Nysa Kłodzka – Operation of the Cascade Reservoirs During a Flood

All reservoirs in the Nysa Kłodzka cascade are so-called “wet reservoirs,” meaning they serve a retention function under normal conditions. In critical situations, changes in water retention levels at these sites are possible. The decisions in this regard are made by Wody Polskie (Polish Waters) and approved as part of the work of the crisis management team. Let’s look at the events that occurred between September 11 and 16, when extreme rainfall and the need to intercept a flood wave took place.

Cascade of reservoirs on the Nysa Kłodzka river

The cascade of reservoirs on the Nysa Kłodzka consists of four reservoirs: Nysa, Otmuchów, Kozielno, and Topola. They perform the following basic functions to varying degrees:

• reduce maximum flow during flood surges;
• increase low flow during periods of atmospheric drought;
• provide water for economic purposes.

How does a retention reservoir work?

The capacity of a retention reservoir is usually divided into reserved capacity, usable capacity, and flood reserve. Effective management of the flood reserve involves capturing the peak of the flood wave while its base is passed through without stopping. This allows for control of the flood wave, provided access to precise meteorological and hydrological forecasts is available. Regulation is done through discharge devices, which modify the characteristics of the wave—reducing the peak, extending its base, and reducing its volume after passing through the reservoir. The reservoir operates according to the water management instructions.

What happened at the reservoirs between september 11 and 16?

Of course, we asked Wody Polskie about this and checked the announcements. Indeed, on Wednesday, September 11, no hydrological warnings were in place in southern Poland. However, based on forecasts, Wody Polskie units—various regional water management authorities—were informed about the expected heavy rainfall, which could exceed 100 mm. They were also advised to check hydrotechnical structures.

On September 12, starting from noon, based on updated weather forecasts, Wody Polskie increased water outflows from three reservoirs: Otmuchów (from 6.0 m³/s to 20 m³/s) and Nysa (from 10 m³/s to 35 m³/s), tripling the discharge. At that time, the water level at the Bardo gauge on the Nysa Kłodzka (Kłodzko Basin) indicated the lower range of average water levels.

Warning levels were exceeded only on Friday, September 13, at 7:00 p.m. During this time, renovation work was ongoing at the Otmuchów reservoir. Despite this, it was successfully used for flood control purposes—all construction equipment was removed, and the reservoir was prepared to handle the incoming flood wave.

In two days—September 12 and 13—the reservoirs in the Nysa Kłodzka cascade had almost 180 million m³ of secured flood reserve. Unfortunately, on September 13, meteorological and hydrological forecasts radically changed. It was still expected that the flow would remain within the river channels.

In reality, from September 12 to 16, the total maximum rainfall in the Kłodzko Basin exceeded 300 mm. According to Wody Polskie, models at that time indicated that floodwaters in the basin would locally exceed a 0.5% flow probability (which means a once-in-200-year event), significantly more than earlier forecasts predicted.

By Saturday, the situation worsened. Due to heavy rainfall and rapid water level rises, the operation of the Nysa Kłodzka cascade reservoirs switched to flood mode. As reported by Wody Polskie, two significant malfunctions occurred—at the Stronie Śląskie and Topola reservoirs (a cascade reservoir). The failure at the latter did not pose a threat to the towns below, as the flood wave was intercepted by Otmuchów and Nysa reservoirs.

Wody Polskie reports that the reservoir control aimed to lower flood levels below the Nysa Kłodzka cascade as much as possible (particularly in the cities of Nysa, Skorogoszcz, and Lewin Brzeski) and to prevent the flood waves from the Kłodzko Basin and the Odra River from overlapping in Wrocław and other towns below the Nysa Kłodzka’s mouth.

Main photo: RZGW Wrocław