Our stories from ZAGREB and UMAG, CROATIA

The basin of a watercourse called Umaški potok lies in the north-western part of Istria, on the territory of the towns of Umag and Buje, between the Dragonja river basin in the north and the Mirna river basin in the south. The Umaški potok basin has a surface area of app. 29 km2, whereas the hydrological basin is slightly bigger – app. 40 km2. The basin follows the direction in which the terrain spreads (SE-NW) and has two characteristic units in terms of relief and hydrogeology: the downstream part of the valley relief type and the upstream part with hills. The upstream part of the basin is mostly covered with meadows, pastures and forests. Due to dense vegetation, run-off is delayed despite steep terrain slopes. A few small settlements have been built here, mostly on hilltops and elevations. The major share of the valley part is tilled agricultural land which has a high water infiltration capacity, as the result of which surface run-off occurs only after long-lasting heavy rainfall. Urbanization is not marked; this region has mostly small settlements, with the exception of the town of Umag which is a marked tourist location. Surface flows can be followed in the intermittent flow of Umaški potok.

Hydraulic engineering facilities on the Umaški potok watercourse

Flood events occur very rarely, but with strongly felt consequences. The two key characteristics of this pilot area are the karst phenomena, which are to a significant extent present in the major part of Istria and cause a significantly more complex run-off system, and the presence of a complex impact of seawater level fluctuations on the run-off, both on a daily scale and on a scale of several years. In the context of climate change and a trend of rising seawater level, the problem of pluvial floods in areas like this carries additional weight.

Flooded areas (2019 flood)

Flood consequences (2019)

The first and key step in risk analysis is the development of a flow model – from the development of an appropriate terrain model, the selection of the relevant rainy episodes to the comparison with the recorded interventions and definition of the assets exposed to the risk. The preparation was done using GIS tools, and the simulation of a 2D flow was done using the HEC-RAS software.

Digital elevation model of the Umaški potok basin

Scope of the Umaški potok basin pilot area

In order to achieve further improvements, for the coming period it is recommended to develop LiDAR surveys for the entire territory of Croatia and to conduct centralized, systematized and regular collection of data about the existing and planned infrastructure, facilities, (change of) land use, etc. in order to be up-to-date and available to the interested parties. Continued and systematized collection of flood reports from the public and voluntary fire brigades and other competent institutions is also recommended, with clearly defined data to be collected, followed by the creation and management of a database to develop tools for taking action timely.

Concerning model improvement in the field of meteorological data, detailed regionalization of short-lasting rainfall throughout Croatia is recommended. Furthermore, a correlation needs to be established between the measured meteorological indicators, topographic site conditions and the resulting flood events, serving as the basis for model calibration and risk analysis. Once established and verified, the model needs to be maintained, with each new project and the development and implementation of the spatial plans evaluated through the established model.