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Evaluation of Turbidity Control Alternatives at Schoharie Reservoir

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.

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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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