Flood protection design
Memories of recent major flooding events in the German state of Thuringia are still fresh, motivating efforts to protect the population from future flooding. One such project is the current modernisation of the Straussfurt flood storage reservoir – in particular, the massive structure that retains the water at one end and controls its release. Since it was commissioned in 1961, the structure has played a crucial flood management role in the Gera and Unstrut catchment areas of the state.
Sustainable construction and greater flood safety: Planning to optimise flood protection and preparing for extreme rainfall events is a high priority in Thuringia. Following a comprehensive assessment of the flood storage reservoir and its structures, Inros Lackner and Tractebel Hydroprojekt GmbH were commissioned to modernise the main floodwater retention structure in Straussfurt, adapting it to current requirements. The aim of the overall project, which includes nine subprojects/elements, is to increase the reservoir’s top water level by 1 metre and the normal floodwater storage volume by 10 million cubic metres – a substantial increase on the current capacity of approximately 18.6 million cubic metres. The project involves the upgrading of existing dykes and the construction of new ones, and also the renewal and improvement of the pumping stations and of the dam structure – the heart of the system – for which Inros Lackner is responsible.
A particular challenge is the inevitable alkali-silica reaction in the concrete of the dam structure, which causes unstoppable swelling and thus damages the structure. This made it necessary to replace a four-section reinforced-concrete substructure equipped with twopart sluice gates, which is integrated into the main dam and regulates the flow of the Unstrut river. Considering the complex hydraulic and structural conditions, a special focus was placed on the design and construction of this key element.
Construction technology and hydraulic design considerations: The construction of the replacement dam structure is very demanding in terms of construction technology and implementation – especially considering that the functionality of the flood protection facility must be maintained throughout the construction period. The new structure is being built at the existing location, with the facility remaining in service to control the flow of the Unstrut river.
For the duration of the construction work, a complex auxiliary structure is required for the excavation pit support. A steel sheet-pile wall is designed for the hydrostatic head of up to 11.6 metres associated with flood design case BHQ1 which applies for the construction period. Diagonal struts, each connected via a pile cap to two vertical micro-piles, transmit the forces into the subsoil. Water inflow into the excavation pit is minimised by extending the protection down to the Keuper level. The construction is being carried out in two phases.
“In the future, we will be placing an even stronger focus on aspects such as sustainability and cost-effectiveness in our planning and design work. On this project, we are especially dedicated to integrating these topics in a comprehensive way. We make use of all the know-how throughout our network of offices to achieve the best possible project success, for the public and the environment.” (Jördis Wünsche, Head of Hydraulic and Maritime Engineering)
3D numerical analysis: A robustly designed stilling basin on the downstream side of the structure dissipates the water’s kinetic energy and calms the flow. In order to ensure a hydraulically efficient and economical solution, 3D hydronumerical calculations covering various stilling basin designs and load cases were carried out using FLOW-3D software. It was concluded that placing two rows of obstruction blocks would enable the structure’s dimensions to be reduced compared to initial expectations and would optimise energy dissipation with a uniform release of the water downstream, even at high flow rates.
“The construction of the new dam structure while the reservoir is still partially filled – and while flood protection still requires to be maintained – is like ‘open heart surgery’. What is unique about this project is that the excavation pit linings required for the two construction phases will continue to serve as integral parts of the dam. This demands detailed collaboration between structural designers, hydraulic engineers, subsoil experts and project planners.” (Stefan Buhr, Project Manager)
Sustainability and life-cycle analysis: In planning and designing the structure, careful attention was paid not only to initial construction costs and ongoing maintenance and durability considerations but also to the use of resources over the entire life of the structure. In order to take a holistic and long-term view of the structure, a life-cycle analysis (LCA) of various preliminary designs was thus carried out. In the case of the wing walls that are integrated into the main dam, consideration of calculated CO2-equivalents enabled the best option from a structural, economic and sustainability point of view to be identified in the preliminary design stage. This demanding hydraulic engineering project involving the construction of a replacement dam structure for the Straussfurt flood storage reservoir is an important step towards improving flood protection in Thuringia.