Advanced Modeling Tools for Evaluating Catskill Turbidity Control Alternatives New York City Department of Environmental Protection NYWEA Watershed Science and Technical Conference September 17, 2008 ▪ West Point Grantley Pyke, P.E. Hazen and Sawyer Steve Effler, Ph.D., P.E. Upstate Freshwater Institute Daniel Sheer, Ph.D., P.E.HydroLogics David WarneNYCDEP

2 I. Study Overview -Issue & Objectives -Alternatives Evaluated II. Reservoir Modeling Framework -Reservoir Water Quality Models -Reservoir System Operations Model III. Evaluation of Alternatives -Water Quality Performance IV. Conclusions Outline

3 Catskill Turbidity Control Study Issue: Issue: –Major storm events in the Schoharie & Ashokan watersheds lead to periodic elevated turbidity levels in the Catskill system Overall Study Goal: Overall Study Goal: –Evaluate turbidity control alternatives at Ashokan that can: a) reduce the frequency of high turbidity levels entering Kensico, and b) reduce the need for alum treatment at Kensico Why Interesting? Why Interesting? –Demonstrates how models are critical for sound decision- making in complex systems

44 Esopus Creek Flow and Alum Treatment Events 46 BG in 12 days 44 BG in 13 days 46 BG in 11 days

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

6 Ashokan Reservoir Turbidity Control Alternatives Alt. 1: West Basin Outlet Structure: To release water from the W Basin and reduce turbidity load into the E Basin ~$250M Alt. 4: Upper Gate Chamber Modifications: To improve selective withdrawal capability ~$65M Alt. 2: Dividing Weir Crest Gates: To increase W Basin detention storage ~$140M Alt. 3: E Basin Diversion Wall: To reduce short-circuiting into Upper Gate Chamber ~$130M Alt. 5: New East Basin Intake: To withdraw from a location less susceptible to high turbidity ~$330M Alt. 6: Catskill Aqueduct Improvements and Modified Operations: a) West Basin Drawdown b) Waste Channel Operation c) Catskill Aqueduct Improvements ~$90M

7 Performance Evaluation Approach How will an alternative improve Catskill turbidity control under the full range of conditions that the system will experience? Water Quality Depends on forcing conditions Depends on forcing conditions Depends how reservoirs and aqueducts are operated (feedback effects) Depends how reservoirs and aqueducts are operated (feedback effects) – Extent of drawdown – Timing of diversions & releases – Reservoir balancing 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, storage levels, drought status Water Quality from Schoharie & Ashokan 2-D Models Operations from Reservoir System Model (OASIS)

8 Schoharie, Ashokan, and Kensico Reservoir Water Quality Models (W2) Developed by UFI using CE-QUAL-W2 framework Model development & testing supported by: Continuous Automated Monitoring – –Continuous automated in-stream and in-reservoir monitoring – –Detailed Daily Depth Profiles – –Temperature, Specific Conductance, Beam Attenuation Coefficient, Optical Backscatter Process Studies – –Particle Settling Velocity – –Sediment Resuspension – –Downward Sediment Flux – –Internal Wave Characterization – –Wave Pressure Measurements Storm Event Gridding/Profiling – –Detailed vertical profiles of reservoir water quality collected during and after storm events – –Lateral and longitudinal transects Sensitivity Analysis –sensitivity of Ashokan model predictions to model drivers and model parameters –Robust analysis of sensitivity of Ashokan model predictions to model drivers and model parameters

9 OASIS Model Mass-balance reservoir system operations model Mass-balance reservoir system operations 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

10 Operating Rules coded into Operations Control Language: OASIS System Model Physical Data –Storage – Elevation curves –Spillway rating curves –Head-discharge functions for tunnels/aqueducts –Reservoir storage zones Operating Rules –Stream releases –Reservoir balancing –Operating preferences

11 OASIS Modeling Approach: Historical Inflows combined with System Scenarios System Scenario Demand Regulations Infrastructure Operating Rules x x Historical Inflows Daily, for each stream reach and reservoir Represents range of future inflows Performance Water Supply Water Quality   Model includes net daily inflow to each reservoir and stream reach for the period Objective is NOT to model history and recreate what happened Objective is to use historical inflows as an indicator of the range of inflows that could occur in the future

12 Daily Simulation: 1948 – 2004 (57 yrs)Daily Simulation: 1948 – 2004 (57 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 OASIS-W2 Linked Model Schoharie W2 OASIS Model of NYC Reservoir System & Delaware River Basin Kensico W2Ashokan W2

13 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 – 2004 (57 yrs) n = 20,728 days CE-QUAL-W2 Schoharie Reservoir Ashokan Reservoir

14 What constitutes a reasonable simulation of operations? –Similar patterns of drawdown and refill –Conservative operations re: water supply reliability –Simulated operations are feasible and implementable OASIS operating rules have been reviewed by DEP Ops Staff and found to provide a sound and realistic simulation of system operations Linked OASIS-W2 Model Performance Comparison with Historical Operations Goal: demonstrate that the model provides a reasonable simulation of how the NYC water supply system is operated OASIS assumptions: –1/1/87 - 9/30/04 –Current baseline operating rules –Historical demand levels Exact match is not expected due to changes in model drivers over the comparison period: –Model reflects current regulations –Model reflects current operating rules –Model reflects current infrastructure capacities –Model has all system components in service

15 Comparison with Historical Operations System Storage Levels

16 Comparison with Historical Operations Catskill Reservoir Elevations

17 Comparison with Historical Operations Catskill Turbidity Levels

18 Evaluation of Alternatives Alternatives Alt 1: West Basin Outlet Structure(2000, 4000, 6000 mgd release capacity) Alt 2: Dividing Weir Crest Gates(4’ crest gate on Dividing Weir) Alt 3: East Basin Diversion Wall**(750’, 1700’, 2400’ wall; 3-D simulations) Alt 4: Upper Gate Chamber Modifications(East only, East + West) Alt 5: East Basin Intake(Single- and Multi-level intakes) Alt 6: Catskill Aqueduct Improvements and Modified Operations –West Basin Drawdown –Waste Channel Operation(1200 mgd release capacity) –Catskill Aqueduct Improvements Alternatives 1- 5 in combination with Alternative 6 Water Quality Performance Measures # Days with Ashokan Diversion Turbidity > 8 NTU # Days when Alum Treatment could be required

19 # Days Ashokan Diversion Turbidity > 8 NTU

20 # Days When Alum Treatment Could be Required

21 Proposed Alternative 6: West Basin Drawdown Current practice: typically divert water from the East Basin Proposed practice: divert water from West Basin during periods of low turbidity to develop/maintain a void Reduces chance of spilling turbid water from West to East Basin during storm events –Provides slight reduction in alum treatment days; large events still require alum treatment Operating rules can be further refined with planned Operations Support Tool (OST) –OST would link near-real time water quality data with OASIS-W2 platform and allow look-ahead simulations –OST expected to provide system-wide benefit – help balance water supply reliability, water quality, environmental objectives

22 Proposed Alternative 6: Waste Channel Operation Existing Waste Channel can be used to release water from Ashokan to Esopus Creek Current practice: not used under normal operations Proposed practice: routine use of Waste Channel for turbidity control –Helps reduce turbid spill into East Basin –Could be used during storm events, or in anticipation of peak flows based on Esopus forecast data –Provisional operating rules could be further refined/improved with the OST Current NYCDEP actions –Acquisition/restoration of low-lying portions of the Ashokan Field Campus –Valve improvements at Lower Gate Chamber to restore original 1200 mgd release capacity –Operations Support Tool

23 Proposed Alternative 6: Catskill Aqueduct Modifications Current practice: DEP must maintain minimum flow of 275 mgd in Cat. Aq. to keep outside community (OC) taps submerged –Operation at lower flow rates requires installation of stop shutters at up to 5 locations –Cumbersome, time-consuming, emergency operation –Cannot be implemented as a routine turbidity control measure Proposed practice: Improve DEP’s ability to operate the Catskill Aqueduct at minimum flow rates during turbidity events 3 main options: 1.Improvements to stop shutter facilities 2.Improvements to OC taps 3.Connection with Delaware Aqueduct Shaft 4 -Further study required to select among options

24 Shaft 4 Connection Catskill and Delaware Aqueducts cross, but are not connected Shaft 4 of Delaware Aqueduct was designed for a future connection Benefits Divert Delaware water into Cat. Aq. during turb events –Reduce/eliminate Ashokan diversion, while maintaining supply to outside communities –Improved WQ to OC’s Greater operational flexibility Improve system reliability

25 Summary of Phase III Study Findings Reducing Catskill diversions is the most effective way to reduce alum treatment –Catskill Aqueduct Improvements & Croton WTP will dramatically increase DEP operational flexibility Releases from Ashokan West Basin prior to or during major events also provide substantial benefit Planned Operations Support Tool will bolster DEP ability to optimize operations for water quality & water supply reliability Powerful modeling framework enabled a robust, performance-based evaluation of alternatives –Captures feedback between system operation and reservoir water quality –Captures a wide range (57 years) of forcing conditions to represent long-term performance –Worthwhile evaluating alternatives using a robust modeling framework before making major capital decisions

26 Question & Answer

27 Operations during Catskill Turbidity Events: System Diversions Catskill Aqueduct diversion turbidity > 8 NTU –Minimize diversions from the Catskill System –Increase diversions from the Delaware and Croton Systems Minimum diversion from the Catskill System –Baseline Operations: Cut back to 275 mgd (minimum flow needed to satisfy outside community demands without installing stop shutters at 5 locations along the Catskill Aqueduct) –Alt. 6 Catskill Aqueduct Improvements (and combined alts): Cut back to the minimum flow possible while still satisfying NYC and outside demands Maximum flow from Delaware System:890 mgd Maximum flow from Croton System:160 mgd (Current Conditions) 290 mgd (Post-Croton Filtration)

28 Operations during Catskill Turbidity Events: Alum Application Alum application at Kensico is approximated using a simple rule based on Catskill Aqueduct turbidity load: –Alum on when load > 5,000 mgd*NTU –Alum off when load < 4,000 mgd*NTU (5-day running average basis) Alum rule is a simple surrogate for the actual decision-making process, which is more complex and depends on: –Delaware System turbidity levels –Overall system status –Time of year –Extent of stratification in Kensico –‘Look-ahead’ modeling of Kensico diversion turbidity levels Alum rule is a simple surrogate and should be interpreted as days when alum application could be required –Focus on relative performance of alternatives

29 Alt 1: West Basin Outlet Structure Operating Rules Physical alternatives: –Outlet Structure with weir elevation 585’  3 max capacities evaluated: 2000, 4000, 6000 mgd (actual flow is head-dependent) –Outlet Structure with multi-level withdrawal capability (4000 mgd)  Operated to withdraw from strata with highest turbidity water Rules –Short-term: release the amount expected to spill into the East Basin, based on 2-day AHPS* inflow forecast –Long-term (snowpack management): release the amount needed to maintain void equal to half the snowpack volume –Outlet Structure operated only when 1200 mgd release capacity via Waste Channel is insufficient to meet the above objectives *AHPS = Advanced Hydrologic Prediction Service; provides 24 and 48hr forecasts of Esopus Creek flow at Coldbrook

30 Alt 6: Catskill Aqueduct Improvements and Modified Operations: Operating Rules Mod Ops 1: West Basin Drawdown –Make diversions from W. Basin whenever turb < 5 NTU and E. Basin has more than 1’ freeboard Mod Ops 2: Optimize Operation of Existing Waste Channel –1200 mgd capacity (after planned valve repairs) –Short-term: release the amount expected to spill to the E. Basin –Long-term: maintain void equal to half the snowpack volume Catskill Aqueduct Improvements –Improve ability to readily operate at flows <275 mgd while still meeting outside community demands –Turbidity > 8 NTU: reduce Catskill diversion to the minimum flow possible while still satisfying NYC and outside demands

31 Catskill Aqueduct Profile

32 Sensitivity and Uncertainty Analysis for 2D Ashokan Reservoir Model Examined sensitivity of predictions to model drivers and model parameters Parameters/drivers examined included: –Hydrodynamic model coefficients (n = 7 parameters) –Thermal model coefficients (n = 3) –Turbidity sub model coefficients (n = 3) –Model boundary conditions / drivers (n = 10) –Bathymetry (n = 3) Representation of variability in drivers more important than uncertainty in model parameters Dominant source of uncertainty for performance of alternatives is Esopus Creek Q-Tn relationship –Evaluated multiple turbidity loading scenarios; developed Monte Carlo approach to accommodate inter-event variability