Brine Dilution Analysis for DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA

Transcription

1 Brine Dilution Analysis for DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA by Scott A. Jenkins, Ph.D. and Joseph Wasyl Submitted by: Scott A. Jenkins Consulting Kalapana St Poway CA Submitted to: K. Scott Jackson, Program Manager DeepWater Desal, LLC 7532 Sandholdt Rd. Suite 6 Moss Landing, CA Draft 7 March; Revised 25 March 2014; 2 nd Revision 11 April 2014; 3 nd Revision 23 June 2014

2 2 TABLE OF CONTENTS ABSTRACT ) Introduction ) Discharge and Dilution Compliance Issues ) Technical Approach ) Model Initialization ) Diffuser Dilution Analysis ) Conclusions ) References APPENDIX-A: Diffuser Turbulence Mortality Estimates..104

3 ABSTRACT: We evaluate the dilution and dispersion of combined brine and thermal effluent discharged from DeepWater Desal s proposed Monterey Bay Regional Water Project (MBRWP) at Moss Landing, CA. This problem was studied by numerical simulation of worst case and long term scenarios using a combination of hydrodynamic models: 1) a near field mixing zone model, Visual Plumes certified by the U.S. Environmental Protection Agency and the California State Water Resources Control Board for use in ocean outfall design; 2) a fully 3-dimensional far field dispersion model SEDXPORT that is a processed-based, stratified flow model with the complete set of littoral transport physics including tidal transport, wind &wave induced transport and mixing; and 3) two commercially computational fluid dynamics models (CFD) known as COSMOS/ FLowWorks for hydraulic design and Star-CD, Version 3.1, with QUICK space discretization for first order up-winding of the diffuser turbulence equations. We consider two potential offshore discharge sites located along the existing Moss Landing Power Plant (MLPP) oil pipeline route; one located in 35 m of local water depth (deep discharge site) at UTM coordinates N, E; while the other is located in 25 m of water depth (intermediate discharge site) at UTM coordinates N, E. At these two potential discharge sites, the study addresses hyper-salinity toxicity relative to existing and proposed discharge limits. The discharge/dilution strategy proposed in the MBRWP project description is based on high velocity diffusers. Our analysis is based on a proxy diffuser design utilizing a five-jet linear diffuser oriented orthogonal to the shoreline and consisting of five discharge risers emerging from a manifold and fitted with duckbill diffuser nozzles. Five such nozzles will discharge a maximum of 5.45 mgd each of brine, or an ultimate maximum total discharge of mgd of brine 3

4 at ppt end-of-pipe salinity. (These figures represent ultimate maximum brine source loading to the receiving waters. When operating at preferred efficiency, the SWRO will operate at 46% recovery, producing 26.1 mgd of brine with an average end-of-pipe salinity of 63.7 ppt). Potential hyper-salinity toxicity arising from the project discharge is evaluated relative to two regulatory discharge limits. Existing discharge limits applied to fully permitted desalination projects in Southern California have been based on the 0.3 TUa objective of Requirement III.C.4(b) of the present version of the California Ocean Plan, as it would apply to a Zone of Initial Dilution (ZID), typically a circle with 1,000 ft. (305 m) radius. However, a recently released study by the California Water Resources Control Board (SWRCB) Science Advisory Panel (The Brine Panel) has suggested amendments to the California Ocean Plan based on a numeric water quality objective that would limit brine discharges from ocean desalination to no more than 5% over ambient ocean salinity at the outer edge of a Regulatory Mixing Zone measuring 100 m (328 ft) in radius around the discharge, (referred to as The 5% Rule). In practice, The 5% Rule is significantly more stringent than Requirement III.C.4(b), and is used as the defining compliance criteria of this study. A question regarding implementation of the 5% Rule which remains uncertain is how ambient salinity is defined. In Monterey Bay, ambient salinity varies seasonally by as much as 8%. The technical approach to evaluating project compliance with discharge limits considers both worst case scenarios at the two potential discharge sites, as well as long term variability of the diffuser-induced dilution fields. Worst case scenarios consider three predominant hydrographic periods of Monterey Bay, namely: 1) the upwelling period, 2) the oceanic period dominated by relaxation states, and 3) the Davidson Current period. The long term analysis is based on 4

5 8,149 modeled generated by feeding the models a matrix of 7 controlling variables assembled from the oceanographic period record. All three hydrographic periods are embedded in this period of record. From the worst case simulations, we conclude that the intermediate discharge site has a slight advantage in dilution performance over the deep discharge site; but has a potential for occasional re-circulation of a small amount of brine at the proposed Tenera intake site (at UTM coordinates N, E). During periods of northward flowing currents (the predominant current direction, particularly during the upwelling period), the brine footprint on the seabed that corresponds to 5% above ambient salinity is 7% smaller at the intermediate site than at the deep water discharge site. This foot print is 2 % smaller at the intermediate site during occasional episodes of southward flowing mean currents (occurring during relaxation and Davidson current periods). This advantage is attributable to differences in wave induced mixing at the two sites, where the scrubbing action of oscillatory waves is more intense at the shallower intermediate site than at the deep water discharge site. These wave mixing effects are especially important in the high-energy wave environment of the Monterey Bay region. The intermediate site also has the advantage of a shorter pipeline run with associated lower capital, operating and maintenance cost. Total head requirements for the 36 inch pipeline and the 5-jet linear diffuser are 14.4 psi over ambient ocean pressure at the depth of the deep water discharge; and 11.1 psi over ambient ocean pressure at the depth of the intermediate discharge. However, during relatively rare and brief periods of southward flowing currents (typically occurring during the relaxation period or Davidson current period), there is less than a 1% chance for a potential re-circulation of 1% of the brine discharged from the intermediate site. The actual re-circulation is likely to be 5

6 6 much less, because the brine plume has dispersed as a thin bottom spreading layer of no more than 5 cm thickness at the proposed Tenera intake site, and the intake structure will probably stand several meters above the seabed. Regardless, the potential for brine re-circulation arising from the intermediate discharge site might tip a cost-benefit analysis of the two sites in favor of the deep discharge site, despite the higher operating pressure requirements of that site. At the intermediate MBRWP discharge site, long-term simulations find the median salinity at the outer limit of the Regulatory Mixing Zone is ppt, well within The 5% Rule requirements when using heated source water from the data center; and 99.9% of the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over long-term average salinity which is ppt). Maximum salinity at 100 m from the intermediate discharge site in any direction is ppt, exceeding the 5% Rule by only 0.7% when based long term average ambient salinity. (If the 5% Rule based on daily salinity, then this over-limit result exceeds the 5% Rule by only 0.3%). The probability of occurrence of this over-limit case is only 0.1%, or about 1 day in 3 years. The median salinity at the outer limit of the Regulatory Mixing Zone at the deep discharge site is ppt, also well within The 5% Rule requirements when using heated source water from the data center; and 99.8% of the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over long-term ambient mean). Maximum salinity at 100 m from the deep discharge site in any direction is ppt, exceeding the 5% Rule by only 1.3% when based long term average ambient salinity. (If the 5% Rule based on daily salinity, then this overlimit result exceeds the 5% Rule by only 0.8%). The probability of occurrence of this of this over-limit is only 0.2%, or about 1 day every 1.4 years. The magnitude of these occasional over-limit model results at either discharge site are within sampling error of standard oceanographic

7 temperature/conductivity measurements used for determination of practical salinity units (psu). The probability of recurrence of these over-limit values is equally negligible, on the order of one day every few years. Although NPDES permits are generally written for the specific conditions of each discharger, most allow for occasional over-limit monitoring results, as a provision for sampling errors. In addition, over-limit outcomes at the MBRWP discharge site are even less likely than indicated by the model results because the SWRO facility will operate most of the time under preferred efficiency conditions. The modeled results were obtained for the ultimate maximum brine discharge rate of mgd with 48% recovery; whereas the most efficient operating point for the MBRWP reverseosmosis facilities is at 46% recovery ratio with about 1 mgd less brine discharge than what produced the over-limit model results. Therefore, we conclude the diffuser dilution strategy at both the deep water and intermediate MBRWP discharge sites satisfies any of the presently permitted or potential future dilution standards for all foreseeable long-term ocean conditions at MBRWP Moss Landing. There is the possibility that the data center and the SWRO facilities at MBRWP will not come on-line at the same time, and consequently the source water for SWRO facilities would be unheated during some portion of the initial operating period. Hydrodynamic simulations of brine plume dilution arising from this the start-up scenario find slightly larger, more frequent over-limit discharge results at both the deep and intermediate discharge sites. However the preponderance of outcomes easily satisfies the 5% Rule, and the over-limit cases are still not statistically significant, particularly sense these cases represent start-up transients. 7

8 8 Brine Dilution Analysis for DeepWater Desal, LL, Monterey Bay Regional Water Project at Moss Landing, CA By Scott A. Jenkins, Ph.D. and Joseph Wasyl 1.0) Introduction: This is a hydrodynamic analysis of a diffuser-based discharge strategy prepared for Deepwater Desal LLC, (DWD), who is developing the Monterey Bay Regional Water Project (MBRWP) at Moss Landing, California, (CCRWP, 2013). At full build-out, the MBRWP will discharge an ultimate maximum mgd of heated brine through a sea-floor located diffuser at the end of a dedicated 36 inch diameter pipeline at depth of approximately 25 m to 35 m, approximately 1 to 1.5 miles offshore at the head of the Monterey Bay submarine canyon (Figure 1). The brine is the by-product of a seawater reverse osmosis desalination ("SWRO") plant at the MBRWP, operating at 48% maximum recovery and producing discharge concentrate having an average end-of-pipe salinity of ppt. (These figures represent ultimate maximum brine source loading to the receiving waters. When operating at preferred efficiency, the SWRO will operate at 46% recovery, producing 26.1 mgd of brine with an average end-of-pipe salinity of 63.7 ppt, see Table 1 ). The salinity of the brine may vary by as much as 8% from this average level, based on long-term variations of ambient ocean salinity in Monterey Bay shown in Figure-2. Most of this variation is due to salinity depression occurring during floods of the regional rivers, principally the Pajaro and Salinas Rivers (Figure 2). The temperature of the brine effluent will be heated to 20 0 C over ambient, ( 20 0 C) because the source water is used to cool a data center before entering the = Tb

9 Figure 1: Bathymetry of Monterey Bay in meters MSL, showing approximate location of the diffusers for the Deepwater Desal Monterey Bay Regional Water Project ("MBRWP") at Moss Landing. 9

10 Figure 2: Long-term salinity variation typical of the Central California, Monterey Bay. 10

11 95, 96, 105 SWRO plant. Temperature data from nearby Tenera and MBARI moorings show pronounced seasonal variations in ambient ocean temperatures. The maximum recorded daily mean temperature was o C during the summer of the 1997 El Niño and the minimum falling to 9.25 o C during the winter of the La Niña. The mean ocean temperature at 30 m depth is o C. On a percentage basis, the natural variability of the temperature of coastal waters in the vicinity of the MBRWP is significantly greater than that of salinity (on the order of T / T = 72% vs. S / S = 8%). These SWRO operating points and ambient ocean properties determine the nature of the brine plume, i.e., whether it sinks or floats once discharged into the receiving water. If the brine plume floats, its density is less than the receiving water, and it is referred to as a buoyant plume. If it sinks, the brine is denser than the receiving water, and it is referred to as a dense plume. The density of the receiving water and the density of the brine discharge b are both a function of temperature, T, and salinity, S, and the density contrast between these two,, will determine the net buoyancy of the plume and control its dynamics in the nearfield of the diffuser. Delta-T and Delta-S effects on density of the brine plume are quantified by the equation of state expressed in terms of the specific volume, 1/ b b or: b 11 d b 1 1 dtb ds T S b (1) The factor / T, which multiplies the differential temperature changes, is known as the coefficient of thermal expansion and is typically 2 x 10-4 per o C for seawater;

12 the factor / S multiplying the differential salinity changes, is the coefficient of saline contraction and is typically -8 x 10-4 per part per thousand (ppt) where 1.0 ppt = 1.0 g/l of total dissolved solids (TDS). For a standard seawater, the specific volume has a value = cm 3 /g and for Monterey Bay water the specific volume averages about cm 3 /g. If the percent change in specific volume by equation (1) is less than zero, then the brine is heavier than the receiving water, and lighter if the percent change is greater than zero. The heat load generated by the data center will likely be insufficient to achieve more than a Tb = 8 0 C to 10 0 C. This will result in a brine discharge temperature of nominallyt b = 16 0 C resulting from 35k mg/l feed concentration at 15 0 C with R.O. recovery ranging between 42%, 46%, and 48% for year 0. Table 1 indicates that the brine salinity will range between 59.3 ppt and 66.2 ppt for these R.O. recoveries, while the density of the brine will range between g/cm 3 and g/cm 3 when it exits the diffuser nozzles, and will be about 1.9 % to 2.4% denser than the receiving water, which typically has a density of g/cm 3. Consequently the brine plume will be a dense plume that will sink in the receiving water, based on the projected operating points. 12 Table 1: Brine Salinity and Density for Projected Operating Points SWRO Recovery, % Brine Salinity, ppt* Brine Density, g/kg, 16C Yr 0 Pressure, psi 42% , % , % , * practical salinity units in part per thousand, Includes isobaric energy recovery, ERI PX seawater 10C, 35 ppt = 1,027 g/kg seawater 60C, 63.7 ppt = roughly 1,027 g/kg

13 13 2.0) Discharge and Dilution Compliance Issues Hyper-salinity toxicity: This is the primary regulatory driver for how much initial dilution by turbulent mixing and entrainment the diffuser should be designed to achieve. The Project Description submitted by Deepwater Desal, LLC, to the CPUC suggests an initial dilution of 38 to 1.This is probably overly aggressive and requiring more powerful diffuser jets and higher operating pressures than necessary to meet either present or potential discharge limits on brine discharge. Recently, the California State Water Resources Control Board convened a 5- member science advisory panel (The Brine Panel) to produce a study that could provide a scientific basis for amending The California Ocean Plan in order to establish water quality objectives for brine discharges from ocean desalination plants. 1 The Brine Panel concluded that there is surprisingly limited scientific data on hyper-salinity tolerance of marine organisms on which a regulatory standard could be based at any particular site. Hyper salinity tolerance is extremely dependent on the mix of locally relevant species and on the seasonal variability of ambient ocean salinity. The hyper salinity tolerance data that does exist primarily focuses on short term lethal exposures (acute toxicity) using a laboratory method referred to as Whole Effluent Toxicity Testing (WET); while data on longer term sub-lethal exposure (chronic toxicity) is virtually non-existent (Sub-lethal exposures cause injuries that typically limit growth or reproduction). Given this paucity of data, The Brine Panel concluded that until additional data became available, it is better to err on the side of caution with a very conservative water quality objective that would limit brine discharges to 5% over ambient ocean salinity at the limit of a Regulatory Mixing Zone measuring 100 m (330 ft) in radius around the discharge point (referred to as The 5% Rule). 1 However the

14 scientific basis for this cautious recommendation was hyper-salinity tolerance data for sea grasses living in territorial waters of Spain, 9, 14, plants with associated marine communities that do not live in California waters. WET lab testing on locally relevant species in Southern California found chronic toxicity thresholds to be in the neighborhood of 20% over ambient ocean salinity. 22,51,52,55 Regardless, a question still remains regarding how ambient salinity should be defined in the implementation of the 5% Rule; whether ambient salinity is the long term average value for the receiving water, or an instantaneous (daily) measurement. We will consider the implications of both definitions in our dilution analysis. In the absence of clear scientific consensus on the appropriate hyper-salinity tolerance levels for the Monterey Bay marine communities, we proceed in the present analysis by addressing two regulatory discharge compliance questions in parallel: 1) Will the discharge strategy of MBRWP satisfy the 0.3 TUa objective of Requirement III.C.4(b) of the present version of the California Ocean Plan as it would apply to a Zone of Initial Dilution (ZID)? This was the dilution standard on which discharge permits were issued to the Carlsbad and Huntington Beach desalination projects by the Regional Water Quality Control Boards of the San Diego and Santa Ana Regions. And, 2) will the MBRWP discharge strategy satisfy suggested amendments to the California Ocean Plan based on the 5% rule? In the present study we seek the appropriate initial dilution parameters to be applied to the MBRWP diffuser design such that both of these regulatory standards are met without subjecting the marine life that is entrained by the diffuser jets (principally eggs larvae and juvenile fish) to excessive turbulent shear stress and strain rates (see APPENDIX-A). Generally, for moderate RO recovery ratios, (less than 50%), the 5% Rule is more stringent. However, for very high end-of-pipe discharge 14

15 salinities (greater than double concentrated) the present 0.3 TUa objective of Requirement III.C.4(b) is more challenging for compliance ) Technical Approach The technical approach evaluates worst case scenarios and long term variability of dilution fields associated with the MBRWP. This analysis involved the use of hydrodynamic transport models driven by historic wave, current, and wind and water mass data for known events. The production of brine by-product by the proposed desalination demonstration facility was overlaid on these events to determine the potential range of variability in dilution outcomes. Analysis of brine dilution issues uses a combination of hydrodynamic models: a near field mixing zone models, Visual Plumes certified by the U.S. Environmental Protection Agency and the California State Water Resources Control Board for use in ocean outfall design; and a fully 3-dimensional far field dispersion model SEDXPORT that is a processed-based stratified flow model with the complete set of littoral transport physics including tidal transport, and wind &wave induced transport and mixing, (cf. Jenkins and Wasyl 2007, 2008). SEDXPORT is used to predict the trajectory of the brine plume following initial dilution in the nearfield of the diffuser. SEDXPORT modeling system has been extensively peer reviewed by 8 independent experts and can be found in the public records of the State Water Resources Control Board, the California Coastal Commission and the Cities of Carlsbad, Huntington Beach and Moss Landing. SEDXPORT was also employed in the dilution studies for desalination projects by Los Angeles Department of Water and Power (Jenkins and Wasyl, 2005) and by the West Basin Municipal Water District (Jenkins and Wasyl, 2008b). SEDXPORT has also been incorporated into the Coastal Evolution Model available on-line at the Digital

16 Library of the University of California (Jenkins and Wasyl, 2005b; Additional details of the SEDXPORT model along with many of its important algorithms are found below. Analysis of diffuser turbulence mortality and turbidity issues issues will use a third type of hydrodynamic model belonging to a class of models known as computational fluid dynamics (CFD). We use three different CFD models: 1) The basic hydraulics of the offshore discharge pipeline, riser and diffuser internal flow simulations were performed using commercially available hydraulics design software known as COSMOS/ FLowWorks. 2) The subsequent turbulence kinetics 16 of the discharge plume was evaluated using a v 2 f mode computational fluid dynamics model, Star-CD, Version 3.1, with QUICK space discretization for the mean flow and first order up-winding of the turbulence equations. 70,71 It was used herein to compute the jet core velocities, shear stress, strain rates. 3) Diffuser induced seedbed scour and sediment re-suspension processes are modeled with the Vortex Lattice Scour Model. This process-based model was developed at Scripps Institution of Oceanography for the Office of Naval Research, and its algorithms have been published in the peer-reviewed literature (Jenkins and Inman, 2006; Jenkins et al. 2007). Because wave and current transport processes are predominant in the analysis of brine dilution and dispersion at the Moss Landing site, SEDXPORT is the primary analysis tool. SEDXPORT has been built in a modular computational architecture (Jenkins and Wasyl, 2007). The modules are divided into two major clusters: 1) those which prescribe hydrodynamic forcing functions; and, 2) those which prescribe the mass sources acted upon by the hydrodynamic forcing to produce dispersion and transport. The cluster of modules for hydrodynamic

17 forcing ultimately prescribes the velocities and diffusivities induced by wind, waves, and tidal flow for each depth increment at each node in the grid network. The finite element current model, TIDE_FEM, (Jenkins and Wasyl, 1990; Inman and Jenkins, 1996) will be employed to evaluate the tidal currents at Moss Landing. TIDE_FEM was built from some well-studied and proven computational methods and numerical architecture that have done well in predicting shallow water tidal propagation in Massachusetts Bay (Connor and Wang, 1974) and along the coast of Rhode Island, (Wang, 1975), and have been reviewed in basic text books (Weiyan, 1992) and symposia on the subject, e.g., Gallagher (1981). TIDE_FEM employs a variant of the vertically integrated equations for shallow water tidal propagation after Connor and Wang (1975). These are based upon the Boussinesq approximations with Chezy friction and Manning s roughness. The finite element discretization is based upon the commonly used Galerkin weighted residual method to specify integral functionals that are minimized in each finite element domain using a variational scheme, see Gallagher (1981). Time integration is based upon the simple trapezoidal rule (Gallagher, 1981). The computational architecture of TIDE_FEM is adapted from Wang (1975), whereby a transformation from a global coordinate system to a natural coordinate system based on the unit triangle is used to reduce the weighted residuals to a set of order-one ordinary differential equations with constant coefficients. These coefficients (influence coefficients) are posed in terms of a shape function derived from the natural coordinates of each nodal point in the computational grid. The resulting systems of equations are assembled and coded as banded matrices and subsequently solved by Cholesky s method, see Oden and Oliveira (1973) and Boas (1966). The hydrodynamic forcing used by TIDE_FEM 17

18 18 is based upon inputs of the tidal constituents derived from Fourier decomposition of tide gage records. Tidal constituents are input into the module TID_DAYS, which resides in the hydrodynamic forcing function cluster. TID_DAYS computes the distribution of sea surface elevation variations in Monterey Bay based on the tidal constituents derived from the tide gage station at Santa Barbara, NOAA # Forcing for TIDE_FEM is applied by the distribution in sea surface elevation across the deep water boundary of the computational domain. Wave driven currents will be calculated from wave measurements by the Coastal Data Information Program (CDIP) arrays and/or buoys. These measurements will be back refracted out to deep water to correct for propagation and shoaling effects between the monitoring sites and Moss Landing. The waves were then forward refracted onshore to give the variation in wave heights, wave lengths and directions throughout the nearshore around the Moss Landing coast. The numerical refraction-diffraction code used for both the back refraction from these wave monitoring sites out to deep water, and the forward refraction to the Moss Landing coast site is OCEANRDS. This code calculates the simultaneous refraction and diffraction patterns of the swell and wind wave components propagating over bathymetry replicated by the OCEANBAT code. OCEANBAT generates the associated depth fields for the computational grid networks of both TID_FEM and OCEANRDS using packed bathymetry data files derived from the National Ocean Survey (NOS) depth soundings compiled by GEODAS. The structured depth files written by OCEANBAT are then throughput to the module OCEANRDS, which performs a refraction-diffraction analysis from deep water wave statistics. OCEANRDS computes local wave heights, wave numbers, and directions for the swell component of a two-component, rectangular spectrum.

19 19 The wave data are throughput to a wave current algorithm in SEDXPORT which calculates the wave-driven longshore currents, v(r). These currents were linearly superimposed on the tidal current. The wave-driven longshore velocity, v(r), is determined from the longshore current theories of Longuet-Higgins (1970). Once the tidal and wave driven currents are resolved by TIDE_FEM and OCEANRDS, the dilution and dispersion of brine and backwash constituents is computed by the stratified transport algorithms in SEDXPORT. The SEDXPORT code is a time stepped finite element model which solves the advection-diffusion equations over a fully configurable 3-dimensional grid. The vertical dimension is treated as a two-layer ocean, with a surface mixed layer and a bottom layer separated by a pycnocline interface. The code accepts any arbitrary density and velocity contrast between the mixed layer and bottom layer that satisfies the Richardson number stability criteria and composite Froude number condition of hydraulic state. The SEDXPORT codes do not time split advection and diffusion calculations, and will compute additional advective field effects arising from spatial gradients in eddy diffusivity, (the so-called gradient eddy diffusivity velocities after Armi, 1979). Eddy mass diffusivities are calculated from momentum diffusivities by means of a series of Peclet number corrections based upon TSS and TDS mass and upon the mixing source. Peclet number corrections for the surface and bottom boundary layers are derived from the work of Stommel (1949) with modifications after Nielsen (1979), Jensen and Carlson (1976), and Jenkins and Wasyl (1990). Peclet number correction for the wind-induced mixed layer diffusivities are calculated from algorithms developed by Martin and Meiburg (1994), while Peclet number corrections to the interfacial shear at the pycnocline are derived from Lazara and Lasheras (1992a;1992b). The momentum

20 diffusivities to which these Peclet number corrections are applied are due to Thorade (1914), Schmidt (1917), Durst (1924), and Newman (1952) for the windinduced mixed layer turbulence and to Stommel (1949) and List, et al. (1990) for the current-induced turbulence. The proposed MBRWP discharges a combined effluent of heat from a data center and concentrated seawater from a sea water reverse osmosis facility. We must solve for the dispersion and dilution of both heat and salt. For the sea salts, SEDXPORT solves the eddy gradient form of the advection diffusion equation for the water column density field: 20 u t 2 0V ( t) da (2) where u is the vector velocity from a linear combination of the wave and tidal currents, is the mass diffusivity, is the vector gradient operator and is the water mass density in the nearshore dilution field; and 0 is the density of the water discharged by the discharge at a flow rate V (t). The density of the discharge is a function of the salt and heat flux from the data center, where the heat flux is given as: Q 2 u Q H Q H Q Q0V ( t) da t dq C p dt where H is the heat diffusivity and C p is the heat capacity of seawater at constant pressure. and Q 0 is the heat discharged by the data center at a flow rate (t) (3) V.

21 Solutions to the density field of the discharge plume from the outfall are calculated from equations (1-3) by SEDXPORT, from which computations of local discharge salinity, S ( x, y, z), can be made using equation (1). The salinity field of the discharge plume can be used to solve for the dilution factor D BWW ( x, y, z) of the combined brine/ thermal effluent from the data center according to: 21 D BWW ( x, y, z) S o S o (4) S( x, y, z) where S o is the ambient seawater salinity in ppt, and S ( x, y, z) is the local salinity in the discharge plume from the model solution in ppt. Model solutions will find a significant variation in the salinity with water depth, z. Therefore we introduced a depth averaged dilution factor, H 1 D( x, y, H) D( x, y, z) dz (5) H( x, y) 0 Where H H( x, y) h is the local water depth, h is the local water depth below mean sea level and is the tidal amplitude. Solutions for the density and concentration fields calculated by the SEDXPORT codes from equations (1)-(3), are through put to the dilution codes of MULTINODE to resolve dilution factors according to (4) and (5). These codes solve for the dilution factor (mixing ratio) for each cell in the finite element mesh of the nearshore computational domain based on a mass balance between imported exported and resident mass of that cell. The diffusivity,, in (1) controls the

22 strength of mixing and dilution of the seawater and storm water constituents in each cell and varies with position in the water column relative to the pycnocline interface. Vertical mixing includes two mixing mechanisms at depths above and below the pycnocline: 1) fossil turbulence from the bottom boundary layer, and 2) wind mixing in the surface mixed layer. The pycnocline depth is treated as a zone of hindered mixing and varies in response to the wind speed and duration. Below the pycnocline, only turbulence from the bottom wave/current boundary layer contributes to the local diffusivity. In the nearshore, breaking wave activity also contributes to mixing. The surf zone (zone of initial dilution) is treated as a line source of turbulent kinetic energy by the subroutine SURXPORT. This subroutine calculates seaward mixing from fossil surf zone turbulence, and seaward advection from rip currents embedded in the line source. Both the eddy diffusivity of the line source and the strength and position of the embedded rip currents are computed from the shoaling wave parameters evaluated at the breakpoint, as throughput of OCEANRDS. Altogether there are eight primary variables that enter into the SEDXPORT solutions for the simultaneous dispersion and dilution of the brine and backwash constituents from the MBRWP. These eight variables may be organized into forcing functions and boundary conditions. The forcing function variables affect the strength of ocean mixing, ventilation and available dilution volume in shallow water. These include: * Waves * Ocean Water Levels (tides and sea level anomalies) * Currents * Winds 22

23 The boundary condition variables control the source strength (concentrated sea salts) and background conditions. Some of these variables change daily (primary boundary conditions), while others vary slowly in time (stationary boundary conditions). The primary boundary conditions are: * Combine Flow Rates of data center and SWRO facility * Ocean Salinity * Discharge Temperature * Ocean Temperature The local bathymetry typically has a seasonal variation inshore of closure depth (about 15 m depth). In the following sub-sections, concurrent 24 year long records (the longest available period of record for all of the eight controlling variables) are reconstructed. These long-term records contain 8,149 consecutive days of daily mean values between 1983 and 2008, depending on the number of unfilled data gaps. Certain of these data sets do not span the period of record. In this case the known data were folded back over time, (preserving seasonality in the folding process), in order to obtain full-length 24 year long time series to be input to the model ) Model Initialization Long-term monitoring of ocean properties in the coastal waters surrounding Moss Landing and the greater Monterey Bay has been on going for about 30 years. These data were accessed from a variety of data archives, including a few NPDES monitoring reports filed with the Regional Water Quality Control Board; the California Coastal Water Quality Monitoring Inventory maintained by the California Department of Water Resources; the Coastal Data Information Program

24 (CDIP); the United States Geological Survey, (USGS), National Water Information System; the California Data Exchange Center; National Geophysical Data Center (NGDC); National Oceanic and Atmospheric Association (NOAA), National Data Buoy Center; the National Ocean Service Water Level Observation Network; Monterey Bay Aquarium Research Institute (MBARI); and the California Spatial Information Library. In attempting to reconstruct 24-year long, continuous, unbroken records of all the controlling variables for the dilution and dispersion modeling problem, certain gaps were found in some of the data bases obtained from these sources. These gaps were filled either by developing proxy records from surrogate sites having similar coastal morphology, or by folding the existing records over the gaps while preserving characteristic seasonality ) Bathymetry: Bathymetry provides a controlling influence on all of the coastal processes that affect dispersion and dilution. The bathymetry consists of two parts: 1) a stationary component in the offshore where depths are roughly invariant over time; and 2) a non-stationary component in the nearshore where depth variations do occur over time. The stationary bathymetry generally prevails at depths that exceed closure depth, which is the depth at which net on/offshore sediment transport vanishes. Closure depth is typically -12 m to -15 m MSL in the Monterey Littoral Cell, [Inman et al. 1993]. The stationary bathymetry was derived from the National Geophysical Data Center GEODAS digital database. Gridding is by latitude and longitude with a 3 x 3 arc second grid cell resolution in a 1201 X 1201 farfield grid yielding an xy-computational domain of km x 86.5 km. The farfield grid mesh is indicated by the black lines in Figure 3. A nearfield grid was constructed with a 1 x 1 arc second grid cell resolution in a 601 X 601 numerical array, yielding an xy-computational domain of km x 18.57

25 Figure 3: Farfield grid with 3 x 3 arc second grid cell resolution in a 1201 X 1201 farfield grid yielding an xy-computational domain of km x 86.5 km. 25

26 26 km,(figure 4). The farfield grid (Figure 3) computes the effects of regional scale refraction and circulation due to the shallow banks of the Bay and inner continental margin (Figure 5). The nearfield grid (Figures 4) is nested inside the farfield grid and is used to calculate the brine dilution and dispersion and potential recirculation to the intake. For the non-stationary bathymetry data inshore of closure depth (less than -15 m MSL) nearshore and beach surveys were conducted by the US Army Corps of Engineers in 1985, 1990, and These surveys were used to update the GEODAS database for contemporary nearshore and changes. 4.2) Intake and Discharge Structures: The MBRWP will utilize a refurbished, existing oil pipeline for the Moss Landing Power Plant (MLPP) (Figure 6). A new intake site is proposed by Tenera 105 at UTM coordinates: N, E. The intake pipeline will convey source water from this location to an existing onshore wet well located at the MLPP. From the wet well the source water will be pumped to an approximately 110-acre site (referred to as the Tank Farm Parcel ) located along Dolan Road, east of the MLPP, where the SWRO plant and data centers will be constructed. The discharge pipeline for brine concentrate from the SWRO plant and raw seawater from other uses in the Project, will be routed via a new pipeline from the site to a submerged discharge below the euphotic zone using the same approximate route as the existing intake pipeline (Figures 4 and 6). At full build-out, the MBRWP will discharge mgd of heated brine through a sea-floor located diffuser at the end of new pipeline, which is sized to be 36 inch diameter and extend to either one of two potential discharge sites. The two discharge sites are located along the existing MLPP oil pipeline

27 Figure 4: Nearfield computational domain. Nearfield grid constructed with a 1 x 1 arc second grid cell resolution in a 601 X 601 numerical array yielding an xycomputational domain of km x km.utm coordinates in meters are shown for the deep and intermediate discharge sites along the existing MLPP oil pipeline shown in red. 27

28 Figure 5: Farfield refraction/diffraction computation for Monterey Bay province for the storm of, 5 December Deep water significant wave height = 5.78 m; significant period = 18.2 sec; predominant direction = 265 deg. (CDIP, 2008). 28

29 29 route, one located in 35 m of local water depth (deep discharge site) at UTM coordinates N, E; while the other is located in 25 m of water depth (intermediate discharge site) at UTM coordinates N, E, see Figure 4. Pipeline run to the deep water discharge site is approximately 2,416 m (7,927 ft.), as measured from the shoreline; while the pipeline run to the shallower intermediate site is 1,738 m (5,703 ft.). Estimated frictional losses in the pipeline to the deep water site are 11.9 psi; while pipeline head losses along the shorter run to the intermediate discharge site are 8.6 psi. The brine/thermal effluent will be discharged via diffusers to assure rapid and thorough mixing with ambient seawater. We pose a five-jet linear diffuser located orthogonal to the -25m and -35m MSL isobaths on the north bank of the Monterrey Submarine Canyon. The linear diffuser consists of five discharge riser/diffuser structures at 10 m spacing that extend above the seabed from a manifold pipe. To minimize the likelihood of snagging drifting kelp and bottom debris, and to reduce seabed scour around the manifold and discharge riser/diffuser structures, it would be advisable to bury the manifold under the seabed. For the same reasons, this design employs the fewest numbers of discharge riser/diffuser structures that are needed for adequate dilution performance without incurring excessive turbulence mortality (see APPENDIX-A). A physical example of such a linear diffuser is shown in Figure 7. The discharge manifold consists of a 54 inch diameter pipe buried below the seafloor (Figure 8) that delivers a maximum of 5.45 mgd the brine to each of 5 diffuser risers. Each riser is fitted with a Tideflex duckbill nozzle angled upward at a 60 degree angle. The duckbill nozzles are selfadjusting to variable flow rate to maintain optimal jet nozzle diameter, and each

30 Figure 6: Location of the abandoned fuel oil pipeline. The proposed route of brine discharge pipeline is shown in red and the feedwater intake pipeline is shown in green, terminating at an intake site proposed by Tenera,

31 Figure 7: Physical example of the proposed linear diffuser concept posed for the DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA. Five discharge riser at 10 m spacing will extend above the seabed from a buried manifold pipe. Each riser pipe is fitted with a Tideflex duckbill nozzle. 31

32 Figure 8. Dimensional drawing and 3-d SolidWorks model of one of the five discharge riser/diffuser structure to be used in the DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA. 32

33 duckbill stands 7 ft above the seafloor atop it s riser (Figure 8) to isolate the discharge nozzles from burial effects and to protect against damage from bottom debris moving about in the wave and current surge. The Tideflex duckbill nozzles are sized converging restrictors inside the 16 inch diameter riser pipes in order to produce discharge velocities of u 0 = 5 m/s. The engineering drawings of the discharge riser/diffuser structures in Figure 8 were gridded into a series of lattice panels to form a nearfield grid. Figure 9 shows a cross-section simulation of a discharge flow of mgd design passing (into the page) through the buried 36-inch feeder pipe that distributes 5.45 mgd to the Port-1 riser/diffuser structures. Maximum discharge velocity is 5 m/sec (16.4 ft/sec). Total head requirements for the 36 inch pipeline and the 5-jet linear diffuser are 14.4 psi over ambient ocean pressure at the depth of the deep water discharge; and 11.1 psi over ambient ocean pressure at the depth of the intermediate discharge. Some head losses are apparent at the junction of the feeder pipe with the riser pipe. The simulation in Figure 9 was based on discharge into a perfectly quiet ocean with no waves or current motion in the receiving water. 33

34 Figure 9. Still water simulation of discharge flow through riser/diffuser structure proposed for the DeepWater Desal, LLC, Monterey Bay Regional Water Project at Moss Landing, CA. Discharge flow for Port-1 of the mgd design. 34

35 35 4.3) Wave Climate: Waves are the principle driving mechanism of mixing and current ventilation in the very near-shore region off Moss Landing. This wave dominated region consists primarily of the surfzone but extends seaward into the wave shoaling zone a few surf zone widths beyond the point of wave breaking. The waves also create local mixing due to the orbital motion around the intake and discharge structures. Waves are among the most difficult of the controlling variables to get long unbroken records. The availability of wave data in the Monterey Bay and Central California region limited the period of record for this long term model analysis to Waves have been routinely monitored at several local and regional locations by the Coastal Data Information Program, (CDIP, 2008). These data are supplemented by ocean observations with Datawell buoys by the National Data Buoy Center (NDBC). The nearest CDIP and NDBC directional wave monitoring sites are: a) CDIP, Harvest, California Station ID: 071 Location: N; W Water Depth: 549 m b) CDIP, Point Reyes, California Station ID: 029 Location: N; W Water Depth: 301 m c) CDIP, Coquille River Inner, Oregon Station ID: 037 Location: N; W Water Depth: 64 m

36 d) National Data Buoy Center, Monterey, CA Station Location: N; W Site elevation: sea level Water depth: 2115 m 36 Air temp height: 4 m above site elevation Anemometer height: 5 m above site elevation Barometer elevation: sea level Sea temp depth: 0.6 m below site elevation These data sets possessed gaps at various times due to system failure and a variety of start ups and shut downs due to program funding and maintenance. The undivided data sets were pieced together into a continuous record from and entered into a structured preliminary data file. The data in the preliminary file represent partially shoaled wave data specific to the local bathymetry around each monitoring site. To correct these data to the nearshore of Moss Landing, they are entered into a refraction/diffraction numerical code, back-refracted out into deep water to correct for local refraction and bay sheltering, and subsequently forward refracted into the immediate vicinity of MBRWP and Moss Landing. Hence, wave data off each monitoring site was used to hindcast the waves at MBRWP and Moss Landing coastal region. The backward and forward refractions of CDIP data to correct it to MBRWP and Moss Landing were done using the numerical refraction-diffraction computer code, OCEANRDS. The primitive equations for this code are lengthy, so a listing of the FORTRAN codes of OCEANRDS appear in Jenkins and Wasyl, (2005), Appendix B. These codes calculate the simultaneous refraction and diffraction patterns propagating over a Cartesian depth grid. A regional outer grid (Figure 3) was used in the back refraction calculations to correct for continental margin effects, while a high resolution inner grid (Figure 4) was used for the forward

37 refraction over the local bathymetry at the MBRWP and the nearfield of Moss Landing (Figure 10). OCEANRDS uses the parabolic equation method (PEM), Radder (1979), applied to the mild-slope equation, Berkhoff (1972). To account for very wideangle refraction and diffraction relative to the principle wave direction, OCEANRDS also incorporates the high order PEM Pade approximate corrections modified from those developed by Kirby (1986a-c). Unlike the recently developed REF/DIF model due to Dalrymple, et al. (1984), the Pade approximates in OCEANRDS are written in tesseral harmonics, per Jenkins and Inman (1985); in some instances improving resolution of diffraction patterns associated with steep, highly variable bathymetry such as found near the Monterey Submarine Canyon (Figure 10). These refinements allow calculation of the evolution and propagation of directional modes from a single incident wave direction; which is a distinct advantage over the more conventional directionally integrated ray methods which are prone to caustics (crossing wave rays) and other singularities in the solution domain where bathymetry varies rapidly over several wavelengths. An example of a reconstruction of the wave field throughout the Monterey continental margin region is shown in Figure 5 using the back refraction calculation of the CDIP data from the NDBC buoy # Wave heights are contoured in meters according to the color bar scale and represent 6 hour averages, not an instantaneous snapshot of the sea surface elevation. Note how the shelf bathymetry induced longshore variations in wave height throughout the Monterey Bay region. Figure 11 shows the significant wave heights inside Monterey Bay offshore of Moss Landing, with corresponding periods and directions, resulting from the series of back-refraction calculations for the complete CDIP and NBDC data sets at Δt = 6 hour intervals over the period of record. The data in 37

38 38 Figure 11 are values used as the deep water boundary conditions on the nearfield grid (Figure 4) for the forward refraction computations into the MBRWP and Moss Landing Harbor region (like those in Figure 10). The deep water wave angles in Figure 11c are plotted with respect to the direction (relative to true north) from which the waves are propagating at the deep water boundary of the nearfield grid (Figure 4). Inspection of Figure 11a reveals that a number of large swells of 6 m to 8 m heights have affected the Monterey Bay and Moss Landing region nearly every year during the period of record. Waves of this size create powerful wave induced currents and vigorous mixing when scrubbing over the seabed and intake and discharge structures. The largest local swells computed from our analysis of the period of record resulted from the storm of 5 December 2007 that produced 10 m high shoaling waves at various locations in and around Moss Landing and the greater Monterey Bay region. (Figures 5 and 10) Examining Figure 10 in closure detail, it is noted how the refraction effects over local bathymetry create areas to north and south of the discharge where heights increase to 7.4 m. In these areas, the bay and shelf bathymetry has focused the incident wave energy and these regions of intensified wave energy are referred to as bright spots. The increased wave heights in these bright spots increase the mixing and turbulence generated over the seabed boundary layer and by oscillatory wakes of the intake riser and discharge diffuser structures. This increases the mixing and dilution rates of the brine that might spread into the bright spots. Conversely, the dark areas in Figure 10 in the immediate nearfield of the intake and discharge are areas where wave heights have been diminished and are termed shadows. The shadow area over the intake and discharge sites at Moss Landing is caused by the refraction effects of the Monterey Submarine Canyon. The waves propagate onshore up the deep water center axis of the canyon

39 Figure 10: Nearfield refraction/diffraction pattern for the storm of, 5 December Deep water significant wave height = 5.78 m; significant period = 18.2 sec; predominant direction = 265 deg. (CDIP, 2008). 39

40 40

41 little loss of energy, and then refract onto the side walls of the canyon, creating the shadow area. Generally, wave induced currents (drift) predominate in the nearshore where wave shoaling effects are maximum. Wave induced currents increase with increasing wave height and remain significant over a nearshore domain extending four to five surf zone widths seaward of the shoreline. They flow longshore in the direction of longshore wave energy flux (down-drift). These longshore currents increase with increasing wave height and obliquity and flow away from bright spots in the local refraction pattern (Figure 10) and converge on shadows. This convergence results in a compensating seaward flowing current within the shadow known as a rip current. Even though the dilution of brine by mixing may be less in a shadow, dilution by rip current advection (ventilated dilution) will be increased. Moreover, these rip currents tend to disperse a brine plume discharged in a shadow zone away from the shore and into deeper water. As a net result, shadows like that found directly over the MBRWP project site can sometimes be areas of enhanced overall dilution. Refraction patterns of the type shown in Figure 10 were generated for each of the 8,149 deep water wave events in Figure 12 between 1983 and The resulting arrays of local wave heights, periods and directions were throughput to SEDXPORT for continuous dilution modeling. The time variation in the local shoaled wave height over the MBRWP project site is shown in Figure 12a. Average shoaled wave heights over the discharge were 2.1 m (1.07 m). The maximum shoaling wave height was 7.4 m in The minimum shoaling wave height in the period of record was 0.3 m over the discharge, used in the worst-case mixing scenarios for dilution modeling. 41

42 Figure 12: Period of record of historic forcing functions at Moss Landing used in time-stepped simulations of brine dilution at the MBRWP project site. 42

43 4.4) Ocean Water Levels & Tidal Oscillations: The local water column depth over the MBRWP discharge is nominally 110 ft (33.5 m) relative to mean lower low water (MLLW), and only 40 ft (12.2m) over the proposed MBRWP intake. Spring tidal ranges can reach as high as 8.9 feet or 8% of the water column at the discharge and 22% of the water column at the intake. Hence tides can vary the local water volume around the discharge that is available for dilution. The nearest ocean tide gage station is at Monterey (NOAA # ). Tidal constituents for the Laplacian tidal equations were obtained from a Fourier decomposition of ocean water levels measured by this tide gage, yielding 8,149 days of water level variation and tidal forcing between 1983 and From these reconstructions we obtain the daily high and low water levels plotted in Figure 12b. These water elevation time series were throughput to SEDXPORT for continuous dilution modeling. Maximum daily high water levels off MBRWP for this period were m MSL, while the average daily high water level was m MSL. Minimum daily low water levels off MBRWP were m MSL, while the average daily low water level was m MSL. The minimum daily tidal range for worst-case dilution modeling was neap tide during a Syzygian tidal epoch. = 0.32 m which corresponded to a 4.5) Current Forcing: While waves dominate the dilution and dispersion of storm water and effluent discharge in the inshore domain, the currents control dilution and dispersion in the offshore domain, particularly in the immediate vicinity of the MBRWP intake and MBRWP discharge. Currents in Monterey Bay are comprised of a complex regime that includes three well known hydrographic periods, namely: 1) the upwelling period, 2) the oceanic period dominated by relaxation states, and 3) the Davidson Current period. We provide current forcing to the SEDXPORT model for these three synoptic current regimes from a spectral 43

44 decomposition of current data measured in the vicinity of the MBRWP intake. The current measurements were performed during the Tenera entrainment study 105 near the Monterey Submarine Canyon at N and W, in local water depths of 30 m during the period October 13, 2012 to June 13, The seasonal gaps in these data were supplemented by additional ADCP current measurements on the MBARI M0 mooring from August 15, 2010 through September 3, 2011 (95, 96). The MBARI M0 mooring is located approximately 9 km (5.6 mi) to the northwest of the Tenera ADCP mooring. Figure 13 gives the synoptic circulation pattern for Skogsberg s upwelling period. This is a period when the North Pacific High rebuilds and establishes long fetches of northwesterly winds that induce coastal upwelling along the central California coast. This upwelling-favorable wind condition is subject to diurnal, geostrophic intensification due to local coupling with a thermal low that rebuilds daily over the land, (Foster, 1993; Petruncio, 1993). During these conditions, an upwelling front typically forms across the mouth of Monterey Bay with a cyclonic gyre inshore of the front inside the Bay (the Monterey Bay Gyre); and an anticyclonic eddy offshore of the upwelling front that is influenced by the California Current meander system, (Paduan and Rosenfeld, 1996; Ramp, et al., 1997; and Collins et al., 1996). Figure 14 gives the auto-spectra of the current speed at the Tenera mooring during the proxy upwelling current regime measured 15 February 2008 to 13 June The largest spectral peak in the current auto spectra (fundamental) is centered on a diurnal frequency lunar-solar diurnal tidal constituent at 5 f x 10 Hz that is very close to the K1 1 5 f x 10 Hz. The next largest K1 peak in the auto spectra occurs at the semi-diurnal frequency (second harmonic of the fundamental) of the M2 principal lunar semi-diurnal tidal constituent, 44

45 45 5 f x 10 Hz. This suggests that the currents at the MBRWP intake and M 2 discharge sites are tidally dominated, including both the barotropic tides (surface tides) and baroclinic internal tides. The tidal current ellipses of the M2 barotropic tides become flatted and elongated on a shore parallel axis near Moss Landing. This results in a predominantly north-south, back and forth current oscillation Figure 13: Canonical circulation pattern of the upwelling period in Monterey Bay with a cyclonic gyre inshore of the upwelling front and an anti-cyclonic gyre offshore of the front; (from Enriquez, 2004).

46 46 across the banks and side-slopes of the Monterey Submarine Canyon. The baroclinic internal tides propagate northward and onshore during a rising tide in the vicinity of the Monterey Submarine Canyon, and offshore on a falling tide, resulting in a net northward drift as evident in the Tenera current measurements. If the current forcing is purely tidal, then the diurnal spectral peak in Figure 14 should be smaller than the semidiurnal spectral peak, which it is not. Something else must be contributing to the large diurnal peak in Figure 14; and the most likely explanation is a wind-driven current component that has a diurnal fluctuation in response to daily heating of the land. The winds that are

47 subject to this diurnal intensification are north-westerlies in geostrophic balance with an onshore pressure gradient set up by the thermal low over the land. The diurnal fluctuation in wind stress during the upwelling period force an upwelling front and its complementary eddy system, as shown in Figure 13, to spin up during the day and spin down at night. The upwelling front originates from Pt. Ano Nuevo, spreading southward across the mouth of the Bay as the upwelling state evolves over time. Usually the upwelling front bifurcates near the southern end of the mouth of the bay, as another upwelling region forms off Pt. Sur (Rosenfeld et al., 1994; Traganza et al. 1981). A cyclonic eddy inshore of the upwelling front commonly occupies a large portion of Monterey Bay, where it is known as the Monterey Bay Gyre (Tseng, et al., 2003). It produces northward flow in the neighborhood of Moss Landing, resulting in the northward bias in the Tenera current measurements. The diurnal spin-up and spin-down of the Monterey Gyre adds additional energy and frequency spreading to the diurnal peak in Figure 14. (The anti-cyclonic eddy off shore of the upwelling front in Figure 8b is believed to be part of the California Current meander system and occasionally moves into the Bay during the relaxation state when upwelling subsides with a reduction in wind stress at the seasonal ending of the upwelling period, Enriquez, 2004). It should be remembered that the circulation pattern shown in Figure 13 is a canonical representation, and that the size, intensity and organization of the eddy system can be quite variable. Typically these patterns have life spans on the order of only 1-2 weeks, and that radar data often show less well-ordered systems embedded throughout the evolution of an upwelling seasonal period, (Lermusiaux, 2003, Rosefeld, et al., 1994). There are additional features in the current spectra in Figure 14 that cannot be explained by the principal barotropic and baroclinic tidal constituents. The third 47

48 largest spectral peak in Figure 14 is a diurnal third harmonic at a frequency 5 f x 3 10 Hz. This is believed to be a baroclinic shelf resonance formed by a resonant triad from the M2 and K1 barotropic tidal constituents interacting with the shelf bathymetry around the Monterey Submarine Canyon. In this case, the third harmonic completes a triad forced by the sum of the frequencies of the M2 and K1 components of the triad, that is: 48 5 f f M f = x 10 Hz 3 2 K1 The spectra in Figure 14 also contains two other sub harmonics with roughly 3 day and 5 day periods of oscillation. These periods correspond to time scales typical of relaxation states embedded in the upwelling period, associated with transient but often energetic circulation features such as mushroom heads and anomalous pools on the inshore side of the upwelling front, (Lermusiaux, 2003). The synoptic current pattern for the relaxation (oceanic) period is shown in Figure 15, and the auto spectra of current speed at the MBARI-M0 mooring is shown in Figure 16 for the proxy relaxation periods of August and September in 2010 and This spectra contains many of the same tidal spectral peaks as found during the antecedent upwelling period in Figure 14 because the tidallydriven barotropic current oscillations are independent of seasonal circulation patterns; although the baroclinic internal tides become weaker with reduced stratification in late summer and early fall. The spectra in Figure 16 show the expected relative size relationship between the diurnal and semi-diurnal spectral peaks, (with the diurnal K1 peak being smaller than the semi-diurnal M1 peak). However, there is a second semi-diurnal peak in Figure 16 that corresponds to the

49 Figure 15: Canonical circulation pattern of the relaxation (oceanic) period in Monterey Bay with an anti-cyclonic eddy from the antecedent upwelling migrating into the bay; (from Enriquez, 2004). 49

50 50 S2 principal solar semi-diurnal constituent at frequency 5 f x S 2 10 Hz. The S2 peak is more energetic than the M2 peak in Figure 16, even though it should be otherwise if the semi-diurnal oscillation were due purely to the barotropic tidal components. One hypothesis suggests the second harmonic of the K1 tides is causing a barotropic enhancement at S2 tidal frequencies through non-linear Reynolds stresses in the coastal boundary layer, since: 5 2 f K1 = x 10 Hz f S 2

51 Longuet- Higgins (1970) showed analytically that cross-shelf phase shifts in the Reynolds stresses of the frictional coastal boundary layer of a progressive tide produce both second harmonics, and rectification of the tidal oscillation. The rectification is relatively weak, but strengthens during spring tides and produces northward flowing currents of 4-5 day duration during spring tides. Other suspected tidal related spectral peaks in Figure 16 are the baroclinic resonant triads associated with the M2 and K1 barotropic tides interacting with the shelf bathymetry (shelf resonances). Here again we find a diurnal third harmonic that is a resonant triad forced by the sum of the frequencies of the M2 and K1 5 components, f3 f M 2 fk1 = x 10 Hz. We also have a second resonant triad at forced at the difference frequency of the M2 and K1 tides: f d 5 f f = x 10 Hz M 2 K1 In addition, there is a diurnal sub harmonic that is presumably another kind of nonlinear interaction of the primary tidal constituents with the bathymetry. This subharmonic is apparently a trapped shelf mode (edge wave, see Guza, 1978) of the progressive K1 barotropic tide obliquely incident on the Monterey Bay shelf, having a frequency that is ½ the K1 tidal oscillation, or: 1 f x 10-6 Hz 2 1 / 2 f K 51 Despite the numerous tidal spectral peaks in Figure 16, the highest energy levels during this proxy oceanic period are found in a broad peak at the red end of the spectra, where peak energy is occurs at 8.75 day periodicity, or f r = x 6 10 Hz. This frequency band is in the typical range of the periodicity of relaxation states, when the upwelling front breaks down allowing the offshore anti-cyclonic eddy (cf. Figure 13) and other features (meanders) of the California Current system to move into the bay. Since the tidal current oscillations near the MBRWP intake

52 52 and discharge are simple back and forth on a shore-parallel axis, the rather strong southward bias of the measured MBARI current direction during the relaxation period must be the result of these offshore current features which occur during the relaxation states and intrude into the Bay from the California Current meander system, (Tracy, 1990). Enriquez (2004) presents high-frequency radar-derived maps during relaxation states that show intrusion into the Bay of anti-cyclonic eddies from the California Current, thereby displacing the cyclonic Monterey Bay Gyre offshore and toward the north. This action forms a vigorous vortex dipole whose circulation causes strong boundary intensification along the northern shore of the Bay that subsequently discharges into offshore waters, see Figure 15. This type of relaxation state circulation would induce net southward transport in the neighborhood of the MBRWP intake and discharge, and for certain alignments of the vortex dipole in Figure 15, this circulation could cause very strong net southward transport. Indeed the strongest currents measured at any time during the 2-year current monitoring at the MBARI-M0 mooring occurred during the proxy relaxation segments, when maximum longshore current reached u max = 67.9 cm/ sec (1.3 knots) toward the south, and the mean longshore current was u =3.2 cm/sec (0.06 knots) also toward south in the neighborhood of Moss Landing. The vortex dipole circulation pattern in Figure 15 should be regarded only as a canonical pattern for the relaxation states that occur during the oceanic seasonal period. The oceanic periods are typically populated by many complex current features scavenged by the Bay from meanders of the California Current system, some of which are considerably less well-ordered than what is shown in Figure 13. These include meandering baroclinic jets with periodicity of 8-10 days, large filaments and squirts on time scales of 6 days to weeks (Lermusiaux, 2003). These circulation phenomena probably account for the low frequency energy of the

53 spectra in Figure 16 found between the 8.75 day spectral peak and the residual low frequency shoulder of that peak extending out to 5.4 weeks ( f x Hz). The synoptic current pattern for the Davidson Current period is shown in Figure 17 and the auto spectra of current speed at the Tenera mooring measured for the proxy Davidson Current period is shown in Figure 18. This spectra appears to be tidally dominated with largest spectral peak (fundamental) occurring at the M2 principal lunar semi-diurnal tidal constituent, 5 f f x 10 Hz. The other 1 M 2 principal barotropic and baroclinic tidal constituents are also represented among the prominent spectral peaks in Figure 18, including the K1 lunar-solar diurnal tidal constituent at 5 f x 10 Hz and a second semi-diurnal peak K1 corresponding to the S2 principal solar semi-diurnal constituent at frequency 5 f x S 2 10 Hz. The K1 diurnal peak is smaller than the M2 semidiurnal peak, as would be expected from a linear combination of the barotropic tidal constituents; but the S2 peak is again larger than what would be expected from such a simple linear tidal system. This was also found for the oceanic period in Figure 16, except the S2 peak remains smaller than the M2 peak during the proxy Davidson Current period in Figure 18. Regardless, we offer the same explanation for the higher than expected energy levels in the S2 peak; namely: the second harmonic of the K1 tides in this neashore environment is causing a barotropic enhancement at S2 tidal frequencies through non-linear Reynolds stresses from the coastal boundary layer. In addition, the broad shoulder of the low frequency spectral peak in Figure 18 contains some energy corresponding with the frequency of some of the longer period barotropic tidal constituents, principally the Mf lunar

54 Figure 17: Canonical circulation pattern of the Davidson current period in Monterey Bay, typically accompanied by onshore Ekman transport with coastal downwelling; (from Tseng, 2004). 54

55 55 fortnightly constituent with 13.6 day periodicity and frequency fmf x 7 10 Hz. As with the other seasonal periods, we also find evidence of baroclinic tides in Figure 18 due to a resonant triad from the M2 and K1 barotropic tidal constituents interacting with the shelf bathymetry around the MBRWP intake; 5 producing a diurnal third harmonic at f3 f M 2 fk1 = x 10 Hz. The measured Tenera current directions during the proxy Davidson Current period shows a northward bias as was found during the other upwelling period. This is in keeping with the tidal dominance of the spectra in Figure 18. It also suggests that the influence of the Davidson Current and other quasi-steady

56 circulation patterns are very weak at the nearshore location of the MBRWP intake and discharge. This is born out in the canonical Davidson current circulation pattern in the vicinity of Monterey Bay in Figure 17. Here the north-westward flowing Davidson Current is shown to have displaced the equator-ward California Current in an inshore zone within about 100 km of the coastline. The vortex dipole is still present in the immediate neighborhood of Monterey Bay, but the cyclonic eddy inside the Bay (Monterey Bay Gyre) is very weak and centered near the mouth of the Bay. Consequently, the net northerly drift in the near shore off Moss Landing is very weak during the canonical Davidson Current circulation pattern and subject to disruption by offshore current structures that intrude into the Bay. Such disruptions appear as the broad low-frequency spectral peak in Figure 18 that has two additional oscillations with 14 day and 20 day periodicity and do not appear to be tidally related. Ramp et al. (1997) and Swensen and Miller (1996) identify meander phenomena in the Davidson Current with this kind of periodicity and have related episodes of brief but vigorous southward flowing currents in the inner Bay to them. 4.6) Wind Mixing: Winds provide mixing in the surface layer above the thermocline that typically extends down to depths of m. Winds also provide wind drift which although weak can bridge the gap between the offshore tidally dominated regime and the inshore wave-dominated regime. Offshore winds were obtained from NOAA buoy # The limited record available from this source was extended by the collection of historical wind data compiled in US Surface Airways Data available from the National Climate Data Center document library (NCDC, 2004). The closest NCDC Surface Airways monitoring location relative to MBRWP is Monterey International Airport. Here, human observations of surface winds were collected and archived by NCDC beginning 1 January 1964 until 28 56

57 February 1997, after which wind observations were taken by means of the Automated Surface Observing System (ASOS). Combining these two data bases, a continuous surface wind record was assembled for the period as shown in Figure 12d. These daily mean wind data were throughput to SEDXPORT for continuous dilution modeling. The minimum daily mean wind speed is 0.17 m/s and was used for the worst case dilution model analysis. The long term record in Figure 12d shows a well defined inter annual (seasonal) modulation of daily mean winds, with a three to seven year intensification associated with El Niño. Maximum mean daily wind is 17.7 m/s and the average is 5.7 m/s. 4.7) Ocean Salinity: Ocean salinity variation exerts a modulating effect on the concentration of sea salts discharged from the desalination plant. The proposed desalination facility (MBRWP) will discharge a maximum of mgd of brine by-product at a maximum end-of pipe salinity of ppt at 48% RO recovery. Figure 19a shows the variation in daily mean salinity in the coastal waters off MBRWP derived from NPDES monitoring data of the Moss Landing Power Plant outfall. Gaps in these daily records were filled by salinity monitoring data from the Tenera and MBARI Monterey Submarine Canyon moorings. Inspection of Figure 19a indicates that the ocean salinity varies naturally by 8.2% between summer maximums and winter minimums, with a long term average value of parts per thousand (ppt). Maximum salinity was ppt during the 1998 summer El Nino when southerly winds transported high salinity water from southern waters where higher evaporation rates occur. Minimum salinity was about ppt during the 1993 winter floods. 4.8) Ocean Temperature: The ocean temperature affects the buoyancy of the discharge through the coefficient of thermal expansion in equation (1) and the temperature contrast with the brine /thermal effluent. This buoyancy effect is 57

58 58

59 59 calculated by the specific volume change of the discharge relative to the ambient ocean water. The buoyancy of the plume exerts a strong effect on the mixing and rate of assimilation of the sea salts and backwash constituents by the receiving waters. We use the average of temperature records from NPDES monitoring data of the Moss Landing Power Plant outfall with gaps filled by temperature monitoring data from the Tenera and MBARI moorings. The resulting 8,149 point record of daily mean ocean water temperatures is plotted in Panel-b of Figure 19. These temperature data were throughput to SEDXPORT for continuous dilution modeling. A pronounced seasonal variation in these temperatures is quite evident with the maximum recorded daily mean temperature reaching o C during the summer of the 1997 El Niño and the minimum falling to 9.25 o C during the winter of the La Niña. The mean temperature was found to be o C. On a percentage basis, the natural variability of the temperature of coastal waters in the vicinity of the MBRWP is significantly greater than that of salinity (on the order of DT = 72% vs DS = 8%. ( Tb 4.8) Combined Data Center and SWRO Discharge Temperature The temperature of the brine effluent will be heated to 20 0 over ambient, = 20 0 C) because the source water is used to cool a data center before entering the SWRO plant. Differences between the ocean and discharge temperatures results in buoyancy effects that alter the dispersion of brine, per equations 1-3. Figure 19c gives the estimated maximum daily discharge temperatures based on this operating scenario folded seasonally to complete a proxy for the period of record. The estimated maximum discharge temperature is 44.5 o C; and the average discharge temperature is estimated to be 37.6 o C. These temperature data were throughput to SEDXPORT for continuous dilution modeling. Moss

60 Landing discharge temperatures are generally 20 o C warmer than ocean temperatures, further augmenting the positive buoyancy of these fresh water discharges. This positive buoyancy is diminished by the addition of brine. However, the equation of state (equation 1) teaches that it takes 4 o C of discharge temperature increase to offset 1 ppt of salinity increase in order for a hot discharge to become positively buoyant ) Diffuser Dilution Analysis: We use several approaches here. First we do a worst-case analysis of two diffuser sites using the proxy 5-jet linear diffuser concept for mgd of combined brine and thermal effluent. The two discharge sites are located along the existing MLPP oil pipeline, one located in 35 m of local water depth (deep discharge site) at UTM coordinates N, E; while the other is located in 25 m of water depth (intermediate discharge site) at UTM coordinates N, E, see Figure 4. The worst case analysis considers at least one overlapping example of the three of the synoptic current regimes at both sites. We follow worst-case analyses with a long term dilution analysis at these two candidate sites for all current regimes embedded in the 24.5 year simulation period of record. It was found that the nearfield jet dynamics and the farfield current regime dominate the dilution outcomes, and that the worst case outcomes have probability of occurrence of significantly less than 1%. 5.1) Defining Worst-Case Brine Dilution Scenarios: Overlapping 24.5 year long records of the primary forcing functions and boundary condition variables are plotted in Figures 2,11,12, and 19 for the Moss Landing/Monterey Bay region. These records contain 8,149 consecutive days between 1983 and We adopt a commonly used approach in environmental sciences of assessing

61 potential impact in terms of a worst case scenario. We pose this worst case by searching these long-term records for historical events that match a criteria for worst-case. The criteria for worst case are based on the simultaneous occurrence of the high salinity and temperature in the receiving water during periods of low mixing and advection in the local ocean environment. The low mixing/ advection conditions arise during each of the three synoptic current regimes, upwelling period, relaxation period, and Davidson Current period, due to the combinations of oceanographic and meteorological conditions detailed in Section 4.5. We overlay on these three worst case environmental scenario the operating parameters for the SWRO system in Table-1. Table 2 summarizes the worst case criteria applied to each controlling variable in the computer search of the historic record. The computer search of 24 year-long digital records of winds, waves and currents, , generated the three worst case combinations of model variables listed in Tables 3-5 that occur during the upwelling periods, the relaxation periods and the Davidson current periods of the historic record. Ocean conditions represented by the Tables 3-5 did not persist in the long term records for more than a week. However, in the worst case model simulations found in the first half of Section 5.3, these conditions were perpetuated for 30 days to verify the stability of the computed results and to insure that all possible cumulative effects had reached steady state. Historically, the recurrence of worst-case environmental extremes is about 1 week every 3 to 7 years, commensurate with the dominant ENSO frequencies. By perpetuating Table 3-5 worst-case conditions in the model for 30 continuous days the recurrence interval is actually rendered more unlikely, with a recurrence probability of about 1 month every 13 to 31 years. 61

62 62 Table 2: Search Criteria for Worst Case Brine Dilution Scenario Variable Search Criteria Impact Significance Discharge Salinity Maximize Higher discharge salinity results in less initial dilution in the pipe, and reduces the dilution efficiency of the diffuser Ocean Salinity Maximize Higher salinity leads to higher initial concentrations of sea salts and backwash constituents from desalination Ocean Temperature Ocean Water Levels Maximize Higher ocean temperature leads to higher density contrast between nearfield brine/thermal effluent and farfield receiving water Minimize Lower water levels result in less dilution volume in the nearshore and consequently lower dilution rates Waves Currents for each of three current regimes Winds Combined Effluent Delta-T Minimize Smaller waves result in less mixing in bottom boundary layer of shoaling zone, weaker oscillatory vortices shed from discharge riser, weaker waveinduced currents, and consequently less near-bottom dilution Minimize Irrespective of current regime, weaker currents result in less advective dilution Minimize Weaker winds result in less surface mixing and less dilution in both the inshore and offshore Maximize Greater Delta-T reduces discharge plume buoyancy and increases plume sea-surface broaching tendencies.

63 63 Table 3: Input Parameters for Worst-Case Upwelling Period 1) MBRWP discharge flow rate = mgd 2) MBRWP discharge salinity = % recovery 3) MBRWP discharge temperature = C 4) Ocean salinity = ppt 5) Ocean temperature = C 6) Wave height = 0.5 m 7) Wave period = 9 sec 8) Wave direction = ) Wind = 0.47 knots 10) Tidal range = 0.53 m (Syzygian spring/neap cycle) 11) Relaxation Period spectral peaks, per Figure 14 Table 4: Input Parameters for Worst-Case Relaxation Period 1) MBRWP discharge flow rate = mgd 2) MBRWP discharge salinity = % recovery 3) MBRWP discharge temperature = C 4) Ocean salinity = ppt 5) Ocean temperature = C 6) Wave height = 0.3 m 7) Wave period = 8 sec 8) Wave direction = ) Wind = 0.17 knots 10) Tidal range = 0.32 m (Syzygian spring/neap cycle) 11) Relaxation Period spectral peaks, per Figure 16

64 64 Table 5: Input Parameters for Worst-Case Davidson Current Period 1) MBRWP discharge flow rate = mgd 2) MBRWP discharge salinity = % recovery 3) MBRWP discharge temperature = C 4) Ocean salinity = ppt 5) Ocean temperature = C 6) Wave height = 0.6 m 7) Wave period = 12 sec 8) Wave direction = ) Wind = 0.17 knots 10) Tidal range = 0.62 m (Syzygian spring/neap cycle) 11) Relaxation Period spectral peaks, per Figure 18

65 5.2) Nearfield Dilution: Nearfield dilution results are independent of the discharge site. Figure 20 gives a Visual Plumes one-dimensional simulation of nearfield dilution of brine for one of the five 5.45 mgd, diffuser jets posed in Section 4.2. Visual Plumes has no wave or tidal current transport physics, so the Figure 20 result represents dilution in a perfectly quiet ocean with no ambient motion; and considers no effects from any of the three synoptic current regimes. This is the receiving water condition required for the implementation of the daily maximum acute toxicity receiving water quality objective of 0.3 TUa (acute toxicity units), per Requirement III.C.4(b) of the California Ocean Plan as well as the implementation plan for the 5% Rule (Jenkins, et. al., 2012). In Figure 20 discharge salinity is shown in red and scaled against the right hand axis as a function of radial distance outward from one diffuser jet nozzle, typical of 5 diffuser jets; 1 ea. per discharge riser. Brine discharge is produced by 5 such diffuser nozzles discharging a combined total flow rate of 5 x 5.45 mgd ea. = mgd total brine discharge at ppt end-of-pipe brine salinity. Ambient ocean salinity is 33.5 ppt, the average from the worst-case Tables 3-5. Inspecting the Visual Plumes dilution results in Figure 20, reveals that a single Tideflex diffuser nozzle dilutes brine 20 to 1 (5% over ambient salinity = 35.2 ppt) at a distance of about 6 m from the point of discharge. This requires the diffuser array to entrain a total of about 518 mgd over that same distance, or about mgd per jet.. These diffuser nozzles will dilute the brine to only 1.6% over ambient ocean salinity at a distance of 10.7 m from the point of discharge. While a one-dimensional Visual Plumes simulation of a single diffuser jet appears from Figure 20 to easily satisfy 65

66 Figure 20: Visual Plumes one-dimensional simulation of still water dilution of brine for the 5-jet linear diffuser concept described in Section 4.3. Discharge salinity (red, right hand axis) as a function of radial distance outward from one typical of 5 diffuser jets; 1 ea. per discharge riser. Dilution factor (blue, left hand axis) as a function of radial distance outward from one typical diffuser jet. Each diffuser jet discharging 5.45 mgd from a Tideflex duckbill nozzle Total discharge: 5 nozzles x 5.45 mgd ea. = mgd total brine discharge at ppt end-ofpipe. 66

67 the discharge requirements of the present version of the California Ocean Plan, as well as the proposed 5 % rule amendment; the decisive question is whether a 3- dimensional simulation that accounts for the interaction of simultaneous discharge from all 5 diffuser jets will satisfy these requirements in the nearfield as well. Figures 21 a & b give salinity contour plots delineating the 3-dimesional dispersion of the mass of new water introduced into the nearfield of the receiving water by mgd of brine at ppt end-of-pipe salinity discharged from the combined 5-jet linear diffuser at the intermediate site in 25 m local water depth. The salinity contour color bar scale is on the left hand side of each figure, and the cross-shore and longshore distance scales are shown in meters measured around the perimeter of the nearfield grid. Comparing Figure 21 with Figure 20 it is clear that there is a cumulative or additive discharge effect among the 5 diffuser jets that retards dilution over distance relative to the one-dimensional Visual Plumes simulation in Figure 20. The combined discharge plumes from the 5 jets produces a bottom spreading layer at distances as far as 30 m -100 m from the discharge riser complex (cf. Figure 32, Section 5.4). The spreading layer appears as areas of light blue halos around the discharge risers in the salinity contours in Figure 21. However, none of the five discharge plumes in Figure 21a broach the sea surface at the intermediate (shallowest) site, where local water depth is -25 m MSL. The discharge plumes originate from the duckbill (Tideflex) nozzles that are at an elevation of 2.1 m above the seabed at the intermediate site. 67

68 a) Brine Plumes in Vertical Cross Section of Water Column 68 b) Brine Plumes in Oblique View Above Bottom Plane Figure 21: SEDXPORT 3-dimensional brine plume simulation of dilution/ dispersion for the 5-jet linear diffuser at the intermediate site in 25 m water depth. Total discharge: 5 nozzles x 5.45 mgd ea. = mgd of brine at ppt endof-pipe.

69 69 5.3) Worst-case Brine Dilution at the Deep Discharge Site: Figure 22 gives the salinity field on the sea floor resulting from the worst case dilution scenario at the MBRWP deep water discharge site (UTM coordinates N, E) during the upwelling period. The salinity field is averaged over a 24 hour period. We focus on mapping bottom salinity for worst case assessment because the brine/thermal effluent is denser than the receiving water, and the brine plume collapses onto the seafloor and forms a hyper-saline bottom spreading layer (Jenkins. et. al., 2012). In all such figures to follow, bottom salinity contours are shown in white and labeled in parts per thousand (ppt), and the continuous salinity distribution of the plume is based on the color bar scale appearing at the bottom of each figure with high end-of-pipe salinity appearing as reds, yellows and greens, and higher salinity approaching the ambient ocean salinity of ppt contoured in blues and purples. The inner core of the hyper-saline bottom spreading layer in Figure 22 is 40 ppt,(20% over ambient salinity) but covers an area of only 0.6 acres. The plume spreads along the bottom in two directions: it is pushed upslope towards the north by the northward currents associated with the Monterey Bay Gyre, (typical of the upwelling period); but an arm of the dense plume also spreads offshore following the bottom gradients along the north slope of the Monterey Submarine Canyon. The interior region of this plume where the bottom salinity decays away from the diffuser to 5% above the ambient salinity (33.52 ppt) is 6.9 acres. This is 11 % smaller than to the area of the Regulatory Mixing Zone under the proposed 5% Rule which would cover 7.8 acres over a circular area of 100m radius. Bottom dilution factors for the brine/thermal effluent are shown in Figure 23 for the worstcase upwelling solution at the deep water discharge site. In the figures that follow,

70 Figure 22: Salinity field on the sea floor resulting from the worst case dilution scenario at the deep discharge site during the upwelling period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 70

71 Figure 23: Dilution field on the sea floor resulting from the worst case dilution scenario at the deep discharge site during the upwelling period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 71

72 72 the dilution fields are contoured in base-10 log according to the color bar scale at the bottom of each plot, with a scale range that spans from 10 0 to We are particularly concerned about the dilution factor of the raw concentrate in the water column at the edge of the Regulatory Mixing Zone of the 5% Rule, where the dilution factor must be at least 20 to 1, (log 10 = 1.3), at a distance of 100 m in any direction from the diffuser. The average distance from the diffuser in Figure 23 where the log 10 = 1.3, (20 to 1 dilution), is only 94 m, in compliance of the 5% Rule for the worst-case dilution scenario at the deep water discharge site during the upwelling period. While the footprint of the dilution field in Figure 23 may appear massive, the model has carried the outer boundary of the dilution field to the 10 million to 1 contour where the salinity is 1 part in 10 billion over ambient salinity, an infinity below any quantifiable detection limit for any salinity anomaly. Figure 24 gives the salinity field on the sea floor resulting from the worst case dilution scenario at the MBRWP deep water discharge site during the relaxation period. The salinity field is averaged over a 24 hour period, where ambient ocean salinity is ppt. Here we find that the southward current drift in the red end of the relaxation period spectra (Figure 16), has nudged the brine plume down-slope into the Monterey Submarine Canyon. This dispersion is aided by the gravitational forces of the net negative buoyancy of the combined brine/thermal effluent, that flows as a gravity current into the head of the canyon. Due to this gravity assisted dispersion, the transport distances of the 5% over ambient salinity contour (35.2 ppt) exceed 100 m. The area of the plume in which salinity is 35.2 ppt or greater is 8.2 acres, or an average radius of 103 m, slightly greater than the perimeter of the Regulatory Mixing Zone, and about 19% greater than the worst-case at the deep water discharge site during the upwelling

73 Figure 24: Salinity field on the sea floor resulting from the worst case dilution scenario at the deep discharge site during the relaxation period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 73

74 period. This happens because the brine plume thins as it flows downslope into the Monterey Canyon during the relaxation period; as opposed to thickens as it is pushed upslope on the north rim of the canyon during the upwelling period. These large down-slope excursions of the brine plume during the relaxation period greatly increase the footprint of the bottom dilution field in Figure 25. Generally the axis of the dilution field tends to follow the 25 m and 35 m isobaths, the depth of the initial diffuser discharge, with a slight down-canyon bias to the dilution footprint. However these qualitative details occur a dilution levels of 1 to 10 million to 1, well below quantifiable detection limits. The salinity field on the sea floor for the worst case dilution scenario during the upwelling period is shown in Figure 26 at the MBRWP intermediate discharge site (UTM coordinates N, E, in 25 m of water depth gives). Again, the salinity field is averaged over a 24 hour period. Bottom salinity contours are shown in white and labeled in parts per thousand (ppt), and the continuous salinity distribution of the plume is based on the color bar scale appearing at the bottom of each figure with high end-of-pipe salinity appearing as reds, yellows and greens, and higher salinity approaching the ambient ocean salinity of ppt contoured in blues and purples. The inner core of the hyper-saline bottom spreading layer in Figure 26 is 42 ppt, but covers an area of only 0.4 acres. The principle spreading of the plume is northward under the influence of the Monterey Bay Gyre, (typical of the upwelling period). Cross-shelf spreading of the plume in Figure 26 is significantly less than what was found at the deep discharge site (Figure 22) because the bottom gradients of the north slope of the Monterey Submarine Canyon are substantially less at the shallower intermediate site. The plume is elongated but narrow due to the limited 74

75 Figure 25: Dilution field on the sea floor resulting from the worst case dilution scenario at the deep discharge site during the relaxation period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 75

76 Figure 26: Salinity field on the sea floor resulting from the worst case dilution scenario at the intermediate discharge site during the upwelling period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 76

77 cross-shelf spreading and the interior portion where the bottom salinity decays away from the diffuser to 5% above the ambient salinity (33.52 ppt) is only 6.4 acres, 17.5% less than to the area of the Regulatory Mixing Zone under the proposed 5% Rule. Bottom dilution factors for the brine/thermal effluent are shown in Figure 27 for the worst-case upwelling solution at the intermediate discharge site. Again, the dilution fields are contoured in base-10 log according to the color bar scale at the bottom of each plot, with a scale range that spans from 10 0 to The average distance from the diffuser in Figure 27 where the log 10 = 1.3, (20 to 1 dilution), is only 90.8 m, in compliance of the 5% Rule for the worstcase dilution scenario at the intermediate discharge site during the upwelling period. As discussed earlier, the large dynamic range of the dilution field gives the impression that the discharge plume is massive, when it really is not, as the sea salts in brine are not a toxin. None the less, the dilution field from the shallower intermediate site does touch the shoreline in Figure 27, north of the MBRWP shore facilities. Shoreline dilution factors are 1000 to 1, well below quantifiable detection limits for salinity anomalies. Figure 28 gives the salinity field on the sea floor resulting from the worst case dilution scenario at the MBRWP intermediate discharge site during the Davidson current period. The salinity field is averaged over a 24 hour period, where ambient ocean salinity is ppt. Here we find that brine plume has been spread southward across the head of the Monterey Submarine Canyon by the southward flowing currents associated with the red end (14 day 20 day) of the Davidson current period spectra (Figure 18). At the south end of the plume, some down-canyon spreading is evident, but mostly the brine/thermal effluent spreads along shelf between the -15 m and -130 m depth contours, because the bottom 77

78 Figure 27: Dilution field on the sea floor resulting from the worst case dilution scenario at the intermediate discharge site during the upwelling period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 78

79 Figure 28: Salinity field on the sea floor resulting from the worst case dilution scenario at the intermediate discharge site during the Davidson Current period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt. 79

80 gradients are smaller in the neighborhood of the shallower intermediate site. The area of the plume in which salinity is 35.2 ppt or greater is 8.0 acres, or an average radius of m, slightly greater than the perimeter of the Regulatory Mixing Zone, and about 25% greater than the worst-case at the intermediate discharge site during the upwelling period. Again the difference in how bottom slopes interplay with the heavy brine/thermal effluent between these two different current regimes seems to be the explanation. During the upwelling period in Figure 26, the northward currents are acting to push the heavy brine/thermal effluent upslope, whence gravity limits the extent of plume spreading; whereas the plume follows a relatively flat or slightly down-sloping bathymetry under the influence of the locally southward flowing currents during the Davidson current period. These larger southward excursions during the Davidson current period result in the relatively large footprint of the bottom dilution field in Figure 29. Generally the axis of the dilution field tends to follow the 20 m isobaths, the depth of the initial diffuser discharge, with a slight down-canyon bias to the dilution footprint. Where the dilution field touches the shoreline south of the MBRWP shore facilities, dilution factors are 1 million to 1, aka, a non-detect. Generally the worst-case simulations indicate that the intermediate site has a slight advantage in dilution performance. During northward flowing mean currents (upwelling period), the brine footprint on the seabed that corresponds to 5% above ambient salinity is 7% smaller at the intermediate site than at the deep water discharge site. This foot print is 2 % smaller at the intermediate site during southward flowing mean currents (relaxation and Davidson current periods). However, there is one feature among the worst case scenarios that presents a minor concern. That is the small amount of brine re-circulation to the proposed Tenera 80

81 Figure 29: Dilution field on the sea floor resulting from the worst case dilution scenario at the intermediate discharge site during the Davidson Current period. MBRWP diffuser discharge rate = mgd at ppt. Ambient ocean salinity = ppt 81

82 intake in Figure 29 during the worst case Davidson Current simulation for the intermediate discharge site. Here, brine dilution is only 100 to 1 at the Tenera intake, which suggests a potential re-circulation of 1%. However actual recirculation is likely to be much less, because the brine plume has dispersed as a thin bottom spreading layer of no more than 5 cm thickness at this distance from the diffuser, and the intake structure will probably stand several meters above the seabed. Moreover, this worst-case outcome has a probability of occurrence of less than 1%, (cf. Section 5.4). Regardless, the potential for brine re-circulation arising from the intermediate discharge site might tip a cost-benefit analysis of the two sites in favor of the deep discharge site, despite the higher operating pressure requirements of that site ) Long-Term Dilution Analysis: The long-term analysis of the salinity and minimum dilution of the combined brine/thermal effluent is summarized in the probability density and cumulative probability functions plotted in Figures 30 a & b. These probabilities represent the outcomes form 8,149 modeled solutions of combined discharge over the period of record The matrix of 7 controlling variables from Figures 2, 11, 12, and 19 were sequentially fed to the dilution model at common time steps over the period record length, producing 8,149 modeled outcomes. Hydraulic parameters for this computation are: 1) MBRWP discharge flow rate = mgd; 2) MBRWP discharge salinity = % recovery; and 3) MBRWP discharge temperature = C over instantaneous ambient ocean temperature. Model output for each individual time step (day) were compressed to records of maximum salinities along a concentric circular contour of 100 m radius centered on the either the deep water or intermediate discharge diffusers and defining The Regulatory Mixing Zone. Along

83 Figure 30: Probability density and cumulative probability of daily mean of maximum seafloor salinity at 100 m from the discharge (limit of the Regulatory Mixing Zone under the 5% Rule) for (a) deep water discharge site; and (b) intermediate discharge site. Based on 8,149 modeled outcomes using heated source water from the data center. 83

84 this circular contour, separate 24.5-yr histograms of the variation in maximum seabed salinity were computed and displayed graphically as probability density and cumulative probability in Figures 30 a & b to identify long term persistence of brine plume salinity level for inferring potential biological impact (including chronic toxicity). Figure 30 gives the probability density and cumulative probability of daily mean of maximum seafloor salinity at 100 m from the deep and intermediate discharge sites, (outer limit of the Regulatory Mixing Zone under the 5% Rule), as computed for brine plumes using heated source water from the data center with a delta- T of 20 0 C. At the deep discharge site (Figure 30a), the median salinity at the outer limit of the Regulatory Mixing Zone is ppt, well within The 5% Rule requirements; and 99.8% of the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over longer term average salinity of ppt, cf. Figure 2). Maximum salinity at 100 m from the deep discharge site in any direction is ppt, exceeding the 5% Rule by only 1.3% when based long term average ambient salinity. (If the 5% Rule based on instantaneous salinity, then this over-limit result exceeds the 5% Rule by only 0.8%). The probability of occurrence of this of this over-limit is only 0.2%, or about 1 day in 1.4 years. At the intermediate MBRWP discharge site, Figure 30 b shows that the median salinity at the outer limit of the Regulatory Mixing Zone is ppt, also well within The 5% Rule requirements; and 99.9% of the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over ambient salinity). Maximum salinity at 100 m from the intermediate discharge site in any direction is ppt, exceeding the 5% Rule by only 0.7% when based long term average ambient salinity. (If the 5% Rule based on instantaneous salinity, then this over-limit result exceeds the 5% 84

85 85 Rule by only 0.3%). The probability of occurrence of this over-limit case is only 0.1%, or about 1 day in 3 years. The magnitude of these occasional over-limit model results at either discharge site are within sampling error of standard oceanographic temperature/conductivity measurements used for determination of practical salinity units (psu). The probability of recurrence of these over-limit values is equally negligible, on the order of one day every few years. Although NPDES permits are generally written for the specific conditions of each discharger, most allow for occasional over-limit monitoring results, as a provision for sampling errors. In addition, over-limit outcomes at the MBRWP discharge site are even less likely than indicated by the model results because the SWRO facility will operate most of the time under preferred efficiency conditions. The modeled results were obtained for the ultimate maximum brine discharge rate of mgd with 48% recovery; whereas the most efficient operating point for the MBRWP reverse-osmosis facilities is at 46% recovery ratio with about 1 mgd less brine discharge than what produced the over-limit model results. Therefore, we conclude the diffuser dilution strategy at both the deep water and intermediate MBRWP discharge sites satisfies any of the presently permitted or potential future dilution standards for all foreseeable long-term ocean conditions at MBRWP Moss Landing. There is the possibility that the data center and the SWRO facilities at MBRWP will not come on-line at the same time, and consequently the source water for SWRO facilities would be unheated during some portion of the initial operating period. Unheated source water (delta- T = 0 0 C) produces denser brine plumes that dilute more slowly as a consequence of lower discharge trajectories and retarded entrainment rates. Figure 31 gives the probability density and cumulative probability of daily mean of maximum seafloor salinity at the outer

86 Figure 31: Probability density and cumulative probability of daily mean of maximum seafloor salinity at 100 m from the discharge (limit of the Regulatory Mixing Zone under the 5% Rule) for (a) deep water discharge site; and (b) intermediate discharge site. Based on 8,149 modeled outcomes using unheated source water from the data center. 86

87 edge of the Regulatory Mixing Zone (100 m from the deep and intermediate discharge sites) when the brine plume results from unheated source water. At the deep discharge site (Figure 31a), the median salinity at the outer limit of the Regulatory Mixing Zone is ppt, slightly higher than with the heated source water, but well within The 5% Rule requirements. The percentage of the 8,149 modeled solutions that are less than or equal to 35.1 ppt (5% over longer term average salinity of ppt, cf. Figure 2) is reduced to 97.9% with unheated source water; but still a healthy percentage. Maximum salinity at 100 m from the deep discharge site in any direction has increased to ppt, exceeding the 5% Rule by 2.3% when based long term average ambient salinity. (If the 5% Rule based on instantaneous salinity, then this over-limit result exceeds the 5% Rule by only 1.9%). The probability of occurrence of this of this over-limit is only 0.3%, or about 1 day in 0.9 years. If unheated source water is used at the intermediate MBRWP discharge site, Figure 31 b shows that the median salinity at the outer limit of the Regulatory Mixing Zone increases slightly to ppt, also well within The 5% Rule requirements; and 99.2% of the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over ambient salinity). Maximum salinity at 100 m from the intermediate discharge site in any direction is ppt, exceeding the 5% Rule by only 1.8% when based long term average ambient salinity. (If the 5% Rule based on instantaneous salinity, then this over-limit result exceeds the 5% Rule by only 1.3%). The probability of occurrence of this over-limit case is only 0.3%, or about 1 day in 0.9 years. As expected from the physics of dense plumes, the start-up scenario whereby the MBRWP reverse-osmosis facilities may be required to use unheated 87

88 source water for a brief period of time gives slightly larger, more frequent overlimit discharge results at both the deep and intermediate discharge sites. However the preponderance of outcomes easily satisfies the 5% Rule, and the over-limit cases are still not statistically significant, particularly sense these cases represent start-up transients. From the both worst case simulations in Section 6.5, and the long-term simulations above, we conclude that the intermediate discharge site has a slight advantage in dilution performance over the deep discharge site. This advantage is attributable to differences in wave induced mixing at the two sites. The last phase of dilution down to the limit of the 5% Rule occurs on the seafloor, where the brine plume disperses as a turbulent bottom spreading layer, appearing in the right-hand half of Figure 32). Much of the dilution in the bottom spreading layer is promoted by the scrubbing action of oscillatory wave motion. This wave scrubbing action is promoted by bottom friction and turbulent mixing in the oscillatory bottom boundary layer; and is stronger in shallow water due to wave shoaling, and vanishingly small in deep water, particularly at depths on the order of 35 m to 40 m. These wave mixing effects are especially important in the high-energy wave environment of the Monterey Bay region ) Conclusions: We evaluate the dilution and dispersion of combined brine and thermal effluent discharged from DeepWater Desal s proposed Monterey Bay Regional Water Project (MBRWP) at Moss Landing, CA. This problem was studied by numerical simulation of worst case and long term scenarios using a combination of hydrodynamic models: 1) a near field mixing zone model, Visual Plumes certified by the U.S. Environmental Protection Agency and the California State Water

89 89 Figure 32: Laboratory flow visualization by laser-induced fluorescence of the trajectory and subsequent bottom spreading layer of a dense plume from a diffuser. 106 The turbulent bottom spreading layer appears between horizontal reference points -25 and -70 on the right hand portion of the figure. Resources Control Board for use in ocean outfall design; 2) a fully 3-dimensional far field dispersion model SEDXPORT that is a processed-based, stratified flow model with the complete set of littoral transport physics including tidal transport, wind &wave induced transport and mixing; and 3) two commercially computational fluid dynamics models (CFD) known as COSMOS/ FLowWorks for hydraulic design and Star-CD, Version 3.1, with QUICK space discretization for first order up-winding of the diffuser turbulence equations. We consider two potential offshore discharge sites located along the existing Moss Landing Power Plant (MLPP) oil pipeline route; one located in 35 m of local water depth (deep discharge site) at UTM coordinates N, E; while the other is located in 25 m of water depth (intermediate discharge site) at UTM coordinates N, E. At these two potential discharge sites, the

90 study addresses hyper-salinity toxicity relative to existing and proposed discharge limits. The discharge/dilution strategy proposed in the MBRWP project description is based on high velocity diffusers. Our analysis is based on a proxy diffuser design utilizing a five-jet linear diffuser oriented orthogonal to the shoreline and consisting of five discharge risers emerging from a manifold and fitted with duckbill diffuser nozzles. Five such nozzles will discharge a maximum of 5.45 mgd each of brine, or an ultimate maximum total discharge of mgd of brine at ppt end-of-pipe salinity. (These figures represent ultimate maximum brine source loading to the receiving waters. When operating at preferred efficiency, the SWRO will operate at 46% recovery, producing 26.1 mgd of brine with an average end-of-pipe salinity of 63.7 ppt). Potential hyper-salinity toxicity arising from the project discharge is evaluated relative to two regulatory discharge limits. Existing discharge limits applied to fully permitted desalination projects in Southern California have been based on the 0.3 TUa objective of Requirement III.C.4(b) of the present version of the California Ocean Plan, as it would apply to a Zone of Initial Dilution (ZID), typically a circle with 1,000 ft. (305 m) radius. However, a recently released study by the California Water Resources Control Board (SWRCB) Science Advisory Panel (The Brine Panel) has suggested amendments to the California Ocean Plan based on a numeric water quality objective that would limit brine discharges from ocean desalination to no more than 5% over ambient ocean salinity at the outer edge of a Regulatory Mixing Zone measuring 100 m (328 ft) in radius around the discharge, (referred to as The 5% Rule). In practice, The 5% Rule is significantly more stringent than Requirement III.C.4(b), and is used as the defining compliance criteria of this study. A question regarding implementation of the 5% Rule which 90

91 remains uncertain is how ambient salinity is defined. In Monterey Bay, ambient salinity varies seasonally by as much as 8%. The technical approach to evaluating project compliance with discharge limits considers both worst case scenarios at the two potential discharge sites, as well as long term variability of the diffuser-induced dilution fields. Worst case scenarios consider three predominant hydrographic periods of Monterey Bay, namely: 1) the upwelling period, 2) the oceanic period dominated by relaxation states, and 3) the Davidson Current period. The long term analysis is based on 8,149 modeled generated by feeding the models a matrix of 7 controlling variables assembled from the oceanographic period record. All three hydrographic periods are embedded in this period of record. From the worst case simulations, we conclude that the intermediate discharge site has a slight advantage in dilution performance over the deep discharge site; but has a potential for occasional re-circulation of a small amount of brine at the proposed Tenera intake site (at UTM coordinates N, E). During periods of northward flowing currents (the predominant current direction, particularly during the upwelling period), the brine footprint on the seabed that corresponds to 5% above ambient salinity is 7% smaller at the intermediate site than at the deep water discharge site. This foot print is 2 % smaller at the intermediate site during occasional episodes of southward flowing mean currents (occurring during relaxation and Davidson current periods). This advantage is attributable to differences in wave induced mixing at the two sites, where the scrubbing action of oscillatory waves is more intense at the shallower intermediate site than at the deep water discharge site. These wave mixing effects are especially important in the high-energy wave environment of the Monterey Bay region. The intermediate site also has the advantage of a shorter pipeline run with 91

92 associated lower capital, operating and maintenance cost. Total head requirements for the 36 inch pipeline and the 5-jet linear diffuser are 14.4 psi over ambient ocean pressure at the depth of the deep water discharge; and 11.1 psi over ambient ocean pressure at the depth of the intermediate discharge. However, during relatively rare and brief periods of southward flowing currents (typically occurring during the relaxation period or Davidson current period), there is less than a 1% chance for a potential re-circulation of 1% of the brine discharged from the intermediate site. The actual re-circulation is likely to be much less, because the brine plume has dispersed as a thin bottom spreading layer of no more than 5 cm thickness at the proposed Tenera intake site, and the intake structure will probably stand several meters above the seabed. Regardless, the potential for brine re-circulation arising from the intermediate discharge site might tip a cost-benefit analysis of the two sites in favor of the deep discharge site, despite the higher operating pressure requirements of that site. At the intermediate MBRWP discharge site, long-term simulations find the median salinity at the outer limit of the Regulatory Mixing Zone is ppt, well within The 5% Rule requirements when using heated source water from the data center; and 99.9% of the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over long-term average salinity which is ppt). Maximum salinity at 100 m from the intermediate discharge site in any direction is ppt, exceeding the 5% Rule by only 0.7% when based long term average ambient salinity. (If the 5% Rule based on daily salinity, then this over-limit result exceeds the 5% Rule by only 0.3%). The probability of occurrence of this over-limit case is only 0.1%, or about 1 day in 3 years. The median salinity at the outer limit of the Regulatory Mixing Zone at the deep discharge site is ppt, also well within The 5% Rule requirements when using heated source water from the data center; and 99.8% of 92

93 the 8,149 modeled solutions are less than or equal to 35.1 ppt (5% over longterm ambient mean). Maximum salinity at 100 m from the deep discharge site in any direction is ppt, exceeding the 5% Rule by only 1.3% when based long term average ambient salinity. (If the 5% Rule based on daily salinity, then this over-limit result exceeds the 5% Rule by only 0.8%). The probability of occurrence of this of this over-limit is only 0.2%, or about 1 day every 1.4 years. The magnitude of these occasional over-limit model results at either discharge site are within sampling error of standard oceanographic temperature/conductivity measurements used for determination of practical salinity units (psu). The probability of recurrence of these over-limit values is equally negligible, on the order of one day every few years. Although NPDES permits are generally written for the specific conditions of each discharger, most allow for occasional over-limit monitoring results, as a provision for sampling errors. In addition, over-limit outcomes at the MBRWP discharge site are even less likely than indicated by the model results because the SWRO facility will operate most of the time under preferred efficiency conditions. The modeled results were obtained for the ultimate maximum brine discharge rate of mgd with 48% recovery; whereas the most efficient operating point for the MBRWP reverse-osmosis facilities is at 46% recovery ratio with about 1 mgd less brine discharge than what produced the over-limit model results. Therefore, we conclude the diffuser dilution strategy at both the deep water and intermediate MBRWP discharge sites satisfies any of the presently permitted or potential future dilution standards for all foreseeable long-term ocean conditions at MBRWP Moss Landing. There is the possibility that the data center and the SWRO facilities at MBRWP will not come on-line at the same time, and consequently the source water for SWRO facilities would be unheated during some portion of the initial 93

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104 APPENDIX-A: Diffuser Turbulence Mortality Estimates: 104 On the scale of the tiny, fragile organisms that live in the sea (ichthyoplankton, eggs, larvae and juvenile fish), dilution is a violent act. The dilution performance described in Section 5 using a diffuser requires a very high discharge velocity (5 m/s), with very high-energy turbulent eddies generated in the receiving waters by those discharge velocities. The Brine Panel appointed by SWRCB expressed concern for the potential for diffuser turbulence to cause injury and mortality to small organisms in the receiving water, and specifically recommended further studies of this issue 1. While this concern has not yet led to expert recommendations for specific amendments to control or mitigate for turbulence mortality arising from brine dilution, we have quantitatively assessed the potential marine life impacts associated with turbulent shear stresses induced by the MBRWP diffuser. The most comprehensive experimental measurements on turbulence mortality to date were performed at the Pacific Northwest National Laboratory, using toxicology protocols. 43,44 These experiments appear to be the closest known proxy of diffuser induced injury to aquatic organisms on prototypic scales, with the advantage of having been performed under highly controlled conditions. Four different species of freshwater juvenile fish measuring 100 mm in length and 2 cm in width were exposed to a submerged jet having exit velocities of 0 to 21.3 m/s, providing estimated exposure strain rates up to 1,185/s. Turbulence intensity in the area of the jet where fish were subjected to shear was minimal, varying from 3% to 6% of the estimated exposure strain rate. Video images were acquired using high-speed cameras positioned to view the fish as they exited a deployment tube and entered the shear zone of the diffuser jet. There was no

105 apparent size-related trend in susceptibility to entrainment by turbulent eddies into the high shear region of the diffuser jet 44. Injuries and mortalities increased for American shad at strain rates greater than 397/s (corresponding to a jet velocity 5 m/s ); and for all species of fish at strain rates greater than 495/s (a jet velocity u 6.1 m/s ). The portion of the test population experiencing sub-lethal injuries or worse was found to obey the following best-fit analytic relation: where K i exp 0 1 K c 2 (6) 1 exp 0 1 2, represents the proportion of the population incurring sub-lethal (minor) injury; u / r is the strain rate, and 0, 1, 2 are empirical best-fit parameters derived from Tables 1-3 in (44). These results can be transposed to a function of shear stress based on the mixing length of the largest turbulent eddies varying with distance x from the jet nozzle according to the relation 54 l ( x / d) 4 / 3, where d = 6.4 cm is the jet diameter. The experimental setup of the deployment tube 44 resulted in the fish entering the shear zone of the diffuser jet at a distance of one body length, x = 10 cm, downstream of the jet nozzle. Based on these experimental dimensions, equation (6) can be restated in terms shear stress as: 105 u 1 / 2 exp 0 1 K c 1 / 2 (7) 1 exp where ( l 1/ ) ; and ( l 2 ) The shear stress dependence of sub-lethal injury factor due to exposure of 100 mm long juvenile fish to a turbulent diffuser jet are plotted as the black curve

106 in Figure 33 based on equation (7), and for the jet velocities reported in (44) and (45). These results show LC-10 values (10% of the population incurring sublethal injury or worse) corresponding to shear stresses of between 1500 dynes/cm 2 and 1600 dynes/cm 2 (150 to 160 Pa), which is consistent with the earlier results from the Chesapeake and Delaware Canal Project 20, 33. We will refer to shear stresses that correspond with LC-10 values as critical shear stress for sub-lethal injury threshold, represented as c. Extrapolation of these experimental results 44 to brine diffuser applications have been criticized 45 on the bases that most of the data was obtained for jet velocities greater than typically used in brine diffusers. By conventional engineering practice, diffuser jet discharge velocities are typically u 3 m/s to 5 m/s. However, this criticism neglects the fact that sizes of the freshwater fish species in these experiments were 10 to 100 times larger than the juvenile fish and ichthyoplankton entrained in the marine environment of an ocean desalination discharge. Dynamic scaling laws for size adaptation of natural swimmers and flyers allow these data 44 to be extrapolated to the relevant species sizes and diffuser discharge velocities of the ocean environment. The basis for this scaling principle is the well-known 2/3-Power Law, that has been a principle of classical fluid dynamics dating back to Galileo Galilei in the 1500 s and has been applied to both natural swimmers and fliers for decades. 56,57 The 2/3-Power Law teaches that the size of swimming or flying organisms is limited by its tensile strength, or the maximum stress that a membrane or epidermis can withstand while being stretched or pulled before failing or breaking. Tensile strength balances shear stress at the onset of injury whence decreases with size as c 2 / 3. Since we already know the shear stresses that cause injury for a given size of size juvenile 106

107 fish, mm after (43) and (44), we can rescale those results for any 107 arbitrary size of smaller organism, i, by : 1 / 3 2 / 3 where ; and 1 1 i 0 1 / 2 exp 0 1 K i(, i ) 1 / 2 (9) 1 exp 2 2 i The blue, cyan and green curves in Figure 33 plot sub-lethal injury factors due to exposure to diffuser jet turbulence for typically sized juveniles, larvae and eggs of marine species, based on rescaling the laboratory results from (43) and (44) using the 2/3 Power Law. The rescaled results show that LC-10 values for a 10 mm juvenile marine fish drop to a critical jet shear stress of c = 350 dynes/cm 2 (70 Pa); while LC-10 for a 1mm size egg or larvae of a marine organism would occur at a critical jet shear stress of c = 75 dynes/cm 2 (7.5 Pa). The latter values for sub-lethal thresholds are expectedly less than the lethal shear stresses for eggs of striped bass and white perch measured during the Chesapeake and Delaware Canal Project. 20, 33 Sub-millimeter sized ichthyoplankton would, by the same scaling procedure, be expected to incur LC-10 injury levels at a critical jet shear stress of the order c = 16 dynes/cm 2 (1.6Pa). It is notable in Figure 33 that the LC-10 for 10 mm juvenile marine fish occurs at jet discharge velocities of 4.2 m/s while the LC-10 for 1 mm eggs and larvae occurs at jet discharge velocities of about 2.5 m/s, both within the operating domain of conventional ocean diffuser systems. In this regard, an important concern with the eggs larvae and sub-millimeter sized ichthyoplankton, is that they cannot swim and have no means of avoiding or escaping the entrainment velocities of an ocean diffuser jet, (Figure 34).

108 108 Figure 33: Sub-lethal injury factor reported in (43) and (44) re-scaled for relevant sizes of marine juvenile fish, larvae and eggs using the 2/3-Power Law. The critical shear stress, c, for sub-lethal injuries of the 100 mm juvenile fish reported in (43) and (44) was incurred at a distance x = 10 cm downstream of the jet nozzle. Smaller shear stress occur in the jet further downstream as the turbulent eddies mix the jet momentum into the interior of the surrounding fluid. Consequently the non-swimming eggs, larvae and ichthyoplankton can be injured at greater distances away from the jet nozzle than the swimming juvenile fish. An important factor in the decay of jet induced shear stress over distance is the proximity of the bottom plane of the seabed, particularly in the case of diffusers.

109 Figure 34: Hydrodynamic simulation of outflow and entrainment regions in the nearfield of a diffuser and riser with TideFex duckbill nozzle. Five such nozzles discharge 5.45 mgd each of brine or a total of mgd total brine discharge at ppt end-of-pipe salinity. 109

110 The seabed influences both the coherent entrainment flow patterns (Figure 34) as well as the extinction rate of the jet shear stress by the action of bottom friction. Laboratory and field measurements of shear stresses in a turbulent jet near the seabed were reported in (60), yielding the following relation for shear stress decay with distance x from the jet nozzle: d 120 u0 Re j (10) x where u 0 is the jet discharge velocity at x = 0, d is the jet diameter at the discharge 110 nozzle, and Re u d j 0 / is the jet Reynolds number. The distance downstream from the jet nozzle X c where the jet-induced shear stress decays to the LC-10 injury threshold c for some particular size of organism can be written: X c d Re c j 2 u (11) while the critical radius of the plume, r c, where the jet-induced shear stress decays to the LC-10 injury threshold c was found to be about rc X c / 3. The diffuser jet induced shear flow structure shown in Figure 34 produces internal shear stress fields evaluated in the receiving water with the Star-CD, Version 3.1 CFD model, as shown in Figure 35 for one of the five Tideflex diffuser nozzles that comprise the hypothetical linear diffuser array. Numerical integration of the 3-dimensional shear stress field around the diffuser allows computations of the stress-integrated injury factor, K ~ i that accounts for the percentage of organisms entrained by the diffuser jet that suffer lethal and sub lethal injuries. Because jetinduced injury is a function of both the shear stress and the sizes of the organism

111 111 Figure 35: Simulation of shear stress field using the Star-CD, Version 3.1 CFD model showing the nearfield of a diffuser fitted with a TideFlex duckbill nozzle. Nozzle sized for the 5% Rule dilution standard using a discharge velocity u 0 = 5 m/s. Five such nozzles discharge 5.45 mgd each of brine or a total of mgd total brine discharge at ppt end-of-pipe salinity.

112 after equation (8); and because the shear stress around the diffuser varies in 3- dimensional space ( x, y, z), this computation is performed from the total derivative of K (, ) according to: i ~ K (, ) i ~ K(, ) d Ki ~ dx x Ki ~ dy y Ki ~ dz z (12) 112 Where ~ K is evaluated from equation (9) for the median size organism = i 2.4 mm; and shear stress gradients ( x, y, z Star-CD, Version 3.1 CFD model. ) are evaluated from the The 5% Rule will require the 5-jet linear diffuser to entrain 518 mgd of dilution water in order to satisfy the = 20 dilution standard. To achieve these high dilution factors in the nearfield, the discharge velocities of each jet must be increased to u 0 = 5 m/s. These higher discharge velocities produce the shear stress field shown in Figure 35, where shear stresses reach 500 dynes/cm 2 (50 Pa), well above the lethal level for 1-mm sized eggs and larvae 20,28, and certainly injurious to 10 mm juvenile fish (cf. Figure 33). Applying numerical integration of shear stress gradients in Figure 35 to equation (12) yields the following percentage of entrained organisms in 518 mgd of dilution water that would suffer lethal or sublethal injuries due to a 5-jet linear diffuser operating under the 5% Rule: ~ ~ K(, ) K i (, ) d = or 16.8% (5 u 0 = 5 m/s ea.) (13) Using the diffuser at the desalination plant in Perth Australia as a comparable example 21, where u 0 = 4.1 m/s, it was estimated by analytic and empirical relations that 23% of the entrained organisms would be subject to damaging turbulence. 106 None the less, the more modest estimate in equation (14)

113 yields a substantial turbulence impact under the 5% Rule, whereby ~ ~ I K C Q = 272,601 larval fish would suffer lethal and sub-lethal injuries dilution i dilution per day due to diffusers diluting brine in the receiving waters offshore of the MBRWP desalination project; where the concentration of organisms in 113 Q dilution = 518 mgd of entrained dilution water is C ~ = 3,132 organisms per mgd of dilution water per day. 105 This number would increase to 373,204 larval fish that would suffer lethal and sub-lethal injuries per day with the alternative estimated impact factor of 23% after (106). These estimates do not take into consideration the relative reduction due to depth demonstrated in the Tenera entrainment study. 105 Instead they assume a more conservative (greater) concentration of larval fish than the potentially true concentration. Because of this these entrainment estimates most likely overestimate the number of larvae potentially entrained by the diffuser. Because the Tenera entrainment study did find that there is a marked decrease in the abundance with increasing water depth off Moss Landing, this would favor the deep water discharge site as the preferred site in terms of minimizing potential diffuser induced injury and mortality. It has been suggested that marine organisms can merely avoid the diffuser turbulence and swim away because entrainment velocities associated the large high energy turbulent eddies are only on the order of 2 cm/sec for diffuser jets in the u 0 = 4.1 m/s to 5 m/s range. 106 However, eggs and larvae can t swim, and subcentimeter juvenile in most cases can t perform sustained swimming speeds of 2 cm/s in any one direction; nor would they necessarily be able to know which direction to swim toward to avoid a diffuser jet. Diffuser jets are very narrow, sharp edged, high-velocity current streams that have no naturally occurring counterpart in the ocean.