INSTRUMENTAL OBSERVATIONS OF UNSATURATED ZONE HYDROLOGY AND SLOPE STABILITY - PUGET SOUND COASTAL BLUFFS

Shallow landslides and debris flows often occur during heavy rainfall on steep bluffs that form much of the coastline of Puget Sound. A simplified stratigraphic model of the bluffs includes glacial outwash sand overlying glaciolacustrine silt mantled by a shallow colluvial layer. Soil moisture conditions within the bluffs differ markedly between the dry and rainy season and these differences have a pronounced effect on shallow landslide occurrences.

In October 2001 the U.S. Geological Survey, in cooperation with the Burlington Northern Santa Fe Railway and their consultant, Shannon and Wilson Inc. instrumented hillslopes near Edmonds and Everett, WA to monitor in near-real time the subsurface hydrologic response to rainfall. Here we report on the monitoring results from the winter season of 2003 – 2004 at the Edmonds site. The instrumentation at Edmonds consists of two water-content profile probes that measure volumetric water content at eight depths (0.2 – 2m), two tensiometer nests with sensors at 6 depths (0.2 – 1.5m), two rain gauges, and one soil-temperature probe.

During the early part of the 2003, winter season soils were dry and wetting fronts moved slowly in response to rainfall. In mid-October, record 24-hr rainfall (5.02 in at Seattle-Tacoma airport) produced few debris-flows. Pre-existing, very dry conditions (e.g. 12 % water and –1.2 m water at 0.8 m depth) impeded infiltration and resulted in very small pore pressure increases. This storm produced 2.93 in of rain in 32 hr at Edmonds and the moisture content at 2 m depth increased only 3% over six days. As soil moisture increased throughout the winter season, pore pressure and moisture content at depth responded much more rapidly to heavy rainfall. In mid-November 3.58 in of rain fell in 29 hr resulting in a 4% increase in moisture content at 2 m depth in only one day. Average pre-storm pressure-head measurements show a difference of ~0.2 m of water for shallow tensiometers and less than 0.1 m of water at depth between the early and late rainy season. Water content at 2 m depth differs by 5% in the same period.

We expect ongoing analysis of these data combined with laboratory tests and physical models to better define conditions that result in shallow landslides on coastal bluffs of Puget Sound. Preliminary data is available at http://landslides.usgs.gov/pugetrt/index.html.