Chapter 4. Secondary Currents '

Chapter 4. Secondary Currents ’

Tidal currents dominate the flow patterns in most First, the falling rain drops gather horizontal momentum coastal areas. Nevertheless, there are secondary currents because they are carried along at some fraction ofthe wind set into motion through a variety of forces that lead to speed. When they strike the surfaceof the sea this momen- variations in circulation quite distinct from the cyclic tidal tum is then imparted to the water which, like the wind oscillations. The estuarine-type circulation discussed in drag itself, causes the water to be driven forward in the Chapter 2, for example, is a secondary flow pattern main- direction of the wind. In addition, rain drops splashing tained by horizontal differences in water temperature and onto the sea produce an increase in the roughness of the salinity, for which discernible fluctuations in speed occur surface, which effectively enhances the wind drag. Lastly, over periods of weeks to seasons. Wind-generated cur- the natural tendency of the wind speed to diminish to zero rents are secondary flow that may alter speed and direc- very close to the water surface due to friction is partly tion over time spans as short as a few hours. offset by rain drops. Because they lose only 10-20% of Although at any particular instant the presence of their horizontal speed as they fall through the lower few secondary currents may be completely hidden by the metres of the atmosphere, rain drops may actually be stronger tidal flow, their influence can be quite moving faster than the air and, therefore, transfer to it pronounced over long periods of time and can be of some of their momentum. This in turn strengthens the considerable importance in determining the oceanogra- wind close to the sea surface and increases its force on the phy of a particular coastal area. water. Wind-generated surface waves produce a weak transport of water in the direction of the waves called the Wind Drift Stokes drift. This is not a wind-current, but is associated with the fact that orbital motions under a wave are not The direct effect of the wind’s drag on a nearly completely closed, and allow the water to advance slightly smooth water surface is felt only in the top few cen- forward with the passage of each wave. The speed of this timetres.This thin layer is then made to move down-wind drift is less than 1/10 the wind-drift and is usually unimpor- at about 3% of the wind speed. (Smooth is used here in tant. the aerodynamic sense in that the wind conforms to any Winds may generate more subtle circulationswithin bumps on the sea surface and does not break up into the surface waters. For instance, the streaks of foam and turbulent patches.) In a 5 m/s wind a thin “skin” of water surface debris that align as windrows along the direction would move at approximately 15 cm/s (3% of 5 m/s), but of the wind are associated with cell-like, circling patterns would have no effect on a boat with a discernibledraught. in the water at right angles to the wind direction (Fig. The shallow penetration of the wind drag can be readily 4.1). Looking downwind, the water to the right of the observed in a pond or sheltered bay where small sub- windrow is circulating anticlockwise while that to the left merged particles, such as pollen, can be seen drifting just beneath the water surface with the wind, while suspended particles a metre or more below the surface remain almost L WINDROINS motionless. The ability of the wind to produce currents to greater wind depths is significantlyenhanced if it is putting energy into -drift surface gravity waves (wind waves). Waves effectively increase the wind drag by increasing the roughness of the surface, and make it more difficult for air to flow smoothly over the water. Under these conditions, approximately 40% of the wind energy goes into the waves, of which 5% is lost to breaking crests in the form of whitecaps. The increased wind drag, together with the momentum trans- ferred to the water by whitecapping, leads to substantially deeper wind drift. As the amount of energy in the waves themselves depends on the wind duration and speed as well as its fetch (the unrestricted length of water surface over which the wind blows), the speed and extent of the wind current will depend on these factors also. Therefore, the state of the sea, and not the wind directly, determines the speed and depth of penetration of these currents. Rain, especially heavy rain, associated with storm FIG.4.1. Cellular circulation patterns associated with windrows. Foam and surface debris gather in streaks where currents oftwo Langrnuir cells windsmay further augment the wind‘s ability to drive a converge. Combined effect of separate wind drift (+) and Langmuir surface arrent. This can happen in a number of ways. cells produccs corkscrewlike flow pattern aligned in direction ofwind.

- 71 - is circulating clockwise; surface debris gathers or con- the normal tidal streams for a few days following the verges where these two circulations meet to produce sink- passage of a storm front. Current speeds commonly reach ing water. These so-called Langmuir circulations combine 25 cmls off the west coast of Vancouver Island and at the with the stronger currents generated in the wind direction entrance to Queen Charlotte Sound, with a high degree of to produce an overall downwind water motion that some- coherence between the current motions over tens to hun- what resembles a corkscrew. The spacing between adja- dreds of kilometres. cent windrows is roughly equal to twice the length of the dominant surface gravity waves. Although the exact reason for the formation of Lang- Relaxation Currents muir circulations is not completely understood, evidence As well as driving surface drift currents, winds can now suggests they result from a complicated interaction indirectly affect other types of flow. Where the wind drift between the Stokes drift assocated with the waves, and the is restricted by a lee shore, persistent onshore winds will wind-driven current directly formed by the wind drag, or cause the water to pile up against the coast and tilt the sea alternatively, through the action of breaking waves and the surface, an effect that can be simulated by blowing on a wind-driven current. Whatever the cause, a number of bowl of water. the same token, offshore winds will observed features of these circulations can be of practical By produce a sea-level tilt by moving water away from the use. For example, the tendency ofoil slicks to collect along coast. On a large enough scale this can lead to the forma- the convergence bands has been successfully exploited in tion of storm surges on low-lying coasts mentioned earlier cleaning up oil spills at sea. And, as lines of greatest wind- in connection with tides. When such winds weaken or directed surface drift are found along windrows, whereas reverse direction, the raised (or depressed) water surface lines of weakest wind-drift lie midway between adjacent at the shore will seek its equilibrium level. The relaxation windrows, yachtsmen have a natural indicator to set their currents associated with this readjustment of water level course. Clearly, best advantage on a downwind run is may persist for hours or days, depending on the area riding along a windrow. When beating to windward it is affected, and result in perceivable deviations of the surface best to stay midway between the dominant windrows as currents from those expected on the basis of tides and much as possible. local winds alone. Wickett (1973) suggested that the un- Perhaps some of the most important types of wind- usually strong southerly currents of nearly 1.5 mls (3 kn) induced secondary flows are those associated with near observed at a drilling rig at the southern end of Hecate vertically propagating disturbances called inertial (or gy- Strait on Sept. 25,1968, were due to such currents after a roscopic) waves. Generated within the upper ocean by period ofstrong onshore winds. In this particular case, the abrupt changes in wind direction, these inertial currents outtlow was apparently augmented by 7 cm of rain on the are rotary flows whose direction constantly changes over a eastern shore of the Strait and by runoff from adjacent specified period oftime, somewhat akin to the rotary tidal inlets. streams discussed in the previous Chapter. Unlike tidal In partially enclosed basins like harbors and certain streams inertial currents are invariably circularly polarized mlets, disturbance of the surface level by passing storms in that the current vector always rotates clockwise (north- can set up oscillations that cause currents to slosh back and ern hemisphere) and maintains a uniform speed over a forth several times before the system returns to equi- single rotation. Put another way, the tip of the current librium. Sea level and current oscillations of this kind, vector, in the absence of other types of current, traces out called seiches, have periods of minutes to hours and ampli- a circle (see Fig. 3.30b). (In the southern hemisphere, the tudes of 5-10 cm, depending on the depth and geometry sense of rotation is counterclockwise.) Set in motion by a of the basin and the nature of the disturbing mechanism. pulse of momentum from the winds, the currents are Generally speaking, the seiche-currents attain maximum maintained by a balance between the rightward turning speeds midway between the two opposite directions of tilt effect of the Coriolis force (northern hemisphere) and the (see Fig. 3.25). Seiches can also be initiated by tides, centripetal (or centrifugal) force due to the curvature of tsunamis, and other types of oceanic waves at the mouth the water's path. Therefore, the time for the current to of the basin. In many instances the exact cause of observed swing once round the circle, the inertial period, is deter- seiche activity is not clearly understood. mined by the local value of the earth's vertical component of rotation. At about 50" lat., this corresponds to 1% h, part way between 12Y2 and 25 h periods of the tides. (At Slippery Water 30" latitude the inertial period equals the diurnal period of the tides, about 24 h.) As noted in the discussion of inlets and other bodies For reasons that are only partly understood, inertial ofwater that receive considerable amounts of river runoff, motions appear to be mainly confmed to the upper 100 m a relatively thin brackish layer often overrides the deeper or less of the ocean's surface and undergo rapid attenua- oceanic water. For yachtsmen who want to use surface tion after only a few periods of oscillation amounting to currents to advantage, the presence of this layer can be of several days. They tend to occur in open regions of the fundamental importance. In the absence of such vertical oceans away from the interference of coastal boundaries stratification, only the top few centimetres are dragged and not in confined basins such as the Strait of Georgia or along by wind directly. With vertical stratification, the Juan de Fuca Strait. Not surprisingly, inertial currents are whole brackish layer, many metres thick, can be made to highly intermittent, often appearing suddenly to disrupt slide downwind like a lubricated "slab" with only a mini-

- 72 mum of resistance from the deeper water beneath (Fig. speed. These are shown graphically in Fig. 4.3, which 4.2). In the open ocean also, wind-mixing of relatively indicates for example that a 15-m/s (30 kn) wind can warm, low-salinity surface water can, over a period of move a brackish layer 10 m thick downwind at about 0.6 many days, lead to the formation of a thin layer of com- m/s (1.2 kn). The thinner the layer the greater the speed paratively light water over large areas of the sea surface. In for a particular wind. Anyone using this relationship, the open ocean, the slab may also be set into inertial however, should bear in mind that wave action can destroy motion, whereby a large region of water moves in a the layered structure of the water, particularly if the winds circular path without change of orientation. are strong or the upper layer is thin (less than 1 m or so). The phenomenon of “slippery water” was used suc- cesshlly by British sailors in the 1968 Summer Olympics in Acapulco, Mexico. To quote David Houghton, the Ekman Spiral meteorological advisor to the British crews, “For the first In open areas of the ocean, frictional effects associ- two weeks the sea surface. . . behaved just like the slippery ated with turbulent motions assist in transmitting the layer which had been postulated. In fact it was too good to influence of the wind downward through the sea surface. be true. The surface water moved almost directly down- The relationship between wind and current that is often wind at speeds of up to about 2 knots, depending on how used, though with some reservation, is based on the the- long the wind had been blowing from that particular ory of the Swedish oceanographer, Ekman (1905), who direction. When the wind dropped to a calm the water attempted to explain the behavior of the ice pack in the continued in the same direction with little change in speed Arctic Ocean. As Fridtjof Nansen observed in his famous and it took a wind from the opposite direction from 24 to 1893 expedition when his specially designed ship the 36 hours to stop the water and get it moving the other Fram was locked into the ice for 3 yr, the arctic ice drifted way.” It is possible to estimate the speed attainable by a at about 45” to the right of the wind at a few percent of the brackish layer by rough approximations for a given wind wind speed. According to Ekman’s explanation, a com- bination of wind drag, internal friction, and the Coriolis WIND force were responsible for this behavior. He further showed that, with increasing depth, the wind-induced hi current should turn more and more away from the direction of the wind and decrease in speed, to produce the so- called Ekman spiral (Fig. 4.4). Present information indicates that the angle for surface currents is closer to 20” to the right of the wind at about 2-3% of the wind speed, provided that (1) the water is actively “roughed up’’ by wind-wave action; (2) the water depth exceeds the maximum depth of direct wind influence, around 100 m; and (3) the flow is not confined horizontally by the presence of land. If the water is shallower than the depth of wind influence or close to shore, the entire water column tends to move downwind with a speed that decreases with

FIG.4.2. Slippery water. Wind drags along slab oflow-density brackish water overlying a deep layer of more dense salinc water. Friction at interface ultimately limits speed of slab (see Fig. 4.3). I I I I

FIG. 4.4. Classic Ekman spiral. Arrows and their projection onto lower surface (broken line) indicate direction and relative strength ofcurrent at CURRENT SPEED in cmls various depths in relation to surface wind in northern hemisphere. (In FIG. 4.3. Maximum down-wind speed a layer of brackish water can southern hemisphere currents are to left ofwind.) Depth, d, varies but is theoretically attain for a specified layer thickness and wind speed. (From typically between 30 and 100 m in open ocean. Deeper currents are weak Waldichuk 1957) and in the opposite direction to wind.

- 73 - depth. In light airs with little or no wave action the differences in mid-ocean associated with the poleward downwind flow becomes mostly confined to the top few decrease in the solar heating of the earth's surface are centimetres. largely responsible for the slow currents below the depths As is so often the case with simple explanations for of large-scale wind influence (500-1000 m). Known as the generation of currents, the full Ekman spiral is rarely the thermohaline (heat-salt) circulation these density-in- observed in the ocean, either because conditions aren't duced flows appear to play an equal role to the wind in quite right or because other effects overshadow it. Nev- maintaining the motion of the sea. Moreover, density ertheless, the direction and speed of the surface current currents that sink and then flow northward from the within the limits of the theory's applicability appear to be shallow continental seas of Antarctica are responsible for good enough as a first approximation to wind influence in the formation of the slowly drifting bottom waters below the open, deep-sea regions of the ocean. depths of 3000 m in the Atlantic and Pacific oceans. As a The current associated with the Ekman spiral, the final example, the tongue of warm salty Mediterranean Ekman drift, is thought to play a vital role in establishing water that flows through the Straits of Gibraltar contrib- the large wind-driven circulation patterns of the world utes significantly to the immediate-depth circulation in oceans. Briefly, the mechanism works as follows. First the the North Atlantic between 20-40" N. wind sets up the Ekman drift pattern with currents to the Under certain circumstances, density currents can be right of the wind in the upper 100 m or so of the ocean pronounced and create atypical circulation patterns. One surface (northern hemisphere). (In the southern hemi- of the most dramatic examples of this on the B.C. coast sphere, drift currents are to the left of the wind.) Then, occurs in the Rupert-Holberg Inlet system at the north- because of the north-south changes in the strength of the ern end of Vancouver Island (Fig 4.5).Oceanographic Coriolis force and horizontal variations in the wind speed observations in this area have shown that intense vertical and direction, some regions of the world oceans experi- mixing of the flood over the sill within Quatsino Narrows ence a long-term convergence of the surface water while frequently results in water that is more dense than that of other regions experience a divergence of these waters. In Rupert Inlet, into which it eventually flows. Driven by large areas where the top layers are diverging there must this horizontal difference in density, the flood current is be a slow upward flow from beneath to compensate for then able to dive down the side of the basin as a rapidly the horimntal spreading of the water. The upward motion moving density current with speeds in excess of 1 m/s (2 in turn induces horizontal currents of its own within the kn). Sweeping along the bottom of the inlet, the current top 1000 m or more of the water column. The currents form into the cyclonic gyres so Characteristic of the major oceans. Similarly, convergence of the surface waters by long-term influence of the winds necessitates downward flow, and the formation of the equally characteristic anticyclonic gyres. The cyclonic subarctic gyre in the North- east Pacific Ocean, for example, is associated with the counterclockwise winds of the Aleutian low-pressure system that dominates this region much of the year. The anticyclonic North Pacific gyre west of California is associated with the equally persistent clockwise winds of the North Pacific high-pressure system. Density Currents These currents are formed when a volume of water with a different density than its surroundings seeks its appropriate level. That is, it seeks a depth where the water above is lighter (fresher or warmer) and the water under- neath is heavier (saltier or colder). Although such currents are generally slow and, therefore,, have little direct effect on boaters, they are extremely important to the long-term dispersal ofpollutants and to maintaining the great variety of flora and fauna in deep portions of certain basins. Without the continual replenishment of the deep and intermediate depth waters in the coastal regime, the dissolved oxygen in these waters would soon be used up by aquatic plants and animals and decaying materials. If this were to happen, most life in the deeper waters would eventually disappear. Despite their sluggish nature, density currents also make an important contribution to circulation patterns in FIG. 4.5. Map of the Rupert-Holberg Inlet system northwest coast, the world oceans. It is now believed that the small density Vancouver Island. (From Govette and Nelson 1977)

- 74 - no data -___ 8 -1 20 rn

100 rn XI0

8 :L 145 rn 0 30 3 4 5 6 7 8 9 AUGUST SEPTEMBER

FIG.4.6. Measured currents in Rupert Inlet immediately north ofQuatsino Narrows (see Fig. 4.5).Plots show variation in current speed at various depths, August-September 1976. In late summer, positively buoyant jetlike flood current is confined to near-surface depths; in autumn, as surface waters become more dense, entrant jet becomes negatively buoyant and can penetrate to bottom. (Courtesy D. Stucchi) displaces the less dense water upward, and causes an in- inlet begins to appear at deeper and deeper depths until crease in the salinity and dissolved oxygen content of the finally it reaches the bottom, having adjusted its level of deep portions of the basin. This process appears to be intrusion into Rupert Inlet in accordance with changing particularly well established during spring and summer, density (Fig. 4.6).It takes roughly 2 h for the core of the when precipitation and runoff are low and dense water of density current to reach the bottom, corresponding to the oceanic origin is present in Quatsino Sound. During the time required for the flood to transport cold saline ocean fall and winter, the larger runoff and precipitation lead to water in Quatsino Inlet to the vicinity of the Narrows. decreased density contrasts between the tidally mixed In many respects, the strong density current in water of Quatsino Narrows and Rupert Inlet, so there is Rupert Inlet during the flood is beneficial. Unlike some more intermittent formation of the bottom density cur- inlets on the coast (e.g.Saanich Inlet), there is a constant rent. (Ebb currents in Rupert Inlet are always weak.) renewal of bottom water and no stagnation or depletion There are other aspects of density current structure of oxygen. Unfortunately, Rupert Inlet has been used worth noting. At the beginning of the flood, for instance, since 1971 as a submarine tailings dump for the copper- the water mixed in Quatsino Narrows is water that went molybdenum wastes of Utah Mines, presently over out with the preceding ebb. Because its density is essen- 30,000 t/day, which it was argued, would gather placidly tially the same as that of the upper layer of Rupert Inlet, it on the bottom and not affect the aquatic environment. must at first begin to flood into Rupert Inlet as a surface The decision to use Rupert Inlet in this manner was clearly flow. (Maximum ebbs and floods of the surface current at based on ignorance. Subsequent studies have shown that the northern end of Quatsino Narrows can reach 3 m/s). the density current that sweeps along the bottom on the As the tide continues to rise, however, more and more flood picks up large quantities of fine tailings and carries comparatively dense water is brought in from the oceanic them to the surface along the northern side of the basin, side of the Narrows, with a subsequent increase in the where they become the light colored water seen in aerial density of the mixture produced near the sill. As a result, photographs (PI. 10). From there, the currents distribute the core of the high-velocity flood stream entering the the suspended wastes over large areas of the inlet before

- 75 - Simply stated, the water daceover which there is a hip amos heric pressure (that is, higher than nod) will tend to Ee depressed, whereas that over which there is a low atmospheric pressure will tend to be elevated Thus, any difference in the pressure between two regions will produce a tilt in sea level toward the region with the lowest atmosghericgresyre: For this to occur, however, there must e a re smbuuon of water via a current from the region of high pressure toward the region of low pressure (Fig. 4.8). Thespeedofsuchacurrentisusu~ysmalland distributed over the entire depth of the water column. Suppose a barometer at Campbell River registered a high pressure of 1010 mb when a barometer on SamIsland in the southern Saait of Georgia registered a low of 990 mb, an uncommon but not unlikely situation. The tilt of the water level along the Strait of Georgia would then become about 20 un or about 1 un for each 1 mb difference in pressure. But even if this difference were to build up over a short period of only 1 day and then hold steady, the current generated would only be around a few centimema per second. The same is true of currents generated when the pressure systems break down and the water surface once again becomes level. Nevertheless, in the deep ocean the amount of water transport associated with small sea surface slopes can be extremely large when the total depth is taken into consideration, and they are, therefore, important to the redisnibution of water in the 0 world oceans. (Due to the lis effect, the direction of ikml large-scale mean ocean currents is perpendicular to the sea surface slope.)

Low PRESSURE FIG.4.7. Distribution of mne railings on bottom of Rupert and HolbcrgInletat n\vdiffcrentdmcs, (A) March 1972 and (B) May-June 1974. The effluent (large concentrations of capper, manganese, chm- mium, mc, molybdenum, lead, ctc.) is normally diluted with xanmer and then discbarged at a depth of 50 m !.ia a marine oudall located inuned~atclywest of ?Jarrow Island Since dumprng bcgvl rn October 1971 tailings haw brm redistributed over a large maof bornby tidal currents. (Fmm Goyme and Nelson 1977) they again settle to the bottom (Fig. 4.7). Although there is no unequivocal evidence as yet to indicate a detrimental effea on marine life, the fine suspended pamculate matter of the wastes destroys the clarity of the water, reduces the amount of sunlight pcneuating the surface waters, accel- of marine equipment, and may even s of its own by altering the density of seawater. If nothing else, the situation of Utah Mines emphasizes the need for comprehensive oceanographic studies prior to granting pollution permits. Fro. 4.8. Longitudinal cross-secuon along Strat of Georgia from Dixovery Pass to Boundary Pass. Cbangc in sea-sdacc slopc prcduccd by along-strait difference in atmospheric prasure, exaggerated for il- Sea-Slope Currents lustraove purpows. hsrho\r. duection of (d)~rrm~ that x- company alteration in slope; broken lme is equilibrium sea level. The extent to which the difference in atmospheric pressure between widely spaced regions in the ocean influ- ences the curtens is not well known. Presumably such Jets and Eddies motions are of negligible importance to navigation, although they may have long-term dfects yet to be dis- Although they are primatidy associated with tidal covered. currents, jets and eddies are secondary flows in that they

- 76 - tend to be localized phenomena, which do not persist during periods of large discharge and large ebbs. As with throughout a complete tidal cycle. tidal jets, the momentum of the entrant water is Tidal streams from a narrows or passage may main- quickly diffused by the processes of lateral spreading, tain their inerna for many kilometres in the wider basin vertical mixing, and friction. This is particularly true of into which they flow. Under such conditions, they appear small rivers whose seaward influence is rapidly lost a short as a jet of water with large unidirectional speeds relative to distance from the ddta front. the surrounding water. Currents attain maximum speeds Intense jets dtedwith tidal passcs are gendy along the axis of the jet and decrease rapidly toward either accompanied by eddies and small whirlpools, which form side, where the flow becomes more irregular (Fig. 4.9). where the rapidly flowing water of the jet “N~s”against Tide rips accompany these jets whenever they oppose the the slower moving waters on either side. When viewed in propagation of surface waves. Tidal currents that flood the downsueam direction the eddies rum in a dockwise into the Strait of Georgia through Porlier, Active, and sense on the right hand side of the jet and in a coun- Boundary passes take the form of spreading jets and can terclockwise direction on the left hand side of the jet (see penetrate to about 2 km.There also is a tendency for these Fig. 3.27). These eddies often detach themselves from the entrant flows to follow curved trajectones as they encoun- edge of the mainstKam and wander into the surroundmg ter the tidal flow of the Strait. The jetlike flood of up to regions where they decay. 100 cm/s (2 kn) that enters the northem end of Johnstone Backeddies form downstream of promontories Strait from Weynton Passage penetrates the entire 400-111 where the water has been cut off from the main current, or depth and moves diagonally to the shoreline. The jet along the sides of strong tidal channels due to the retard- flooding out of Porlier Pass often turn northward to hug ing effect of the shoreline. In certain cases, such flow the coast of Valdes Island. reversals may intensify into narrow jets along the banks of Rivers enter coastal waters in the form of surface jets the channel, accompanied by overfalls of sharp drops in whose direction is affected by bathymetry, local tidal cur- the water level in the direction of flow. In Seymour Nar- rents, and the Coriolis force. In the main arm ofthe Fraser rows during a flood, strong northward counter flows are River, entrant speeds can be as great as 2.5 m/s (5 kn) found along both sides of the channel. There is a similar

Fic 4 9 Snongcbbcurrcnr flow southud through Dodd Narrows from Nonhumbcrlmd Chmncl nru Namuno. Aug 13.1%2 Flow IS rclanrclv lurunar (smooth) rhrough neck of inarron 5 but bccomcr. ~rhulentin broadrr basin to south (Courrcsq R H Hcrlinvcam)

- 77 - flow reversal to the north of Capc Mudge on a flood that Spit, a shoal that covers an area greater than the island, has become a favorite fishing haunt of thc Campbell River mail bc partlv maintained bv thesc cddics, which would sport fishermen. There is a highly visible nearshore coun- allow transpbrtcd sand from the upstream cliffs to be tcrflow in Active Pass just east ofMathcw Point on the ebb dumped into the morc quicsccnt watcrs behind thc island that is often emphasized by a change in the nature of the on thc ebb. (Howcvcr, the prcscncc of three prominent surface waves. spits along Cordova Channel to the wcst of James Island Onc of the more pronounccd backcddies in B.C. attests to prcdominantly northward littoral drift in this waters is between Victoria Harbour and Race Rocks dur- rcgion.) As a final example, thcrc is a strongcountcrclock- ing the flood. The main portion of the eastward tidal wise cddy during the flood at thc southcrn cnd of Haro stream continues toward Haro Strait rather than turning Strait (SCC Fig. 11.13). Surfacc currcnts associated with northward into thc harbor. A particularly interesting ex- this cddv, measured in 1979 by hydrographic launches ample of a backeddy is south of James Island near Sidney and airciaft-tracked floats, mav bc in exccss of 75 cm/s during an ebb. Aerial photographs show that large eddies (1.5 kn), and arc partly responsible for the confused and pccl off both sides of the downstream end of the island variable tidal currents that charactcrize the rcgion. and curve into the shadow zone behind (PI. 11). James

- 78 -