Evaluating a 2D image-based computerized approach for measuring riverine pebble roundness

The geometrical characteristics of pebbles are important features to study transport pathways, sedimentary history, depositional environments, abrasion processes or to target sediment sources. Both the shape and roundness of pebbles can be described by a still growing number of metrics in 2D and 3D or by visual charts. Despite new developments, existing tools remain proprietary and no pebble roundness toolbox has been available widely within the scientific community. The toolbox developed by Roussillon et al. (2009) automatically computes the size, shape and roundness indexes of pebbles from their 2D maximal projection plans. Using a digital camera, this toolbox operates using 2D pictures taken of pebbles placed on a one square meter red board, allowing data collection to be quickly and efficiently acquired at a large scale. Now that the toolbox is freely available for download,1 an evaluation is needed to help end users sample, process and interpret the data. We first assess the influence of pebble position and of the resolution and enhancement of pictures and errors associated with the sampling size. Second, we run experimental flume tests of roundness trends that confirm the great potential of the toolbox for river geomorphic diagnoses as well as for targeting sediment sources. We also validate the correlation between roundness metric trends and particle abrasion.