| Abstrakt: |
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 fraction of sand and silt for different discharges, and utilized an 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 likely 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), an increased dune height and length is observed, possibly caused by the hiding‐exposure effect, resulting in enhanced mobility of the coarser material. However, weakly cohesive fine silt suppresses dune height and length, possibly caused by reduced sediment mobility. Finally, in the transition from dunes to upper stage plane bed, there are indications that the bed becomes unstable and dune 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. Underwater bedforms, such as dunes, are often found on the bed of rivers and deltas. These rhythmic undulations have specific shapes and sizes, and they affect how water flows. When the bed of the river is made up of sand, we can predict the dune height and length. However, mixtures of different‐sized sediments are common in rivers, and it is unknown how this impacts the geometry of the dunes. Therefore, we did experiments in a flume, a laboratory facility to simulate a river, and we tested different sediment bed mixtures. We found that replacing part of the base material with non‐cohesive fine particles leads to longer dunes, likely caused by increased mobility of the base material. However, for weakly cohesive fine particles, the effect was the opposite, and the dunes became shorter, probably due to the limited mobility of the sediment. Finally, we observed that under high flow conditions, the bed became unstable and different dune shapes occurred. We found that the friction the water experiences is not directly impacted by the sediment bed mixtures, but is mostly affected by the shape of the bedforms. An increased dune length due to a larger fraction of finer, non‐cohesive material in a sand bed, implies an increased mobility of the sandA decreased dune size due to a larger fraction of finer, weakly cohesive silt in a sand bed, implies a decreased mobility of the sandSediment bed composition indirectly affects hydraulic roughness by altering bedform geometry An increased dune length due to a larger fraction of finer, non‐cohesive material in a sand bed, implies an increased mobility of the sand A decreased dune size due to a larger fraction of finer, weakly cohesive silt in a sand bed, implies a decreased mobility of the sand Sediment bed composition indirectly affects hydraulic roughness by altering bedform geometry |
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