Variation of internal sedimentological parameters by CT-Scan measurment during sediment transport

Category Sedimentology
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Long, Bernard
Holding Date 08 September 2008

The variation of the density of a sediment bed during sediment transport is a key measurement of the determination of the different evolution phases of the genesis of the internal architecture of sand ripples. The aim of this study was to define densitometric parameters for sediment transport using a CT-scanner to determine the effect of flow velocity and grain size on roughness coefficient during its migration under a stationary flow regime inside a vertical tunnel flume on the way to determine the variation of the shear stress coefficient over a sand ripple field under sediment transport.
Computed axial tomography (CAT) scanning, originally a medical technique, has been adapted in geosciences (Duliu 1999, Ketcham and Carlson 2001, Mees et al. 2003), and use to measure sand layers in sedimentological studies. The Quebec City research laboratory, dedicated to none-medical applications, is built around a Siemens Somatom Volume Access CT-Scan and will be modified soon with a Somatom Sensation 64. The tunnel flume, designed by Teeter and Pankow (1989), passes trough the mobile gantry of the CT Scan. The lower part of the tunnel is filled with Ottawa sand and the flow regime is controlled by a Doppler flow. In this experiment, each image of 512x512 pixels has voxel size of 0,391 x 0,602 mm.
The evolution of the different internal phases, determined previously, are analysed in 4D, during the transport. The suspended transport zone (STZ), is separated from the upper bedload transport zone (UBTZ) by the frontier density point (FDP). The density of the FDP is function of the sand dune position and the UBTZ, limited at the base by maximum density point (MDP), represent a transition zone between suspended load and bedload. The shape and the thickness of this zone vary along the sand ripple. Bellow the MDP, the middle bedload transport zone (MBTZ) correspond to the end of the bedload transport. The lower bedload transport zone (LBTZ) corresponds to an erosional transition of the upper part of the foreset laminae developed on the lee-side of the ripple.
This work opens the possibility for characterization of sediment transport in 3D. In vertical profiles of the sand ripple, four zones were defined: the STZ, the UBTZ, the MDTZ and the LBTZ. The passage between the STZ and the UBTZ is continuous and is determined by a frontier density point on the curve of vertical density concentration. The density related to this frontier point changes along the surface of the ripple. This is probably due to the intensity of the turbulence at the ripple surface. The MDTZ thickness is determined by the inflection in the slope how indicates an activity in the lower part of the UBTZ. The different stages of transport have been measured successfully for a single grain size and flow velocity.