Mapping Vegetation Type and Slope Hydrology: Implications for Landslide Susceptibility on Waud Bluff, Portland, OR.

In steep river bluffs with sandy, alluvial soils, roots are a primary contributor to slope stability. The Waud Bluff, a 50 m tall river bluff adjacent to the University of Portland, is composed of fine- to coarse-grained deposits from the Missoula Floods. The Bluff has an array of different vegetation zones, including grassland, wooded areas, and non-native underbrush. Over a small area of approximately 118 square kilometers, we investigated how the different vegetation zones and the roots of the plants that comprise them control the hydrology and slope stability of the Waud Bluff. For each flora assemblage, we characterized the tensile root strength, size, and vegetation density from soil pits and aerial photography. Rain gages and soil moisture meters were placed in each zone to look at how the hydrology compares between areas with similar soil types but dissimilar vegetation. If the roots in grass-intensive areas are indeed shallower and provide less of a resisting force than roots in areas that are woodland dense, then we hypothesize that the landslide potential in those areas would be higher. This work complements ongoing USGS data collection in the West Hills of Portland, which have a different soil type but are only 5.4 km from our study area, and hence have similar climatic forcing. Through our work, we can also measure the hydrology of areas where landslides have previously occurred. These results will help us in creating a predictive landslide map and characterizing the relationship between the soil type and vegetation of the Waud Bluff. The data gathered can be helpful in informing future development plans on the bluff by University of Portland and mitigating hazard for the freight railway that runs at the base of the Bluff.