State Water Resources Research Institute Program
Project ID: 2006VT25B
Title: Evaluating Quantitative Models of Riverbank Stability
Project Type: Research
Start Date: 3/01/2006
End Date: 2/29/2008
Congressional District: First
Focus Categories: Sediments, Models, Geomorphological Processes
Keywords: Riverbank stability, erosion, fluvial geomorphology, Vermont, slope stability model, soil shear strength, pore pressure
Principal Investigators: Dewoolkar, Mandar M.; Bierman, Paul
Federal Funds: $ 14,679
Non-Federal Matching Funds: $ 56,276
Abstract: The Vermont Department of Environmental Conservation recognizes that streambank erosion could be one of the most important nonpoint sources of sediment and phosphorus entering streams, rivers, and lakes, and thus one of the largest contributors to the impairment of surface water quality and aquatic habitat in Vermont. The Lake Champlain Basin Program also considers streambank erosion to be a potentially important source of phosphorus loading to Lake Champlain. Therefore, it is only logical that Vermont as well as other states are expending significant funds and effort to control streambank erosion. In Vermont, decisions concerning which stream banks require restoration are typically based on the State?s fluvial geomorphic model, which is comprised of three increasingly detailed, however still mostly qualitative, assessment phases.
The objective of the proposed study is to gain a fundamental understanding of physics and mechanics of riverbank instability in Vermont. Our quantitative evaluation will be based on in-depth geotechnical analysis of bank stability in order to understand and thus be able to predict what makes some banks stable and other banks fail over both time and changing river and groundwater conditions. The resulting quantitative models will incorporate measured soil strength parameters as well as real bank geometries and failure processes observed in the field, a major advance over the current state of knowledge. The semi-quantitative evaluation will be similar; however, soil strength parameters will be empirically correlated to index properties, which are cheaper and less time consuming to determine than soil cohesion and friction angle. We will use regression analysis to establish this correlation and to estimate its robustness.
Considerable progress has been made in the first year of the project since March 2006. In consultation with the Vermont Agency of Natural Resources, it was decided that three types of streams/rivers will be studied, i.e. large low-gradient, smaller low-gradient, and smaller higher gradient. Since the research methods involve a variety of in-situ soil testing, laboratory experiments, and multiple types of sensors, we decided to focus on one stretch of Winooski River to ensure that all methods and sensors would work satisfactorily. The Winooski River is a large low-gradient alluvial river. A borehole shear testing device was acquired and successfully used in the field to measure in-situ shear strength parameters of soils. Procedures for conducting cross-sectional surveys of streams were established. A variety of sensor systems and associated remote data logging system were developed. A rain gage, multiple pore pressure transducers, multiple tilt switches were installed at one cross-section on a bank of Winooski River. Additional seven cross-sections were selected with varying degree of perceived instability for conducting borehole shear tests and collecting soil samples for laboratory testing. Currently, development of devices for determining in-situ grass root strength and soil erosion is underway. Also, laboratory testing on collected soil samples and limit equilibrium based slope stability modeling are also underway.
This work will have substantial impact on the understanding of bank stability and sediment input to Vermont streams. Once we understand these processes quantitatively at specific sites, we will then be able to make more broad-ranging predictions of stream and riverbank behavior under a variety of different conditions because we can use models for extrapolating to different sites and changing conditions over time. In particular, we will be able to use our models to predict bank stability response to channel evolution over time and space as well as draw conclusions about erosion hazards. Thus, our work should be applicable far beyond the sites we study and should allow us to make predictions about basin-scale behavior of streambanks.
Progress/Completion Report, PDF