New spillway at the Esch-sur-Sûre dam in Luxembourg

The 50 m high arch dam of Esch-sur-Sure, built on the River Sure between 1956 and 1957, is located 1.2 km upstream of the town of Esch-sur-Sure in the north west of Luxembourg (district of Diekirch). The catchment area of the dam is 430 km and the reservoir has a total storage capacity of 59 Mm, is mainly used for water supply, power generation, flood control, flow regulation and recreational activities. The dam is not equipped with a surface spillway. Floods are currently discharged using two gated bottom outlets, with a max. capacity of 225 m/s each. This capacity is not sufficient to meet current flood safety requirements. The risk of a gate failure or an inadequate gate operation leading to an artificial flood result in rather unsatisfactory conditions of the current flood management. Consequently, the construction of a new surface spillway was considered necessary. The design of the spillway should: 1. fulfil the dam hydraulic safety requirements according to modern standards and in particular to guarantee the evacuation of the 10,000 year return period flood of 650 m/s assuming that one outlet fails to operate (n-1 condition); 2. improve flood detention of the reservoir; 3. improve flood protection at the town of Esch-sur-Sure. The study of various alternatives finally led to a new spillway design, which consists of building two labyrinth weirs discharging into a spillway tunnel passing under the left dam abutment. The design discharge of the new spillway is 400 m/s. The increase of the total discharge capacity allows the maximum water level of the reservoir to be raised by 1 m in order to increase the flood detention capacity by approximately 3.5 Mm. Therefore, more than 35% of the reservoir capacity will be allocated for flood mitigation during the winter (floods period). The feasibility project will be submitted to the authorities for approval by the end of 2007 and the go ahead is expected in the near future.