Restoring sediment fluxes downstream of large dams: The case of the Lower Ebro River

September 22nd, 2014

Dr. Albert Rovira & Dr. Carles Ibáñez, IRTA-Aquatic Ecosystems, Catalonia, Spain

The construction of dams produces a number of social benefits. But, in producing these benefits, dams also alter the natural balance of sediment flow in rivers by impounding sediment within and upstream of the reservoir and discharging clean water downstream1. Sediment retained in reservoirs also leads to the disruption of the transport continuity while reducing the land-ocean sediment transfer. Under these conditions the morphological system is dramatically altered, leading to the imbalance between the fluvial and the marine processes that cause coastal retreat and land subsidence. A clear example is the case of the Ebro River, located in the North-East of the Iberian Peninsula (Figure 1).

The sediment transport of the lower Ebro River (drainage basin 85,530 km2) is being altered by the Mequinensa and Riba-Roja reservoir system constructed at the end of the 1960s. As a result, the lower Ebro River and its delta are facing a severe sediment deficit which is leading to a progressive change of the river channel morphology and sediment transport dynamics2,3, the degradation of the fluvio-deltaic system4, and the dramatic reduction of fluvial sediment inputs to the delta5. In the long-term, a significant elevation loss of the delta plain due to subsidence and sea level rise is expected, with the prediction that 45% of the emerged delta will be under mean sea level by the end of this century6. Under these conditions, the sediment supply required to maintain the delta is estimated to be about 1.3 x 106 tonnes/yr, but considering the predictions of sea level rise for the year 2100 the value could be about 2.1 x 106 tonnes/yr7.

Confronted with this situation, the Catalan Autonomous Government has developed a management plan (SedMa), whose main goal is to achieve sustainable management of the Ebro River and its delta through an integrated management of water, sediment and habitats, following the European Union Water Framework Directive requirements. The successful implementation of the Plan requires an integrated approach including long-term research, intense consultation with local representatives and organisations, as well as the coordination of different administrative bodies where the stakeholders, administrations and experts are represented.

The SedMa plan mainly consists of the restoration of the sediment flux of the lower Ebro River by means of the removal of the sediment trapped behind the dams, and the effective transport of the by-passed sediment to the river mouth and delta plain through an ecological engineering approach8. Three major elements constitute the framework of the SedMa plan: i) the application of some kind of technology to remove and by-pass the sediment stored in the dams; ii) the definition of a specific flow regime to transport the sediment from the river to the delta, including periodical pulses (floods); and iii) the establishment of a controlled system to deliver part of the sediment to the delta plain.

Different options to mobilise the sediment stored into the Riba-Roja reservoir (e.g. generation of flushing floods; construction of a by-pass system; mechanic dredging, etc.) were analysed; and the ‘flushing flood’ method was found to be the most suitable10. This method consists in partially or totally emptying the reservoir in order to erode the stored sediments, and evacuate them through the bottom outlets by using the water column pressure (in the first case) or by temporally restoring the water flow through the reservoir bed (in the second case)1. The application of this method in the Ebro River is focused on the removal of the sediment stored in the Riba-Roja reservoir by: 1) emptying of the Riba-Roja dam. At this stage, sediment located close to the dam bottom out-level outlets can partially be removed by using the water column pressure; (2) generation of discharges from Mequinensa reservoir. Sediment stored into Riba-Roja dam should be remobilised by water erosion; (3) closure of Riba-Roja bottom outlets and refilling of the reservoir. Prior to flushing operations, a detailed analysis of the required operations of the reservoir system, and the associated economic costs (i.e. opportunity and marginal) and environmental impacts, has to be undertaken.

Once the sediment is transferred downstream of the reservoirs and transported by the river, it has to be deposited in the appropriate areas of the delta. The transport and deposition of sand to the mouth area (and silt to the delta front) can be achieved without technical intervention. However, some human intervention is needed to deliver part of the sediment transported from the river to the delta plain. This could be achieved by means of using the two irrigation canals that are diverting part of the water and sediment transported by the river through the delta plain. This system was designed at the end of the 19th century and is widely used by local farmers to transform the natural wetlands into rice fields by increasing land elevation with fertile sediments. This practice continued until the construction of the Mequinensa and Riba-roja dams7, 9. When considering the whole delta (excluding beaches) it has been estimated that about 1.3 x 106 tonnes/yr of sediment would be needed to compensate relative sea-level rise; this is 10 times more the present sediment load but 20 times less the pre-dam sediment load8. It has been estimated that recovering about 20% of the original load (5-6 x 106 tonnes/yr) and supplying about 20% of this load to the delta plain, a vertical accretion of 1 cm/yr could be achieved11.Vertical accretion can be also increased by means of stimulating wetland plant productivity and organic soil formation12, 13, and this alternative is being evaluated as a complementary measure to compensate relative sea level rise.

Overall, the sustainability of the lower Ebro River and its delta could only be guaranteed by the implementation of a new reservoir management concept with the allocation of an appropriate liquid and solid flow regime. The determination of this flow regime requires taking into account a number of processes essential for the system’s functioning and specific requirements for sediment transport (i.e. pulses) in order to avoid the loss of geomorphic functionality of the river and of the delta. However, the SedMa plan is a non-mandatory document and it has to be approved by the Confederación Hidrográfica del Ebro (CHE) (Ebro River Basin Authority) who has the full competences and the legal responsibility of the water and sediment management of the whole Ebro basin. Meanwhile, discharges released from reservoirs are designed as a function of hydropower production and water demand (i.e. irrigation cycle), without taking into account the hydromorphological and ecological needs of the river and delta. Furthermore, alternatives to the SedMa plan have not been yet evaluated.

References:

Morris GL, Fan J (1998): Reservoir sedimentation handbook. McGraw-Hill, New York Guillen J, Palanques A (1992) ‘Sediment dynamics and hydrodynamics in the lower course of a river highly regulated by dams: The Ebro River.’ Sedimentology 39, 567-579. Tena A., Batalla R.J., and Vericat D. (2012) ‘Reach-scale suspended sediment balance downstream from dams in a large Mediterranean river.’ Hydrological Sciences Journal, Vol. 57(5), pp. 831-849. Ibáñez C., Alcaraz C., Caiola N., Rovira A., Trobajo R., Alonso M., Duran C., Jiménez P.J., Munné A., and Prat N. (2012) ‘Regime shift from phytoplankton to macrophyte dominance in a large river: top-down versus bottom-up effects.’ Science of the Total Environment, Vol. 416, pp. 314-322. Jiménez JA, Sánchez-Arcilla A (1993) ‘Medium-term coastal response at the Ebro Delta, Spain.’ Marine Geology 114, 105-118 Ibáñez C., Sharpe P.J., Day J.W., Day J.N., and Prat N. (2010) ‘Vertical Accretion and Relative Sea Level Rise in the Ebro Delta Wetlands (Catalonia, Spain).’ Wetlands, Vol. 30, pp. 979-988. Ibàñez C, Canicio A, Day JW, Curcó A (1997) ‘Morphologic development, relative sea level rise and sustainable management of water and sediment in the Ebre Delta, Spain.’ J Coastal Conservation 3, 191-202 Rovira A. and Ibáñez C. (2007) ‘Sediment Management Options for the Lower Ebro River and its Delta.’ Journal of Soils and Sediments, Vol. 7(5), pp. 285-295. Gorría H (1877) ‘Desecación de las marismas y terrenos pantanosos denominados de los alfaques.’ Technical report. Ministerio de Agricultura, Madrid Martín-Vide JP, Mazza de Almeida GA, Helmbrecht J, Ferrer C, Rojas Lara DL (2004) ‘Estudio técnico-económico de alternativas del programa para corregir la subsidencia y regresión del delta del Ebro.’ Technical Report (unpublished) Ibánez C, Day JW, Reyes E (2013) ‘The response of deltas to sea-level rise: natural mechanisms and management options to adapt to high-end scenarios.’ Ecological Engineering, Vol. 65, pp. 122-130. DeLaune RD, Jugsujinda A, Peterson GW, Patrick WH (2003) ‘Impact of Mississippi River freshwater reintroduction on enhancing marsh accretionary processes in a Louisiana estuary.’ Estuarine, Coastal and Shelf Science 58, 653, 662 Mendelssohn IA, Kuhn N (2003) ‘Sediment subsidy: Effects on soil-plant responses in a rapidly submerging coastal salt marsh.’ Ecological Engineering, Vol. 21, 115-128

Albert Rovira: Degree in Geography (University of Barcelona, 1995). Doctor in Geography (University of Barcelona, 2001). Postdoctorate in the University of California at Berkeley (2004-2006). Since 2006 senior researcher of the Institute of Research & Technology of Food & Agriculture (IRTA) in the Aquatic Ecosystems Area. Specialized in sediment transport and hydraulics, river restoration and water management, with more than 20 years of research experience. Carles Ibáñez: Degree in Biological Sciences (University of Barcelona, 1986). Doctor in Biology (University of Barcelona, 1993). Postdoctorate in the Laboratory of Ecology of Fluvial Systems (CNRS), Rhône Delta, France (1993-1994). Since 2005 Chief of the Aquatic Ecosystems Area of the Institute of Research & Technology of Food & Agriculture (IRTA). Member of the Council for Sustainable Management of Water of the Catalan Government. Specialized in ecology of rivers, estuaries and wetlands, with 24 years of research experience mainly carried out in the Ebro Delta, but also in the Mississippi (USA), Rhone (France) and Po (Italy) deltas.

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