Date of this Version
The overall objective of this study was to evaluate the potential impacts of bank stabilization on the morphologic processes in the Missouri River with a particular emphasis on the formation and persistence of habitat bars. This investigation addresses the following four open water reaches of the Missouri: (1) Fort Peck Dam to vicinity of Yellowstone River (304 kilometers); (2) Garrison Dam to Lake Oahe (127 kilometers); (3) Fort Randall Dam to the Niobrara River (58 kilometers); and (4) Gavins Point Dam to Ponca (93 kilometers). This report provides an additional tool for designers and managers to use when developing and assessing bank stabilization projects.
A detailed geomorphic, hydrologic, and sediment transport analysis of each study reach was conducted. A total of 655 sediment samples from the banks, bed, bars, islands and tributaries were collected and analyzed. The percent of the bank material greater than the bed material size for each reach ranged from about 21% in the Fort Randall Reach to 60% in the Garrison Reach with the Fort Peck and Gavins Point Reaches both being about 48%. Each study reach was divided into individual Geomorphic Reaches (GR), and a sediment budget was calculated for each GR as well as for the entire study reach. The sediment budget was calculated using comparison of historical aerial photography and cross-sectional data from the late 1960s to 1998. From the sediment budget, the percent of the total bed material load comprised of material supplied from the banks was calculated. The percent bank contribution varied considerably from GR to GR, ranging from as low as 3% to as high as 58%. The overall study reach bank contribution percentages are: (1) Fort Peck Reach - 17%; (2) Garrison Reach - 13%; (3) Fort Randall Reach - 8%; and (4) Gavins Point Reach - 24%. Tables are provided that show the percent reduction in bank material supply resulting from various stabilization schemes ranging from stabilizing 10%,20%, etc. up to 100% of the eroding areas for each GR.
The supply of sediment from the banks is only one factor that affects bar morphology. The three primary factors, identified in this study, necessary for the formation and persistence of bars are a supply of suitably sized sediment, a local channel geometry (channel width) and a stability status (aggradation, degradation, or equilibrium) that allows and promotes bar existence. In a system such as the Missouri River, where there is an abundant supply of material, the local geometry is probably the dominant factor with respect to bar morphology. As a consequence, when considering the potential impacts of a proposed bank stabilization scheme, the investigator can not just focus on one factor, but rather must consider a number of factors. Each bank stabilization project should be evaluated on a case by case basis in an engineering-geomorphic investigation that identifies and quantifies the impacts of channel width, reduction in sediment supply, and existing stability of the reach. Guidance for the evaluation of these factors is provided.