Incision due to gravel mining: Modeling a case study from the Gállego River, Spain

Historical information on river degradation was used in a case study on the Gállego River, a tributary of the Ebro River, in northeastern Spain. The Gállego drains about 4000 km2 of the southern slopes of the Pyrenees. Good channel surveys since the 1940s allowed a comparison of longitudinal bed profiles over time. Over this period about 1 million m3 of gravel was mined according to the archival records. The volume of alluvium lost due to incision in the same period was 2 million m3. This imbalance is tentatively explained by a budget model based on a bedload equation and an algorithm to determine whether the effective bedload transport is controlled by the transport capacity or by the supply of sediment. It appears that the incision process has changed the magnitude of the shear stresses on channel bed. As the river became deeper, the channel could accommodate higher discharges without overbank flooding. The results obtained from a second model based on diffusion equation for the bed elevation compared well with the field data. This model is based on the hypotheses of steady uniform flow regarding water and sediment conservation.