Water Quality Modeling used to Inform Operational Decisions for the NYC Water Supply:

1. Water Quality Modeling used to Inform Operational Decisions for the NYC Water Supply: A Ten Year Retrospective Mark S. Zion, Donald C. Pierson, Elliot M. Schneiderman and Adao H. Matonse New York City Department of Environmental Protection NYC Watershed/Tifft Science and Technical Symposium West Point, New York September 18-19, 2013

2. Introduction • Elevated turbidity can be an issue for the Catskill System of the NYC water supply during and after periods of high streamflow. • Informed operations during event periods can help to mitigate the impacts of turbidity on the water supply • Over the last decade, DEP has developed and applied an extensive suite of water quality and water system models which are used to analyze turbidity transport in the Catskill System. • This presentation describes the models and illustrates how the models have been applied to help inform operational decisions to maintain high quality water at supply intakes.

3. System Description • DIVIDING WEIR • DIVIDING • WEIR • GATE • GATE • HOUSE • EAST BASIN • Kensico • WEST • BASIN • RELEASE • CHANNEL • CATSKILL • AQUEDUCT • Ashokan Reservoir Options

4. Model Description Catskill Influent • Esopus Creek Delaware Influent • Dividing • Weir Catskill Effluent • Spillway • Catskill • Aqueduct • Release • Channel Delaware Effluent CEQUAL-W2 Two Dimensional Reservoir Models Ashokan Kensico • CEQUAL-W2 Models adapted to simulate turbidity transport by Upstate Freshwater Institute (UFI).

5. Model Description Catskill Influent Delaware Influent Catskill Effluent Delaware Effluent CEQUAL-W2 Two Dimensional Reservoir Models Example Longitudinal Profiles Kensico Temperature Turbidity Profile

6. Model Description • OASIS system model combined with CEQUAL-W2 Reservoir Models • OASIS Model used to simulate aqueduct flows and reservoir releases and spills • Combined OASIS-W2 is backbone of Operations Support Tool (under development)

7. Model Development History • Late 1990s: Two Dimensional water quality models (CEQUAL-W2) developed as part of FAD modeling (UFI). • Early 2000s: W2 models connected using linked reservoir software (UFI). • Mid 2000s: W2 models integrated with OASIS system model for analyses of Catskill Turbidity Control Alternatives. (Hazen & Sawyer/Hydrologics/UFI) • Mid 2000s: W2 model improvements including use of multiple settling rates for turbidity causing particles, improved resuspension processes and expanded historical time series. (UFI) • Mid-Late 2000s: OASIS/W2 model used to evaluate alternatives for Catskill Turbidity Control Program. (Hazen & Sawyer/Hydrologics/UFI) • Late 2000s-Early 2010s: Development of Operations Support Tool (OST) including the integrated OASIS/W2 model along with improved inflow forecasts and data integration. (Hazen & Sawyer/Hydrologics/UFI) • Other Modeling Applications: • Long Term Planning • Recommendations for operating rules • Catskill Turbidity Control Program • Delaware Basin Flexible Flow Management Program (FFMP) • Climate Change Studies

8. Model Description – Application Method Flows Reservoir Storage Effluent Turbidity Reservoir Water Quality Model (CEQUAL-W2) Initial Conditions (Reservoir Water Temperature/Turbidity Profiles; Reservoir Storage Levels; Snowpack) OASIS Water System Model Meteorologic Input (Temp., RH, Solar Rad) Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

9. Model Description – Application Method Flows Reservoir Storage Effluent Turbidity Reservoir Water Quality Model (CEQUAL-W2) Initial Conditions (Reservoir Water Temperature/Turbidity Profiles; Reservoir Storage Levels; Snowpack) OASIS Water System Model Meteorologic Input (Temp., RH, Solar Rad) • Current reservoir water surface elevations • Limnological surveys • Data from automated profiles of water column temperature and turbidity • Snow survey results Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

10. Model Description – Application Method • Forecast based on historical record or weather service analysis • Multiple traces representing many potential future input time series Flows Reservoir Storage Effluent Turbidity Reservoir Water Quality Model (CEQUAL-W2) Initial Conditions (Reservoir Water Temperature/Turbidity Profiles; Reservoir Storage Levels; Snowpack) OASIS Water System Model Meteorologic Input (Temp., RH, Solar Rad) Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

11. Model Description – Application Method Flows Reservoir Storage Effluent Turbidity Reservoir Water Quality Model (CEQUAL-W2) Initial Conditions (Reservoir Water Temperature/Turbidity Profiles; Reservoir Storage Levels; Snowpack) OASIS Water System Model Meteorologic Input (Temp., RH, Solar Rad) Multiple time series results representing each of the forecast traces Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

12. Model Description – Application Method • Example of Position Analysis • Model run for 57 inflow traces based on historical flow and meteorologic record (1948-2004) • Statistics of traces can be translated to • cumulative probability function

13. Modeling Applications History Number of Modeling Analyses by Year Number of Modeling Analyses by Month

14. Sample Analysis – Ashokan Reservoir Model Simulations Late February 2010 • Large storm at the end of January 2010 filled the Ashokan Reservoir and elevated turbidity in the West Basin of the reservoir. • If another large storm event were to occur in late February, the West Basin would spill to the East Basin, creating elevated East Basin turbidity. • A series of CEQUAL-W2 reservoir model simulations and OASIS system model simulations were performed to understand how the use of the Ashokan release channel reduces the risk of higher turbidity water in the West Basin spilling over the dividing weir and entering the East Basin. Esopus Creek Inflow • Esopus Creek • Dividing • Weir • Spillway • Catskill • Aqueduct • Release • Channel Esopus Creek Turbidity

15. Sample Analysis – Ashokan Reservoir Model Simulations Late February 2010 Position Analysis Traces Release Channel = 0 MGD Cumulative Probability Fraction of Traces with Turbidity > 10 NTU on or before date East Basin Withdraw Turbidity (NTU) Release Channel = 350 MGD Simulation Date - 2010 Release Channel = 0 MGD Release Channel = 350 MGD Probability of turbidity exceeding 10 NTU by end of three month simulation period reduced from near 95% to about 35% Simulation Date - 2010

16. Sample Analysis – Ashokan Reservoir Model Simulations Late February 2010 Cumulative Probability First Date of Spill from West Basin to East • Using release channel significantly delays and reduces risk of spill from West Basin to East Basin. • Delay and reduction in spill reduces risks of elevated turbidity in East Basin • Delay and reduction in West to East Spill also reduces risk and amount of spill from East Basin. Simulation Date - 2010 Release Channel = 0 MGD Release Channel = 350 MGD

17. Model Description – Application Method • Kensico Sensitivity Model Application Flows Effluent Turbidity Initial Conditions (Reservoir Water Temperature/Turbidity Profiles) Reservoir Water Quality Model (CEQUAL-W2) Meteorologic Input (Temp., RH, Solar Rad) Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

18. Model Description – Application Method • Kensico Sensitivity Model Application Flows Effluent Turbidity Initial Conditions (Reservoir Water Temperature/Turbidity Profiles) Reservoir Water Quality Model (CEQUAL-W2) • Current reservoir water surface elevations • Limnological surveys • Data from automated profiles of water column temperature and turbidity Meteorologic Input (Temp., RH, Solar Rad) Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

19. Model Description – Application Method • Kensico Sensitivity Model Application Flows Effluent Turbidity Initial Conditions (Reservoir Water Temperature/Turbidity Profiles) Reservoir Water Quality Model (CEQUAL-W2) Meteorologic Input (Temp., RH, Solar Rad) • Forecast based on historical record or weather service analysis • Multiple traces representing many potential future input time series Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

20. Model Description – Application Method • Kensico Sensitivity Model Application Prescribed constant time series based on expected turbidity and potential flow rates from upstream reservoirs (via aqueducts) Flows Effluent Turbidity Initial Conditions (Reservoir Water Temperature/Turbidity Profiles) Reservoir Water Quality Model (CEQUAL-W2) Meteorologic Input (Temp., RH, Solar Rad) Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

21. Model Description – Application Method • Kensico Sensitivity Model Application Flows Effluent Turbidity Initial Conditions (Reservoir Water Temperature/Turbidity Profiles) Reservoir Water Quality Model (CEQUAL-W2) For each combination of fixed turbidity and flow inputs the model produces multiple traces of turbidity results based on the meteorologic input traces Meteorologic Input (Temp., RH, Solar Rad) Turbidity Time Series Input (Forecast) Model Initial Conditions (Snapshot) Results

22. Sample Kensico Analysis October 2010 Esopus Creek Inflow Catskill Influent (1.5 NTU) (20 or 40 NTU) (1150, 1050 or 1000 MGD) (50, 150 or 250 MGD) Delaware Influent Catskill Effluent (400 MGD) Delaware Effluent (800 MGD) • Large event in October produced large input of turbidity to Ashokan Reservoir. There was a large impact on Ashokan diversion turbidity and stop shutters were employed to reduce flow to Kensico Reservoir. • Kensico 2D reservoir model used to determine effects of elevated turbidity and various Catskill Aqueduct flow rates on Kensico Reservoir effluent. Esopus Creek Turbidity

23. Sample Kensico Analysis October 2010 Simulated Catskill Effluent Turbidity Catskill Aqueduct Influent Turbidity Catskill Aqueduct Inflow 50 MGD 150 MGD 250 MGD 20 NTU 40 NTU These runs indicated that if Catskill influent turbidity was above 20 NTU flow rate should be reduced to 150 MGD. If Catskill influent turbidity rises to about 40 NTU for an extended period, then flow should be reduced to about 50 MGD.

24. Conclusions • Over the last decade DEP has developed and applied an extensive suite of water quality and water system models to analyze turbidity transport within the Catskill System Reservoirs. • DEP’s water quality models have been effectively used to help inform operational decisions to minimize use of alum and turbidity at water supply intakes. • Development and application of these models continues under the Operations Support Tool project.