Integrated Water Quality–Water Supply Modeling to Support Long-Term Planning W

Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E217 Journal - American Water Works Association Peer-Reviewed

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. Johnstone5

1Hazen and Sawyer, Baltimore, Md. 2Hazen and Sawyer, New York, N.Y. 3HydroLogics, Columbia, Md. 4Upstate Freshwater Institute, Syracuse, N.Y. 5New York City Department of Environmental Protection, Grahamsville, N.Y.

New York City’s unfiltered Catskill System of reservoirs simulate performance of structural and nonstructural turbidity provides high-quality water to help meet demands for more control alternatives under realistic operations and over a than 9 million people. Following extreme storms, however, broad array of hydrologic conditions. Results indicated that these reservoirs periodically experience high levels of turbidity modest improvements to the existing infrastructure and that may require alum addition before the water enters the modified system operations could control turbidity and reduce city’s distribution system. A study was conducted to evaluate the need for costly capital improvements. This study provides measures to control turbidity and reduce the need for alum a useful framework for other utilities to follow in developing treatment. The study involved development and application analytical tools to help them meet both current and future of a linked water supply–water quality modeling platform to water supply challenges.

Keywords: decision support system, reservoir operation, simulation model, turbidity, water quality, water supply

The New York City Department of Environmental Protection turbidity export from the Catskill System and ultimately to (NYCDEP) manages a system of reservoirs and controlled lakes reduce the frequency and duration of alum treatment required to supply water to more than 9 million consumers in the city and at Kensico Reservoir. This study serves as a demonstration of the surrounding communities (Figure 1). Water flows by gravity the value of models in water supply planning and operations from reservoirs in the Catskill, Delaware, and Croton watersheds and provides other water supply utilities with a framework for via the Catskill, Delaware, and New Croton aqueducts to down- carrying out similar studies to improve system performance and stream balancing reservoirs and ultimately to the city’s distribu- guide capital planning decisions. tion system to meet a total demand of more than 1 bgd. Opera- tion of the system requires complex balancing of multiple background objectives, chief among them the need to maintain a reliable, New York City’s Catskill System. The Catskill System includes the high-quality supply of drinking water for NYCDEP customers. Schoharie Reservoir, Shandaken Tunnel, and both the west and Because of the high quality of its upstate reservoirs and ongoing east basins of Ashokan Reservoir (Figure 1). Approximately 30 watershed protection efforts, New York City is one of only a few to 40% of New York City’s average annual demand is met by the major water suppliers in the United States with a filtration avoid- Catskill System; the Delaware and Croton systems historically ance determination (FAD). Although water quality typically is have met 50–60% and up to 10% of demand, respectively. high, the Catskill System reservoirs periodically experience epi- Schoharie Reservoir is fed by a 314-sq-mi watershed and deliv- sodes of elevated turbidity following major storms. During peri- ers up to 615 mgd to the west basin of Ashokan Reservoir via the ods of elevated turbidity in the Catskill System, the city must rely Shandaken Tunnel, which releases into Esopus Creek. Diversions more heavily on the higher-quality Delaware supply. In extreme from the Shandaken Tunnel are subject to a State Pollutant Dis- cases the city must add alum to the Catskill supply immediately charge Elimination System permit aimed at minimizing turbidity upstream of Kensico, the terminal reservoir for the Catskill and levels and maintaining cold temperatures in the Esopus Creek, Delaware systems, to reduce turbidity levels. which is a state-listed class A stream (NYSDEC, 2008; NYCRR, This article describes the results of a study conducted to 1991). Esopus Creek drains a watershed of 200 sq mi and flows evaluate operational and structural alternatives to minimize into the west basin of Ashokan Reservoir. Water from Ashokan

2013 © American Water Works Association Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E218 Journal - American Water Works Association Peer-Reviewed

west basin. Water can enter the east basin from the west basin FIGURE 1 New York City water supply system 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 Environ- mental 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 Water- shed 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 dis- Reservoir is conveyed via the Catskill Aqueduct to Kensico Res- infectant application . . . ” (USEPA, 1989). The FAD prioritizes ervoir, where it typically mixes with water from the Delaware development of turbidity control alternatives that provide maxi- System before being disinfected and conveyed to the city. mum turbidity reduction throughout the system without the Turbidity in the Catskill System. From a public health perspective, addition of coagulants or other chemicals. 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 disinfec- FIGURE 2 Ashokan Reservoir tion. (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 occasion- ally 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

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Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E219 Journal - American Water Works Association Peer-Reviewed

FIGURE 3 Historical alum usage, Ashokan and Kensico diversion turbidity, and Esopus Creek stream ows (1987–2008)

Alum on Esopus Creek flow Catskill Aqueduct diversion turbidity Kensico diversion turbidity

10,000

1,000 w— mgd 100 u or Flo 10 rbidity — nt

Tu 1.0

0.1 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 Year

Project Approach Turbidity control alternatives. Following an initial phase of cost One component of the CTCP was evaluation of operational and feasibility screening, a set of turbidity control alternatives and infrastructure alternatives for controlling turbidity transport was selected for evaluation using the system modeling frame- from Ashokan Reservoir to Kensico Reservoir via the Catskill work. These alternatives generally represented four mechanisms Aqueduct. Alternatives were evaluated using an innovative water for controlling turbidity in the Catskill System: supply–water quality system modeling framework that accounts • removal of turbid water via optimized reservoir releases, for the feedback between reservoir water quality and reservoir • enhanced settling and reduced short-circuiting, system operation. • selective diversions of the highest-quality water available, and The system modeling–decision support approach to water sup- • reduced diversions from affected reservoirs during the ply planning, reservoir and watershed management, and water/ occurence of turbidity episodes. wastewater treatment selection has been applied successfully, Table 1 categorizes the various alternatives by mechanism; both in near-term operations support and long-term planning Table 2 provides a summary of the six alternatives evaluated in contexts (Hamouda et al, 2009; Zhu & McBean, 2007; Xu et al, the study. 2006; Olsson et al, 2003; Percia & Oron, 1997; Meister & Ker- sten, 1994; Simonovic, 1992; Harris, 1984; Sheer, 1980). Applica- tions 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 Turbidity in the Catskill system is derived from natural sources. The watershed the evaluation of turbidity control alternatives while explicitly is underlain by glacial silts and clays, and major storms can destabilize stream accounting for the multiobjective, complex nature of the city’s banks, mobilize minimally armored stream beds, and suspend the underlying water supply system. clay particles that cause turbidity.

2013 © American Water Works Association Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E220 Journal - American Water Works Association Peer-Reviewed

TABLE 1 Summary of conceptual turbidity control mechanisms

Mechanism Description Alternative

Optimized releases Remove turbid water from Ashokan Reservoir upstream of the Catskill Aqueduct intake by making releases 1, 6b to Esopus Creek using existing release channel or upgraded infrastructure. Enhanced settling Increase residence time within the west basin of Ashokan Reservoir for particle settling. 2, 3, 6c Minimize spill to the east basin. Reduce short-circuiting in the east basin to the Catskill Aqueduct intake. Selective diversions Enhance the multilevel intake capability at the existing west and east basin intakes. 4, 5a, 5b 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. Minimized diversions Minimize diversions of Catskill System water during turbidity events. 6a 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.

Analytical framework. An innovative, robust modeling frame- This analysis used a daily time-step model of the entire New work was developed and applied in this project to evaluate per- York City reservoir system, including the adjoining lower Dela- formance of the turbidity control alternatives. The major com- ware River Basin (whose status affects release decisions from ponents of the modeling framework were a water supply system the city’s Delaware reservoirs); this model was linked to W2 simulation platform1 and mechanistic two-dimensional water models of the Schoharie, west basin Ashokan, east basin Asho- quality models for key reservoirs, based on the US Army Corps kan, and Kensico reservoirs. The W2 models were adapted and of Engineers’ widely used CE-QUAL-W2 (referred to here as W2), calibrated to include simulation of turbidity transport through a two-dimensional, laterally averaged, hydrothermal transport the reservoirs, as described elsewhere (Gelda & Effler, 2007a–c). framework (Cole & Wells, 2002). Together, these components The linkage between the simulation platform and the W2 mod- composed the linked water supply–water quality tool used to els was designed such that for each simulation day, the water evaluate alternatives. supply model decided what quantities of water to release to The water supply simulation platform is a software program streams and divert for water supply from all reservoirs, based that realistically simulates the routing of water through a water on the operating objectives and Catskill System water quality at resources system based on user-specified operating objectives the beginning of that day. Release and diversion quantities were (Figure 4). Operation of the system is represented by a series of then reported to the W2 models, which in turn simulated water goals and constraints. Goals are operating rules that the model quality through the reservoirs based on release and diversion 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 The New York City Department of Environmental Protection maintains period, resulting in a time series of reservoir release and diversion an active stream management program that supports local development decisions that simulate realistic operation of the entire water of subbasin-level stream management plans and implements stream supply system over the range of hydrologic conditions. restoration and protection projects at priority locations.

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from the baseline operations scenario and the east basin diversion TABLE 2 Summary of Ashokan Reservoir turbidity control wall alternative. alternatives Simulation scenario. The analysis was based on a long-term Alternative Description simulation, driven by roughly 57 years (Jan. 1, 1948, to Sept. 30,

1 Construct new west basin release structure to provide 2004) of historical hydrologic data for the entire NYC reservoir enhanced release capacity upstream of Catskill system and the Delaware River Basin. (Such historical operations Aqueduct intake (2,000–6,000 mgd capacities and single- and multilevel outlet structures were evaluated). data must be processed to produce the hydrologic record in the form required for the water supply simulation platform. At the 2 Install crest gates on Ashokan dividing weir to increase time of the analysis described here, these processed data existed storage in west basin to capture storm runoff and increase residence time for settling. through 2004. Because of the computational effort required to do the processing, the database is updated roughly every few years.) 3 Install diversion wall in the east basin around the Catskill These historical data were used as a representation of the range Aqueduct intake to minimize short-circuiting of west of hydrologic conditions for the system in order to evaluate tur- basin spill to the east basin intake (750–2,400 ft lengths were evaluated). bidity control alternatives over an array of historical conditions (including very low flow, very high flow, and normal periods), 4 Modify current west and east basin intakes to provide thus providing a more robust analysis than one in which only a multilevel withdrawal capability. selected set of conditions drives the comparison. Water quality

5 Construct new east basin intake at downstream location. drivers for the model (e.g., instream temperature and turbidity, (a) Single-level intake air temperature, solar radiation) were estimated based on avail- (b) Multilevel intake able historical hydrologic and meteorological data. Although the simulations were driven by historical hydrologic 6 Optimize operation of existing and planned infrastructure conditions, the intent was not to recreate history. Rather, the simula- using model simulations. tions were meant to approximate the effects of current and modified (a) Optimize diversions to the Catskill Aqueduct using existing and planned infrastructure and implement operations under existing and planned infrastructure conditions. improvements to the Catskill Aqueduct to allow for Historical turbidity drivers. Turbidity inputs to the Schoharie and reduced diversions. Ashokan reservoirs from Schoharie and Esopus creeks were (b) Optimize operation of existing release channel (600–1,200 mgd capacity) for turbidity control. essential drivers for the long-term simulations. Because turbidity (c) Optimize Ashokan operations by proactively drawing the west basin down to provide buffer for capturing storm runoff. FIGURE 4 Screenshot of the water supply simulation model, with zoom of Catskill system quantities determined by the supply simulation platform. Simu- lated 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 three- dimensional 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 Blue triangles represent reservoirs, yellow ovals represent junctions supply platform and W2, it was assumed that for days between (e.g., where two streams come together), and red rectangles represent episodes (i.e., days in which there was no predicted alum use demand nodes (locations where water is removed from the system, typically representing water supply demands). under baseline conditions), there was no difference in alum days

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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 Stop shutters on the Catskill Aqueduct are manually installed to provide basins are separated by a dividing weir with operable gates. Opening the adequate submergence for outside community taps during a turbidity dividing gates during a turbidity episode helps to reduce short-circuiting episode–driven reduction in flow in the Catskill Aqueduct below 275 mgd. of turbid spill over the dividing weir to the east basin intake for the Catskill Aqueduct (lower left). levels in the creeks were not available for most of the 57-year rules to operate the system in a realistic and reliable way under historical period, they were estimated for the entire simulation the conditions of interest. Baseline operating rules in the model period on the basis of correlation between recent flow and turbid- were developed with extensive input from NYCDEP managers ity measurements (Figure 5). Flow–turbidity regressions for and operators to ensure that they generally followed existing Schoharie and Esopus creeks were developed in order to estimate water supply operations and reasonable approximations of turbidity inputs to Schoharie and Ashokan reservoirs for the operations under future infrastructure scenarios, including infra- entire long-term simulation period. Long-term simulations were structure related to conceptual turbidity control alternatives. 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 FIGURE 5 Paired stream ow and turbidity measurements varied among these simulations, the relative performance of for Esopus Creek turbidity control alternatives did not. Results given here corre- 2003–08 Observations spond to a best-fit deterministic relationship between Esopus 10,000 Creek flow and turbidity, based on roughly five years of con-

tinuous monitoring data at 15-min intervals (Figure 6). u Performance measures. Relative performance of turbidity nt y— control alternatives were evaluated primarily on the basis of 1,000

simulated Catskill Aqueduct diversion turbidity and the pre- rbidit dicted frequency and duration of alum treatment. In model e Tu simulations, alum treatment was assumed whenever the Catskill erag Aqueduct turbidity load (flow × turbidity) exceeded a threshold 100 y Av

of 5,000 mgd × ntu, roughly corresponding to available his- il torical data on the commencement of alum treatment. In reality, d Da the NYCDEP’s decision to add alum is based on a number of 10

factors not represented in the model; however, the threshold eighte used in the current study was determined to be adequate for -w comparing relative performance of alternatives. Flow Model simulations. Linked model simulations were conducted to evaluate the relative performance of turbidity control alternatives. 1 1001,000 10,000 Simulated operating rules. The water supply–water quality Esopus Creek Average Daily Flow—mgd linked model was programmed with scenario-specific operating

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Under the baseline operating rules, the model’s response to Results elevated turbidity levels (defined for modeling purposes as The linked model simulation results for the turbidity control Catskill diversion turbidity exceeding 8 ntu) was to reduce diver- alternatives analysis are summarized in Figures 6 and 7 with sions from Ashokan Reservoir to 275 mgd, roughly correspond- respect to the key performance metrics: percent of simulation days ing to the level required to maintain supply to outside communi- in which alum treatment occurs and percent of simulation days in ties that draw from the aqueduct. Although this did not which Catskill Aqueduct turbidity exceeds 8 ntu. In Figures 6 and completely represent current NYCDEP capabilities and practice 7, turbidity control alternatives are grouped by primary turbidity (e.g., installation of stop shutters), it did provide a reference point control mechanism. In addition to the stand-alone alternatives, from which to measure the relative performance of the turbidity various combinations of alternatives were evaluated and are shown control alternatives. Because Catskill water is diluted by Kensico in Figures 6 and 7 as combined mechanisms. Reservoir storage and diversions from the Delaware System, the Of the stand-alone alternatives, the alternatives with the great- regulatory limit of 5 ntu for diversions from Kensico Reservoir est predicted reduction in the number of days of alum addition was not exceeded by the 8-ntu threshold in diversions from the (Figure 6) included those that minimized diversions to the Catskill Catskill System. Rather, the 8-ntu level from Ashokan Reservoir Aqueduct during turbidity episodes (alternative 6a, ~ 70% reduc- represented conditions consistent with the potential for turbidity tion) and those that relied on reservoir releases to remove turbid levels of concern in Kensico Reservoir. water from the west basin upstream of the dividing weir (alterna- Turbidity control operations for the infrastructure alternatives tive 1, ~ 50% reduction; alternative 6b, ~ 30% reduction). Alter- (alternatives 1–5) comprised these baseline operating rules plus native 5, a new east basin intake at a downstream location, alternative-specific rules (e.g., rules for operating the crest gates provided some reduction in the number of alum days (roughly in response to hydrologic and turbidity conditions). Rules for 10% for a single-level intake and 15% for a multilevel intake). each alternative were developed on the basis of intermediate Other alternatives provided little or no benefit in the number of model simulations and discussions with NYCDEP Bureau of alum days. Similar results were observed with respect to diver- Water Supply staff in order to evaluate alternatives using the most sions exceeding 8 ntu (Figure 7). realistic and sensible operations (Table 3). Combining the three components of alternative 6 provided a Table 3 summarizes the major alternative-specific operating 96% reduction in the total number of days of alum addition, rules used for these simulations. These rules do not necessarily nearly eliminating the need for alum treatment for the 57-year represent actual operations of the various facilities but rather are simulation period (Figure 6) and substantially reducing the intended as reasonable approximations of operation of the vari- frequency of diversions that exceeded 8 ntu over the simulation ous turbidity control options. On implementation of an alterna- period (Figure 7). Slight additional reductions in days of alum tive or set of alternatives, NYCDEP will develop actual operating addition were observed for the alternatives that optimized rules based on such factors as institutional experience, infrastruc- releases or provided selective withdrawal capabilities along with ture limitations, and output from model simulations. the combined alternative 6 (Figure 6). In general, however,

TABLE 3 Summary of alternative-specific operating rules examined in model simulations

Alternative(s) Summary of Operating Rules

Alternative 1—New west basin release structure Make releases from the west basin whenever: (a) the forecast indicates a large inflow over the next 24–48 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. Alternative 2—Dividing weir crest gates Raise crest gates seasonally to provide an additional ~ 4 bil gal of storage in the west basin during the winter–spring refill season. Alternative 3—East basin diversion wall There are no modified operations associated with this alternative. Alternative 4—Multilevel withdrawal capability Optimize diversions to the Catskill Aqueduct by selecting the intake elevation(s) with the lowest available turbidity. Alternative 5—New east basin intake 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. Alternative 6a—Optimize Catskill Aqueduct diversions. 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. Alternative 6b—Optimize use of existing release Optimized operations of the existing release channel are the same as those for Alternative 1. channel. Alternative 6c—Maintain a void in the west basin. 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.

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FIGURE 6 Simulated frequency of days requiring alum treatment during model simulation period (1948–2004) for various alternatives and baseline

Optimized Alternative 1: 1.8Alternative 6b: 2.6 releases Baseline: 3.9

Alternative 5a: 3.5 Enhanced Alternative 3: 3.8 settling Alternative 6c: 3.8 Alternative 2: 4.0 m echanis M Selective Alternative 5b: 3.3 diversions Alternative 4: 3.8 Control bidity Tur Minimize Alternative 6a: 0.9 diversions

Alternative 1 + 6a + 6b + 6c: 0.1 Alternative 5b + 6a + 6b + 6c: 0.1 Alternative 2 + 6a + 6b + 6c: 0.2 Combined Alternative 3 + 6a + 6b + 6c: 0.2 mechanisms Alternative 4 + 6a + 6b + 6c: 0.2 Alternative 6a + 6b + 6c: 0.2

012345 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 these four categories, providing a useful framework for describ- substantial additional reduction in alum addition because the ing why some alternatives were successful and others were not. few turbidity episodes that could not be mitigated completely Optimized releases. Alternatives that provided turbidity control by alternative 6 were related to extremely large runoffs that are by releasing turbid water upstream of the Catskill Aqueduct particularly difficult to control. included alternative 1 and alternative 6b, optimization of the With respect to the number of simulation days in which existing release channel for turbidity control. These alternatives Catskill Aqueduct diversions exceeded a turbidity of 8 ntu, all of provided substantial improvement (30–50% reduction in simu- the alternatives except the east basin diversion wall were pre- lated days of alum addition) by removing turbidity from the dicted to provide slight incremental improvement over alternative system and thereby reducing turbidity transfer from the west 6 (Figure 7). Similar to the alum treatment results, the most sig- basin to the east basin. nificant additional improvement over alternative 6 was accom- However, during episodes of highest turbidity, releasing water plished by adding capacity to release water from the west basin. at the maximum simulated release capacity was still insufficient to completely eliminate turbid spill to the east basin. Although the Discussion predicted duration of alum treatment was substantially reduced, As described previously, the turbidity control alternatives the need for alum was not eliminated altogether. (The total of 367 evaluated in this research represented four major (nonexclusive) alum days does not signify that there were 367 individual episodes categories of turbidity control mechanisms. The following sec- requiring alum addition; roughly 150 of these alum days arose tions discuss the performance of the alternatives with respect to from the single largest episode over the 57-year simulation period.)

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FIGURE 7 Simulated frequency of days with Catskill Aqueduct diversion turbidity greater than 8 ntu during model simulation period (194 8–2004) for various alternatives and baseline

Optimized Alternative 1: 3.7 Alternative 6b: 4.9 Baseline: releases 6.8

Alternative 3: 6.8 Enhanced Alternative 6c: 6.8 settling Alternative 2: 6.9 Alternative 5a: 7. 1 m echanis M Selective Alternative 5b: 6.0 diversions Alternative 4: 6.2 y Control idit Tu rb Alternative 6a: 5.4 Minimized diversions

Alternative 1 + 6a + 6b + 6c: 1.8 Alternative 5b + 6a + 6b + 6c: 2.4 Alternative 2 + 6a + 6b + 6c: 2.5 Combined Alternative 3 + 6a + 6b + 6c: 2.8 mechanisms Alternative 4 + 6a + 6b + 6c: 3.0 Alternative 6a + 6b + 6c: 3.7

0 12345678910

Model Simulation Days—% of Simulation Period

Enhanced settling. As described previously, the optimized release peak turbidity levels, diversion turbidities generally remained well alternatives reduced the amount of turbid spill to the east basin by above 8 ntu during significant turbidity episodes. removing turbid water from the west basin. Although they did not Although these alternatives 2, 3, and 6c demonstrated some ben- provide the benefit of removing turbidity completely from the efit at the leading edge of some turbidity episodes, the benefit gener- system, alternative 2 and alternative 6c both sought to increase the ally was short-lived and provided little overall improvement in the residence time of turbidity-causing particles in the west basin and predicted frequency of alum addition. In general, these alternatives reduce the amount of turbid spill to the east basin, thereby reduc- underperformed because during the major episodes for which alum ing the turbidity near the Catskill Aqueduct intake. These alterna- addition was predicted, the west basin fills up rapidly, quickly filling tives were not predicted to provide substantial performance, with any void created in the west basin and overtopping the weir. Fur- the dividing weir crest gates providing no improvement over the thermore, the small delay in spilling to the east basin afforded by baseline and the west basin drawdown providing little improve- these alternatives simply did not provide adequate time for turbidity- ment with respect to days of alum addition. causing particles to settle, likely because of the small size and poor Similarly, alternative 3 would shift flow patterns such that spill settling characteristics of the majority of these particles. from the west basin was routed further into the east basin, pre- Selective diversions. Alternatives 4 and 5 attempt to selectively venting short-circuiting to the Catskill Aqueduct intake and divert the water with the lowest turbidity into the Catskill Aque- increasing residence time for settling. As noted previously, the east duct during a turbidity episode. basin diversion wall does not generally provide substantial ben- Alternative 4 provided some improvement over the baseline with efit in terms of alum addition. Despite substantial reductions in respect to the number of days when Catskill Aqueduct diversion

2013 © American Water Works Association Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E226 Journal - American Water Works Association Peer-Reviewed 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 contrib- uted 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 In this closeup, water spills over the Ashokan Reservoir dividing weir from the take the system off line completely while continuing to provide west basin to the east basin. service to the outside communities that draw from the aqueduct. In contrast to the other alternatives, this option provided a direct (OST)—which uses an upgraded version of the water supply reduction of the flow (and therefore the turbidity load) entering simulation–W2 linked model. This alternative will improve the Kensico Reservoir. department’s ability to reduce Catskill diversions during turbid- Alternative 6a provided roughly a 75% reduction in the fre- ity episodes to the minimum level necessary to meet NYC and quency of alum addition. In particular, this alternative provided outside community water demands and guide the use of the substantial benefits during the major episodes in which turbidity existing release channel to help control turbidity without com- loads were sufficiently high that none of the in-reservoir alterna- promising water supply reliability. tives provided a benefit. For these episodes, shutting down (or Furthermore, this alternative represents a cost-effective drastically reducing the flow in) the Catskill Aqueduct for an approach to turbidity control, given that the results of the cur- extended period of time was the only way to successfully reduce rent study indicated that the much more costly infrastructure the turbidity load exported to Kensico Reservoir. alternatives were not likely to provide much additional turbidity Combined alternatives. The combined alternative 6 (release chan- control benefit. Accordingly, NYCDEP is planning a connection nel optimization, west basin drawdown, and minimized Catskill between the Catskill Aqueduct and shaft 4 of the Delaware Aqueduct diversions) provided a 96% reduction in the predicted Aqueduct to divert water from the Delaware System into the number of alum addition days over the 57-year simulation period, Catskill Aqueduct. This connection could be used to supply suf- far surpassing the performance of any of the stand-alone alterna- ficient Delaware water to the Catskill Aqueduct to maintain tives. It also reduced the number of predicted alum treatment days adequate submergence for outside community taps, while still over the 57-year simulation period to roughly 30 days, compared minimizing diversions from Ashokan Reservoir. Under this with more than 800 days for the baseline. The majority of alum option, diversions from Ashokan would be reduced to the min- treatment days for the combined alternative 6 occurred during imum necessary to satisfy any remaining system demand not met the largest storm event in the simulation period (October 1955, by diversions from the Delaware and Croton systems as well as with a 30-bil-gal inflow over two days). minor demands from the two outside community systems above Turbidity episodes of this magnitude, although rare, cause high the shaft 4 connection. turbidity levels throughout the system and are difficult to mitigate OST. Implementation of the state-of-the-art OST (currently under any of the alternatives. For episodes of lesser magnitude, the under development by NYCDEP) will enhance the depart- combined effects of the various turbidity control mechanisms at ment’s ability to implement and refine the modified operating play (primarily optimized releases and reduced diversions) are rules developed under the CTCP (and help to minimize the additive, indicating that an optimal approach would seek to reduce need for alum application in general). Like the analytical turbidity levels in Ashokan Reservoir while simultaneously reduc- framework used to evaluate alternatives in the current study, ing diversions when levels are high. Accordingly, little additional the OST at its core has both the water supply simulation benefit with respect to alum addition was provided by operating model to address water quantity and the W2 water quality alternatives 1–5 in combination with the combined alternative 6. models to incorporate near-real-time turbidity data from the Selected approach for implementation. On the basis of these Catskill System. The OST does not replace the judgment and results, NYCDEP has proposed as a long-term turbidity control experience of NYCDEP operators and managers but provides measure the implementation of optimization of release-channel them with a powerful analytical tool to help them better operating rules and near-term planned infrastructure, i.e., com- understand the risks and benefits of alternative operating bined alternative 6; this measure would be supported by a cut- policies and ultimately to provide support for both short-term ting-edge computer system—the Operations Support Tool system management and long-term planning.

2013 © American Water Works Association Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E227 Journal - American Water Works Association Peer-Reviewed

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

2013 © American Water Works Association Weiss et al | http://dx.doi.org/10.5942/jawwa.2013.105.0043 E228 Journal - American Water Works Association Peer-Reviewed

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