INSIGHTS ON SHALLOW LANDSLIDE INITIATION BASED ON HYDROLOGIC MONITORING IN THE ATLANTIC HIGHLANDS, NEW JERSEY

Shallow landslides are a recurring hazard on the coastal bluffs of the Atlantic Highlands region of New Jersey. Such landslides are typically triggered by increases in shallow soil moisture and pore-water pressure caused by sustained intense precipitation. Ongoing monitoring efforts seek to quantify the hydrologic conditions necessary for shallow landslide initiation on these bluffs. Precipitation, persistent downslope movement of a recent landslide, and pore-water pressure and soil moisture in the landslide deposit and in slope colluvium within an older landslide source area are currently being monitored.

Continuous monitoring from August 2016 through October 2017 allowed comparison of the seasonal variation in shallow soil moisture and pore-water pressure in the slope colluvium and landslide deposit. During the monitoring period, generally elevated soil moisture and pore-water pressure conditions existed from late fall into early summer. Rainfall totals for the two most substantial storms in late March and January were 78 and 66 mm, respectively. The hydrologic responses to the March storm resulted in annual peak shallow soil moisture and pore-water pressure values. The elevated pore-water pressure in the slope colluvium was sustained at relatively high levels through early June. In contrast, pore-water pressure decreased more rapidly in the landslide deposit before rebounding in response to a late April storm. During drier soil moisture conditions in summer and early fall, the hydrologic response was greater in the landslide deposit than in the slope colluvium, but peak levels remained generally below those during the period of elevated soil moisture. A 24-hr storm on October 8-9, 2016, near the end of a dry period, resulted in 48 mm of rainfall and seasonal peak pore-water pressure in the landslide deposit. The same storm resulted in a minor hydrologic response in the colluvium. A cable extension transducer on the west flank of the landslide deposit recorded about 10 mm of incremental horizontal displacement since September 2015, and most periods of movement corresponded to storms where rainfall totals exceed 25 mm. Additional monitoring will further define the hydrologic conditions leading to shallow landslide initiation.