Effects of Removal of Riparian Vegetation on Levee Stability on the Sacramento River
Abstract
A new policy of the US Army Corps of Engineers requires that all levee vegetation be removed from federal levees in California. This directive requires levees to be cleared of all vegetation to preserve channel capacity and allow access for inspection and repair. The case for leaving vegetation in place on levees has largely been an environmental one, with concerns regarding removal of habitat and aesthetics. However, stability factors should also be considered. A previous study by Shields and Gray (1992) investigated the effects of vegetation on sandy levee integrity along the Sacramento River, just one such river that is affected by this vegetation-removal policy. Their study showed that even low root concentrations allowed for more stable bank conditions under worst-case conditions for bank stability. In the years since this initial study, modeling of root-reinforcement and streambank stability has improved greatly. This study used geotechnical data collected along the Sacramento River to model the effects of woody and herbaceous vegetation on levee stability using the Bank Stability and Toe Erosion Model developed at the National Sedimentation Laboratory and the root-reinforcement model, RipRoot. Model runs were carried out for a 4 m high levee with 2H: 1V and 3H: 1V slopes, and vegetation growing at different locations on the levee. Levee material was assumed to be a homogeneous, sandy soil, with very low cohesion (0.84 kPa). Three hydrologic conditions were modeled: baseflow conditions, peak of hydrograph, and the most critical bank condition during the receding limb of a hydrograph. Roots were assumed to grow perpendicular to the soil surface, with the additional cohesion due to roots only being added to soil layers in which the roots extended beyond the potential shear surface in that layer. Values for root-reinforcement were calculated using the RipRoot model, using typical root densities, depths and tensile strength measurements for different riparian species measured at sites across the USA. Values of 3, 15, and 20 kPa were added to the banks to represent young saplings, bunch grasses and mature trees respectively. Results showed that the levees were stable without the reinforcing effect of vegetation under all conditions, except under drawdown conditions which are the most critical. In those cases, root-reinforcement increased levee- stability significantly. A 2H: 1V levee had a factor of safety of 0.33 under these conditions without vegetation and a 3H: 1V levee a value of 0.54 (values <1 are unstable). With the addition of vegetation to the levee sides, factor of safety values increased to >1 under all conditions. Reinforcement added by mature trees and bunch grasses provided highest factor of safety values of up to 8.16 and 5.13 for 2H: 1V and 3H: 1V slopes respectively. The findings suggest that root-reinforcement of levees should be taken into account before complete removal of vegetation is carried out along rivers such as the Sacramento River. In cases where levees are composed of largely uncohesive materials, root-reinforcement provides significant support to the soil matrix, whilst additionally reducing shear stresses acting on the soil from flowing water and protecting the levee from rainfall impact and runoff. In deciding the case for removal of levee vegetation, these positive influences of vegetation should be weighted carefully against the desire for increased channel capacity and any possible negative influences of plant roots on levee integrity.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H31J..05P
- Keywords:
-
- 1815 Erosion;
- 1825 Geomorphology: fluvial (1625);
- 1856 River channels (0483;
- 0744);
- 1860 Streamflow;
- 1894 Instruments and techniques: modeling