Fine sediment in mixed sand-silt environments impacts bedform geometry by
altering sediment mobility
Abstract
Geometric characteristics of subaqueous bedforms, such as height, length
and leeside angle, are crucial for determining hydraulic form roughness
and interpreting sedimentary records. Traditionally, bedform existence
and geometry predictors are primarily based on uniform, cohesionless
sediments. However, mixtures of sand, silt and clay are common in
deltaic, estuarine, and lowland river environments, where bedforms are
ubiquitous. Therefore, we investigate the impact of fine sand and silt
in sand-silt mixtures on bedform geometry, based on laboratory
experiments conducted in a recirculating flume. We systematically varied
the content of sand and silt for different discharges, and utilized a
UB-Lab 2C (a type of acoustic Doppler velocimeter) to measure flow
velocity profiles. The final bed geometry was captured using a line
laser scanner. Our findings reveal that the response of bedforms to an
altered fine sediment percentage is ambiguous, and depends on, among
others, bimodality-driven bed mobility and sediment cohesiveness. When
fine, non-cohesive material (fine sand or coarse silt) is mixed with the
base material (medium sand), the hiding-exposure effect comes into play,
resulting in enhanced mobility of the coarser material and leading to an
increase in dune height and length. However, the addition of
weakly-cohesive fine silt reduces the mobility, suppressing dune height
and length. Finally, in the transition from dunes to upper stage plane
bed, the bed becomes unstable and bedform heights vary over time. The
composition of the bed material does not significantly impact the
hydraulic roughness, but mainly affects roughness via the bed
morphology, especially the leeside angle.