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

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

3 Catskill Turbidity Control Study Issue: 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: Overall Study Goal: comprehensive analysis of engineering and structural alternatives: –Schoharie: reduce turbidity levels entering Esopus Creek –Ashokan: reduce turbidity levels entering Catskill Aqueduct

4 Cannonsville 450 mi 2 97 BG 4.7 mi 2 /BG Rondout 95 mi 2 50 BG 1.9 mi 2 /BG Neversink 93 mi 2 36 BG 2.6 mi 2 /BG Pepacton 372 mi BG 2.6 mi 2 /BG Schoharie 314 mi 2 20 BG 16.0 mi 2 /BG Ashokan 257 mi BG 2.0 mi 2 /BG West Delaware Tunnel East Delaware Tunnel Neversink Tunnel Shandaken Tunnel Delaware Aqueduct Catskill Aqueduct Esopus Creek Delaware System Catskill System

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

6 Schoharie Creek Gilboa Dam Schoharie Reservoir Shandaken Tunnel Intake

Catskill Turbidity Control Study: Summary of Phases 7 Phase I Study (Dec 2004): Screening analysis of potentially feasible Schoharie and Ashokan alternatives Phase II (Schoharie) Study (Sept 2006): Detailed evaluation of Schoharie alternatives Selected for Implementation: Modified Operations Operations Support Tool (OST) Implementation Plan/ Supporting Analyses (July 2009): Conduct uncertainty & sensitivity analyses (including Schoharie Creek turbidity loading) Schoharie Crest Gates / Low Level Outlet Re-evaluate alternatives using updated linked model Phase III (Ashokan) Study (Dec 2007): Detailed evaluation of Ashokan alternatives Selected for Implementation: Modified Operations Catskill Aqueduct Improvements OST Implementation Plan/ Supporting Analyses (July 2008): Conduct uncertainty & sensitivity analyses (including Esopus Creek turbidity loading)

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

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 on forcing conditions Depends how reservoir is operated (feedback effects) Depends how reservoir is operated (feedback effects) – Extent of drawdown – Timing of withdrawals Schoharie Operations Depends on water quality Depends on water quality Depends on Catskill conditions Depends on Catskill conditions – Ashokan storage, Esopus flow Depends on system conditions 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 Daily Simulation: 1948 – 2008 (61 yrs)Daily Simulation: 1948 – 2008 (61 yrs) Daily Turbidity Predictions at Schoharie-Ashokan-KensicoDaily Turbidity Predictions at Schoharie-Ashokan-Kensico Daily Release and Diversion Decisions throughout the SystemDaily Release and Diversion Decisions throughout the System Schoharie W2 OASIS Model of NYC Reservoir System & Delaware River Basin Kensico W2Ashokan W2

11 Schoharie Reservoir Water Quality Model 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 1 m thick vertical layers 17 longitudinal segments

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

13 Operating Rules coded into Operations Control Language: Key Components of OASIS Model 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

14 OASIS-W2 Linked Model How are the Models Linked? OASIS Model NYC Reservoir System & Delaware River Basin What is the most reliable way to move water around the system? Daily Diversion & Release Decisions Diversions from Schoharie Reservoir Diversions from Schoharie Reservoir Diversions from Ashokan Reservoir Diversions from Ashokan Reservoir Releases from Ashokan West Basin Releases from Ashokan West Basin Operation of Ashokan Dividing Weir Gates Operation of Ashokan Dividing Weir Gates Alum application at Kensico Alum application at Kensico Daily Water Quality Info Turbidity (& Temp) at the Intake Turbidity (& Temp) at the Intake What water quality is available for withdrawal? Daily Simulation 1948 – 2008 (61 yrs) n = 22,189 days CE-QUAL-W2 Schoharie Reservoir Ashokan Reservoir

Phase II SA Model Updates: OASIS-W2 Linked Model Various improvements to the linked model conducted in Phase II SA 15 Phase IIPhase II SA Simulation Period ~57 years (1/1/1948-9/30/2004)~61 years (1/1/1948-9/30/2008) System Operations System balancing based largely on recent historical operations data Revised balancing scenario accounts for probability of refill, Croton WTP, Catskill water quality Delaware River Basin Rules Rev. 1FFMP (Dec. 2007) Ashokan Operations No explicit water quality-based rules; no Ashokan W2 models; no representation of Ashokan dividing weir gates Explicit simulation of Ashokan hydraulic controls (dividing weir, waste channel) and water quality Schoharie Infrastructure Pre-Gilboa Dam notch (spill elev. 1130’) Includes notch and crest gates (spill elev. 1125’/1130’) No low-level outlet Includes low-level outlet (operated for snowpack management under baseline) Shandaken SPDES Baseline not SPDES-compliant Baseline is SPDES-compliant (DEP adoption of Ph II Mod Ops) Absolute 15 NTU turbidity limit (assumes 0 NTU in Esopus) Delta-15 NTU turbidity limit (Esopus turbidity based on flow-turbidity regression) Schoharie Creek Flow-Turbidity Relationship Single line, flow-based regression (1) Single-line, flow-based regression (2) Multi-line, event-based regression

Phase II SA Model Updates: OASIS-W2 Linked Model Various improvements to the linked model conducted in Phase II SA Longer simulation period –Added ; ~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 16

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

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 18

Modified Operations: Reduced Diversions 19 Condition Phase II (Modified Ops) Phase II SA (Reference Run) Reduce diversions to maintain 160 mgd Esopus MCF Turb15 NTU (Absolute)Delta-15 NTU Temp70°F Turbidity Shutdown (No Diversion) 100 NTU 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

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 20

Implementation of Modified Operations: Operations Support Tool DEP proceeding with development of OST; completion in

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 22 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

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 23 LocationNo. of Levels Intake Invert Elevation (ft) Top Level 2 nd Level 3 rd Level Drain Level Site Site Site 1.5 or Current Intake 4 (Site 1.5) + 1 (Site 3)

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

Turbidity and Temperature Performance: Stand-Alone Alternatives 25

Turbidity and Temperature Performance: Combined Alternatives 26

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

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) 28

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 29

30Conclusions 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