Resuspension as a Source of Turbidity in a Water Supply Reservoir Emmet M. Owens, Rakesh K. Gelda, Steven W. Effler Upstate Freshwater Institute, Syracuse NY Donald C. Pierson New York City Dept. of Environmental Protection, Kingston NY Watershed Science & Technical Conference New York Water Environment Assoc. West Point, NY September 2009 New York City Department of Environmental Protection Bureau of Water Supply Water Quality

Schoharie Reservoir Diverts water from Schoharie Cr. (Mohawk R. basin) into Shandaken Tunnel, Esopus Cr., Ashokan Res., and Catskill Aqueduct

Schoharie Reservoir long, narrow shape; steep bottom slopes deep, thermally stratified short residence time; function is primarily diversion large watershed; 9/17/99 (Hurricane Floyd) reservoir rose 9.8 m (32 ft.) in 24 hrs episodes of elevated turbidity driven by runoff events, exacerbated by reservoir drawdown Schoharie Creek Manor Kill Bear Kill Intake to Shandaken Tunnel Scale in km m 30 m Dam

Modeling Goals understand factors leading to historical turbidity events contribute to design features of potential structural turbidity control alternatives allow evaluation of turbidity control alternatives: structural and operational

Monitoring Program stream inflows: USGS reservoir outflows & operations: NYCDEP local meteorology: NYCDEP routine temperature and turbidity monitoring: tributaries, water column and withdrawal: NYCDEP event-based monitoring: Schoharie Creek (robotic); water column (robotic and manual gridding): UFI (Sept – Dec. 2005).

Historic Reservoir Drawdown Median annual drawdown = 17 m (56 ft.) monitoring period: 2 full reservoir years, 2 with significant drawdown

Schoharie Reservoir Turbidity Model state variable is turbidity Tn (an optical property) while there is no conservation principle for Tn, it is treated as if it is mass (good empirical evidence for doing so) turbidity model considers following processes: turbidity loading, deposition, transport, export, and resuspension

Model Framework: CE-QUAL-W2 (W2) two-dimensional approach assumes that temperature and turbidity are uniform over width of the basin hydrothermal component of model previously applied by UFI model enhanced by UFI to simulate turbidity and resuspension (W2Tn)

Early Model Testing assumed that stream loading is the only source of particles and turbidity resulted in underprediction of observed Tn in water column and withdrawal during certain runoff events underprediction was greater during periods of reservoir drawdown

Resuspension Relationship Focus on field measurements to validate model predictions of shear stress

Two Sources of Motion Causing Resuspension Stream Inflow – high current velocity near mouth of Schoharie Creek during runoff events Waves – oscillatory motion associated with wind-driven surface waves

Full Reservoir, Low Streamflow: Large A, Small Q Small V (Deposition) Drawdown, High Streamflow: Small A, Large Q Large V (Resuspension) A A Drawdown Reservoir cross sections near creek mouth under two conditions Velocity (V) = Streamflow (Q) Area (A) Resuspension Due to Stream Inflow

Shear Stress relationship  =  g V 2 /C B 2 g = acceleration of gravity

Resuspension Zones Intake Schoharie Cr. Bear Kill 1.Resuspension in inflow region due to Schoharie Cr runoff events

Hydrodynamic Monitoring E.A. Cowen, Cornell Univ. T-RDI 1200 KHz Workhorse Monitor ADCP Nortek Vector ADV Temperature loggers Aug. – Sept. 2004

Observed and Predicted Bed Stress

Wave-Induced Resuspension fetch < 1500 m; wave heights < 30 cm (small) due to small waves, wave-induced bed stress and resuspension occur where depth < 1 m (narrow strip along lee shore) effect of drawdown: sediments that are in a depositional environment at full reservoir are exposed to resuspension during drawdown

Resuspension Zones Intake Schoharie Cr. Bear Kill 1.Resuspension in inflow region due to Schoharie Cr runoff events 2. Wave Resuspension at Shoreline (SW, W, NW winds dominant)

October 2001 Severe Drawdown (19 meters; 62 ft.) Gatehouse and Intake Structure

Surface Wave Model Donelan/GLERL model used to simulate waves and associated bottom motion and bed stress Previously applied to coastal ocean, large estuaries, Great Lakes; first application to small lake or reservoir 1 measurements of wave height and period made with submerged pressure sensors were used to validate the model 1 Owens, E.M Observation and simulation of surface waves in two water supply reservoirs. Jour. of Hydraulic Engr. 135(8):

Surface Wave Model Validation Oct.-Nov. 2002

Drawdown Conditions – 2002 Example simulations follow

Model Performance: Oct 2002 Red: no resuspensionWhite: all resuspension Green: inflow resuspension

Probability that Withdrawal (Tunnel) Turbidity is less than X (days Tn > 10 NTU) Sept Dec 2005

Conclusions tributary input is generally the dominant source of turbidity to Schoharie Reservoir resuspension near creek mouth caused by runoff events can be an important contributing source, particularly during drawdown wave-driven resuspension is source to surface waters, and is a minor contributing source of turbidity turbidity model accurately represents these two resuspension processes