Currents in submarine canyons

Deep-Sea ltesearch, 1914, Vol. 21, pp. 691 to ~06. Pergamon ][Zrus. ltrinted ~n (~reat Brlta~n.

Currents in submarine canyons

F. P. SHEPARD,* N. F. MARSHALL* and P. A. McLouGHLIN*

(Received24 October 1973; in revised form 1 April 1974; accepted 1 April 1974)

Abstract--Earlier work indicated that currents move alternately up and down the floors of submarine canyons with greater average speeds and longer duration downcanyon. Now we find there are roughly synchronous movements up to at least 34 m above the canyon floors. The speeds decrease somewhat with height above the canyon floors and upcanyon flows are more significant. The patterns of up- and downcanyon flow at adjacent stations usually can he matched, indicating that the currents are related to internal waves. Crosscanyon flow occurs, particularly during periods of strong crosscanyon winds and usually has a definite sequence of repetition related to the tidal cycle. The strongest and longest transverse flows were in the broadest floored canyon. During storms with strong onshore winds, there are violent downcanyon flows that have carried current meters with them and eroded the floors, but the speeds are unknown.

INTRODUCTION SINCE 1968, we have been recording currents in submarine canyons, mostly along the California coast (St-n~ARD and MARSHALL, 1973a). In the first four years, most o f the records were f rom current meters suspended 3.6 m above the bottom, and all records were during conditions of relatively low winds, small wave heights, and little land run-off. Under these conditions, currents in the narrow V-shaped canyons alternate between down- and upcanyon flows with cycles varying from about 30 rain to approximately 12.5 h. Usually the longer periods were at the greater depths. The current speeds were all less than 36 cm s -1 and irregular, unlike the gradual build-up and decline of tidal currents in narrow straits. Greater average velocities were in the downcanyon direction. There was no indication in the early records that the velocities were influenced by the relative size of the waves and wind speeds during those comparatively calm conditions. However, a current meter disappeared during a storm, indicating the probability of strong currents.

We have now obtained records during periods of stormy seas. To examine the third dimension of the currents, we have taken simultaneous records at various heights above the bot tom and placed current meters at several positions along the canyon axes to find the relationship among separated stations (SI~PARO, MARSHALL and McLouGrILr~, 1974). Our new records indicate that the close relationship between current direction and canyon axis observed in the early records was not as constant as we had supposed and, particularly during periods of strong winds, much crosscanyon motion has been detected.

We have also measured currents in the canyon off the Congo River where there is a large land run-off.

*Geological Research Division, Scripps Institution of Oceanography, University of California, La Jo]la, California 92037, USA.

691

692 F.P. SHePARD, N. F. MARSHALL and P. A. McLoUGHLIN

CURRENT METER RECORDS Savonius rotor current meters developed by ISAACS, REID, SCHICK and

SCHWARTZLOSE (1966) have been used. The meters are lowered over the axes of canyons and set to return to the surface at predetermined times, when a small explosion releases the anchoring weights. After surfacing, a radio signal is transmitted and a flashing light operates. During operation, the meters are maintained at positions above the bottom by floats with considerable lifting power. Since the height is determined by the length of line above the bottom weight, the distance above the bottom is decreased slightly by strong currents. Several current meters can be used simul- taneously by attaching them at intervals along the line below the floats.

M E T H O D S O F P L O T T I N G R E S U L T S

Tapes provide continuous recordings of the current direction and indicate the speed by the spacing of tick marks, which are made after a pre-set number of rotations (2 to 256) of the rotor. The speed and direction have been read at intervals (2-5-15 rain) appropriate for the recording speed. These readings and the canyon trend are then computerized. The results include: (1) the velocities in relation to up- and downcanyon vectors computed by multiplying the vector by the cosines for the angle of divergence from the axial trend (Fig. 1); (2) velocities in crosseanyon vectors computed by multiplying the vector by the cosine of the angle of divergence from the normal to the canyon trend (Fig. 2); (3) absolute velocities in the up- and downcanyon quadrants (Fig. 3); (4) the cumulative total movement (net transport) during the period of the record (Fig. 4); and (5) the true direction and speed for each measured interval (Fig. 5). Tidal curves from nearby stations are superimposed on graphs l to 3.

COMPARISON OF CURRENTS NEAR AND WELL ABOVE CANYON FLOORS Our choice of distance above the bottom for the current meters was based originally

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Fig. 2. Sequence of crosscanyon components of current velocities adjusted for divergence from the direction normal to canyon axes.

A. Crosscanyon flows of the 375-m Hueneme Canyon Sta. at 3 m above bottom, showing relation of the strong southeasterly flows to early stages of successive flood tides.

B. Crosscanyon flows in the 357-m station of Santa Cruz Canyon at 29 m above bottom.

on finding as much as possible about transport of sediment and canyon erosion. Because of the movement of kelp and other material along the canyon floors and because of danger of damaging current motors on the bottom, motors wore placed betwaen 3 and 4 m above the floor for most of our early observations. Later, we added meters at various distances up to 88 m above the floor, placing the meters on the same line. In two operations we used three meters, and in six others two meters in La Jolla, Redondo, Santa Cruz, Carmel, and Monterey canyons (Fig. 6; Table 1). Included in Table 1 is a comparison of a record in Redondo Canyon whore the instrument was 88 m above the 274-m-deep floor. At this station, the current meter at 3 m failed, but b~ause the meter at 88 m was the only one so far above the canyon floor, we have averaged the 3-m records from the adjacent upcanyon and downcanyon stations and have compared the averages. Because some averages were meaningless, we have left blanks in the table.

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A. The four current meter records o f two Carmel Canyon stations showing similarity o f the records from different heights above bottom.

B. The records at the 206-m La Jolla station showing similarity o f the curves at 3.6 and 11 m above the bot tom but only a poor resemblance to the curve at 34 m.


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c. Net flows in the 357-m Santa Cruz Canyon Sta. 3 and 29 m above bottom. Indicates a general change in trend after strong winds changed from southeast to northwest about 30 h after the

record started. d. Net flows in Carmel Canyon at 205- and 348-m stations. Note alternating crosscanyon and

downcanyon flow at the 348-m station 3 m above bottom.

Table 2. Current speed and time in relation to height above canyon floor.

No. greater No. greater No. ties No. greater No. greater- No. ties near floor top meter near floor top meter

Maximum speed downcanyon Average speed downcanyon 7 3 1 7 3 1

Maximum speed upcanyon Average speed upcanyon 5 5 1 8 2 1

Although the comparisons are inadequate for good statistical analysis, the table gives some idea of the relation of the currents near the canyon floor to those farther above the floor. Our results suggest that the currents are generally slower at greater heights above the bottom (Table 2).

The percentages of time in which the current was flowing down-, up-, and crosscanyon show a clear relationship to the distance above the bottom. The near-floor current meter showed more downcanyon currents in seven of the comparisons and fewer in four. The crosscanyon flows, somewhat surprisingly, occurre~i for larger

698 F .P . SHEPARD, N. F. MARSHALL and P. A. McLouGm.IN

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A. The plot shows large crosscanyon flows in Huoneme Canyon at the 375-m station. Fastest currents are only approximate.

B. Records from Carmel Canyon at 205- and 348-m depths. Note the unusual relationship of what appears to be upcanyon current at the 348-m station 3 m above bottom (se. also Fi$. 7).

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percentages of time near the canyon floor in eight comparisons and smaller in three. We had anticipated that some of the currents at higher levels would be the reverse

of those near the bottom, but the directions at the various heights coincide to a considerable degree and even the speeds are closely similar. One means of comparison is to find the percentage of current peaks in both up- and downcanyon directions that approximately coincide at the two levels (Table 1). Figure 2 gives an example of the best agreement (Carmel Canyon) and tbe one with the least (La Jolla Canyon, Sept. 1972). In the best example, the records at both stations are remarkably similar, both in direction and speed. Even the example with the least agreement shows some similarity between the 3.6- and 34-m currents. In this three current-meter plot, the

700 F . P . SHEPARD, N. F. MARSHALL and P. A. McLOuoHLIN

fastest downcanyon current was at about the same time at all three heights, and 46 % of the downcanyon peaks greater than 8 cm s -~ coincide. That is far better agreement than would come from superimposing random curves from the records. Furthermore, at these two levels in La Jolla Canyon, only during 19% of the time did the currents flow in opposite directions.

The net flow diagrams (Fig. 4) show that of the ten comparisons of stations with averages in Table 2 (omitting Redondo Canyon at 274 m), eight have net movement in the same (up- or downcanyon) direction. Even for the one case where the net direction was opposite (Fig. 4a) there was a considerable agreement of the times and directions of peak flows.

CAUSE OF CROSSCANYON CURRENTS

In earlier work, during average weather conditions, we failed to observe any relationship between the surface wind direction and the divergence of current direction from the canyon axes. Our recent records, some made during moderately stormy conditions with winds up to at least 40 km h -1, show much greater crosscanyon flows. Current meters operated at three stations in the relatively wide-floored Hueneme Canyon during a period of winds from the west of about 40 km h -1. The canyon extends north and south (Fig. 7), but all the records show a net movement to the east (Fig. 4b). The polar plot for the 374-m station (Fig. 5A) shows fast crosscanyon currents to the southeast and a wide spread of directions between 106 and 285 °, with little if any concentration in the south-southwest (downcanyon) direction. Although most of these differences were in the direction of the wind, they also show a relationship to the tides (l 2-h 26-min interval), because most of the periods of strong southeasterly flow occurred between the time of peak low and peak high tide at Hueneme (Fig. 2A). A similar but less striking relationship was observed earlier at the same station. These indications of water circulation from Santa Barbara Basin across Hueneme Canyon into Santa Monica Basin were also found by GORSLrm~ (1970).

Similar indications come from observations at heights of 3 and 29 m at a station in Santa Cruz Canyon. The wind was from 25-40 km h -1 from the south and southeast for the first third to half of the period and from the northwest for the remainder. The net flow (Fig. 4c) displays rather clear directional changes that coincide approximately with these wind shifts, particularly for the higher meter. The strong crosscanyon flows correspond with the tidal curve (Fig. 2B), occurring mostly during the early flood stage.

Another case, which may have some relation to wind direction, was in the Carmel Canyon records. During most of this period, the wind was about 40 km h -~ from the south. The polar plots twice indicate deflection in a northerly direction, At one station, the northerly deflection was at the higher meter, but at the other station it was at the lower meter (Figs. 4d, 5B). The difference at the two stations may be because the topography around the deeper station probably resembles that shown in Fig. 8, which would cause the upeanyon flow to be diverted by the topographF just downcanyon from the station. The wind may have deflected the flow still farther north. The upper meter, being well above the bottom, was less aff~t~l by the jog in the canyon. At the shallower station, the general west-northwest axial direction con- trolled both up- and downcanyon flows, with much more control near the bottom,


Fig. 8.

Carmel Sta.32 Lo 1 548m 3mAB

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> Down canyon flow - - - e Up canyon flow

Suggested flow pattern to explain crosscanyon currents at the 348-m Carmel Canyon Sta.

and was affected more by wind-induced currents farther above the bottom. The crossflows are related to the local tidal cycle.

TIME R E L A T I O N S H I P S OF C U R R E N T P A T T E R N S FOR A D J A C E N T S T A T I O N S

In earlier work, we sometimes placed two current meters along the axis of a canyon, but the curves for up- and downcanyon flows could only be matched when the stations were close together. Recent data show better correlations (StmPARD, MARSHALL and McLOU~HLIN, 1974). Except where one of the current meters was nearshore and less than 100 m deep, we can match the records by moving them time- wise. Thus, comparing the records for the two adjacent stations in Monterey Canyon by moving the starting point for the 357-m station to a position 84 rain later than for the 384-m station, the records match well (Fig. 9A). This suggests that alternating up- and downcanyon flows are related to internal waves and, in this case, the wave sequence arrived 84 rain later at the shallower station; in other words, the waves were moving upc~tnyon. Six of these comparisons show an upcanyon propagation, but in another the propagation was downcanyon (Fig. 9B). Where there were simultaneous observations at three stations in Hueneme and Monterey canyons, the propagation was upcanyon. The downcanyon propagation was based on a rather poor fit of curves and would require more measurements for verification.

The speed of advance of these internal waves along the canyon axes can be determined by comparing the distance between stations and the difference of time of arrival. (Fig. 9B). The speeds are not considered absolutely accurate, particularly where the fit of the curves was not convincing. The best fits---Carmel, Monterey deep stations, and Hueneme deep stations---show rates of 83, 25, and 50 cm s -1, averaging close to 50 cm s -1. This is about twice as fast as internal waves move in over the shelf near Mission Beach, San Diego (LAFOND, 1966).

The downc~nyon advance of internal waves in Santa Cruz Canyon may be related to the geographic position and the consequent transfer of water across the sill between

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internal waves moved up the canyon at a phase velocity of about 25 cm s -x. B. Time comparisons between adjacent stations in the canyons showing separation distances, depths of the meters, estimated phase velocities, and directions. The dasl-~d line repre~'nts the

uncertain fit from the Santa Monica Canyon stations.

Santa Rosa and Santa Cruz islands. The head of Santa Cruz Canyon is in this sill (Fig. 7), suggesting that the source of the internal waves is in Santa Barbara Basin.

TIDES IN RELATION TO CURRENT REVERSALS The up- and downcanyon cycles for our earlier records were generally much


shorter than the 12 h 26 rain of a typical tidal cycle. However, the new data indicate that the cycles at most of the stations deeper than 250 m approximate the lunar half day or the lunar day (25 h), and spectral analysis shows the tidal period to be an im- portant or dominant factor. For example, the average cycle in Carmel Canyon at 205 m is about 3.5 h, but six out of seven of the highest upcanyon peaks are separated by approximately 12.5 h (Fig. 1).

One might expect upeanyon currents during rising tides and downoanyon during ebbs, but our data show only one such instance. At the mouth of the Congo Estuary, perfect agreement with the tides at a nearby station was observed (Sam,~.D and E ~ R e , 1973). Tides in canyons may not coincide with those at nearby tidal stations, so there may be more agreement than our records indicate.

CURRENT SPEEDS RELATED TO STORM CONDITION Earlier data showed no relationship between wave height and wind velocities,

but there were no observations during periods with more than 28-kin h -1 winds. Now we can compare observations at the same stations under both average and stormy conditions. In Carmel Canyon, currents were considerably stronger during a period of rough seas and strong winds. Maximum upcanyon currents were 28.4 cm s -1 during stormy weather and 20.0 cm s -~ under average conditions. The maximum downcanyon currents were 31.6 cm s -~ in the stormy weather and 19.3 cm s -1 during ordinary weather. Average upcanyon currents were 12.5 cm s -~ during stormy weather, 5.1 cm s -~ for normal conditions, and average downcanyon currents were 14.9 cm s -1 for stormy weather and 7.6 cm s -~ under normal conditions.

In Hueneme Canyon, there were relatively high waves during both periods of observation, but during the second the winds were much higher. In the latter period, the currents were too fast to be measured accurately,* but the mean speed was observed. The currents were faster during the strong winds, and high-sp~d cross- currents occurred only during the stormy period (Fig. 2A).

The average currents during a storm of December 2, 1972, at La JoUa (SI-mPARO and MARSHALL, 1973b) were not high during its early stages (Fig. 10), but a current of 50 cm s -1 was recorded just before the cunent meters stopped recording, probably when they became wrapped in kelp. Later, the meters were swept downcanyon, suggesting that even higher speeds developed.

Indirect evidence of high-speed currents during storms comes from our loss of current meters on two occasions when there were onshore winds in excess of 46 km h-k INMAN (1970) also lost a current meter during such conditions. Currents measured by GENNESSEAUX, GUIBOUT and LACOMBE (1971) in Var Canyon off the Vat River in the French Riviera had velocities up to 100 cm s -1. This was during storm conditions but could have been related to concurrent river floods, which carried abundant sediment to sea. To date, we have no good current records from California canyons during rain storms. During the March 1938 flood, Shepard had a sediment trap in La Jolla Canyon with partitions and a compass to show directions of the derivation of sediment. He found as much sediment coming from the downcanyon direction as from upcanyon, suggesting that the currents at that time were alternating as usual and not predominantly downcanyon.

*The record was run with only four rotations for each tick mark, and some of the unusually fast currents caused too much crowding of the ticks to be read.

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CAUSES OF CANYON CURRENTS

The recent increase in our knowledge of currents in submarine canyons makes it possible to speculate with more assurance concerning the underlying causes of these currents than was possible in an earlier report (SH~y.v and MARSHALL, 1973a). There are at least two types of currents in the canyons. Under ordinary conditions, the currents alternately flow up- and then downcanyon with speeds that are usually less than 40 cm s-L Rarely, perhaps once a year, strong currents flow down the canyons with snflicient power to cut trenches into the sediments and to transport large blocks of rock (SHePARD and MARSHALL, 1973a; SH~PARD, 1973, p. 308).

We think that normal currents are caused by some type of internal wave or tide that progresses along the floors of canyons with a phase velocity of about 50 cm s -x, considerably greater than the actual speeds of up- and downcanyon currents (SH~PARV, M ~ L L and McLoUGHLIN, 1974). Evidence that points to internal waves as the cause is primarily their resemblance to the internal waves on the adjacent continental shelves (LAFOND, 1966). Some of the latter are of tidal period (SUMMERS and EMERY, 1963). Internal waves are also known to progress mostly shoreward with phase velocities that are slightly slower than those in the canyons, where one would expect a concentration of energy. The approximately synchronous up- and downcanyon movements that occur to heights of 30 m or perhaps even 90 m above the floor are also characteristic of internal waves. The strong influence of the tides is seen in all the deeper water stations, but it is also recognizable among the shorter period cycles in the canyon heads (Fig. 1). The normal currents are sufficiently strong (~18 cm s -x) during peak flows to transport fine sand along the axes of canyons. Because the more frequent and faster flows are mostly downcanyon and because downcanyon movement is facilitated by gravity, net transport from these currents is largely seaward.

Little has been learned from our program about the occasional strong downcanyon flows, presumably turbidity currents, aside from the discovery that current meters may be carried away during storms with onshore winds.

Crosscanyon currents, at first thought to be of minor importance, now prove to be siLmificant during some periods of strong crosscanyon winds. The crosscanyon flows are related to the tidal cycles, but they are also in the general direction in which the wind is blowing. This relationship was also observed by CANNON, LAIRD and RY^N (1972) in the broad trough coming out of Juan de Fuca Strait. The wind appears to be particularly influential where submarine valleys are relatively broad, as in Hueneme Canyon. Some crosscanyon flows, as in Carmel Canyon (Fig. 6), are probably related in part to the sharp curves in canyon axes, but the wind direction appears to play some role. Helical flows may be set up by the wind as they are in streams (En~sTFJN and LI, 1958). More information is needed to understand these flows across the canyon axes.

Acknowledgements--Supported by NSF Grant GA-19492 and by ONR contract Nonr-2216(23). Ship time was given us on Velero I V by the Hancock Foundation of the University of Southern Californm and on the Aeania by the U.S. Naval Postgraduate School at Monterey, California. The cooperation of David E. Drake, Donn S. Gorsllne, and Robert E. Andrews is gratefully acknowledged. We also appreciate the field work of Gary G. Sullivan and the ideas contributed by C. S. Cox, both of Sm'ipps Institution of Oceanography.

706 F.P. SHEPARD, N. F. MARSHALL and P. A. McLOUGHLIN

REFERENCES CANNON G. A., N. P. LAmD and T. V. RYAN (1972) Currents observed in Juan de Fuca

submarine canyon and vicinity, 1971. National Oceanic and Atmospheric Administration Technical Report ERL-252-POL 14, 57 pp.

EINS'~tN H. A. and HHON LI (1958) Secondary currents in straight channels. Transactions of the American Geophysical Union, 39(6), 10851-088.

GENN~SS~.Atrx M., P. Gtnaotrr and H. L^co~aE (1971) Enregistrement de courants de tur- bidit6 darts la vall~ sous-marine du Var (Alpes-Maritimes). Compte rendu hebdomadaire des s~ances de l'Acad~mie des sciences, Paris, 273, 2456-2459.

GORSUN~ D. S. (1970) Report of a reconnaissance survey of the hydrographic characteristics of the Hucaerae-Mugu shelf to evaluate ground-water leakage from submarine exposures of coastal acquifers, June-July, 1970. University of Southern California, Geology Rep. 70-6, Los Angeles, 21 pp.

IN~N D. L. (1970) Strong current in submarine canyons. Abstracts, Transactions of the American Geophysical Union, 51(4), 319.

ISAACS, J. D., J. L. REID, JR., G. B. StruCK and R. A. SCHWARTZLOSE (1966) Near-bottom currents measured in 4 kilometers depth off the Baja California coast. Journal of Geo- physical Research, 71(18), 4297-4303.

LAFoND E. C. (1966) Internal waves. In Encyclopedia of oceanography, R. W. FAmaRIDOE, editor, Reinhold Publishing Corporation, New York, p. 402.

SHEPARD F. P. (1973) Submarine geology, third edition, Harper & Row, New York, 517 pp. SI~PARD F. P. and K. O. EMERY (1973) Congo Submarine Canyon and fan valley. Bulletin

of the American Association of Petroleum Geologists, 57(9), 1679-1691. SnEPARD F. P. and N. F. MARSHALL (1969) Currents in La Jolla and Scripps Submarine

Canyons. Science, 165, 177-178. SHEPAnD F. P. and N. F, MARSHALL (1973a) Currents along floors of submarine canyons.

Bulletin of the American Association of Petroleum Geologists, 57(2), 244-264. SHEPARD F. P. and N. F. MAaSHALL (1973b) Storm-generated current in La Jolla Submarine

Canyon, California. Marine Geology, 15(1), MIg-M24. SHEPARD F. P., N. F. MARSHALL and P. A. McLoHonLIN (1974) "Internal waves" advancing

along submarine canyons. Science, 183(412!), 195-198. S ~ H. J. and K. O. E~RY (1963) Internal waves of tidal period off Southern California.

Journal of Geophysical Research, 68(3), 827-839.

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Currents in submarine canyons