Submitted to: Environmental and Water Resources Institute World Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 4/9/2003
Publication Date: 6/15/2003
Citation: POLLEN, N., SIMON, A. THE EFFECT OF EVAPOTRANSPIRATION ON STREAMBANK PORE WATER PRESSURES. PROCEEDINGS, EWRI-ASCE, WORLD WATER & ENVIRONMENTAL RESOURCES CONGRESS. 2003. 10 P. CD-ROM. Interpretive Summary: Streambank vegetation has a number of effects on the stability of the bank. Some of these effects are due to mechanical strengthening from the roots of the plants, and some are due to the way that vegetation alters the movement of water at the soil surface and within the soil. Most attempts to quantify the effects of streambank vegetation on bank stability concentrate on the effect of the roots rather than the hydrologic effects. Variations in evapotranspiration between different species can affect the stability of the bank by different amounts, which is important, especially during the spring when bank failures are most likely. Results from the Goodwin Creek field site suggest that the soil moisture varies under different types of vegetation (mature trees, 18-month-old pine trees, and Switch Grass). A more detailed study was then set up to look at the effects of different tree species (Eastern Sycamore, River Birch, and Black Willow) by planting young saplings in separate pots of soil, and measuring the soil moisture underneath them. The data from these young trees was compared to those at the Goodwin Creek field site to assess the effects of the different vegetation types on streambank stability. Results so far suggest that the hydrologic reinforcement provided by streambank vegetation was important, and should be included in calculations to quantify the potential reinforcing effects of vegetation.
Technical Abstract: Riparian vegetation has a number of mechanical and hydrologic effects on streambank stability. Most attempts to quantify these effects focus on the mechanical strengthening provided by the addition of roots to the soil matrix, and ignore the effect that evapotranspiration by the vegetation has on pore-water pressures within the streambank. Variations in evapotranspiration potential between riparian species have important implications for streambank stability, especially during the critical spring period when streambank failures are most likely. Results from nests of tensiometers installed at Goodwin Creek, MS, have shown that different soil matric suction values occur under different vegetative plots (mature mixed deciduous trees, 18-month-old Pine saplings, Switch Grass, and an open plot with just a turf coverage). Following these initial results, the evapotranspiration potentials of different riparian species were isolated by setting up a controlled experiment to grow young trees (Eastern Sycamore, River Birch, and Black Willow), in separate soil monoliths, each instrumented with tensiometers at 30 cm and 70 cm depths. The data from the tensiometers under the trees and grass were compared to values from control monoliths containing no vegetation. The results therefore suggest that the hydrologic reinforcement provided by riparian vegetation was important, and should be included in calculations that seek to quantify the potential reinforcing effects of vegetation.