Groundwater levels, soil moisture, precipitation, and slope movement in the coastal bluffs of the Atlantic Highlands, New Jersey, 2015-2016

Seasonal variations in vegetation, rainfall, and soil moisture conditions have the potential to impact the slope stability of locally forested coastal bluffs in the Atlantic Highlands of New Jersey. Both the seasonality and rainfall amounts of the two types of storms that induce shallow landslides in the area vary considerably. Most of the documented historical landslides are the result of heavy rainfall caused by late summer-fall tropical cyclones. The majority of the remaining documented landslides are related to spring noreasters and total storm rainfall amounts for these storms are generally lower than the rainfall amounts for the tropical cyclones. In order to assess how conditions that may affect the potential for shallow landslide initiation vary seasonally, we are monitoring shallow groundwater levels and soil moisture, precipitation, and slope movement. Our monitoring is located at two sites of previously documented historical landsliding. The Mount Mitchill Scenic Overlook (MMSO) site is located on a slope interpreted to be the main scarp of a deep-seated, rotational landslide or slump that occurred in April 1782. The scarp slope is covered by a veneer of colluvium that has accumulated over the past few centuries. Shallow landslides have been documented nearby on the bluff and other evidence for shallow landslide movement includes scars on the upper slope and convex lobes lower on the slope. The Ocean Boulevard Bridge (OBB) site has experienced recurrent episodes of shallow landslide movement including in 2007 and May 2012. This data release presents the time series data from instrumentation installed at MMSO and OBB for an initial monitoring period that began on June 18, 2015, and lasted through July 31, 2016. The instrumentation includes two observation wells, five soil moisture probes, two rain gauges, and a cable extension transducer. At MMSO, we monitored shallow groundwater level in the lower part of the bluff, rainfall, effective rainfall beneath the deciduous forest canopy, and shallow soil moisture at three locations on the lower and middle parts of the bluff. At OBB, we monitored the shallow groundwater level in the upper bluff and movement along the west flank of the May 2012 landslide where a displaced soil block forms the upper part of the landslide deposit. The data release mostly presents the output for each sensor type as recorded on the datalogger. In some cases the output requires conversion to engineering units and we provide all the necessary factors, values, and equations to facilitate these conversions. Calibrated groundwater levels are also included in this data release. Our initial monitoring occurred during a drier-than-normal period in the region. The two most significant storms during this monitoring period included a five-day storm complex that lasted from September 29, through October 3, 2015, and a blizzard on January 23, 2016. Our rain gauges are not designed to accurately measure the snow water equivalent of snow fall, and recorded no precipitation on January 23, 2016. Data included in this release support an interpretive paper published in the conference proceedings of the 3rd North American Symposium on Landslides held in Roanoke, Virginia in June 2017.