Impact of maintenance of floodplains of the Vistula River on high water levels on the section from Włocławek to Toruń

Abstract

This article describes the methodology of hydraulic calculations to estimate the water levels in open channels for steady gradually varied flow. The presented method has been used to analyse the water level on the Vistula River from Wloclawek cross-section to Torun cross-section. The HEC-RAS modelling system has been used for parameterization of the river channel and floodplains, as well as for flow simulation. The results obtained have been the basis for assessing the impact of maintenance of floodplains on water level during maximum discharges. DOI: 10.12736/issn.2300-3022.2013308 Introduction The analysed section of the Vistula (Wisla) River, with a length of 59.7 km, is located between Wloclawek cross-section below the dam, at 675.0 km, and the water gauge cross-section in Torun, located at 734.7 km (Fig. 1). The average river channel slope is approx. 0.15 ‰. Cross-sections of the river valley on the examined section are characterised by diverse geometry, where main channel and clearly outlined floodplains may be distinguished in most of the analysed cross-sections. The width of the main channel ranges from a few hundred metres to approx. 1 km, while the extent of floodplains (terrace) reaches up to 7 km. The vast part of the area is covered with forests, meadows, pastures and agricultural land. The flow capacity of the river channel and adjacent floodplains is affected by the cross-sectional geometry, the bottom slope and the type of cover of land that is directly related to flow resistance. The type of material which the main river channel is made of and the type of cover of floodplains, which affects the roughness of the river valley, may be characterised by Manning’s roughness coefficient [4]. According to [2], the average value of that parameter in the main channel is approx. 0.04 in the analysed area. The minimum value of roughness coefficient in the floodplain is 0.023 for the land covered with grass, while the maximum value of 0.12 is characteristic for the land covered with bushes and trees [4]. High values of roughness coefficient, adopted on the basis of the analysis of aerial photographs and site visits, prove the poor maintenance of floodplains, which leads to reduction of the flow capacity in the river channel, thus contributing to an increase in water level at the same flow rate, as compared to a well-maintained channel. In the analysed river section the levees of 2nd class are located on the left bank between 683.3 and 689.8 km and between 708.0 and 718.2 km of the river course, which corresponds to a total length of 22.9 km. The levees on the right bank are situated over Fig. 1. Schematic diagram of the analysed section of the Vistula River D. Gąsiorowski, M. Szydlowski | Acta Energetica 3/16 (2013) | 112–116