Integrated water quality water supply modeling to support long-term planning

Transcription

1 E217 Integrated water quality water supply modeling to support long-term planning W. Joshua Weiss, 1 Grantley W. Pyke, 1 William C. Becker, 2 Daniel P. Sheer, 3 Rakesh K. Gelda, 4 Paul V. Rush, 5 and Tina L. Johnstone 5 1 Hazen and Sawyer, Baltimore, Md. 2 Hazen and Sawyer, New York, N.Y. 3 HydroLogics, Columbia, Md. 4 Upstate Freshwater Institute, Syracuse, N.Y. 5 New York City Department of Environmental Protection, Grahamsville, N.Y. New York City s unfiltered Catskill System of reservoirs provides high-quality water to help meet demands for more than 9 million people. Following extreme storms, however, these reservoirs periodically experience high levels of turbidity that may require alum addition before the water enters the city s distribution system. A study was conducted to evaluate measures to control turbidity and reduce the need for alum treatment. The study involved development and application of a linked water supply water quality modeling platform to simulate performance of structural and nonstructural turbidity control alternatives under realistic operations and over a broad array of hydrologic conditions. Results indicated that modest improvements to the existing infrastructure and modified system operations could control turbidity and reduce the need for costly capital improvements. This study provides a useful framework for other utilities to follow in developing analytical tools to help them meet both current and future water supply challenges. Keywords: decision support system, reservoir operation, simulation model, turbidity, water quality, water supply The New York City Department of Environmental Protection (NYCDEP) manages a system of reservoirs and controlled lakes to supply water to more than 9 million consumers in the city and the surrounding communities (Figure 1). Water flows by gravity from reservoirs in the Catskill, Delaware, and Croton watersheds via the Catskill, Delaware, and New Croton aqueducts to downstream balancing reservoirs and ultimately to the city s distribution system to meet a total demand of more than 1 bgd. Operation of the system requires complex balancing of multiple objectives, chief among them the need to maintain a reliable, high-quality supply of drinking water for NYCDEP customers. Because of the high quality of its upstate reservoirs and ongoing watershed protection efforts, New York City is one of only a few major water suppliers in the United States with a filtration avoidance determination (FAD). Although water quality typically is high, the Catskill System reservoirs periodically experience episodes of elevated turbidity following major storms. During periods of elevated turbidity in the Catskill System, the city must rely more heavily on the higher-quality Delaware supply. In extreme cases the city must add alum to the Catskill supply immediately upstream of Kensico, the terminal reservoir for the Catskill and Delaware systems, to reduce turbidity levels. This article describes the results of a study conducted to evaluate operational and structural alternatives to minimize turbidity export from the Catskill System and ultimately to reduce the frequency and duration of alum treatment required at Kensico Reservoir. This study serves as a demonstration of the value of models in water supply planning and operations and provides other water supply utilities with a framework for carrying out similar studies to improve system performance and guide capital planning decisions. background New York City s Catskill System. The Catskill System includes the Schoharie Reservoir, Shandaken Tunnel, and both the west and east basins of Ashokan Reservoir (Figure 1). Approximately 30 to 40% of New York City s average annual demand is met by the Catskill System; the Delaware and Croton systems historically have met 50 60% and up to 10% of demand, respectively. Schoharie Reservoir is fed by a 314-sq-mi watershed and delivers up to 615 mgd to the west basin of Ashokan Reservoir via the Shandaken Tunnel, which releases into Esopus Creek. Diversions from the Shandaken Tunnel are subject to a State Pollutant Discharge Elimination System permit aimed at minimizing turbidity levels and maintaining cold temperatures in the Esopus Creek, which is a state-listed class A stream (NYSDEC, 2008; NYCRR, 1991). Esopus Creek drains a watershed of 200 sq mi and flows into the west basin of Ashokan Reservoir. Water from Ashokan

2 E218 FIGURE 1 New York City water supply system Reservoir is conveyed via the Catskill Aqueduct to Kensico Reservoir, where it typically mixes with water from the Delaware System before being disinfected and conveyed to the city. Turbidity in the Catskill System. From a public health perspective, elevated turbidity in drinking water is an overall indicator of water quality and is of concern primarily because of its potential effect on the disinfection process, e.g., shielding of microbial contaminants during ultraviolet (UV) and/or chlorine disinfection. (Historically, disinfection of Kensico effluent with chlorine has served as the primary means of disinfection of Catskill and Delaware water; NYCDEP is currently constructing a UV disinfection facility.) Turbidity in Ashokan Reservoir and the Catskill Aqueduct is normally < 5 ntu. However, historically infrequent major storms erode naturally occurring silt and clay deposits in stream banks and channels in the Schoharie and Ashokan Watersheds, which can lead to elevated turbidity levels in Schoharie and Esopus Creeks, resulting in periods of elevated turbidity in Schoharie Reservoir, the Shandaken Tunnel Diversion, Ashokan Reservoir, and occasionally in Catskill Aqueduct diversions to Kensico Reservoir. The design of Ashokan Reservoir as a two-basin system separated by a dividing weir offers a substantial buffer against the export of turbidity to Kensico Reservoir (Figure 2). Inflow from the Esopus Creek enters the northwestern-most section of the west basin. Water can enter the east basin from the west basin through gates in the dividing weir or by spilling over the weir. During a turbidity episode, the west basin can provide some buffer as a settling basin, increasing the residence time and subsequent settling of turbidity-causing particles before the water is transferred to the east basin for withdrawal. Diversions into the Catskill Aqueduct intake structure can be made from either basin. Standard operation is to make diversions from the east basin, which typically has higher-quality water. In the past, sustained periods of elevated turbidity in Ashokan Reservoir have required treatment with aluminum sulfate (alum) in the Catskill Aqueduct just upstream of Kensico Reservoir (Figure 3) to ensure a safe drinking water supply and maintain compliance with the Surface Water Treatment Rule (SWTR) 5-ntu turbidity limit for diversions from Kensico Reservoir to the city (USEPA, 1989). Although alum is effective at reducing turbidity levels within Kensico Reservoir, NYCDEP seeks to minimize the frequency and duration of alum application in order to minimize potential adverse effects on the aquatic environment. Regulatory context. In its 2002 and 2007 FAD, the US Environmental Protection Agency determined that contingent on the addition of UV disinfection facilities, the Catskill and Delaware water supply systems comply with the requirements for unfiltered surface water systems established in the SWTR and Interim Enhanced SWTR (USEPA, 1998; 1989). To reduce periodic elevated turbidity levels in the Catskill supply, the FAD also requires that NYCDEP implement the Catskill Turbidity Control Program (CTCP), developed as part of the department s long-term Watershed Protection Program (NYCDEP, 2009; 2006; 2001). The essential SWTR turbidity provision identified in the FAD is the requirement that source water turbidity levels cannot exceed 5 ntu immediately prior to the first or only point of disinfectant application... (USEPA, 1989). The FAD prioritizes development of turbidity control alternatives that provide maximum turbidity reduction throughout the system without the addition of coagulants or other chemicals. FIGURE 2 Ashokan Reservoir

3 E219 FIGURE 3 Historical alum usage, Ashokan and Kensico diversion turbidity, and Esopus Creek stream flows ( ) 10,000 Alum on Esopus Creek flow Catskill Aqueduct diversion turbidity Kensico diversion turbidity Turbidity ntu or Flow mgd 1, Year Project Approach One component of the CTCP was evaluation of operational and infrastructure alternatives for controlling turbidity transport from Ashokan Reservoir to Kensico Reservoir via the Catskill Aqueduct. Alternatives were evaluated using an innovative water supply water quality system modeling framework that accounts for the feedback between reservoir water quality and reservoir system operation. The system modeling decision support approach to water supply planning, reservoir and watershed management, and water/ wastewater treatment selection has been applied successfully, both in near-term operations support and long-term planning contexts (Hamouda et al, 2009; Zhu & McBean, 2007; Xu et al, 2006; Olsson et al, 2003; Percia & Oron, 1997; Meister & Kersten, 1994; Simonovic, 1992; Harris, 1984; Sheer, 1980). Applications of systems modeling for water resources analysis date back to pioneering case studies from as early as the late 1960s (Yeh, 1985; Eastman & ReVelle, 1973; ReVelle & Kirby, 1970; ReVelle et al, 1969; Loucks, 1968; Dietrich & Loucks, 1967). The human ability to reason successfully decreases as the mass of information, level of interdependency, and the length of causal interactions increases (Lempert et al, 2003) all typical aspects of long-term water supply planning. The value of the system modeling approach lies in its ability to support complex decision-making under these computationally intense conditions. System models allow for efficient evaluation of the tradeoffs among various objectives (e.g., water quality, water supply reliability, treatment cost, regulatory compliance, and environmental and/or recreational goals) based on predefined rules and system constraints. In the context of turbidity control for New York City, this approach supported the evaluation of turbidity control alternatives while explicitly accounting for the multiobjective, complex nature of the city s water supply system. Turbidity control alternatives. Following an initial phase of cost and feasibility screening, a set of turbidity control alternatives was selected for evaluation using the system modeling framework. These alternatives generally represented four mechanisms for controlling turbidity in the Catskill System: removal of turbid water via optimized reservoir releases, enhanced settling and reduced short-circuiting, selective diversions of the highest-quality water available, and reduced diversions from affected reservoirs during the occurence of turbidity episodes. Table 1 categorizes the various alternatives by mechanism; Table 2 provides a summary of the six alternatives evaluated in the study. Turbidity in the Catskill system is derived from natural sources. The watershed is underlain by glacial silts and clays, and major storms can destabilize stream banks, mobilize minimally armored stream beds, and suspend the underlying clay particles that cause turbidity.

4 E220 TABLE 1 Summary of conceptual turbidity control mechanisms Mechanism Description Alternative Optimized releases Enhanced settling Selective diversions Minimized diversions Remove turbid water from Ashokan Reservoir upstream of the Catskill Aqueduct intake by making releases to Esopus Creek using existing release channel or upgraded infrastructure. Increase residence time within the west basin of Ashokan Reservoir for particle settling. Minimize spill to the east basin. Reduce short-circuiting in the east basin to the Catskill Aqueduct intake. Enhance the multilevel intake capability at the existing west and east basin intakes. Construct alternative east basin single-level or multilevel intake downstream of the current intake location to allow for selective diversion of the highest-quality water. Minimize diversions of Catskill System water during turbidity events. Various planned system improvements would allow for faster and more reliable reduction in Catskill diversions at the onset of turbidity events, reducing turbidity export until conditions improve. 1, 6b 2, 3, 6c 4, 5a, 5b 6a Analytical framework. An innovative, robust modeling framework was developed and applied in this project to evaluate performance of the turbidity control alternatives. The major components of the modeling framework were a water supply system simulation platform 1 and mechanistic two-dimensional water quality models for key reservoirs, based on the US Army Corps of Engineers widely used CE-QUAL-W2 (referred to here as W2), a two-dimensional, laterally averaged, hydrothermal transport framework (Cole & Wells, 2002). Together, these components composed the linked water supply water quality tool used to evaluate alternatives. The water supply simulation platform is a software program that realistically simulates the routing of water through a water resources system based on user-specified operating objectives (Figure 4). Operation of the system is represented by a series of goals and constraints. Goals are operating rules that the model will attempt to satisfy to the extent possible (e.g., maintain reservoir A at full storage for as long as possible during system drawdown ). Constraints are rules that the model cannot violate under any condition (e.g., the hydraulic capacity of aqueduct B is 500 mgd ). The modeling platform and its predecessors have been used extensively to support water resources planning and management in a variety of watersheds across the United States and worldwide (McCrodden et al, 2010; Sheer & Dehoff, 2009; Palmer, 2008; Pearsall et al, 2005; Randall et al, 1997; Sheer et al, 1992; Lettenmaier & Sheer, 1991). For each simulation time step, the modeling platform expresses the various goals and constraints as a linear objective function for which the optimum solution (i.e., the set of reservoir diversions and releases that satisfy as many of the goals as possible, according to a hierarchy of priorities, while not violating any constraints) can be determined mathematically by a linear programming algorithm or solver. The solved upstream release and diversion volumes determined for one time step then become inflows to downstream reservoirs for the subsequent time step. In this way, the model proceeds daily through the simulation period, resulting in a time series of reservoir release and diversion decisions that simulate realistic operation of the entire water supply system over the range of hydrologic conditions. This analysis used a daily time-step model of the entire New York City reservoir system, including the adjoining lower Delaware River Basin (whose status affects release decisions from the city s Delaware reservoirs); this model was linked to W2 models of the Schoharie, west basin Ashokan, east basin Ashokan, and Kensico reservoirs. The W2 models were adapted and calibrated to include simulation of turbidity transport through the reservoirs, as described elsewhere (Gelda & Effler, 2007a c). The linkage between the simulation platform and the W2 models was designed such that for each simulation day, the water supply model decided what quantities of water to release to streams and divert for water supply from all reservoirs, based on the operating objectives and Catskill System water quality at the beginning of that day. Release and diversion quantities were then reported to the W2 models, which in turn simulated water quality through the reservoirs based on release and diversion The New York City Department of Environmental Protection maintains an active stream management program that supports local development of subbasin-level stream management plans and implements stream restoration and protection projects at priority locations.

5 E221 TABLE 2 Alternative Summary of Ashokan Reservoir turbidity control alternatives Description 1 Construct new west basin release structure to provide enhanced release capacity upstream of Catskill Aqueduct intake (2,000 6,000 mgd capacities and single- and multilevel outlet structures were evaluated). 2 Install crest gates on Ashokan dividing weir to increase storage in west basin to capture storm runoff and increase residence time for settling. 3 Install diversion wall in the east basin around the Catskill Aqueduct intake to minimize short-circuiting of west basin spill to the east basin intake (750 2,400 ft lengths were evaluated). 4 Modify current west and east basin intakes to provide multilevel withdrawal capability. 5 Construct new east basin intake at downstream location. (a) Single-level intake (b) Multilevel intake 6 Optimize operation of existing and planned infrastructure using model simulations. (a) Optimize diversions to the Catskill Aqueduct using existing and planned infrastructure and implement improvements to the Catskill Aqueduct to allow for reduced diversions. (b) Optimize operation of existing release channel (600 1,200 mgd capacity) for turbidity control. (c) Optimize Ashokan operations by proactively drawing the west basin down to provide buffer for capturing storm runoff. quantities determined by the supply simulation platform. Simulated temperature and turbidity values were reported back to the water supply model at the end of the simulation day in order to serve as input for the next simulation day s decisions. In this way, the water supply and water quality models captured the effects of water quality on daily water supply decisions and vice versa, a critical feature for accurate and robust comparison of turbidity control performance among the alternatives. Because the east basin diversion wall improvements alternative required a lateral flow component in order to fully evaluate performance, this analysis relied on a combination of linked model simulations and simulations using a three-dimensional model, the Environmental Fluid Dynamics Code (Tetra Tech, 2002). Although this approach made direct comparison with other alternatives more difficult, it provided a better understanding of the behavior of the flow of turbid water in this critical portion of the reservoir. The computational burden of the threedimensional model limited the ability to simulate the full 57-year period of record. Instead, only the major turbidity episodes resulting in alum treatment in the baseline simulation were modeled. For comparison with the simulation results from the water supply platform and W2, it was assumed that for days between episodes (i.e., days in which there was no predicted alum use under baseline conditions), there was no difference in alum days from the baseline operations scenario and the east basin diversion wall alternative. Simulation scenario. The analysis was based on a long-term simulation, driven by roughly 57 years (Jan. 1, 1948, to Sept. 30, 2004) of historical hydrologic data for the entire NYC reservoir system and the Delaware River Basin. (Such historical operations data must be processed to produce the hydrologic record in the form required for the water supply simulation platform. At the time of the analysis described here, these processed data existed through Because of the computational effort required to do the processing, the database is updated roughly every few years.) These historical data were used as a representation of the range of hydrologic conditions for the system in order to evaluate turbidity control alternatives over an array of historical conditions (including very low flow, very high flow, and normal periods), thus providing a more robust analysis than one in which only a selected set of conditions drives the comparison. Water quality drivers for the model (e.g., instream temperature and turbidity, air temperature, solar radiation) were estimated based on available historical hydrologic and meteorological data. Although the simulations were driven by historical hydrologic conditions, the intent was not to recreate history. Rather, the simulations were meant to approximate the effects of current and modified operations under existing and planned infrastructure conditions. Historical turbidity drivers. Turbidity inputs to the Schoharie and Ashokan reservoirs from Schoharie and Esopus creeks were essential drivers for the long-term simulations. Because turbidity FIGURE 4 Screenshot of the water supply simulation model, with zoom of Catskill system Blue triangles represent reservoirs, yellow ovals represent junctions (e.g., where two streams come together), and red rectangles represent demand nodes (locations where water is removed from the system, typically representing water supply demands).

6 E222 An aerial view shows Ashokan Reservoir during a turbidity episode. The west basin is on the upper left and the east basin is on the lower right. The two basins are separated by a dividing weir with operable gates. Opening the dividing gates during a turbidity episode helps to reduce short-circuiting of turbid spill over the dividing weir to the east basin intake for the Catskill Aqueduct (lower left). Stop shutters on the Catskill Aqueduct are manually installed to provide adequate submergence for outside community taps during a turbidity episode driven reduction in flow in the Catskill Aqueduct below 275 mgd. levels in the creeks were not available for most of the 57-year historical period, they were estimated for the entire simulation period on the basis of correlation between recent flow and turbidity measurements (Figure 5). Flow turbidity regressions for Schoharie and Esopus creeks were developed in order to estimate turbidity inputs to Schoharie and Ashokan reservoirs for the entire long-term simulation period. Long-term simulations were conducted using several different empirical relationships between turbidity and flow in order to better understand the effects of uncertainty in the historical turbidity levels on the modeled performance evaluation results. Although absolute performance varied among these simulations, the relative performance of turbidity control alternatives did not. Results given here correspond to a best-fit deterministic relationship between Esopus Creek flow and turbidity, based on roughly five years of continuous monitoring data at 15-min intervals (Figure 6). Performance measures. Relative performance of turbidity control alternatives were evaluated primarily on the basis of simulated Catskill Aqueduct diversion turbidity and the predicted frequency and duration of alum treatment. In model simulations, alum treatment was assumed whenever the Catskill Aqueduct turbidity load (flow turbidity) exceeded a threshold of 5,000 mgd ntu, roughly corresponding to available historical data on the commencement of alum treatment. In reality, the NYCDEP s decision to add alum is based on a number of factors not represented in the model; however, the threshold used in the current study was determined to be adequate for comparing relative performance of alternatives. Model simulations. Linked model simulations were conducted to evaluate the relative performance of turbidity control alternatives. Simulated operating rules. The water supply water quality linked model was programmed with scenario-specific operating rules to operate the system in a realistic and reliable way under the conditions of interest. Baseline operating rules in the model were developed with extensive input from NYCDEP managers and operators to ensure that they generally followed existing water supply operations and reasonable approximations of operations under future infrastructure scenarios, including infrastructure related to conceptual turbidity control alternatives. FIGURE 5 Flow-weighted Daily Average Turbidity ntu 10,000 1, Paired stream flow and turbidity measurements for Esopus Creek Observations ,000 10,000 Esopus Creek Average Daily Flow mgd

7 E223 Under the baseline operating rules, the model s response to elevated turbidity levels (defined for modeling purposes as Catskill diversion turbidity exceeding 8 ntu) was to reduce diversions from Ashokan Reservoir to 275 mgd, roughly corresponding to the level required to maintain supply to outside communities that draw from the aqueduct. Although this did not completely represent current NYCDEP capabilities and practice (e.g., installation of stop shutters), it did provide a reference point from which to measure the relative performance of the turbidity control alternatives. Because Catskill water is diluted by Kensico Reservoir storage and diversions from the Delaware System, the regulatory limit of 5 ntu for diversions from Kensico Reservoir was not exceeded by the 8-ntu threshold in diversions from the Catskill System. Rather, the 8-ntu level from Ashokan Reservoir represented conditions consistent with the potential for turbidity levels of concern in Kensico Reservoir. Turbidity control operations for the infrastructure alternatives (alternatives 1 5) comprised these baseline operating rules plus alternative-specific rules (e.g., rules for operating the crest gates in response to hydrologic and turbidity conditions). Rules for each alternative were developed on the basis of intermediate model simulations and discussions with NYCDEP Bureau of Water Supply staff in order to evaluate alternatives using the most realistic and sensible operations (Table 3). Table 3 summarizes the major alternative-specific operating rules used for these simulations. These rules do not necessarily represent actual operations of the various facilities but rather are intended as reasonable approximations of operation of the various turbidity control options. On implementation of an alternative or set of alternatives, NYCDEP will develop actual operating rules based on such factors as institutional experience, infrastructure limitations, and output from model simulations. Results The linked model simulation results for the turbidity control alternatives analysis are summarized in Figures 6 and 7 with respect to the key performance metrics: percent of simulation days in which alum treatment occurs and percent of simulation days in which Catskill Aqueduct turbidity exceeds 8 ntu. In Figures 6 and 7, turbidity control alternatives are grouped by primary turbidity control mechanism. In addition to the stand-alone alternatives, various combinations of alternatives were evaluated and are shown in Figures 6 and 7 as combined mechanisms. Of the stand-alone alternatives, the alternatives with the greatest predicted reduction in the number of days of alum addition (Figure 6) included those that minimized diversions to the Catskill Aqueduct during turbidity episodes (alternative 6a, ~ 70% reduction) and those that relied on reservoir releases to remove turbid water from the west basin upstream of the dividing weir (alternative 1, ~ 50% reduction; alternative 6b, ~ 30% reduction). Alternative 5, a new east basin intake at a downstream location, provided some reduction in the number of alum days (roughly 10% for a single-level intake and 15% for a multilevel intake). Other alternatives provided little or no benefit in the number of alum days. Similar results were observed with respect to diversions exceeding 8 ntu (Figure 7). Combining the three components of alternative 6 provided a 96% reduction in the total number of days of alum addition, nearly eliminating the need for alum treatment for the 57-year simulation period (Figure 6) and substantially reducing the frequency of diversions that exceeded 8 ntu over the simulation period (Figure 7). Slight additional reductions in days of alum addition were observed for the alternatives that optimized releases or provided selective withdrawal capabilities along with the combined alternative 6 (Figure 6). In general, however, TABLE 3 Summary of alternative-specific operating rules examined in model simulations Alternative(s) Alternative 1 New west basin release structure Alternative 2 Dividing weir crest gates Alternative 3 East basin diversion wall Alternative 4 Multilevel withdrawal capability Alternative 5 New east basin intake Alternative 6a Optimize Catskill Aqueduct diversions. Alternative 6b Optimize use of existing release channel. Alternative 6c Maintain a void in the west basin. Summary of Operating Rules Make releases from the west basin whenever: (a) the forecast indicates a large inflow over the next h, (b) the west basin is currently spilling water with elevated turbidity, or (c) the void space in Ashokan is less than half of the current snowpack water equivalent volume in the watershed. Do not make releases when the receiving stream is nearing the flood action level. Raise crest gates seasonally to provide an additional ~ 4 bil gal of storage in the west basin during the winter spring refill season. There are no modified operations associated with this alternative. Optimize diversions to the Catskill Aqueduct by selecting the intake elevation(s) with the lowest available turbidity. For a multilevel intake, make diversions to the Catskill Aqueduct from the intake elevation(s) with the lowest available turbidity. Otherwise there are no modified operations associated with this alternative. Reduce diversions to the Catskill Aqueduct to the minimum flow needed to meet demand whenever turbidity exceeds a threshold value. Increase use of the Croton and Delaware systems to the extent possible. Optimized operations of the existing release channel are the same as those for Alternative 1. Make diversions to the Catskill Aqueduct from the west basin whenever turbidity is acceptable in order to create and maintain a void in the west basin to capture stormwater runoff.

8 E224 FIGURE 6 Simulated frequency of days requiring alum treatment during model simulation period ( ) for various alternatives and baseline Optimized releases Alternative 1: 1.8 Alternative 6b: 2.6 Baseline: 3.9 Turbidity Control Mechanism Enhanced settling Selective diversions Alternative 5b: 3.3 Alternative 4: 3.8 Alternative 5a: 3.5 Alternative 3: 3.8 Alternative 6c: 3.8 Alternative 2: 4.0 Minimize diversions Alternative 6a: 0.9 Combined mechanisms Alternative 1 + 6a + 6b + 6c: 0.1 Alternative 5b + 6a + 6b + 6c: 0.1 Alternative 2 + 6a + 6b + 6c: 0.2 Alternative 3 + 6a + 6b + 6c: 0.2 Alternative 4 + 6a + 6b + 6c: 0.2 Alternative 6a + 6b + 6c: Model Simulation Days % of Simulation Period Performance of the baseline scenario (current infrastructure and operating rules) is indicated by the dashed line. Values to the left of the line indicate improvement in performance over the baseline. Black dots represent performance of specific alternatives, with shading showing the performance range for a given turbidity removal mechanism. combining other alternatives with alternative 6 did not provide substantial additional reduction in alum addition because the few turbidity episodes that could not be mitigated completely by alternative 6 were related to extremely large runoffs that are particularly difficult to control. With respect to the number of simulation days in which Catskill Aqueduct diversions exceeded a turbidity of 8 ntu, all of the alternatives except the east basin diversion wall were predicted to provide slight incremental improvement over alternative 6 (Figure 7). Similar to the alum treatment results, the most significant additional improvement over alternative 6 was accomplished by adding capacity to release water from the west basin. Discussion As described previously, the turbidity control alternatives evaluated in this research represented four major (nonexclusive) categories of turbidity control mechanisms. The following sections discuss the performance of the alternatives with respect to these four categories, providing a useful framework for describing why some alternatives were successful and others were not. Optimized releases. Alternatives that provided turbidity control by releasing turbid water upstream of the Catskill Aqueduct included alternative 1 and alternative 6b, optimization of the existing release channel for turbidity control. These alternatives provided substantial improvement (30 50% reduction in simulated days of alum addition) by removing turbidity from the system and thereby reducing turbidity transfer from the west basin to the east basin. However, during episodes of highest turbidity, releasing water at the maximum simulated release capacity was still insufficient to completely eliminate turbid spill to the east basin. Although the predicted duration of alum treatment was substantially reduced, the need for alum was not eliminated altogether. (The total of 367 alum days does not signify that there were 367 individual episodes requiring alum addition; roughly 150 of these alum days arose from the single largest episode over the 57-year simulation period.)

9 E225 FIGURE 7 Simulated frequency of days with Catskill Aqueduct diversion turbidity greater than 8 ntu during model simulation period ( ) for various alternatives and baseline Optimized releases Alternative 1: 3.7 Alternative 6b: 4.9 Baseline: 6.8 Turbidity Control Mechanism Enhanced settling Selective diversions Minimized diversions Alternative 5b: 6.0 Alternative 4: 6.2 Alternative 6a: 5.4 Alternative 3: 6.8 Alternative 6c: 6.8 Alternative 2: 6.9 Alternative 5a: 7.1 Combined mechanisms Alternative 1 + 6a + 6b + 6c: 1.8 Alternative 5b + 6a + 6b + 6c: 2.4 Alternative 2 + 6a + 6b + 6c: 2.5 Alternative 3 + 6a + 6b + 6c: 2.8 Alternative 4 + 6a + 6b + 6c: 3.0 Alternative 6a + 6b + 6c: Model Simulation Days % of Simulation Period Performance of the baseline scenario (current infrastructure and operating rules) is indicated by the dashed line. Values to the left of the line indicate improvement in performance over the baseline. Black dots represent performance of specific alternatives, with shading showing the performance range for a given turbidity removal mechanism. Enhanced settling. As described previously, the optimized release alternatives reduced the amount of turbid spill to the east basin by removing turbid water from the west basin. Although they did not provide the benefit of removing turbidity completely from the system, alternative 2 and alternative 6c both sought to increase the residence time of turbidity-causing particles in the west basin and reduce the amount of turbid spill to the east basin, thereby reducing the turbidity near the Catskill Aqueduct intake. These alternatives were not predicted to provide substantial performance, with the dividing weir crest gates providing no improvement over the baseline and the west basin drawdown providing little improvement with respect to days of alum addition. Similarly, alternative 3 would shift flow patterns such that spill from the west basin was routed further into the east basin, preventing short-circuiting to the Catskill Aqueduct intake and increasing residence time for settling. As noted previously, the east basin diversion wall does not generally provide substantial benefit in terms of alum addition. Despite substantial reductions in peak turbidity levels, diversion turbidities generally remained well above 8 ntu during significant turbidity episodes. Although these alternatives 2, 3, and 6c demonstrated some benefit at the leading edge of some turbidity episodes, the benefit generally was short-lived and provided little overall improvement in the predicted frequency of alum addition. In general, these alternatives underperformed because during the major episodes for which alum addition was predicted, the west basin fills up rapidly, quickly filling any void created in the west basin and overtopping the weir. Furthermore, the small delay in spilling to the east basin afforded by these alternatives simply did not provide adequate time for turbiditycausing particles to settle, likely because of the small size and poor settling characteristics of the majority of these particles. Selective diversions. Alternatives 4 and 5 attempt to selectively divert the water with the lowest turbidity into the Catskill Aqueduct during a turbidity episode. Alternative 4 provided some improvement over the baseline with respect to the number of days when Catskill Aqueduct diversion

10 E226 turbidity exceeded 8 ntu. However, little improvement was seen in the number of days of alum addition, because most of the significant turbidity episodes occurred during unstratified reservoir conditions (i.e., fall, winter, and spring). Alternative 5 relocates the intake further downstream in the east basin, providing some additional protection against turbidity entering from the west basin. A new east basin intake (both single-level and multilevel) provided some benefit in reducing the total number of days of alum addition, particularly for small- and medium-sized turbidity episodes. However, for the larger episodes that contributed the majority (~ 90%) of the simulated days of alum addition, turbidity levels were reduced initially, but performance converged with the baseline simulations after several weeks. Minimized diversions. Optimization of planned infrastructure upgrades (alternative 6a) would provide NYCDEP with the capability to substantially reduce Catskill System diversions or even take the system off line completely while continuing to provide service to the outside communities that draw from the aqueduct. In contrast to the other alternatives, this option provided a direct reduction of the flow (and therefore the turbidity load) entering Kensico Reservoir. Alternative 6a provided roughly a 75% reduction in the frequency of alum addition. In particular, this alternative provided substantial benefits during the major episodes in which turbidity loads were sufficiently high that none of the in-reservoir alternatives provided a benefit. For these episodes, shutting down (or drastically reducing the flow in) the Catskill Aqueduct for an extended period of time was the only way to successfully reduce the turbidity load exported to Kensico Reservoir. Combined alternatives. The combined alternative 6 (release channel optimization, west basin drawdown, and minimized Catskill Aqueduct diversions) provided a 96% reduction in the predicted number of alum addition days over the 57-year simulation period, far surpassing the performance of any of the stand-alone alternatives. It also reduced the number of predicted alum treatment days over the 57-year simulation period to roughly 30 days, compared with more than 800 days for the baseline. The majority of alum treatment days for the combined alternative 6 occurred during the largest storm event in the simulation period (October 1955, with a 30-bil-gal inflow over two days). Turbidity episodes of this magnitude, although rare, cause high turbidity levels throughout the system and are difficult to mitigate under any of the alternatives. For episodes of lesser magnitude, the combined effects of the various turbidity control mechanisms at play (primarily optimized releases and reduced diversions) are additive, indicating that an optimal approach would seek to reduce turbidity levels in Ashokan Reservoir while simultaneously reducing diversions when levels are high. Accordingly, little additional benefit with respect to alum addition was provided by operating alternatives 1 5 in combination with the combined alternative 6. Selected approach for implementation. On the basis of these results, NYCDEP has proposed as a long-term turbidity control measure the implementation of optimization of release-channel operating rules and near-term planned infrastructure, i.e., combined alternative 6; this measure would be supported by a cutting-edge computer system the Operations Support Tool In this closeup, water spills over the Ashokan Reservoir dividing weir from the west basin to the east basin. (OST) which uses an upgraded version of the water supply simulation W2 linked model. This alternative will improve the department s ability to reduce Catskill diversions during turbidity episodes to the minimum level necessary to meet NYC and outside community water demands and guide the use of the existing release channel to help control turbidity without compromising water supply reliability. Furthermore, this alternative represents a cost-effective approach to turbidity control, given that the results of the current study indicated that the much more costly infrastructure alternatives were not likely to provide much additional turbidity control benefit. Accordingly, NYCDEP is planning a connection between the Catskill Aqueduct and shaft 4 of the Delaware Aqueduct to divert water from the Delaware System into the Catskill Aqueduct. This connection could be used to supply sufficient Delaware water to the Catskill Aqueduct to maintain adequate submergence for outside community taps, while still minimizing diversions from Ashokan Reservoir. Under this option, diversions from Ashokan would be reduced to the minimum necessary to satisfy any remaining system demand not met by diversions from the Delaware and Croton systems as well as minor demands from the two outside community systems above the shaft 4 connection. OST. Implementation of the state-of-the-art OST (currently under development by NYCDEP) will enhance the department s ability to implement and refine the modified operating rules developed under the CTCP (and help to minimize the need for alum application in general). Like the analytical framework used to evaluate alternatives in the current study, the OST at its core has both the water supply simulation model to address water quantity and the W2 water quality models to incorporate near-real-time turbidity data from the Catskill System. The OST does not replace the judgment and experience of NYCDEP operators and managers but provides them with a powerful analytical tool to help them better understand the risks and benefits of alternative operating policies and ultimately to provide support for both short-term system management and long-term planning.

11 E227 An interim version of the OST is currently in use to support real-time decision-making in evaluating possible responses to increased turbidity levels (e.g., how long the Catskill Aqueduct can be taken off line before turbidity improves and/or effects on supply reliability become unacceptable). Consistent with the general benefits of the system modeling framework, the OST facilitates such what-if analyses, greatly increasing the ability of managers and operators to make timely decisions while maximizing their understanding of the alternatives and associated risks. This is especially important given the increasingly scrutinized and political climate surrounding such decisions. A key component of the OST is a network to monitor water quality that includes automated depth-profiling instruments at Schoharie, Ashokan, and Kensico reservoirs. These nearreal-time water quality data, combined with current stream flow data and hydrologic forecasts, are fed into the water supply W2 modeling framework and facilitate forecasting of near-term turbidity levels within each reservoir. This allows NYCDEP to simulate operation of the system in a probabilistic, look-ahead mode and test the effects of turbidity control measure decisions on water quality and reservoir storage levels in the coming weeks or months. At Ashokan Reservoir this capability is currently being used to support refinement and implementation of release-channel operating rules. At Kensico Reservoir the OST will improve the department s ability to forecast influent turbidity levels and minimize alum application without compromising water quality or water supply reliability. Finally, the expanded forecasting, monitoring, and analytical tools will support NYCDEP in system planning and operations well beyond turbidity issues. The OST has already become the foundation of a landmark agreement among the five Delaware River Supreme Court Decree Parties (Delaware, New Jersey, Pennsylvania, New York State, and New York City) in which the tool is used to determine the availability of water for conservation release from the city s Delaware System reservoirs (DRBC, 2011). In addition, the OST is being used by NYCDEP to evaluate system vulnerabilities to the effects of climate change and other future uncertainties. As more utilities begin to address these kinds of issues, analytical tools similar to those developed and applied in the current study will be necessary to support robust, cost-effective solutions. Conclusion This study demonstrated the value of a linked reservoir water quality simulation model for the purposes of long-term planning. This tool was used to evaluate a set of turbidity control alternatives for their effectiveness under an array of possible hydrologic conditions using realistic system operations. This approach helped to ensure a robust analysis that accounted for the feedback between system operation and the resulting water quality conditions. Furthermore, the model allowed for an evaluation that considered the human decision-making process as a key variable in implementing long-term operating rules. This analysis led to NYCDEP s decision to forgo costly (e.g., in the range of several hundred million dollars) and less-effective capital improvements in favor of modest improvements to the existing infrastructure and improved monitoring and analytical support for real-time operating decisions. The department has begun to use interim versions of the OST to support routine and episode-based management decisions, including support for operations during several recent turbidity episodes. In addition, the OST s hydrologic forecasting component has become the basis for an improved release program for New York City s Delaware System reservoirs. Continued use and development of the OST (with a final version slated for release in late 2013) will continue to demonstrate the benefits of this type of analytical tool to support water supply management in a rapidly changing environment. Acknowledgment The authors acknowledge the excellent team of engineers and scientists who collaborated on this study, including staff from Hazen and Sawyer, New York, N.Y.; Upstate Freshwater Institute (contribution 311), Syracuse, N.Y.; HydroLogics, Columbia, Md.; and the New York City Department of Environmental Protection, Grahamsville, N.Y. This work was conducted as part of a joint venture between Hazen and Sawyer and Gannett Fleming, Camp Hill, Pa. About the authors W. Joshua Weiss (to whom correspondence should be addressed) is senior principal engineer at Hazen and Sawyer, 1 South St., Ste. 1150, Baltimore, MD 21202; jweiss@hazenandsawyer.com. He has been with Hazen and Sawyer for eight years, working primarily on supporting water supply planning and operations for the city of New York and other clients. Weiss is active in the firm s Water Resources Management and Applied Research Groups and currently is the task leader for Implementation of New York City s Operations Support Tool, a probabilistic decision support system for the city s water supply system that will help guide long-term planning and near-term operations for the system. He holds a BCE degree from the Georgia Institute of Technology in Atlanta, Ga., and MSE and PhD degrees from The Johns Hopkins University in Baltimore, Md. Grantley W. Pyke is a senior associate at Hazen and Sawyer in Baltimore. William C. Becker is vice-president at Hazen and Sawyer in New York, N.Y. Daniel P. Sheer is president of HydroLogics, Columbia, Md. Rakesh K. Gelda is an environmental engineer at Upstate Freshwater Institute, Syracuse, N.Y. Paul V. Rush is deputy commissioner, Bureau of Water Supply, and Tina L. Johnstone is director of Operations, Bureau of Water Supply, at the New York City Department of Environmental Protection, Grahamsville, N.Y. Peer Review Date of submission: 05/07/2012 Date of acceptance: 01/09/2013

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