Application of a Deterministic Bank-Stability Model to Design a Reach-Scale Restoration Project

Sediment is one of the leading contributors to water-quality impairment in the United States and streambank erosion has been found to be the dominant source of sediment in many watersheds with heightened sediment loads. Goodwin Creek is a typical incised channel in northeastern Mississippi (4.7 m-deep) that yields about an order of magnitude more suspended sediment than stable, "reference" streams in the Mississippi Valley Loess Plains ecoregion. Periodic channel surveys in conjunction with dating of woody vegetation growing on the channel banks and bars in an actively eroding meander bend between 1977 and 1996 were used to determine an average migration rate of about 0.5 m/y over the period. Channel processes and sediment-transport rates were monitored in detail between 1996 and 2006 and showed a similar migration rate. Because of continued land loss in adjacent agricultural fields by mass failure of the streambanks, a restoration project was designed to stabilize the banks and to protect a road running parallel to the bendway. Bank retreat occurs by interactions between hydraulic forces acting at the bed and bank toe and gravitational forces acting on in situ bank material. To provide a stable alternative, analysis of the restored configuration needed to address both hydraulic erosion and geotechnical stability. This was accomplished using a Bank-Stability and Toe-Erosion model developed at the USDA-ARS National Sedimentation Laboratory. The proposed design was limited to 1:1 bank slopes due to the proximity of the road and included longitudinal stone-toe protection and bendway weirs to counter basal erosion by hydraulic shear. Worst-case geomorphic conditions under the proposed design were simulated by modeling (1) typical, annual high flows (3 m-deep) to evaluate the amount of bank-toe erosion that would occur, and (2) geotechnical stability where groundwater levels were high and flow had receded to low-flow conditions in the channel (drawdown case). Results showed that the bank would still be unstable at 1:1 under the drawdown case but that the addition of specific riparian vegetation on the slope would stabilize the bank even under worst-case conditions. The design was, therefore, implemented and constructed in a week.