Background Deposition. S. China Sea. Study Region. Brunei. Borneo. R. z 2. z 1. ~100 m. y. Levees grow with self-similar form. Each Deposit has constant Taper. Cumulative Taper increase with T. Individual deposit taper decreases with T. Cumulative Taper increase with T. km. 0. 5.
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Levees grow with self-similar form
Each Deposit has constant Taper. Cumulative Taper increase with T.
Individual deposit taper decreases with T. Cumulative Taper increase with T.
Morphodynamics of Levees Built by Turbidity Currents: Observations and ModelsOS23B-1662
Straub, K.M., EAPS, MIT, 77 Massachusetts Avenue, Cambridge, MA [[email protected]]
Mohrig, D., Department of Geological Sciences, The University of Texas at Austin
Importance of Levees to Seascape Models
- Primary element of self-formed channels
- Faithful recorders of channel history (channel bed subject to strong erosion & deposition).
- Record vertical structures of currents
- Levees connect channel to the overbank.
Levees are the primary element of self-formed channels, are faithful recorders of channel history, and connect channels to their overbank surface yet little is known about their morphodynamics. Using an industry-grade 3D seismic survey we have studied a submarine network of channels located offshore Brunei Darussalam. We have mapped the seafloor and a shallow regional surface beneath the network of interest. The subsurface horizon defines the geometry of a scarp and slide plane associated with a mass-failure event that reset the margin to an unchannelized state. A map of deposit thickness created by differencing the seafloor and subsurface horizons was used to create plots of deposit thickness as a function of distance from a channel thalweg for channels of varying relief. Levee steepness increased from 0.01 m/m to 0.05 m/m as channel depth increased from 5 to 50 m, but this trend rolled over to a near constant steepness value of 0.05 m/m for channels greater than 50m in depth. A similar trend of levee steepness vs. local channel depth was observed in a reduced scale laboratory experiment. We model levee growth using a simple advection settling model for currents with multiple grain sizes and a vertical sediment concentration profile defined by the Rouse equation. This model reproduces the field and laboratory observations of levee growth and suggests that the most important parameters controlling levee deposition rates and steepness are the degree of channel confinement and the vertical structure of the suspended-sediment concentration profile.
Submarine channels offshore Brunei are bounded by prominent levees. We use an industry-grade seismic cube to
unravel their growth history
1200m water depth
200m water depth
Horizontal Data Resolution = 25 m by 25 m
Vertical Resolution ~ 5 m
Regional Overbank Deposition
Seismic Dip Line
Extent of Channel Overbank Deposition
Horizontal Boundary of Subsurface Horizon
~ 200 m
Seismic Strike Line
How does Levee Morphology Change as
Channel Relief = R
Depositional taper = ∆z/y
y ≈ 1 – 2 channel widths
~ 50 m
How does bulk levee taper and taper of individual flow event deposits change as channel relief increases??
Detachment Surface Slope Map
Levee taper was defined from linear regression best-fit lines through plots of average cumulative deposit thickness vs. distance from channel.
Increasing Water Depth
What causes roll-over in trend with increasing relief?????
4 Current Properties Determine Change in Levee Taper as a function of deposition
We use a laboratory study and an advection-settling model to explore the influence of these 4 parameters
Support for our research was provided by Brunei Shell Petroleum and Shell International Exploration and Production Inc.
Additional funding provided by the National Center for Earth-Surface Dynamics, an NSF Science and Technology Center
Comparison of Brunei and
Reduced scale laboratory experiments provide data needed to describe DYNAMICS that are missing in field scale studies of channel morphology.
Parameters & Scaling for Channelized Turbidity Current
Rate of levee taper increase is greatest at low values of Relief/Reliefmax for both channels that increase in relief through time (Brunei) and channels that decrease in relief through time (lab experiments).
Governing Dimensionless Parameters
Approximate Dynamic Similarity
(Fr)M = (Fr)P , (p)M = (p)P
Re ≥ 6400, ensuring turbulent flow conditions
Widthchnl = 77.0cm
Depthchnl = 10.0cm
Lengthchnl = 300cm
U = 6.5cm/s
H = 10.0cm
T = 576 sec
D50 = 2.9×10-3cm, m silt
Widthchnl = 770m
Depthchnl = 50m
Lengthchnl = 3000m
U = 2.2m/s
H = 100m
T = 2.8hr
D50 = 1.0×10-2cm, vf sand
Levee Growth Model
Observations from Brunei and lab experiments suggests taper of individual beds comprising levee is influenced by degree of current confinement
We couple a suspended sediment concentration profile defined by a Rouse equation to an advection-settling scheme. As levee deposition occurs, channel relief increases causing progressive confinement of current
Levee Deposit Properties
Bathymetry (T = 0)
Deposition rate is influenced by the near bed concentration and settling velocity of each (i) particle class
C. I. = 6.5 mm
Spatial change in levee deposit taper is greater than change in gradient of deposit grain-size with distance from levee-crest
Deposit (T8 – T0)
Turbidity Current Properties
Near bed concentration at a given location, x, on the levee is determined by concentration at a height in current that advects at a rate, Uy, and settles at a rate, Ws.
C. I. = 2.5 mm
Best-fit Parameters to
Roll-over in trend of levee taper as a function of channel relief occurs once heavily stratified portion of concentration profile is confined in channel
Elevation of heavily stratified and high sediment concentration, lower portion of the turbidity current above levee crest results in rapid growth of levee thickness and taper
Jim Buttles (University of Texas at Austin) provided additional help in conducting experiments