A PROVISIONAL RAINFALL THRESHOLD FOR ABUNDANT LANDSLIDES IN THE GREATER PITTSBURGH REGION DERIVED FROM RAINFALL CHARACTERISTICS OF MAJOR STORMS AND BEDROCK GROUNDWATER LEVEL DATA

Abundant landslides related to heavy rainfall during major storms in the Greater Pittsburgh region of western Pennsylvania have caused millions of dollars in property damage and adversely impacted transportation corridors and other infrastructure. In June 1972 and September 2004, heavy rainfall from the waning stages of two hurricanes, Agnes and Ivan respectively, caused numerous and widespread landslides in the region. In 2012, rainfall from post-tropical cyclone Sandy also induced multiple, but less widespread landslides. Field investigations and documentation of landslide occurrences following Sandy revealed a transition from shallow to deep-seated landslides and rock-slope failures through the progression of the multiday storm and evidence for incipient slope failure and smaller secondary landslides near primary slope-failure sites. The latter observations suggest that the rainfall totals during Sandy were near the threshold for the onset of abundant landslides. During Sandy, most of the documented landslides occurred after cumulative rainfall exceeded 76 mm and the total storm rainfall (TSR) value of 95 mm is inferred to approach the threshold for the onset of abundant landslides. A provisional rainfall threshold was derived using rainfall characteristics from historical storms from 1926 through 2012 where TSR exceeded the critical rainfall values that occurred during Sandy. News media accounts were used to identify which of the historical storms resulted in numerous or significant damaging landslides.

Groundwater level data from a USGS observation well completed in a red bed-containing section of the landslide-prone, Pennsylvanian age Glenshaw Formation provide additional insight into hydrologic conditions that influence landslide occurrence. More than half (55%) of the historical storms between 1971 and 2015 that resulted in landslides occurred within 60 days of the peak annual groundwater level. In contrast, most (73%) storms with no documented landslides occurred within 80 days of the annual minimum groundwater level. The observed transition over multiple days in landslide depth and type observed during Sandy corresponds with the gradual storm-induced rise in bedrock groundwater levels.