ROLE OF GROUNDWATER AND SURFACE WATER INTERACTIONS ON SLOPE STABILITY IN THE STONY CLOVE CREEK, CHICHESTER, NY

Stony Clove Creek is a tributary of the Esopus Creek, which flows into the Ashokan Reservoir, one of several reservoirs that provide drinking water to New York City. The New York Department of Environmental Protection wants to avoid the implementation of a filtration plant for this water supply, but the turbidity of the water is an issue. The study site on the Stony Clove Creek is contributing to the turbidity issues, as the instability of the slope causes large amounts of sediment to enter the creek. The slope instability is likely influenced by the interactions of groundwater and surface water in the area.

To determine the hydraulic conductivity and transmissivity in the underlying aquifer, slug and bail tests were carried out on four monitoring wells in the study area. These tests were analyzed with the Hvorslev Method. Nineteen water samples were collected from surrounding surface water bodies, the Stony Clove Creek, and all six monitoring wells. These samples were analyzed for chemical concentrations using flame atomic absorption spectroscopy, ion chromatography, as well as Vernier and HACH probes. Soil samples were collected from sixteen sites in the study area and a particle size analyzer was used to determine the distribution of clay grain sizes in each sample.

The results indicate that the slope failure occurs in two forms: rotational failure and subsidence. The rotational failure is caused by the fluctuation of water level in the Stony Clove Creek. As the water level decreases from a high flow period, the bank sediments lose support, causing failure. The subsidence is caused by increases in pore pressure. With higher pore pressure, the resistive forces of the sediments decrease, leading to a scenario of “quick-clay.” Quick-clay is a layer of sediments that begins to flow as if liquid, flowing out from under other layers and causing subsidence. Large storms such as Hurricane Irene and Tropical Storm Lee caused high flow in the Stony Clove Creek, as well as increase in pore pressure in the surrounding sediment due to high rainfall. The significant change in the shape of the bank in the last year can be attributed to these storms.