The doctoral thesis discusses the different theoretical and practical aspects of flood waves modeling and propagation of coarse incoherent materials (debris flow modeling) and fine-grained cohesive material (mud flow modeling) as two potentially significant natural phenomena in the middle and lower part of the Rječina River. This doctoral thesis was based on the collection of meteorological, hydrological and geotechnical data, and consequently the development of physical and mathematical models for different movement scenarios of flood water waves, debris and mudflow propagation on the Rječina River, from the position of Valići accumulation to the urban part of the Rijeka City, including possible result scenarios of these activities. Research methods are based on the input data collected from terrain measurements in the defined area. The database contains basic information on the current state of Grohovo landslide area and Rječina River, including their nature, size, location and history, all available information on previous research (photographs, historical documents, reports, feedback analysis of areas, etc.), as well as real-time information from the built-measuring devices (meteorological stations, Mini and Baro Diver instruments, ombrografs, limnigrafs, satellite, radar, ADCP flow meter, etc.) to track changes at the location. Research methods include surface and underground research (measuring of groundwater levels in the area of the Grohovo landslide), 2D numerical modeling of flood waves and debris flows propagation (application of Smoothed Particle Hydrodinamics method (SPH) and SOLFEC (Solver for Finite Element Computation) simulation program based on Non Smooth Contact Dynamics - NSCD), and GIS (Geographic Information System) technology. Based on the input of hydrological and geological data, numerical models for debris flow and mudflow propagation were created. In developing numerical models, most commonly used empirical methods of the erosion law (Egashira, Hungr, and Voellmy erosion law) have been applied, as well as the implementation of the new erosion law. Based on that were given the guidelines for hazard and risk analysis of the flash-flood and debris flow effects on the investigated area. With the help of the debris and mudflow simulation display we quantified the volume of deposited materials, the flow velocity, the depth of the deposited materials, as well as run-out distance of debris and mudflow on the analyzed area for several possible scenarios of catastrophic phenomena. Some of the scenarios processed and analyzed in this doctoral thesis are related to the consequences of the Valići dam collapse (due to input flysch sliding mass into the accumulation), the formation of a tidal wave and its propagation to the urban part of the Rijeka City, as well as the possibility of forming the debris flow propagation in the middle part of the Rječina River due to mixing of water from the accumulation and initiating the sliding mass on the unstable flysch slopes. This analysis enables the quantification of individual input parameters that initiate the formation of debris flow and mudflow. The relationships of defined parameters allow the establishment of correlations between geomorphological and hydrogeological conditions, as well as the specifics of terrain conditions with the parameters of the soil characteristic for the formation of debris flow and mudflow. The definition of critical geomorphological and hydrogeological parameters of soil that encourage the emergence of debris flow and mudflows in flysch area enabled the assessment of hazards and hazard mitigation measures for the investigated area. In this regard a rough risk analysis of possible consequences on the middle and lower part of the Rječina River was conducted, whereby the main elements of risk were defined. Additionally, the causes of possible occurrence of natural hazards were described, as well as guidelines for their rehabilitation and preservation of the possible hazard consequences in the future.