Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 1/6/2001
Publication Date: N/A
Interpretive Summary: Vegetation is often used to strengthen streambanks, and to improve environmental quality. Vegetation has several beneficial effects, such as mechanically reinforcing the bank, and drying it out. However, it also has potential negative effects, by increasing the delivery of water deep in the soil during rainstorms. We have monitored soil moisture under three different vegetation treatments (trees, long clump grass and bare ground) on an unstable streambank in Northern Mississippi over one year. In addition we have measured the strength added to the soil by vegetation roots. We have used the data in a computer model that calculates the stability of the streambank. We find that during the summer the tree cover dries out the soil at depth so that bank stability is greatly increased, compared with streambanks covered in grass or bare earth. However, rainfall in spring led to more rapid wetting under the tree cover than under the other covers. A large storm in April caused the bare streambank to collapse, while the other sections of bank were stable. At this time our model predicts that the streambank under the grass cover was one and a half times more stable, and the tree-covered bank two times more stable, than the bare bank. From our data we are able to say that this effect is due to transpiration by the trees the previous summer. This research suggests that three covers are potentially effective in strengthening streambanks.
Technical Abstract: Stabilization of streambanks using riparian vegetation offers numerous potential benefits, and some potential problems. These can be separated into mechanical and hydrological effects and though widely discussed, they are rarely quantified. Impacts are monitored and modeled for unstable streambanks on Goodwin Creek, Mississippi. Mechanical reinforcement has been quantified through measurements of root tensile strength, used to calculate increased soil cohesion. Hydrological effects of vegetation are monitored using interception plots and tensiometers under three covers: a cropped grass 'control' cover, clumps of eastern gamma grass, and a deciduous woody-vegetation stand. The measurements have been applied in a numerical model to assess the individual and combined effects of vegetation on streambank stability. On April 4th, 2000 prolonged rainfall at the field site caused bank failure at the control cover plot. The resulting factor of safety (Fs) values (incorporating both hydrological and mechanical effects) were 1.04, 1.64 and 2.18, respectively. Results show that the main contribution of the woody-vegetation to bank stability during the study period was hydrological rather than mechanical, and that the buildup of matric suction at depth during the summer moths persisted throughout the winter and spring. The beneficial hydrologic effect of woody vegetation may not be the case during wetter winters; more rapid loss of matric suction beneath woody-vegetation suggests that this bank may become less stable than the control or clump grass treatments due to enhanced infiltration of rainfall.