Evaluation of the functionality of seepage prevention structures of dam bodies after a long period of operation – a case study of Otmuchów and Nysa reservoirs

The article reviews the technology of construction and functioning of watertight elements of hydrotechnical embankments. A case study of two reservoirs located in the middle course of the Nysa Kłodzka River is described: (1) Otmuchów reservoir functioning since 1933, (2) Nysa reservoir functioning since 1971. The effectiveness of the applied impermeable screens, i.e. a clay screen of the Otmuchów reservoir and a concrete screen of the Nysa reservoir, was assessed and compared. Data from piezometers located at the height of the dam crest along its entire profile were analysed for measurements taken in the years 2020–2022. Indicators such as the distribution and depth of the water level in the dam body and changes in pressure in piezometers under the influence of water level fluctuations in the reservoir were considered. In addition, the scale and frequency of repair and maintenance activities carried out on both facilities were analysed, especially in aspects of the dam body sealing and the need to repair the upstream slope structures, which are directly exposed to filtration and wave action. It was found that, despite the long period of operation of both facilities, they maintain proper water tightness, and the depth of the water level in the embankments reacts only slightly to fluctuations in the amount (height) of stored water. This correlation is more evident closer to the middle part of the dam of the Nysa reservoir, where an abrupt increase in damming height caused a vertical movement of the water table in the embankment, but with a delay of about a week. It was pointed out that both sealing technologies adopted were associated with the need for numerous revitalisation measures, mainly due to dynamic degradation of the stone paving in the area of fluctuations in the level of dammed water in the case of the Otmuchów reservoir and loss of watertightness at expansion joints of concrete slabs protecting the upstream slope of the Nysa reservoir dam.