Author
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Bankhead, Natasha |
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Submitted to: Catena
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 5/15/2006 Publication Date: 7/7/2006 Citation: Pollen, Natasha. 2006. Temporal and spatial variability in the root-reinforcement of streambanks: Accounting for soil shear strength and moisture. Catena 69 (2007): 197-205. doi: 10.1016/j.catena.2006.05.004. Interpretive Summary: Streambank vegetation has a number of effects on bank stability, which affect the amount of sediment that reaches the channels. Estimates of the effects of roots in the soil of streambanks have commonly been calculated using models that simply sum root tensile strengths and consider these as an add-on factor to soil strength. A major problem with these models is that the effect of variations in soil moisture and other soil properties are ignored as root strengths are considered to be the same in any soil conditions. In reality, during failure of a streambank, some roots break, and some roots are pulled out of the soil whole. In this paper an equation to predict the frictional resistance of root-soil bonds was tested against field data collected at Long Creek, MS, under two soil moisture conditions. The root pullout equations were then included in the root-reinforcement model, RipRoot, and bank stability model runs for Goodwin Creek, MS, were carried out in order to look at the effects of variations in soil shear strength and rooting density, on streambank stability. Results showed that at low root diameters roots were more likely to be pulled out of the soil whole, but at higher root diameters roots were more likely to break. The threshold diameter between root pullout and root breaking varied with soil shear strength, with increasing soil shear strength leading to a greater proportion of roots failing by breaking instead of pullout. Root-reinforcement estimates were shown to reflect changes in soil shear strength, for example, caused by variations in soil moisture. Resulting stability values for the bank during the period modeled ranged from 1.36 to 1.74 with 1000 grass roots/m2, compared to a range of 0.97 to 1.37 for the non-vegetated bank. Root-reinforcement was shown to increase bank stability under the entire range of soil moisture conditions modeled. However, the amount of root reinforcement varied in both space and time as determined by soil geotechnical properties and soil moisture. Technical Abstract: Riparian vegetation exerts a number of mechanical and hydrologic controls on bank stability, which affect the delivery of sediment to channels. Estimates of root-reinforcement of soils have commonly been attained using perpendicular root models that simply sum root tensile strengths and consider these as an add-on factor to soil strength. A major limitation of such perpendicular models is that the effect of variations in soil moisture and bank geotechnical properties on root-reinforcement are omitted as root tensile strengths are considered to be independent of soil type and moisture. In reality, during mass failure of a streambank, some roots break, and some roots are pulled out of the soil intact; the relative proportions of roots that break or pull out are determined by soil moisture and shear strength. In this paper an equation to predict the frictional resistance of root-soil bonds was tested against field data collected at Long Creek, MS, under two soil moisture conditions. The root pullout equations were then included in the root-reinforcement model, RipRoot, and bank stability model runs for Goodwin Creek, MS, were carried out in order to examine the effects of spatial and temporal variations in soil shear strength and rooting density, on streambank factor of safety. Model results showed that at low root diameters breaking forces exceeded pullout forces, but at higher root diameters pullout forces exceed breaking forces. The threshold diameter between root pullout and root breaking varied with soil shear strength, with increasing soil shear strength leading to a greater proportion of roots failing by breaking instead of pullout. Root-reinforcement estimates were shown to reflect changes in soil shear strength, for example, brought about by variations in soil matric suction. Resulting FS values for the bank during the period modeled ranged from 1.36 to 1.74 with 1000 grass roots/m2, compared to a range of 0.97 to 1.37 for the non-vegetated bank. Root-reinforcement was shown to increase bank stability under the entire range of soil moisture conditions modeled. However, the magnitude of root reinforcement varied in both space and time as determined by soil geotechnical properties and soil moisture. |
