Vegetated buffer strips are widely advocated and applied as a Best Management Practice in riparian environments. This chapter presents data on the mechanical properties of riparian species commonly found or planted in riparian environments and assesses their impact on streambank stability for a number of typical scenarios using the ARS Bank Stability Model. The ARS Bank Stability Model is used to assess the relative contributions made by the new and old species tested on bank stability in several scenarios, and to explore the consequences of root growth rates and mobilization. Vegetation affects streambank stability through mechanical and hydrologic processes, and the effects are both beneficial and detrimental. The chapter also evaluates the effects of plant age on root reinforcement and estimates the difference between potential and actual root strength contributions to soil strength and streambank stability.

Grass hedges are dense, erect, vegetative barriers made of large-stemmed grass that slow runoff and reduce erosion. Research has shown that well-established hedges can remain erect against flows that pond to depths of 0.4 to 0.5 m. We hypothesized that planting grass hedges every 0.45 m vertically within a gully would allow the entire gully bed to be protected by low-velocity backwater areas during high flows. To test this we established a series of hedges in a number of concentrated flow channels during 2000. Two of the channels were previously eroded spillways cut into in compacted fill; and all flow was diverted from these channels during the hedge establishment period. The other channels were located at the margin of floodplain fields adjacent to an incised stream channel, Little Topashaw Creek, in Chickasaw County, MS, USA. Natural rainfall runoff was allowed to pass through most of the Topashaw channels during hedge establishment, and various techniques were used to keep transplanted hedge grasses from being washed away during their establishment period. We gauged the discharge through some of these channels and monitored resulting damage. We plan to introduce flow into these three channels during February and March, 2002, using synthetic trapezoidal-shaped hydrographs with peak discharge rates of 0.042, 0.085, and 0.170 m 3 s -1 , flow rates that are similar to those observed flowing through gullies into Little Topashaw Creek. We will monitor pore water potential that contributes to mass failure of soil blocks and record the extent and mode of erosion of the hedge-lined channel during the controlled inflow tests.

Riverbanks are usually stabilized by reinforcing the soil, installing horizontal drains or by regrading the slope. Though well established, these techniques incur problems including ground disturbance, inability to drain deep within the bank, loss of land and expense. A potential alternative is to increase bank stability by actively lowering the water table using submerged pumps, reducing positive pore-water pressure and promoting the development of matric suction. This approach is suitable in critical locations such as bridge abutments, where rapid bank stabilization is required, or where deep drainage is needed. It also has potential as a medium term technique to stabilize banks until vegetation or other reinforcing measures have had time to take effect.

Riparian vegetation has many well-documented beneficial properties for river restoration and bank stabilization. However, the hydrologic effects of vegetation on bank stability are more ambiguous, poorly quantified, and potentially detrimental in some cases. This paper presents results from an experiment in which the hydrologic effects of different riparian covers were quantified in relation to bank stability. Geotechnical and pore-water pressure data from streambank plots under three riparian vegetation covers (bare, grass and trees) were used to drive the ARS Streambank Stability Model, and the resulting factor of safety (Fs) was broken down into its constituent parts to assess the contribution (beneficial or detrimental) of individual hydrologic and mechanical effects. Data on the mechanical effects of vegetation were obtained in an associated project reported in a companion paper. Canopy interception was relatively insignificant (approximately 3%) especially during the crucial winter and spring period when most bank failures occur. Interception remained insignificant during the summer as most rain fell in high intensity events that exceeded the storage capacity of the canopy many times over. However, transpiration under the tree cover had a very significant impact on pore-water pressure which persisted through the winter and spring in some cases.