Case study:The San Marcos weir removal

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Location: 42° 35' 55", -5° 34' 2"
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Project overview

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Status Complete
Project web site
Themes Environmental flows and water resources, Flood risk management, Hydropower, Urban
Country Spain
Main contact forename Rincón Sanz
Main contact surname Gonzalo
Main contact user ID
Contact organisation
Contact organisation web site
Partner organisations
Parent multi-site project
This is a parent project
encompassing the following
projects
No
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Project summary

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The San Marcos weir is located in the city of León. The weir has been operating during the last 50 years, accumulating large amounts of upstream sediments and generating a strong incision downstream of the dam, which requires a specific management.

Works: partial demolition of the weir, decreasing its height by about 2.5 meters. Its complete demolition was not viable because it is a danger for the bridge sited upstream the weir, which has a high historical and cultural value. A fish pass was also installed to facilitate the movement of the fish species in the river. The process of demolition has to take care of the bridge placed upstream, so that the cost of the works became more expensive.

Benefits:

- Decreased risk of flooding in the city of León. With the adopted solution, the danger of flooding decreases considerably, regarding the previous situation in which the river would overflow for a return period of 131 years, with the projected solution, there is no risk of overflow for a 494 m3 /s flow, corresponding to the period of return of 500 years.

- The materials removed from the weir were used to fill the pool produced downstream the weir (approximately 6 meters deep), diminishing the erosion of the river bed (600m3 removed from which they used 500m3 to fill the pool).

- The longitudinal connectivity was enhanced by the construction of a fish pass, improving the potential habitat for native fish species such as the Brown trout (Salmo trutta) and the Northern Iberian spined-loach (Cobitis calderoni).

- Local populations are usually against the use of public funding to remove dams. This removal action supposes a measure of awareness for the population of the city of León.

- Increasing awareness of local people about the natural conditions of a river.

Monitoring surveys and results

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This action is very significant because it has been carried out in the urban section of a medium-sized city, which means that the local population become accustomed to the fact that these actions are carried out and are beneficial for both the river and the inhabitants of the city.

The movement of the sediments has provided a more natural image to the river and the native riparian vegetation is regenerating, creating a more natural fluvial landscape.

Furthermore, the extraordinary flood of 25 years of return period occurred a year after the demolition would have caused a flood in part of the city if this elimination had not been done. It is therefore found that the removal of this weir has been beneficial to the local population.

On the contrary, the longitudinal connectivity will need more time to recover due to the great number of obstacles located in that section of the river.

Lessons learnt

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With this action, an extraordinary flood was prevented (the year after the dam removal) that would have flooded part of the city of León.

The realization of this removal in an urban area is a great success since it increases the awareness of the society because they will demand this type of actions when seeing the results.

The location of the weir near a bridge forced to use more costly removal techniques, which increased the final price of the project.


Image gallery


Image of the Bernesga River before (left side) and after (right side) the works of removal. (CIREF).
Images of the area during and after demolition of the weir. Photo credits: José Ignacio Santillán.
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Catchment and subcatchment



Site

Name
WFD water body codes
WFD (national) typology
WFD water body name
Pre-project morphology
Reference morphology
Desired post project morphology
Heavily modified water body No
National/international site designation
Local/regional site designations
Protected species present No
Invasive species present No
Species of interest
Dominant hydrology
Dominant substrate
River corridor land use
Average bankfull channel width category
Average bankfull channel width (m)
Average bankfull channel depth category
Average bankfull channel depth (m)
Mean discharge category 10 - 100 m³/s
Mean annual discharge (m3/s) 23
23 m³/s
23,000 l/s
Average channel gradient category
Average channel gradient
Average unit stream power (W/m2)


Project background

Reach length directly affected (m)
Project started 2013/07/01
Works started
Works completed
Project completed 2013/10/01
Total cost category 100 - 500 k€
Total cost (k€) 424
424 k€
424,000 €
Benefit to cost ratio
Funding sources

Cost for project phases

Phase cost category cost exact (k€) Lead organisation Contact forename Contact surname
Investigation and design
Stakeholder engagement and communication
Works and works supervision
Post-project management and maintenance
Monitoring



Reasons for river restoration

Mitigation of a pressure Flood risk management, Barriers to fish migration, Riparian development
Hydromorphology Continuity of sediment transport, Continuity for organisms, Width & depth variation
Biology
Physico-chemical
Other reasons for the project Continuous maintenance costs (dredging, removal of river vegetation...).


Measures

Structural measures
Bank/bed modifications Creation of fish pass, Partial weir removal
Floodplain / River corridor
Planform / Channel pattern
Other
Non-structural measures
Management interventions Materials removed from the weir were used to fill the pool produced downstream the weir, diminishing the erosion of the river bed
Social measures (incl. engagement)
Other


Monitoring

Hydromorphological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Biological quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Physico-chemical quality elements

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative

Any other monitoring, e.g. social, economic

Element When monitored Type of monitoring Control site used Result
Before measures After measures Qualitative Quantitative


Monitoring documents



Additional documents and videos


Additional links and references

Link Description
http://europe.wetlands.org/publications/river-fragmentation-analysis-spain/ A link on the Wetlands International website to a document that describes the project.
http://www.ecohidraulica.com/ All information on this page is copied from the document mentioned above which is written by Ecohidráulica, S.L.

Supplementary Information

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References

Confederación Hidrográfica del Duero (2013). El Ministerio de Agricultura, Alimentación y Medio Ambiente reducirá parcialmente el azud existente aguas abajo del Puente de San Marcos sobre en el río Bernesga (León). Press release from 30/ 07/2013.

Confederación Hidrográfica del Duero (2013). El Ministerio de Agricultura, Alimentación y Medio Ambiente reducirá parcialmente el azud existente aguas abajo del Puente de San Marcos sobre en el río Bernesga (León). Press release from 28/10/2013.

Headwaters Economics (2016). Dam removal: case studies on the Fiscal, Economic, Social and Environmental benefits of dam removal.

Industrial Economics Inc. (2015). Economic & Community Benefits from Stream Barrier Removal Projects in Massachusetts (MA). Massachusetts Department of Fish and Game, Division of Ecological Restoration.

Santillán Ibánez, J.I. (2015). Eliminación del azud urbano de San Marcos (León) en el río Bernesga. Buena práctica para recuperar la continuidad del curso fluvial. II Actas del Congreso Ibérico de Restauración Fluvial (Restauraríos). Pamplona.