Results of hydrologic monitoring of a landslide-prone hillslope in Portland’s West Hills, Oregon, 2006–2017

Abstract

Cover. The photograph shows terrain and vegetation in the vicinity of the monitoring site in Portland's West Hills, Oregon. The camera was facing northeast and located about 50-meters southeast of the hydrologic monitoring site. The solar panel and a rain gage can be seen in the foreground. The treeless area is a scar from a recent landslide; the zone of depression was filled with gravel to mitigate the disturbed topography. For more information on the USGS-the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment-visit https://www.usgs.gov or call 1-888-ASK-USGS. Results of Hydrologic Monitoring of aFor an overview of USGS information products, including maps, imagery, and publications, visit https://store.usgs.gov.Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as°F = (1.8 × °C) + 32.Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as°C = (°F -32) / 1.8. DatumVertical coordinates are referenced to the North American Vertical Datum of 1988 (NAVD 88).Altitude, as used in this report, refers to distance above the vertical datum.Horizontal coordinate information is referenced to the Universal Transverse Mercator, Zone 10, meters. Results of Hydrologic AbstractThe West Hills of Portland, in the southern Tualatin Mountains, trend northwest along the west side of Portland, Oregon. These silt-mantled mountains receive significant wet-season precipitation and are prone to sliding during wet conditions, occasionally resulting in property damage or casualties. In an effort to develop a baseline for interpretive analysis of the groundwater response to rainfall, an automated monitoring system was installed in 2006 to measure rainfall, pore-water pressure, soil suction, soil-water potential, and volumetric water content at 15-minute intervals. The data show a cyclical pattern of groundwater and moisture content levels-wet from October to May and dry between June and September. Saturated soil conditions tend to last throughout the wet season. These data show the hydrologic response of the monitored area to rainfall and provide insight into the dynamics of rainfall-initiated landsliding. This report details the monitoring methods and presents data

Developing Hydro-Meteorological Thresholds for Shallow Landslide Initiation and Early Warning

“…Whereas thresholds are optimized objectively with receiver operating characteristics (ROC), different threshold variables and skill statistics used for optimization can influence the utility of said threshold for a landslide alert system, depending on end-user objectives, requirements and data availability. To demonstrate the implementation and utility of our approach we leverage landslide inventories and telemetered near-real-time measurements of rainfall and hillslope hydrologic response for two landslide prone areas in the Pacific Northwest of the United States: The Seattle-Everett railway, Washington [24] and the City of Portland, Oregon [25]. In both Seattle and Portland, shallow translational slope failures are a common and destructive form of landslide hazard during the winter rainy season (November-April), but uncommon during the drier half of the year (May-October).…”

“…The Seattle-Everett railway monitoring site (Figure 2) and the Portland Hills monitoring site (Figure 3) are both located in steep, densely forested terrain with a mantle of colluvial soil that is typically involved in shallow slope failures [24,25]. Mean annual precipitation of around 889 mm along the Seattle-Everett railway and around 937 mm within the City of Portland, with precipitation falling mainly during the winter months of December through February in the form of rainfall or rapidly melting snow.…”

“…Initial monitoring in the City of Portland began in 2006 November, but some breaks in the time series prevent the rainfall and subsurface data from being used in tandem. Instead, our analysis uses monitoring data from 13 November 2009 to 23 January 2016 [25], during which time a total of 93 landslides were recorded within the City of Portland (Table S2) by the Oregon Department of Geology and Mineral Industries' (DOGAMI) statewide landslide inventory database [26]. Further information on the two monitoring sites, including geologic site characterization, study area maps and instrument configuration can be found elsewhere [24,25].…”

“…Instead, our analysis uses monitoring data from 13 November 2009 to 23 January 2016 [25], during which time a total of 93 landslides were recorded within the City of Portland (Table S2) by the Oregon Department of Geology and Mineral Industries' (DOGAMI) statewide landslide inventory database [26]. Further information on the two monitoring sites, including geologic site characterization, study area maps and instrument configuration can be found elsewhere [24,25]. Interactive maps of the study locations and the current rainfall and hillslope hydrologic conditions are available on the U.S. Geological Survey (USGS) Landslide Hazards Program monitoring sites for Seattle and Portland [27,28].…”

“…The degree of clustering may depend on the selected timescales for the recent and antecedent variables. Measured variables reflecting subsurface hydrologic conditions such as pore-water pressure, water table depth, or soil moisture define the underlying factors that actually cause and trigger landsliding [22][23][24][25], which is reflected in various approaches that attempt to use rainfall to calculate subsurface hydrological conditions [9][10][11][12][13][14][15][16][17]. However, it is difficult to quantify the non-linear response of soil moisture to antecedent rainfall.…”