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Coastal Transport Processes

Noble, Xu, Rosenfeld, Largier, Hamilton, Jones, and Robertson, 2003, Huntington Beach Shoreline Contamination Investigation, Phase III: U.S. Geological Survey Open-file Report 03-62, version 1.0

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1. Background
2. Hypotheses
3. Objectives and Methods
4. Measurement Program
5. Major Findings:
5.1. Surfzone Bacterial Contamination Patterns
5.2. Outfall Plume Tracking
5.3. Coastal Transport Processes
6. Transport Processes
7. Conclusions
8. References
9. Acknowledgements


6. Coastal Transport Processes and Their Relationship To Significant Surfzone Bacterial Patterns

The temporal relationships between the three categories of surfzone bacterial exceedance events defined earlier, section 5.1.2, and the various coastal oceanographic processes were used to determine the probability that significant levels of bacteria from the outfall were transported to the beach.

6.1. Subtidal Cross-shelf Transport, Newport Canyon
Occasionally, the along-isobath subtidal currents in the near shore region flow in the opposite direction to currents over the middle and outer shelf. This provides a current regime in which plume material could be transported downcoast towards the Newport Canyon and then transported up coast by the reversed nearshore currents. Because this plume water is relatively close to shore, other coastal ocean processes may carry this water to local beaches.

In order to address this possibility, a transport pathway through Newport Canyon was postulated. The pathway assumed downcoast currents always hug the shelf edge and transport water and suspended materials toward the canyon (Figure 18.  Hypothetical transport of effluent plume through Newport Canyon and table Figure 18 - 544KB PDF file ), If the downcoast flow is persistent, water from near the outfall can reach the canyon. We hypothesize that canyon upwelling will then transport this subsurface plume water onto the adjacent shelf. If upcoast flows exist at the time, this plume water will then be transported to the nearshore water off Huntington Beach. The possibly contaminated water will continue to remain offshore of Huntington Beach as long as the transport pathway remains intact. Various nearshore, cross-shore transport processes could bring the bacteria to the beach. This is a very idealized pathway that favors transport of plume material through Newport Canyon. It ignores the fact that the plume often exits the shelf in the downcoast direction and moves into much deeper water. It also ignores the possibility that canyon upwelling may not exist when the pathway is in existence.

We assessed the currents along the pathway and found that water from near the outfall could have reached Newport Canyon 7 times during the summer of 2001 (Figure 18.  Hypothetical transport of effluent plume through Newport Canyon and table Figure 18 - 544KB PDF file ). The nearshore upcoast flow was persistent enough in 4 of these events that plume water could have been carried to Huntington Beach. During the postulated time windows when the plume water would have been offshore of Huntington Beach, only one surfzone measurement category was above the AB411 standards. Broadening the window by one day on each side yielded two more possible contamination days. However, these events, which occurred on the first day of an expanded window, were at the end of an earlier 3 day beach contamination event. Hence it is unlikely they were associated with the Newport Canyon pathway.

The potential for transport through Newport Canyon did not commonly occur in the summer 2001. The pathway was only open for 7-13% of the days in the study period for the actual and expanded windows, respectively. The actual and expanded windows were associated with only 1 to 3 of the 42 days when either a Type 1-3 contamination event occurred during the study period from June to October 2001.

6.2. Diurnal and semidiurnal transport pathways
As individual mechanisms, the diurnal wind-driven and barotropic tidal oscillations can move cold sub-thermocline water toward the shore, although the distance of the excursions is relatively small. The excursion of these motions separately is insufficient to move plume water into the nearshore in less than two days, unless assisted by some other transport mechanism. Furthermore, the portion of plume water most likely to be transported by the diurnal seabreeze mechanisms is from the top of the plume, which has much smaller bacterial concentrations than the core of the plume. Any piece of plume water transported by these mechanisms will mix with the receiving water nearshore, resulting in further dilution of bacterial concentrations.

Historically, the beaches are most likely to be contaminated by bacteria during spring tides. It is not likely that this contamination is predominately caused by cross-shelf transport of the outfall plume by large internal tides. The strongest internal tides did not tend to occur during spring tides over the 4-month study period.

6.3. Coastal-ocean event transport pathways
Even though average diurnal and semidiurnal currents separately may not bring the plume to shore, these processes may reinforce each other and carry the outfall plume toward the beach. The largest cold-water pulses, forced by a combination of diurnal and semidiurnal processes, occurred near the end of July 2001; these excursions were still less than 4 km (half the distance from outfall to shore). In an effort to relate the appearance of these cold events to the occurrence of bacterial concentrations exceeding AB411 standards at the beach, a time series of cool, near-shore events was constructed (Figure 19. Nearshore surface and bottom temperatures and the 2-week average of temperature at the top of the offshore effluent plume computed from a plume model. Note that there are several instances when the bottom temperatures are similar to those at the top of the plume. Figure 19 - 275KB PDF file ). A cool event was defined to be when the energetic cross-shore current pulses brought water colder than 12 oC into 30 m (98 ft) water depth or colder than 13 oC into 15 m (49 ft). A cool event was also defined as when the nearshore temperature was as cold or colder than the temperature at the top of the offshore effluent plume, irrespective of a transport pathway. The July 23-26 cooling events, as judged by both of these criteria, were the largest cooling events in the summer of 2001.

The series of cool events was compared to the dates when types 1, 2 and/or 3 contamination events were found along the local beaches. Most of the nearshore cooling events did not coincide with contamination events (Figure 20.  Days on which bacterial events occur are shown together with cruise days; and occurrence of cold water.  The day's higher high water as measured at Los Angeles is shown at the bottom. Figure 20 - 202KB PDF file ). Only 2 of the 17 cooling events occurred on days when AB411 standards were exceeded in the surfzone in the 3N-12N region. Most of the large cooling events at the end of July happened after a nearly week-long period of beach contamination.

6.4. Sediment transport pathways
Sediment transport was one of the hypothesized mechanisms that could bring fine particles that may have outfall bacteria sorbed onto them to the beach. However, there is no apparent correlation between resuspension and beach contamination events (Figure 17.  Time-series of water clarity (top panel) and estimated bedshear stress (bottom panel). The transmissometer was fouled after early August. The vertical bands in the bottom panel designate the beach contamination events.  Shear stress values within or above the horizontal band are able to resuspend very fine sands and silts. Figure 17 - 174KB PDF file ). The current flows during the few periods when beach contamination events did coincide with high estimated shear stress could not carry this material to shore.

6.5. Nearshore transport pathways
Water temperature in the surfzone was used to diagnose transport pathways associated with high bacteria events. If any routes for transporting sub-thermocline (and thus potentially wastewater plume) waters through the nearshore, either natural or associated with AES, were a primary cause of beach contamination events, then one would expect an association of contamination events with specific water temperatures. If cold-water intrusions mediated contamination events, then one would expect high bacteria counts to be associated with colder waters. Alternatively, if the power plant mediated contamination events, then one would expect high bacteria counts to be associated with warmer waters. None of these associations is apparent in the data. The results of the 2002 study of bacteria in the AES plant intake and discharge are awaited to better resolve the viability of that transport pathway.

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U.S. Department of the Interior, U.S. Geological Survey, Western Region Coastal and Marine Geology
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Maintained by: Michael Diggles
Created: January 23, 2003
Last modified: July 7, 2005 (mfd)