Abstract
This paper summarises twelve years of research undertaken on the lower River Ebro (NE Spain). The research has been undertaken to achieve a fundamental understanding of the fluvial processes in this large Mediterranean river affected by flow regulation. Dams in the whole basin and, in particular, in the lower section of the river alter the Ebro’s flow and sedimentary regimes, thus affecting its physical and ecological functioning. Data obtained on hydrology, sediment transport and river channel dynamics (i.e. particle mobility, bed structure) have aided the design and implementation of flushing flows since 2002; the main objectives of these artificial releases were the control of excess growth of aquatic vegetation (i.e. macrophytes) and the maintenance of certain sedimentary activity in the channel downstream from the Mequinenza-Ribarroja-Flix Dam complex. Results highlight the sedimentary disequilibrium of the river, with the current sediment load equating to 1% of that estimated at the beginning of the 20th century in the absence of dams and under different land uses. Channel platform changes show that the channel has become more stable since dams were closed and a significant reduction of sediment availability has occurred, together reinforcing the sedimentary disequilibrium of the river. Monitoring of flushing flows has demonstrated their high sediment transport capacity. This suggests that, in combination with other restoration measures such as sediment injections, flushing flows could greatly benefit the riverine ecosystem, reducing macrophyte stands and maximizing the sediment delivery to the lowermost river reaches.
Highlights
Rivers in natural conditions tend to maintain a long-term morphosedimentary equilibrium, whereby the amount of sediment coming from the erosion zones is equivalent to the amount deposited in the lowland zones (e.g. Williams and Wolman, 1984)
Water released from the dam may still be competent to move sediment and, under this circumstance, a sediment deficit exists and river degradation may occur through bed incision, channel narrowing and/or bed coarsening (e.g. Surian and Rinaldi, 2003; Surian and Cisotto, 2007; Vericat et al, 2006)
In this study we considered the lower Ebro as the river reach located downstream of MRFDC (Fig. 1C)
Summary
Rivers in natural conditions tend to maintain a long-term morphosedimentary equilibrium, whereby the amount of sediment coming from the erosion zones is equivalent to the amount deposited in the lowland zones (e.g. Williams and Wolman, 1984). The Ebro River Basin Management Plan indicates the need of implementing “River restoration measures based on environmental criteria” to improve (i) the morphological status and (ii) the sediment management of the lower river downstream from the dams. Against this legislative background, scientists from the University of Lleida in collaboration with the Ebro Water Authorities (CHE) and Endesa Generación SA, the hydropower company operating the dams, have been working since 2002 to design, model, implement and monitor flushing flows Associated processes have been analysed and, second, how knowledge generated from this work has informed the development FFs and aided adaptation of their design to the evolving needs of a complex system such as the Ebro
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