HISTORICAL 3D MODELING OF EROSION IN STONY CLOVE CREEK FOR SEDIMENT VOLUME ANALYSIS USING GIS

The New York City Department of Environmental Protection identifies Stony Clove Creek as a chronic supplier of suspended sediment to the city’s water system. Stony Clove Creek, a tributary of Esopus Creek, raises concerns because it feeds the Ashokan Reservoir, a major supplier of drinking water to New York City which has no filtration system to deal with high sediment loads. In the Stony Clove, there is a bowl-shaped erosional feature called “Stony Clove Creek at Chichester Site 2” which began forming around 1970 after anthropogenic straightening of the creek just downstream. The erosion of Site 2 seems to be occuring from a combination of: (1) high-energy flow at the cutbank and (2) slumping due to excessive groundwater flow through exposed fine-sediment glacio-lacustrine clays and clay-rich tills. Understanding how this feature grew, how quickly it grew, and how much sediment it has contributed to the turbditiy issue will give insight into further stream straightening projects and management of similar erosion sites in the watershed.

3D modeling of this feature was conducted in ArcGIS using aerial photos from 1959 to present as well as field-based GPS points and elevation measurements. Seven 3D triangulated irregular network (TIN) models of Site 2 were created. Analysis was done through a combination of methods including on-screen digitizing of aerial photos, mapping GPS points, interpreting LiDAR data, compiling precipitation and discharge measurements, and collecting historical information from library resources. The erosional feature first appears in 1980 and has grown at an increasing rate. From 1970 to present, approximately 2.4 million cubic feet of sediment have entered the creek and traveled downstream. This signifcant contribution of fine sediment poses a threat to the NYC drinking water system and the lower Esopus Creek; sites for potentially similar erosional features should be identified and mitigated against further erosion.