THE RELATIONSHIP OF LANDSLIDE OCCURRENCE IN THE EASTERN UNITED STATES TO WET AND DRY CYCLES INDICATED IN HISTORICAL ANNUAL PRECIPITATION RECORDS

The temporal distributions of historical landslide events were compared with historical annual precipitation records that range from 128 to 186 years in length at six sites in the eastern United States. The objective of this study is to develop a method of predicting the temporal probability of landslide movement based on historical precipitation data. The records reveal a consistent pattern of wet and dry cycles. Wet cycles are characterized by an abundance of wet years, periods of successive wet years, and increased variability in annual precipitation. At four sites, wet cycles occur in both the early part and at the end of the record that are statistically indistinguishable. Cumulative excess precipitation during the wet cycles ranges between 1 and 5.6 times the mean annual precipitation. Landslide movement may be triggered during wet cycles by a gradual rise in pore-water pressures caused by infiltration of excess precipitation (deep-seated landslides) and by more frequent record precipitation events (shallow landslides).

The 186-year-long historical annual precipitation record at Albany, New York, begins with a wet cycle that spans 66 years between 1826 and 1891, and ends with a similar ongoing wet cycle that began in 1971. Mean annual precipitation and variance for the two wet cycles exceed that in the intervening dry cycle by about 20 and 54 percent, respectively. Consistent with the higher variability in annual precipitation, the ten wettest years on record occurred during the two wet cycles. Cumulative excess precipitation for the two wet cycles is 5.2 and 3.1 times the mean annual precipitation, respectively.

Preliminary analysis of more than 300 documented landslides in New York suggests landslide frequency increases near the ends of the wet cycles and periods of successive wet years. The two largest, deep-seated landslides in New York history occurred in years 23 (1993) and 41 (2011) of the ongoing wet cycle, consistent with a gradual rise in pore-water pressures that resulted in instability. Fourteen of 15 documented landslide events between 1836 and 2011 in nearby Troy, New York, also occurred in one of the two wet cycles. Further development of robust landslide event archives is prerequisite for defining the temporal probability of landslide movement using historical precipitation data.