a wide range of parameter spaces unobtainable in the field alone.