The photographic sampling to characterize the grain size distribution of sediments on gravel bed rivers

The characterisation of the particle size distribution along a river is an hard task and usually involves the work of many experienced operators. Al- though that, it is very difficult to obtain a clear description of the different granulometric patches constituting, for instance, a big gravel bar. This is the result of the chosen spatial sampling schemes and of the used method for collecting samples. Pebble count (grid or transect line) methods, fre- quently used in gravel and cobble riverbeds, are time consuming procedures to perform and give results concerning the spread area where the samples were collected. Photographic methods can improve the quality and the resolution of the sampling efforts because they allow the collection of large samples (hun- dreds of stones) over little areas (the dimension of the picture). The poster shows the results of a study finalised to define a criterion and a method to collect photographic samples. A field survey was carried out concerning the collection of grid-by-number and photographic samples. The data have been used to compare the results of the different methods. In particular we decided to concentrate our efforts to define criteria to collect photographic samples. Obviously, this is strictly related to the method we used to automatically obtain the granulometric data from the photographs but also to the type of used photographic camera, focal length and resolution.