A SLOW - MOVING SLUMP IN AN URBAN AREA AS A CASE STUDY IN DETECTION OF MASS MOVEMENTS FOR URBAN PLANNING

The Mile High Drive landslide is an active slump located in a residential area of Utah County, Utah above the Bonneville shoreline on the northeast bench of Provo. Movement of the 150 by 200 m slump since 1998 has caused significant damage to several homes, including one that was condemned and demolished, as well as damage to streets and other features. UVU Department of Earth Science students have tracked movement of the slump since 2004 using RTK GPS phase-differential survey equipment to locate individual marked points on the slide relative to off-slide reference points to a precision of ±1.3 cm. With four markers moving in unison on the slide, precision increases to ±0.55 cm at 90% confidence. The slide moved 32.0 cm down slope towards the SW between January 2004 and September 2008. Movement has consisted of long periods of relatively slow motion (e.g., 0.20 cm/month from 6/29/06 to 7/12/07) punctuated by shorter periods of faster movement (e.g., minimum 2.45 cm/month between 3/2/06 and 6/29/06). Slow movement correlates to periods of average to below average precipitation, whereas more rapid movement correlates to months - long periods of above average precipitation.

This slow - moving yet damaging slump is an opportunity to study methods for detecting mass movements relevant to urban planning. Slide – related morphology reflects a balance between slide motion and recovery processes such as erosion and soil formation. During periods of slow movement the recovery processes act quickly enough that the slump does not produce any morphologic features indicative of activity. Furthermore, cracks and enlargement of scarps produced by episodes of faster movement may become obscured within 1 to 2 years of slow movement. Consequently field inspection of landslide morphology may be insufficient to identify the slump as active. However, high-precision surveying is capable of detecting the slide's motion. RTK GPS can resolve, at 90% confidence, 1.3 cm displacement of a single marker on a slide, and 0.55 cm or less of displacement when several markers are located on an active slump. At the slowest sustained velocity of the Mile High Drive slide, these displacements accumulate in approximately 8 and 3 months, respectively. Resolution is also increased by more frequent surveys that can reveal consistent trends of slow motion