Bedload Sediment Flux Research Articles

Pinpointing the key parameters in estimating the bed-load sediment flux in reservoir engineering

When a dam is constructed on a river to store water, sediments transported by the water flow are also stored and reservoir capacity is gradually reduced by sediment accumulation. Prediction of sediment distribution in reservoirs is an important issue for dam designers to determine the reservoir active storage capacity, outlet sill elevation, dam stability, recreational facilities, and backwater conditions. The main objective of this study is to develop the most reliable parameters of sedimentation that are directly or indirectly influencing in the equations and measured dataset. For validation of the proposed parameters, data of 40 reservoir sets gathered from different reliable sources, rather than focusing entirely on bed-load equations. Artificial neural network (ANNs) method was used to validate this study. Several graphs and statistical analysis were presented to emphasize the influencing effect of those parameters that were detected by ANNs and are directly controlling the error in the bed-load sediment flux using measured particles datasets.

Evaluating the controls of shear stress, sediment supply, alluvial cover, and channel morphology on experimental bedrock incision rate

We explored the dependence of experimental bedrock erosion rate on shear stress, bed load sediment flux, alluvial bed cover, and evolving channel morphology. We isolated these variables experimentally by systematically varying gravel sediment flux Qs and water discharge Qw in a laboratory flume, gradually abrading weak concrete “bedrock.” All else held constant, we found that (1) erosion rate was insensitive to flume‐averaged shear stress, (2) erosion rate increased linearly with sediment flux, (3) erosion rate decreased linearly with the extent of alluvial bed cover, and (4) the spatial distribution of bed cover was sensitive to local bed topography, but the extent of cover increased with Qs/Qt (where Qt is flume‐averaged transport capacity) once critical values of bed roughness and sediment flux were exceeded. Starting from a planar geometry, erosion increased bed roughness due to feedbacks between preferential sediment transport through interconnected topographic lows, focused erosion along these zones of preferential bed load transport, and local shear stresses that depended on the evolving bed morphology. Finally, continued growth of bed roughness was inhibited by imposed variability in discharge and sediment flux, due to changes in spatial patterns of alluvial deposition and impact wear. Erosion was preferentially focused at lower bed elevations when the bed was cover‐free, but was focused at higher bed elevations when static alluvial cover filled topographic lows. Natural variations in discharge and sediment flux may thus stabilize and limit the growth of roughness in bedrock channels due to the effects of partial bed cover.

Influence of Vessel Passages on Tidal Creek Hydrodynamics at Aransas National Wildlife Refuge (Texas, United States): Implications on Materials Exchange

Abstract The impact of commercial vessels on coastal hydrodynamics and sediment movement has been measured and simulated in shipping channels and rivers. However, little empirical data exist on the temporal variation in vessel-generated forces in addition to the ecological implications for natural waterways such as tidal creeks and bayous. Along the U.S. Gulf of Mexico coast, these ecosystems may be highly susceptible to unnatural currents created by commercial vessel traffic. We sought to characterize and quantify the impact of large vessel traffic in the Gulf Intracoastal Waterway (GIWW) on tidal creek hydrodynamics at Aransas Wildlife Refuge (ANWR). Creek water level was monitored at 1-minute intervals in three different tidal creeks at ANWR during the summer of 2004. We also conducted preliminary measurements of bedload sediment flux associated with vessel passages. Vessel-induced fluctuations in water level in at least one site were the equivalent or greater than the diurnal tidal range (about 0.1 m)...

Bed Load Sediment Transport in an Ephemeral Stream and a Comparison with Seasonal and Perennial Counterparts

Bed load sediment flux in an ephemeral channel, the Nahal Yatir, is shown to be a comparatively simple function of stream power and to reach levels that are several orders of magnitude higher than maxima measured at similar levels of stream power in perennial counterparts. Channel average submerged unit flux rates are recorded as high as 4.3 kg s−1 m−1, while at the center of the channel, the highest rate recorded is 6.5 kg s−1m−1. Transport efficiency is at least an order of magnitude higher than in other channels for which there are comparable data and, on average, as much as 400 times that of Oak Creek. These differences are explained by the fact that the bed of the Yatir is not armored. It is surmised that the unvegetated nature of this desert watershed provides ample supplies of sediment of all sizes and that this, together with the rapid recession of the flash flood hydrograph and the extended periods of no flow, discourages the development of an armor layer. The flux rates are not sediment supply‐limited, as they are in perennial channels. Nahal Yatir and Oak Creek represent two ends of a spectrum, between which come seasonal and less well armored perennial streams. Transport efficiency is shown to vary considerably for each stream and from one stream to another, suggesting that it may not be possible to incorporate it easily into bed load equations in order to improve levels of prediction.

A cellular model of braided rivers

A BROAD sheet of water flowing over non-cohesive sediment typically breaks up into a network of interconnected channels called a braided stream (Fig. 1). The dynamics of such networks are complex; channels migrate laterally, split, rejoin and develop bars, with the flow shifting unpredictably from one part of the network to another. Many processes are known to operate in a braided river1–3, but it is unclear which of these are essential to explain the observed dynamics. We describe here a simple, deterministic numerical model of water flow over a cohesionless bed that captures the main spatial and temporal features of real braided rivers. The patterns arise from local scour and deposition caused by a nonlinear dependence of bedload sediment flux on water discharge. Although the morphology of the resulting network depends in detail on the sediment-transport rule used in the model, our results suggest that the only factors essential for braiding are bedload sediment transport and laterally unconstrained free-surface flow.