HYDROLOGIC RESPONSE ASSOCIATED WITH SHALLOW SLOPE FAILURES DURING A CLOUDBURST STORM IN THE ATLANTIC HIGHLANDS, NEW JERSEY

Shallow slope failures have been recurring hazards on the coastal bluffs of the Atlantic Highlands, New Jersey for over a century. Most documented landslides are typically triggered by long-duration rainfall from late summer/early fall tropical cyclones or nor’easters in late fall through early spring, and involve slope failure of sandy colluvium, historical landslide deposits, or fill. Hydrologic and slope movement data from two continuous monitoring sites in the Atlantic Highlands provide insight into the hydrologic response of soil moisture and pore-water pressure from storms, as well as the stability of the slope colluvium and recent landslide deposits. A rare, short-duration cloudburst storm on August 7, 2018 exceeded previously developed preliminary critical rainfall thresholds and resulted in multiple shallow slope failures, including runoff- or erosion-induced shallow landslides, debris flows, and sand flows. Monitoring data indicate the duration of infiltration is largely controlled by antecedent moisture conditions, water table depth, and the saturated hydraulic conductivity and thickness of the sandy deposits overlying low permeability glauconitic units. Soil moisture increased rapidly in response to the cloudburst until peak rainfall intensity exceeded saturated hydraulic conductivity, resulting in decreased recharge to the water table and a subsequent muted response of deeper pore-water pressure. These results differentiate the hydrologic response within the sandy colluvium to the cloudburst from that of storms of longer duration and less intensity.