Storm-generated current in La Jolla Submarine Canyon, California

Marine Geology, 15 (1973): M19-M24 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

Letter Section S t o r m - g e n e r a t e d c u r r e n t in L a J o l l a S u b m a r i n e C a n y o n , C a l i f o r n i a

FRANCIS P. SHEPARD and NEIL F. MARSHALL

Geological Research Division, University of California, Scripps Institution of Oceanography, La Jolla, Calif. (U.S.A.] (Accepted for publication June 14, 1973) ABSTRACT Shepard, F.P. and Marshall, N.F., 1973. Storm-generated current in La JoUa Submarine Canyon, California. Mar. Geol., 15: M19-M24. Current meters were operating in La JoUa Submarine Canyon at 200 m depth during a period of high seas and onshore winds up to 62 km/h (34 knots). The meters were subsequently extracted from a kelp tangle by use of a deep-diving vehicle 0.5 km downcanyon from their emplacement position. The records show a downcanyon speed up to 50 cm/sec, considerably higher than any of our numerous earlier measurements. This was followed by an abrupt termination of data, evidently due to being engulfed in seaward-moving kelp masses. The record may provide evidence of the initial stages of a turbidity current. The conditions for such a current were provided by the piling up of water at the canyon head by the unusually strong onshore wind.

INTRODUCTION Daly (1936) first suggested turbidity currents as the cause of submarine canyons. Since then there has been considerable supporting evidence, at least of the importance of such currents in canyon development, but velocities attained by these currents have had to be estimated largely from theoretical considerations (Inman, 1963; Middleton, 1966a, b; Komar, 1969). Speeds up to 90 k m / h have been attributed to them by sequences of cable breaks following earthquakes (Heezen and Ewing~ 1952), but these excessive speeds have been questioned (Shepard, 1963; Menard, 1964). Attempts to measure currents on the canyon floors so far have failed to confirm the existence o f high-speed flows, although Inman (1970) had a current meter in the head of Scripps Canyon (California) during an onshore blow with a large surf, and measured a velocity of 160 cm/sec of downcanyon flow before the connecting wire broke and the current meter was swept down the canyon. Also, diving in Rio Balsas Canyon, Mexico, during stormy conditions, Reimnitz (1971) observed a downcanyon flow along the b o t t o m estimated to be about 70 cm/sec.

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CURRENT METERS DRAGGED DOWNCANYON During numerous measurements of canyon-floor currents in La Jolla Canyon and elsewhere along the West Coast (Shepard and Marshall, 1973), we have emplaced the free-fall current meters mostly during conditions of low-wave energy. In one case, we installed a current meter just before an onshore blow. This meter failed to surface after the allotted interval, although three floats were recovered much later along the shoreline to the south. In early December 1972, we placed two current meters on the same line to operate at 2 m an.d 4 m above the floor of La Jolla Canyon at approximately 200 m depth. Three days after installing the meters, an early season storm developed high-surf conditions and, according to the Navy tower records off nearby Mission Beach, onshore wind gusts attained velocities up to 62 km/h (34 knots) (Fig. 1). These were exactly the conditions that led to the loss of the other meters, so we were not surprised when the current meters failed to surface on schedule. A few days later, we obtained the use of the small deep-diving submersible "Nekton" from General Oceanographics. Diving in the canyon, we passed the area where the current meters had been dropped and saw no sign of the apparatus. Proceeding downcanyon for 0.5 km, we observed a large mass of kelp on the canyon floor, and rising out of it was the rope held up by one of our recovery floats. Grabbing the float with the manipulator arms, the "Nekton" proved to have sufficient lifting power to pull the current meters out of the sediment and kelp tangle. The anchoring weight had been dropped by the action of the explosive release that was set to go off ten days after installation. The current meters and the records were not damaged by the transportation downcanyon. Up to the time of the storm, the 2-m current record was normal for La Jolla Canyon, with predominating directions alternating between 320 ° downcanyon and 140 ° upcanyon. However, the 4-m current meter, which was partially tangled in its own mooring lines, as observed during recovery, showed in its record currents alternating between 60 ° and 160 ° . Because the changes correspond very closely in time with the 320 ° and 140 ° , respectively, of the 2-m record, coupled with the obvious channeling effect of a canyon, and have a close resemblance in velocity (Fig. 1), we are convinced that these data are useful and comparable to the 2-m data. NATURE OFRECORDS Both records indicate that there was a relatively strong downcanyon flow a few hours after the highest recorded wind velocities (Fig. 1). In Fig. 1, speeds are averaged for 5-min intervals, but in Fig.2, they are averaged for 1-min intervals for the last 25 min. The current speed (shown by number of marks placed on the record, each representing eight rotations of the Savonius rotor), reached 50 cm/sec at 2 m and somewhat less at 4 m (Fig.2). The marks stopped after about 10 min (end of records in Fig. 1 and 2). No further marks were made for two hours. We assume that the halt in the record was due to drifting kelp that tangled the rotors. After two hours, more marks indicate that the kelp must have been

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Fig.1. Relation of up- and downcanyon flow velocities at 2 m and 4 m above bottom. Assumption for 4-m record made as noted in text. Speeds averaged for 5-rain intervals. Wind speeds for the storm period compared in right-hand portion of record. See also Fig.2.

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Fig.2. Current speeds averaged for each minute just before the seaward-movingkelp clogged the rotors. Higher peak speed than in Fig.1 due to averagingeach minute. temporarily removed, but shortly afterwards kelp drifted onto the rotors again and no more marks were made for the remainder of the record. The records of the two meters show some interesting differences during the time just before the kelp interfered (Fig. 1 and 2). In Fig. l, the current record at 4 m shows a change from upcanyon direction to downcanyon, 5 - 1 0 min before the reversal in the record from 2 m. The highest velocities were observed at about the same time at 4 m and 2 m (Fig.2). The current surge at 4 m built up quickly to its greatest strength at first and died out gradually; whereas, at 2 m, the downcanyon current was slow to build up at first and then shows two spurts of high current, each lasting about 2 min, followed by an abrupt termination of data. Fig. 1 shows a record that is typical of the normal alternating up- and downcanyon flows in submarine canyons (Shepard and Marshall, 1973) up to 1200 h, when the downcanyon current reached 27 cm/sec, which is unusually high. This burst of speed occurred shortly before the highest wind gusts in nearby Mission Beach area. The record that follows shows an unusual amount of downcanyon current until one brief but large upcanyon flow at 2000 h. Then two hours of extremely small current followed with one moderate upcanyon surge and a short episode of no current preceeded a record downcanyon flow at 2325 h (Fig.2). The downcanyon transport of the current meters must have followed. Presumably, this last surge represented the early stage of a turbidity current that had built up gradually as the result of the surf beat induced by the strong onshore wind. Thus, it may show the sequence of current conditions preceeding and during the early stages of a turbidity current. Obviously, more records will be necessary to justify any such conclusion.

LETTER SECTION

M23

NEWLY ERODED CHANNEL While in the " N e k t o n " looking for the current meters, another indication o f fairly strong downcanyon currents was discovered. The floor of the canyon was found to have a channel about 5 m wide cut in the fill, with steep walls somewhat less than 1 m in height. Marshall spotted an aluminum cocktail-pudding can protruding from one wall o f this fill. This was recovered and found to be of a type first manufactured about two and a half years ago. Therefore, the fill was of very recent origin. Presumably, this channel was excavated by the current that carried our current meters downcanyon, although other weaker storms had occurred during this early winter period. CONCLUSIONS We can conclude from the available evidence that after some lag the heavy surf and powerful onshore wind probably developed a downcanyon flow, apparently as the result of piling up of water along the shore with a return underflow that may have stirred up enough sediment to produce a turbidity current. In this case, there is no evidence of currents of the high velocities often attributed to turbidity currents. However, it is very possible that the currents increased after the kelp stopped the rotors, but a much higher velocity probably would have carried away the kelp mass in which the current meters were buried, and very likely would have swept the canyon clean of debris and sediment, leaving rock b o t t o m . This was seen nowhere. ACKNOWLEDGEMENTS This work was made possible by the National Science Foundation Contract GA-19492 and the Office of Naval Research Contract Nonr-2216(23). Gary Sullivan and Pat McLoughlin participated in the field work and also the data reduction. REFERENCES Daly, R.A., 1936. Origin of submarine canyons. Am. J. Sci., 31(186): 401-420. Heezen, B.C. and Ewing, M., 1952. Turbidity currents and submarine slumps, and the Grand Banks earthquake. Am. J. Sci., 250: 849-873. lnman, D.L., 1963. Ocean waves and associated currents. In: F.P. Shepard (Editor), Submarine Geology. Harper and Row, New York, N.Y., 2nd ed., pp.138-140. Inman, D.L., 1970. Strong currents in submarine canyons. Abstr. Trans. Am. Geophys. Union, 51(4): p.319. Komar, P.D., 1969. The channelized flow of turbidity currents with application to I~onterey deep-sea fan and channel. J. Geophys. Res., 74(18): 4544-4558. Menard, H.W., 1964. Marine Geology of the Pacific. McGraw-Hill, New York, N.Y., pp.207-210. Middleton, G.V., 1966a. Experiments on density and turbidity currents, 1. Motion of the head. Can. J. Earth Sci., 3: 523-546. Middleton, G.V., 1966b. Experiments on density and turbidity currents, 2. Uniform flow of density currents. Can. J. Earth Sci., 3: 627-637.

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Reimnitz, E., 1971. Surf-beat origin for pulsating bottom currents in the Rio Balsas Canyon, Mexico. Geol. Soc. Am. Bull., 82: 81-90. Shepard, F.P. (Editor), 1963. Submarine Geology. Harper and Row, New York, N.Y., 2nd ed., pp.339-343. Shepard, F.P. and Marshall, N.F., 1973. Currents along floors of submarine canyons. Bull. Am. Assoc. Pet. Geol., 57(2): 244-264.