Catastrophic floods in Moldova in the summer of 2008
By Valeriu Cazac, Director of the State Hydrometeorological Service
Floods are one of the most frequent and most common natural hazards, causing considerable human and material losses in both developed and developing countries. They represent 37 per cent of the total number of natural hazards (Centre for Research on the Epidemiology of Disasters, Université Catholique de Louvain, Belgium).
An analysis of floods over the last century shows an increase in the damage they cause, one of the main reasons being population growth and intensification of economic activity in the zones at risk.
Global warming and the inevitable increase in the use of river valleys will contribute further to the intensification of the frequency and destructive power of floods. A major task therefore is the development of appropriate measures for flood management with a view to minimizing loss of life from flooding and to finding ways of managing floodplains in support of sustainable development.
Moldova is situated in a physical-geographical region prone to floods of the large rivers of Nistru (Dniester) and Prut, the most destructive being in 1941, 1955, 1969, 1974, 1980 and the summer of 2008.
Moldova has 57 natural lakes and about 3 400 reservoirs, including 90 with a volume of more then 1million m3. Small lakes with a surface of 0.2 km2 are more numerous. The largest water reservoirs are: Costesti-Stînca (735 million m3) on the Prut river and Dubasari (277.4 million m3) on the Nistru river.
There are two types of natural floods: those caused by spring snowmelt and rain and those by heavy rainfall during the warm period of the year. The dangerous increase of water level of the Nistru and Prut rivers (within Moldova) is caused by high quantities of water from precipitation or rapid snowmelt in the upper river basins (in Ukraine and Romania).
The main destructive factor of the floods is the water flow characterized by high level and speed, especially when levees or dams fail during high water events. In such cases, a rapid increase of water level occurs and the neighbouring areas are flooded.
In July/August 2008, Ukraine, Romania and Moldova were affected by one of the most destructive floods in 200 years. In the period 22-28 July 2008, in the western part of Ukraine, where the upper courses of the Nistru and Prut rivers are located, the total quantity of precipitation constituted 63-260 mm, that constitutes up to 3 times the monthly norm.
In the northern part of the country and in some central and southern parts, the total amount of precipitation for 10 days constituted 85-185 mm or 440-800 per cent of the decadal norm. Over the rest of the territory, the total amount of precipitations constituted 15-70 mm or 100-420 per cent of the decadal norm.
The largest amount of 225 mm fell in Ocnita, exceeding the decadal norm 10 times, a fact registered for the first time during the whole period of instrumental measurements.
As a result, the high waters from July to August 2008 on the Nistru and Prut rivers were historic events (see the table below). The exceptionally high levels of the Nistru and Prut rivers over the last 40 years, compared to the multiannual mean data, are presented in Figure 1 and 2.
On the river Nistru, near the town of Zalesciki (Ukraine), high waters reached the level of 3.3-7.3 m comparing to the level before the flood, the maximum discharge constituted 5 410 m3/s and, on the hydrological post of Hrusca (Moldova), 3 362 m3/s.
On the Prut river, near the town of Cernauti (Ukraine) high waters reached the level of 7.9 m comparing to the level before the highwater, the maximum discharge constituted 3 890 m3/s. On the hydrological post of Sireuti (Moldova) the maximum discharge constituted 4 560 m3/s.
On the Nistru river, volume of the Novodnestrovsc reservoir reached 3 000 million m3 (3.0 km3). To show how dangerous this water volume is, we should specify that it is equivalent to the actual volume of the reservoir and to 1/3 of the Nistru liman total volume.
The water passing through the Novodnestrovsc reservoir almost exceeded the limits of its possibilities. As a result, the maximum outflow downstream was 3 300-3 400m3/s on 27 July, at 16h00. Two days later, the maximum outflow decreased to 2 955 m3/s and was maintained at this level until 31 July.
A critical situation occurred together with the propagation of the water flow on the Nistru river segment situated in Moldova as the flood protection infrastructure, that was built in 1970, were equipped for a drainage volume of 2 600 m3/s. Overflow in the localities near the Nistru river was therefore unavoidable.
Water discharge on Nistru and Prut rivers
As a result of the discharge that reached the Dubasari reservoir, an exceptional situation occurred. On 27 July, water was released from the reservoir downstream with a discharge of 2 100-2 500 m3/s, increasing to 2 850 m3/s, that constituted a flood risk to the Nistru river segment from the town of Dubasari to its mouth.
On 2 August, the water level in the Dubasari reservoir reached the critical level of 29.11 m, while the maximum acceptable level is 30.0 m.
The flood continued several days after the flood protection infrastructure broke.
The flood of the Prut river developed in the same way as that of the Nistru river, but the flow and water evacuation parameters were different. The high water wave formed in the mountain part of the Prut river basin, and the maximum discharge was 3 890 m3/s (town of Cernauti, 27 July). On the same day, the Costesti-Stînca reservoir started to release water downstream at a rate of 620 m3/s, increasing to 1 400 m3/s on 30 July. The reason for the increase was the sudden rise of the water level in the reservoir up to 98.25 m (the maximum acceptable level being 99.5 m). In order to save the dam, several localities were inundated. The increase of water level in the neighboring areas continued until 5 August.
Exceptional and catastrophic floods occurred and caused serious material damage (preliminary estimates indicate losses of US$ 120 million) to houses, roads, agricultural lands, wells (about 3 000) and water-collection systems in all 22 districts located on the Nistru and Prut floodplains. More than 3 000 animals drowned and 8 473 ha of agricultural land were flooded, including 4 980 ha of pasture. In total, 1 183 houses were flooded and 7 851 people evacuated. The highest number of flooded houses was registered in Briceni (293), Causeni (283), Anenii Noi (213) and Criuleni (145).
Analysis of the hydrometeorological data led to the following conclusions:
During the high waters in July/August 2008, the State Hydrometeorological Service continuously provided the local authorities, relevant institutions, economic sectors and the population with operative and qualitative information on hydrological monitoring, including the evolution of the high waters (hydrological informative notes, forecasts, bulletins and warnings). The land surface periodically affected by floods in Moldova is 20 per cent, i.e. more then 600 000 ha.
Some 10 per cent of the dams and hydrological constructions are in poor condition and represent a serious danger for the neighbouring localities. More then 168 localities with a total surface of 1 300 km2 and about 160 000 residents, of which 625 are rural localities, 31 districts and three towns, are at risk of flooding.
The building of dams on the Nistru and Prut rivers for the regulation of the hydrological regime of the river to reduce the flood risk, for hydropower as well as for irrigation, led to some serious ecological problems. The flow speed and the thermal regime of the water changed. In 1965, before the Novodnestrovsc dam was built, the mean yearly temperature of the Nistru river was 9.9°C and in Duasari lake 10.3°C. After the dam was built (in 1987) the temperature decreased to 8.8?C. In summer months (of the last decades) the water temperature has not risen above 18°C; in 1965, it was 23°C. These thermal changes led to a decrease in rate of the physical, biochemical and biological processes that are important in the natural cycle of water autopurification.
These dams also are an obstacle for the migration to the sea of suspended particles of sand and gravel that cause the accumulation of considerable quantities of mud in the reservoirs. These aquatic deposits contain organic compounds, heavy metals, etc. More than half of the Dub?sari reservoir (Nistru river) consists of mud. The sediments may cause secondary water pollution if favourable conditions occur (modification of pH, temperature, ionic force, etc.).
In order to forecast, prevent and reduce the consequences of the floods, it is necessary to carry out the following:
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