Mathematical Model and Simulations of Water Wave Propagation in the Southern Part of River Gacka

In the region of Lika (Croatia) rivers Lika and Gacka intersect at the "Sumecica" junction. A dam separates that junction from the downstream part of the natural watercourse of Gacka, particularly from the "Gornja Svica" Lake. The waters collected at that junction flow through the "Sumecica - Gornja Sivca" artificial open channel towards the Hydroelectric Power Plant “Senj”. If there is more water then it is needed in the power plant, the dam is raised and part of the water flows through the natural watercourse. Exactly this is a possible cause of flooding in the town of Otocac. In fact, the coasts of the natural watercourse of Gacka between “Sumecica” junction and “Gornja Svica” lake are very low. So, the houses built in the south Gacka riverbasin get flooded even in the case of smaller water wave propagations.

The task of our team was to create a complete mathematical model for the south Gacka, simulate water wave propagations in the present state of the watercourse and investigate possible solutions of the flooding problem. We built the model within our software using extensive geodetic data (see for example triangulation for the "Sumecica" junction and triangulation the "Gornja Svica" lake) and verified it by comparison with measurements.

First we investigated a possible regulation of the natural watercourse of Gacka downstream of the “Gornja Svica” lake. Our simulations showed the effects of such a regulation would be minor. So, such a regulation was rejected as a potential solution. Also, we helped in the determination of the flooded areas in the riverbasin of Gacka (upstream, middle and downstream sections) if a dam would have been built on the exit of the “Gornja Svica” lake.

Then our team proposed the idea of a bypass channel between "Sumecica - Gornja Svica" open channel and "Gornja Svica" lake. We modified accordingly the mathematical model of the system of open channels and natural watercourses. New simulations showed that even in the worst possible scenario: a hundred-year-return-period water wave propagation and power plant not working, there would be no flooding. Thus, a well-dimensioned bypass channel is the complete solution for the flooding problem in the south Gacka riverbasin.

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