Effects of Herbaceous Riparian Vegetation on Streambank Stability
Riparian vegetation is widely thought to stabilize stream banks, but this effect has rarely been quantified. We evaluated how riparian vegetation affects bank erodibility along the South Fork of the Kern River at Monache Meadow. Our methodology combines remote sensing estimates of bank migration rates with field measurements of bank strength. Our results demonstrate that vegetation communities significantly affect bank erodibility of a meandering montane meadow stream.
The South Fork Kern River at Monache Meadow is an ideal location for studying vegetation effects on bank erosion. The stream bank soils are relatively homogenous, but are colonized by two distinct vegetation communities: a dry meadow community dominated by sagebrush and non-native grasses, and a wet meadow community dominated by rushes and sedges. We measured rates of channel migration in the wet meadow versus dry meadow by analyzing four decades of aerial photographs. Over 40 years, the stream channel migrated laterally up to 100 meters in the dry meadow, but less than 10 meters in the wet meadow. We used these measurements in combination with a numerical model of flow to calculate an erodibility coefficient that characterizes bank migration potential independent of channel curvature.
To complement this remote sensing analysis, we made field measurements of channel geometry, bed and bank material grain size, and the in-situ strength of vegetated bank soils. Measurements of how bank shear strength varies with vegetation community and density enabled us to use geotechnical models of bank stability to assess modes and frequencies of bank failure. By increasing the tensile strength of bank soils, wet meadow riparian vegetation increases the stable width of an undercut bank by a factor of 10. In-situ stabilization of failed blocks by wet meadow sedges and rushes limits the frequency of block failure and removal to approximately one in five years, as opposed to up to 10 failures per year in the dry meadow.
We assessed the impact of channel incision on bank stability. Our analysis offload frequencies and flow depths using the BEC-RAS model compared pre- and post- incision patterns of bed shear stress and overbank flow. We observe a correlation between vegetation communities and stream characteristics, including bank height and frequency of overbank flows. We hypothesize that channel incision may convert wet meadow to dry meadow, with significant impacts on streambank stability.