Meandering in Gravel-Bed Rivers
- Author(s): Braudrick, Christian Arthur
- Advisor(s): Dietrich, William E
- et al.
It is surprising that gravel-bed rivers meander. In laboratory settings using cohesionless sediment transported as bedload, braiding inevitably emerges. The easily entrained outer bank sediments lead to relatively rapid bank erosion, the inner bank bar reaches it shallowest depth towards the center of the channel, the flow diverges, and braiding arises. These observations have led to the inference that meandering requires some bank strength to slow outer bank erosion and allow the inner bank to keep pace with outer bank retreat. Commonly it has been suggested that vegetation can provide that outer bank strength. While this view has led recently to numerical modeling incorporating vegetation strength, no studies had successfully created meandering rivers in the laboratory such that controlling mechanisms could be explored. Nor has there been an effort to delineate under what general conditions gravel bedded meanders are found in the field. These knowledge gaps are particularly important to stream restoration work because the creation of morphologically-stable gravel-bedded meanders is a common goal.
This dissertation uses flume experiments and a compilation of field data to explore the conditions that support meandering in gravel-bed rivers. Using alfalfa sprouts as model vegetation, sand as scaled-down gravel, and a lightweight plastic as scaled-down sand, I created for the first time a self-maintaining, laterally migrating meandering river with cutoff loops in a 6.1-m wide, 17-m long laboratory flume. The channel, 0.4 m wide, had a sinuosity of 1.1-1.2 and transported sediment with a median size of 0.78 mm. The sinuosity of the channel increased as bends grew, decreased as the channel cut off, and was regenerated while the channel maintained a steady width. In this experiment, we found that a steady bankfull flow was sufficient to sustain a meandering planform, and that application of higher peak flows caused the channel to widen because bank erosion outpaced bar growth. Coarse sediment was exchanged between eroding banks and the next bar downstream with little net downstream flux, consequently to prevent aggradation and avulsion at the upstream end of the experimental reach required turning off the coarse sediment feed. The input of fine sediment was crucial for blocking chute channels (a locus for braiding), filling point bars downstream of the bend apex, and plugging abandoned channels following cutoff. Hence, sustained growth of bends and development of meandering requires more than just sufficient bank strength to slow the outer bank erosion rate.
A compilation of 166 gravel-bedded meanders from the literature shows that gravel meanders primarily occur in lower gradient reaches extending from mountain ranges, high-elevation valleys, and in humid areas influenced by glaciation or glacio-fluvial outwash. Unexpectedly, analysis of Google Earth imagery showed that about 1/3 of the dataset lacked cutoff scars and other evidence of active migration. Gravel bedded meanders with median surface grain size> 10 mm had low Shields stress, particularly for meanders with cutoffs and without islands. Gravel bedded meanders with median surface grain sizes < 10 mm had higher Shields stresses that were transitional between coarser gravel bedded meanders and finer sand bedded meanders. Calculation of potential sediment transport rates show that for 16 of the 18 gravel bedded meanders with migration rate data, the sediment flux from bank erosion exceeds this transport capacity, suggesting that bank erosion is a primary source of sediment. Collectively this shows that gravel bedded meanders can be classed into different types that reflect the relative migration rates and associated Shield stress and that generally gravel-bedded meanders are associated with low Shield stresses and relatively low sediment supply.
Because of the importance of sediment supply in meandering rivers, I also investigated the morphological effects of doubling supply to a self-formed sinuous channel with 9 bends that was fixed in place. Five of these bends developed bar-pool topography. The doubling of supply caused the channel to steepen by 33%, and the topographic response of bars was limited to shoaling of two pools as they filled with coarse sediment. One of the bars also extended upstream. Three other bends had limited changes to an increase in supply, likely due to the fixed walls that supported wide bars and narrow, deep pools. The maximum aggradation was greater than a channel depth, indicating that for a similar supply increase in a freely migrating channel would lead to avulsion. This experiment supports further the interpretation that gravel-bedded meanders are associated with relatively low sediment supply.
The final experiment replicated our first alfalfa experiment but used a constant discharge and sediment supply with much denser alfalfa, in order to explore the effect of increased bank strength on channel morphodynamics. The denser alfalfa led to a narrower channel, but the migration rate was similar to our first experiment. As the channel migrated, associated decreases in the water surface slope caused the style of migration to change from focused on the entire bend to focus at the bend apex, creating skewed bends. As the bend skewness increased, the sediment flux at the flume outlet decreased, and most of the sediment flux out of the experimental reach was the lightweight plastic (model sand) rather than the coarse sediment supply. This created a channel with a sinuosity of 1.6, but then the channel aggraded and cut off, decreasing the sinuosity (to 1.2) and a corresponding increased slope. Following the cutoffs, the sediment transport rates and channel migration rates increased. The steady flow discharge and dense alfalfa limited cutoffs until aggradation altered floodplain hydrology to suppress alfalfa growth and create relatively weak paths through the floodplain.
Taken together these studies show that for gravel bedded rivers to meander three conditions are needed: 1) additional bank strength (here from vegetation) to slow erosion rates' 2) low sediment supply and correspondingly low Shields stresses to prevent aggradation and avulsion as sinuosity increases; and 3) fine sediment to fill chutes and the downstream end of bars. Variable peak discharges are not necessary but may contribute to cutoffs that maintain a steeper slope able to transport the sediment supplied to the reach. These observations suggest that creating a self-maintaining laterally migrating gravel-bedded river as a restoration outcome will require an assessment of likely coarse and fine sediment supply as well as the ability of vegetation to provide sufficient bank strength.