Chapter 8 A decade of sediment transport measurements in a large Mediterranean river (the Tordera, Catalan Ranges, NE Spain)

Abstract

The sediment yield of the Mediterranean Tordera River (894 km2) has been computed from field measurements of runoff, suspended sediment and bedload transport. Measurements were carried out during 1990s at three different points within the Tordera basin: the Upper Tordera (35 km2), the Arbucies River (106 km2), and the Lower Tordera (785 km2). Almost one thousand samples of suspended load and bedload were used to estimate the total yield of two catchments representative of the main catchment lithologies, the Upper Tordera (metamorphic) and Arbucies (plutonic), and the sediment load at the entrance of the Lower Tordera reach. The annual load in suspension passing the downstream Lower Tordera section was estimated at 32,500 t, giving a specific sediment yield of 41 t km−2 yr−1. Most of the sediment is supplied by the granitic catchments at a rate of 60 t km−2 yr−1, three times higher than that of the metamorphic areas (20 t km−2 yr−1). Annual bedload transport through the Lower Tordera section was estimated at 29,000 t yr−1, giving a specific sediment yield of 37 t km−2 yr−1. The catchments underlain by plutonic rocks supply more bedload (27 t km−2 yr−1) than the metamorphic areas (22 t km−2 yr−1). The total load at the Lower Tordera section was estimated at 61,600 t yr−1, which gives a specific sediment yield of 78 t km−2 yr−1. The total sediment load from the granitic catchments is 88 t km−2 yr−1 and doubles the sediment contribution from the metamorphic areas (41 t km−2 yr−1). Most sediment is transported during short periods of time. In the Lower Tordera, 65% of the total load is carried in 10% of the time, in the Arbucies River the proportion raises to 82% and in the Upper Tordera it reaches almost 90%, for the same time duration. The plutonic areas appear to be the main source of sediment, both in suspension and as bedload, accounting for 90% (ca. 56,000 t) of the total annual load of the entire basin. The data indicate that the Lower Tordera shows a similar pattern of sediment yield to that described for Mediterranean semi-arid regions, whereas values estimated in the Arbucies river (mainly for bedload) and, especially, in the upper part of the catchment agree with data reported for more humid, Mediterranean mountainous environments.