San Francisco Bay Area – Marin County (BALT2) Site near San Rafael, CA

Science Center Objects

Recent Conditions

The instruments for this site were installed in the winter of 2010 and are used to monitor and detect changes in local conditions.  Soil water content and ground water pressure are monitored at two nests on the same hillslope.  Data for the site include:

Data are recorded and updated every 5 minutes and displayed on graphs.

Project Background

Landslides in the San Francisco Bay area (SFBA) of California impact people, infrastructure, and the environment, and are commonly induced by intense or prolonged rainfall associated with strong winter storms.  Both shallow (<3 meter) and deep (≥3 meter) landslides can occur.  Typically, shallow landslides occur as a result of high-intensity precipitation when the ground is already nearly saturated with water from previous storms.  These types of events can occur during the height of the rainy season, from December through February.  Deep-seated landslides can also occur during these times, but are also often triggered during the spring months (March through May), when water from rainfall has had a chance to make its way to deeper soil and bedrock.

The U.S. Geological Survey, in cooperation with local and state government organizations, operates several shallow landslide monitoring sites in the SFBA for research purposes.  The goal of this research is to identify thresholds for widespread shallow landsliding and related high-mobility debris flows based on in-situ soil moisture conditions.  This information is being used to develop landslide warning criteria with the National Weather Service.  Monitoring data includes cumulative rainfall, rainfall intensity, soil water content, groundwater pressure, air temperature, and relative humidity.  Infiltration of water from rainfall increases soil water content, cumulatively saturating the soil profile.  However, periods without rainfall can lead to drying of the soil.  If the soil profile is nearly saturated, additional intense precipitation can increase groundwater pressures which can, in turn, induce landslide movement.

steep dry grassy slopes in foreground with populated valley in background

The USGS “BALT2” site is located in Marin County, north of San Francisco, and consists of dataloggers monitoring the hillslope in the central part of the image.  This slope is typical for many in the Marin County region that have generated mobile debris flows in the past.  Evidence of two recent landslides are shown in the fore- and middle-ground.  The area consists of grasslands over an approximately 1-meter-deep sandy soil layer, underlain by sandstone. (Public domain.)

BALT2 Landslide Monitoring Site - Marin County, north of San Francisco

The USGS “BALT2” monitoring site showing datalogger enclosures, solar panel (far right), air temperature and relative humidity sensor (left, white), and rainfall gauge (far left, brown and white). (Public domain.)