water Article The Catastrophe of the Niedów Dam—The Causes of the Dam’s Breach, Its Development, and Consequences Stanislaw Kostecki 1 and Robert Banasiak 2, *   Citation: Kostecki, S.; Banasiak, R. The Catastrophe of the Niedów Dam—The Causes of the Dam’s Breach, Its Development, and Consequences. Water 2021, 13, 3254. https://doi.org/10.3390/w13223254 Academic Editor: Laurens M. Bouwer Received: 11 October 2021 Accepted: 14 November 2021 Published: 17 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Faculty of Civil Engineering, Wroclaw University of Science and Technology, 50-370 Wroclaw, Poland; stanislaw.kostecki@pwr.edu.pl 2 Institute of Meteorology and Water Management—National Research Institute, 01-673 Warszawa, Poland * Correspondence: robert.banasiak@imgw.pl; Tel.: +48-713-200-230 Abstract: 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 m 3 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. Keywords: the Lusatian Neisse River; the Niedów dam; flood; dam breach; 2D flood routing 1. Introduction The number of dams for storing and supplying water is increasing worldwide due to the growing demand from towns, agriculture, industry, or power generation. Dams also play an important role in reducing the risk of flooding. Apart from the substantial benefits to society provided by dams, there is also an inherent and growing risk of dam failure. This results in flooding that can cause serious material and environmental damage and loss of life. The failure of a dam could have occurred due to technical faults during the design and construction stages, the aging of the structure, miss-operation, and climate change that resulted in the altering of meteorological and hydrological patterns [1–3]. The International Commission on Large Dams (https://www.icoldchile.cl/boletines, accessed on 15 October 2021) [4,5], later referred to as Commission, has reported 176 failures among the 17,406 registered dams in the world. According to the Commission, the failure rate for embankment dams is higher than for concrete dams. It also revealed, in the case of embankment dams, that overtopping failure is the most common cause of failure when compared with other types of failures, such as piping and slope failure. Analysis of the dams’ failure plays a key role in understanding the mechanisms of such disasters [6]. This, in turn, enables more accurate methods of forecasting failures, as well as ways to prevent them, to be developed. These actions are also a great help for administration bodies when preparing flood hazard maps and contingency plans, which allow for a quick and effective response to disasters [7]. However, obtaining detailed data on the course of such an event is difficult, because on the one hand, the activities of the services in a hazardous situation Water 2021, 13, 3254. https://doi.org/10.3390/w13223254 https://www.mdpi.com/journal/water