Semi-diurnal transport in 6 months

Transport during the two turbidity currents

count*m2

[an arbitrary unit that can be converted to g/m]

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Diurnal transport in 6 months

Subtidal transport in 6 months

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• The mean and tidal transport at the mooring site can be divided into three layers: 30-40 m immediately below the canyon rims, the canyon interior, and the bottom 60 m near the canyon floor.

• Transports in the top and bottom layers are nearly one order of magnitude greater than in the interior layer. The diurnal transport is minimal compared to the much greater semi-diurnal or subtidal transport.

• While the semi-diurnal transport in both the top and bottom layers are upcanyon, the subtidal transport has opposite directions in the two layers.

• Transports due to turbidity currents are limited to the bottom 20 m near the canyon floor.

• The transport during two turbidity current events, a combined duration of less than 3 hours, is in the same order of magnitude of oscillatory transport integrated over the whole 6-month deployment. A single two-hour event is more than double the mean transport through the cross-section of the whole thalweg for the entire 6-months.

SUMMARY

Reference:Xu, J.P., P.W. Swarzenski, M.A. Noble, A.C. Li (2010) Event-driven sediment flux in

Hueneme and Mugu submarine canyons, southern California. Marine Geology, 269,

74-88. doi:10.1016/j.margeo.2009.12.007

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Turbidity Current Transport Turbidity Current Transport

<u*c>, count * cm/s

u, cm/s c, count u, cm/s c, count

−8 −7 −6 −5 −4 −3 −2 −1 0 1x103<u>*<c>, count * cm/s

12/05/2007 eventDuration: 55 min.

12/05/2007 event, Duration: 55 min.

2/25/2008 eventDuration: 120 min.

2/25/2008 event, Duration: 120 min.

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5 min. interval

color code:red - start of eventblue - end of event

5 min. interval

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SEDIMENT TRANSPORTBY

TURBIDITY CURRENTS

Energetic sediment transport events resembling turbidity currents were observed four times during the 6-month period (two of them shown here). The maximum speed of these turbidity currents reached almost 300 cm/s.

Integrate the bottom 20 m of the <uc> profiles then multiply the duration of the events (T) to obtain the sediment transport,

W = (∫<u c> dz) T

in an arbitrary unit of count*m2, the transport for the two listed events are respectively

12/05/2007 event: 1.72e+6 2/25/2008 event: 5.52e+6

When a calibration becomes available to convert ‘count’ to g/m3, the unit for W is g/m - flux per unit width of the thalweg.

Using the depth-averaged estimate of u=1.5 m/s, c=2 kg/m3 (Xu et al, 2010), the 2-hour long event on 2/25/2008 transported a total of 69,000 metric tons (46,000 m3) of sediment downcanyon through the bottom 20 m (turbidity current thickness) of the 160 m wide thalweg at the mooring site.

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Oscillatory Transport

count * cm/s

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/s)2

Auto Spectra of Along-Canyon Velocity

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Auto Spectra of ADCP Backscatter

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Co-Spectra

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Semi-Diurnal

Diurnal

Subtidal

Semi-Diurnal: 1/13 < f < 1/11 cph

Diurnal: 1/25 < f < 1/23 cph

Subtidal: f < 1/25 cph

down-canyon up-canyon

OSCILLATORY TRANSPORT

The co-spectra of along-canyon tidal currents and sediment concentrations indicate an upcanyon transport at the dominant semi-diurnal frequency, contradicting the asymmetry of the semi-diurnal currents near the canyon floor that appears to favor downcanyon transport. This discrepancy seems to be caused by the phase lag between tidal currents and the timing of the turbid plumes.

Like in the mean transport, there is a layered structure in which both the magnitude and direction of transport vary with water depth. Transport in three distinctive layers are computed separately:

W = (∫F dz) Twhere T=6 months.

Laye

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Hourly Currents Semidiurnal (M2)

Northern Velocity, cm/s

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Along-Canyon Velocity, Low pass (Subtidal)

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Mean Velocity

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Canyon Rims

Above the canyon rim (26 m) the sub-tidal currents are mainly along the shoreline (not shown here). From below the rims to about 120 m the subtidal are dominated by down-canyon cur-rent. From 120 m down to the canyon floor the flow struc-ture is more com-plex. Since the mag-nitude of the subtidal flow near the canyon floor is rather small (<5 cm/s), the time averages tend to be insignificant.

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Mean Backscatter

Blue - excluding the four turbidity events

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MEAN TRANSPORTTransport by the mean current in the entire thalweg (60 m deep, 350 wide) is down-canyon. Using a depth-averaged estimate of u=0.01 m/s, c=0.01 kg/m3 (Xu et al, 2010), the total sediment transport during the 6-month deployment is 32,700 metric tons (22,000 m3), less than one half of the transport by one 2-hr turbidity current event.

Modes of Suspended Sediment Transport in Hueneme Submarine Canyon, Southern CaliforniaPosterW-07

Jingping Xu U. S. Geological Survey, Menlo Park, CA jpx@usgs.gov

0 2 4 6 8Distance, km

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V.E. 20x

up-coast down-coast

Santa Clara River

CANYON MOORINGDeployment: Sep. 2007 - Mar. 2008Water depth: 188 mInstruments: RDI 300 KHz ADCPs

INTRODUCTION

Submarine canyons are known as preferential conduits for transporting sediment and other particles from coastal waters to ocean basins, therefore are key players in the source-to-sink system. In this study, two ADCPs (one upward-looking and another downward) were deployed in Hueneme Canyon for 6 months (September 2007 – March 2008) at a water depth of 188 m. The measured velocity profiles and acoustic backscatter intensities are used to characterize the different modes of suspended sediment transport along the canyon. The co-spectra of along-canyon tidal currents and sediment concentrations are used to estimate the oscillatory transport along the canyon. The primary goal of the study is to obtain “order-of-magnitude” estimates of along-canyon suspended sediment transport due to three modes: mean, tidal, and turbidity currents.