Aplicación de la fotografía al análisis granulométrico de ríos de gravas

Grain-size distribution (GSD) and roughness of gravel-bed rivers are studied by means of vertical photography. Testing area is located in the Ribera Salada River basin (Catalan Pre-Pyrenees), where fifteen reaches were surveyed and sampled. Methods are direct sampling by means of a sampling grid, and indirect measurements using surface photography (scale 1:10) and remote photography (1:45). On one side, results indicate that surface photography is a reliable technique to estimate particle size from 10 mm and fully applicable to derive the whole grain-size distribution. Adjustment of this method to direct measurements reaches 95% and errors are mainly due to underestimation. On the other, remote photography is a reliable tool in order to obtain grain-size distributions for particles larger than 45 mm in diameter, the D25 of the Ribera Salada GSD. Adjustment between methods for particles between D25 and D100 is very high (95%) in truncated distributions at 45 mm, and errors are due to underestimation. However, without truncation, error increases up to 30% for particles larger than 64 mm. River-bed roughness can also be estimated using data on particle size from photographs, with a degree of adjustment between the three methods close to 95%.