Sediment concentrations, flow conditions, and downstream evolution of two turbidity currents, Monterey Canyon, USA

Deep-Sea Research I 89 (2014) 11–34

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Deep-Sea Research I journal homepage: www.elsevier.com/locate/dsri

Sediment concentrations, flow conditions, and downstream evolution of two turbidity currents, Monterey Canyon, USA J.P. Xu a,n, Octavio E. Sequeiros b, Marlene A. Noble a a b

U.S. Geological Survey, Santa Cruz, CA 95060, USA Shell Global Solutions International, Kessler Park 1, 2288 GS Rijswijk, The Netherlands

art ic l e i nf o

a b s t r a c t

Article history: Received 27 September 2013 Received in revised form 25 March 2014 Accepted 5 April 2014 Available online 13 April 2014

The capacity of turbidity currents to carry sand and coarser sediment from shallow to deep regions in the submarine environment has attracted the attention of researchers from different disciplines. Yet not only are field measurements of oceanic turbidity currents a rare achievement, but also the data that have been collected consist mostly of velocity records with very limited or no suspended sediment concentration or grain size distribution data. This work focuses on two turbidity currents measured in Monterey Canyon in 2002 with emphasis on suspended sediment from unique samples collected within the body of these currents. It is shown that concentration and grain size of the suspended material, primarily controlled by the source of the gravity flows and their interaction with bed material, play a significant role in shaping the characteristics of the turbidity currents as they travel down the canyon. Before the flows reach their normal or quasi-steady state, which is defined by bed slope, bed roughness, and suspended grain size, they might pass through a preliminary adjustment stage where they are subject to capacity-driven deposition, and release heavy material in excess. Flows composed of fine (silt/ clay) sediments tend to be thicker than those with sands. The measured velocity and concentration data confirm that flow patterns differ between the front and body of turbidity currents and that, even after reaching normal state, the flow regime can be radically disrupted by abrupt changes in canyon morphology. Published by Elsevier Ltd.

Keywords: Turbidity currents Sediment transport Grain size Monterey canyon

1. Introduction Turbidity currents are sediment-laden gravity flows that are capable of transporting large amount of material across continental margins. Large scale turbidity currents can run out for hundreds, even thousands of kilometres offshore (Walker, 1978; Wynn et al., 2002; Pirmez and Imran, 2003) to produce large submarine fans that in geological time have become host of hydrocarbon reserves (e.g. Weimer and Llink, 1991; Weimer and Slatt, 2006). Aside from the economic importance, turbidity currents in the ocean pose serious hazards to submarine communication cables (Heezen and Ewing, 1952; Hsu et al., 2008) that modern-day society and commerce are depending on. For these reasons, turbidity currents have been the interest of many investigations in the past century (Sequeiros, 2012, Table 1). Ever-improving instrumentations have advanced our study of field turbidity currents from simple event detection 30 years ago to today's quantitative characterization (Prior et al., 1987; Xu et al., 2002; Puig et al., 2003; Canals et al., 2006; Khripounoff et al., 2012), however, our understanding of most

n

Corresponding author. Tel.: þ 1 831 426 7426. E-mail address: [email protected] (J.P. Xu).

http://dx.doi.org/10.1016/j.dsr.2014.04.001 0967-0637/Published by Elsevier Ltd.

dynamic and sediment parameters of turbidity currents still largely rely on laboratory experiments (Sequeiros et al., 2010a and reference therein) or numerical models (e.g. Parker et al., 1986; Necker et al., 2002; Huang et al., 2005, 2007; Cantero et al., 2007, 2008). Because of the destructive nature of turbidity currents, sediment concentration and sediment grain size are very difficult to measure directly in the field. In particular it is difficult for moorings and instrumentation to survive the front of the currents that is both fast and destructive. After the front passes, the body of the flow, which may travel at a different speed depending on the bed slope (Keulegan, 1957; Middleton, 1966a, 1966b), tends to reach the normal flow conditions, also known as quasi-steady state or equilibrium conditions, defined by morphology and suspended sediment characteristics (Sequeiros, 2012). In normal conditions, changes in the flow properties in the downstream direction are limited to the effects of the gradual entrainment of ambient water and entrainment/deposition of suspended sediment. Velocity remains reasonably constant as a result of the balancing effects from a declining density and an increasing current thickness due to ambient fluid entrainment. These conditions mostly refer to the body of the current, not to the front. Normal conditions have been achieved under laboratory conditions with constant bed slope and controlled input rates (e.g. García, 1993, 1994; Sequeiros et al., 2010a).

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J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Table 1 Nominal parameters of the three mooring in Fig. 1. Canyon Sensor orientation heights (MAB)

Instruments

36146.300 N; 260 121157.790 W

201

320 190 67

T/Tr T/Tr ADCP

R2 1020

36146.820 N; 260 122100.810 W

1341

310 180 70 67

T/Tr T/Tr T/Tr ADCP

R3 1445

36143.170 N; 530 122100.750 W

101

165 70 67 16

T/Tr T/Tr ADCP T/Tr

Site

Water depth (m)

Position

R1

820

Thalweg width (m)

preliminary findings of the turbidity current events (Xu et al., 2004) and a normalized analysis of the flow velocity profiles (Xu, 2010). The aim of the present paper is to investigate the hydraulic and sedimentary behavior of field turbidity currents whose fundamental parameters are undoubtedly the best guidelines for future physical experiments or numerical models. Here we presents the data, analyze, and results of two turbidity currents that display distinctive sedimentary and hydraulic properties. Flow structures, grain sizes and sediment concentrations and their temporal and spatial variation during the flows' downstream evolution along the Monterey Canyon are presented. Lessons learnt with respect to the practicality of designing a comprehensive monitoring program and its implication to regional and international applications are discussed.

2. Instruments and methods In natural canyons with changing slopes that create gradual accelerations they are more difficult to attain and maintain for long stretches. While the down-slope change of sediment properties, e.g., grain size decreases toward the lateral and distal margin of the turbidities (deposits by turbidity currents), has been well established from coring data (Dennielou et al., 2006; Armitage et al., 2010; Babonneau et al., 2010; Mas et al., 2010; Bourget et al., 2013), outcrop observations (Komar, 1985; Talling et al., 2007; Sumner et al., 2012), and physical or numerical experiments (Lüthi, 1981; Groenenberg et al., 2009; Abd El-Gawad et al., 2012; Huang et al., 2012; Pyles et al., 2013), our understanding of the spatial change of sediment properties within the body of turbidity currents are still limited to laboratory experiments (Gladstone et al., 1998; Mulder and Alexander, 2001; Kubo, 2004; Sequeiros et al., 2009) and numerical models (Felix, 2002; Huang et al., 2005, 2007; Nasr-Azadani et al., 2013). The coarsest grains were found within the head of the flows, and there was a vertical gradient of both concentration (decreasing upward) and sediment grain sizes (finer upward) within the body of turbidity currents (García and Parker, 1993; McCaffrey et al., 2003). Similarly, Felix et al. (2005) and Alexander et al. (2008) demonstrated that the highest concentrations were within the head as well as within the dense basal layers of turbidity currents. Vertical gradients within the body and the head-body difference are attributed to the balance between the upward directed turbulence and particle settling (Middleton and Southard, 1984; Baas et al., 2005). Using a 2-D numerical model to describe the interaction between velocity, turbulence, and sediment distribution, Felix (2002) showed a clear dependency of flow properties upon sediment grain sizes: low vertical, high horizontal gradients of sediment concentration for fine-grained turbidity currents that decelerate slowly; and high vertical, high horizontal concentration gradients for coarse-grained flows that decelerate rapidly. Salaheldin et al. (2000) model of non-uniform sediment grains numerically verified a finding in an earlier flume experiment (Gladstone et al., 1998): increasing the content of fines (clay and silt) in a turbidity current enhances both the mobility and sand-carrying capacity of the flow. In 2002, a field study aimed at monitoring turbidity currents was conducted in Monterey Canyon. Because it was specifically designed for studying field gravity flows in a submarine canyon environment, the flow and sediment data collected within as well as above the turbidity currents are by far the most complete data set (Meiburg and Kneller, 2010). Among all the oceanographic and sediment time-series this study collected, in-situ sediment concentrations and sediment grain size are the most unique that have not been achieved by previous field investigations. This is the third paper based on this field study, following the introduction and

2.1. The 2002 field experiment The data are from a year-long (December 2002–November 2003) deployment of three moorings placed at water depths of 820, 1020, and 1445 m (Fig. 1, Table 1). Each mooring was deployed in the center of the thalweg so that the current meters could record the maximum flow velocity in each cross-canyon section. A total of 13 instrumented packages (Fig. 2) were mounted on the moorings at 16–740 m above bed (MAB). Each package included one Aanderaa RCM current meter, one Seabird Seacat or Microcat that measured temperature and conductivity from which salinity was computed, one Seatech 25-cm path-length transmissometer that measured light transmission (turbidity), and one Anderson-type sediment trap. On packages C, G, and L (Fig. 2) the RCM current meters were replaced with downward-looking RDI ADCPs (Acoustic Doppler Current Profilers) that measured velocity profiles from  60 MAB to the canyon floor. There was no current meter on packages D, H, and M. The ADCPs (all 300 kHz, one Narrow Band model on package C, one Broad Band model on G, and one Workhorse model on L) were configured to record a velocity profile by averaging 60 one-second pings every hour on the hour. The vertical bin size of the ADCPs was 2 m, providing  30 data points in each velocity profile. The RCM current meters recorded flow speed and direction every 20 min. Temperature, conductivity, and light transmission were recorded at a higher rate – intervals ranged from 5–20 min. Each sediment trap consisted of a baffled collecting funnel with a 0.25 m2 opening on the top and a 5cm diameter, 1-m long polycarbonate tube liner attached to the bottom of the funnel. To prevent bioturbation of the collected sediment, a supersaturated solution of seawater and sodium azide was slowly introduced to the top of the tube liner via a dripping device. In 5 of theses sediment traps (C, D, G, H, and M) an “intervalometer” (Rendigs et al., 2009) mounted inside the funnel dispensed a small amount of Teflon beads onto the sediment surface inside the liner, providing a manual time mark every 20 days. The distribution of floatations on each of the three moorings were designed such that all sediment traps stayed upright during recovery so no sediment was lost from the traps. There were two acoustic releases on each mooring: a primary one just above the anchor and a backup unit below the ADCP. The three moorings were deployed on 5–6 December 2002 (GMT) and scheduled for recovery on 25 November 2003. On 19 November 2003, the R1 mooring surfaced because of a broken mooring wire that left instrument package D unrecovered. The backup acoustic release on the R2 mooring had to be applied due to a failure of the primary release, resulting in the loss of instrument package H on the bottom of the R2 mooring. Mooring R3 was retrieved without any problem.

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

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-122°

-121°45'

37°

Water depth, m

0

-500

R1

-1000

R2 R3

-1500

-10000

-5000

0

5000

Distance, m

10000

n yo an C el qu So

RIN

R1 R2 sRIN

R3

Ca r

m

0

5

Ca ny on

36°30'

el

10km

Fig. 1. Location map of the study area showing the three USGS moorings positioned respectively at 820 m (R1), 1020 m (R2), and 1445 m (R3) water depths in the Monterey Submarine Canyon. The inset at the upper left corner shows the cross-canyon section at the mooring sites; black dots represent the relative positions of RCM current meters, and the small circles denote the positions of downward looking ADCPs. RIN (295 m) and sRIN (203 m) near the canyon head are MBARI's deployments.

Both Microcat and Seacat sensors in each instrument package recorded water conductivity and temperature from which salinity and density were computed (Morgan, 1994). The transmissivity, measured by the transmissometers, were converted to beam attenuation via cp ¼  ðln ½T r =T 0 Þ=L

ð1Þ

where Tr is measured transmission in voltage, T0 is transmission voltage through clear water obtained in pre-deployment calibration, L is the path length of the transmissometer (0.25 m in this case), and cp is beam attenuation in m  1. The beam attenuation can be used to compute the sediment concentration if grain size data are also available (Xu et al., 2002). 2.2. Grain sizes and sediment concentration A total of eleven sediment traps were successfully recovered. Upon retrieval it was found that sediment traps B, C, F, G, K, L, and M were all overfilled. In addition to a full tube liner, variable amounts of sediment were found in the collecting funnels. The four traps located above 200 MAB (A, E, I, and J) were not full. The sediment tubes were photographed and described before being placed in a refrigerated

storage. In January 2004, X-radiographs were taken using a portable veterinarian X-ray machine (Fig. 3). In February 2004, these sediment tubes were logged by a Geotek multi-sensor core logger for p-wave velocity, gamma bulk density, magnetic susceptibility, acoustic impedance, fraction porosity, and electrical resistance. The sediment tubes were then subsampled by extruding the sediment out of the top of the tube liner. In February 2004, the three traps on the top of the moorings (A, E, I) were sampled in 5-cm increment for dichlorodiphenyltrichloroethane (DDT) analyses, which consumed the whole samples therefore left no sample for grain size. In August 2004, sediment tubes K, L, and M were sampled in 1-cm increment, primarily for grain size analysis. In June 2012 the rest of the sediment tubes (B, C, F, G, and J) were subsampled in 2–4 cm increments. Sediment samples used for grain size analysis were initially placed in beakers overnight with a solution of hydrogen peroxide to remove organics and to begin the process of sample dispersion. Following digestion, samples were washed twice and rinsed to remove solubles and then wet-sieved into three fractions: coarse sand or gravel (42 mm), sand-silt (2–0.063 mm) and the fines or mud fraction (material finer than 0.063 mm). All grain size samples were processed using a Beckman Coulter LS230 laser diffraction particle size analyzer.

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J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

I (740) Mooring

700

Inst. Assem. A

R1 ( 8 2 0 m )

600

C D

R2 (1020 m)

E

300

F

170

G

74 69

500

H

R3 ( 1 4 4 5 m )

Distance above floor (m)

B

Height interval ( MAB) sensors min. C/T 5 300 trans. 10 RCM 20 C/T 5 170 trans. 10 RCM 20 C/T 5 74 trans. 5 69 ADCP 60 5 C/T 16 trans. 10

400 J (400)

16

I

740

J

400

K

170

L

73 68

M

16

C/T trans. RCM C/T trans. RCM C/T trans. ADCP C/T trans.

5 10 20 5 5 20 5 5 60 5 10

C/T RCM C/T/P RCM C/T/P trans. RCM C/T trans. ADCP C/T trans.

5 20 20 20 5 5 20 5 10 60 5 10

2002 2003 DEC JAN

FEB

MAR

APR

MAY

JUN

JUL

AUG

SEP

OCT

NOV

FAILED FAILED LOST LOST

LOST LOST

300 E (300)

A (300)

Instrument Package 200

Sed. Trap F (170)

K (170)

B (170)

Transmissometer C/T sensor

100 L (73) ADCP (68)

G (74) ADCP (69)

C (74) ADCP (69)

M (16)

H (16)

D (16)

R3 (1445m)

R2 (1020m)

R1 (820m)

Fig. 2. Diagram of the three moorings. A total of 13 instrument packages (A through M) were deployed on the three moorings. The numbers in parentheses next to each package are their heights (meters above bottom). The drawing at the lower right corner shows the configuration of a typical package. Detailed compositions of each package and their data recovery are listed in the table at the upper right corner.

The results from these analyses are shown in Fig. 3, which plots the vertical variation of composition in 4 sediment traps, and Fig. 4 that plots the more detailed grain size distribution of 7 sediment traps. Sediment concentrations were obtained from the beam attenuation coefficient, cp using a combined calibration relationship (Fig. 5) based on two laboratory results (Xu et al., 2002; Ochiai and Kashiwaya, 2010) that are respectively derived from fine and coarse material (see Appendix I).

3. Field turbidity currents This paper focuses on the two turbidity current events recorded on December 17 and 20 2002 respectively, and examines the

similarity as well as differences of the two events in terms of their temporal and spatial scales, origins, sediment contents, and sediment transport. Figs. 6–8 plot the 10-day time-series of alongcanyon velocity, pressure, temperature, and beam attenuation coefficient from the three moorings. Clear contrast between the two events can be found in the following aspects. (1) The downcanyon speed of the turbidity current on December 20 (hereafter TC2) was much faster than the speed of turbidity current on December 17 (hereafter TC1). TC2's maximum speed near the canyon floor was nearly 3 times that of TC1. In addition, all three mooring were either moved toward deeper water (see pressure record in Fig. 6) or severely tilted for a period of time (see pressure record in Figs. 7 and 8) presumably by TC2's strong drag force. (2) The reversal of temperature gradient (temperature increase

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

15

Grain % top

0

0

100

50

100

0

50

0

100

100

100

90

90

90

80

80

80

80

70

70

70

70

60

60

60

60

50

50

50

50

40

40

40

40

30

30

30

30

20

20

20

20

10

10

10

90

Trap Core Length, cm

50

100

100

Sand Silt Clay

50

T Jan. 3

10 T Dec. 14

T Dec. 14

T Dec. 14

bottom

0

T Nov. 24

M

100

0

0

L

G

C

Fig. 3. Grain size data from 4 sediment traps C (R1, 70 MAB), G (R2, 70 MAB), L (R3, 70 MAB), and M (R3, 16 MAB). Next to each line plot are the photograph and X-radiograph of tube liner from each sediment trap. Each X-radiograph is a mosaic of several shorter sections that were taken separately with a small X-ray machine. Seams between X-ray sections are visible because of different amount of exposure. In general, brighter shade indicates coarser material. The ruler in the photographs is 100 cm long. Time marks are also shown in sediment traps C, G, and M, each was equipped with an intervalometer.

towards seabed) during TC1 was much more profound than that in TC2 (Figs. 7 and 8) (3) Counter-intuitively, the weaker TC1 exerted its effect to a much higher elevation in both temperature and sediment concentration, and these effects lasted as long as, if not longer than, that of TC2 (Figs. 7 and 8). Table 2 lists the best estimate of TC1 and TC2 arrival time at each mooring site. TC1's arrival time at R2 and R3 are obtained from the anomalies in transmissometer and/or temperature sensor whose sampling interval was 5 min. The arrival times of TC2 are estimated from pressure anomalies recorded on all three moorings with sampling intervals of 5 min (on R3 mooring) and 20 min (on R1 and R2). Since turbidity currents flowed close to the canyon floor, the arrival times at R3 are deemed more accurate because the sensors on this mooring were closer to the canyon floor (16 MAB). The instrument packages at this height were not recovered at R1 and R2 mooring. In the following sections we will describe in more details of the variations of temperature, velocity profile, and sediment concentration at each of the three moorings during each of the two turbidity current events. As shown in Fig. 2, sensors from each instrument package were placed in slightly different elevations. To simplify descriptions and discussions below, we from now on in this paper group the data into four nominal elevations: 300 MAB (instrument packages A, E), 170 MAB (instrument packages B, F, K), 70 MAB (instrument packages C, G, L), and 16 MAB (instrument package M). 3.1. Temperature and conductivity Changes of water temperature at the R2 and R3 moorings induced by TC1 were plotted in Figs. 9 and 10A. The arrival of TC1 substantially raised the temperature of the water column from 170 MAB down to the canyon floor, reversed the vertical temperature gradient (warmer water beneath colder water), and caused

high-frequency fluctuations that lasted for many hours. The magnitude of temperature increase was greater at R2 than at R3. The entire water column below 170 MAB was raised by at least 2 1C at R2, whereas the same magnitude of increase at R3 only occurred 16 MAB. The temperature gradient reversal persisted for about 10 h at both sites, but the TC1's impact in the temperature field was seen for 20 h before pre-event conditions returned. It is worth noting that at the beginning of TC1's arrival the temperature 170 MAB at both sites experienced a rapid decrease of almost 0.5 1C before rising, a phenomenon similar to observations in laboratory experiments (Felix and Peakall, 2006) who attributed to the ambient water pushed upward by the front of the turbidity current. This type of temperature drop was not seen at either 16 MAB or 70 MAB. There was no apparent effect of TC1 in the temperature 300 MAB or above. The variation of measured conductivity (not shown) mimicked the temperature. The net effect of increasing temperature and conductivity is a reduction of potential density. Calculations using a standard formula (Morgan, 1994) showed that potential density were reduced, with respect to the ambient water, by 0.4 and 0.3 kg/m3 at 70 and 170 MAB respectively at the R2 mooring, and by 0.4, 0.25, and 0.1 kg/m3 at 16, 70, and 170 MAB respectively at the R3 mooring. This seemingly suggests a buoyancy reversal and instability; however, the high sediment concentration brought to the water column by the turbidity current was more than adequate to compensate this density deficit. In comparison, TC2's impact on the temperature field was weaker in magnitude, smaller in vertical scale, and probably shorter in duration (Figs. 11–13A). The arrival of TC2 did raise the temperature 70 MAB and below at both R2 and R3 sites, even though it was for only 6 h at R2. The largest change was an increase of 1 1C at 16 MAB at the R3 site. The increase at 70 MAB was probably less than 0.5 1C (Fig. 12A). The strange spikes of decreasing temperature at the R1 site, where measurements at

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J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Size distributions of sediment trap samples 60 50

30

K

30

F

25

25

40

20

20

30

15

15

20

10

10

10

5

5

0 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

0

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

30

60

L

50

0

25

25

20

20

30

15

15

20

10

10

10

5

5

0 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

30

G

40

0

B

C

0 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

-1 0 1 2 3 4 5 6 7 8 9 10 11 12 13

Grain size, phi

Grain size, phi

Percentage, %

60 50

M

top

K

F

B

70 MAB

L

G

C

16 MAB

M

170 MAB

40 30 20 10 0

bottom 0

2 3 4 5 6 7 8 9

0

3

Grain size, phi

R3

R2

R1

K, L, M: 35 size bins; B, C, F, G, J: 53 size bins.

Fig. 4. Grain size distribution of the sediment trap samples. In each panel, multiple distribution curves are stacked in sequence: red lines are sample from the bottom of the trap and blue lines are from the top. Traps K, L, and M were subsampled every 1 cm; Traps B, C, F, and G were subsampled every 2–4 cm. For clarity, only every other sample is shown in the plots. Scales of both x-and y-axis are identical in all panels. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

both 16 and 70 MAB were unavailable, were determined to result from the swing motion of the R1 mooring due to the high drag of fast TC2 based on the fact that (1) the temperature drop was so brief that the RCM temperature sensor sampling at a 20-min interval missed the signal altogether (red and cyan line in Figs. 11A); and (2) the R1 mooring was likely dragged downcanyon to a location where the water depth was  20 m deeper (Fig. 6B). 3.2. Flow speeds and directions The speed and direction of the two turbidity currents are from the ADCP measurements because the RCMs 170 MAB on each mooring showed no sign of impact by TC1 or TC2. The integral thickness of the turbidity currents,, based on a depth-averaged velocity (García, 1994) was also calculated from the hourlysampling ADCP data. At the R2 site, TC1 was about 20 m in thickness and 60 cm/s in maximum speed. A second episode of bottom-hugging flow of 40 cm/s appeared after a 2-h hiatus. From its first arrival to eventual demise, including both episodes, TC1 lasted for about 11 h (Fig. 9B and C). At the R3 site, TC1 retained its two-pulse pattern but appeared to have increased in both speed and duration

(Fig. 10B and C). The maximum speed grew to 75 and 60 cm/s respectively for the 2 episodes, and it lasted for about 3 h longer than at the R2 site. The most interesting feature of TC1's current data is its flow direction. For the first two hours at the R2 site, TC1 flowed cross-canyon to the North rather than along-canyon (Fig. 9C). This is discussed later as direct evidence that TC1 was originated in Soquel Canyon. In comparison, TC1 flowed exclusively downcanyon at the R3 site (  1701) as one might expect (Fig. 10C). TC2 was a much faster turbidity current. Its highest velocity measured at the R1 site was 155 cm/s (Fig. 11B and C). Because the hourly-sampling ADCP missed the very beginning of the event when the highest down-canyon velocity was assumed to occur, TC2's velocity was likely much greater than 155 cm/s. During the 8 h of its existence at the R1 site, the current within the bottom 40-m flowed downcanyon ( 1601), more or less in the opposite direction of the current in the upper layer represented by the RCM data 170 MAB (Fig. 11C), while TC2's speed gradually decreased till completely diminished. At the R2 site (Fig. 12B and C), TC2's flow pattern was almost identical except for its slightly faster speed (160 cm/s) and shorter duration (6 h). Similarly, TC2 flowed exclusively downcanyon (  501) at the R2 site even though the

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

tidal current (Xu and Noble, 2009) switched to upcanyon in the middle of the event (Fig. 12C). At the R3 site (Fig. 13B and C), TC2 continued to grow faster (180 cm/s) even though the flow also

17

lasted for 6 h. Similar to TC2's behavior at the two upstream sites, its speed monotonically decreased as the front, body, and tail of the turbidity current sequentially passed through the R3 mooring. 3.3. Water turbidity

Along-canyon velocity, cm/s

Fig. 5. Beam attenuation coefficients obtained from two different calibrations are plotted together against sediment concentration. Calibrations of Xu et al. (2002) are shown in 3 different colors, each represents a specific grain size group and a calibration relationship. Note that the LV-H110 sensor from is saturated because of its lower sensitivity range (Ochiai and Kashiwaya, 2010).

50 0 -50 -100 -150 -200

16

17

18

19

20

At the R2 site, TC1's arrival immediately saturated the transmissometer 70 MAB, which stayed saturated for nearly 5 h (Fig. 9D). After the saturation period the cp remained high with large variations that were presumably due to the intense flow fluctuations in the water column (Felix et al., 2005), which was also seen in the temperature record. Both the water turbidity and the intensity of flow fluctuations progressively subsided with time, but at 24 h after TC1's arrival the water turbidity was still greater than the pre-event values. The rapid rise of cp 170 MAB occurred  20 min after the cp 70 MAB started to rise. For the first 3 h it was nearly saturated and a cp value of 33 m  1 (the highest possible value without total saturation) was recorded. As expected, both the absolute value and the amplitude of the fluctuation of cp at 170 MAB subsided much faster than its counterpart at 70 MAB. The water turbidity 300 MAB was also raised but the impact of TC1 at this height appeared to be minimal. At the R3 site, the cp values at 70 and 170 MAB and their highfrequency variations (Fig. 10D) are similar to those at the R2 site. The transmissometer at 16 MAB, only available at this site, was saturated by TC1 for almost 15 h. Between 18:30 and 20:00 on December 17 the water at both 70 and 170 MAB appeared to have cleared out, even though the near-bed water was still very turbid. This gap coincided with the speed hiatus between the two episodes of TC1 (Fig. 10B and C).

21

22

23

24

25

Pressure dbar

30 20 +516 dbar

10

+657 dbar

Temperature o C

0

6 300 MAB

4

70 MAB

170 MAB

2

Attenuation coeff., m-1

8 6 300 MAB

4

170 MAB

2 0

16

17

18

19

20

21

22

23

24

25

26

Dec 2002 Fig. 6. Time-series plots of measurements at R1 mooring. (A) Along-canyon current velocity recorded by a downward-looking ADCP sampling hourly with a vertical bin interval of 2 m. The ‘jet’ color map is used for plotting the 30 lines, with warm colors (red) representing the velocity at the top of the 60-m water column and cool colors (blue) the velocity near the canyon floor. Negative velocities point downcanyon direction. Note that TC1 did not pass through this mooring site. (B) Pressure anomaly (with mean value removed) measured by the two RCM current meters on R1 mooring. Note the simultaneous 20-dbar increase at both sensors at the time of TC2 event on December 20. The “saw-tooth” perturbations are within the precision range of the low-resolution pressure sensors therefore treated as noise. (C) Time-series of measured temperature at 300, 170, and 70 MAB (the package at 16 MAB was lost). The ADCP-measured temperature at 70 MAB is hourly while the other two are every 5 min. (D) Timeseries of beam-attenuation computed from the transmissometer data (in voltage). Due to instrument failure and loss, only the data from 300 and 170 MAB are available for this mooring. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Along-canyon velocity, cm/s

18

50 0 -50 -100 -150 -200

Pressure dbar

17

18

19

20

21

22

8 6 4 2 0

23

+723 dbar

24

25

+855 dbar

Temperature oC

7 300 MAB

6

170 MAB

70 MAB

5 4 3

Attenuation coeff., m-1

40 300 MAB

170 MAB

70 MAB

20 0

17

18

19

20

21

22

23

24

25

Dec 2002 Fig. 7. Same as Fig. 6 but for the R2 mooring. Note the change of scales in some panels. (A) Both turbidity currents are clearly shown in the current velocity records. (B) There is no pressure perturbation associated with TC1, but the impact of TC2 on the pressure records is obvious. Whether the 2-dbar difference between the two normalized pressure records after TC2's passage is real or artefact is uncertain because it is within the 2-dbar accuracy range of the low resolution pressure sensors. (C) The arrival of the low-speed TC1 caused huge increase of the temperature at two sensors (170 and 70 MAB), with the lowest sensor at 70 MAB increasing the most (  5 1C). In contrast, the high-speed TC2 barely raised the temperature at 70 MAB. The sensor at 16 MAB was lost. (D) The impact of both turbidity currents on the beam attenuation (sediment concentration) was obvious. But like in the temperature records, the weaker TC1 raised the beam attenuation much more than the stronger TC2, especially at the two sensors on the top (300 and 170 MAB).

The water turbidity and the vertical structure of TC2 are somewhat different in terms of magnitudes and spatial distribution. At the R2 and R3 sites where the data of TC1 and TC2 can be compared (Fig. 9D vs. 12D; Fig. 10D vs. 13D) it is seen that TC2's plume (above the integral thickness) is not only thinner, its measured beam attenuation values at a given height, e.g. 70 or 170 MAB, are also lower than TC1's. Interestingly, TC2's plume grew bigger (thicker) and more turbid (compare Figs. 12 and 13D) during its transit from R2 to R3, seemingly related to TC2's increasing speed downcanyon (Figs. 11–13B).

4. Discussions 4.1. Sediment concentration within the turbidity currents Table 3 shows the details of estimated sediment concentration within the body of the two turbidity currents. The measured beam attenuation coefficient is the result of the collective light impedance by the multiple grain size fractions of the suspended sediments, cp ¼ ∑ki C i ¼ C∑ki pi

ð2Þ

and the total concentration can then be estimated with C ¼ cp =∑pi ki

ð3Þ

where pi is the percentage of the three fractions (do3 phi, d¼ 3–4 phi, d44 phi) derived from grain size analysis of the sediment trap samples (Table 3), and ki is the calibration parameter from Xu et al. (2002) for the three grain size fractions (k[1,2,3] ¼ [6.29, 9.17, 142.86]). It is necessary to point out that only the TC1 concentrations

at the R3 mooring are calculated using parameters (grain size and cp) that were both measured inside (at 16 MAB) the body of the turbidity current. Values measured at 70 MAB for at least one of the two parameters have to be used in all other cases in Table 3, thus some uncertainties within these calculated concentrations are inevitable. These concentration estimates are only the highest values the transmissometers were able to measure before their maximum range was exceeded. It is almost certain that the concentrations at the peak of the turbidity currents, when the transmissometers were saturated, were much greater. These “peak” concentrations can be estimated (Appendix II) with the measured peak speed and the speed corresponding to the maximum measurable concentration. At the R3 mooring, saturation of the transmissometer 70 MAB ended at 16:00 on December 17 2002 for TC1 (Fig. 10D) and at 06:00 on December 20 2002 for TC2 (Fig. 13D). The measured bulk (depth-averaged) speed at these two time points were respectively 0.21 and 0.38 m/s with the flow thickness respectively at 40 and 55 m (Table 1A in Xu, 2010). These are the velocities and thicknesses that correspond to the concentrations, 0.32 and 0.79 kg/m3, or 0.01% and 0.03% respectively (Table 3, [6F and 6S], hereafter 6F is the chess-style locator, column 6 and row F), that are directly calculated from the measured maximum cp (Table 3, [6E and 6R]) from instrument package M (16 MAB), which was inside the body of the turbidity currents. At the R3 mooring, the highest bulk speed measured by the ADCP at the beginning of the events while the transmissometer was saturated were 0.57 and 1.08 m/s respectively for TC1 and TC2, with the corresponding thickness at 57 and 31 m (see Table 1A in Xu, 2010). Substituting these values into Eq. (A.10) derives the highest concentrations at the times of peak flows at mooring R3, 1.70 kg/m3 [0.06%] for TC1 and 11.37 kg/m3 [0.43%] for

Along-canyon velocity, cm/s

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

19

50 0 -50 -100 -150 -200 17

18

19

20

21

22

23

24

25

26

Pressure dbar

4 +1047 dbar

3

+1278 dbar

2 1

Temperature oC

0

6 740 MAB

400 MAB

170 MAB

70 MAB

16 MAB

4

Attenuation coeff., m-1

2

30 20

170 MAB

70 MAB

16 MAB

10 0

17

18

19

20

21

22

23

24

25

26

Dec 2002 Fig. 8. Same as Fig. 6 but for the R3 mooring. Note the change of scales in some panels. (A) The down-canyon velocity of both TC1 and TC2 appears to have increased from R2 to R3 site. (B) The short spike of pressure coincided with TC2. Note that the two pressure sensors on this mooring are high-resolution ones. (C) Temperature records at five elevations are shown. Temperature increases due to turbidity currents are the greatest at the sensor closest to canyon floor (16 MAB). Like at the R2 mooring, neither TC1 nor TC2 appears to be able to affect the water above 170 MAB. (D) The water column was more turbid at R3 mooring than at R2 mooring during both turbidity currents. Again, at 170 MAB the weaker TC1 raised the beam attenuation much more than the stronger TC2. Table 2 Morphological parameters of Monterey Canyon, and time of arrival of the two turbidity currents at each mooring site. Each turbidity current arrived on or within one sampling interval (5–20 min) prior to the time listed in the table. The frontal speeds (Uf) are based on the interval between arrival times at the mooring sites.

Morphology Water depth Floor slope Thalweg width Streamwise (thalweg-following) distance from coast

[m] [deg.] [m/m] [m] [m]

TC1 Arrival time, t0 Frontal speed, Uf Flow duration, T Depth-averaged speed of TC body, U Integral flow thickness of TC body, h Depth-averaged volume concentration, C Densimetric Froude number, Fd

[GMT] [m/s] [h] [m/s] [m] [–] [–]

TC2 Arrival time, t0 Frontal speed, Uf Flow duration, T Depth-averaged speed of TC body, U Integral flow thickness of TC body, h Depth-averaged volume concentration, C Densimetric Froude number, Fd

[GMT] [m/s] [h] [m/s] [m] [–] [–]

a b

RIN

R1

R2

R3

295 2.3 4.0% 160 8250

820 1.7 2.9% 260 27,270

1020 1.7 2.9% 260 34,150

1445 2.9 5.1% 530 45,000

08:15 0.90 7 0.02 10 [6 þ4] 0.35 52.5 0.02% 0.95a

11:35 13 [7þ 6] 0.57 57.1 0.04% 0.99

00:50

01:40

23:38 6.107 0.59

00:30 5.737 1.43 7 1.13 23.9 1.36% 0.51b

3.62 7 0.36 5 0.91 38.2 0.11% 1.13

5 1.08 31.1 0.16% 1.24

Flow affected by collision at the junction of Soquel and Monterey Canyons, thus not probably in normal conditions. Normal conditons probably not reached due to the deposition being capacity-driven at this stage.

TC2 (Table 3, [5F and 5S]). Both values are several times greater than the maximum concentrations that the transmissometers were able to measure. It is worth noting, however, that although it is reasonable to assume that the densimetric Froude number Fd remains

unchanged during a period of several hours when the transmissometers were saturated (analogous to the similarity analysis in Sequeiros et al., 2009), the above estimated concentration values would accordingly be higher (lower) if Fd were smaller (larger) at

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Temperature, oC

170 MAB

18

04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 00 01 02 03 04 05

Dec 17 2002

Height, MAB

7

6

5

4

3

Speed, cm/s 60

Direction, deg.

Dec. 17 2002 08:00

40

60 40

20

20

0 0

20

40

60

-180 -135 -90 -45 -4

0

45

0 90 135 180

Height, MAB

20

09:00

10:00

11:00

12:00

300 MAB

70 MAB

13:00

14:00

Beam Attenuation coef. m-1 40

30

20

10

0 18

16:00

17:00

18:00

19:00

300 MAB

170 MAB

70 MAB

00 02 04 06 08 10 12 14 16 18 20 22 00 02 04 06 08 10 12 14 16 18 20 22

Dec 17 2002

15:00

20:00

Fig. 9. Time-series plots of (A) temperature, (B) vertical profiles of ADCP-measured speed and (C) direction, and (D) beam attenuation coefficient during the TC1 event on December 17 2002, measured at the R2 mooring. The grey bars in (C) show the orientation of the canyon at the mooring site (zero degree is true north). The large black dots in (C) are the current direction measured by an RCM at 170 MAB. All panels in (B) and (C) share the same scales that are only shown in the first hour. Colored lines in (A) and (D) represent measurements at different heights. There are two temperature sensors at each height: 300 MAB (red, blue); 170 MAB (green, cyan); 70 MAB (yellow, magenta). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

18

Direction, deg.

Dec. 17 2002 11:00 20

40

60

80

80 -135 -90 -45 -180

0

45

60 40 20 0 90 135 180

12:00

13:00

14:00

15:00

16:00

17:00

740 MAB

400 MAB

170 MAB 70 MAB 16 MAB

19

04 06 08 10 12 14 16 18 20 22 00 02 04 06 08 10 12 14 16 18 20 22 00 02

Dec 17 2002

Height, MAB

6

5

4

3

2

Speed, cm/s 60 40 20 0 0

Height, MAB

Temperature, oC

21

18:00

Beam Attenuation coef. m-1 30

25

20

15

5

0

18

20:00

21:00

22:00

23:00

Dec. 18 2002 00:00

01:00

170 MAB

70 MAB

16 MAB

19

23 01 03 05 07 09 11 13 15 17 19 21 23 01 03 05 07 09 11 13 15 17 19 21 23 01 03 05 07 09

Dec 16 17 2002

10

19:00

02:00

Fig. 10. Same as Fig. 9 but for the TC1 event measured at R3 mooring. Note the changes of scale in (A), (B), and (D). There are two temperature sensors at 740 MAB (red, blue).

22

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Temperature, oC

60

Direction, deg. 60

Dec. 20 2002 00:00

40

40 20

20

20 21 22

0

0

20 40 60 80 100 120 140 160 180 200

0

0 180

Height, MAB

Height, MAB

16 17 18

2002

6

Speed, cm/s

01:00 00 01 02

20

04

02:00

06 07 08 12

170 MAB

10 11

04:00

Beam Attenuation coef. m-1 8

6

4

2

0

2002 21 01

20

06:00 07 11

07:00 17 21

08:00 01

21

170 MAB

07 11

Fig. 11. Same as Fig. 9 but for the TC2 event measured at R1 mooring. Note the changes of scale in (A), (B), and (D). There are two temperature sensors at each height: 300 MAB (blue – high-resolution, high-frequency data from microcat; red – low-resolution, low-frequency data from RCM); 170 MAB (green – high-resolution, highfrequency data from microcat; cyan – low-resolution, low-frequency data from RCM). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

23

Temperature, oC Direction, deg.

2002

60

Height, MAB

00

20

40

Dec. 20 2002 00:00

40

01 02

20

20

04

0 0

20 40 60 80 100 120 140 160 180 200

0

Height, MAB

4

Speed, cm/s 60

0 180

06

01:00

07 08 10 11 12

02:00

14 18

170 MAB

17

70 MAB

16

Beam Attenuation coef. m-1 40

20

10

0

2002

04:00 22 01

20

04 07 10 16 22

06:00

21

01 04 07 10

07:00 170 MAB

70 MAB

16 22

Fig. 12. Same as Fig. 9 but for the TC2 event measured at R2 mooring. Note the changes of scale in (A), (B), and (D). There are two temperature sensors at each height: 300 MAB (red, blue); 170 MAB (green, cyan); 70 MAB (yellow, magenta). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

24

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Temperature, oC 2002 01

20

Height, MAB

60

Direction, deg. 60

Dec. 20 2002 01:00

40

40

20

20

07

0

20 40 60 80 100 120 140 160 180 200

0

Height, MAB

6

4

2

Speed, cm/s

180

11

02:00

17 21 01

21

07

04:00 740 MAB

400 MAB

170 MAB

70 MAB 16 MAB

11

Beam Attenuation coef. m-1 20

10

0 20 21

07:00

08:00

170 MAB

70 MAB

16 MAB

18 20 22 00 02 04 06 08 10 12 14 16 18 20 22 00 02 04 06 08 10 12 14 16 18 20

2002

06:00

Fig. 13. Same as Fig. 9 but for the TC2 event measured at R3 mooring. Note the changes of scale in (A), (B), and (D). Temperatures were measured at 5 different heights: 740 MAB (red, blue); 400 MAB (green); 170 MAB (cyan); 70 MAB (magenta); and 16 MAB (yellow). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

25

Table 3 Sediment concentration, in both kg/m3 and %, within the body of turbidity currents. R1 TC1 Sediment trap % ( o3 phi) % (3–4 phi) % ( 44 phi) Peak Maximum cp Concentration Maximum velocity, um Elevation of maximum velocity, zm Depth-averaged velocity, U Shear velocity, un Fall velocity, ws Rouse Number Concentration, 16 MAB (inside TC)

Recorded Max.a

[1/m] [kg/m3] [%] [m/s] [m] [m/s] [m/s] [m/s] [kg/m3] [%]

TC2 Sediment trap % ( o3 phi) % (3–4 phi) % ( 44 phi)

R3

5 55 40

2 40 58

[1/m] [kg/m3] [%] [m/s] [m] [m/s] [m/s] [m/s] [–] [kg/m3] [%]

A B C

Peak

Recorded Max.

Peak

Recorded Max.

0.88, 0.03% 0.59 13 0.35 0.043 0.0053 0.25 1.26, 0.05%

33 0.53, 0.02% 0.39 7 0.27 0.031 0.0053 0.34 0.87, 0.03%

1.70, 0.06% 0.74 8 0.57 0.058 0.0053 0.18 1.7, 0.06%

28 0.32, 0.01% 0.39 5 0.21 0.033 0.0053 0.32 0.32, 0.01%

b

75 23 2 Peak

Max. cp Concentration Maximum velocity, um Elevation of maximum velocity, zm Depth-averaged velocity, U Shear velocity, un Fall velocity, ws Rouse number Concentration, 16 MAB (inside TC)

a

R2

c

c

15 65 20

Recorded Max.

Peak

Recorded Max.

Peak

37.9, 1.43% 1.46 9 1.13 0.112 0.0175 0.31 60.2, 2.27% 1

30 3.10, 0.12% d 0.44 7 0.39 0.035 0.0175 0.99 13.5, 0.51% 2

5.61, 0.21% 1.45 11 0.91 0.110 0.01 e 0.19 7.37, 0.28% 3

30 0.85, 0.03% 0.54 15 0.41 0.038 0.01 e 0.52 1.82, 0.07% 4

E F G H I J K L M

N O P

Recorded Max.

e

11.37, 0.43% 1.78 5 1.08 0.150 0.01 0.13 11.37, 0.43% 5

28 0.79, 0.03% 0.50 7 0.38 0.040 0.01 0.50 0.79, 0.03% 6

f

f

R S T U V W X Y Z

a Here the ‘Recorded Max.’ concentrations are the measured concentrations converted from the highest possible cp values. The ‘Peak’ concentrations are the extrapolated values using equation (A.10), corresponding to the peak velocities. b Both the sediment trap (grain size data) and the transmissometer (for max. cp) are 70 MAB (instr. package G). c Both the sediment trap (grain size data) and thes transmissometer (for max. cp) are 16 MAB (instr. package M). d Sediment trap data is from 70 MAB (package C); transmissometer failed, so the max. cp is an assumed value for total occulsion of the sensor. e Sediment from TC2 cannot be discerned from either of sediment traps at 70 or 170 MAB so they are both assumed to have been filled by TC1. Here the low bound value, same as at R3, of the weighted mean calibration coefficient is used. f Grain size data for TC2 is from sediment trap L at 70 MAB.

the onset of the turbidity current. Furthermore, because the hourly sampling ADCP very likely missed the head/front of the turbidity currents, these peak values are from the body, not the head, of the turbidity currents. For moorings at R1 and R2, the concentration values shown in Table 3 are not the concentration within the body of the turbidity currents because they were based on the measurements from a transmissometer located 70 MAB, which is likely above the flows (Xu, 2010). Thus a vertical concentration profile needs to be known in order to obtain the concentration within the body of the flows, at 16 MAB. Theoretical and experimental studies (e.g. Stacey and Bowen, 1988; García, 1994; Altinakar et al., 1996) and limited field data (e.g. Chikita, 1989; Normark, 1989) have suggested that the vertical concentration profiles have patterns similar to those of shear flows and obey a power law known as the Rouse equation: C=C a ¼ ðz=z0 Þ  b

ð4Þ

where C is concentration at elevation z, Ca is the near-bed concentration at a reference height z0, and b ¼ ws =ðκun Þ is the Rouse number, ws is the sediment fall velocity, un is the frictional velocity and κ is the von Karman constant assumed to be equal to 0.41. Eq. (4) is valid for supercritical flows. In subcritical flows the concentration remains roughly constant below the height of the velocity maximum (Sequeiros, 2012). In the cases of TC1 and TC2,

16 MAB is greater than the height of the velocity maximum zm (Table 3, rows H and U); therefore Eq. (4) should provide a valid estimate even if the flows were subcritical. By definition the concentrations at 70 and 16 MAB, C70 and C16, both fall on the same profile given by the Rouse equation, thus C16 can be easily calculated from C70 that is already known: C 16 ¼ C 70 ð70=16Þb

ð5Þ

With known median sediment grain sizes, the fall velocities are calculated using a formula by Jiménez and Madsen (2003). The friction velocity can be calculated with the ‘law of the wall’ when the boundary layer (from floor to the maximum velocity in a measured velocity profile) is logarithmic (Altinakar et al., 1996; Kneller et al., 1999; Sequeiros et al., 2010b): un ¼

um κ lnð30zm =ks Þ

ð6Þ

where um is the maximum velocity and zm is the elevation of the velocity maximum in the vertical profile; ks is the Nikuradse roughness of the canyon floor that can be readily estimated when bedform information is available. For a sea bottom full of sandwave bedforms like the floor of the Monterey Canyon (Xu et al., 2008; Paull et al., 2011), a bedform roughness model (e.g., Nielsen, 1992;

26

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

Xu and Wright, 1995) may be used, 2

ks ¼ mη =λ

ð7Þ

where m is an empirical constant that ranges from 8 to 28. For the average bedform height, η¼1.5 m, and bedform length, λ¼ 50 m, which were observed near the head (o300 m water depth, Xu et al., 2008), a roughness length of ks ¼0.23–1.26 m is obtained. For the average bedforms in deeper part of the canyon (Paull et al., 2011), η¼2 m, and λ¼ 80 m, the roughness length ks ¼ 0.4–1.4 m. For simplicity, a low-bound value of ks ¼ 0.4 m is used in the calculations below. The friction velocities, Rouse numbers, and the extrapolated concentrations C16 are listed in Table 3. At mooring R2, C16 are up to 2 times greater than C70, with concentrations, especially the peak concentration, in TC2 significantly higher than in TC1 (Table 3). At the R1 mooring, the extrapolated concentrations C16 are much higher: its peak concentration (Table 3, [1Z]) is 4–5 times greater than that of mooring R2 and R3; and the maximum measurable concentration (Table 3, [2Z]) is more than an order of magnitude greater. The main reason for these high concentrations is due to the high Rouse number (Table 3) that results from the much coarser sand grain found in the sediment trap at mooring R1 during the TC2 event. The fact that sands were deposited into a sediment trap 70 MAB (trap C in Fig. 3) suggests that TC2's head was likely much thicker than the integral thickness of its body (Table 2). Laboratory experiments (Middleton, 1966a; Gladstone et al., 2004; Choux et al., 2005; Felix et al., 2005) and numerical models (Felix, 2002; Nasr-Azadani et al., 2013) either qualitatively or quantitatively demonstrated this characteristic of turbidity currents, especially in deep-water environments. McCaffrey et al. (2003) also noted that the coarsest grain sizes were found in the head of the flow. It needs to be borne in mind that, because of instrument failures, transmissometer saturation was “assumed” to take place 70 MAB on mooring R1 during the passing of TC2. This not withstanding, a turbidity current whose sediment concentration is greater than 2% is still considered rare (Sequeiros, 2012). 4.2. Origins of the turbidity currents The lack of either velocity or temperature anomaly at the R1 mooring on December 17 2002 dictated that TC1 did not pass through the R1 site prior to being recorded at R2 and R3 sites. This suggests that TC1 originated somewhere in the canyon between the 820 m and 1020 m sites. In the following, we will try to determine TC1's origin by analyzing its flow properties observed at the R2 mooring at 1020 m. Prior to the TC1 arrival, the average temperature measured by instrument package G (70 MAB on mooring R2) was 4.3 1C. The highest temperature during TC1 was 6.5 1C. Several CTD casts collected during the deployment cruise in 2002 and recovery cruise in 2003 showed that, on average, the 6.5 1C water resided about 450–500 m below water surface (Fig. 14). Hence the water and sediment in TC1 must have come from a source at a maximum water depth of 500 m. Given the intensive mixing with ambient water, TC1's initial water temperature could have been greater than 6.5 1C, implying that TC1 could have originated from a source at a much shallower water depth. There are two possible sources: slumping of canyon walls upstream of the R2 site, and/or material from the Soquel Canyon, which joins the Monterey Canyon about 500 m upstream of the R2 site (Fig. 1). The northward crosscanyon flow at the beginning of TC1 (Fig. 9B and C) appears to suggest the south canyon wall near the R2 site as the source of TC1. But the topmost rim on the south side of the canyon is at water depths ranging from 800 to 1000 m where water temperature was less than 5 1C (Fig. 14), therefore a slide/slumping from the southern canyon wall near the R2 site could not be the source of water/sediment mixture that had a temperature of 6.5 1C or higher.

Soquel Canyon is then the most likely source of TC1. It is reasonable to propose a sequence of events that led to the TC1 event: (1) A gravity flow (TC1) was generated at the middle to upper reach of Soquel Canyon above water depth 500 m. What triggered TC1 remained unknown but submarine landslides on the canyon walls or even on the canyon floor due to its high longitudinal gradient of 3–61 (Paull et al., 2011) were the main suspects. (2) As TC1 flowed down Soquel Canyon where mud drapes are ubiquitous, it could have entrained more fine-grained sediment and accelerated due in part to the unusually high floor slope. (3) Because Soquel Canyon intersects Monterey Canyon in almost a perpendicular angle (Fig. 1), TC1 would have to flow right into the facing wall on the south side of Monterey Canyon, so the turbid plume was thrown up into the water column by the tremendous flow fluctuation and mixing. (4) TC1 was deflected from the south wall and then flowed northward, crossing the R2 mooring at 1020 m water depth. The cross-canyon flow was recorded by the downward looking ADCP, and the very turbid water containing fine material from Soquel Canyon saturated the transmissometers at 170 MAB and reached as high as 300 MAB (Fig. 9D). In contrast to TC1, the sediment source of TC2 is clearly from the upstream of the Monterey Canyon because of its record at the R1 mooring. In addition, record of damaged or displaced instrument platforms that deployed by Monterey Bay Aquarium Research Institute (MBARI) at 203 and 295 m water depths in Monterey Canyon (Table 3 in Paull et al., 2010) also coincided with TC2's passing at the R1 mooring. Particularly, data from the MBARI instrument at 295 m (RIN) accurately showed the time of a disturbance at 23:38 on December 19 (derived by adding 8 h to 03:38PM December 19 PST reported in Paull et al., 2010). The timing correspondence between the RIN and the 3 USGS moorings strongly suggests a sequence of “sightings” of the same event (TC2). TC2 started at or near the head of the Monterey Canyon sometime near the end of December 19 2002; arrived at the 203 m site (sRIN)some time later and displaced/buried an instrument platform (still unrecovered as of today, Paull et al., 2010); arrived at the 295 m site (RIN)and displaced another platform at 23:38 December 19; arrived at R1 mooring less than 30 min later at 00:30 December 20 and moved on to R2 and R3. Even though we are still unable to pinpoint the exact location of the sediment source for TC2, the “pure sands” collected in sediment trap C (Fig. 3), whose grain size distribution (Fig. 4) is very similar to the beach sands in Monterey Bay (Paull et al., 2005), suggests a source from the head region of the Monterey Canyon, possibly by accumulation of littoral sand transport into the canyon head (Best and Griggs, 1991) that increases exponentially with storm waves (Puig et al., 2003). Eke et al. (2011) demonstrated that breaching near the canyon head, where the slope of the pile of littoral sands exceeds a critical value in an event like a local storm, is a feasible process that initiates a turbidity current. Localized submarine landslides (Smith et al., 2007) is another possible source of sand at the head region. California is an earthquake prone region, but because there was no occurrence of measurable quakes in the region in December 2002, storm waves were the most plausible triggers to possible submarine landslides. Storm waves are capable of driving large amounts of littoral sand into the canyon head (Inman et al., 1976) and generating localized liquefaction (Ishihara and Yamazaki, 1984) that can potentially lead to landslides or breaching. Fig. 15 is a time-series plot of the meteorological data measured by NODC Buoy 46024 outside Monterey Bay (36147'7″N, 122128'9″W) and river discharge measured at two stream gauges along the Salinas River. Prior to the TC1 event, there were two low-pressure systems in the region that brought high southerly winds on December 14 and 16. Wave height increased following each of the two low-

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Fig. 14. Vertical profiles of water temperature and density computed from 4 separate CTD casts collected in December 2002 and November 2003 in the vicinity of mooring sites.

pressure systems, but the measured waves were more signatures of swells than of local storm waves. The directions of these waves were uniformly from the west, different from the southerly winds, and the wave periods were more than 15 s. By the time of the TC1 event on December 17 the winds had dissipated but the long swells sustained at 7.5 m in height and 16 s in period. The storm system prior to the TC2 event was different. Even though the barometric pressure of the December 19 system was not as low as the previous two, its wind speed was much higher and the wind directions were more southeasterly. Of more significance were the different wave characteristics during this storm. Their short wave periods (5–10 s) and southeasterly direction (same as the winds) were typical signatures of local storm waves rather than of Pacific swells. It is worth noting that Buoy 46042, the only station that had wave measurement available in December 2002, is more than 50 km offshore. It is unclear how representative these wave data are with respect to the wave condition inside Monterey Bay or at the head of the Monterey Canyon. This not withstanding, the Weather Section of a regional newspaper, San Jose Mercury News, named the 7-day period of December 14–21 a “spectacular stormy week”. The National Weather Service issued flood watch and heavy surf advisory for every day of the week, but December 15 and 19 saw the greatest punch of the storms when wind speed hit 80 mph in the Santa Cruz Mountains and waves of 25–30 feet high smashed over the 25-foot-high jetties of the Santa Cruz Harbor, whose senior harbor master called the conditions “probably some of the roughest seas we have seen in 5–10 years, about as bad as you can get”. Clearly the two storm systems during the week (Fig. 15) were the most likely cause of both turbidity currents. For TC2, it can be imagined that a chain reaction took place followed the December 19 storm. (1) High storm waves significantly increased the littoral sand transports converging from both sides of the canyon head; (2) Beach sands piled up at the head of the canyon, probably on the top of some littoral deposit brought by previous storms, and the front slope of the pile increased with

time; (3) As the slope increased toward the angle of repose, the cyclic loading (Ishihara and Yamazaki, 1984) from the high waves helped to make the pile unstable. At a certain critical point the pile collapsed either due to breaching (Eke et al., 2011) or landsliding (Smith et al., 2007); (4) A turbidity current was initiated. While traveling downcanyon, TC2 damaged MBARI's instrument platforms at 203 m and 295 m, dumped some beach sands into the sediment traps at R1 mooring (820 m), passed through the R2 and R3 moorings where it registered a speed of nearly 2 m/s, and dissipated further down canyon and finally died off somewhere in deep water.

4.3. Evolution of the turbidity currents This section discusses and attempts to interpret the transformation of the two turbidity currents during their transit along the canyon, and compare their differences and similarities exhibited in the process. Table 2 summarizes the key parameters of the two turbidity currents observed/estimated at the three mooring sites (plus MBARI's RIN station that only provided the timing of TC2). The depth-averaged speed, volume concentration and flow thickness are values measured at the peak of the flows, which represent the body, rather than the front for the turbidity currents because the hourly sampling ADCPs missed the very front of the flows. Flow durations are the number of hours shown in Figs. 9–13 during which persistent density flow profiles existed. Clearly, flow durations are much shorter than the persistent turbid plumes that tend to linger for many more hours after the flow ceased to exist (panels C and D in Figs. 9–13). Because each of the two turbidity currents was recorded at multiple locations along the canyon a frontal speed can be estimated with the known longitudinal streamwise distances: Uf ¼ΔL/Δt, where ΔL is the distance between two moorings, Δt is the differential of arrival times at the two locations. The smallest sampling interval on each mooring was 5 min, thus the calculated frontal speeds have an error margin

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Fig. 15. Meteorological data from NDBC Buoy 46042. The two grey bars marked the times of the two turbidity currents. (A) air pressure and water temperature; (B) wind speed and wind direction; (C)wave height, wave period, and wave direction; and (D) water level measured at Monterey Harbor and discharge measured at two gages along Salinas River.

of 5–10%, except for TC2 between R1 and R2 where the error can be potentially as high as 25%. It took TC1, which originated in the Soquel Canyon, 3 h to flow from R2 to R3, where TC1 grew slightly thicker and faster while doubled its sediment concentration (Table 2). Depth-averaged flow speed, measured by the downward-looking ADCP, increased slightly, from 0.35 m/s at R2 to 0.57 m/s at R3. Similarly, the flow duration increased from 10 h at R2 to 13 h at R3. In fact, the two consecutive flow pulses in TC1 (Figs. 9 and 10) appeared to have simply translated from R2 to R3, only with their durations expanded respectively from 6 to 7 h for the first pulse and 4–6 h for the second pulse. This phenomenon of turbidity current stretching (growing temporally longer downstream) was also seen in laboratory experiments (McCaffrey et al., 2003; Choux et al., 2005), who attributed to the assumption that the front of the flow traveled more rapidly than the body and/or tail of the same flow. There are more data points in TC2 for examining its transformation along the canyon because TC2 was recorded at 4 locations (Table 2). The frontal speeds are given in a range that account for the 5-min uncertainty of Δt. It seems to show that TC2's frontal speed decreased substantially past mooring R2. This slow-down is not reflected in the measured depth-averaged speeds at the three moorings, which are essentially the same magnitude at  1.0 m/s, 3–5 times smaller than the estimated frontal speed. The flow duration of TC2 decreased downcanyon while the flow thickness increased. The volume concentration was the highest at R1, in fact

roughly an order of magnitude greater than either R2 or R3. This dramatic decrease was closely associated with the grain size observation in the sediment trap samples. R1 was the only site where medium/fine sands (2–3 phi) deposited into the sediment traps. Presumably the coarse sediment settled out in transit between R1 and R2, effectively reduced the concentration and speed of TC2 further downcanyon, similar to the experiment results of Stix (2001). This processes is categorized as dissipative turbidity current (Parker, 1982; Parker et al., 1986; Sequeiros et al., 2009). The measured velocity and concentration have a direct impact on calculating the densimetric Froude number, which differentiates supercritical flows from subcritical ones. Previous works used different approaches in making arguments on the flow conditions of deep-water turbidity currents but they most agreed that turbidity currents in canyons were supercritical (Fd 41.0). Komar (1971) argued that turbidity current in submarine canyons are supercritical as long as the canyon floors are steeper than a critical range around 1%. Similarly, Sequeiros et al. (2010a) Froude-number scaled laboratory experiment demonstrated that turbidity currents in general, and TC2 of Monterey Canyon in particular, should indeed be supercritical providing the bed slope is steeper than 1%. The calculations of densimetric Froude number using Eq. (A.5) require both the depth-averaged velocity U and depth-averaged volume concentration C. The values of U are known but depthaveraged C is not, so an ad-hoc approach is employed to estimate

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

the depth-averaged C. Because the dynamics of a sedimentcarrying flow dictates that the concentration is in general proportional to the velocity squared, the ratio of the depth-averaged velocity to the maximum velocity in the profile, [U/um]2 (Table 3), can be assumed to be equivalent to the ratio of the depth-averaged concentration to the peak concentration at 16 MAB, C/C16 (Table 3, [3M, 5M, 1Z, 3Z, 5Z]). This relationship is more or less in conformity with the empirical analysis, [um/U]E Fd 1.3 and [Cm/C] EFd 2.9 (Eq. 24 and 26 in Sequeiros, 2012). Such estimated depthaveraged concentrations are listed in Table 2, along with the computed densimetric Froude number (Fd). It is necessary to point out that ranges of error exist in most of data in Tables 2 and 3 due to instrumental accuracy and, more importantly, the uncertainties propagated in the chains of assumptions and calculations. For instance, if we treat the depth-averaged error velocity (  2 cm/s) as the standard deviation of the horizontal velocity measurements (Teledyne RDI Library & Reference Center Glossary, http://www. rdinstruments.com/glossary.aspx), the error of U is approximately 5% for TC1 and 2% for TC2. In turn, this leads to an error of approximately 10% (square of the error for U) for C during TC1 and 4% during TC2. Therefore, even though the estimated Fd values for TC1 are less than unity (0.95 and 0.99 at R2 and R3 sites respectively), they are so close to unity that it is not impossible that TC1 was in fact critical or even supercritical, within the margins of error. Furthermore it is believed that the critical densimetric Froude number of a subaqueous gravity flow can depart from unity depending on the degree of entrainment of ambient fluid and the density variation in the currents (Huang et al., 2009). Because this margin of error cannot be precisely quantified, the discussions here will be based on the face values, which dictate that (1) TC1 was subcritical at R2, and (2) TC1 was transitioning to be critical/supercritical while flowing downstream toward R3. The apparent contradiction with arguments that turbidity currents in submarine canyons are supercritical (Komar, 1971) can perhaps be explained as follows. Originated from a submarine landslide or wall failure in the upper reach of Soquel Canyon, TC1 was probably supercritical while traveling downstream inside the Soquel Canyon whose longitudinal gradient is as high as 61 (Paull et al., 2011). As TC1 entered the Monterey Canyon it ran directly into the southern canyon wall at the junction (Figs. 1 and 9). This collision was effectively like a “forced hydraulic jump” that changed the flow state of TC1 from supercritical to subcritical. Not only was the speed of the reflected flow (toward north) substantially reduced, the flow also became thickened and the turbid plume was raised up in the water column to as high as 300 MAB. As this subcritical current flowed downstream toward the R3 mooring, it would tend again to reach its equilibrium, normal state that is ultimately determined by the slope and roughness of the bed. For the channel slope between R2 and R3, the normal state is most probably supercritical. At mooring sites R2 and R3, TC2 was supercritical, with Fd ¼ 1.13 and 1.24 respectively (Table 2), indicating that TC2 reached an equilibrium, quasi-steady state in this stretch of the canyon (Sequeiros et al., 2010a). However, TC2 was apparently subcritical (Fd ¼0.51) at the R1 mooring despite the fact that it registered the highest depth-averaged velocity (U¼ 1.13 m/s). The direct cause of the much smaller Fd value was TC2's high concentration at R1, which was more than an order of magnitude greater than its concentrations at R2 or R3 (Tables 3, [1,3,5Z] or [2,4,6Z]), resulting from the profoundly higher content of coarse sediments (Table 3). Because of the high settling velocity and Rouse number (Table 3, [1X, 1Y]) associated with the coarse sediment, it is conceivable that the C16 and the depth-averaged concentration C were overestimated. But even if the concentration at 70 MAB (C70 ¼1.43%), which is deemed more accurate because C70 is a ‘true measurement’, were used in calculating the depth-averaged concentration,

29

it still results in a densimetric Froude number smaller than unity (Fd ¼0.64). This suggests that TC2's concentration was so high and its sediment so coarse that the flow simply had no capacity to carry them and achieve steady state when it arrived at site R1. As TC2 flowed further downcanyon, however, the excess material settled out so the flow reached the quasi-steady state prior to reaching site R2. Turbidity currents may not reach normal state if steady deposition of suspended sediment leads to velocity and Froude decrease in the downstream direction (Sequeiros, 2012). This happens if at a certain time and place the concentration and/ or grain size distribution of the suspended sediment is too high and/or coarse for the flow to keep it suspended. This results in a capacity-driven deposition of suspended material in the proximal areas that might be followed by a normal condition stage if the current manages to adjusts itself, before entering a stage of competence-driven deposition more distally (Komar, 1970; Hiscott, 1994; Kneller and McCaffrey, 2003). Gravity flows with relatively coarse suspended material are more prone to be subject to this capacity-driven behavior, regardless of the nature of the process that triggers the flows (Piper and Normark, 2009). Channel width widens by a factor or two between sites R2 and R3. Even if currents might tend to reach normal state in this stretch, flow discharge and sediment flux can increase substantially. For instance, as the channel width doubles, TC2 peak velocity increases by  20%, flow thickness decreases by  20%, and sediment concentration increases by  45%. This suggests that discharge has increased by a factor of 2, and sediment flux by a factor of 2.9. It is worth noting the difference between the ADCP-measured velocities, U, and the calculated front speed, Uf. In spite of the error bars resulting from the uncertainty due to the 5-min sampling interval, Uf is still as large as 5 times greater than U (Table 2), a discrepancy also recognized in a similar mooring deployments in the Congo Canyon (Vangriesheim et al., 2009). Interestingly, if the values of Uf in Table 2 were used to compute the densimetric Froude number instead, they would have resulted in Fd ¼2.44 for TC1 and Fd ¼[2.75, 7.14, 4.16] for TC2 at the three sections of the canyon. Admittedly, these values are somewhat misleading because they are derived with the “front/head” velocities but with “body” concentrations since the “front/head” concentrations were unknown. The behavior of a turbidity current's front/head (Keulegan, 1957, 1958; Middleton, 1966a; Britter and Simpson, 1978; Garcia and Parsons, 1996; Sequeiros et al., 2009) is very different than that of the turbidity current's body (Middleton, 1966b; Komar, 1971; Stacey and Bowen, 1988; Bo Pedersen, 1980; Altinakar, 1993; García, 1994; Sequeiros, 2012). It has been found that the head of a turbidity current in deep water (i.e. current thickness«water depth) can be thought to be in permanent “critical” state so its Fd is close to unity regardless of the slope (e.g. Middleton, 1966a). After the head/front passes, the body rapidly stabilized to a quasi steady state condition (normal condition) that depends exclusively on the bed slope, roughness, and grain size in suspension (Sequeiros, 2012). Thus it is important to differentiate the parameters belonging to the body and those belonging to the head/front in Froude calculations. Empirically, the known frontal speed Uf can be used to estimate the concentration in the head of the turbidity pffiffiffiffiffiffiffiffiffiffi currents by applying Middleton (1966a) formula U f ¼ 0:75 g 0 H h . In the case of TC2, using the same density values in Eqs. (A.8) and (A.9) and assuming Hh ¼ 70 m (because it deposited sands into a sediment trap 70 MAB), a frontal speed of 6 m/s (see Table 2) would have required a volume concentration of 6% in the head of TC2, which is several times greater than the concentration in the body (Table 2). Secondary flows are well known phenomena in meandering channels (Keevil et al., 2006; Parsons et al., 2010). When the nearbed secondary flow in a bend is directed inward, i.e. toward the

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inner bank, it is called normal; and when directed outward it is called reversed. The near bed data is not accurate enough to prove the existence of secondary current reversal or otherwise. Based on recent developments on secondary currents of underflow in meandering channels (e.g. Fig. 25 in Abad et al., 2011) it is plausible that TC2 was more prone to exhibit near bed secondary current reversal than TC1 due to higher Froude numbers. In summary, the two turbidity currents described here, despite being recorded less than 3 days apart in the same submarine canyon, are fundamentally different (Table 4). TC1 was a slow, thick flow composed of finer grains, typical traits of a muddy turbidity current. We suggest that the forced hydraulic jump it encountered after a head-on collision with the canyon wall not only made TC1 markedly thicker, but also locally changed the state of the flow to Froude-subcritical. In contrast, TC2 was a fast, thin, supercritical flow composed of coarser grains, typical behavior of a sandy turbidity current. Composition of the flows, namely grain sizes, clearly played the key role in defining the different characteristics of the two turbidity current, conforming with findings from previous laboratory experiments (Stix, 2001; Gladstone et al., 2004; Sequeiros et al., 2009). In the 10þ km canyon stretch between R2 and R3, both TC1 and TC2 appear to be in an “autosuspension” or even a mildly self-acceleration mode (velocity and concentration of the body gradually increasing downstream, Parker, 1982; Sequeiros et al., 2009). It is not implausible to argue that they could keep going downcanyon as long as the bed slope and the channel (thalweg) width remain roughly the same. Paull et al. (2011) high resolution mapping shows that these conditions are likely satisfied for another 10 km downstream to 1800 m water depth, where a rather strong turbidity current was also recorded in May 2010 (Xu et al., 2013). 4.4. Lessons learned The cases presented in this paper are arguably one of the best data sets that were ever collected in field turbidity current studies (Khripounoff et al., 2003; Vangriesheim et al., 2009; Cooper and Andrieux, 2012). It is probably the only data set with measurement of both concentrations and grain sizes of sediment particles inside a turbidity current. However, this data set also showed us its incompleteness and hopefully provided guidance for future improvement. Here are several lessons we have learned. 4.4.1. Field experiment in wet seasons Although turbidity currents in Monterey Canyon can occur any time of the year, existing data (Paull et al., 2003; Xu et al., 2004, 2013) suggest that turbidity currents are more likely to be recorded in winter season, December in particular, than in any other months. Despite lack of direct evidence linking turbidity currents with wave induced submarine landslides, the close timing associations shown in this paper or elsewhere (Paull et al., 2003) suggest a high probability of catching an event in Monterey Canyon if instruments were deployed in the December wet season when high waves and precipitations in the region tend to take place. Under the same assumption, field instruments such as moorings can be retrieved shortly after a big weather event so the time window can be further constrained. Being able to shorten the deployment will afford much flexibility in terms of data collection planning (see below).

4.4.2. Canyon head experiment Turbidity currents recorded in Monterey Canyon so far mostly coincided with storm weather, but more detailed field experiment is needed to determine the processes that trigger a turbidity current at

Table 4 Characteristics of the two turbidity currents. Properties

TC1, December 17 2002

TC2, December 19–20 2002

Speed of the front Speed of the body Duration Sediment

Slow,  1 m/s Slow,  0.5 m/s Long-lasting, 10–13 hours Smaller grain size, low concentration

Size

Thicker flow (50 þ m); Bigger plume (300 m high)

Type

Borderline subcritical but with possible supercritical zones, Autosuspension, quasi-steady?

Fast, 4–6 m/s Fast,  1 m/s Short-lived, 5–7 h Larger grain size, higher concentration at early stages upstream Thinner flow (30 m); Smaller plume (100 m high) Supercritical, Dissipative, Surge-like?

the canyon head. A feasible experiment would include synchronous observations of oceanographic and sedimentary parameters on the shelf, in the surfzone, and inside the canyon. Surfzone and shelf stations on both sides of the canyon head monitor the propagation and refraction of surface waves that are responsible for coastal water level change, wave energy dissipation, and littoral sediment transport near the canyon head. In addition to the synchronous recording of flow conditions along the canyon that are necessary to constrain the correlation between the surfzone/shelf and canyon sediment dynamics, the experiment would also need to mobilize a highresolution multibeam mapping at the canyon head, e.g., one before and one after a storm, to detect the location and size of possible submarine landslide during the storm. Although it is still extremely difficult, to be able to measure the in-situ geotechnical properties (e.g., pore pressure change) of the canyon sediment would be a significant improvement. 4.4.3. Faster instrument sampling Field instruments are mostly battery-limited, so the longer the deployment, the slower the sampling frequency needs to be. This is the reason that the ADCPs in the 2002 experiment were set to sample once per hour. If the deployment were for one month (in December) instead of one year, the likelihood of ADCPs recording the peak velocity of the turbidity currents during their passage would be substantially greater. Similarly, in a month-long deployment, the intervalometers in the sediment traps can be set to dispense a time marker daily or shorter instead of tri-weekly. This would substantially improve the accuracy of associating the grain size sequence with the various phases of a turbidity current. 4.4.4. Saturated optical instruments Ironically most of the concentration values in Table 3 are not the results of direct measurement even though there were transmissometers both inside and above the passing turbidity currents. This is because the instrument signals were saturated during the peak of the flows. Signals of optical (and acoustic) instruments, regardless of their working principles (backscatter or transmission, e.g.), vary profoundly not only with concentrations but also with particle sizes. Older instruments like the transmissometers in this study only have a single gain setting so they tend to be saturated when concentration becomes too high or particles in the flow are too fine in sizes. To overcome this problem, instruments with shorter path-length and/or dynamic gain settings that automatically switch to appropriate gain range in response to sudden changes in either concentration or grain size are required. Another measure that could substantially improve

J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

the accuracy of sediment concentration is to capture samples of the turbidity currents containing both water and sediment in bottles at selected time (e.g., when peak velocity occurs). However, this is extremely difficult to do, and it has not been done in field environments.

4.4.5. Velocity measurements ADCPs measure current velocity by measuring the Doppler shift of sound backscattered from the moving particles in a flow. To authors' best knowledge, there is no published report on ADCP's performance in an environment of very high concentration, e.g., the 5–6% during TC2. Therefore there are reasons to suspect that the ADCP-measured speed of a high-concentration turbidity current may not be the true speed of the flow. May this be in part responsible for the discrepancies between the U and Uf in Table 2? This type of uncertainty can be solved by conducting a series of laboratory calibration where ADCPs are tested with a matrix of different concentrations and velocities in a controlled flume. Moreover, deploying a traditional travel-time-based current meter (e.g., MAVS, see Hogg and Frye (2007)) concurrently with ADCPs is another way of crosschecking the accuracy of velocity measurements.

31

instruments at the “right” locations is as important as knowing the bathymetry and morphology details of the study area. Both turbidity currents occurred during a “storm week” in the Monterey Bay area. In particular, TC2 took place following a day of a 10-year storm that brought the highest waves and rainfalls. Pounding surface waves not only cause orders-of-magnitude increase of littoral sediment transport that converge to the head of Monterey Canyon from both sides, but also increase the probability of liquefaction of the existing sediment already accumulated in shallow water at the canyon head. There is no data on the direct contribution of rainfalls to the formation of turbidity currents in Monterey Canyon, but it is conceivable that high precipitations and the consequential greater sediment yields and discharges in local rivers help delivering sediment, directly or indirectly, to the canyon head. In general, a canyon of close proximity to the coastline that receives direct feed of river sediment or littoral transport (e.g., Gaoping Canyon of southwest Taiwan, Nazare Canyon of Portugal, Capbreton Canyon and Var Canyon of France, and Congo Canyon in Africa) is a good natural laboratory for future studies of turbidity currents and their triggering mechanisms.

Acknowledgment 5. Conclusions In summary, this paper studied the velocity and sediment properties of two turbidity currents recorded in Monterey Canyon in December 2002. Sediment concentrations and particle grain sizes were, for the first time, measured within the body of field turbidity currents. The highest instantaneous velocity of the two flows was 1.9 m/s, but this was far less than the frontal speed of the turbidity currents that was as fast as 6.1 m/s. The discrepancy is most likely due to ADCP's hourly recording rate that missed the highest speed at the moment of the passing of the flow's front. Flow thickness of the two turbidity currents calculated by depth integration was 30–50 m (Xu, 2010). Turbid plumes of the flows, however, reached as high as 300 MAB in the water column. The highest sediment concentration of the two turbidity currents was 2.27%. Sediment particles in TC2, which originated from the head of Monterey Canyon, were much coarser than that of TC1, which started from the upper reach of Soquel Canyon. Medium sands (2–3 phi) were collected in a sediment trap 70 MAB during the passing of TC2. Sediment composition is not only key to resolving sediment concentration from field measurements of optical or acoustic instruments, but also an important parameter in determining the properties of the turbidity currents. In the case of the “silty” TC1, its plume lingered much longer at all three moorings because of the finer particles and smaller fall velocities. As a result, most sediment traps on R2 and R3 moorings were filled full with TC1 sediment. Consequently, the coarser sediment from the “sandy” TC2 could only be found in sediment traps on mooring R1, where TC1 did not pass through, and the trap L at 70 MAB on mooring R3. The data also evidently showed that the “silty” TC1 was thicker and slower than the “sandy” TC2, but this alone is not enough to assert “turbidity currents of finer material is slower and thicker” as a general rule. Canyon morphology in the pathway of turbidity currents is of importance in changing the flow properties. The fact that TC1 grew thicker slowed down and thus became subcritical at the R2 site indicates a hydraulic jump, which could conceivably occur when TC1 exited Soquel Canyon and ran head-on into the southern wall of Monterey Canyon. In contrast, TC2, which did not experience this hydraulic jump, stayed thinner, faster, and supercritical. In field studies, therefore, placing a limited number of

The project was supported by the USGS Coastal and Marine Geology Program and the Naval Postgraduate School. The manuscript benefited from insightful discussions with Charlie Paull, the late Bill Normark, and Leslie Rosenfeld, and careful reviews from Jessie Lacy and two anonymous reviewers from the Journal. Angela Lam, Kurt Rosenberger, and Jerome Li helped preparing and processing the sediment and current meter data. The authors also thank our field crew: Marinna Martini, Jonathan Borden, Joanne Ferreira, Marla Stone, Rick Rendigs, Kevin O’Toole, Hal Williams, Andree Ramsey, Fred Bahr and the crews of R/V Pt. Sur and R/V Shana Rae for their excellent, and in some cases heroic, work at sea.

Appendix I. Grain-size dependent calibrations The attenuation coefficients measured by all transmissometers are very sensitive to the grain size diestribution of the suspended particles. For fine particles such as clay and silts (grain diameter 44 phi), the optical beam tends to be completely attenuated (saturated) at a concentration of about 0.5 kg/m3 (Bunt et al., 1999; Nittrouer et al., 1986), even though Xu et al. (2002) were able to calibrate the transmissometer at concentrations as high as 0.9 kg/m3. For sandy sediment (grain diametero3 phi), much higher concentration can be tolerated but the difficulty is to keep the coarse grains in a homogeneous suspension. To the authors' best knowledge, Ochiai and Kashiwaya (2010) is the only published work that calibrated sand size particles with concentration as high as 10 kg/ m3. Despite the differences (calibration devices, sediment concentrations and grain sizes used) between Xu et al. (2002) and Ochiai and Kashiwaya (2010), they were both based on the same optical principle, the Lambert–Beer Law: I ¼ I 0 expð KCLÞ

ðA:1Þ

where I and I0 are respectively the transmittal and incident light intensity; C is sediment concentration, L is path length of the optics, and K is a constant. A simple manipulation can translate the quation (A.1) to KC ¼ lnðI 0 =IÞ=L

ðA:2Þ

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J.P. Xu et al. / Deep-Sea Research I 89 (2014) 11–34

where KC can also be replaced by the beam attenuation coefficient, cp. Instead of beam attenuation, Ochiai and Kashiwaya (2010) used the light absorbance coefficient, A, in their calibration formula: A ¼ log ðI 0 =IÞ

ðA:3Þ

Thus, a linear relationship between cp and A can be easily established, cp ¼ A= log ½eL ¼ 15:4A

ðA:4Þ

given that Ochiai and Kashiwaya (2010) path length L ¼0.15 m. Fig. 5 overlays the calibration relationships from Xu et al. (2002) on the results by Ochiai and Kashiwaya (2010). It appears to show that the linear calibrations by Xu et al. (2002) can be used in situations where sediment concentration is much higher, e.g., cp ¼30 (A ¼2). Note that the transmissometer was not calibrated with a bulk field sample (Xu et al., 2002). Instead, it was calibrated with three sieved samples with a narrow range of grain sizes. In applying the calibration to the transmissometer data here, it is assumed that the calibration is dependent of grain size only, regardless of source, chemical and mineral compositions. II. Estimating concentrations when instruments are saturated To calculate concentrations when the sensors were saturated, we assumed (1) that the concentration within the body of a turbidity current is proportional to the squared densimetric Froude number, i.e., C p F2d (e.g. Bowen et al., 1984), and (2) that Fd stayed constant during the peak of the turbidity currents when the transmissometers were saturated. The densimetric Froude number is expressed as F 2d ¼ U 2 =g 0 h

ðA:5Þ

where U is the depth-averaged speed, h is the thickness, and g0 is the reduced gravity. If Fd1 ¼Fd2, where the subscript 1 denotes the time when the highest possible values of beam attenuation coefficients were measured (the same time the transmissometer's saturation ceased, columns 2, 4, 6 in Table 3); and subscript 2 denotes the parameters at the peak of the turbidity current; ðU 21 =g 01 h1 Þ ¼ ðU 22 =g 02 h2 Þ:

ðA:6Þ

0

where g ¼gRC. Because submerged specific gravity of sediment, R, is also dependent of concentration, the above relation now becomes C 2 ¼ C 1 ðU 22 h1 R1 Þ=ðU 21 h2 R2 Þ

ðA:7Þ

Since R ¼ ðρs  ρ0 Þ=ρt

ðA:8Þ

and ρt ¼ ρ0 þ ðρs  ρ0 ÞC

ðA:9Þ

where ρs, ρ0, and ρt are respectively the densities of sediment grains, interstitial water and the turbidity current itself. Eq. (A.7) can then be rearranged into C2 ¼

Aρ0 C 1 ρ0 þ BC 1 ð1  AÞ

ðA:10Þ

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