VOLUME II.8 APPENDIX E.4 ANALYSIS OF INITIAL DILUTION FOR OCSD DISCHARGE FLOWS WITH DIVERSIONS TO THE GWRS

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1 VOLUME II.8 APPENDIX E.4 ANALYSIS OF INITIAL DILUTION FOR OCSD DISCHARGE FLOWS WITH DIVERSIONS TO THE GWRS

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3 Analysis of Initial Dilution for OCSD Discharge Flows with Diversions to the Groundwater Replenishment System (GWRS) FINAL REPORT Submitted To Orange County Sanitation District 1844 Ellis Avenue Fountain Valley, CA 9278 By Tetra Tech, Inc Mt. Diablo Blvd., Suite 3 Lafayette, CA December 23, 28

4 EXECUTIVE SUMMARY Initial dilution modeling was performed to evaluate Orange County Sanitation District s (OCSD) discharge into the Pacific Ocean offshore of the Huntington Beach area. In 22, Tetra Tech performed initial dilution modeling to support OCSD s NPDES permit renewal (OCSD 22, Appendix M). Since that time, the Orange County Water District constructed the Groundwater Replenishment System (GWRS) in a joint venture with OCSD, in which OCSD discharge flows of around 65 mgd are diverted to the GWRS, with return flows (brine) of approximately 15 mgd. This change in effluent flow rates and density necessitated updating the initial dilution modeling to determine the influence of these changes on the initial dilution. Visual Plumes was used to model initial dilutions for thirteen months using hourly time-series of discharge flow, current, and density. OCSD effluent flow rates and density data used in the modeling were for the period July 27 through August 28. The GWRS system went on-line in mid-january 28, which resulted in effluent flow rates about 5 mgd lower than previously. The density of the discharge also increased slightly due to high-density return flows from the GWRS (from kg/m 3 to kg/m 3 ). This change in density is small and its effect on initial dilutions is minor compared to the influence of the change in flow rate. The same ocean current and density profiles as used in the 22 modeling were used in this analysis. This allows for a better evaluation of the impact of the reduced discharge flow rate and higher discharge density on the initial dilution without having to consider the influence of differing ocean currents and temperatures. The results of the initial dilution modeling indicate that the lower discharge flow rate due to diversions to the GWRS results in a higher average dilution (352:1) for the 27-8 modeling runs than for the modeling performed in 22 (341:1). The 27-8 modeling runs also have a smaller range of initial dilutions, 142:1 to 1922:1, versus 119:1 to 2411:1 for the modeling performed in 22. Furthermore, higher dilutions are more frequent for the 27-8 modeling runs (post-gwrs) in the lower range of dilutions (<25:1). Correlation plots also show a slight shift to lower flows and higher dilutions for the runs where diversions to the GWRS were occurring compared to the 22 modeling runs. Comparison of initial dilutions over selected weeks of the modeling runs indicates that the influence on initial dilution of the reduced discharge flow rate due to diversions to the GWRS is slightly more pronounced (lower dilutions) during periods of strong stratification. Other plume characteristics showed minor or no changes because of diversions to the GWRS. The altered effluent characteristics resulted in no change to the time to complete the initial dilution and the mixing length. The plume rise was only slightly affected by the change in discharge characteristics due to diversions to the GWRS. The plume did not rise as high with the smaller discharge flow. However, the plume was predicted to surface in the 27-8 modeling runs during the same periods as in the 22 modeling runs. i

5 TABLE OF CONTENTS EXECUTIVE SUMMARY... i 1. INTRODUCTION Site Description Report Format MODEL INPUT AND DESCRIPTION Model Description Outfall Description and Diffuser Hydraulics Summary of Input Data DILUTION MODELING RESULTS SUMMARY AND CONCLUSIONS REFERENCES Appendix A Density Calculation... A-1 ii

6 LIST OF FIGURES Figure 1-1. Orange County Sanitation District Outfall Location Figure 2-1. Definition of Visual Plumes Output Parameters Figure 2-2. Histogram of Discharge Flow Rates... 4 Figure 2-3. Histogram of Discharge Flow Rates for the 27-8 (both Pre- and Post- GWRS) and Modeling Runs Figure 2-4. Time Series of Discharge Flow for the 27-8 Modeling Runs Figure 2-5. Current Speed and Direction in the Vicinity of the Orange County Sanitation District Outfall, July 1, : to August 16, 2 8: GMT Figure 2-6. Ambient Density and Temperature Fields in the Vicinity of the Orange County Sanitation District Outfall, July 1, : to August 16, 2 8: GMT Figure 3-1. Histogram of Flux-Averaged Dilution as Predicted by Visual Plumes Figure 3-2. Histogram of Depth of Plume Rise as Predicted by Visual Plumes Figure 3-3. Histogram of Top of Plume as Predicted by Visual Plumes Figure 3-4. Histogram of Bottom of Plume as Predicted by Visual Plumes Figure 3-5. Histogram of Mixing Zone Length as Predicted by Visual Plumes Figure 3-6. Time Series of the Hourly Visual Plumes Modeling Results Along with Current Speed in the Vicinity of the Outfall for the 27-8 and Modeling Runs Figure 3-7. Time Series of the Plume Rise, Depth of Top and Depth of Bottom Predicted by Visual Plumes for 27-8 Modeling Runs Figure 3-8. Scatter Plot of Current Speed Versus Mixing Zone Length for the 27-8 Modeling Runs Figure 3-9. Scatter Plot of Mixing Zone Length and Flux-Averaged Dilution for the 27-8 Modeling Runs Figure 3-1. Weekly Snapshots of Flux-Averaged Dilution, Current Speed, and Discharge Rate... 2 Figure Histogram of Time to Complete Initial Dilution as Predicted by Visual Plumes for the 27-8 and Modeling Runs Figure Correlation of Flux-Averaged Dilution Figure Correlation of Mixing Zone Length Figure Correlation of Discharge Flow Rates LIST OF TABLES Table 2-1. Orange County Sanitation District Outfall Characteristics Used in Visual Plumes Table 3-1. Statistics of the Time-Series Dilution Modeling Results Table 3-2. Monthly Average Plume Characteristics iii

7 1. INTRODUCTION Orange County Sanitation District (OCSD) discharges approximately 25 million gallons per day (mgd) of highly treated wastewater from its outfall located at 55 m depth and approximately 8.2 km (5.1 miles) offshore of the Huntington Beach area, just north of the Santa Ana River. In 22, Tetra Tech performed initial dilution modeling to support OCSD s NPDES permit renewal (OCSD 22, Appendix M). During that study, Tetra Tech used Visual Plumes to model initial dilutions for two years (June 1999 through June 21) using hourly time-series of discharge flow, current, and density. Since that time, the characteristics of OCSD s effluent have changed, resulting in a need to update the initial dilution modeling. In a joint venture with OCSD, the Orange County Water District constructed the Groundwater Replenishment System (GWRS) in which secondary effluent is highly treated before being used for a seawater intrusion barrier and groundwater recharge. This system has a capacity of 7 mgd of net recycled water and the GWRS discharges up to 14 mgd of brine from its reverse osmosis treatment train directly to OCSD s outfall. The GWRS became fully operational in January of 28. Tetra Tech performed modeling to evaluate the effect of the altered effluent characteristics due to the GWRS system on the initial dilution. 1.1 Site Description The OCSD outfall has a 1.8 km long, multi-port diffuser most of which is directed along the trend of the local isobaths (Figure 1-1). The outfall discharges treated wastewater into coastal waters on a wide continental shelf area (San Pedro Shelf) of the Southern California Bight. The shelf is bordered on the south by the steep continental slopes of the San Pedro Basin. Just to the east of the outfall, the shelf decreases in width so that deep water at the head of the Newport Canyon is located only a few kilometers from the shoreline at Newport Beach (Figure 1-1). Circulation on the San Pedro shelf is complex with strong currents sustained over periods of several days to weeks in both the up- and down-coast directions (i.e. towards northwest and southeast, respectively). Observations by Hamilton et al. (21) have shown important contributions to local currents from tidal and sea-breeze driven circulation. The temporally and spatially varying current fields over the shelf and slope transport and disperse the wastewater plume. 1.2 Report Format This report contains information and calculations to update the initial dilution modeling to evaluate the influence of the GWRS. Section 2 describes the model and input data used in the initial dilution modeling. In Section 3, the results from the initial dilution modeling are presented, including a comparison to the results from the modeling performed in 22. Section 4 provides a summary of this report and references are provided in Section 5. 1

8 2. MODEL INPUT AND DESCRIPTION Initial dilutions were calculated using the Visual Plumes model (Frick et al., 22). This section provides a description of the model and the data used in the modeling effort. Source: USGS Figure 1-1. Orange County Sanitation District Outfall Location. 2

9 2.1 Model Description Visual Plumes is a Windows-based mixing zone modeling application developed and supported by the U.S. Environmental Protection Agency (EPA). Visual Plumes includes multiple initial dilution models that simulate single and merging submerged plumes in arbitrarily stratified ambient flow. Specifically, the Roberts-Snyder-Baumgartner (RSB) model, renamed NRFIELD within Visual Plumes, was used for the initial dilution modeling work. NRFIELD is an empirical model for multi-port diffusers based on the experimental studies on multi-port diffusers in stratified currents described in Roberts, Snyder, and Baumgartner (1989a, b, and c) and subsequent experimental works. NRFIELD was used to estimate the flux-averaged and centerline dilution, the height of rise of the plume (Z max ), the top height of the plume (Z top ), the bottom height of the plume (Z bot ), and mixing zone length (X i ). These parameters are illustrated in Figure 2-1. The minimum initial dilution is also known as the centerline dilution, and is the smallest value of dilution in the plume at the end of the near field. The flux-averaged dilution is the reciprocal of the volume fraction of effluent in the plume, and for the NRFIELD model is a constant, calculated as 1.15 times the centerline dilution. The model requires the following types of inputs: Outfall characteristics (port diameter and spacing, diffuser length and depth, diffuser orientation, and effluent density) Time series of discharge rates from the outfall Time series of vertical density profiles Time series of current speeds and directions Figure 2-1. Definition of Visual Plumes Output Parameters. 2.2 Outfall Description and Diffuser Hydraulics The OCSD outfall discharges approximately 7,25 m offshore. The marine section of the outfall has a length of 6,523 m and terminates in a 1,829 m multi-port diffuser, which ranges in diameter from 1.83 to 3.5 m. The diffuser is L-shaped, with the last 945 m section oriented to follow the 3

10 6 m contour. The diffuser ports are in an opposed configuration with port pairs spaced 7.32 m apart. The ports range in diameter from.75 to.15 m and discharge horizontally to the two sides of the diffuser. Additionally, there are three.152 m diameter ports in the flapgate structure for increasing pipe flow velocity toward the end of the diffuser. The Visual Plumes model requires a straight diffuser with same-sized port diameters; therefore, the OCSD outfall was represented as such in the model. The Visual Plumes model also assumes that the port velocity is the same for all ports; however, this is not true for the OCSD diffuser. In the 22 initial dilution modeling effort (OCSD 22, Appendix M), the equal-velocity assumption was evaluated using the diffuser hydraulics model, PLUMEHYD (Baumgartner et al., 1994) and determined to be acceptable. Although the distribution of flows has changed (Figure 2-2), the range of flow rates is not significantly different; therefore, the previous modeling of diffuser hydraulics is assumed to be valid data data Frequency Discharge Flow Rate (mgd) Figure 2-2. Histogram of Discharge Flow Rates. 2.3 Summary of Input Data The OCSD outfall is represented in Visual Plumes by a straight diffuser with same-sized port diameters. Outfall characteristics used in Visual Plumes are summarized in Table 2-1. The port spacing is input as half the real spacing since Visual Plumes assumes all the ports are on one side of the diffuser; however, when running NRFIELD, which is based on T-riser experiments with ports on both sides, Visual Plumes makes the necessary adjustments to this convention. The density and temperature of the effluent are assumed to be constant. The density of the discharge was calculated from the discharge temperature and salinity (see Appendix A). The salinity 4

11 representing the effluent with the return flow from the GWRS, 1.7 parts-per-thousand (ppt), was based on the value provided for Scenario 4 (Full Secondary with GWR) in the OCSD 1999 Strategic Plan. The effluent temperature remained the same as used in the 22 initial dilution modeling effort. The effluent density for this analysis (997.9 kg/m 3 ) is slightly higher than was used in the 22 modeling (997.2 kg/m 3 ). Table 2-1. Orange County Sanitation District Outfall Characteristics Used in Visual Plumes. Parameter Value Port diameter.9 m Port elevation.1 m Vertical discharge angle deg Horizontal port discharge angle 7 surv-deg Number of ports 53 Port spacing 3.64 m Port depth 54.6 m Effluent density kg/m 3 Effluent temperature 26.9 deg C Hourly data for outfall discharge rates, vertical density profiles, current speed and direction were input into Visual Plumes. Outfall discharge rates were provided by OCSD from their CRISP Historian system. Initial dilution modeling was performed using hourly time-series of discharge flow for the period July 1, 27 7: GMT through August 16, 28 8: GMT. One period in the flow data appears suspect. On November 29, 27, from 11: to 23:, the flow rates are reported to be constant at 22 mgd. Then for the 7 hours immediately following that period (November 3, 27 : through 7:), the flow rate is reported to be. Because it appears to be erroneous, this period (November 29, 27 11: GMT through November 3, 27 7: GMT) was removed from the analysis. The histogram of outfall discharge rates for the period modeled is presented in Figure 2-3 along with the discharge rates for the initial dilution modeling performed in 22. For the 27-8 modeling runs, the diversions to the GWRS system began mid-period, assumed to be January 12, 28. Therefore, the data is shown separately on the histogram as pre-gwrs and post-gwrs. As shown in the figure, the post-gwrs flows are shifted to the left, indicating that they are smaller than the pre-gwrs flows and the flows from the modeling performed in 22 ( ). A time series of the discharge flow rates is shown in Figure 2-4. From the time series, a drop in the discharge flow of about 5 mgd can be observed starting in mid-january 28. This is when the GWRS system went on-line and flows around 65 mgd started to be diverted to the GWRS, with return flows of approximately 15 mgd. In order to evaluate changes in initial dilution without needing to take into account differing currents and density profiles in the ocean, vertical density profiles and current speed and direction were extracted from the density and current information used in the 22 initial dilution analysis for the period that corresponds to the same time of year (July 1, : GMT through August 16, 2 8: GMT). More information about the density and currents, including Visual Plumes corrections for instabilities in the density profiles, can be found in the OCSD 24 NPDES Permit Application (OCSD 22, Appendix M). Current speed and direction data for the period used in this analysis are presented in Figure 2-5. The figure demonstrates that the 5

12 current flows primarily along an axis that is slightly tilted from the E-W axis, which indicates that the current runs alongshore. Current speeds are generally less than 2 cm/s, but reach as high as 4 cm/s on occasion. The ambient temperature and density fields are shown in Figure Pre-GWRS 27-8 Post-GWRS data Frequency Discharge Flow Rate (mgd) Figure 2-3. Histogram of Discharge Flow Rates for the 27-8 (both Pre- and Post-GWRS) and Modeling Runs. Pre-GWRS Post GWRS Average (Pre-GWRS) Average (Post-GWRS) Discharge Flow Rate (mgd) Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Date Figure 2-4. Time Series of Discharge Flow for the 27-8 Modeling Runs

13 Figure 2-5. Current Speed and Direction in the Vicinity of the Orange County Sanitation District Outfall, July 1, : to August 16, 2 8: GMT. 7

14 (a) Density Expressed as Sigma-T (b) Temperature (Degrees C) Figure 2-6. Ambient Density and Temperature Fields in the Vicinity of the Orange County Sanitation District Outfall, July 1, : to August 16, 2 8: GMT. 8

15 3. DILUTION MODELING RESULTS For the 27-8 modeling runs, the flux-averaged dilutions were calculated on an hourly basis using the discharge flow rate and density for the period July 1, 27, 7: GMT through August 16, 28, 8: GMT and the receiving water density and current information from the 22 initial dilution analysis (July 1, : GMT through August 16, 2 8: GMT). This interval included a range of conditions from strong stratification in summer to weak stratification in winter, and possible episodes of upwelling. Along with flux-averaged dilutions, five additional parameters were reported: centerline dilution, the height of rise of the plume (Z max ), the top height of the plume (Z top ), the bottom height of the plume (Z bot ), and mixing zone length (X i ). These parameters were illustrated previously in Figure 2-1. While Visual Plumes reports the rise height (Z max ), top height (Z top ), and bottom height (Z bot ) as height from the diffuser, those parameters are hereafter represented in this report as depth from the water surface. The dilution modeling results are presented for two modeling runs: the results from the current modeling runs (27-8) and the results from the previous modeling runs ( ). Furthermore, the current modeling results are broken down into two parts, the period before diversions to the GWRS began (pre-gwrs) and the period after diversions to the GWRS were occurring (post-gwrs). The point at which the diversions to the GWRS began is assumed to be January 12, 28 based on estimation from the change in discharge flow data. The focus of the comparison is on the post-gwrs period where the results are based on changes in effluent flow, and to a lesser extent effluent density, from the diversions to the GWRS. Table 3-1 presents the minimum, maximum, and mean of the initial dilutions calculated by the model along with the statistics for the other plume characteristics. The statistics are shown for the 27-8 modeling runs split into pre- and post-gwrs periods. Also shown in the table are the statistics from the modeling runs. Note that in the table, the height of rise, top height, and bottom height of the plume are shown measured from the water surface. The distributions of these parameters are shown as histograms in Figures 3-1 through 3-5. The flux-averaged dilution for the 27-8 modeling runs ranged from a low of 142:1 to a high of 1922:1. The range is smaller than the range for the modeling runs (119:1 to 2411:1). The minimum flux-averaged dilution for the 27-8 modeling runs occurred during a period of strong stratification (June) which was the same time period as the minimum occurred in the modeling runs. For the 27-8 modeling runs, the flux-averaged dilutions averaged 368 (pre-gwrs) and 352 (post-gwrs). Although the post-gwrs runs shows a lower mean dilution, the histogram (Figure 3-1) indicates that smaller dilutions for the pre-gwrs runs are more frequent than for the post-gwrs runs during some of the smaller ranges (i.e., between 2 and 25). The top of the plume averaged a depth of 27-3 m for all modeling runs with the height of rise averaging a depth of m (depth of minimum dilution). Both of these depths varied widely; the depth of the top of the plume ranged from the surface to nearly 43 m and the depth of the height of rise extended from the surface to approximately 47 m. The bottom of the plume had the least depth variation, extending from just above the diffuser at 54 m to approximately 41 m. In general, the depth of the rise of the plume (and its top and bottom) were slightly larger for the post-gwrs runs. During the 27-8 modeling runs, the plume rises into the upper 1 m of the water column less than 1 percent of the time compared with about 2 9

16 percent for the modeling runs. The length of the mixing zone averaged about 68 m for the post-gwrs period compared to 72 for the modeling runs. Table 3-1. Statistics of the Time-Series Dilution Modeling Results. Flux-Averaged Dilution Mean Median Minimum Maximum 27-8 Pre-GWRS Post-GWRS Centerline Dilution 27-8 Pre-GWRS Post-GWRS Depth Below Surface of Top of Plume (m) 27-8 Pre-GWRS Post-GWRS Depth Below Surface of Plume Rise (m) 27-8 Pre-GWRS Post-GWRS Depth Below Surface of Bottom of Plume (m) 27-8 Pre-GWRS Post-GWRS Mixing Zone Length (m) 27-8 Pre-GWRS Post-GWRS Figure 3-6 shows the time series of flux-averaged dilution and the mixing zone length for both the 27-8 and modeling runs along with current speed (used for both periods). For the modeling runs, the data that correspond to the period with the same current and density used for the 27-8 modeling is shown (July 1, : GMT through August 16, 2 8: GMT). As demonstrated the figure, the results for the two modeling runs are similar. The dilutions and mixing zone length fluctuate similarly for both sets, though the dilutions for the post-gwrs runs are slightly higher than the dilutions for the runs. In general, periods of high dilutions were correlated with periods with high current speeds and weak stratification. 1

17 Pre-GWRS 27-8 Post-GWRS Relative Frequency More Flux-averaged dilution Figure 3-1. Histogram of Flux-Averaged Dilution as Predicted by Visual Plumes. 11

18 Figure 3-2. Histogram of Depth of Plume Rise as Predicted by Visual Plumes. 12

19 Figure 3-3. Histogram of Top of Plume as Predicted by Visual Plumes 13

20 Figure 3-4. Histogram of Bottom of Plume as Predicted by Visual Plumes. 14

21 .25 Relative Frequency Pre-GWRS 27-8 Post-GWRS Mixing Zone Length (m) Figure 3-5. Histogram of Mixing Zone Length as Predicted by Visual Plumes. Figure 3-7 shows the time series of top, bottom and rise heights for the 27-8 modeling runs. This figure indicates that the effluent plume can, under certain conditions, reach the water surface. For the 27-8 modeling runs, the plume was predicted to surface (defined as coming within 2.5 m of the water surface) three times during winter months during intervals of especially weak stratification. These are the same periods during which plume surfacing was predicted in the modeling runs, indicating that the change in the discharge due to GWRS diversions does not affect surfacing during these periods of weak stratification. When the plume surfaced, the dilution ranged from 4:1 to more than 14:1. Similar to the modeling runs, for most of the time interval modeled in the 27-8 modeling runs, the top of the plume remained 2 m or more below the water surface, with the most concentrated part of the plume centered between 32 and 44 m. The bottom of the plume showed the least amount of variation and was centered at about 51 m. As with the modeling runs, for the 27-8 runs the mixing zone length was strongly correlated with current speed (Figure 3-8) and there is a strong correlation between the mixing zone length and dilution, with long mixing zone lengths associated with large dilutions (Figure 3-9). 15

22 Flux-Averaged Dilution Pre-GWRS 27-8 Post-GWRS Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Month Mixing Zone Length (m) Without GWRS 27-8 Pre-GWRS 27-8 Post-GWRS Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Month Current Speed (cm/s) Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Figure 3-6. Time Series of the Hourly Visual Plumes Modeling Results Along with Current Speed in the Vicinity of the Outfall for the 27-8 and Modeling Runs. Depth (m) Jul Aug Sep Oct Nov Dec Jan Feb Mar Apr May Jun Jul Aug Date (GMT) Plume Rise (Pre-GWRS) Top of Plume (Pre-GWRS) Bottom of Plume (Pre-GWRS) Plume Rise (Post-GWRS) Top of Plume (Post-GWRS) Bottom of Plume (Post-GWRS) Figure 3-7. Time Series of the Plume Rise, Depth of Top and Depth of Bottom Predicted by Visual Plumes for 27-8 Modeling Runs. 16

23 Mixing Zone Length (m) Pre-GWRS Post-GWRS Current Speed (cm/s) Figure 3-8. Scatter Plot of Current Speed Versus Mixing Zone Length for the 27-8 Modeling Runs. 25 Pre-GWRS Post-GWRS Flux-averaged Dilution Mixing Zone Length (m) Figure 3-9. Scatter Plot of Mixing Zone Length and Flux-Averaged Dilution for the 27-8 Modeling Runs. Table 3-2 contains the monthly averages of initial dilutions, the depth below the surface of the top of the plume, bottom of the plume, and height of rise, and mixing zone lengths for both the 27-8 and modeling runs. The results between the two scenarios are similar; the months of December and January have higher flux-averaged dilutions and mixing zone lengths, since stratification is typically weaker in the winter, leading to higher dilutions. The exception is for the month of May, where the flux-averaged dilution is much higher for the 27-8 modeling 17

24 runs. This is due to a period of weak stratification during the last half of May 2. This effect is not as obvious for the modeling runs since both May 2 and May 21 were included in those runs. Flux- Averaged Dilution Table 3-2. Monthly Average Plume Characteristics. Centerline Dilution Depth Below Surface of Top of Plume (m) Depth Below Surface of Plume Rise (m) Depth Below Surface of Bottom of Plume (m) Mixing Zone Length (m) January February March April May June July August September October November December Notes: 1 For the 27-8 modeling period, represents a combination of pre- and post-gwrs discharge flows. 2 For the 27-8 modeling period, represents post-gwrs discharge flows only. 3 For the 27-8 modeling period, represents pre-gwrs discharge flows only. Weekly snapshots of initial dilution, current speed, and discharge rate are presented in Figure 3-1 for both the 27-8 and modeling runs. One week was selected from each season. The discharge flow and average dilution for the months of September and December are very similar for the two sets of runs. This corresponds to the period before the GWRS system was fully operational. In March and June, the discharge flow rates are consistently smaller for the 27-8 modeling runs, as the diversions to the GWRS are fully operational during this time. In March, the dilutions are similar for the two scenarios, with the values for the 27-8 runs being slightly higher. For June, a period of stronger stratification, a larger difference (though still small) between the average dilutions between the two sets of modeling runs is observed. Because the receiving water currents and temperature are the same for both modeling runs, this indicates that the impact on initial dilution of reduced discharge flows due to diversions to the GWRS are slightly more pronounced during periods of stronger stratification. For each of the weeks, the flux-averaged dilution demonstrates a positive correlation to the current speed and an inverse correlation to the discharge flow. This indicates that the difference in effluent density between the and 27-8 modeling runs is insignificant compared to the difference in effluent flow rates. A histogram of the time to completion of initial dilution, as predicted by Visual Plumes, is presented in Figure 3-11 for both the 27-8 and modeling runs. The time to

25 complete the initial dilution is similar for both sets of runs. Typically the time to completion of initial dilution is 3-6 minutes, and is always less than 25 minutes, confirming that modeling using an hourly timestep is appropriate. Several correlations were performed to compare the results of the initial dilution modeling for the modeling runs with results for the modeling runs. For the modeling runs, the data that correspond to the period with the same current and density used for the 27-8 modeling runs was used (July 1, : GMT through August 16, 2 8: GMT). Correlations were plotted for the flux-averaged dilution (Figure 3-12) and mixing zone length (Figure 3-13). As shown on the plot of flux-averaged dilution, the data are uniformly scattered around the 1:1 line. However, for the runs post-gwrs, the data are slightly skewed higher than the 1:1 line, indicating that the dilutions are generally slightly larger for post-gwrs 27-8 runs than for runs. The mixing zone lengths show a fairly uniform scatter centered around the 1:1 line with little difference between the pre- and post-gwrs runs. A correlation plot of discharge flow rates (Figure 3-14) shows a fairly uniform scatter around the 1:1 line, but with the post-gwrs flows representing the lower of these flows. This illustrates that the flow rates during this period are lower than they were in the runs due to the diversions to the GWRS. 4. SUMMARY AND CONCLUSIONS Initial dilution modeling using Visual Plumes was performed for the OCSD discharge for the period July 1, 27 through August 16, 28. During this period, diversions to the GWRS system began on approximately January 12, 28, which resulted in effluent flow rates about 5 mgd lower than previously. In order to see the effect on the initial dilution of the change in discharge characteristics from diversions to the GWRS, the receiving water current and density data for the modeling performed for June 1999 through June 21 was used. Results of the current initial dilution modeling were compared to the modeling runs. The results of the initial dilution modeling show that flux-averaged dilutions for the 27-8 modeling runs have a higher average (352:1) than for the runs (341:1). Also the range of dilutions for the 27-8 runs was smaller, 142:1 to 1922:1, compared to 119:1 to 2411:1 for the modeling runs. During the 27-8 modeling runs, higher dilutions are more frequent in the lower dilution ranges than during the modeling runs. Correlation plots also show a shift to lower flows and higher dilutions for the 27-8 post- GWRS runs compared to the modeling runs. Other plume characteristics (time to complete initial dilution, mixing length, and plume rise) showed minor or no differences between the 27-8 modeling runs and the runs. 19

26 Average Dilution (1999) Pre-GWRS Average Dilution (27) Discharge Flow Rate (1999) Pre-GWRS Discharge Flow Rate (27) Current Speed Average Dilution (1999) Pre-GWRS Average Dilution (27) Discharge Flow Rate (1999) Pre-GWRS Discharge Flow Rate (27) Current Speed Average Dilution or Discharge Flow Rate (mgd) Sep 7-Sep 8-Sep 9-Sep 1-Sep 11-Sep 12-Sep 13-Sep Current Speed (cm/s) Average Dilution or Discharge Flow Rate (mgd) Dec 7-Dec 8-Dec 9-Dec 1-Dec 11-Dec 12-Dec 13-Dec Date Date Average Dilution (2) Post-GWRS Average Dilution (28) Average Dilution (2) Post-GWRS Average Dilution (28) Discharge Flow Rate (2) Post-GWRS Discharge Flow Rate (28) Discharge Flow Rate (2) Post-GWRS Discharge Flow Rate (28) Current Speed Current Speed Average Dilution or Discharge Rate (mgd) Mar 7-Mar 8-Mar 9-Mar 1-Mar 11-Mar 12-Mar 13-Mar Date Current Speed (cm/s) Average Dilution or Discharge Flow Rate (mgd) Jun 7-Jun 8-Jun 9-Jun 1-Jun 11-Jun 12-Jun 13-Jun Date Figure 3-1. Weekly Snapshots of Flux-Averaged Dilution, Current Speed, and Discharge Rate. 2

27 .35.3 Relative frequency Time to complete initial dilution (hr) Figure Histogram of Time to Complete Initial Dilution as Predicted by Visual Plumes for the 27-8 and Modeling Runs. 2 Flux-averaged dilution :1 Pre-GWRS Post-GWRS Flux-averaged dilution Figure Correlation of Flux-Averaged Dilution. 21

28 5 1:1 Mixing Zone Length (m) with GWRS Pre-GWRS Post-GWRS Mixing Zone Length (m) Without GWRS Figure Correlation of Mixing Zone Length :1 Discharge Flow Rate (mgd) Pre-GWRS Post-GWRS Discharge Flow Rate (mgd) Figure Correlation of Discharge Flow Rates. 22

29 5. REFERENCES Baumgartner, D.J., W.E. Frick, and P.J.W. Roberts PLUMEHYD model. Distributed with DOS PLUMES. Standards and Applied Science Division, Office of Science and Technology, Oceans and Coastal Protection Division, Office of Wetlands, Oceans, and Watersheds, Pacific Ecosystems Branch, ERL-N U.S. Environmental Protection Agency. Frick, W.E., P.J.W. Roberts, L.R. Davis, D.J. Baumgartner, J. Keyes, and K.P. George. 22. Visual Plumes, version 1.1. Environmental Research Division, NERL, ORD, U.S. Environmental Protection Agency, Athens, Georgia. Hamilton, P., J.J. Singer, E. Waddell and G. Robertson. 21. Circulation Processes on the San Pedro Shelf. Proceeding: MTS 21 Conf., November, 21, Honolulu, Hawaii, 8 pp. OCSD. 22. NPDES Permit Application. Orange County Sanitation District, California. December 2, 22. Roberts, P.J.W., W.H. Snyder, and D.J. Baumgartner. 1989a. Ocean outfalls I: submerged wastefield formation. ASCE Journal of Hydraulic Engineering No. 1. pp Roberts, P.J.W., W.H. Snyder, and D.J. Baumgartner. 1989b. Ocean outfalls II: spatial evolution of submerged wastefield. ASCE Journal of Hydraulic Engineering No. 1. pp Roberts, P.J.W., W.H. Snyder, and D.J. Baumgartner. 1989c. Ocean outfalls III: effect of diffuser design on submerged wastefield. ASCE Journal of the Hydraulic Engineering No. 1. pp

30 Appendix A Density Calculation The density of the effluent is calculated from the specified temperature and salinity using the following procedure. In oceanography, it is customary to refer to seawater density in sigma units. In the cgs system, density units are g/cm 3. To convert from density to sigma-t form, the number one is subtracted from the density and the result multiplied by 1. Knudsen (191) provided the first relationship between the density of sea water at o C and salinity, measured at the sea surface (so pressure effects are neglected). Neumann and Pierson (1966) give this relation as: σ = 1 (ρ S. 1) = S.482 S S 3 The effect of temperature on seawater density is incorporated in a complicated empirical relationship reported by Hill (1962). The sigma form density at a specific temperature, T ( o C) is: in which σ T = 1 (ρ S.T 1) = (σ )[1 A(T) + B(T)(σ.1324)] C(T) A(T) =.1(.1843 T T T) B(T) =.1(.1667 T T T) 2 (T 3.98) (T + 283) C(T) = 53.57(T ) and S and T are the salinity (parts-per-thousand) and temperature ( o C) of the water, respectively. The above formulas are considered accurate to within.2 σ T units (Hill, 1962). The effects of pressure are neglected. The density, ρ S.T, calculated for the OCSD effluent is.9979 g/cm 3. References Hill, M.N The Sea. Volume 1. Wiley, New York. 864p. Knudsen, M Hydrographical tables. G.E.C. Gad, Copenhagen. 63p. Neumann, G. and W.J. Pierson Principles of Physical Oceanography. Prentice Hall Inc., Englewood Cliffs, N.J. A-1