CHAPTER 9. NEARSHORE CIRCULATION AND TRANSPORT PATHWAYS
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Chapter 1. Introduction and Background
Chapter 3. Surfzone Bacteria Patterns
Chapter 4. Subtidal Circulation Pathways
Chapter 5. Newport Canyon Transport Pathway
Chapter 7. Tidal Transport Pathways
Chapter 8. Sediment Resuspension and Transport near the OCSD Outfall
Chapter 9. Nearshore Circulation and Transport Pathways
This chapter addresses the circulation in the nearshore and the possibilities for outfall-contaminated water to be transported into the surfzone (Figure 9-1). The previous chapters have addressed patterns and processes over the shelf in the vicinity of the outfall and inshore over the shelf. In addition to being transported from the outfall on the outer shelf to the nearshore, wastewater plume waters need to be transported through the nearshore and into the surfzone before their bacterial load will be detected at the beach sampling stations and before associated pathogens will pose a threat to the health of swimmers and surfers. In this study, the “nearshore” is understood as that region where the water depth is less than the thermocline depth over the shelf–so that, in the absence of thermocline motions, the water would be weakly stratified and uniformly warm. By this definition, the nearshore off Huntington Beach typically extends out to about 15-20 m in summer and a bit deeper during the fall. Cross-shore transport within the surfzone and across the breaker line was not addressed directly in this study.
In spite of the massive effort in sampling fecal indicator bacteria off Huntington Beach in 2001, the intensity of sampling of bacteria away from the shoreline is inadequate to address patterns and processes that exhibit small space and time scales (this is particularly true in the nearshore where space and time scales are smallest). The approach of investigating onshore transport of cold waters, adopted here, is based on the observation that the wastewater plume is mixed into cold sub-thermocline waters and trapped beneath the thermocline (Chapter 10). Model and observations indicate that the mixed wastewater plume is at temperatures below 14°C. As nearshore waters are much warmer in summer and fall (18-24°C) (Figure 9-2), the presence of cold water nearshore indicates onshore transport of sub-thermocline water and the possibility of plume waters being transported into the nearshore.
The approach in this chapter, then, is to investigate when and where and how often such cold water is observed in the nearshore and to identify and characterize the transport processes, including possible mixing and dilution enroute. This addresses the possibility of wastewater being carried through the nearshore and into the surfzone, but it does not address whether there were any such events. The latter issue is addressed in Chapter 3, where an attempt is made to link observed bacteria events along the shoreline with cold-water transport events.
There are two primary reasons why one does not expect to see this sub-thermocline cold water moving into the nearshore on a more regular basis.
(1) Polarization: Currents directed perpendicular to the shore get weaker as one approaches the shore, going to zero at the shoreline (unless they are very short-lived, such as waves, or very localized, such as rip currents). Horizontal excursions of cold water are thus short, being limited by the strength and duration of onshore currents.
(2) Potential energy: Substantial energy is required to lift the dense cold water up from below the thermocline to the surface. The vertical excursions of sub-thermocline water are thus limited by the energy available (relative to the depth and strength of the thermocline/pycnocline). No potential energy is required to move cold water inshore to the depth where the thermocline intersects the bottom (e.g., 20 m), irrespective of how close this is to shore, but substantial potential energy is required to raise this cold water up as it intrudes into the nearshore, irrespective of the horizontal excursion from the 20-m isobath to the surfzone.
Where cold water enters the nearshore and surfzone over an extended length of the beach, it does so through onshore flow of cold water. However, 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. This is clearly seen in spectra of depth-averaged current velocities from nearshore moorings (Figure 9-3a) and in the orientation of barotropic tidal ellipses (Chapter 7), and is further discussed by Carrillo et al (2004). While depth-averaged cross-shore and alongshore currents at HB07 exhibit similar strength for periods up to 2-3 days, cross-shore tidal and diurnal currents at HB05 and HB03 are about half the strength of alongshore currents, and inshore at HBN2 tidal-diurnal cross-shore currents are several times weaker than alongshore. Inshore of 10 m, cross-shore tidal currents are very weak (about 1 cm/s, e.g., at AES2) corresponding to horizontal excursions of order 0.5 km and 1 km for semidiurnal and diurnal tides, respectively. Inshore of 10 m, only short-lived depth-averaged cross-shore currents (periods of a few hours or less) are as strong as alongshore currents. However, stronger cross-shore currents are observed nearshore in the presence of stratification, where vertical shear allows surface water to flow in one direction while near-bottom water flows in the opposite direction, with no net transport into the coast. This can be seen in spectra of current shear (Figure 9-3a). For all moorings offshore of 10 m, cross-shore and alongshore current shear is comparable in the tidal-diurnal band and at higher frequencies, although cross-shore flows are a bit stronger in the internal wave band (particularly nearshore). But, closest inshore at AES2, even sheared cross-shore flows are much weaker than alongshore flows, due to the shallowness of the water column (which limits vertical shear), in addition to the proximity of the shoreline. A summary of the relative weakening of cross-shore flow nearshore (i.e., polarization) is given in Figure 9-3b. In the nearshore, alongshore currents are stronger and more persistent.
Stratification is thus key to allowing cross-shore transport in the nearshore, and cold water will generally be transported into the nearshore as a near-bottom intrusion, with concurrent offshore flow near-surface. This intrusion of cold water actually creates or strengthens the stratification in a water column that would have been isothermal or weakly stratified. The presence of strong thermal stratification and the associated cross-shore shear flows in the nearshore are thus largely due to near-bottom intrusions of colder water. These intrusions are primarily associated with three processes that extend from offshore into the nearshore:
· internal tides (Chapter 7),
· wind-driven daily oscillations (Chapter 6), and
· subtidal upwelling effects (Chapter 4).
In addition to shoreward intrusions of cold water through natural processes driven by tidal or wind forcing, human activity may be important in the form of the cooling water pumped into and discharged from the Huntington Beach power plant. The power plant draws in cool near-bottom ocean water at an intake on the 8-m isobath and discharges heated water at an outfall on the 6.5-m isobath (Figure 9-4). The water discharged is warmer than the surface ocean water and thus rises to the surface and forms a thermal plume floating on the ambient ocean water. This thermal plume spreads radially from the discharge point and may easily enter the surfzone as well as being easily moved by winds, e.g., onshore by afternoon sea breezes. Thus, the cooling water system offers a way to lift up cold water (that may be contaminated) as well as allowing an onshore shear flow.
9.2. Cold-water Intrusions into the Nearshore
The presence of cold water at the 15-m mooring HB03 has been explained in terms of subtidal upwelling (Chapter 4), onshore movement of near-bottom water due to the daily sea-breeze wind cycle (Chapter 6), and onshore movement of near-bottom water due to internal tides (Chapter 7). These time scales of cold-water intrusion are evident in Figure 9-2 and in spectra of near-bottom temperatures in the nearshore (Figure 9-5a). An example of subtidal intrusion was observed between September 10 and 17, when colder temperatures persisted in the nearshore for several days, concurrent with a general uplift of the thermocline over the whole shelf (Chapter 4). At other times, nearshore water was warm, and cold-water intrusions were brief, occurring once or twice a day. However, in the nearshore the diurnal signal was most evident (Figure 9-2), with only brief periods where the semidiurnal signal was strong (e.g., at HBN2 on September 3-4). The increasing importance of diurnal fluctuations near-surface and nearshore is also illustrated in Figure 9-5b. Semidiurnal fluctuations are strongest on the thermocline, being weak above the thermocline, whereas diurnal fluctuations are strong in the thermocline and throughout the surface layer, specifically nearshore.
In Chapter 4 subtidal flow and associated cooling of the shelf is described. These subtidal upwelling events also flood the nearshore with cooler water (for example, in Figures 4-14, 6-8, and 6-9); one can see the concurrence of cool periods at HB01 (10-m isobath) with cool periods offshore. The minimum temperatures in the nearshore during these persistent cool periods is much the same as the minimum temperatures associated with tidal-diurnal cold-water intrusions and the absence of stratification results in weak tidal-diurnal variability during these periods. These events take a few days to set up the cold nearshore temperatures, as deeper water is slowly advected onshore. Further, it is shown in Chapter 4 that these subtidal cold events are associated with periods of southward flow over the shelf. So, while the shelf is inundated in cold water and the 14oC isotherm may reach the nearshore (Figure 4-14), this cold water has typically come from further north.
Short-lived intrusions of cold water occur diurnally and semidiurnally in the nearshore (Figures 9-2 and 9-5b), with low temperatures being observed for a few hours before the cold water drains back down to thermocline depths and the nearshore is again filled with warmer surface water. These cold-water intrusion events are due to a combination of semidiurnal internal tides (Chapter 7) and the daily wind-driven oscillation (Chapter 6).
At the latitude of Huntington Beach it is only the semidiurnal tides that can generate internal tides. This semidiurnal tidal signal is strong offshore, specifically exhibiting a temperature signal at thermocline depths (Figure 9-5b) and a near-bottom cross-shore velocity signal at moorings offshore of the 10-m isobath (Figures 9-3a and 9-6). These internal tide waves can be expected to run up to depths shallower than the still thermocline, resulting in near-bottom intrusions of cold water in the nearshore. This was observed at 10 m, but coherent run-up events are rare at 5 m. Time series of near-bottom velocities at nearshore moorings (Figure 9-6) illustrate the breakdown of the internal tide signal. At the 15-m mooring, a clear signal was observed with a cross-shore velocity signal of amplitude of about 10 cm/s, whereas at the 6.5-m mooring there is no obvious semidiurnal signal.
This demise of the semidiurnal signal is also seen in spectra for velocity (Figure 9-3a and 9-3b). Inshore of 10 m there is a rapid dissipation of cross-shore flow energy in the internal wave frequency band (Figure 9-7): an average energy density of 6.7 J/m at the 10-m isobath is reduced to only 4.2 J/m at the 6.5-m isobath.
Given the regular onshore winds in the afternoon (“sea breeze”) and weaker or offshore winds after midnight (“land breeze”) (Figure 9-8), one may expect a regular wind-forced onshore movement of surface water and nearshore thermocline depression during the afternoon and evening, followed by a rebound of the thermocline, onshore movement of cold bottom water, and offshore movement of surface water after midnight. As this cycle would be phase-locked to the time of day, a canonical-day pattern was calculated for temperatures, currents, and wind in the nearshore (Figure 9-8). The “canonical day”, or average day, is obtained by averaging all values from the same time of the day; this 24-hour series of average values then defines the canonical, average, or typical daily pattern. The nearshore wind pattern is calculated from 2002 data, as there were no shoreline wind data for 2001 and the sea-breeze cycle should be similar from year to year (at least in phase, if not strength). Coldest water is found in the nearshore during the early morning (Figure 9-8), preceded by a period of onshore flow and onshore wind. While this cycle is statistically weak (large standard deviation), a consistent cycle is seen at all nearshore moorings, and this cycle matches the expected onshore swash of cold bottom water following the weakening and reversal of the sea breeze. Further, this cycle matches the diurnal patterns observed at offshore moorings (Chapter 6).
While the diurnal cycle in nearshore temperatures may at first appear to match daily warming and nighttime cooling, typical daytime surface heat fluxes (100-1000 W/m) are too weak to account for the strength and depth of warming, and this direct warming effect would not account for the associated diurnal pattern of currents (Figures 9-3a and 9-8). Nevertheless, in the surfzone this surface heating may drive a significant thermal cycle of amplitude of 1-2°C, as this water is shallow (assuming that water is resident in the surfzone for a few hours); given this, one cannot interpret the canonical day pattern in the surfzone as evidence of a diurnal cold-water intrusion, as one can for the nearshore outside of the surfzone. Likewise, one must interpret this signal with care at moorings near the power plant thermal discharge. There is a day-night variation in thermal discharge from the power plant which results in a diurnal cycle in near-surface stratification at mooring T5; however, this power plant effect is limited to the vicinity of the power plant outfall and the associated heat flux is too weak to effect the large-scale nearshore pattern observed. Outside of the surfzone and away from the power plant discharge, this diurnal pattern suggests a regular pathway into the nearshore, but it should be noted that the associated velocities are weak (order 2 cm/s) and the horizontal excursion is thus short (order 1 km). In other words, this diurnal swash may be able to move cold water through the nearshore, but it can only do so if the cold water is already in the outer nearshore at the beginning of the cycle (i.e., inshore of the 15-m isobath).
Both the internal tide run-up and the wind-driven swash of cold water into the nearshore suggest that observed temperature changes in the nearshore should be explained primarily by the onshore advection of cold water from further offshore. This argument has been made by Boehm et al (2002), who found that cross-shore advection dominated the observed temperature changes at the 18-m isobath in data from the summer of 2000. However, this thermal balance breaks down in the nearshore. Ignoring a number of terms that should be of secondary importance (like mixing) and expecting vertical velocities to be zero near-bottom, one can expect a simple thermal balance: tT + u.xT + v.yT ~ 0, where u and v are onshore (x) and upcoast (y) velocities, T is temperature, and t is time. Consistent with Boehm et al (2002), we find that changes in temperature tT correlate well with cross-shore advection u.xT at the 15-m isobath (HB03 mooring). This correlation is dominated by the semidiurnal internal tide signal, which typically dissipates through the nearshore (Figures 9-3a, 9-5a, 9-5b, 9-6, and 9-7), and the thermal balance becomes more complex. In the nearshore, the wind-driven diurnal signal dominates (Figures 9-3a, 9-5a, and 9-5b), but it is not a simple cross-shore swash. Both alongshore and cross-shore components are important in nearshore diurnal currents (Figures 9-3a and 9-6), due to a combination of wind-driven baroclinic flows and tidal barotropic flows. The result is a thermal balance in which both alongshore and cross-shore terms are important (Figure 9-9). At 10 m there is still a correlation (r2 = 0.4) between tT and u.xT, but at 6.5 m the alongshore advection v.yT does a better job of explaining temperature changes tT, but even for this the correlation is weak and marginally significant (r2 = 0.2).
But more notable than the shift from cross-shore to alongshore dominance in the nearshore thermal balance is the increase in the residual, due to the presence of temperature and velocity structures that are smaller than those resolved by the mooring array (and perhaps due to the importance of other terms neglected from the simple thermal balance). Figures 9-3a and 9-5a indicate that there are more high-frequency (and probably small-scale) features in cross-shore velocities nearshore than further offshore.
The above discussion addressed the onshore-offshore advection of cold water by internal tides and wind-driven diurnal swash. In this to-and-fro motion some cold water may remain in the nearshore so that there is a net flux of cold water to the nearshore over a tidal-diurnal cycle (e.g., this would result from mixing during an intrusion into the nearshore). However, this pumping may happen due to cross-shore motions at any frequency and it is best to explore it by calculating cross-spectra between cross-shore velocity and temperature. There is zero net flux if velocity and temperature are in quadrature, a maximum onshore flux of warm water if velocity and temperature are in phase, and a maximum onshore flux of cold water if in anti-phase. In the outer nearshore, near-bottom cross-shore velocity and temperature are typically well correlated in the diurnal and semidiurnal frequency bands, with phase differences such that velocity lags temperature by close to 90° (Figure 9-10), corresponding to cold water swashes with minimal net flux. This correlation decreases in the inner nearshore (inshore of 10 m). Similarly, near-bottom alongshore velocities are well correlated with temperature, but mostly in the diurnal frequency band. Again, the temperature-velocity phase difference is in quadrature, with minimal net flux. These results suggest that mixing is secondary to the to-and-fro advection of cold water into the nearshore on diurnal and semidiurnal time scales.
While there are statistically significant peaks in near-bottom velocity and temperature spectra (and a canonical day pattern), these tidal-diurnal peaks are notably weak inshore of 10 m, and do not simply account for observed temperature changes through cross-shore advection of a cross-shore temperature gradient. Nevertheless, there are clear cold-water intrusion events observed in the nearshore from time to time, with some intrusions propagating through the entire nearshore and into the surfzone (Figure 9-11). The form and penetration of the cold-water intrusion into the nearshore depends on the incident feature and the conditions in the nearshore. The most striking events appear to have the form of internal bores, intruding into the nearshore with a marked thermal front and exhibiting propagation speeds (0.05-0.10 m/s) consistent with that of an internal bore (Figure 9-12). Further, the water velocity within the cold-water feature exceeds the speed of propagation of the feature, as expected within a bore. This characteristic indicates that some of these nearshore features are capable of transporting water shoreward as they propagate. Lagged correlations between near-bottom thermistors in the nearshore indicate similar propagation speeds on average (0.08 m/s between 10- and 6-m isobaths, and 0.03 m/s between 3- and 0.5-m isobaths).
The thermistors deployed in the surfzone were at 0.5 m below mean lower low water (MLLW) and remained submerged during the entire deployment period; however, they were at times under less than 0.5 m of water (low tide) and at times in as much as 3 m of water (high tide), mostly being under about 1.5 m of water. It is expected that the surfzone is mixed and close to isothermal most of the time, except when there is an active intrusion event (Smith and Largier, 1995), so this tidal variation in water level is not expected to (and does not appear to) have a major influence on the observed temperatures. In Figure 9-13, one can see a clear diurnal cycle that is also represented in canonical day calculations (Figure 9-8). During warm weather, it is reasonable to expect a day-night cycle due to diurnal surface heating with an amplitude of order 1-2°C for surfzone residence of a few hours. This appears to explain the smooth background diurnal signal in records plotted in Figure 9-13, but cannot explain the brief cooling events observed on specific days (e.g., days 228-233), when water temperatures may drop over 2°C in an hour. Examining the event on day 232 (Figure 9-11), one can see that it is associated with a cold-water intrusion that has propagated through the nearshore and into the surfzone, indicating that this cold water is advected into the surfzone. On some other days, cold-water intrusions are observed penetrating to the mooring at the 2.5-m isobath and then presumably mixing into the surfzone as the surfzone thermistors show cooling a few hours later. However, on many days there is no clear penetration of the surfzone by cold-water intrusions (e.g., days 234-236).
While there are clear examples of cold-water intrusions penetrating the surfzone, it should be noted that these “cold” temperatures are never less than 17.5oC and well above the temperature of water found in the wastewater plume. Further, the horizontal excursions associated with these cold-water intrusions are short and can only import water from just beyond the nearshore itself. Nevertheless, an analysis of shoreline bacteria data is presented in Chapter 3, where an association between contamination events and cold-water events is sought.
9.3. The Possible Effects of the AES Corporation Power Plant
It has been suggested that the AES Corporation power plant may explain the observed contamination of the beach between surfzone stations 3N and 9N, because it is found in exactly this vicinity. There are three primary possibilities:
1. A source: The power plant is a source of bacteria, or a conduit through which contaminated runoff enters the ocean.
2. A perturbation: The inflow, entrainment, and outflow of cooling water from the power plant perturbs natural coastal flows in a way that enhances onshore transport of bacteria.
3. A pathway: The cooling water flow offers a pathway by which contaminated cold-water intrusions can be transported to the surfzone.
The power plant draws in cooling water at the 8-m isobath, cycles it through the plant for about 20 minutes, and then discharges heated water at the 6.5-m depth contour (Figure 9-4). Unfortunately, the power plant was not operated during May and June 2001, resulting in a minimal flow rate for cooling water and zero discharge of heat–i.e., the discharge was the same temperature as the intake (Figure 9-14). Once the power plant came back online in early July, the discharge was about 15°C above the ambient ocean and intake temperature. However, all through July flow rates were low (about 120 mgd) and only increased to 240 mgd in August and to 360 mgd in October, by which time the summer was over and the experiment almost complete. This precluded observations of power plant thermal effects in summer 2001. Nevertheless, a study of the power plant effects was commissioned by the California Energy Commission in 2002 (the CEC-KOMEX study). This review has focused on the source and pathway issues, and a report is due in mid-2003. Consequently, the discussion of power plant effects on beach bacteria contamination is premature here and it will be brief.
Some preliminary bacteria and salinity data collected by MBC in 2001 indicate that low-salinity water in the plant may have high levels of fecal indicator bacteria, presumably from a land runoff source. The volume, nature, and existence of a specific possible source is unknown and examining this was one of the primary aims of the CEC-KOMEX study in 2002.
The withdrawal flow of water to the intake, the entrainment of water into the discharge jet, and the enhanced stratification due to the thermal plume are thought to alter the strength and penetration of cold surges in the vicinity of the power plant. However, late summer comparison of near-bottom velocity and temperature data on 10-m moorings at HBN2 (offshore of the intake), HBN3 (2 km upcoast from HBN2), and AES3 (0.5 km downcoast from HBN2), when the power plant was operating, revealed no apparent differences in the strength of flow nor in the intensity of temperature drops between these three moorings along the 10-m isobath. Not only are the intake and discharge effects expected to be localized, but the strength of the cold-water intrusions are already weakening by the 10-m isobath and unlikely to be enhanced by a localized power plant effect.
The extent of the thermal plume is illustrated in Figure 9-15 by the distribution of average temperatures. While the power plant discharge is 15°C above the ambient, this warm water quickly mixes into the receiving waters and appears to be diluted more than ten-fold in the near-field. Near-surface temperatures at moorings nearest to the discharge are 0.5-1.0°C higher than further afield (corresponding to a weak density effect), and the spatial extent of this effect is limited to a few hundred meters.
The idea that has received most attention to date has been the possibility that the power plant can draw in sub-thermocline water as it surges up to 8 m of water depth, and then be pumped into the cooling water system and entrained into the discharge jet (a combined flow of order 5000 mgd). These waters then form a thermal plume at the surface with little obstacle to this plume moving ashore. In fact, the discharge process results in radial spreading that would impact the surfzone; further, the regular onshore sea breeze every afternoon may transport this thin surface layer into the surfzone. Clearly, colder water is found at the 8-m isobath than in the surfzone (Figure 9-2), and a thermal plume forms and may enter the surfzone easily. Thus, the transport possibility does exist, but its role in beach contamination depends on the presence of contaminated water at the intake/discharge locations. While there was one observation of contaminated water in the power plant intake and discharge during the preliminary 2001 sampling by MBC, this was not during a cool event. Intense sampling of both the intake and discharge is a primary focus of the CEC-KOMEX study in 2002; this should allow a better evaluation of this issue. However, it is worth noting that there were regular beach contamination events during May-June 2001, a period during which the power plant was not operating, indicating that this mechanism was not playing a role in those events (Chapter 3). Further, if the thermal plume is an important part of this pathway, one would expect to see maximum contamination in the late afternoon and evening, during and following the afternoon sea-breeze maximum, and after the daytime maximum plant heat discharge. Historical observations reveal a maximum in the morning (Figure 3-27).
Cold-water intrusions penetrate the outer nearshore due to the action of internal tide run-up and wind-driven diurnal swash. In the inner nearshore the internal tide appears to have dissipated and the diurnal signal dominates. At times cold-water intrusions penetrate the surfzone, although temperatures are only relatively cool (always above 17.5°C). At the same time, cold-water intrusions are entrained into the power plant thermal plume, which may easily penetrate the surfzone. Either route offers a possible pathway for near-bottom waters to enter the surfzone, and for bacterial concentrations to enter the surfzone if they are initially present immediately outside of the surfzone (i.e., where the thermocline intersects the bottom). If either of these transport pathways are a regular cause of beach contamination, one should see an association between bacteria concentrations and water temperature in the surfzone. Concurrent bacteria and temperature data are plotted in Figure 9-16, and no obvious association of high bacteria concentration with either warm water or cold water is apparent. This may be because contamination events are occasional and do not occur during most cold-water or warm-water events. The examination of individual contamination events is presented in Chapter 3.
Boehm, A.B., B.F. Sanders, and C.D. Winant, 2002. Cross-shelf transport at Huntington Beach; Implications for the fate of sewage discharged through an offshore ocean outfall. Environmental Science and Technology, v. 36, p. 1899-1906.
Carrillo, L., J. Largier, M. Noble, P. Hamilton, and L. Rosenfeld, 2004. Polarization of low-frequency currents and the importance of internal waves in near-coastal waters off Huntington Beach, California. To be submitted to Journal of Geophysical Research.
Smith, J.A. and J.L. Largier, 1995. Observations of nearshore circulation: rip currents. Journal of Geophysical Research, v. 100 (C6), p. 10967-10975.
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/2004/1019/chap9.html
Maintained by: Mike Diggles
Created: September 24, 2004
Last modified: October 06, 2004 (lh)