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Recharge from intermittent flow
Published in Ian Simmers, I. Simmers, J.M.H. Hendrickx, G.P. Kruseman, K.R. Rushton, Recharge of Phreatic Aquifers in (Semi-) Arid Areas, 2017
In floods the stream bed scours to depths of 1 m or more and very high concentrations of bed material, comprising boulders and gravel as well as sand, silt and clay, are transported. Whenever there is a significant change in flow velocity the sediment composition changes; coarse material is replaced by finer material. There is thus a general sorting of sediment along the stream bed with the finest material coming to rest at the end of slopes during the rising limb of a flood. As the force of a flood gradually subsides and the flood recedes from the alluvial fan, deposition replaces scour and the stream bed is rebuilt, when first coarse and gradually finer material falls out. Eventually the clay fraction comes to rest and may clog the stream bed. The next heavy flood, however, destroys this mud cake and renews the stream bed infiltration capacity.
Three-dimensional flow structures driven by turbulence
Published in Iehisa Nezu, Hiroji Nakagawa, Turbulence in Open-Channel Flows, 2017
Table 5.2 provides a summary of the turbulent structures of cellular secondary currents in wide fluvial channels. Non-homogeneity of bed roughness, as well as lateral undulation of the bed, causes non-homogeneities in the bed shear stress. In turn, this non-homogeneity of shear stress creates anisotropic turbulence, which generates streamwise vorticity and cellular secondary currents. The reverse process may also occur. If cellular secondary currents exist beforehand, longitudinal ridges and troughs are formed even in uniform bed material because of the lateral variation of bed shear stress. In addition, if the bed material is composed of a mixture of sizes, finer sediments are swept away by high shear stress in downflow regions, leaving a strip of coarser sediment exposed to the flow. That is, cellular secondary currents will generate beds consisting of alternating finer and coarser strips.
Topography and Bathymetry
Published in Jochen Aberle, Colin D. Rennie, David M. Admiraal, Marian Muste, Experimental Hydraulics: Methods, Instrumentation, Data Processing and Management, 2017
Jochen Aberle, Colin D. Rennie, David M. Admiraal, Marian Muste
At the surface of a gravel bed river, the flow causes sorting of the bed material. Finer material is more likely to be washed out or transported – whereas coarser material is more likely to be deposited and, in turn, prevents fines from being washed out. Thus, a coarser layer forms at the bed surface. Detailed, partly different explanations for the formation and presence of a coarse-grained surface layer are given by Carling and Reader (1982), Parker et al. (1982), Dunkerley (1994) and Dietrich et al. (1989). If the surface material shields the subsurface material from the flow it is often called pavement or an armoring layer (Sutherland, 1987). However, finer material can be transported and deposited on top on the coarsened surface layer – when such finer material is supplied from upstream. Thus, an armor layer in turn can be locally covered by finer material. Figure 4.A.3.1 illustrates this phenomenon.
Mining pit migration of an alluvial channel: experimental and numerical investigations
Published in ISH Journal of Hydraulic Engineering, 2020
Bandita Barman, Arup Kumar Sarma, Bimlesh Kumar
Sediment transport and morphological evolution of channel bed depend on the interaction of bed material with flow. For simulation of bed profile hydrodynamic equations can be incorporated with sediment continuity equations by using decoupled, semicoupled, and coupled methodology. Coupled methodology is suitable for alluvial river with rapid bed evolution (Cao et al., 2002) whereas, semicoupled method is more flexible and appropriate for bed load dominant flow. For bed load-dominant flow the time scale of flow and channel bed evolution processes may be different, therefore, fully coupling of flow and sediment transport may not be necessary (Wu 2004). Non equilibrium sediment transport model is observed to be more realistic since sediment transport in natural rivers is generally not in a state of equilibrium due to the variation of flow conditions and channel properties (Wu 2004; Cao et al. 2005). Sand mining is a phenomenon where rapid interaction of bed sediment with flow occurs. In this case equilibrium sediment transport model is not appropriate for calculation of mining pit migration (Chen et al. 2010). However, in case of numerical modeling of mining pit migration, empirical formulation is required for closure of governing equations for both equilibrium and non-equilibrium model and may add certain amount of uncertainty to either of the models (Cao et al. 2004).
Spatial total load rating curve for a large river: a case study of the Tigris River at Baghdad
Published in International Journal of River Basin Management, 2020
Ammar A. Ali, Nadhir A. Al-Ansari, Qusay Al-Suhail, Sven Knutsson
Watershed soil erosion, migrating stream channels and human interventions are the main sources of fluvial sediments (Vanoni 2006). When these sources of sediment are unlimited, and the capacity of streams are sufficient to mobilize it, sediment transport will continue (Friend 1993). Construction of large dams traps sediment from the watershed causing aggradation upstream of the dam whilst the released water from the dam is almost sediment-free; causing degradation of the river downstream of the dam (Meade and Moody 2010, Kondolf et al. 2014, Issa 2015). Total flow downstream is also often diminished as barrages on rivers divert part of the river flow away thereby reducing transport capacity downstream. Therefore, permanent regulation schemes on river systems and/or climatic changes, particularly in regions of low precipitation, will disturb the water flow and sediment supply. This will subsequently lead to changes in the river course, its dimensions and characteristics and will negatively impact the supply of sediment which may lead to under-fit conditions as in the case of the Tigris River in Baghdad (Ali 2016). A supply limited condition is often the case for most natural rivers (Hickin 1995), so, bed-material is the main source of the sediment load that affects their morphology, erosion, deposition and migration (Turowski et al. 2010). Considering the Tigris River in Baghdad as a supply limited river is one of the issues that was addressed in this research.
Acoustic sampling effects on bedload quantification using acoustic Doppler current profilers
Published in Journal of Hydraulic Research, 2020
Slaven Conevski, Massimo Guerrero, Axel Winterscheid, Colin D Rennie, Nils Ruther
The distribution of the bedload through alluvial streams contributes in shaping of the river morphology; thus, it is one of the key components in many branches of river management. Understanding the bed-material mechanics is critical for bedform dynamics and the management of heavily exploited rivers (e.g. ship-navigable waterways). Therefore, bedload transport data are a fundamental requirement for proper management of engineering practices in complex river systems. However, measurement series of bedload transport are seldom available and sometimes considered as statistically unreliable.