Quantifying the Morphology and Growth of Levees in a Submarine Tributary Channel System Offshore Brunei Darussalam

Straub, Kyle M.¹, David Mohrig² (1) Massachusetts Institute of Technology, Cambridge, MA (2) University of Texas at Austin, Austin, TX

A network of submarine channels sharing planform attributes similar to river systems is located on the present-day continental slope offshore Brunei Darussalam. We mapped the seafloor and a shallow regional surface beneath the network of interest using an industry-grade 3D seismic survey. 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 was created by differencing the seafloor and subsurface horizon and this thickness data is used to unravel the growth of self-formed, leveed channels. With thickness and topographic data we have determined sedimentation trends; particularly relative rates of levee and overbank sedimentation as a function of lateral distance from the nearest channel centerline. We also use the deposition map to quantify the relationship between channel relief and levee taper. Levee steepness increases from 0.010 m/m to 0.050 m/m with a growth in channel depth from 5 to 50 m. Steepnesses increase at an ever diminishing rate for the deepest channels with sections of channel up to 72 m deep having a levee taper of only 0.056 m/m. We model levee growth using a simple advection settling model for currents with a vertical sediment concentration profile defined by the Rouse equation. This reproduces field and additional laboratory observations of levee growth and suggests that the most important parameters controlling levee morphodynamics are the degree of channel confinement and the vertical structure of suspended-sediment concentration profiles.