Entrainment of Muddy Sediment From Immobile Gravel River Beds

Abstract This study examines the transport of fine muddy sediment through a gravel‐bed reach. In such cases, mud is often conceptualized as a size fraction that interacts little with the bed (i.e., as washload). However, previous studies have shown that mud can deposit within gravel beds. Based on this prior work, it has been hypothesized that mud entrains from the bed only once the surface gravel layer is mobilized. In this paper, we show that, at least in the streams we studied, mud can be re‐entrained into suspension without mobilization of the gravel. Additionally, the flow condition at which mud begins to be mobilized was found to depend on the distance between the tops of the uppermost gravel clasts and the upper surface of fine sediment filling pore spaces within the gravel bed (i.e., gravel embeddedness). Data supporting these findings were obtained using a newly developed erosional flume applied to relatively fine gravel creek beds ( mm) near Blacksburg, VA. Data from the erosional flume also allow for calculation of a mud erosion rate. Analysis of the erosion data suggests that classic sand‐based entrainment equations can be modified with a hiding function to predict the erosion rate of loosely compacted surface mud. These results provide additional components of a framework for modeling mud transport within gravel‐bed streams and support conceptualizing mud transport as a “hop load” rather than a washload even in channelized reaches.