Deposit geometry and grain sorting in gravel braided rivers

English Abstract:

Braided river dynamics and sedimentary processes involve complex sequences and patterns of erosion/deposition and channel migration. The different grain size, shape or thickness of the deposition layers developed during these processes create a vertical structure within the bed which vary with the river dynamics and influence the bedload transport processes. Quantitative analysis of these interrelations between sedimentary characteristics, river morpho-dynamics and bedload transport need further development. Field measurements can be used to document vertical structure on the river bed and point out layers boundaries, but the time scale and deposition dynamics is not available. We investigated the dynamic formation of the vertical bed structure using a small-scale physical model of a braided channel. The braided channel was developed at a slope of 1.5% and discharge of 2.1 l/s. The grain size distribution in the model is an approximately 1:30 scaled distribution of medium fluvial gravel with median size 1.3mm (D10 0.3 mm, D90 4mm). Vertical stereo images (with pixel resolution of approximately 1mm) of the dry bed were taken at one hour intervals over 40 hours of flume running time during which a large area of the river was re-worked. DEMs were derived photogrammetrically with mean elevation error of about 4mm. Surface grain size was mapped using calibrated relationship with picture texture value and sieved grain size. Over the 40 hour period the range of elevation and grain size variations over the river bed can be used to analyse deposits dynamics, deposit structure and grain size variation within the river bed.