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

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

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

6. 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. 7 Model Development History Mid 2000s: W2 models integrated with OASIS system model for analyses of Catskill Turbidity Control Alternatives. (Hazen & Sawyer/Hydrologics/UFI) 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). Other Modeling Applications: Long Term Planning Recommendations for operating rules Catskill Turbidity Control Program Delaware Basin Flexible Flow Management Program (FFMP) Climate Change Studies 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)

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

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

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

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

12. 12 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 Model Description – Application Method Example of Position Analysis

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

14. 14 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 Turbidity Sample Analysis – Ashokan Reservoir Esopus Creek Dividing Weir Catskill Aqueduct Release Channel Spillway

15. 15 Model Simulations Late February 2010 Position Analysis Traces Release Channel = 0 MGD Release Channel = 350 MGD Cumulative Probability Fraction of Traces with Turbidity > 10 NTU on or before date East Basin Withdraw Turbidity (NTU) 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% Sample Analysis – Ashokan Reservoir

16. 16 Model Simulations Late February 2010 Cumulative Probability First Date of Spill from West Basin to East Simulation Date - 2010 Release Channel = 0 MGD Release Channel = 350 MGD 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. Sample Analysis – Ashokan Reservoir

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

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

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

20. 20 Reservoir Water Quality Model (CEQUAL-W2) Meteorologic Input (Temp., RH, Solar Rad) FlowsTurbidity Effluent Turbidity Initial Conditions (Reservoir Water Temperature/Turbidity Profiles) Time Series Input (Forecast) Initial Conditions (Snapshot) Model Results 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)

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

22. 22 Esopus Creek Inflow Esopus Creek Turbidity Catskill Influent Catskill Effluent Delaware Effluent Delaware Influent 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. (20 or 40 NTU) (50, 150 or 250 MGD) (1150, 1050 or 1000 MGD) (1.5 NTU) (400 MGD) (800 MGD) Sample Kensico Analysis October 2010

23. 23 October 2010 Simulated Catskill Effluent Turbidity 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. Catskill Aqueduct Inflow Catskill Aqueduct Influent Turbidity 150 MGD250 MGD50 MGD Sample Kensico Analysis

24. 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.