Landslide Movement

Cumulative stretching across the main scarp zone in 2008. Changes in the rate of movement occurred in May and September 2008.

The landslide is currently active and has been continuously moving at a very slow rate since at least January 2008. Measurements indicate that different parts of the landslide are moving at slightly different rates. The landslide is moving to the northwest, toward Valley View Drive. In 2008, the landslide moved approximately 7 to 9 inches.

The UGS monitored ground deformation at several locations in on the landslide in 2008 to estimate the approximate movement amounts. In the head of the landslide, stretching (points on the ground get farther apart) occurs due to landslide movement. In the toe of the landslide, shortening (points on the ground get closer together) occurs.

The plot above shows that about 8.9 inches of cumulative stretching (the amount that occurs over a period of time from an initial measurement date) occurred across the main scarp zone of the landslide in 2008. The amount of cumulative stretching is likely a close estimate of the total movement of the head of the landslide in 2008.

Change in the Rate of Movement

The rate of landslide movement changed abruptly twice during 2008, slowing in late spring and increasing in fall. A gradual increase in the rate of movement also occurred in late February. The rate of movement was relatively constant in between these brief periods in which the rate of movement changed. The timing of the fall increase varied in the upper and lower parts of the landslide, occurring three months earlier in the upper part than in the lower.

Between May and September, the rate of movement in the head of the landslide averaged approximately 0.12 inch per week and was relatively constant. In September, the average rate of movement increased to 0.23 inch per week, increasing by a factor of about 2.

At the toe of the landslide, the average rate of movement was about 0.05 inch per week between May and December 2008, a period of about 210 days. However, during about a 50-day period between December 2008 and January 2009, the average rate of movement was about 0.17 inch per week. Thus, the average rate of movement increased by more than a factor of 3 beginning in December.

Ground-Water Levels

Fluctuations in ground-water level in observation well P-5 in the toe of the landslide, March 2001 to November 2008. Gaps due to periods when observation well was inaccessible.

UGS geologists last measured ground-water levels in November 2008. In two of the four wells (P-4 and P-5), the levels were the highest measured since 1998. In observation well P-4, the ground-water level rose by about 3.4 feet between June (the seasonal low level) and November. In observation well P-5 in the toe of the landslide, high ground-water levels have persisted throughout 2008. The ground-water level in this well has risen about 21 feet since March 2001.

Changes in the seasonal peak ground-water levels in observation wells P-3 and P-4 between 1999 and 2008.

Fluctuations in the seasonal peak ground-water level (SPGWL) provide a relative measure of the landslide’s stability. The plot above shows the fluctuations in the SPGWL in observation wells P-3 and P-4 (both in Springhill Drive) since 1999. The high SPGWLs since 2005 coincide with an increase in annual movement, which ranges from about 4 (in 2005) to 9 inches (in 2008).