The Effects of Vegetation on Stream Bank Erosion

Abstract

Riparian buffers are promoted for water quality improvement, habitat restoration, and stream bank stabilization. While considerable research has been conducted on the effects of riparian buffers on water quality and aquatic habitat, little is known about the influence of riparian vegetation on stream bank erosion. The overall goal of this research was to evaluate the effects of woody and herbaceous riparian buffers on stream bank erosion. This goal was addressed by measuring the erodibility and critical shear stress of rooted bank soils in situ using a submerged jet test device. Additionally, several soil, vegetation, and stream chemistry factors that could potentially impact the fluvial entrainment of soils were measured. A total of 25 field sites in the Blacksburg, Virginia area were tested. Each field site consisted of a 2nd-4th order stream with a relatively homogeneous vegetated riparian buffer over a 30 m reach. Riparian vegetation ranged from short turfgrass to mature riparian forest. Multiple linear regression analysis was conducted to determine those factors that most influence stream bank erodibility and the relative impact of riparian vegetation. Study results indicated that soil erosion is a complex phenomenon that depends primarily on soil bulk density. Freeze-thaw cycling, soil antecedent moisture content, the density of roots with diameters of 0.5 mm to 20 mm, and the interaction of soil pore water and stream water had a significant impact on soil erodibility and critical shear stress, depending on the soil type. Riparian vegetation had multiple significant effects on soil erodibility. In addition to reinforcing the stream banks, the streamside vegetation affected soil moisture and altered the local microclimate, which in turn affected freezethaw cycling. This study represents the first in situ testing of the erodibility of vegetated stream banks; as such, it provides a quantitative analysis on the effects of vegetation on stream bank erosion, relative to other soil physical and chemical parameters. It also represents the first measurements of the soil erosion parameters, soil erodibility and critical shear stress, for vegetated stream banks. These parameters are crucial for modeling the effects of riparian vegetation for stream restoration design and for water quality simulation modeling.