The use of photogrammetry based on UAVs allows to survey long distances of riverbanks with a resolution capable of identifying detailed bank erosion features and quantifying retreat rates with an accuracy of centimetres. The identification of these features would be missed in slower and more expensive GPS profiling. Moreover, moving terrestrial laser scanning would require significant greater times and costs for comparable results.

We experimentally tested the effect of a large range in helical flow intensity and sediment mobility on equilibrium transverse slopes, which resulted in a function for slope effects depending on bed state and sediment transport mode, that deviates from linear functions with sediment mobility in literature. Furthermore, we obtained basic relations for sorting patterns as a function of transverse slope and sediment mobility.

Lateral sorting (= bend sorting) is observed in  natural meanders, where the inner and outer  bend are fairly fine and coarse, respectively  (e.g. Julien and Anthony, 2002; Clayton and  Pitlick, 2007). This is caused by the mass  differences between grains on a transverse  slope, leading to coarser grains being dragged  down net more than finer grains (Ikeda et al.,  1987). The slope of the transverse bed  influences the degree of bend sorting greatly.  Also vertical sorting occurs. Grainflows at  the lee side of dunes result in a net fining  upward trend (Kleinhans, 2005). The degree of  vertical sorting depends on flow velocity and  sediment mixture characteristics.  Most previous studies focused on the  development of the transverse slope using  small ranges of uniform sediment, so spatial  sorting was absent. Yet, it was argued that  there is a feedback mechanism between bend  sorting and the transverse slope (Ikeda et al.,  1987). So, it is of key importance to attain  better understanding of how sorting in river  bends comes about, which can improve current  numerical models.  It is the objective of this study to examine  experimentally the effect of transverse bed  slope and sediment mobility on spatial sorting  of bed load in a meander. 

We experimentally tested the effect of a  largerange in helical flow intensity and  sedimentmobility on equilibrium transverse  slopes,which resulted in a function for slope  effectsdepending on bed state and  sedimenttransport mode, that deviates from  linearfunctions with sediment mobility in  literature.Furthermore, we obtained basic  relationsfor sorting patterns as a function of  transverseslope and sediment mobility