The aim of the study was to determine an effective variant of operation of the Golina polder to reduce flood hazard in the Warta River valley. We implemented a trial-and-error method for the development of computational variants. Our approach was based on staged analyses of alternatives, which took into account different locations and parameters of the inlet and outlet-controlling structures. Various control scenarios for flooding and draining the polder were also considered. A hybrid hydrodynamic model consisting of a 1D part for the main river area and a 2D part for the polder area was used for calculations. The model was built based on a digital elevation model (1 m resolution) and channel sections of the Warta River. The calibration was based on data collected during the flood in 2010 from the water gauge Slawsk (located directly at the Warta River) and water surface elevation measurements carried out in the polder area. Alternatives in subsequent stages were determined based on the results of previous stages, as well as experiences from the 2010 flood and consultations with the Regional Water Management Board in Poznan. The performance of the alternatives was evaluated according to six criteria that described the effectiveness of the polder operation in terms of its practical use and effectiveness in reducing flood hazard. The results showed that our approach made it possible to identify an effective variant of polder operation. Additional calculations were also performed to determine the magnitude of flows for which flooding of the polder should be considered to reduce downstream flooding.

The assessment and mapping of riverine flood hazards and risks is recognized by many countries as an important tool for characterizing floods and developing flood management plans. Often, however, these management plans give attention primarily to open-water floods, with ice-jam floods being mostly an afterthought once these plans have been drafted. In some Nordic regions, ice-jam floods can be more severe than open-water floods, with floodwater levels of ice-jam floods often exceeding levels of open-water floods for the same return periods. Hence, it is imperative that flooding due to river ice processes be considered in flood management plans. This also pertains to European member states who are required to submit renewed flood management plans every six years to the European governance authorities. On 19 and 20 October 2022, a workshop entitled “Assessing and mitigating ice-jam flood hazard and risk” was hosted in Poznań, Poland to explore the necessity of incorporating ice-jam flood hazard and risk assessments in the European Union’s Flood Directive. The presentations given at the workshop provided a good overview of flood risk assessments in Europe and how they may change due to the climate in the future. Perspectives from Norway, Sweden, Finland, Germany, and Poland were presented. Mitigation measures, particularly the artificial breakage of river ice covers and ice-jam flood forecasting, were shared. Advances in ice processes were also presented at the workshop, including state-of-the-art developments in tracking ice-floe velocities using particle tracking velocimetry, characterizing hanging dam ice, designing new ice-control structures, detecting, and monitoring river ice covers using composite imagery from both radar and optical satellite sensors, and calculating ice-jam flood hazards using a stochastic modelling approach.

The paper includes data derived from airborne laser scanning (LiDAR) to develop a 2-D hydrodynamic model of water flow in the middle reach of the Warta River between Nowe Miasto nad Wartą and Solec. Particular attention was paid to problems associated with the conversion of a digital terrain model (DTM) for the purpose of hydraulic modelling. LIDAR data were also used in the development process of a numerical roughness model by means of the so-called vegetation stratification. The results of model calculations in the form of maximum water surface elevations of the river were compared with water surface elevations derived from levelling carried out in the field. On this basis the verification of the model results was performed. The possibilities of using data from airborne laser scanning to develop a model and assess roughness coefficients of the floodplains were also examined.

In the paper usefulness of one dimensional model for restoration purposes was investigated. The sediment model of downstream part of the Flinta River (Western Poland) was prepared using HEC-RAS 5.0.0. The set of deflectors, which was implemented as constrained cross-sections, was analyzed. Computations were carried out for three formulas for intensity of sediment transport: Yang, Ackers-White and MPM-Toffaleti. Two geometry variants were tested: without modifications of the channel and with deflectors. Moreover, within geometry with structures, two variants of markers for mobile beds were distinguished, where at the location of structures erosion was allowed (variant A) or blocked (variant B). The obtained results showed different responses of each formula in relation to inflow hydrographs, and impact of deflectors on sediment transport processes. At the bank opposite to structures obtained erosion while, in the location of structures, there was erosion (variant A) or deposition (variant B) of sediment. Additionally, analysis of maximum values of shear stresses indicated high peak values for variant B. Basically, results were consistent with other studies with deflectors however, the size and length of the scour, which are crucial during designing these structures, were not obtained. For this reason, one dimensional model was considered as insufficient to planning restoration measures, as simplistic information about impact of structures was not enough. The main practical output of our research is showing the limits in application ID models to analyze simple river management hydraulic structures.

The presented research analyses the impact of deflectors on sediment transport processes. The basis for analysis is a hydrodynamic model of downstream part of the Flinta river. The geometry reproduction was performed using spatial data: digital elevation model (DEM) and cross-sections of considered reach. The computations were calculated in HEC-RAS 5.0.1, a common software used to calculate water surface profiles and sediment transport. In the research, two calculation variants were analysed: (1) with initial geometry of channel, and (2) with geometry after introducing deflectors. In order to take into account uncertainty, five scenarios of 10-year flow hydrographs were tested. To calculate the intensity of sediment transport, the Engelund-Hansen formula was used. The results suggest a possible initiation of local scours near the structures.