Case study:The San Marcos weir removal

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Location: 42° 35' 55.41" N, 5° 34' 1.55" W

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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	Francisco Martínez
Main contact surname	Capel
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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Image gallery

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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.

Catchment and subcatchment Select a catchment/subcatchment Site Edit 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) 2323 m³/s <br />23,000 l/s <br /> Average channel gradient category Average channel gradient Average unit stream power (W/m2) Project background Edit project background Reach length directly affected (m) Project started Works started Works completed Project completed Total cost category 100 - 500 k€ Total cost (k€) 424424 k€ <br />424,000 € <br /> 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 Edit reasons for 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 Edit Measures Structural measures Bank/bed modifications Floodplain / River corridor Planform / Channel pattern Other Non-structural measures Management interventions Social measures (incl. engagement) Other Monitoring Hydromorphological quality elements Edit Hydromorphological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Biological quality elements Edit biological quality elements Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Physico-chemical quality elements Edit 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 Edit Other responses Element When monitored Type of monitoring Control site used Result Before measures After measures Qualitative Quantitative Monitoring documents Upload monitoring documents Additional documents and videos Upload additional documents Additional links and references Edit links and references Link Description Supplementary Information Edit Supplementary Information 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.