Mathematical Model for Flooding in Lower Part of River Rjecina

The river Rjecina is 18.3km long, with an orographic catchment area of the size of 218km2 and it is located in the western part of Croatia. In its lower part it flows through the city of Rijeka and into the Adriatic Sea. The downtown and the most economically significant industries are situated next to the watercourse of river Rjecina and its estuary. In the history this river endangered the city many times by its floods. Traditional principles of riverbed regulation and protective flood-dam construction are only partially acceptable as flood prevention measures because of the urbanized environment. For the purpose of solving these problems, Company "Croatian waters", which is the state enterprise in charge of water management and implementation of flood protection and control measures, engaged our team in the activities toward flood prediction and key factor analysis of flood occurrence and intensity.

In the first phase we used city maps to generate triangulation with geodetic data for the entire computational domain, i.e., for all the city areas that can be flooded. Also, we analyzed very extensive hydrological data (see measurements in some characteristic months like May and December) that were available to us, to find what are the typical water wave propagations in river Rjecina. Furthermore, we used these measurements for the calibration of the Manning’s friction factor and obtained very satisfying accordance between computation and measurements (see comparison for lower, medium and higher values of discharges).

Then, in the second phase our team applied the developed mathematical model for the key factor analysis. First group of simulation was about the sea level influence on water wave propagation and flooding. These simulation demonstrated that sea influence diminished with the transition from lower to higher water discharge and did not particularly change the distribution of the flooded areas in the downtown of Rijeka (see details around Skoljic and Delta).

The second group of simulations was concerned with the influence of bridges over Rjecina. There are actually seven bridges in the downtown of Rijeka and we virtually removed these bridges in the mathematical model to establish how much they increase the risk of flooding. The result was that they were a minor flood risk factor.

In the two additional groups of simulations we investigated effects of a potential riverbed regulation and of a hypothetical sediment settlement. In particular, we showed that riverbed sediment caused a significant modification in water level distribution even for smaller discharges and much stronger floodings (see details at Autotrolej, Korzo and Delta). Finally, we established minimal flooding discharges for the existing state of the riverbed, for the potentially regulated riverbed and for the riverbed with that hypothetical sediment settlement. These results furthermore confirmed our conclusions about key flood risk factors.

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