Experimental Study of Embeddedness Across Gravel Mobility Conditions: Effects of Fine Sediment Supply and Size

Abstract

Fine sediment accumulation in the interstitial spaces of a gravel surface layer can adversely impact benthic ecosystem and alter the mobility of the gravel bed. One of the most common metrics used to quantify the presence of fine sediment in a stream is embeddedness, defined as the vertical extent to which a particle is buried by fine sediment. Embeddedness is typically thought of as a site-specific quantity that must be measured in the field. However, recent research has suggested that embeddedness might be reasonably predicted using river hydraulics principles and remote sensing data. Yet the processes that relate flow conditions to embeddedness, particularly when the bed is mobile, are not well understood. This laboratory study presents results of 45 experiments conducted in a water and sediment recirculating flume. Three gravel-sand size mixtures were tested, with increasing initial amounts of sand present in the bed for each mixture. For each set of conditions, experiments were conducted under three gravel mobility conditions: submobilization, low mobility, and high mobility. Overall, the results show that the average embeddedness increase with increasing initial amount of sand in the bed. This pattern was observed across all mobility conditions, indicating that the amount of fine sediment supply exerted a first-order control on embeddedness in the context of a closed system, where sediment supply is defined by the initial amount of sand in the bed. In contrast, flow conditions, represented by the transport-scaled Shields stress, did not show a clear trend with embeddedness. One exception to this behavior is that a decrease in embeddedness was observed around the reference bed shear stress for two mixtures in two sediment size combinations. Sand size also influenced embeddedness, with the larger sand size generally producing higher embeddedness than the smaller size.