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Major Findings 5.3

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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Abstract
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

Contacts

5. Major Findings (Continued)


5.3. Coastal transport Processes

5.3.1. Subtidal Transport
When currents associated with the tides and sea breeze are removed from the records, the remaining flows are substantial and play the major role in the longer term dispersal of the outfall plume. These low frequency or subtidal flows have patterns that are caused by circulation processes over the larger scale coastal region of the southern California Bight. In the summer, there is a substantial down-coast (towards San Diego) mean current over the San Pedro shelf that has a maximum near the surface on the outer shelf. This down-coast flow decreases in magnitude with depth and toward shore. It is generally unrelated to local winds over the shelf. However, there is some indication that these subtidal fluctuations in along-coast flow are related to wind forcing off central Baja California (via coastal trapped wave propagation). Figure 10. Mean subtidal velocity vectors at the indicated depths for Figure 10 (176KB PDF file) shows the mean current vectors for the period of summer 2001. Surface mean currents of 10 to 12 cm/s (0.2 mph) on the outer shelf can transport material down-coast a distance of order 10 km (6 miles) in one day.
At depths below about 70 m (230 ft) over the San Pedro continental slope, seaward of the outfall, the flow is predominantly up-coast (towards Palos Verdes) and constitutes an undercurrent. This undercurrent occasionally rises to shallower depths for a few days and floods the outer shelf with up-coast currents. Figure 10. Mean subtidal velocity vectors at the indicated depths for Figure 10 (176KB PDF file) shows that at depths below about 30m (98 ft) on the outer shelf, the mean flows are directed weakly up-coast.

Imposed on this mean flow pattern are fluctuations with periods of 7 to 20 days that have a magnitude similar to the mean flow. They are largest at the surface and on the outer shelf and decrease with depth and towards shore. In the upper layers, the currents are parallel to the general trend of the coastline, but in the lower layers the currents follow the trend of the depth contours (isobaths). Shorter period (7 day) fluctuations dominate in the near shore (depths < 15 m (49ft)). Maximum currents can exceed 60 and 30 cm/s (1.3-0.7 mph) on the outer and inner shelf, respectively. Given the long periods of the fluctuations, the low frequency surface currents can be set in the same along-coast direction for periods on the order of weeks.

The fluctuations across the shelf and with depth are generally all in the same direction at any given time. However, in the near shore, alongshelf current fluctuations are found to be closely related to the alongshore winds and somewhat disconnected from currents on the middle and outer shelf. This means that flows in the near shore may be in the opposite direction to flows over the middle and outer shelf. This provides a current regime in which plume material could be transported down coast towards the Newport Canyon by flows over the outer shelf, and then be transported up coast by the reversed nearshore currents.

At the depths of the outfall plume on the outer shelf (~30 to 55 m (98 to180 ft)), the combination of weak mean currents and large along-shelf fluctuations leads to the plume being equally likely to be transported up- or down-coast during this summer period. These subtidal currents are the most effective of all the transport mechanisms for dispersing the main body of the plume. This dispersal is primarily along the isobaths rather than across the shelf. The subtidal cross-shelf currents are small and apparently not a major factor in the movement of cold water towards shore. This is evident by the relative stability of the cross-shelf distribution of temperature at longer periods. However on occasion, usually when the flow regime changes from up- to down-coast or vice versa, the lower layer isotherms can make relatively abrupt movements to and from the near shore over periods of a few days. The maximum subtidal cross-shelf excursion was on the order 2 to 3 km (1.2 to 1.9 miles) over a period of 2 days. These longer term changes in the temperature field influence the cross-shelf transport by internal tides and the sea breeze, because they are the background upon which the shorter period excursions operate. For example, when warm water extends to greater depth (August and September), the internal tide is less likely to lead to cold-water intrusions into the nearshore.

5.3.2. Diurnal and Semidiurnal transport

5.3.2.1. Seabreeze and diurnal currents
The moored observations of current and temperature exhibit strong diurnal (24 hour period) fluctuations, with a daily temperature minimum observed over the inner shelf in the early hours of the morning. These fluctuations are forced by diurnal winds and influenced by tidal currents. Wind records at coastal airports (e.g., John Wayne Airport) describe a clear diurnal wind cycle, with maximum onshore winds (sea breeze) in the afternoon (Along-shore and on-shore wind components at the indicated meteorological buoy and airports. Wind components use the oceanographic convention of direction towards. Figure 11 - 97KB PDF file ). Away from the coast, at mooring HB07, winds over the outer shelf also exhibit a clear diurnal signal. However, alongshore diurnal winds are stronger than cross-shore.

This regular diurnal wind drives regular diurnal movements of water towards and away from the coast. The wind forces the surface water directly and the near-surface currents (uppermost 5m(16 ft)) follow the wind at the offshore buoy. This causes an onshore movement of near-surface waters in the afternoon. At the same time, there is an associated offshore movement of water at depth, typically between 10 and 30 m (33 and 98 ft) below the shallow summer thermocline. This pattern results in near shore maximum temperatures in the late afternoon or early evening. At nighttime this flow structure reverses and near-surface water moves offshore while deeper cold waters move onshore, resulting in the observed temperature minimum around dawn. This wind-driven diurnal oscillation thus moves cold sub-thermocline water across the shelf and provides a regular mechanism for transporting plume waters towards the shore. The vertical structure in the diurnal cross-shore currents is summarized in Figure 12. Depth profiles of cross (U) and along (V) shore velocities from a frequency domain EOF analysis of diurnal period currents for the indicated moorings. Figure 12 (323KB PDF file), which shows velocity maxima near-surface and at depth with a sharp change in phase around 5-10m (16 -33 ft). Over the outer shelf, the wind-driven diurnal currents die off well above the seabed. However, in water depths less than about 30 m (98 ft), this wind-driven diurnal circulation interacts with the bottom.

Transport in both near-surface and deeper layers must go to zero at the shoreline. Thus, over the inner shelf there is a vertical flux associated with these cross-shore flows, resulting in upward movement during the night and downward movement during the day. At times when the diurnal wind-forced fluctuations are in phase with cross-shore internal tidal fluctuations, the transport of material across the shelf is enhanced.

5.3.2.2. Semi-diurnal currents
The semi-diurnal (12 hour period) tidal currents are composed of two distinct current fields. The individual barotropic tidal constituents have a constant amplitude with time and a uniform velocity with depth. Within a tidal band, such as the semidiurnal band, the separate semidiurnal tidal constituents M2 and S2 can reinforce or partially nullify each other. This causes the spring/neap cycle that is most clearly seen in sea level records. The individual constituents of the baroclinic, or internal, tidal currents have amplitudes that vary over periods of days to a week or two. They also have a pronounced shear with depth. Usually, internal tidal currents near the surface and near the bed flow in opposite directions.

The tidal currents have strong oscillatory motions which result in little net movement over a tidal cycle. An ellipse (Figure 13. M2 tidal current ellipses for barotropic and baroclinic tides at the bed. The strongest tidal curretns flow parallel to the long axis of the ellipse. Figure 13 - 224KB PDF file ) can describe the speed and direction of the tidal currents over a cycle. The strongest currents flow parallel to the long axis of the ellipse. The length of the major axis represents the strength of the flow. For semidiurnal tides, the currents 3 hours later are weaker and flow parallel to the ellipse‰s minor axis. Six hours later, the currents again flow parallel to the long axis of the ellipse, but in the opposite direction of the previous flow. Twelve hours later, the currents return to their original direction and speed.

On the outer shelf, barotropic tidal current ellipses are aligned halfway between the along and cross-shelf directions (Figure 13. M2 tidal current ellipses for barotropic and baroclinic tides at the bed. The strongest tidal curretns flow parallel to the long axis of the ellipse. Figure 13 - 224KB PDF file ). Internal tidal current ellipses are aligned approximately perpendicular to the coast. Clearly, each average cross shore tidal current constituent, with speeds around 2-3 cm/s (0.7-1.ft/s), will not move plume water or suspended effluent material from where it is discharged at the shelf break to the nearshore. Water is transported onshore by an average flood tide about 0.4 km, an insignificant portion of the distance from the outfall to the shore. If all tidal constituents reinforce each other, tidal currents can flow across the shelf with combined speeds of 5-10 cm/s. Thus water is transported onshore by an average flood tide 0.9 to 1.5 km (0.6 to 0.9 mile), less than 1/5 of the distance from the outfall to the shore. When the tidal currents ebb, the average tidal current will carry the water about the same distance offshore, causing little net movement of suspended material. As one nears the coast, cross-shelf transport by tidal currents is even more limited because barotropic tidal currents flow parallel to shore (Figure 13. M2 tidal current ellipses for barotropic and baroclinic tides at the bed. The strongest tidal curretns flow parallel to the long axis of the ellipse. Figure 13 - 224KB PDF file ); only internal tidal currents can transport water and suspended material toward the coast.

However, the semidiurnal internal tidal currents do not have constant amplitude over the summer months. They can be larger or smaller than their average amplitude for periods of days to weeks. It is interesting to note that the times when internal tides are large are not tied to the spring/neap cycle. If daily wind or other diurnal forcing reinforces one of the two semidiurnal current pulses, the internal tides may also have a pronounced mixed tidal signal. That is, every other tidal current pulse can be noticeably larger than the preceding pulse. During the summer of 2001, large internal tidal currents appeared at the beginning and end of July and in late August. Cross-shore current speeds near the bed at the end of July were larger than 10 cm/s.

5.3.2.3. Cross-shore transport events in coastal ocean currents
Occasionally, diurnal or tidal processes cause large shoreward movements of cold water. The 14oC isotherm is usually found within 30 m (98 ft) of the sea surface. Because it is nearly the same temperature as water at the top of the plume, one can use this isotherm to estimate cross-shore excursions of this portion of plume water. The primary shoreward excursions are due to a combination of subtidal, diurnal seebreeze and tidal movements. The diurnal excursion of the 14oC isotherm is at most 3 km (1.6 miles) (Figure 14. 3-HLP time series of the offshore distance from the shoreline, along the main transect, of the indicated isotherms where they intersect the bed. No line indicates the isotherm has surfaced. The top panel is the along and across-shelf wind components from HB07. Figure 14 - 205KB PDF file ). Although this onshore transport is rapid and a significant fraction of the distance from the outfall to the beach (7 km (4.5 miles)), it is followed by an equally strong offshore transport. While a single diurnal oscillation can transport waters from the top of the plume to and from the mid-shelf regions, no piece of this plume water can move onshore in less than 2-3 diurnal cycles (i.e., 2-3 days). Further, any piece of water from the top of the plume will mix with receiving waters nearshore, resulting in further dilution of any possible plume bacteria carried shoreward by these excursions.

At any one time, a large shoreward movement of cold water along the bed may occur because the seabreeze, diurnal and semidiurnal tidal forcing have the proper phasing to reinforce the cross-shore excursions due to the separate processes alone. The largest series of these events occurred near the end of July. A close examination of one of these strong cross-shore current pulses shows that they can move cool water along the seabed from the shelf break toward the coast ( Figure 15. Temperature profiles for the rising, peak and falling portion of a cold pulse event on July 24. Figure 15 - 96KB PDF file ). During a 7-hour period (half a semidiurnal tidal cycle) in the late night and early morning hours, 12 oC water (which represents the core of the effluent plume) migrated about 4 km across the shelf, from 40 m (131 ft) water depths to about 15 m (49 ft). Simultaneously, warm surface waters were displaced offshore. About 6 hours later, the cool water moved back offshore and the warm surface waters returned. This pattern repeated itself for several days.

It should be noted that water and suspended materials near the shelf break were transported along, as well as across, the shelf. The alongshore currents in this pulse had speeds of 10-30 cm/s. Hence, water and suspended materials from the shelf break were displaced up or down coast as they moved toward or away from the beach.

5.3.3. Nearshore Cross-Shore Transport
As one approaches the shoreline, the strength of cross-shore transport weakens, primarily because it is not possible to move water through this coastal boundary. Barotropic tides (e.g., Figure 13. M2 tidal current ellipses for barotropic and baroclinic tides at the bed. The strongest tidal curretns flow parallel to the long axis of the ellipse. Figure 13 - 224KB PDF file ) and wind-driven currents are directed parallel to the shoreline and transport in the cross-shore direction is weak or absent. Off Huntington Beach, there are two primary routes that may allow the ready transport of cold plume waters through the nearshore into the surfzone:
  ‡ Stratified shoreward intrusion of cold water from the nearshore region;
  ‡ Entrainment of plume water by the power plant, with discharge as a nearshore thermal plume.

Cold water intrusions into the nearshore The constraint of the coastal boundary is mitigated where there is stratification. Cross-shore currents can persist closer to the shoreline, as it is possible for near-surface water to be flowing in one direction while near-bottom water flows in the opposite direction (with no net transport into the coast). Thus, cold water present at 15-m (49 ft) water depth may surge into the shallow nearshore and surf zone, as observed in water depths of 2.5 m (8 ft) and less (Figure 16. Temporal variations in near-surface and near-bottom temperatures at mooring in 15m, 10m, 7.5m, 2.5m, and 0.5m. Figure 16 - 93KB PDF file ). Low temperatures persist for a few hours before these cool waters are replaced again by the warmer surface waters. While these cold-water intrusion events can be due to many processes, including internal tides, the 24-hr diurnal period dominates the nearshore record. The specific form and penetration of the cold-water intrusion into the nearshore depends on the nature of the forcing and on ambient stratification, but many events evolve into internal bores. These bores intrude into the nearshore as a marked thermal front and with a propagation speed of 5-10 cm/s (taking an hour or two to move from 5m (16 ft) water depth into the surf zone). On occasion, these intrusions move right into the surfzone, resulting in surfzone temperature drops of up to 3oC in an hour (Figure 16. Temporal variations in near-surface and near-bottom temperatures at mooring in 15m, 10m, 7.5m, 2.5m, and 0.5m. Figure 16 - 93KB PDF file ). However, the coldest water observed in the surf zone during August and September is 17.5oC, well above the 14oC temperature taken as an indicator of the possible presence of plume waters.

AES power plant
Due to its location, the AES power plant has been suspected of directly contributing to the contamination of the beach between 3N and 9N. The power plant pumps in cooling water at 8.4 m (27 ft) water depth and then discharges heated water at 6.5 m (21 ft) water depth. Three potential roles have been identified.
  a. The thermal stratification could enhance cold-water intrusions into the nearshore: During the period of plant operation in late summer (including both reduced cooling water flows in July-August and stronger flows in September-October), there is no evidence of a difference in the currents at 10 m offshore of the power plant and at a comparable 10-m location 2 km (1.2 miles) further upcoast. Further, the calculated heat load and observed thermal plume stratification are not sufficient to significantly alter the stratification and cold-water intrusions dynamics.
  b. The pumping of cooling water through the power plant enhances shoreward transport of bottom water from 8.4 m (27 ft) water depth: As is evident in Figure 16. Temporal variations in near-surface and near-bottom temperatures at mooring in 15m, 10m, 7.5m, 2.5m, and 0.5m. Figure 16 (93KB PDF file), colder water reachesthe depth of the power plant intake and discharge (see thermistor at 7.5 m (25 ft) water depth) more often than it intrudes into the surf zone (see thermistors at 2.5 m (8 ft) and 0.5 m (1.6 ft) water depths). Once entrained by the intake or discharge jet, this potentially contaminated cold water forms part of the surface thermal plume that is observed nearshore and can be expected to move into the surf zone through buoyant spreading or under the action of afternoon winds and other ambient processes. Surf zone temperature observations showed limited effects of the thermal plume, indicating that these waters had been well mixed before impacting the beach (dilutions of at least 10-fold). While the power plant was not operated in early summer, the observation of beach contamination events during that time period indicates that the power plant does not play an essential role in beach contamination.
  c. The power plant may act as a source or conduit for bacteria moving from land to ocean. The role of the power plant in beach contamination is presently the subject of a California Energy Commission study, aiming to assess this question through extensive sampling of bacteria concentrations in the plant during 2002. The possible role of the plant in the transport of wastewater to the beach is being assessed through concurrent sampling of intake water temperatures, salinity, ammonia and fecal bacteria concentrations.

5.3.4. Sediment Resuspension
Hourly data of wave heights, wave periods and near-bed current velocities measured at site HB07, about 1 mile west of the outfall diffuser at 60 m of water, are used to calculate bed shear stress for the summer of 2001 (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 estimated bed shear stress intermittently surpassed the critical shear stress of 0.5 dyne/cm2 (Gardner, 1989) and caused resuspension of the fine material (very fine sands and silts) from the bed. Some estimated amplitudes were even occasionally higher than a more conservative critical value of 1.0 dyne/cm2, (Maa et al., 1993). Both water clarity data and video footages taken during such two events in June and July confirmed the occurrences of resuspension and therefore the validity of the estimated values of bed shear stress.

Whether the resuspended sediments can be brought to the nearshore and then to the surfzone depends on the flows on the shelf. During the three events when beach contamination and bed resuspension coincide (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 flows at all three stations across the shelf were generally along-shore. Therefore, most suspended material near the outfall would have been transported up- or down-coast along the shelf break, not across the shelf to the beach or surfzone.

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U.S. Department of the Interior, U.S. Geological Survey, Western Region Coastal and Marine Geology
URL of this page: http://pubs.usgs.gov/of/2003/of03-62/objectives.html
Maintained by: Michael Diggles
Created: January 23, 2003
Last modified: March 10, 2005 (mfd)