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In the Yangtze River, the Three Gorges Dam (TGD) has altered the flow‐sediment regimes, resulting in channel degradation and bed surface coarsening, which have been reported in recent years. However, there has been no systematic study on the impacts of the TGD on the downstream gravel–sand transition (GST). Based on detailed field observations and numerical modelling, this study fills this gap and reveals significant adjustments in the GST and the leading causes. The results show that after operation of the TGD began, the GST migrated 49.5 km downstream during 2003–2010 due to a reduction in the suspended‐load supply (grain size < 0.5 mm) and then remained stable from 2010 to 2015 under the control of a braided river morphology. In addition, as channel degradation continued, the steepening bed profile upstream of Chenjiawan and the lowering of the downstream water level caused some gravel within the GST to become more frequently mobile during low‐flow periods (discharge <15,000 m3/s). Combined with the effects of the TGD‐induced prolonged low‐flow season and reduced frequency of large floods (discharge ≥ 40,000 m3/s), the GST exhibited a poorly understood phenomenon in which gravel was mainly transported downstream during low‐flow periods. However, these processes affected only localized sediment movement, contributing little to overall GST migration in the Yangtze River. The results also show that when a low sediment supply persists long term, additional GST migration after transient downstream migration depends primarily on the river morphology. This article has implications for the channel regulation and evolutionary processes of GSTs below dams. [ABSTRACT FROM AUTHOR]

Sediment load and budgets are a fundamental component of the process-based hydromorphological framework developed by the REFORM project, and are needed to accurately assess the current condition of a river, its sensitivity to change, and its likely future evolutionary trajectory. This paper presents an evaluation of three different methods for estimating both bedload sediment transport and bed-material budget within river channels, using the Middle Loire River as a case study. The first method is based on the stream power concept and does not need any hydraulic calculations. It yields estimates of the sediment transport in the same order of magnitude as measurements but poor results for the bed-material budget in terms of magnitude and tendency. For the second method, hydraulic parameters are computed using the Manning-Strickler equation (or a 1D hydraulic model for steady flow). It provides useful indicators for understanding river dynamics but does not yield significant improvements compared to the first method. The third method uses 1D numerical software for water flow and river bed evolution. It yields the most accurate results for both sediment transport and bed evolution but requires more data and overall more work to construct the model. Guidance is provided on the amount of data required, the competence needed to build the models, and the predictive capability of each of the methods. [ABSTRACT FROM AUTHOR]

The results of an experimental study on transport of suspended wash load through a coarse-bed stream are presented. The experiments were conducted under different concentrations of fine suspended sediment (wash load of uniform size, 0.064 mm diameter) and with three different coarse-bed sediments: two having uniform sizes and one with nonuniform size distribution. For any equilibrium concentration of wash load in suspension, a definite proportion of the wash material was observed to be present within the bed material. No difference is found in this regard between wash load and suspended load transport. Therefore, the relationship, as stated by Samaga et al., for the parameter representing sheltering—exposure and interference effects in the suspended load transport of nonuniform sediments was applied in a modified form by using the present data and the data collected from the literature. [ABSTRACT FROM AUTHOR]

It has been the conventional assumption that wash load is supply limited and is only indirectly related to the hydraulics of a river. Hydraulic engineers also assumed that bed-material load concentration is independent of wash load concentration. This paper provides a detailed analysis of the Yellow River sediment transport data to determine whether the above assumptions are true and whether wash load concentration can be computed from the original unit stream power formula and the modified unit stream power formula for sediment-laden flows. A systematic and thorough analysis of 1,160 sets of data collected from 9 gauging stations along the Middle and Lower Yellow River confirmed that the method suggested by the conjunctive use of the two formulas can be used to compute wash load, bed-material load, and total load in the Yellow River with accuracy. [ABSTRACT FROM AUTHOR]

Identification of coarse sediment connectivity down river-channel systems is important for understanding the linkages between river reaches, the influence of sediment sources on channel morphology and the mechanisms and liability to propagation of morphological change. A conceptual framework is developed in which several types and degrees of connectivity are proposed. It is hypothesised that sediment connectivity varies with sediment sources and with stream competence to transport the coarse fractions. A method of identifying the status of reaches is developed, based on field mapping of morphology and sedimentology. The analysis is exemplified by two channels: one an ephemeral channel in SE Spain; the other a perennial channel in NW England. Questions arise over whether reaches that lack channel stores transfer coarse sediment. Three scenarios are suggested: (1) lack of flux due to lack of competence of the reach; (2) flushing through of sediment due to high competence; and (3) potential transport of coarse sediment but exhaustion or lack of availability. Changes in connectivity will take place over different time scales depending on the nature of exchange and time scales of storage. Short-term variations in connectivity can be identified by repeated mapping. [Copyright &y& Elsevier]

Sedimentological studies of coarse-grained alluvial rivers reveal patterns of bed material sorting at a variety of spatial scales ranging from downstream fining over the length of the long profile to the vertical segregation of a coarse surface layer at the scale of individual particles. This article reviews the mechanisms that sort bed material by size during sediment entrainment, transport and deposition and discusses some of the inter-relationships that exist between patterns and processes of sediment sorting at different spatial and temporal scales. At initiation of motion, sorting can arise from the preferential entrainment of the finer fractions from the heterogeneous bed sediments. Bedload grain-size distributions are modified during transport as different size fractions are routed along different transport pathways under the influence of nonuniform bed topography and associated flow patterns, and during deposition as the variable pocket geometry of the rough bed surface and turbulence intensity of the flow control the size of the particles that deposit. The review highlights the poor understanding of the many feedback linkages that exist between patterns and processes of sediment sorting at different scales and the need for a greater awareness of the spatial and temporal bounds of these linkages. [ABSTRACT FROM AUTHOR]