Thanos Papanicolaou
IIHR - Hydroscience & Engineering, The University of Iowa

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Application Note: Sediment Transport

Noncohesive Sediment: Understanding the mechanisms involved in the transport and fate of noncohesive soils in natural channel systems remains an open case in water-related engineering disciplines. The main challenge in studying noncohesive sediments is the complex character of the bed geometry which governs the velocity, as well as the turbulence structure of the flow which in turn, control the sediment-carrying capacity of the flow. Bed geometry is controlled by stochastic processes and subject to drastic changes due to changes in the flow condition. Bedforms, clusters, step and pools are few examples of such complicated bed geometry. In addition, sediment may consist of a wide range of grain sizes.

Cluster and relative submergence studies: Groupings of gravel particles in streams (clusters) have been found to dominate the bed topography in a specific range of flow conditions. Clusters are a ubiquitous feature in gravel-bed streams whose size and shape depends on sediment availability, flow intensity, and sediment properties. Formation of clusters is, in most cases, related to the rapid recession of floods and can be interpreted as it is originated by the impact of poorly sorted particle waves against large, immobile boulders or steps. It was speculated that disintegration of clusters is associated to the episodic action of highly energetic near-bed turbulent events, known as bursting events, which are capable of dislodging particles found within a cluster structure. These cluster microforms have greater stability than individual particles and when present increase the magnitude of fluctuations in bedload transport. A study was conducted to examine the interconnected effects of cluster formation and the near bed flow field. Fine scale mapping of the flow field surround a cluster was performed with a 2D-LDV. Continuity was employed to examine the extent of the changes in the 3D near bed flow structure, e.g., secondary currents, recirculation regions, reattachment lengths, fluid forces exerted on clusters. The formation geometry, bedload transport, and critical bed shear stress for stability of clusters determined from previous studies were examined in light of the flow analysis to gain a more fundamental understanding of processes involved with cluster formation, stability, and effects on bedload transport.
Cluster found to occupy between 10% and 90% of gravel bed streams. Their size depends on the bed packing density, bed topography, flow intensity, and sediment material properties. A study was conducted to quantitatively describe the integration and disintegration processes of cluster structures in a streambed. For this purpose, experiments for three packing configurations were performed in a tilting flume for mild and steep slopes. Painted glass beads of the same size and specific gravity were placed atop a porous bed of identical colorless beads. A video camera was used to monitor the clustering processes occurring at the flume bed. The video images were digitized and the evolution of the clusters will be quantified using image analysis techniques. The analysis of the images via the imaging software provided the variation of the clusters size with time, the time scale during which bed immobile conditions exist, and the role of sediment availability and slope to the size of clusters. The study extended to investigate the role of sediment availability and specific gravity on cluster formation and cluster geometric characteristics (spacing and size), identify the ranges of flow within which clusters start to disintegrate, and test the hypothesis that mono-sized particles can also lead to cluster formation for different flow and sediment availability conditions. Experiments for sediment availabilities were conducted in a flume at incipient and two times the incipient flow conditions. An advanced image analysis technique was employed to track the evolution of cluster microforms. The results of the study showed that sediment availability affects the architecture and size of cluster microforms. The disintegration of clusters is a sporadic process supporting the hypothesis that turbulent bursts cause their disintegration. For the tested flow conditions clusters provide extra stability and delay sediment transport, instead for high sediment availabilities clusters disintegrate. The randomness in sediment properties may not be the only parameter leading to the genesis of clusters. Uniform sediments, under some conditions, can result to cluster formation however; beds consisting of clusters with different specific gravity were more dynamic.
Studies had shown that relative submergence play an important role on the formation and evolution of cluster microforms in gravel bed streams and its implications to bedload transport. It is hypothesized that the relative submergence is an important parameter in defining the feedback processes between the flow and clasts which governs the flow patterns around the clasts, thus directly affecting the depositional patterns of the incoming sediments. To examine the validity of the hypothesis, experimental runs were conducted in a tilting water recirculating laboratory flume. The experimental runs were carefully designed to isolate the role of relative submergence on the genesis and evolution of clusters and to provide a detailed description of the flow mechanisms governing the transport of sediments for high and low relative submergence. For this purpose, fixed arrays of clast-obstacles were placed atop a well-packed bed with uniform size glass beads. During the runs, multifractional uniform size spherical particles were material was fed upstream of the clast section. Imaging analysis software, Large Scale Particle Velocimetry (LSPIV) and an Acoustic Doppler Velocimetry (ADV) were employed to provide unique quantitative measurements for bedload fluxes, clast/clusters geomorphic patterns, and mean flow characteristics in the vicinity of the clusters. The findings of this research showed the effects of relative submergence on the depositional patterns of clusters and bedload rate can be significant, depending on the prevailing flow conditions. For both relative submergences, clasts worked predominately as a sink of the incoming sediments. However, a more careful examination of the results revealed that for low relative submergence (LRS) the overall rate of the exiting sediment was measurably lower than the corresponding rate for high relative submergence (HRS), especially for conditions representing bankfull flow conditions. For high HRS, clusters were formed randomly throughout the test section, although preferential grouping of particles was observed in the wake region of clasts. Clusters in the LRS were either formed at the stoss of a clast or in the inner spacing among neighboring clasts, in the form of ‘dump’ deposits. The clusters for the HRS were rhomboidal or triangular in shape, and for the LRS, the clusters appeared as elongated streaks.