Modeling the spatial distribution of landslide‐prone colluvium and shallow groundwater on hillslopes of Seattle, WA

Abstract

Landslides in partially saturated colluvium on Seattle, WA, hillslopes have resulted in property damage and human casualties. We developed statistical models of colluvium and shallow-groundwater distributions to aid landslide hazard assessments. The models were developed using a geographic information system, digital geologic maps, digital topography, subsurface exploration results, the groundwater flow modeling software VS2DI and regression analyses. Input to the colluvium model includes slope, distance to a hillslope-crest escarpment, and escarpment slope and height. We developed different statistical relations for thickness of colluvium on four landforms. Groundwater model input includes colluvium basal slope and distance from the Fraser aquifer. This distance was used to estimate hydraulic conductivity based on the assumption that addition of finer-grained material from downsection would result in lower conductivity. Colluvial groundwater is perched so we estimated its saturated thickness. We used VS2DI to establish relations between saturated thickness and the hydraulic conductivity and basal slope of the colluvium. We developed different statistical relations for three groundwater flow regimes. All model results were validated using observational data that were excluded from calibration. Eighty percent of colluvium thickness predictions were within 25% of observed values and 88% of saturated thickness predictions were within 20% of observed values. The models are based on conditions common to many areas, so our method can provide accurate results for similar regions; relations in our statistical models require calibration for new regions. Our results suggest that Seattle landslides occur in native deposits and colluvium, ultimately in response to surface-water erosion of hillslope toes. Regional groundwater conditions do not appear to strongly affect the general distribution of Seattle landslides; historical landslides were equally dispersed within and outside of the area potentially affected by regional groundwater conditions.

Effects of watershed land use and geomorphology on stream low flows during severe drought conditions in the southern Blue Ridge Mountains, Georgia and North Carolina, United States

“…This is substantiated by the fact that the amount of watershed colluvium also emerged as an important explanatory variable. This corroborates recent work in Scotland, in which groundwater storage at lower slopes in mountainous headwaters (where colluvium accumulates) was shown to be a major source of base flow [Tetzlaff and Soulsby, 2008], as well as a recent study indicating substantial colluvial water storage in the Cascades [Schulz et al, 2008].…”

Pre-processing algorithms and landslide modelling on remotely sensed DEMs

“…Schulz et al, 2007) or for performing local hydrologic analyses and slope stability assessment (e.g. Montgomery and Dietrich, 1994;Okimura, 1994;Pack et al, 1998;Xie et al, 2004).…”

Climate, Landforms, and Geology Affect Baseflow Sources in a Mountain Catchment

“…Earthflows are characterized by relatively low slope and high surface roughness. Surface roughness can indicate soil thickness (Patton et al, ), which in turn is linked to shallow groundwater (Schulz et al, ), baseflow variability during drought conditions (Price et al, ), and groundwater supply (Tetzlaff & Soulsby, ). Other studies also demonstrate that hillslope characteristics (e.g., slope and roughness) influence baseflow (Jencso & McGlynn, ; McGuire et al, ; Singh et al, ; Zimmer & Gannon, ), particularly during dry conditions (Heidbuchel et al, ).…”

“…Earthflows are characterized by relatively low slope and high surface roughness. Surface roughness can indicate soil thickness (Patton et al, 2018), which in turn is linked to shallow groundwater (Schulz et al, 2008), baseflow variability during drought conditions , and groundwater supply (Tetzlaff & Soulsby, 2008).…”