Summary:

The paper summarises research into the geomorphological and hydrological effects of river training and improvement measures taken 45 years ago in the Nida valley. Positive and negative effects were identified, the latter including an increased flood exposure in half of the study area. The study aimed to find a feasible solution that would mitigate the flood risk. The valley is protected as a landscape park and belongs to a network of environmental corridors of national importance. Prior to the training measures, the River Nida, down from the confluence of the Czarna Nida and Biała Nida, ran in an entirely meandering channel with a minimal gradient causing long-term stagnation of the floodwater in the floodplain. The training and melioration measures were aimed at mitigating the flood risk, accelerating flood water drainage and draining part of the valley that had been used solely as meadows and pastures (Fig. 1). Vast wet marsh areas prone to cyclical channel avulsion were predominant along the braided reaches, such as near Umianowice (Fig. 2). As the engineering project started in the upper river course and continued downstream, the channel reaches directly below the newly deepened reaches became shallower. The process was documented by records of minimum annual water levels and bankfull discharges measured at three water gauges (Fig. 3). Measurements at the water gauge at Brzegi in the upper course, the first to undergo the training measures, indicate a deepening trend from ca. 1960 onwards. Downstream the channel initially began to get shallower, but the trend was fully reversed as the engineering project continued. This pattern of the shallowing channel reach travelling downstream continued until the project was halted near the town of Pińczów in the 1990s. Below the town the channel has become much more shallow, as was indicated during an analysis of its morphometry and a comparison of depths with those in oxbow lakes and cut off branches (Fig. 4). The deepened channel upstream is exposed to just minor flood risk, as indicated by short durations of over bankfull water stages (the Brzegi gauge). While the channel near Pińczów, with its alternating periods of deepening and shallowing, experienced a wide variation in the duration of such water level periods during the last 45 years, the flood risk has been consistently increasing during the last ten years (Fig. 5). Downstream of the town the flood risk is greater than anywhere else in the valley and it is bound to continue to grow as the shallowed channel zone will travel further down the river. Further downstream, near Wiślica, the Nida’s particularly long periods of over bankfull water stages are a result of the backwater effect from the confluence with the River Vistula. There is one way to effectively mitigate or at least to halt the increase of the flood risk in the Nida valley between Pińczów and the confluence that would also be acceptable to the authorities of the local Landscape Parks (Zespół Świętokrzyskich i Nadnidziańskich Parków Krajobrazowych). This is to revitalise the largest wet marsh area in the valley near Umianowice. As soon as this area can resume its role as an effective retention zone for flood water and bedload, the Nida channel shallowing process will stop and might possibly be replaced by downcutting, thus shortening the floodplain flooding in the future.