Although floods as a natural phenomenon have always occurred, cannot be ruled out in the future and are, in principle, beneficial for the “health” of the river and floodplain ecosystems, the flooding caused by them causes millions in damage to the economy and population of Moldova and Ukraine and takes a death toll of dozens of people annually. The local population suffers the most as it is not always timely warned of floods and is not fully trained in how to act in case of floods.
The disastrous floods in 2008 and 2010 in the Dniester Basin reminded once again that the existing flood protection facilities only partially fulfil their functions today. The effectiveness of the modern protection facilities will decline as the water volume of the disastrous floods is expected to rise in the future.
Thus, based on the results of flood simulation modelling in Mohyliv-Podilskyi (which was conducted within the framework of previous Dniester projects), one of the regularly flooded Ukrainian cities in the middle reaches of the Dniester, given a hypothetical 15% increase in the water volume of flood with one percent probability, the maximum water level there would increase by 1.3 meters (13%) and the flooded area would increase by 20% compared to today’s level.
Similar consequences can result from an increase in the water volume of devastating floods in towns and townships in Moldova. In August 2015, the flood risk modelling and mapping in the Dniester delta area was completed for floods with 1% and 0.5% probability (a chance of flood occurring once every one hundred and two hundred years, respectively) and for a disastrous flood taking into account climate change. These works were carried out in cooperation with the Moldova flood protection project financed by the European Investment Bank and prepared by BETA studio, an Italian company.
The Dniester has a highly specific flood regime, featuring up to five flood events annually. During these events, water levels in the river may increase by 3-4 m, and sometimes even more. The Dniester river discharges, recorded during a flood event, are significantly higher than those occurring during a spring high-water period. The significant rises in water levels formed in the Carpathians are attributed to the river channel’s low capacity. The capacity is limited by the steep slopes of the river valley and the narrow floodplain, which is virtually non-existent in some locations.
Img 1.1. Sections of the Dniester basin with increased vulnerability to floods and insufficient capacity to inform the population and local authorities (based on the results of work in small groups)
The main factors determining the flood flow regime of the river valleys in the Pre-Carpathian region, and particularly the Dniester River, are as follows:
- tectonic (endogenous character of orographic and hydrographic pattern, coupled with neotectonic movements);
- climatic (precipitation intensity and river flow pattern);
- geomorphologic (combination of plain surface runoff, channel flow and river valley runoff);
- biotic (proportions of forest cover, meadow vegetation and arable land).
The river network in the upper part of the Dniester Basin is clearly asymmetrical, with the majority of Dniester tributaries flowing from the Carpathian Mountains. In the event of heavy rainfalls or intensive snow melting, spontaneous changes in their water levels can significantly affect the water levels in the Dniester itself. Given that the average river channel slope in this section of the Dniester River is at about 0.5 m/km, with the channel slopes of its tributaries being 2-3 times steeper, it can be concluded that the tributary flows, discharged into the Dniester, cause a backwater effect upstream of the tributary inflow. Such backwater pattern is particularly favourable for valley flooding, especially in the event of simultaneous elevation of water levels in all tributaries or, alternatively, when the water levels start to rise first in the tributaries joining the Dniester further downstream (i.e. in the Svicha and Stryi Rivers).