CHAPTER 7. TIDAL TRANSPORT PATHWAYS

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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

Pulses of cool water occasionally appear 1-2 km offshore of Huntington Beach during the summer months (SAIC, 2001; Boehm et al., 2002). The pulses usually move cool water into and out of the nearshore region once or twice a day for periods of several days. Because the cool-water pulses sometimes have temperatures similar to that of the offshore effluent plume, it was hypothesized that semidiurnal internal tidal currents carried water from the offshore effluent plume across the shelf. The shoaling of these internal tidal currents could then carry the effluent into the nearshore region (Figure 7-1). In an effort to decide whether either diurnal or semidiurnal tidal currents could effectively transport the effluent plume across the shelf in the summer of 2001, we determined the characteristics of the persistent barotropic and the more intermittent baroclinic, or internal, semidiurnal tidal currents for the region. The characteristics of sheared diurnal currents, which are predominantly forced by sea breezes, are characterized here and in Chapter 6. As discussed in the previous chapter, cross-shelf diurnal fluctuations are, for the most part, not internal waves, and, thus, less effective at displacing the thermocline. However, if the semidiurnal internal tide raises the thermocline into the nearshore, directly forced diurnal currents may “push” the colder water further inshore (Section 6.6).

7.2. Barotropic Tides

Barotropic tides are characterized by a pronounced deflection of the sea surface. There was a nearly uniform deflection of sea level over the middle and outer shelf (Table 7-1). Sea-level oscillations at the semidiurnal frequencies were about 50 cm for the M2 and 20 cm for the S2 tidal constituents. The ratio between the M2 and S2 amplitudes was 2.5. This is near the predicted astronomical ratio of 2.1 (Godin, 1972). There was no significant phase difference between these two tidal constituents. There were no direct measurements of pressure at the coast, but sea-level deflections within Los Angeles Harbor were 51 and 20 cm for M2 and S2, respectively. This is within the range of measurements offshore. Sea-level deflections at the two diurnal frequencies were smaller than at the semidiurnal frequencies, nearly 21 and 33 cm across the shelf for the O1 and K1 constituents, respectively. The phase difference between these two constituents was around 20°, with O1 leading.

The beat frequency between the two semidiurnal constituents has a period of 14.8 days. About every two weeks, the M2 and S2 deflections reinforce each other and sea-level deflections have amplitudes of 70 cm. About 7 days later, the amplitude is reduced to 30 cm. This is the spring-neap cycle in sea level. For the diurnal tidal constituents, the beat frequency is 13.7 days. In July, the larger sea-level deflections caused by the individual diurnal and semidiurnal beat frequencies reinforce each other (Figure 7-2). Hence, the highest highs and lowest lows in tidal sea-level oscillations occur in July off Huntington Beach. The repeat cycle between the two-beat frequencies is 182.6 days, exactly one half of a year. Hence, Huntington Beach will have the largest extremes in sea level in July and again in January for the foreseeable future.

Barotropic tidal currents are expected to have constant amplitude and phase over the water column, providing the measurement site is outside the bottom frictional boundary layer. In order to estimate the barotropic tidal currents at sites where currents were measured with an ADCP (sites HB11, HB10, HB08, HB07, HB06, HB05, HB03, and HBN2), the currents in each 2-m bin were high-pass-filtered to remove periods longer than 66 hours. The high-pass-filtered currents at each site were then averaged over the entire water column. The amplitude of the barotropic tides was then calculated from the depth-averaged records at each site using the Foreman tidal analysis programs (Foreman, 1977, 1978).

The barotropic diurnal and semidiurnal currents are energetic components of the spectrum for both along- and cross-shelf currents (Figure 7-3a, 7-3b). The barotropic semidiurnal currents are more energetic than barotropic diurnal currents. At the semidiurnal frequency, the along-shelf currents are more energetic than the cross-shelf currents, with the largest spectral peaks found over the middle and inner shelf. The semidiurnal cross-shelf currents decrease between the middle- and inner-shelf sites. This is expected because the coast (the coastal wall) imposes a zero cross-flow condition at the shore. The along-shelf currents also dominate the diurnal frequency band. The strongest diurnal currents are found over the inner shelf. The cross-shore currents account for only a minor portion of energy in the diurnal band.

The tidal currents have strong oscillatory motions which result in little net movement over a tidal cycle. An ellipse (Figures 7-4a, 7-4b, 7-4c, 7-4d, 7-4e, 7-4f, 7-4g) can describe the speed and direction of the tidal currents over a tidal cycle. The current vectors trace out an ellipse, showing which direction the current is flowing at any phase of the tide.

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 flow 6 hours prior. About 12.42 hours later, the currents return to their original direction and speed.

On the outer shelf, barotropic semidiurnal tidal current ellipses are aligned halfway between the along- and cross-shelf directions (Figure 7-4a). Current speeds along the major axis of the M2 tidal ellipses range between 2-3 cm/s over the outer shelf (Table 7-2). The S2 constituent is usually less than 1 cm/s. Semidiurnal tidal currents increase slightly in amplitude in shallower water depths. Currents along the major axis of the tidal ellipse can reach 5 cm/s over the middle and inner shelf. But the most notable characteristic is that the barotropic semidiurnal tidal ellipses tend to be aligned parallel to the coast in the shallowest water depths (Figure 7-4a).

The barotropic diurnal tides are weaker than the semidiurnal currents, with speeds on the outer shelf of 2 cm/s or smaller (Table 7-2). However, they share the same characteristics as the barotropic semidiurnal tides. Current amplitudes increase towards the shore and tidal current ellipses tend to be aligned with the local isobaths (Figure 7-4b).

7.3. Semidiurnal Internal Tidal Currents

The characteristics of the average amplitude of the internal tidal currents were calculated from the high-pass-filtered ADCP records that had the depth-averaged barotropic tidal constituents removed. Variance-preserving spectra of these records show that the internal tides account for a varying percentage of the semidiurnal tidal energy over the water column (Figure 7-5a, 7-5b, 7-5c, 7-5d). The energy in the semidiurnal surface tides at HB07 is slightly larger than the near-surface internal tidal currents there. However, it is the cross-shore, rather than the along-shore, component that has the largest internal tidal energy, suggesting the internal tidal ellipses are more oriented perpendicular to the isobaths. The energy in the surface and near-bottom internal tides are similar; surface internal tides are only slightly more energetic than internal tides near the bed.

More precise characteristics for the average internal tidal ellipses were calculated at each ADCP bin using the Foreman tidal programs. The semidiurnal internal tides had the expected structure with depth; surface currents were approximately 180° out of phase with bottom currents, even in water depths of 10 m (Table 7-3a, 7-3b). The semidiurnal internal tidal current amplitudes were smaller than the barotropic amplitudes, with speeds along the major axis less than 3 cm/s. However, the orientation of the major axis of the semidiurnal internal tidal ellipse was approximately 90° to that of the barotropic (Figure 7-4c, 7-4d). The strongest semidiurnal internal tides flowed toward or away from the shoreline both near the surface and at the bed.

The semidiurnal internal tidal currents did not have constant amplitude over the summer months. They can be larger or smaller than their average amplitude for periods of days to weeks. A complex demodulation of the energy in the cross-shore semidiurnal internal tidal band shows that the near-surface tidal currents were strong in early July (Figure 7-6). Cross-shore internal tidal currents can have speeds over 10 cm/s near the end of July and are enhanced for several days in August and September. It is interesting to note that the energetic cross-shore semidiurnal tidal currents did not occur during spring tides, as measured by the pressure sensors. In particular, Energetic internal tides were mainly absent from the inner shelf site in the latter portion of the measurement program. As discussed in Chapter 4, this is probably because of the deepening of the thermocline in August and September that leaves the nearshore less stratified than in early July. The patterns are a little more consistent over the outer shelf where maximum internal tidal currents occur at 4-10 days after maximum spring surface tides.

7.4. Vertically-Sheared Diurnal Tidal Currents

The near-surface currents in the diurnal tidal band are dominated by wind forcing, as discussed in Chapter 6. In order to look more closely at the vertically sheared portion of the diurnal band currents, their characteristics were calculated from the high-pass-filtered ADCP records that had the depth-averaged barotropic tidal constituents removed. Because the K1 frequency is very close to 1 cycle per day and the daily wind-forced currents are strongest nearer the surface, this analysis is likely to show the strong influence of sea-breeze currents that are not of astronomical origin.

Over the outer shelf, the K1-sheared currents were largest within 10 m of the surface, with amplitudes of 8-10 cm/s over the outer and mid shelf. More moderate amplitudes, with speeds of 2-4 cm/s, were found within 10-30 m of the surface (Table 7-3c). Near-surface ellipses had moderately circular shapes. Bottom speeds near the shelf break were 1-2 cm/s. As one approached the coastline, near-surface amplitudes remained large over the mid shelf, but near-bottom currents increased as the water depth decreased. Near-bottom currents had nearly the same amplitudes as found in similar water depths at the shelf break. Closer to the coast, the near-surface and near-bottom current ellipses had similar amplitudes and ellipse orientations (Table 7-3c, Figure 7-4e). At the shallowest sites, sheared diurnal ellipses were orientated slightly more along the isobaths.

The sheared current in the O1 tidal band was much weaker than in the K1 band (Table 7-3d, Figure 7-4f). Near-surface current speeds over the outer shelf were 1-3 cm/s. Otherwise sheared current characteristics were similar to those in the K1 band. Near-surface currents were larger than those near the bed over the outer shelf (Table 7-3d, Figure 7-4f). Near-surface and near-bed currents were closer in amplitude in the shallow water near the coast.

7.5. Cross-Shore Transport by Tidal Currents

7.5.1. Event Analysis of Energetic Cross-Shore Tidal Current Pulses

If one assumes that cross-shore currents of the 4 major tidal constituents off Huntington Beach are in phase and reinforce each other, they can flow across the shelf with speeds of 5-8 cm/s below the thermocline on the outer shelf. Water can be transported onshore by this flood tide 0.7 to 1.1 km, which is less than 1/6 of the shelf width. Subsequently, as the tidal currents ebb, the average tidal currents will carry the water about the same distance offshore. On the inner shelf, barotropic tidal currents flow parallel to shore. It is the internal tidal currents, with average cross-shore currents of 2-3 cm/s, that transport water and suspended material toward the coast. Clearly, the average tidal currents will not move suspended effluent material from where it is discharged at the shelf break to the nearshore in a few days or a few tidal cycles.

However, semidiurnal internal tides have larger than average amplitudes for periods of days to a week. During the summer of 2001, the largest cross-shore internal tidal currents occurred in late July (Figure 7-6). A closer examination of this energetic period shows that the pulses have a pronounced mixed tidal signal. That is, every other tidal current pulse can be noticeably larger than the preceding pulse. The effect is compounded if daily wind forcing reinforces the larger of the two pulses. Internal tidal-current speeds near the bed were larger than 10 cm/s. During this same period, cold water was found nearshore. Water colder than 12°C, which is normally found at the shelf break at depths equivalent to that of the core of the effluent discharge plume, is found offshore of Huntington Beach in water depths less than 30 m (Table 7-4).

The large semidiurnal pulses in early and late July and late September show that water cooler than 13°C reaches the 10- and 15-m isobaths (Figure 7-7). It appears that internal tidal pulses, combined with other forcing, can move cool water from below the thermocline toward the coast. A closer examination of a typical large pulse during a 7-hour period (slightly more than half a tidal cycle) shows that 12°C water migrated about 4 km across the shelf, from 40- to about 20-m water depths (Figure 7-8a, 7-8b). 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 in the late-night hours for several days. An analysis of the events, combined with the temperature and salinity properties of the water particles and an indication of where the isotherms intersect the seabed, indicates that over a tidal period nearshore cooler water probably advected from no further away than mid-shelf. In addition, the cooler water in the very nearshore region may have been subthermocline, not bottom water, mixed to the surface by the strong internal tidal pulses as they shoaled in water depths of 15 m. 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 the internal tidal pulses 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.

7.5.2. Cross-shelf Displacements of the 14°C Isotherm at Tidal Periods

The large cross-shore tidal pulses discussed above occasionally move water as cold as 12°C (which represents the core of the effluent plume) from the outer shelf into water depths shallower than 30 m (Table 7-4). We can also track the movement of the 14°C isotherm across the shelf at tidal periods for most of the record. Because 14°C water 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.

If, as discussed above, the semidiurnal internal tide is the primary forcing mechanism for the on and offshore displacements of bottom colder water, there should be a direct relation between the tidal-period cross-shelf current fluctuations and the isotherm displacements. This has been studied by calculating the cross-shelf current component (averaged over the portion of the water column beween the 14°C isotherm and the bottom) on the outer shelf that is required to account for the displacements of the 14°C isotherm. This isotherm is representative of the lower thermocline, and is found at about 30-m depth in July, though it deepens with the thermocline through August and September. The 14°C isotherm is approximated as an impermeable surface, and the implicit assumption is that the tidal cross-shelf flow regime is uniform in the along-shelf direction. The relations between the cross-sectional area, along the main transect and below the 14°C surface, and the depth-mean current component is given by the equation in Section 4.1.5. The method of calculation is the same as discussed in that section except that 3-HLP isotherm depths are used. The results of calculations of U, the cross-shelf transport between 14°C and the bottom at HB07, derived from the rate of change of the cross-sectional area, are given in Figure 7-9, where the cross-sectional area and the onshore position of the 14°C surface where it intersects the bottom, are also given. U is compared to the mean cross-shore component of velocity at HB07, where the ADCP depth bins and the bottom current meter are averaged between the position of the 14°C isotherm and the bottom. The highest correlation was found when the component direction was taken as the minor principal axis direction for the lower layer (Chapter 4), i.e., approximately normal to the local isobaths. It can be seen that the calculated transport velocity and the observed mean current component correspond quite closely, both in the amplitudes of the fluctuations and their phasing.

Both the calculated and observed velocities in Figure 7-9 are predominantly semidiurnal. However, the offshore position of the 14°C isotherm has diurnal as well as semidiurnal signals, particularly in July when the isotherm is closest to the coast. Later in the record, as the isotherm moves deeper and out of the influence of the sea breeze, semidiurnal fluctuations dominate isotherm fluctuations. In July, there are times when the isotherm position is virtually stationary (e.g., at neap tides around July 14 and 29), but the cross-sectional area still has large fluctuations. This implies that, during these periods, the cross-shore semidiurnal lower-layer transports are strongly attenuated on the inner shelf. There are also periods (July 20-27 and August 3-8) where the daily inequality in the semidiurnal current amplitudes produces a more diurnal signal in the area, and even more so in the isotherm position. This appears to be because the smaller amplitude cross-shelf current fluctuations do not proportionally perturb the surface as much as the larger. This is a consequence of the diurnal inequality of the semidiurnal tide. However, these predominantly diurnal fluctuations of the 14°C isotherm position occur when it is present in the nearshore, and therefore may be also influenced by the sea-breeze-forced cross-shelf flows.

The mixtures of diurnal and semidiurnal signals in Figure 7-9 are presented as variance preserving spectra where equal areas under the curve represent equal contributions to the variance (Figure 7-10). The offshore position spectra and the upper-layer cross-shelf currents at HB07 show strong variances in the diurnal and semidiurnal bands, but the lower-layer velocity components, both observed and calculated, are strongly semidiurnal. The arguments of Chapter 6 suggest that the diurnal fluctuations in the upper-layer cross-shelf currents are forced by the sea breeze. We expect that the actual amplitude of the diurnal fluctuation in these currents is much smaller than the measured because the maximum amplitude of the sea-breeze-forced current fluctuations is at the surface, which is not well captured by the ADCP. This current is not measured and therefore can’t cancel the oppositely directed diurnal fluctuations found just above the 14°C isotherm.

The coherence squared between the calculated and observed velocity components are well above the 95% significance level for both diurnal and semidiurnal frequency bands with only small phase differences (Figure 7-10). The energy in the diurnal band is, however, small. For the isotherm position, where the diurnal energy is substantial, the coherence squared with the observed velocity component is less significant in this band indicating other factors are influencing this signal. However, the coherence squared in the semidiurnal band, for these two series, is again well above the 95% significance level. This confirms the dominant internal tide forcing of the lower layer and the onshore excursions of sub-thermocline water.

7.6. Conceptual Model of Tidal Period Circulation

The preceding sections have shown that for periods shorter than about 30 hours shelf circulation processes are a complex mixture of sea-breeze-forced flows with a 24-hour period and internal tide propagation with predominantly semidiurnal periods. These processes strongly influence the cross-shelf current fluctuations, and, therefore, dominate the tidal-period temperature fluctuations because cross-shelf density gradients are much larger than along-shelf gradients. In the along-shelf direction, the barotropic tide is also an important contribution to the current fluctuations. In this section, a conceptual description of the cross-shelf circulation processes at diurnal and semidiurnal periods is attempted, since these current fluctuations strongly affect the short-term transport of pollutants across the shelf into the nearshore.

The sea-breeze-forced flows are confined to layers above 30 m, and consist of surface currents that are in phase with the wind fluctuations and subsurface flows between 10- and 20-m depth that flow in the opposite direction. In the cross-shelf direction, this creates a two-layer flow system, and because the amplitudes decrease from the outer shelf towards the shore, it was surmised that vertical velocity fluctuations become important where these flows encounter the sloping seabed. The temperature fluctuations fit this model with predominantly diurnal fluctuations in the upper layer, with maximum amplitudes occurring in water depths of 10 to 20 m and at depths corresponding to the interface between the oppositely directed cross-shelf flows. The depth of penetration of sea-breeze-forced flows varies with the degree of stratification in the upper layers and the horizontal shears of the subtidal currents. The latter affects whether or not diurnal-period internal waves can propagate. If internal waves can freely propagate, the energy of the flows tends to be higher and penetration depths deeper for similar winds than when the diurnal flows are directly forced.

In contrast to the daily period currents in the upper layer, cross-shelf velocities at tidal periods in the lower layer are dominated by the internal semidiurnal tide. Barotropic semidiurnal tidal currents are present, but they are primarily directed along the trend of the isobaths (Figure 7-4g). Therefore, temperature fluctuations on the outer shelf and below 30 m are predominantly semidiurnal. Using the 14°C isotherm as a surrogate for the lower thermocline and the interface between the upper and lower layers, it was shown that the onshore-offshore excursions of this isotherm could be reasonably accounted for by the fluctuations of lower-layer cross-shelf velocities at the shelf break. Moreover, the semidiurnal temperature fluctuations were approximately out of phase between the shelf break and the nearshore. This means that when the 14°C isotherm moves downward at the shelf break, it moves further onshore and to a shallower position where it intersects with the bottom in the nearshore. Thus, the lower layer appears to slosh backwards and forwards across the shelf, forced by the cross-shelf internal tide. The fluctuations of the inshore edge of the 14°C isotherm often have a diurnal, rather than a semidiurnal, character. It is not clear whether this is caused by a non-linear response to the lower-layer cross-shelf velocity fluctuations, which can have a diurnal inequality similar to the surface tide, or whether, when the lower layer penetrates into the nearshore, it is assisted or taken over by the sea-breeze-forced flows.

A sketch of the sea-breeze and semidiurnal internal-tide-forced cross-shelf flow fluctuations is given in Figure 7-11. This is based on the above analysis and sketches the movements of the isotherms. Since the upper and lower layers have different dominant periodicities, there will be times when the sea-breeze-forced currents below ~10 m, on the inner shelf, may assist or inhibit the onshore slosh of sub-thermocline water toward the shore. This simplified picture of the cross-shelf tidal-period circulation processes is modified by seasonal changes in the depth of the thermocline and its degree of slope, caused by the thermal wind balance with the subtidal along-shore currents. When the thermocline deepens, as it does in August, compared to July, then the lower-layer sloshing is less likely to penetrate into the nearshore, and, because of this, the fluctuations of the inshore edge of the 14°C isotherm will have a more semidiurnal character.

7.7. Relationships Between Cross-Shore Tidal Current Pulses and the Exceedance of AB411 Standards in the Surfzone

In an effort to relate the appearance of the most energetic cross-shore tidal current pulses to the occurrence of bacterial concentrations exceeding AB411 standards at the beach, a time series of cool, nearshore events was constructed. A cool event occurred when the energetic cross-shore current pulses brought water colder than 12°C into 30-m water depth (Table 7-4) or colder than 13°C into 15-m water depth. A cool event also occurred 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 (Figure 7-7). 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 (Figure 7-12) 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 significant contamination events. Only 3 of the 17 cooling events occurred on days when AB411 standards were exceeded in the surfzone. Most of the large cooling events at the end of July happened after a nearly weeklong beach contamination event. In addition, the strongest internal tides did not tend to occur during spring tides (Figure 7-6), which historically is the most likely time the beach is contaminated.

7.8. References

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(9), p. 1899-1906.

Foreman, M.G.G., 1977. Manual for tidal heights analysis and prediction. Institute of Ocean Sciences, Patricia Bay, Sydney, British Columbia, Pacific Marine Science Report 77-10, 97 p.

----, 1978. Manual for tidal currents analysis and prediction. Institute of Ocean Sciences, Patricia Bay, Sydney, British Columbia, Pacific Marine Sciences Report 78-6, 70 p.

Godin, G., 1972. The analysis of tides. University of Toronto Press, Toronto, 264 p.

Hamilton, P., J.J. Singer, E. Waddell, and G. Robertson, 2001. Circulation processes on the San Pedro Shelf. Proceeding, MTS 2001 Conference, November 2001, Honolulu, Hawaii, 8 p.

SAIC, 2001, Strategic Process Study: I, Plume tracking-ocean currents. Final report prepared for Orange County Sanitation District, 61 p.