San Francisco Bay Area - Santa Cruz Mountains, CA (BALT-CZU)

Science Center Objects

Landslides in the San Francisco Bay Area of California impact people, infrastructure, and the environmnent, and are commonly induced by intense or prolonged rainfall associated with strong winter storms.

Recent Conditions

The instruments were installed in the autumn of 2020 after the August 2020 CZU Lightning Complex wildfire and are used to monitor and detect changes in local hillslope hydrologic conditions.  Soil water content, soil water matric potential (soil suction) and ground water pressure are measured at two nests on the same hillslope.  Data for the site include:

Data are recorded every 5 minutes and updated on the graph every 30 minutes.

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.

sparsely-vegetated hillside

The USGS “BALT-CZU” site is located in Santa Cruz County, south of the San Francisco Peninsula, near the town of Brookdale.  The site consists of instrumentation monitoring the hillslope in the central part of the image. This slope is typical of many in the Santa Cruz County region that exhibited moderate to high soil burn severity from the CZU Lightning Complex wildfire. Nearby slopes have generated debris flows in the past. The area consists of a fully burned land surface with only tree trunks remaining. The soil profile consists of a 2.5 cm layer of ashy charred leaf litter and a 0.5 m horizon of gravelly sand underlain by highly weathered diorite bedrock. (Public domain.)

metal instruments on poles on hillside

The USGS “BALT-CZU” monitoring site showing datalogger enclosures, air temperature and relative humidity sensor (left, white), and rain gauge (far left, brown and white).

(Public domain.)