Response of a river sediment budget after historical gravel mining (the Lower Tordera, NE Spain)

The channel of the lower Tordera River (Catalan Coastal Ranges) was intensively mined between 1956 and 1987, at an extraction rate 14 times higher than the replenishment rates. Mining caused incision of up to 2 m over the whole reach, leading to damage of infrastructures and affecting the groundwater system. In this paper we analyse the response of the river sediment budget by means of sediment transport measurements (both in suspension and as bedload) and topographic surveys. Historical data on mining activities was used to verify current values of sediment transport and to locate the main extraction areas in the long profile of the river. This has led to an integrated methodology that can be useful for assessing and managing current and future sediment deficits in heavily mined rivers. In the lower Tordera, a minimum of 3 million cubic metres of sand and gravel were extracted, especially from two gravel pits still visible as concavities in the river's longitudinal profile. Nowadays, 15 yr after mining ceased, the river shows during dry periods a general tendency towards aggradation (3.6 mm yr(-1) on average and up to 12 mm yr(-1) in central sections where mining took place), but a slight long-term tendency to erode the riverbed. Sediment yield at the entrance of the study reach attains 552 000 t yr(-1), while net sediment deposition reaches 16 400 t yr(-1) (78% as bedload), mostly during small floods in dry periods. Estimated time for the river to recover pre-extraction bed-level is of the order of 420 yr. Copyright (c) 2005 John Wiley & Sons, Ltd.