River incision due to gravel mining: a case study

Abstract

Historical information was used to study river degradation in a case study in NE Spain. The accumulation of good topographical information since the 1940s for the Gállego river (a tributary of the Ebro river, which drains about 4,000 km2 of the southern slopes of the Pyrenees) enables a comparison of longitudinal bed profiles. Historical research has also revealed the volume of gravel mined, which amounts to ≈ 1 million m3 compared to the volume of alluvium lost due to incision in the same period, which is ≈ 2 million m3. This unbalance is explained by a simple model based on a bedload equation and an algorithm to determine whether effective bedload transport is controlled by transport capacity or by the supply of sediment. It follows from the analysis that the incision process has changed the magnitude of the shear stresses on the bottom. This is because, as the river becomes degraded, higher discharges fit into the main channel before spilling.