The relative contribution of near-bed vs. intragravel horizontal transport to fine sediment accumulation processes in river gravel beds
2018
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
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