INTERACTION OF HYDRAULIC AND GEOTECHNICAL PROCESSES IN CONTROLLING BANK STABILITY
Watershed Physical Processes Research Unit
2007 Annual Report
1a.Objectives (from AD-416)
To improve our understanding of hydraulic and geotechnical processes operating at the bank toe of retreating streambanks and to use this understanding to develop more accurate numerical algorithms for predicting bank-toe erosion, bank stability and width adjustment in alluvial channels. This research will help to develop strategies for watershed management and to evaluate conservation effects where channel processes are important.
1b.Approach (from AD-416)
Conduct field and laboratory experimentation on fluvial entrainment of bank-toe materials which consists of: (A) improved evaluation of shear stress in the near-bank region; (B) improved evaluation of the critical shear stress of in situ and re-worked bank-toe materials; and (C) analysis of the excess shear-stress equation with archived and newly-acquired data to determine the variability and controls of its exponent.
This report serves to document research conducted under a Specific Cooperative Agreement between ARS and the University of Nottingham. Additional details of research can be found in the report for the in-house project 6408-13000-017-00D, "Integrated Assessment and Analysis of Physical Landscape Processes that Impact the Management of Agricultural Watersheds." Monitoring of field instruments that measure pore-water pressure at the Goodwin Creek Experimental Bendway were completed and results of numerical simulations indicate that lateral seepage forces can be an important control of mass failure along streambanks and at vertical sections of gully headwalls. This has been shown to occur by the enlargement of pathways in the bank or headwall that collapse, leaving an oversteepened section that is subject to failure by gravity. An investigation into the tensile strength of banks and gully headwalls was found to be significantly greater than previously thought and to vary with moisture content. Greatest tensile strength occurred at about 20% moisture content, decreasing under both wetter and drier conditions similar to cohesion. Results provide a means of estimating the resistance of gully headwalls in process-based simulations of gully retreat. Results of an investigation of the variability of geotechnical properties and their effect on estimates of bank stability were further refined to provide a means of establishing a probabilistic framework for estimating the upstream-downstream extent of bank failures. This will provide improved estimates of loadings from bank erosion in channel evolution models by more appropriately quantifying the volume of sediment delivered to the channel by a bank failure. Research results for a journal article submission are being revised. Research findings are critical in modeling soil erosion from streambanks and bank-side gullies from these features. The methods used for monitoring activities for this project included conference calls, e-mails, and site visits.