Summary of Springhill Landslide Movement and Conditions in 2009-2011
This section summarizes landslide conditions from summer of 2009 through fall of 2011. Precipitation for this time span is also discussed.
The landslide was active in the recent past and has been moving at a slow rate since at least January 2008. Measurements indicate that different parts of the landslide were moving at slightly different rates. In general, the landslide moved to the northwest, toward Valley View Drive. Ground deformation measurements were collected from survey markers using either a cloth measuring tape or survey-grade Global Navigation Satellite System (GNSS) instrument, depending on location.
The UGS monitored ground deformation at several locations on the landslide with wooden stakes to estimate approximate movement. At the head or main scarp of the landslide (uppermost part), stretching (points on the ground get farther apart) occurred due to landslide movement. At the toe of the landslide (lowermost part), shortening (points on the ground get closer together) occurred.
The two plots below show the cumulative stretching and shortening (the amount that occurs over a period of time from an initial measurement date) at the main scarp and near the toe of the landslide from June 18, 2009 to August 22, 2011. The main scarp experienced about 40.3 inches of cumulative stretching (at station SHC5).
Measurement of toe movement has been complicated because of the destruction and re-establishment of one of our stakes. Therefore, minimum cumulative toe shortening during this period of time was 20.2 inches.
The table below summarizes total measured ground deformation from June 2009 to August 2011.
Summary of recent landslide ground deformation measurements
|Ground deformation across main scarp zone (SHC5)
|Upslope of Springhill Circle
|Cloth tape and survey stakes
|June 18, 2009 -August 22, 2011
|Minimum ground deformation across toe
Valley View Drive
|Cloth tape and survey stakes
|June 18, 2009 -August 22, 2011
|Survey-grade GNSS instrument
|June 18, 2009 -September 28, 2011
1.1 to 22.3
Change in the Rate of Movement
Monitoring showed consistently slow movement of the landslide since at least 2008. Repeated monitoring of the rate of movement (the rate of change observed between measurement intervals) allows us to assess changes in these movement rates.
At the head of the landslide, very slow movement was recorded from June 2009 to February 2010. The rate of movement then increased until May 2010. Summer and fall of 2010 was again a time of very slow movement followed by a significant increase in the rate beginning in December 2010. This increased rate of landslide movement at the main scarp continued until June 2011 when a rapid decrease was recorded.
At the toe of the landslide, the rate of movement showed a similar general pattern. Toe movement was very slow from June 2009 to late summer 2010, when its rate increased significantly. This heightened rate continued until summer 2011, when toe movement again slowed substantially.
This pattern of landslide movement rates is likely the result of increased pore-water pressures as the water table rose due to an increase in precipitation and is consistent with other Wasatch Front landslides.Generally, landslides along the Wasatch Front experience most of their damaging landslide movement in the spring and early summer months when precipitation and/or snow melt are the greatest and are more active during years with increased precipitation.
UGS geologists have been measuring groundwater levels in at least three observation wells (P-3, P-4, and P-5) at the landslide between June 18, 2009 and August 22, 2011.Of the three wells, P-3 and P-4 showed very little variability over the reporting period with ranges of 3.2 and 1.7 feet, respectively. Well P-5 recorded the greatest variability with a range of 6.4 feet. Well P-5 recorded a seasonal variation in groundwater levels with high levels coming in late spring or early summer and low levels in late summer.
Seasonal Peak Ground-Water Levels
Fluctuations in the seasonal peak groundwater level (SPGWL) provide a relative measure of the landslide’s stability. Between 1999 and 2011, the SPGWLs in three wells in the vicinity of the Springhill landslide typically occurred in the first six months of the year, suggesting groundwater levels rise in response to snowmelt and spring precipitation (generally the wettest months of the year are March through May). A comparison of groundwater levels and precipitation patterns show this same relationship.
The plot below 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 4.0 (in 2005) to 23.0 inches (in 2011).
Comparison of Groundwater Levels and Movement
During the reporting period, the UGS monitored ground deformation at several locations on the landslide to estimate approximate movement amounts. At the head of the landslide, stretching occurs due to downslope landslide movement, and at its toe shortening occurs. The amount of cumulative stretching is likely a close estimate of the total movement of the head of the landslide for this time. Also during this time, monitoring of several observation wells provided groundwater levels for comparison to landslide movement.
The plot below shows a comparison of groundwater levels and landslide deformation from fall 2010 to late spring 2011, during which there was a rise in groundwater levels and a significant increase in the movement rate of the landslide. This relationship between fluctuations in groundwater levels and landslide movement is generally consistent with other landslides along the Wasatch Front.
Precipitation in 2009-2011
Precipitation at the landslide is estimated using data from the nearby Bountiful-Val Verda National Weather Service (NWS) station, approximately 1.5 miles northeast of the landslide. Precipitation between June 18, 2009 and August 22, 2011 was 137 percent of normal and is shown on the plot below. From October 2010 to June 2011, when a significant increase in the rate of landslide movement was observed, precipitation was 179 percent of normal.