Couesnon dam manages water flow

The Couesnon dam controls the flow of water by retaining rising tides and releasing them during low tide, with a maximum discharge of 100 m³/s. This controlled flow helps push sand back toward the open sea, aiding in the restoration of coastal areas around Mont-Saint-Michel.

Restoration of maritime areas

Between 2009 and 2014, nearly 40 hectares of maritime areas were reclaimed within a 1 km radius around Mont-Saint-Michel. This area is nearly six times the size of the island itself, which covers 7 hectares, showing significant environmental recovery.

Cost of the project

The Couesnon dam represents less than one fifth of the total cost of the restoration project. The dam cost 33.90 M€ HT, while the overall project budget was 184.74 M€, highlighting the scale of the investment required.

Historical context of Mont-Saint-Michel

Mont-Saint-Michel has been a UNESCO World Heritage site since 1979, with its bay added in 2007. The island, located about 1 kilometer from the French coast, faced increasing isolation from the sea due to human interventions like polders and dikes built from the 19th century onward.

Environmental modeling and predictions

Modeling by the Sogreah laboratory predicted that the bay would be completely surrounded by grassland by 2042 if no action was taken. The bay receives an additional 3 million cubic meters of sediments annually, leading to a rise in the seabed by 3 cm per year.

Early dam construction

A first dam on the Couesnon was built between 1966 and 1969 to protect the town of Pontorson from sudden water level rises. It had six manually operated flood gates and contributed to the siltation of the river, disrupting its natural flushing during low tide.

Restoration efforts begin

The French government decided in 1995 to restore Mont-Saint-Michel’s maritime environment. A public utility survey was conducted in 2002, and the construction of the new dam took place from 2005 to 2015, with the structure becoming operational in 2009.

Design and function of the new dam

Designed by architect Luc Weizmann, the new dam has a hydraulic width of 78 meters, including two fish locks and eight passes equipped with sector gates. It can store up to 1.2 million cubic meters of water, mixing fresh and seawater based on tidal coefficients.

Operation and sediment management

The dam operates by storing water during high tide and releasing it six hours later, creating a maximum flow of 100 m³/s. This process helps deposit sediments on the sea side and prevents the bay from filling with land, maintaining Mont-Saint-Michel’s unique maritime setting.

Impact on surrounding areas

Between February 2009 and April 2011, maritime areas around Mont-Saint-Michel increased from 125 to 156 hectares. However, land areas decreased by 31 hectares in the same period, illustrating the dynamic nature of the restoration efforts.

Infrastructure changes

The project involved the removal of a road embankment and relocation of a 15-hectare parking lot with 4,150 spaces inland. A 760-meter pedestrian bridge was constructed, allowing access across the dam with views of Mont-Saint-Michel.

Automated control and monitoring

The dam is automated, operating based on tide times, water levels, and turbidity. This ensures efficient sediment management and maintains the ecological balance of the bay, supporting the ongoing restoration of Mont-Saint-Michel’s natural environment.