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Understanding flow-sediment interactions is important for comprehending river functioning. Fine sediment accumulation processes, in particular, have key implications for ecosystem health. However, the amount of fines generated by intragravel flows and later accumulated in gravel streambeds may have been underestimated, as the hydraulic-related driving transport mechanisms in play are not clearly identified. Specifically, the relative contribution of fines from upper vs. lower sediment layers in gravel beds is not well understood. By recreating flooded and dewatered conditions in an experimental flume filled with natural sediment, we estimated such contributions by observing and collecting intragravel transported fines that were later accumulated into a void in the middle of the sediment matrix. Near-bed transport in the upper sediment layers (named Brinkman load ) during flooded conditions accounted for most (90%) of the accumulated fines. Intragravel transport in the lower sediment layers (named Interstitial load ) was the sole source of transport and accumulation during dewatered conditions with steeper hydraulic gradients. Interstitial load accounted for 10% of the total transport during flooded conditions. Although small, such estimations demonstrate that hydraulic-gradient transport in the lower sediment layers occurs in spite of the contradicting analytical assessments. We provide a case study to challenge the traditional approaches of assessing intragravel transport, and a useful framework to understand the origin and relative contribution of fine sediment accumulation in gravel beds. Such knowledge will be highly useful for the design of monitoring programs aiding river management, particularly in regulated rivers. [ABSTRACT FROM AUTHOR]

Fine sediment processes in gravel beds may have significant impacts to overall river ecosystem function. In addition to gravitational deposition, horizontal intragravel transport has been recognized to influence fine sediment accumulation. However, the specific hydraulic mechanisms and origin of fine sediment movement are not clearly identified. The purpose of this study was to investigate key hydraulic drivers and patterns of fine sediment accumulation. Using a conceptual framework to set the scene, we implemented an experimental setup in a gravel lateral bar subject to irregular flow fluctuations in the Kiewa River (Australia). We installed nine sets of sediment collector pairs and piezometers into the gravel. Each pair included one horizontally and one horizontally-vertically perforated collector. Mid-range, rather than peak flows, covering the site in water drove fine sediment deposited in the collectors. We estimated horizontal contribution to final deposition as 59%. Such contribution resulted from shear stresses > 3 N m − 2 promoting streamwise near-bed turbulence at the water-sediment interface during flooded conditions. Despite high subsurface hydraulic gradients, intragravel transport in the lower sediment layers via Darcy flow did not show any influence to fine sediment deposition. Our findings contribute to an improved understanding of fine sediment accumulation processes, key for overall river ecosystem functioning, particularly in regulated rivers. [ABSTRACT FROM AUTHOR]

Abstract: Particles of fine sediment can clog interstitial pore spaces of coarser grained sediment in river beds and thereby impede the exchange of water, dissolved constituents, and particulate matter with consequent ecological impacts. The extent to which fine sediment reduces connectivity between the stream bed and overlying channel is a function, in part, of sediment residence time. Short residence times imply frequent exchange of matter and reduced impact of fine sediment on stream bed habitat, while long residence times indicate that with respect to fine sediment delivery, the stream bed is more isolated from the overlying channel. Here we present a novel technique to quantify the residence time of river bed sediment at various depths over annual and decadal timescales using the fallout radionuclides (FRNs) 7Be (t 1/2 =53days) and 210Pbex (t 1/2 =22.3years). We placed mesh cylinders filled with 7Be-free sediment into a stream bed to quantify the capture of 7Be-tagged particles in the absence of scour and fill. We also took cores to the depth of refusal in alluvial sediment in unregulated and regulated rivers in Vermont and New Hampshire. Sampled watershed areas ranged from 29 to 410km2, and core depths ranged from 19 to 77cm. The 210Pbex activity profiles of cores show that bed sediment is exchanged to the depth of refusal at decadal timescales. In contrast, 7Be activity profiles indicate that fine sediment infiltrating into the bed had residence times ranging from 4 to >300days in unregulated rivers. Cores from a regulated river are notably different—subsurface sediment residence times were always longer than in unregulated rivers at comparable depths, likely owing to restriction of bed mobilization and clogging of bed material by fine sediment. These results suggest that filtration can be an important component of bed material delivery to stream beds, but filtration does not deliver material as deeply into the bed as scour and fill. We find that fallout radionuclides provide direct measurements of time and depth scales of active exchange of particulate matter, which are key controls on hyporheic function. [Copyright &y& Elsevier]

Abstract: Forced riffle-pools occur in gravel-bed rivers where a large nonalluvial element leads to local scour and deposition. To manage rivers where this type of morphology is found and to specify restoration measures that mimic this process, more field data on sediment mobility in forced riffle-pools is needed. The objectives of this study are to describe the spatial variability of sediment mobility and deposition in a developing forced riffle-pool and to use high-resolution flow velocity measurements to explain the observed dynamics. The field site is a forced riffle-pool in Moras Creek, a 6-m-wide gravel-bed stream with a 1.2% bed slope in Quebec, Canada. Topography and the movement of sediment particles equipped with Passive Integrated Transponder (PIT) tags are surveyed through a series of competent floods. Bed shear stress is estimated from near-bed measurements of time-averaged velocity and turbulent kinetic energy. The sediment transport regime of the creek is characterized by partial mobility, exponential distributions of path lengths, and a negative relation between particle size and path length. Full mobility occurs in the center of the pool and over the exit slope where flow is accelerated from the constriction of flow during two events above the bankfull discharge. Partial mobility occurs during the same events over the entrance slope to the pool. Lateral gradients of deposition and mobility suggest that the majority of sediment in motion is routed over the side bar at the entrance to the pool. High levels of turbulence intensity that occur as a result of flow deceleration may explain the removal of finer sediments from the head of the pool. [ABSTRACT FROM AUTHOR]