The catastrophe of the Niedów dam-the causes of the dam’s breach, its development and consequences

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 m 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 5 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 twodimensional 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 10 concerning the consequences of the dam’s failure to be provided.

. The catchment area of the Witka River The structure and geometry of the earth dam are presented in Figure 4. The maximum height of the embankment with respect to the base ground level was 11.6 m. The body of the dam was made of well compacted sand without a clay core. The slope 90 upstream was of a ratio of 1:3, while the slope downstream had a ratio of 1:2.5. Because the sand had a high permeability coefficient of 2.8×10-3 ms −1 , the upstream slope was shielded with a double layer of concrete slabs with dimensions of 1.5×1.5×0.1 m, which were sealed with a bituminous material. The shield from the upstream water was supported by a vertical reinforced concrete cut-off wall, which was reaching down to the basement rock. The downstream slope was covered with humus and grass. In the lower part of the slope, there was a drainage of mixed gravel and stone. The dam's crest was 5 m wide 95 and served as a road made of concrete slabs with asphalt. The power plant and water outlet sections were connected to the dam with abutments. The total volume of the earth dam was ca. 61,000. m 3 The dam was technically supervised regularly, and was stable and in good condition. A number of maintenance and restoration works were executed in the years from 1998 to 2009, including the repair of the steel and concrete structures, the repair of the upstream slope, and the replacing of the road pavement on the top of the dam in 2009. The dam was operated according 100 4 https: //doi.org/10.5194/nhess-2021-199 Preprint. Discussion started: 9 July 2021 c Author(s) 2021. CC BY 4.0 License.  to its documentation, which consisted of five major items: i) guidelines for the operation of the water intake, ii) guidelines for flood management for the reservoir area, iii) technical instruction of the dam's operation during a flood, iv) a manual for gate control, v) a manual for the operation of the power plant.

Meteorological and hydrological conditions
In the period between 6 and 8 of August, 2010, the upper catchment area of the Lusatian Neisse (in Polish -Nysa Łużycka) was  rainfall occurred in the morning between 8 and 9 a.m., with 15-35 mm of rain falling in an hour -locally reaching even 58 mm at the Heinice station. This rainfall statistic corresponds to one-fourth of the yearly rainfall in this mountainous region.

110
Moreover, the hydrological situation deteriorated due to the precedent wet period in the second half of July (with precipitation above the norm), which led to the saturation of the ground and the acceleration of the subsequent run-off. The consequence of such a meteorological situation was the occurance of catastrophic floods on several rivers, including flash floods on the Witka River, the Miedzianka River, and the Lusatian Neisse River (IMGW et al., 2010).  The flow of the Witka river -its name on the Czech territory is the Smeda river -is monitored at four gauge stations. On the Polish section from km 0.0 (river mouth) to km 8.0, there are two stations: Ostróżno (km 7.98) -upstream from the reservoir,  and Ręczyn (km 1.8) -downstream from the reservoir (Fig. 2). On the 7th of August at the Ostróżno gauge station, the highest water level of the flash flood occurred at 16:40. The Ręczyn gauge station was recording the water level until the time of 15:20, and thus until it was destroyed due to the high release of water from the reservoir before the failure of the dam. During the 45-year period of continuously monitoring flow at Ostróżno gauge station, the flood discharges were less than 70 m 3 s −1 , which is still within the limit of bankful flow. There was only one case of higher flow, which occured in August, 2001 and which was 125 equal to 171 m 3 s −1 . That event also featured a rapid ascent and descent of the wave, which is typical for a flash flood. On the 7th of August, the estimated flood rate was 615 m 3 s −1 , but this estimation is still burdened with significant uncertainty.
On the 7th of August, the estimated flood rate was 615 m3s/1. This estimation was difficult, as the water level substantially exceeded the measured range and due to locally wide floodplain. Between the Ostróżno cross-section and the reservoir, there is an increase of the catchment area from 268 to 331 km 2 , including the Koci Potok stream, which also severely flooded and 130 delivered a significant direct inflow to the reservoir. This stream was not monitored, but based on the field survey after the flood, the peak flow rate was estimated at ca. 70 m 3 s −1 .

Dam failure -wash out mechanism and breach characteristics
The water level in the reservoir was controlled according to the operational manual. The procedure involved the gradual elevation of the gate by 0.2 m in order to maintain the desired water level. When the control of one gate was insufficient, 135 the additional gate was also raised by 0.2 m. In the course of this unpreceded water level rise, the opening of the gate was accelerated. During the catastrophic flood, one of the two turbines was undergoing renovation. After the water level exceeded the edge of the repaired gate, the water flowed into the hydroelectric power plant. As a result, the control room was also flooded, the crew was evacuated to the top of the dam, and the power supply was turned off. The crew still tried to open more gates manually from the dam's crest, but were unsuccessful. The overflow started at 17:00 over the left side of the dam near the 140 bank because the crest was slightly inclined towards it. The water passing over the crest caused the erosion, which first occured around the lamp post's foundations, and then on the slope covered with grass, which is documented in Sup. 1. This process took about half an hour and resulted in the gradual disintegration of the road on the dam's crest. The concrete slabs then lost their support and fell due to the washing away of the sand that constituted the dam's body. Remarkably, the concrete slabs, when losing support, broke in a series like chocolate, and were swept away by the intensified flow. Afterwards, another important 145 moment occurred. As the support of the earth embankment vanished, the left training wall flanking the central concrete dam collapsed due to upstream water pressure. This resulted in a further rapid outbreak. This phase was relatively short but intense, and resulted in a torrential flood wave downstream, which is documented in Figure 7. After the next 80 minutes, the left side of the earth dam was almost completely swept away (Fig. 8).
The overtopping of the right dam began approximately 15 minutes after the left one. The breaching in this case developed 150 in a similar, but less dynamic way. The washout started at the central part of the right dam, and evolved towards the right bank of the valley. As a result of the fall of the left abutment, and a fast lowering of the water level in the reservoir, the right bank washing out decelerated. In addition, the concrete slabs resisted failure and worked as a weir for about 20 minutes. It is difficult to explain the origin of this. Possibly, the slabs jammed, or concrete debris temporally hindered the erosion process. Figure   7 shows the hindered breaching of the right side of the dam. Finally, the concrete slabs were washed away. Nevertheless,   Table   2 for the left and right embankment.

18.47
The breach of the left side of the dam finishes, and has a length of 106 m.

18.56
Water level -209.00 m a.s.l. The right side of the dam continues to breach.

19.25
The breach of the right side of the dam is complete. The dam is washed out along the width of 58 m. The reservoir releases the remaining water.

21.00
The reservoir is empty.
the German side and the city of Zgorzelec on the Polish side (the peak of the wave in Zgorzelec was at 6:40 UTC). It was the   (Froehlich, 2008). Considered "length of time needed for the final trapezoidal breach to form, which takes place after the breach initiation phase". d Times from the overflow to the emptying of the reservoir.

Field observations
A field survey was carried out after the flood in order to collect data on the flood wave passage along the Lusatian Neisse 180 river and its major tributaries (IMGW, 2011). A number of eye witnesses of the flood were interviewed, including several local authority representatives. The maximum water elevation marks were sought and fixed for further geodesy recording at over 50 locations, including upstream and downstream of the Niedów dam. In some cases, clear water lines were found on walls in the form of sediment and residue marks or washed out dirt, but in several locations the high water marks were only approximate, Table 3. Comparison between the measured and calculated water levels (for location see Fig. 12

Conclusions
The literature review and the current case study demonstrate that the dam breach mechanism and its prediction is an extensively 280 studied and complex subject. There is a variety of failure modes and possible approaches to quantitatively assess the dynamics and consequences of the dam break. Thus, the study aimed to reconstruct and explain the catastrophic event of the Niedów dam failure in order to contribute to the current database concerning the developments of dam breaches. As a result, a detailed description of the dam breaching mechanism is provided with the final breach parameters, which can be used for statistical analyses or for the development of a model that is based on the description of the physics of this phenomenon. A particular 285 feature of the Niedów dam was the fact that the homogenous embankments made of sand and gravel had a concrete facing, which acted as an impermeable barrier. This, along with the asphalt road on top, substantially affected the process of the washing out of both sides of the embankments, which was remarkably different and longer than what can be expected in the case for homogenous earth embankments. The slower washing away of the dam resulted in a lower peak value of the wave