Effects of grid scale and resulting initial bed disturbance differences on the evolution of braided rivers

<p>Physics-based models have been increasingly developed in recent years and applied to simulate the braiding process and evolution of channel units in braided rivers. Braided rivers are the river network system characterized by the staggered distribution of bars and channels. In the numerical calculation, the grid scale affects the behavior process and morphological description of braided rivers. In this paper, a 2D numerical model is used to simulate the evolution of the braided rivers where the transport of load bed sediment plays a dominant role. In the natural scale braided rivers evolution modeling, the difference of the braided rivers' morphological characteristics under different grid scales is discussed, and the influence of different distribution of topographic disturbance caused by grid scale difference on the morphological characteristics of braided rivers is discussed. The study shows that the grid scale does not affect the description of braided rivers evolution process, and braided rivers evolve in the same way regardless of grid scale (within a reasonable range). However, the statistical characteristics of braided rivers are greatly affected by the grid scale. The braiding index increases as the grid scale decreases, but when the grid scale decreases to a certain extent (20m in this paper), the braiding index no longer increases. The number and average height of bars in braided rivers increase with the decrease of grid scale, and the average area of bar near riverbed also increases with the decrease of grid scale, but the average area of bar near water surface does not change with the change of grid scale. In general, the higher the grid resolution is, the more similar the bar morphology in the numerical model is to that in natural rivers. In addition, the different distribution of topographic disturbance caused by the grid scale difference has an influence on the braiding intensity and the bar morphology of the braided rivers, but the influence degree is much smaller than that caused by the grid scale difference.</p>

Influence of river pattern and sediment input on estuary morphology

<p>Alluvial estuaries are dynamic landscapes that are very sensitive to changes in boundary conditions such as river discharge and sediment supply. A better understanding of the influence of upstream river discharge and sediment input on the development of estuaries under various scenarios requires long-term morphodynamic models, to both predict future changes and improve geological interpretations by storing the stratigraphy. Past 1D model studies have shown that upstream river discharge has a significant effect on the equilibrium bed profile of estuaries, but these studies ignore the effect of 2D bar and channel formation. Using 2D numerical models to predict the development of these systems on the scale of millennia proved to be difficult, since the modelled morphology is very sensitive to the choice in e.g. sediment transport predictor and bed slope effect. In this study, we use the knowledge of previous research that determined best parameter settings for realistic river and bar patterns to model long-term and large-scale estuary morphodynamics in Delft3D. Our objective is to quantify the effects of river discharge and sediment supply on the shape of estuaries and its deposits. Firstly, we systematically varied upstream river width and tidal amplitude to examine the relation between upstream river pattern and estuary dimensions. We quantified e.g. braiding index, bar dimensions, and tidal excursion length. Results show that flood flow velocities and tidal prism are less influenced by river discharge than suggested by 1D models, and are significantly influenced by the braiding index of the river. With relatively high tides, estuary bar patterns depend on tidal amplitude, while with lower tides estuary depth and braiding index are related to upstream river width and discharge. Next steps will include varying discharge to study the effect on the rate of adaptation of the river and estuary, and varying the grain size of the sediment input at the upstream boundary. We will input coarse sediment to explore differences between fluvial deposits and tidal currents, and fine sediment to use the model for research related to biofilm.</p>

A Study on Variation in Channel Width and Braiding Intensity of the Brahmaputra River in Assam, India

The Brahmaputra River flows through Assam, India, for about 670 km along an alluvial valley as a wide braided river. The width of the river varies with time along its course. The braiding intensity of this river is estimated using the braiding index (BI) of Brice (1964), which also changes with space and time along the course of the river. Temporal changes of both width and BI have been studied using topographic maps of 1912–1928 and 1963–1975, and dry season satellite data of 1996, 2000, 2007 and 2009. The mean widths of the Brahmaputra River channel in Assam during 1912–1928, 1963–1975, 1996, 2000, 2007 and 2009 were 5949 m, 7455 m, 7505 m, 8008 m, 8308 m and 9012 m, respectively, confirming an overall increase in width with time. Both the width and variation of width are lowest in four short narrower segments of the river. Three of these segments represent hard points comprising gneissic rock, and one segment is on alluvium comprising cohesive clay. The increase in width is correlated to enormous sediment load produced by the great Assam earthquake of 1950 and large-scale deforestation in the Himalayas. The mean BIs for the Brahmaputra for 1963–1975, 1996, 2000, 2007 and 2009 were 8.59, 8.43, 6.67, 6.58 and 7.70, respectively, indicating in general a decreasing trend up to 2007. The BI showed low variation at the four narrow segments where there is also a minimum variation of the channel width. The BI has increased significantly in the upstream part of the river. Very high fluctuation of discharge (17,000 m 3 / s − 1 in 24 h) and high sediment loads of the Brahmaputra (daily mean sediment discharge of 2.0 million tonnes during monsoon), erodible alluvial banks and high width/depth ratios are the main causes of development of braiding. The interrelationship between channel width and BI of the Brahmaputra shows a positive correlation, indicating an increase in BI with increasing channel width.

Development of Chute Cutoff in the Lower Course of River Mayo-Inne, Yola South, Nigeria

The study investigates the development of chute cutoff in the lower course of River Mayo-Inne, Yola South LGA, Adamawa State, Nigeria. The study employed the integrated approach of Remote Sensing, Geographic Information System, Field Survey, Laboratory Analysis, Oral Interview and Personal Observation in examining the influences of some relevant channel planform parameters (Sinuosity Index, Cutoff Ratio and Braiding Index), land use/land cover, channel bank materials, water stage and channel depth on the development of the chute cutoff over a period of Twenty five years (1990-2015). Results revealed the drastic reduction of Sinuosity Index from 1.57 in 1990 to 1.46 in 2015, changing the channel from meandering to the straight pattern. The analysis of changes in cut-off ratio unveiled the development of chute cutoff in bend II, which ultimately separated the river flow, forming a weak braided channel with a braiding index of 0.43. These developments were attributed to incessant flooding in the study area and floodplain characteristics such as floodplain elevation, bank strength and changes in vegetal cover.

The influence of transverse slope effects on large scale morphology in morphodynamic models

Large-scale morphology is greatly affected by the amount of downslope sediment transport on slopes transverse to the main flow direction, which determines bar length and bifurcation dynamics. Consequently, the transverse slope parameter is a crucial part of morphodynamic models. However, existing models have the tendency to overpredict channel depth and braiding index, and therefore slope effects are often increased when calibrating on existing morphology. The objective of this study is to identify possible causes of the overdeepening of channels in the morphodynamic model Delft3D, and to show how different slope effect parameterisations affect morphology in rivers and estuaries. Results show that the two methods to calculate transverse sediment transport in Delft3D have a significantly different effect on the predicted morphology and this effect is larger for environments with a large braiding index, in combination with the sediment transport predictor of Engelund-Hansen. Results furthermore imply that even when models are initially calibrated on existing morphology, results could significantly differ downstream when calibrated with a different choice of transverse slope option.