# Mathematical model of the junction between Rječina and Dead canal with flooding of the lower part of Rječina

The important characteristic of river Rječina is sediment transport and deposition. Throughout history sediment changed the shape of the lower part of the Rječina riverbed as the sediment accumulated on the left bank so the river changed its course towards the west. After several floods caused great damage for the city during the 19th century, it was decided that an artificial estuary would be made. The old river bed, now called Mrtvi kanal (“dead canal”), was kept as port for small boats. Unfortunately, due to the weak inflow of fresh water, Mrtvi kanal emits unpleasant odors, which is very inconvenient due to the fact that it is located in the center of the city.

The main purpose of this project is to estimate the effects of the connecting channel between the river Rječina and the channel Mrtvi kanal. It is necessary to analyze different variants of connecting channel which should provide Mrtvi kanal with fresh water, and well as take up part of the Rječina discharge, which should in turn reduce flooding danger in lower part of the river Rječina. At the same time, the junction should not jeopardize the current and future use of Mrtvi kanal as a port for small boats.

The first step in this project was gathering and analysis of the existing geodetic and hydrological data for the river Rječina and the mareography of the Rijeka bay. New geodetic measurements were conducted to complete the geometric data needed for flooding simulations and to compare the old geodetic data with the new data, so as to obtain an assessment of the sedimentation dynamics.

One of the two main parts of this project were 2D flooding simulations, as a part of the extension of study “Mathematical Model for Flooding in Lower Part of River Rjecina” (2000). In this study a numerical mesh of 50 000 nodes was used, while the new simulations use over 1 million nodes, as well as updated geodetic and hydrological information.

2D simulation was modelled as an open channel flow, where model included city areas that are exposed to flooding risk. Simulations were done for a number of different flows to determine flooding risk factors for different parts of the city. 1D open channel flow simulations were used as a verification of 2D simulation results and for flow simulations which include geometry of the bridges. In these 1D simulations it was determined for which discharge the bridges are starting to flood.

The other main part of the project were 2D and 3D simulations of the flow in the Rječina - junction - Mrtvi kanal system. The 3D simulations were performed by the use of turbulent Navier-Stokes one-phase model, where the water surface was modeled as wall with the water elevations taken from 2D simulations. 3D simulations provide a more detailed flow pattern which has shown to include areas of strong recirculation zones. Flow simulations were performed for four different variants of the connecting channel (“leaning junction”, “perpendicular junction”, “round junction” and “Y-junction”) and a complex canal system, as proposed by a winning entry of the competition for the urban-architectural concept design for the development of the Rječina estuary area.

Modern visualization with 3D display of flooded areas together with video was produced, together with the interactive visualization model which enables virtual walkthroughs in Rijeka with incorporated junction variants and computed water surface of various simulation scenarios.

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