Martyniuk M. Floods on the rivers of the Vistula basin within Ukraine, the method of determining their characteristics and risk assessment

The dissertation is devoted to research and flood risk assessment during the passage of the maximum runoff of spring and rain floods in the Ukrainian part of the Vistula River basin. In the work, the collection, preparation and analysis of hydrological and meteorological raw data and data of remote sensing of the Earth, used in the calculations of the maximum runoff of spring and rain floods in the studied basin, as well as further hydrological flood modeling of individual areas of the catchment, which have potentially significant flooding risks (APSFR), were performed. The homogeneity and stationarity of data series of the maximum runoff (maximum water discharges and runoff layers) of the spring and rain floods of the rivers of the Vistula River basin were evaluated, the cyclicity of runoff fluctuations was determined, and statistical processing of the initial hydrological data from the beginning of observations to 2015 was performed. Determination of the maximum runoff of the rivers of the Vistula River basin within Ukraine was carried out using the operator model developed by scientists of the Odesa State Environmental University. In order to substantiate the parameters of the regional methodology, the calculation of the characteristics of the maximum runoff of the rare probability of exceeding both spring and rain floods was performed. The results of the calculations are the characteristics of the slope influx (runoff layers of rain and spring floods, the duration of slope influx, and the maximum modules of the slope influx) geographically generalized by the territory of the studied basin using GIS technologies, as well as calculation equations for determining the transformation functions of the maximum runoff of different origin. A well-founded regional method allows determining the characteristics of the maximum runoff of the rare probability of exceeding spring and rain floods in the absence of hydrological observation data in the territory of the studied basin. A comparison of the calculated maximum runoff modules and water dischargres during floods of rare probability of exceedance with the maximum observed and the results of statistical processing showed that the presented method is theoretically and practically justified; the accuracy of the calculations of the characteristics of the maximum runoff of the rare probability of exceedance for spring floods was ± 8.84%, and for rain floods ± 18.3%. The flood risk assessment was carried out for APSFR, which were determined according to the Flood Risk Management Plan in individual territories within the Vistula River Basin District for 2023-2030. The dissertation research analyzed the existing methods and models used in global practice when identifying flood zones. Performed hydrologic modeling of flood zones using HEC-RAS software developed by the Hydrologic Engineering Center of US Army Corps of Engineers. The constructed digital evaluation model, data of measuring works and calculated hydrographs were used as input data for modeling. The digital evaluation model of the study area was developed using QGIS software based on SRTM images obtained from the EarthExplorer resource, as well as data from the HydroSHEDS project conducted by the World Wildlife Fund US to create digital data layers to support large-scale hydroecological research. Estimated hydrographs were built for spring and rain floods according to the methodology given in SNiP 2.01.14-83 and improved by E.D. Hopchenko. and S.V. Avgaytis, while individual parameters are based on the data of the rivers of the studied region of the Vistula River basin. In the work, a comparative analysis of calculated hydrographs with actual ones for characteristic years was carried out, which showed a high convergence of the shape of hydrographs and runoff volumes. Modeling of the inundation area in the territory with the availability of hydrological observation data was carried out on the example of the WGS Rata – Volitsa. A comparison of the inundation area during the passage of the maximum runoff of spring and rain floods was performed according to the calculated and actual hydrograph, which showed a significant dependence of the inundation area on the topography of the area and the nature of the underlying surface. The flood risk assessment was carried out for the Rata - Pristan in the absence of observations of the river runoff. The identified number of objects that fell into the inundation zone, including buildings, residentials, cultural monuments, industrial facilities, agricultural areas, roads and bridges, the magnitude of the risk of flooding is moderate. The presented method can be used for other rivers of the Vistula River basin in order to determine the magnitude of the flood risk of the APSFR and is recommended for the purpose of further implementation of the EU Flood Directive in Ukraine and flood risk management plan within the area Vistula River Basin District for 2023-2030.