REACTIVATION OF SLOPE FAILURES ALONG MEADOW RUN, SHENANDOAH VALLEY, VIRGINIA

Reactivation of slope failures along Meadow Run, Shenandoah Valley, Virginia

Meadow Run is one of numerous gravel bed rivers that drain from the western slopes of the Blue Ridge Mountains into the Shenandoah Valley of Virginia. The arrival of Hurricane Isabel on September 18-19, 2003 brought 174 mm of rain within 24 h, and caused moderate flooding. Two previously studied slope failures in this basin were reactivated during the hurricane. The lower third of the original slope failure was removed by high flows in Meadow Run, thus destabilizing the upper slope. In the 3 months following Hurricane Isabel, multiple pulses of block movement have coincided with periods of significant rain and snowfall. These recurrent slope failures are triggered by undercutting of steep fan edges (~20 m thick) by lateral migration of the mainstem river. The objective of our investigation is to quantify processes and rates of block movement on the reactivated surfaces. The most prominent reactivated slope failure is an earth slump, and consists of unconsolidated saprolitized alluvial- and debris-flow material. The rate of movement of these blocks continues to be measured. One detached block has moved a total of ~2.5 m from its initial position from several rainfall events. New tension cracks continue to form, serving as conduits for surface water to infiltrate and reach the failure plane, thus increasing pore pressure and the likelihood of slope movement. Recent measurements indicate that movement has accelerated due to the snow and ice storm of December 4-5, 2003, along with additional freeze-thaw processes. New scarps uphill are propagating upslope, with a possibility of reaching the main headscarp of the older slope failure. The water that frequently seeps or ponds in the lower third of the slide suggests that the failure plane also behaves as an aquiclude, minimizing infiltration and perpetuating the saturated conditions of the slide material. Field evidence suggests that slope failure by recurrent movement may be common in this landscape. Several intact, yet abandoned stream channels, combined with evidence of incipient stream piracy suggest Meadow Run frequently avulses, eroding the fans at multiple sites over time. This activity indicates that fan edges are susceptible to erosion by the mainstem river.