Sinuosity

Abstract: A series of experiments was performed in a large flume to determine the effect of slope and sediment load on channel patterns. Sediment loads and slopes were closely related, and as slope and sediment loads increased, threshold values of these variables were encountered, at which channel patterns altered significantly. At a very low slope and sediment load, the channels remained straight, but at a discharge of 0.15 cfs, a meandering-thalweg channel formed at slopes greater than 0.002. With increased slope and sediment loads, thalweg sinuosity increased to a maximum of 1.25. At slopes greater than 0.016, a braided channel formed. The model channels responded to increased sediment loads by maintaining steeper gradients and by major channel pattern changes, but at very gentle slopes and at steep slopes, the channel could not be forced to develop a meandering thalweg. These experiments suggest that landforms may not always respond progressively to altered conditions. Rather, dramatic morphologic changes can occur abruptly when critical erosional and (or) depositional threshold values are exceeded. The meandering-thalweg channel was not a meandering channel. A truly meandering channel with a sinuosity of 1.3 formed when a suspended-sediment load (3 percent concentrations of kaolinite) was introduced into the flow. The clay stabilized the alternate bars, and scour and deepening of the thalweg resulted. This in turn lowered the water level at constant discharge, and the alternate bars emerged o t form point bars. A meandering-thalweg channel was thus converted to a meandering channel by the type of sediment load change that has accompanied climatic and hydrologic changes of the recent geologic past.

Abstract: Data on the morphologic and sediment characteristics of stable alluvial rivers of the Great Plains were collected at 50 cross sections. The channel patterns of these rivers were classified into five types: tortuous, irregular, regular, transitional, and straight. Because no clear demarcation existed between each of the types, the pattern of the rivers was described by sinuosity, a ratio of channel length to valley length. The sinuosity ( (P) ) of these rivers is related to the shape of the channels expressed as a width-depth ratio ( F ) and to the percentage of silt and clay in the perimeter of the channel ( M ) as follows: ![Formula][1] ![Formula][2] Sinuous streams are characterized by a low width-depth ratio ( F ), a high percentage of silt-clay in the perimeter of the channel ( M ), a high percentage of silt-clay in the banks (although the banks of straight channels may also contain large amounts of silt-clay), and a lower gradient than straight channels having the same mean discharge. Discharge itself does not appear to affect the sinuosity of streams. Another possible distinction between straight and sinuous streams is in the proportions of the components of total sediment load. In a wide, shallow channel much of the sediment transported is bed-material load. In a narrow, deep channel most of the sediment transported is wash load. On the Great Plains both straight and sinuous streams may flow on the surface of alluvial valley fills at about the same valley slope. The departure of a stream from a straight course down the alluvial valley results from changes in both the caliber of the sediment load and in the relative proportions of bed-material load and wash load during the post-Pleistocene alluviation of these valleys. When during this alluviation the proportion of wash load increased, most probably by a decrease in bed-material load, the stream adjusted itself by decreasing its gradient through the development of a sinuous course. Recent changes in stream sinuosity in response to changes in the proportions of bed load and suspended load support this hypothesis. [1]: /embed/graphic-1.gif [2]: /embed/graphic-2.gif

Abstract: Alluvial rivers respond to valley-slope deformation caused by active tectonics in various ways depending on the rate and amount of surficial deformation and on the type of river. On the basis of experimental results and field examples, hypothetical models of river response to anticlinal uplift and synclinal subsidence were developed for different types of alluvial rivers. An experimental braided channel responded to anticlinal uplift across the channel with degradation and terrace formation in the central part of the uplift. With subsidence, aggradation in the central reach was the main response. Transverse bars developed downstream of the subsidence axis. An experimental meandering channel responded to slope steepening with a sinuosity increase. Bank erosion and point-bar growth occurred downstream of the anticlinal axis and upstream of the synclinal axis. Upstream of the uplift axis and downstream of the subsidence axis, where the slope was flattened, water flooded over bars. Local convexity in longitudinal profiles of the middle Rio Grande, New Mexico, is considered to be formed by a domal uplift. Local aggradation and degradation could be explained by the effect of uplift. The San Joaquin River, California, which is now highly controlled, does not show clear adjustment to the rapid subsidence due to ground-water withdrawal. It shows, however, a channel-pattern adjustment to active tectonic subsidence that has been occurring for a long time. The San Antonio and Guadalupe Rivers in Texas both increase their sinuosity significantly where monoclinal movements steepen valley slopes.