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Evaluation of Turbidity Control Alternatives at Schoharie Reservoir

This study evaluates engineering and structural alternatives to reduce turbidity levels in the Schoharie and Ashokan watersheds in New York City. The study includes reservoir modeling, evaluation of turbidity and temperature control alternatives, and summarizes the findings and conclusions.

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Evaluation of Turbidity Control Alternatives at Schoharie Reservoir

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  1. New York City Department of Environmental Protection Evaluation of Turbidity Control Alternatives at Schoharie Reservoir NYWEA 2009 Watershed Science & Technical Conference September 15, 2009 ▪ West Point W. Josh Weiss, Ph.D., P.E. Hazen and SawyerSteve Effler, Ph.D., P.E. Upstate Freshwater InstituteDavid Warne NYCDEP

  2. Outline • Overview of Catskill Turbidity Control Study • Reservoir Modeling Framework • Turbidity & Temperature Control Alternatives • Evaluation of Alternatives • Summary and Conclusions

  3. Issue: Storm events in the Schoharie & Ashokan watersheds lead to periodic elevated turbidity levels in the Catskill system Overall Study Goal: comprehensive analysis of engineering and structural alternatives: Schoharie: reduce turbidity levels entering Esopus Creek Ashokan: reduce turbidity levels entering Catskill Aqueduct Catskill Turbidity Control Study

  4. DelawareSystem CatskillSystem Schoharie 314 mi2 20 BG 16.0 mi2/BG Shandaken Tunnel Cannonsville 450 mi2 97 BG 4.7 mi2/BG Pepacton 372 mi2 144 BG 2.6 mi2/BG Ashokan 257 mi2 128 BG 2.0 mi2/BG EsopusCreek West Delaware Tunnel East Delaware Tunnel Rondout 95 mi2 50 BG 1.9 mi2/BG Neversink 93 mi2 36 BG 2.6 mi2/BG CatskillAqueduct NeversinkTunnel DelawareAqueduct

  5. Turbidity Sources • Streambank and streambed erosion • Watershed underlain by glacial lake silts and clays • Minimally armored streams • Small particles scatter light efficiently Photo Courtesy NYCDEP

  6. Schoharie Reservoir Gilboa Dam Shandaken Tunnel Intake Schoharie Creek

  7. Catskill Turbidity Control Study:Summary of Phases

  8. Outline • Overview of Catskill Turbidity Control Study • Reservoir Modeling Framework • Turbidity & Temperature Control Alternatives • Evaluation of Alternatives • Summary and Conclusions

  9. Performance Evaluation Approach • Objective of Performance Evaluation: • How will an alternative improve Schoharie water quality under the full range of conditions that the reservoir will experience? • Schoharie Water Quality • Depends on forcing conditions • Depends how reservoir is operated (feedback effects) • Extent of drawdown • Timing of withdrawals • Schoharie Operations • Depends on water quality • Depends on Catskill conditions • Ashokan storage, Esopus flow • Depends on system conditions • seasonal demands, drought status Water Quality from Schoharie 2-D Model Operations from Reservoir System Model (OASIS)

  10. OASIS-W2 Linked Model OASIS Model of NYC Reservoir System & Delaware River Basin Schoharie W2 • Daily Simulation:1948 – 2008 (61 yrs) • Daily Turbidity Predictions at Schoharie-Ashokan-Kensico • Daily Release and Diversion Decisions throughout the System Ashokan W2 Kensico W2

  11. Developed by Upstate Freshwater Institute Mechanistic 2-D water quality model CE-QUAL-W2 platform Simulates temperature & turbidity Loss by Stokes settling – three particle size classes Wave and circulation-driven resuspension Driven by meteorological conditions (e.g., temp, wind speed, solar radiation), inflows and outflows Developed & tested based on detailed monitoring data Documented in numerous peer-reviewed journals Additional data/upgrades/testing conducted in Phase II SA Schoharie Reservoir Water Quality Model 1 m thick vertical layers 17 longitudinal segments

  12. OASIS Model ofNew York City Water Supply System and Delaware River Basin • Reservoir System Operations Model • OASIS - Mass-balance reservoir system model • Developed by HydroLogics • Simulates operation of the reservoir system using goals, constraints, and linear programming • Makes decisions every day about how much water to release from each reservoir in order to meet demands and environmental requirements

  13. Physical Data Storage – Elevation curves Spillway rating curves Head-discharge functions for tunnels/aqueducts Reservoir storage zones Operating Rules Water quality response Reservoir balancing Operating preferences Stream releases Key Componentsof OASIS Model Operating Rules coded into Operations Control Language:

  14. OASIS-W2 Linked ModelHow are the Models Linked? What water quality isavailable for withdrawal? What is the most reliable way to movewater around the system? CE-QUAL-W2 Schoharie Reservoir Ashokan Reservoir OASIS Model NYC Reservoir System & Delaware River Basin • Daily Water Quality Info • Turbidity (& Temp) at the Intake Daily Simulation1948 – 2008 (61 yrs) n = 22,189 days • Daily Diversion & Release Decisions • Diversions from Schoharie Reservoir • Diversions from Ashokan Reservoir • Releases from Ashokan West Basin • Operation of Ashokan Dividing Weir Gates • Alum application at Kensico

  15. Phase II SA Model Updates:OASIS-W2 Linked Model • Various improvements to the linked model conducted in Phase II SA

  16. Phase II SA Model Updates:OASIS-W2 Linked Model • Various improvements to the linked model conducted in Phase II SA • Longer simulation period • Added 2005 -2008; ~61 year record (1/1/1948-9/30/2008) • Revised system balancing rules • Accounts for probability of refill, Croton WTP, Catskill water quality • Updated Ashokan / Catskill Aqueduct operations • Updated Schoharie infrastructure and operations • Crest Gates • Low Level Outlet • Snowpack • Modified Shandaken Tunnel operations

  17. Outline • Overview of Catskill Turbidity Control Study • Reservoir Modeling Framework • Turbidity & Temperature Control Alternatives • Evaluation of Alternatives • Summary and Conclusions

  18. Schoharie Turb/Temp Control Alternatives • Modified Operations • Reduced diversions • Hypolimnetic banking • Low-Level Outlet (LLO) • Multi-Level Intake (MLI) • Site 3 (current intake location) • Site 1.5 (downstream) • Baffle Curtain not studied further due to feasibility and performance

  19. Modified Operations:Reduced Diversions • DEP has adopted Modified Operations evaluated under Phase II Study: • Reduce Shandaken diversions during high turbidity conditions • Subject to water supply constraints (e.g. drought, void) • Updated analyses assume these operations are part of the “baseline” scenario

  20. Modified Operations:Hypolimnetic Banking • Consistent with Phase II Modified Operations • Jun 1 – Sep 15, reduce diversions to just maintain Esopus MCF whenever the 70°F isocline falls below seasonal elevation pattern • Evaluated as proof-of-concept rule, to be refined and implemented with the OST

  21. Implementation of Modified Operations:Operations Support Tool • DEP proceeding with development of OST; completion in 2013

  22. Low-Level Outlet • DEP proceeding with construction of LLO as part of Gilboa Dam Reconstruction; completion in 2014; ~$140M construction cost • Dewater reservoir (during routine O&M or emergency) • Snowpack offset • Proof-of-concept rule for operating the LLO for turbidity control: • Operate LLO when turbidity at the Shandaken Intake exceeds 15 NTU • Subject to various operational constraints • Preliminary evaluation intended to assess performance potential; operation for turbidity control would require further evaluation

  23. Multi-Level Intake • Site 3 and Site 1.5 MLI carried forward from Phase II analysis • Additional option modeled: Site 1.5 MLI plus operation of existing single-level intake at Site 3

  24. Outline • Overview of Catskill Turbidity Control Study • Turbidity & Temperature Control Alternatives • Reservoir Modeling Framework • Evaluation of Alternatives • Summary and Conclusions

  25. Turbidity and Temperature Performance:Stand-Alone Alternatives

  26. Turbidity and Temperature Performance:Combined Alternatives

  27. Outline • Overview of Catskill Turbidity Control Study • Turbidity & Temperature Control Alternatives • Reservoir Modeling Framework • Evaluation of Alternatives • Summary and Conclusions

  28. Summary of Performance Evaluation • Phase II SA verified key findings of the Phase II Study • Modified Operations • Reducing diversions during periods of elevated turbidity is effective • ~2.5% of simulation days over turb threshold (avg. ~9 days/yr) • Hypolimnetic Banking • Can improve control of peak summer diversion temperature • For all alternatives, there are few days in which diversion exceeds 70°F • <1% of simulation days, (avg. ~2-3 days/yr) • Modified Operations adopted by DEP • Full implementation using the OST (development underway)

  29. Summary of Performance Evaluation (cont’d) • Multi-Level Intake • MLI predicted to provide slight incremental turbidity control benefit • ~2-3 days/yr on average, primarily in May-June • No performance difference between Site 3 and Site 1.5 • Performance limited by medium and large events in which the entire reservoir quickly becomes turbid • An MLI at either location can provide good control of peak summer temperatures • Low-Level Outlet • LLO predicted to provide some turbidity control benefit • Proof-of-concept evaluation • Implementation would require additional testing using OST and detailed evaluation

  30. Conclusions • Powerful modeling framework enabled a robust, performance-based evaluation of possible alternatives • Captures feedback between system operation and reservoir water quality • Captures a wide range (61 years) of forcing conditions • Models are critical for sound decision-making in complex systems

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