DYNAMIC STREAM ADJUSTMENT: A STUDY OF WARNER CREEK'S EROSIVE RESPONSE TO LARGE HYDROLOGICAL EVENTS

The New York City Department of Environmental Protection (DEP) must maintain delivery of high quality water to 9 million people. Turbidity from suspended sediment in the unfiltered water supply can interfere with the efficiency of disinfection mechanisms. Warner Creek is a tributary to Stony Clove Creek in the Esopus Creek and Ashokan Reservoir watershed and is identified as a significant source of turbidity due to its interaction with Pleistocene glacial deposits. Bank erosion can entrain glacial silts and clays, causing acute and chronic suspended sediment loading. This study aims to understand the fluvial processes in a glaciated mountainous terrain and evaluate assessment methods used to diagnose conditions to characterize this watershed.

In Summer 2011, SUNY New Paltz REU and DEP evaluated a 10,500 ft. section of Warner Creek through a diagnostic stream assessment to determine reach scale response to two geomorphically significant high flow events (Q>25 year recurrence interval), occurring in the previous 9 months. Baseline geomorphic data was collected by the REU program and DEP in the summer of 2010 before these flow events in the form of a comprehensive stream feature inventory for >90% of the 9 mile stream length. Stream features including eroding banks and fine sediment sources were remapped in 2011 to put the spatial data from 2010 REU into a temporal context. A reach stability study was conducted by Clear Creeks Consulting in between the two large flow events. Bank cross sections and BEHI scores were calculated. Five cross sections in an actively adjusting reach were resurveyed and overlain with the existing data to measure bank retreat. We find that the floods of late 2010 cause extensive erosion, entrainment of large woody debris, and deposition. This resulted in an increasing eroding bankline in some reaches and caused extensive bank retreat from fluvial and consequent geotechnical processes. Values calculated for bank retreat in this study were used to conclude that BEHI is an adequate predictor of erosional hazard. The limitations of this study include error associated with collected GPS data, differences in methodology in surveying techniques and only one year of spatial data being available as baseline information.