Stream Channel Stability

This process-oriented study was organized to investigate three complementary aspects of channel stability including: (a) the nature of channel failure processes; (b) the influences of valley-fill depositional units on these processes; and (c) the properties and distributions of the valley-fill units. The study included the near-surface geologic investigation, investigation of the late-Quaternary valley-fill deposits, and channel morphometric investigations. The properties and distributions of the valley-fill units directly and indirectly influence the nature of channel failure processes. Although gravity-induced failure is the most frequent form of present-day bank instability, the type of gravity failure is dependent upon the properties of the valley-fill units. Both depositional and weathering properties influence the type of failure. The valley-fill units indirectly influence bank stability through their control of groundwater movement and the development of unusually large seepage forces at point-locations along the channels. Bed instability has primarily resulted from upstream migration of knickpoints and the rate of knickpoint migration has been affected by (valley-fill) unit controls. Present drainage systems in the study area are immature; channel morphometry has not adjusted at this time to the new flow regime resultant from cultural and natural changes.

Since effective streambank protection measures are costly to install, a determined effort should be made to use vegetation because it is the most readily available material and is relatively inexpensive to obtain. Vegetation greatly reduces the hydraulic forces on the bank and provides other esthetic and environmental advantages over other methods of stabilization. This report describes a series of streambank vegetative studies located on the channels of Johnson, Goodwin and Peters Creek in Panola County near Batesville, Mississippi. On Johnson Creek, there are four sites; two of them use vegetation in conjunction with bank shaping and structural materials, the other two sites use vegetation in conjunction with structural devices without bank shaping. The site of Goodwin Creek uses vegetation in conjunction with bank shaping--with and without structural materials. The Peters Creek sites use woody vegetation in conjunction with structural devices without bank shaping. Criteria used in the design of the combined vegetative and structural projects are presented along with a detailed description of the project sites. Since these type studies require several years to evaluate, only preliminary results are presented and many of them are based on previous experience.

In a stable alluvial channel, no net erosion or deposition of sediment occurs on the average. The sediment supply rate from upstream is balanced by capacity of the flow to transport the bed material. Any successful channel design must maintain this equilibrium or establish it for channel reaches that are not stable. Design relationships between the bed material transport capacity and the hydraulic variables of flow are based primarily on data from relatively small test channels. Reliable data for equilibrium transport of bed material by flows over about twenty cfs are not adequate to insure that data from small flumes can be extrapolated to prototype designs. An investigation was conducted in the 250-ft long test channel at the USDA Sedimentation Laboratory to obtain additional data on equilibrium transport by flows up to 150 cfs. Data on the transport rates, flow friction factors, and statistical properties of the bed forms were obtained. The results are presented as basic variable correlations with the controlled variables of the experiments, depth and discharge, along with attempts to generalize the relationships by similitude principles. (Author).

The objective of this study is to develop a better understanding of the stochastic structure of the instantaneous boundary shear processes in open channel flows. This report presents new measurements of some stochastic properties of boundary shear stress taken at points spaced over half the wetted perimeter of a smooth open channel, under conditions of essentially constant aspect ratio and Reynolds number. The instantaneous boundary shear stress was measured with hot-film sensors driven by constant-temperature anemometers. The anemometer signals were digitized and analyzed in a high speed computer. The effects of position along the wetted perimeter on the statistical moments and the probabilistic distribution of the instantaneous boundary shear stress are discussed. (Author).