Sediment management options for the lower Ebro River and its delta

Abstract

Past and present management of the Ebro catchment have altered water, sediment and nutrient fluxes of the lower Ebro River and its delta. The construction of the Mequinensa and Riba-Roja dams during the 1960s disrupted the sediment transport continuity. As a result, the lower Ebro River and its delta are facing a reduction in sediment delivery which is estimated to be up to 99% of its original yield (about 3 × 107 t/yr). Consequently, about 45% of the emerged delta will be under the mean sea level by 2100 due to subsidence and sea level rise, whereas the delta coast is retreating at a rate of several meters per year in the mouth area. This paper aims to establish the basis for a sediment management plan directed to offset the effects of the sediment delivery deficit of the Ebro delta. A new management plan named ‘Integrated Plan for the Protection of the Ebro Delta’ (PIPDE) is being presently elaborated in order to develop sustainable management of the Ebro River and delta through an integrated management of water, sediment and habitats, in order to achieve European Union (EU) Water Framework Directive (WFD) requirements. Among other aspects, the plan aims to restore the sediment flux of the lower Ebro River by means of both the removal of the sediment trapped behind the dams and the transport of the sediment downstream of the reservoirs to the river mouth and delta plain. Preliminary studies show that the ‘flushing flood’ method has lower costs and is the most adapted method to transfer the sediments retained into the Riba-Roja dam. The required sediment load to stop coastal retreat in the mouth area and to compensate relative sea level rise represents 10 times the present suspended load (0.021 g/l), but is 10 times lower than the total volume of sediment delivered to the sea under natural conditions. Sediment delivery to the delta plain can be accomplished through the rice irrigation system, a method used prior to dam construction. Problems of achieving a sustainable management of water and sediment are mostly linked to reservoir management. Technical problems associated with by-passing sediments through the reservoirs can be solved through operational and structural changes. Benefits of sediment flow recovery are multiple, including maintenance of the reservoir capacity, possible decline of invasive species, reduction of elevation loss and coastal retreat in the delta. The partial restoration of sediment fluxes in the lower Ebro River and its delta is technically feasible and environmentally desirable, but further detailed studies need to be carried out before the plan can be implemented. Overall, the sustainability of the lower Ebro River and delta can only be guaranteed by the implementation of a new reservoir management concept where the ecological and morphological values, as well as the economical and social values, have to be taken into consideration.