Changes in streambank erodibility and critical shear stress due to subaerial processes along a headwater stream, southwestern Virginia, USA

Despite more than 40 yr of research attributing temporal changes in streambank erosion rates to subaerial processes, little quantitative information is available on the relationships between streambank erodibility (k(d)) and critical shear stress (tau(c)) and the environmental conditions and processes that enhance streambank erosion potential. The study goal was to evaluate temporal changes in k(d) and tau(c) from soil desiccation and freeze-thaw cycling. Soil erodibility and tau(c) were measured monthly in situ using a multiangle, submerged jet test device. Soil moisture, temperature, and bulk density as well as precipitation, air temperature, and stream stage were measured continuously to determine changes in soil moisture content and state. Pairwise Mann-Whitney tests indicted k(d) was 2.9 and 2.1 times higher (p<0.0065) during the winter (December-March) than in the spring/fall (April-May, October-November) and the summer (June-September), respectively. Regression analysis showed 80% of the variability in kd was explained by freeze-thaw cycling alone. Study results also indicated soil bulk density was highly influenced by winter weather conditions (r(2)=0.86): bulk density was inversely related to both soil water content and freeze-thaw cycling. Results showed that significant changes in the resistance of streambank soils to fluvial erosion can be attributed to subaerial processes. Water resource professionals should consider the implications of increased soil erodibility during the winter in the development of channel erosion models and stream restoration designs. (C) 2007 Elsevier B.V. All rights reserved.