Image analysis for measuring the size stratification in sand–gravel laboratory experiments

Abstract

Abstract. Measurements of spatial and temporal changes in the grain-size distribution of the bed surface and substrate are crucial to improving the modelling of sediment transport and associated grain-size selective processes. We present three complementary techniques to determine such variations in the grain-size distribution of the bed surface in sand-gravel laboratory experiments, as well as the resulting size stratification: (1) particle colouring, (2) removal of sediment layers, and (3) image analysis. The resulting stratification measurement method was evaluated in two sets of experiments. In both sets three grain-size fractions within the range of coarse sand to fine gravel were painted in different colours. Sediment layers are removed using a wet vacuum cleaner. Subsequently areal images are taken of the surface of each layer. The areal fraction content, that is, the relative presence of each size fraction over the bed surface, is determined using a colour segmentation algorithm which provides the areal fraction content of a specific colour (i.e. grain size) covering the bed surface. Particle colouring is not only beneficial to this type of image analysis but also to the observation and understanding of grain-size selective processes. The size stratification based on areal fractions is measured with sufficient accuracy. Other advantages of the proposed size stratification measurement method are (a) rapid collection and processing of a large amount of data, (b) a very high spatial density of information on the grain-size distribution, (c) the lack of disturbances to the bed surface, (d) only minor disturbances to the substrate due to the removal of sediment layers, and (e) the possibility to return a sediment layer to its original elevation and continue the flume experiment. The areal fractions are converted into volumetric fractions using an existing conversion model.

Armor breakup and reformation in a degradational laboratory experiment

“…The measurements were taken using the image analysis technique developed by Orrú et al (2014Orrú et al ( , 2016, which is based on color segmentation, i.e., the division of the pixels of an image into color groups (Fig. 3).…”

“…The changes in the bed surface texture were measured during flow using the technique developed by Orrú et al (2014Orrú et al ( , 2016. We examine the stability of an armor under peak flow under a limited sediment supply.…”

A new technique for measuring the bed surface texture during flow and application to a degradational sand‐gravel laboratory experiment

“…Our newly proposed technique for measuring the grain size distribution of the bed surface during flow builds on the technique proposed by Orrú et al . []. The latter allows for measurement of the size stratification of a sand‐gravel deposit after draining the flume.…”

“…Orrú et al . [] combined image analysis, based on color segmentation, with a sediment removal technique to measure the bed surface texture and the size stratification of a sand‐gravel deposit after draining the flume. For a more detailed review on the above image analysis techniques we refer to Orrú et al .…”

“…The technique is an extension and an improvement of the one presented by Orrú et al . []. The changes comprise: (a) an extension of the technique for measuring during flow and an automatic cropping of the images to cover the entire bed surface, (b) a new algorithm that allows for leaving one of the sediment fractions (sand) in its natural color to avoid cohesion‐related issues, (c) the possibility to apply the new technique to sediment mixtures composed of up to 6 grain size fractions.…”