Due to extreme rainfall in 2010 in the Lusatian Neisse River catchment area (in Poland), a flood event with a return period of over 100 years occurred, leading to the failure of the Niedów dam. The earth-type dam constructed for cooling the Turów power plant was washed away, resulting in the rapid release of nearly 8.5 million m³ of water and the flooding of the downstream area with substantial material losses. Here we analyze the conditions and causes of the dam’s failure, with special attention given to the mechanism and dynamics of the compound breaching process, in which the dam’s upstream slope reinforcement played a specific and remarkable role. The paper also describes a numerical approach for simulating a combined flood event downstream from the dam with the use of a two-dimensional hydrodynamic model (MIKE21). Considering the specific local conditions, i.e., wide floodplain, meandering character of the main channel, embankment overtopping, and available data set, an iterative solution of the unsteady state problem is proposed. This approach enables realistic flood propagation estimates to be delivered, the dam breach outflow to be reconstructed, and several important answers concerning the consequences of the dam’s failure to be provided. Finally, the paper presents the reconstruction of the dam that is more resilient to extreme hydrological conditions under changing climate.

Large-scale two-dimensional hydrodynamic modeling at high resolution is still rarely performed because of its high computational cost and the lack of topographical data for some areas. Despite this, such modeling has been performed for the Odra River, the second largest river in Poland. This river has a high potential for flooding, which has been severely experienced many times in history, most recently in 1997 and 2010, when floods caused large losses. Since then, many different types of activities have been executed in order to reduce the risk of flooding. The paper presents a 2D modeling concept created during these activities. Given that the river valley is up to several kilometers wide, and consists of many complex topographical features and hydrotechnical facilities, a cascade of 25 2D models in MIKE21 software was developed. It covers a 600 km long section of the Odra River and an area of 5700 km² in total. A regular grid resolution of 4–6 m was used in the modeling. The models were applied for numerous purposes, first for the elaboration of flood hazard and flood risk maps for larger cities, and then for the verification of historic flood data and stage–discharge relations at gauge stations, as well as the verification of design discharges via flood routing. Other important uses were the evaluation of the effectiveness of flood mitigating works, including the feasibility study for the Racibórz reservoir, and the assessment of flood hazard due to embankment failure or ice jamming. Selected applications, as well as practical aspects of the model’s preparation and use, are presented.

Due to extreme rainfall in 2010 in the Lusatian Neisse River catchment area, a flood event with a return period of over 100 years occurred, leading to the failure of the Niedów dam. The earth-type dam was washed away, resulting in the rapid release of nearly 8.5 million m³ of water and the flooding of the downstream area with substantial material losses. The paper analyses the conditions and causes of the dam’s failure, with special attention given to the mechanism and dynamics of the compound breaching process, in which the dam’s upstreeam slope reinforcement played a remarkable role. The paper also describes a numerical approach for simulating a combined flood event along the Lusatian Neisse River with the use of a two-dimensional hydrodynamic model (MIKE21). The flood event occurred downstream from the dam. Considering the specific local conditions and available data set, an iterative solution of the unsteady state problem is proposed. This approach enables realistic flood propagation estimates to be delivered, the dam breach outflow to be reconstructed, and several important answers concerning the consequences of the dam’s failure to be provided.

Aim of the study: Development of new hydrodynamic numerical tools for simulations of the flow in the Odra River valley for the flood hazard assessment and management Material and methods: Use of the MIKE21 software for models building. Geospatial (ArcGis) topographical data processing and analysis for model inputs and outputs. Extensive use of remote sensing data (digital terrain model and orthophoto maps) combined with traditional data sources. Results and conclusions: A cascade of twenty five MIKE21 models was developed to cover a nearly 600 km long section of the Odra River. The models were used in different studies, including: the assessment of flood hazards with regards to various scenarios, the assessment of flooding due to ice jamming and breaks in embankments, the evaluation of the effectiveness of flood mitigating works within the complex Hydrosystem of the city of Wrocław, the verification of historic flood data and stage-discharge relations, and also the verification of design discharges.

Due to extreme rainfalls in 2010 in the Lusatian Neisse river catchment, a flood event with a return period of over 100 years took place leading to the failure of the Niedów dam. The earth type dam was washed away with a result of a rapid release of nearly 8.5 million m³ of water volume and flooding of the downstream area with substantial material losses. The paper analyses the conditions and causes of the dam’s failure, while special attention is given to the mechanism and dynamics of the compound breaching process. Several empirical formulas for the dam breach prediction are tested with regards to their usefulness in assessing breach dimensions and outflow peak discharge. The paper also describes a numerical approach to simulate the flood propagation downstream the dam with the use of a two-dimensional hydrodynamic model. Considering the specific local conditions and available data set, an iterative, reversed solution of the unsteady state problem is made. This approach enabled to deliver realistic flood propagation estimates, verification of the dam breach outflow, and several important answers on the dam failure consequences.