Resilient flood protection in Breese, Germany
Hydraulic-numerical modelling for flood protection
To secure the town of Breese against flooding in the Elbe backwater area, AFRY was commissioned with the planning as well as hydraulic and hydrological calculations for the backwater dikes.
The town of Breese lies approximately three kilometres aerial distance from the Elbe and is situated in the backwater area of the river. The watercourse of the Stepenitz, a lowland river and right-hand tributary of the Elbe, passes near the town at a distance of about 400 meters.
Flood events in the years 2002, 2006, 2011 and 2013 demonstrated a hazardous situation for the town and thus the flood protection of Breese needed to be improved. The entire flood-defence line was not designed for the current design water level of 7.99 metres at the gauge Wittenberge, and there were still unprotected built areas.
The Brandenburg State Office for the Environment (LfU) commissioned AFRY with the technical planning for the enhancement of the flood protection and the preparation of the associated hydraulic-numerical calculation proofs for the backwater dike in connection with the new construction of the bypass road L11.
AFRY was commissioned with:
• the overall planning and construction supervision of the project
• the modelling and calculation verification of seepage and stability of the dikes
• the structural planning and engineering design for the entire scheme up to the plan-approval decision.
Scope of the project
The construction project was divided into four construction lots: three lots comprised dike construction with about 1,480 metres of dike rehabilitation and approx. 1,720 metres of new dike. The fourth lot involved construction of the bypass road L11 on the dike crown. In sections, the road embankment also serves flood-protection functions.
The freeboard of the dikes is 0.86 m and was verified for the lake-like expansion of the Stepenitz in flood conditions in accordance with German Association for Water Management and Agricultural Engineering information sheet 246/1997 (DVWK-Merkblatt 246/1997).
The backwater dikes were planned with a crown width of 4.0 m. As dike sealing, a geosynthetic clay liner (GCL) is used on the waterside slope, protected by a cover layer. The flood-protection dike in construction lot 3b includes a dike height of approx. 2.50 m and a crown width of 8.50 m along the route of the existing district road.
Hydraulic-numerical calculation proofs
In order to ensure the stability of the flood-protection dikes for the town, extensive calculations and structural-stability proofs were required. As the design event, the backwater water level of the Elbe for a 100-year return interval was used (100-year event). Representative sections of the dike lots were modelled in a hydro-numerical calculation model and cross-section-based structural-stability proofs were performed. To simulate dike seepage and subsurface hydraulics under flood conditions the software GGU-2D-SSFLOW was applied in accordance with German ISO standard (DIN) 19712:2013-01. The simulation model solves the Laplace equation numerically for a vertically planar system using Darcy’s flow approach. A finite-element mesh in the form of triangular elements was used.
The knowledge of hydrogeological conditions was the basis for the structural-stability proofs of the earth-pressure systems, e.g., slope failure, sliding, spreading, uplift, hydraulic base failure as well as various material-transport proofs. Various load cases were considered, including operational loading of the dikes as well as partial loss of safety elements. The structural-stability proofs were carried out using the software GGU-STABILITY, GGU-UPLIFT, and, on occasions, empirical equations for all planned dike sections.
Backwater connections for Stepenitz as compensation and replacement measures
Because additional land was required for the dike construction, and these represented an intervention in nature, compensatory and replacement measures had to be implemented. These included the re-connection or restoration of several historic ox‐bow arms in the Stepenitz flood-plain adjacent to the Elbe. In order to ensure flood-neutrality of the old-arm connections, the affected river sections were modelled in a hydraulic-numerical 2D river model and simulated in various execution scenarios. AFRY was also commissioned with these measures.
We are pleased that with this project we were able to make a valuable contribution to the flood protection of the town of Breese and at the same time, to the enhancement of the Stepenitz in accordance with the goals of the EU Water Framework Directive. As a result the area is much better protected against the increasing flood-hazard from the Elbe, and the prospects for sustainable re-settlement of the European salmon in the Stepenitz have been improved.
Read more here about the technical and environmental planning for two oxbow lake connections to the Stepenitz River.
We are pleased that this project has made a valuable contribution to flood protection in the village of Breese while also enhancing the Stepenitz River in line with the objectives of the EU Water Framework Directive. As a result, the area is now significantly better protected against the increasing risk of flooding from the Elbe River, and the prospects for the sustainable reintroduction of European salmon into the Stepenitz have improved.
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