Case study:Experimental flood in the Ebro river

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Location: 41° 13' 41", 0° 32' 37"
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Project overview

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Status Complete
Project web site
Themes Environmental flows and water resources, Habitat and biodiversity, Hydromorphology
Country Spain
Main contact forename Fernando
Main contact surname Magdaleno
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Project summary

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Flood releases from the dams (a.o.the Flix Dam) were managed by the dam operator (Endesa Generación S.A.) and controlled by the Ebro Basin authority. After two consecutive dry years that led to substantial hydrological and ecological concerns regarding river dynamics and the interaction with human uses, an understanding was reached in 2002 among the hydropower operator, the water authorities, and the scientific community to promote the release of flushing flows. Except for in 2004 and 2005, since then flushing flows have been performed on a regular basis, twice a year (in autumn and spring). The discharged floods have normally required the delivery of about 36 hm3 over 16 h, with peak flows of 900 to 1300 m3 /s (each).

Monitoring surveys and results

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The design and downstream effects of these floods have been monitored and discussed in several studies (Batalla et al., 2006; Batalla and Vericat, 2009). In 2010, the flood hydrograph was slightly modified because of progressive reductions in the efficiencies of the 2002–2009 releases. Recently, Tena et al. (2013) monitored flow and sediment transport, river bathymetry, and geomorphic effects during the flushing flow of May 2008, and Tena et al. (2014) focused on the spatial and temporal dynamics of suspended sediment transport during the 2008–2011 flushing events. Those experiments have made use of different sampling procedures (e.g., sonar backscatter for estimating macrophyte density, or a boardmounted acoustic Doppler current profiler (ADCP) to measure discharge and hydraulics). The sampling reach for many of those works was a 12-km long reach located between the Flix Dam toe and the Ascó gauging station.

Lessons learnt

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The main overall effects on river geomorphology determined by Tena et al. (2013, 2014) highlight the remarkable but irregular effectiveness of flushing flows for macrophyte removal, which reaches 95% in the sub-reaches closer to the dam but decreases substantially downstream. The released flood showed a significant ability to entrain and transport sediment, but limited overall geomorphic impact. Mobilization was primarily limited to fine-medium gravels, whereas bedload rates remained low, likely due to the short duration of the events. The seasonality of the releases was directly related to sediment availability, with higher sediment peaks observed in autumn despite similar flushing peaks. In parallel, different routing velocities were found for discharge and sediments.

Some of these findings can be attributed to the fact that the rate of discharge increase per unit time during flushing flows is an order of magnitude higher than during natural events (Batalla and Vericat, 2009). Therefore, flushing flows show greater transport capacity than the natural floods, despite having a lower magnitude and shorter duration. The authors also concluded that the Lower Ebro River shows evidence of being under geomorphic adjustment 40 years after dam construction.

Most importantly, flushing flows in the Lower Ebro River have been shown to be compatible with hydropower operation. Gómez et al. (2014) calculated and compared the cost of the reduced power generation due to the release of flushing floods with the observed willingness to pay for river restoration programmes. They concluded that the provision of artificial floods had a cost equivalent to a small fraction of the energy delivered to the market and overall annual revenue (0.17% for the two annual flushing floods).


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Catchment and subcatchment



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Project background

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Monitoring



Reasons for river restoration

Mitigation of a pressure Hydropower, Drinking water storage and supply
Hydromorphology Continuity of sediment transport, Structure & condition of riparian/lake shore zones
Biology
Physico-chemical
Other reasons for the project After 2 consecutive dry years there was substantial hydrological and ecological concerns regarding river dynamics and the interaction with human uses.


Measures

Structural measures
Bank/bed modifications
Floodplain / River corridor
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Non-structural measures
Management interventions Experimental flood
Social measures (incl. engagement)
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Monitoring

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Monitoring documents



Additional documents and videos


Additional links and references

Link Description
http://www.sciencedirect.com/science/article/pii/S1462901117301545 All information on this page is copied from this article written by Fernando Magdaleno.

Supplementary Information

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References

Batalla, R.J., Vericat, D., 2009. Hydrological and sediment transport dynamics of flushing flows: implications for river management in large Mediterranean Rivers. River Res. Appl. 25 (3), 297–314.

Batalla, R.J., Vericat, D., Palau, A., 2006. Sediment transport during a flushing flow in the lower Ebro River. In: In: Rowan, J.S., Duck, R.W., Werritty, A. (Eds.), Sediment Dynamics and the Hydromorphology of Fluvial Systems 306. IAHS Publication, Wallingford, UK, pp. 37–44.

Tena, A., Książek, L., Vericat, D., Batalla, R.J., 2013. Assessing the geomorphic effects of a flushing flow in a large regulated river. River Res. Appl. 29 (7), 876–890.

Tena, A., Vericat, D., Batalla, R.J., 2014. Suspended sediment dynamics during flushing flows in a large impounded river (the lower River Ebro). J. Soils Sediments 14 (12), 2057–2069.

Gómez, C.M., Pérez-Blanco, C.D., Batalla, R.J., 2014. Tradeoffs in river restoration: flushing flows vs. hydropower generation in the Lower Ebro River, Spain. J. Hydrol. 518, 130–139.

For more references, please check the link to the article above.