CHAPTER 5. NEWPORT CANYON TRANSPORT PATHWAY

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Chapter 1. Introduction and Background

Chapter 2. Methodology

Chapter 3. Surfzone Bacteria Patterns

Chapter 4. Subtidal Circulation Pathways

Chapter 5. Newport Canyon Transport Pathway

Chapter 6. Sea Breeze

Chapter 7. Tidal Transport Pathways

Chapter 8. Sediment Resuspension and Transport near the OCSD Outfall

Chapter 9. Nearshore Circulation and Transport Pathways

Chapter 10. Spatial and Temporal Patterns of Plume Tracers

Chapter 11. Temporal and Spatial Patterns for Surfzone Bacteria before and after Disinfection of the OCSD Outfall

Acknowledgements

Occasionally, subtidal currents over the outer shelf flow downcoast toward the southeast. Usually, if current speeds are not too strong, these subtidal flows follow isobaths transporting water over the same depths on the shelf. Because the edge of the shelf downcoast of the outfall pipe bends sharply toward the coast, these downcoast subtidal currents may carry suspended materials including bacteria from the plume toward the coast and into Newport Canyon (Figure 5-1). If this contaminated effluent from the plume is upwelled or transported out of Newport Canyon by some oceanographic process and the nearshore currents are also directed upcoast, then a hypothetical pathway exists between the outfall and Huntington Beach.

For this investigation, we used subtidal data from the current meters at moorings along the hypothetical pathway (moorings HB07, HB11, HB13, and HB03) to determine if and when the hypothetical pathway does in fact exist. We compared current speeds at the top, middle, and bottom plume depths along the shelf edge to determine the percentage of time that currents were downcoast and when the pathway was possibly open.

5.2. Methods to Determine When the Pathway Was Open

To define possible contamination events, currents must flow in directions that are along the pathway toward Huntington Beach and flows must persist long enough to carry the water from the outfall along the entire pathway to the nearshore region off Huntington Beach. Six necessary conditions were defined to evaluate this (the first four are shown in Figure 5-1):

1) Currents at HB07 flowed downcoast for a sufficient time to transport water from the outfall to HB11 (5.5 km). Here we assume that the currents always follow isobaths until they reach Newport Canyon, although it is known that downcoast currents do occasionally flow onto the slope when the shelf-break isobaths turn toward the northeast.

2) Once the hypothetical water particles and suspended materials reached HB11, the downcoast currents at that site must have lasted long enough for the suspended material to enter Newport Canyon (3.5 km from HB11).

3) Once in the canyon, the hypothetical plume water was assumed to upwell near HB13. Upcoast alongshore currents at that site need to flow for a long enough period to carry the suspended material to the nearshore region off the Santa Ana River (3.5 km from the canyon).

4) A contamination window first opened when the hypothetical plume water first reached the Santa Ana River and at the same time currents at HB03 flowed upcoast, effectively carrying the water along the nearshore region off Huntington Beach (2.5 km from the river to HB03).

5) The hypothetical window remained open as long as currents at each of the moorings continued to flow along the pathway toward Huntington Beach.

6) The hypothetical window closed soon after currents reversed direction at any of the moorings along the pathway. At this point the pathway was no longer intact and the source of contamination was cut off. We assumed that the potentially contaminated water remaining in the system at the time of reversal would also flow to the beach, provided currents at moorings past the point of cut-off flowed in the direction of Huntington Beach. Actual window closure times were calculated based on this assumption and on when currents would transport remaining hypothetical effluent to HB03.

For the first two legs of transport, currents at three discrete depths along the shelf edge (HB07 at 30 m, 38 m, and 50 m, and HB11 at 30 m, 38 m, and 48 m) were evaluated to determine the fate of contaminated water from the top, core and bottom of the plume. The plume near the shelf edge lies on average from 29 m to about 50 m below the surface (Chapter 10). Currents over these depths often vary in speed and direction in large part because the California undercurrent spills up onto the shelf and carries the deepest shelf waters upcoast while shallower waters flow downcoast.

All rates and distances of transport were calculated by overplotting the current-velocity time-series data and integrating under these curves. Figure 5-2 shows an example of these curves for a period that met the six conditions of transport. Here, a window of hypothetical contamination begins on July 2 and ends on July 4 GMT when possibly contaminated water from the outfall could have been in the nearshore region off Huntington Beach.

Three assumptions bias the results toward defining more contamination events than may actually occur. First, we assumed downcoast currents at the shelf break always hugged the shelf edge and transported water and suspended materials toward the canyon; this ignores the fact that it is known that downcoast currents do occasionally flow onto the slope when the shelf-break isobaths turn toward the northeast. Second, water within Newport Canyon was assumed to always upwell onto the shelf. Upwelling occurred only intermittently within the canyon as shown by the hydrographic surveys (Chapter 10). Last, we assumed sufficient cross-shore transport mechanisms were always available to bring contaminated water ashore from the nearshore region. Typically, alongshore flow predominates in nearshore environments.

5.3. Results

Currents over the shelf were large enough in duration and velocity to transport plume water from the outfall into Newport Canyon eight times during the summer of 2001 (Tables 5-1a, b, c). Upcoast flow in the nearshore was persistent enough to bring plume water to the nearshore by Huntington Beach four of these times for water near the top of the plume and two of these times for water near the core and base of the plume. During the other times currents flowed downcoast at the nearshore moorings, denying the possibility of transport of effluent upcoast to the beach. For the October 17 event, the upcoast flow rates at HB13 were substituted for HB03 data that were not recorded.

5.3.1. Comparison Between Canyon Events and Beach Contamination Events

No windows of hypothetical beach contamination for both the core and bottom of the plume coincided with beach bacteria events. Only one beach contamination day (July 7) corresponded with flow of upper plume water along the canyon pathway. The specific contamination on the beach was a moderate-sized enterococci and total coliform event from stations 0 to 15N (Chapter 3; Figure 3-2) that spanned three days (July 5 to 7). Note that surfzone bacteria samples were not collected on every day that a window of hypothetical contamination was open. For example, during the window from July 1 to 4 sampling occurred on only two days (Tables 5-1a, b, c).

To account for the possible inaccuracies in representing current speeds along the pathway from discrete mooring sites that were stationed a few km apart, the time windows were broadened by one day on each end of the windows. No more events resulted from the expanded windows at core and bottom plume depths. One more beach contamination event coincided with an expanded window for the upper plume waters. This was an isolated type 1 event at station 3N on July 31. Also, the expanded window around the July 7 event included July 6–another day with beach contamination above AB411 levels.

5.4. Discussion

Minor if any causal association existed between the pathway and beach contamination events during summer 2001. The pathway was open for 4-7% of the days in the study period for the actual and expanded windows, respectively (Figure 5-3). The actual and expanded windows were associated with only 1 to 3 of the 42 days when either a type 1, 2 or 3 contamination event occurred during the study period from June to October 2001.

The most concentrated effluent commonly lies at about 39-m water depth near the shelf edge. Water at this depth could possibly be transported from the outfall along the canyon pathway to the beach two times. No events coincided with these open windows. No more events resulted from the expanded windows at core and bottom plume depths, indicating that the most concentrated plume water did not come to shore via the pathway in any significant amount, if at all. The edges of the plume contain the least concentrated effluent because the contamination diffuses more readily into the surrounding seawater. Therefore the events defined in this study represent times when the least contaminated plume water may have reached Huntington Beach.

The hypothetical beach contamination on July 7 from the canyon pathway is unlikely to produce the beach-wide event at that time. This event was spread across most of Huntington Beach for three days at relatively moderate to high levels of contamination. It is not likely that water from the top of the plume contains this level of concentrated pollution. Also, HB13 currents flowed strongly downcoast until the morning of July 6. This implies that the period of contamination was caused by a source that was polluting the beach for a day prior to the arrival of any possible effluent from the canyon. This event also had high enterococci counts. As shown in Chapter 3, enterococci have been associated with sunlight-induced die-off during the day. This suggests that events that span more than one day require a source that supplies contamination to the beach at least daily if not continuously. Another source must be responsible for most if not all of the contamination during this event.

Possible contamination from the canyon on July 31 was relevant after the original window was expanded one day. The event was isolated and small. The sampling coincided with the last few hours of the expanded window of upper plume water reaching the beach. This type 1 event occurred also on August 1, suggesting that another source is responsible for the contamination.

5.4.1. General Flow Patterns for Alongshore Currents near the Outfall

During summer 2001, currents at the top of the plume obtained magnitudes strong enough to reach Huntington Beach via the hypothesized canyon pathway over twice as often as did water from the core and bottom of the plume (155 hours vs. 67 hours). Current histograms for moorings at the shelf edge revealed that currents at lower and middle plume depths on average flowed upcoast while water near the top of the plume flowed downcoast (Figures 5-4a, 5-4b, 5-4c). Note that the largest mean upcoast currents occurred near the bed while water near the top of the plume usually flowed downcoast. The effect was most strongly seen at HB11 where mean speeds of over 3 cm/s upcoast were obtained at the core and bottom plume depths (at 39 m and 47 m). The strength of upcoast flow with depth decreases from HB12 to HB07.

Hamilton (Chapter 4) also showed that the deepest currents over the outer shelf did not flow downcoast on average unlike the currents in the mid to upper water column (Chapter 4; Figure 4-8). For example, mean currents at HB11 at 45 m flowed almost due west on average, away from the canyon.

The California undercurrent likely played a primary role in retarding the flow of core and bottom plume water into Newport Canyon during the summer. The undercurrent flows along the continental slope upcoast and poleward toward and around the San Pedro Bay shelf. It is the dominant current that flows over the slope on the west margin of North America (Pierce et al, 1996). It has been shown to reach speeds as high as 50 cm/s with a mean speed of about 24 cm/s (Noble and Ramp, 2000; Pierce et al., 1996). Pierce et al. (2000) and Barth et al. (2000) also showed that the undercurrent meanders onshore and offshore at promontories off the Northern California coast and that this flow interacts with shelf currents and the deeper California Current.

Where the slope orientation changes abruptly southwest of Newport Canyon the undercurrent spills onto the shelf and cuts the shelf corner, adding an upcoast component to the flow in the deepest shelf waters. This may result in a shearing of the plume-depth water that transports the deepest waters upcoast away from Newport Canyon. The undercurrent would first come onto the shelf near HB11, where the shelf bends. At HB12 and HB07 the mean flow near the bed was upcoast but weaker than HB11. This was probably due to movement of the undercurrent back toward the slope.

This upcoast flow of deeper waters may be more pronounced during summer. Histogram plots for winter 2002, summer 1999, and winter 2000 reveal that upcoast mean flow is stronger during summer for both years of data (Figures 5-5a, 5-5b and 5-6a, 5-6b). Winter currents in both years at the top and core of the plume flowed dominantly downcoast. This indicates that plume water could reach the canyon more often in winter months. The SAIC 1999 mooring is located near the summer 2001 HB12 site. These results corroborate Chelton’s work (1984) that showed maximum speeds of the undercurrent occurred in late summer to early fall off central California.

5.5. Conclusions

The hypothetical pathway of beach contamination via Newport Canyon is known to exist in part; during hydrographic and plume tracking surveys, plume water was measured in Newport Canyon inshore, but not above the canyon rim or alongshore north of the canyon (Chapter 10 and MEC, 2001). Our measurements show that the pathway does exist, but it is rarely intact (5% of the summer 2001). Also, when the pathway was open it coincided with beach contamination events during only 1 of the 4 open-window periods. Therefore, the pathway is an insubstantial source of beach pollution, if at all. The California undercurrent is partially responsible; when the current spills onto the shelf the deepest plume water near the shelf edge is forced upcoast away from the canyon, and probably more so in summer than in winter.

5.6. References

Barth, J.A., S.D. Pierce, and R.L. Smith, 2000. A separating coastal upwelling jet at Cape Blanco, Oregon, and its connection to the California Current System. Deep-Sea Research II v. 47, p. 783-810.

Chelton, D.B., 1984. Seasonal variablility of alongshore geostrophic velocity off central California. Journal of Geophysical Research, v. 89 (C3), p. 3473-3486.

MEC (MEC Analytical Systems, Inc), 2001. Strategic Process Study, Plume Tracking. June 1999 to September 2000. Final Report, Vol. I: Executive Summary.

Noble, M.A. and S.R. Ramp, 2000. Subtidal currents over the central California slope: evidence for offshore veering of the undercurrent and for direct, wind-driven slope currents. Deep-Sea Research II, v. 47, p. 871-906.

Pierce, S. D., R.L. Smith, and P.M. Kosro, 1996. Observations of the poleward undercurrent along the eastern boundary of the mid-latitude Pacific. Transactions, American Geophysical Union, EOS 77 (46), F345.

Pierce, S. D., R.L. Smith, P.M. Kosro, J.A. Barth, and D.C. Wilson, 2000. Continuity of the poleward undercurrent along the eastern boundary of the mid-latitude north Pacific. Deep-Sea Research II, v. 45, p. 811-829.