Hydrologic and Hydraulic effects of Riparian Root Networks on Streambank Stability: Is Mechanical Root-Reinforcement the Whole Story?

Authors: Bankhead, Natasha; Simon, Andrew

Submitted to: Geomorphology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/12/2009
Publication Date: 3/11/2010
Citation: Bankhead, N.L., Simon, A. 2010. Hydrologic and Hydraulic effects of Riparian Root Networks on Streambank Stability: Is Mechanical Root-Reinforcement the Whole Story? Geomorphology. 116:353-362.

Interpretive Summary: Vegetation growing on streambanks can have both positive and negative effects on the stability of a streambank. Previous work has shown that the reinforcement provided by the roots of a plant can be considerable, and can be calculated and included in models that estimate streambank stability. However, roots contained within a streambank also have effects on the amount of water contained within a streambank, and the erodibility of the streambank soil. Whilst these effects of roots have often been discussed it is hard to quantify them and include them in estimations of streambank stability. This paper summarizes experiments carried out to test how much different streambank species affect soil moisture by removal of water from the soil by evapotranspiration. Different trees were planted in isolated soil columns and instruments were inserted into the soil at a depth of 30cm and 70cm to measure soil moisture. The results from the soil columns with trees were then compared to results from soil columns with no vegetation. The removal of water by the plants increased the strength of the soil by a moderate amount in the spring when banks in the study area are most likely to fail, and by a large amount in the summer when the most water was removed from the soil by the plants. A second set of experiments tested the erodibility of soil with and without Switch grass roots. The results of these tests showed that the volume of soil eroded decreased as the volume, mass and length of roots in the soil increased. The volume of soil eroded in the tests with the most roots was just 10% of the volume of soil eroded when no roots were present. The data from the two sets of experiments were used in a streambank stability model to compare whether evapotranspiration (loss of water from the soil both by evaporation and by transpiration from plants), effect of roots on soil erodibility or the reinforcing effect of roots had the most influence on streambank stability. The results showed that changes in soil moisture because of evapotranspiration gave the most benefit to streambank stability, but only during the summer months. During the winter and spring months root-reinforcement was the most important factor for streambank stability. The results also showed that although roots reduce erodibility of the soil by water, this did not have as much influence on streambank stability as the effect of evapotranspiration or reinforcement by roots.

Technical Abstract: Riparian vegetation has a number of effects on the mechanisms by which streambanks fail, some positive and some negative. Previous research has shown that the effect of mechanical root-reinforcement on soil stability can be considerable, and can be successfully quantified and included in streambank stability models. However, root networks contained within a soil-matrix also have effects on the hydrologic and hydraulic processes acting on a streambank, and although these effects are often discussed they have generally been difficult to quantify. This paper summarizes the results of field data collection, laboratory testing and computer simulations carried out to better quantify the effects of riparian vegetation on hydrologic and hydraulic processes occurring along streambanks. First, the evapotranspiration potentials of different riparian species were isolated by setting up an experiment to grow young riparian trees and switch grass in separate soil columns, each instrumented with tensiometers at 30 cm and 70 cm depths, and compared against bare control columns. The hydrological reinforcement provided to the soil from increased apparent cohesion as a result of enhanced matric suction was estimated to range from 1.0 to 3.1 kPa in spring when bank stability was most critical and up to a maximum of 5.0 kPa in the summer. Second, a vertical jet-test device was used to measure rates and volumes of scour in soils permeated by switch grass roots. Results showed that the volume of soil scoured during a test declined non-linearly with increasing root volume, per unit volume of soil, and with increasing root length density (RLD) and increasing root biomass. Calculation of relative soil detachment rates (RSD) showed that with the highest rooting densities measured in the field jet-tests, eroded soil volume was 10 % of that in the tests with no roots. Third, the effects of enhanced matric suction due to evapotranspiration, and decreased soil erodibility because of the presence of plant roots were modeled using BSTEM 5.1 to quantify their effects on streambank Factor of Safety (Fs), and to compare with the effects of mechanical root-reinforcement. The sensitivity analysis showed that the change in soil matric suction due to evapotranspiration provided the greatest potential benefit to Fs but only during the summer months. During the winter and spring months, root-reinforcement remained the most important contributor to Fs. The sensitivity analysis conducted here also showed that whilst roots are capable of reducing the volume of hydraulic scour, the resulting effect on streambank geometry did not increase Fs as much as changes in soil matric suction and/or mechanical root-reinforcement.