Submitted to: Book Chapter
Publication Type: Book / chapter
Publication Acceptance Date: 5/15/2003
Publication Date: 5/10/2004
Citation: Pollen, N., Simon, A., Collison, A. 2004. Advances in assessing the mechanical and hydrologic effects of riparian vegetation on streambank stability. In: Bennett, S.J., Simon, A., Editors. Riparian Vegetation and Fluvial Geomorphology. American Geophysical Union, Washington, DC. p. 125-140. Interpretive Summary: Streambank instability causes problems for the ecology and health of a stream and for users of the stream. Sediment pollutes streams in many areas and affects water quality so a priority for river managers is to stabilize the banks and prevent this sediment being transferred into the stream. Streambank vegetation can protect the banks above the ground by preventing the washing away of sediment, and below the ground the vegetation can help to prevent bank collapse because the vegetation roots strengthen the soil. Plants can also remove water from the soil by evaporation and transpiration and this further strengthens the soil as it is less moist. However, engineers often prefer to use hard materials such as concrete and rocks to protect the banks as these do not change over time. This study looks at past methods used to calculate the strengthening effect of vegetation roots on streambanks and to see what the effects are from roots compared to the effects from evapotranspiration. Results so far show that previous methods used to calculate root strengthening of the soil give values that are too high. The experiments also show that the removal of water by plants can have a large effect on streambank stability.
Technical Abstract: Streambank instability poses a number of economic and ecological problems. As sediment has been reported to be one of the principal contaminants of rivers in many areas, a high priority of river managers is to stabilize streambanks to prevent additional sediment being added to the channels from mass-wasting and fluvial entrainment of the bank material. Riparian vegetation plays a number of roles in the protection of streambanks from erosion by the processes of particle entrainment and mass wasting, and its use in stabilization has a number of possible benefits, but is often neglected in favor of hard stabilization measures that are more easily quantifiable and remain constant over time. This study seeks to investigate the importance of the assumptions previously made in calculations of soil reinforcement by roots, and aims to study how root networks, and the contribution to soil strength made over time varies both mechanically and hydrologically. Results show that previous methods used to estimate root reinforcement may have produced overestimates of up to 91%. Hydrological reinforcement by evapotranspiration may also be important and results show that the pore-water pressures in experimental soil monoliths were reduced by up to 17.8 kPa during the spring of 2002, by trees that were just 2 years old.