Skip Navigation Links Home 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 |
5. Major Findings (Continued)5.3. Coastal transport Processes5.3.1. Subtidal TransportWhen 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). ![]() 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. ![]() 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 ( ![]() 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 ![]() 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 ( ![]() On the outer shelf, barotropic tidal current ellipses are aligned halfway between the along and cross-shelf directions ( ![]() ![]() 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) ( ![]() 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 ( ![]() 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., ![]() ‡ 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 ( ![]() ![]() 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 ![]() 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 ( ![]() 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 ( ![]() |
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)