Climate change and dangerous hydrological processes

Reduction of glaciation began at the end of 19th century, after the peak of the Little Ice Age (Voitkovskiy and Volodicheva, 2004). Now, glaciation is in a regressive phase and the general reduction removed about 20% from the glaciated area during the first 70 years of the 20th century. Thus, there has been a reduction in the area, volume and length of glaciers, and an increase of their number and the altitudes of their snouts. The size of glaciers has continued to change in recent decades (1970-2000), despite an increase in precipitation in the Large Caucasus (of 10-15%) and air temperatures of +1°C. From 1970 to 2000, the area of glaciation was reduced by 12.6%, the volume of ice by 14.9%, and the number of glaciers has increased for 2.4%. Glacier contraction continued over the 30 years and they have on the average reduced in length by 100 m. The increase in precipitation, especially during winter, has resulted in an increase in snowfall and the frequency of snow avalanches during the last 20 years.

Over the last 50 years, a general increase has been seen in the naturally renewed water resources of the region. A consequence of glacial area reduction has been a reduction of the discharges of the rivers that depend on glacial meltwaters. In basins where glaciation is actively degrading, the runoff is increasing. For the rivers with mixed snow-rain and glacial feed, the trend of runoff is mostly significantly positive. The increase of sediment yield has been particularly marked in recent years, reflecting the regressive changes in snowlines and glaciation, increasing the area of high-altitude zones producing huge volumes of sediments and mudflows.

Floods on the rivers of the Terek basin occur in the spring-summer period during intensive snow and ice melting and heavy rainfalls. Extreme catastrophic floods are always connected with the maximum height of the spring flood being accompanied by significant or outstanding rainfall floods. Such floods were observed in the Terek basin in 1931, 1932, 1958, 1960, 1963, 1966, 1967, 1970, 1974, 1998, 2000, 2001 (Taratunin, 2000; Dobrovolskiy and Istomina, 2006) and in 2002. In the mouth of the Terek, high spring and summer floods result in the breaching of the levées and flooding. In the Terek basin, over 200,000 ha of agricultural land, including 84,000 ha of irrigated land, and a population of 140,000 lie in the zone of possible inundation.

As a result of the catastrophic water level rise in 2002, the territories of Kabardino-Balkaria, North Ossetia, Chechnya, Ingushetiya and Dagestan were flooded, 114 people were killed, 300,000 people were injured and over 100,000 people were evacuated. Dozens of thousands of homes, hundreds of municipal buildings, educational establishments and other units were destroyed and damaged. Flood protecting constructions and dams were seriously damaged as well. The financial damage amounted to about $530 million (Lourie, 2002). In the mountain regions of Kabardino-Balkaria, North Ossetia-Alania, Ingushetiya, Chechnya and Dagestan numerous mud flows occurred. Breaching of protection dikes took place at the Kargalinskaya station when the water discharge was 1,530 m3/s. Vast areas were flooded in the delta of the Terek, homes, bridges and other constructions were destroyed, kilometres of protection levées and dams were scoured, great ecological and economic damage was caused in the affected areas (Gorelits et al., 2005). The duration of peak level in the rivers varied from 3 to 288 h. The probability of the maximum discharge during the flood at the lower gauging section of the Terek amounted to 3%. In spite of the fact that greater discharges were observed at the head of the Terek mouth in 1931 and 1967, the flood of 2002 was unique in its duration as well as its discharge and sediment load.

The effects of climate change on flood characteristics has been estimated by Christoforov et al. (2007). Probable climate changes will not affect the magnitude and shape of the flood peaks, so much as their average number and distribution during the year. For the rivers of the Terek basin, the average number of peaks increases during the winter season and decreases in the summer with the rise in air temperature, and may even be reduced to zero with extreme warming. An increase in annual precipitation will lead to a general increase in the number of floods in all seasons for all the rivers studied. Calculations show that the risk of catastrophic floods on the rivers of Northern Caucasus will increase in coming decades with the rise in air temperature.

Owing to the change of climatic conditions in this region, there has also been a change in the level of the Caspian Sea. Over almost 500 years, it fell sometimes quickly, sometimes more slowly, until in 1977 it reached the lowest position at -29.01 m BS (Fig. 1). Then it promptly began to rise (on average by 13 cm/year), reaching a maximum in 1995. As a result, part of coastal territories has been flooded, and conditions of hydraulic interaction between river and sea waters and sediment yield have changed significantly. During the last decade, the level of the Caspian Sea has stabilized. The amplitude of annual fluctuations was 0.4 m. On average, for the period of 1996-2006 the level of Caspian Sea was -27.03 m BS.

As a result of floods, sedimentation and reorganization of the channel network in the delta occurs periodically. Every 60-80 years, the main channel in the delta changes direction considerably. All known cycles of channel orientation change in the delta have coincided with periods of increases in flood numbers (Alekseevskiy et al., 1987).

For the last 500 years in a lower reaches of Terek, seven such catastrophic breaks have taken place. Each of them has led to the occurrence of systems of channels named the Kuru-Terek (16th century), Sullu-Chubutli (17th century), Old Terek (the beginning of 18th century), New Terek

(the end of 18th century), Borozdinskaya Prorva (the beginning of 19th century), Talovka (the end of 19th century) and lastly the Kargalinskiy breach. The most recent cycle in the Terek delta formation occurred during the catastrophic flood of July-August, 1914, at discharges of more than 2,000 m3/s. For some years, the river has developed a channel which carries 80-100% of the discharge. The worst water shortage has been in the cultivated left-bank part of Terek delta. Attempts to block the Kargalinskiy breach continued until 1927. In addition, it was decided to secure the water supplies in this part of delta with an expensive dam and irrigation canals.

The history of the evolution of the Terek delta shows that the most complicated problems in a lower reaches of the river are caused by the high sediment yield and extremely high turbidity of river waters. In conditions of gradual reduction of a bed slope and a water surface the stream loses ability to transport all of its sediments. They deposit in the channel, reducing its carrying capacity. As a result even rather small discharges of water can lead to the flooding of districts adjacent to a river channel. For the observation period 1924-1988, the average suspended sediment discharge in the Terek delta was 588 kg/s, which corresponds to an annual sediment load 18.5 million tonnes. In individual years, the suspended sediment yield was as high as 38 million tonnes and as low as 6 million tonnes.

The increase in damage from dangerous hydrological processes in recent decades in the Terek basin is connected with lack of data about these processes, neglecting the risks, development of areas for the potential occurrence of these processes, mistakes in scientific support of projects, changes in natural and anthropogenic factors, and shortage of funds for the monitoring schemes.

In order to decrease the risks, it is necessary to estimate the size and variability of the components of runoff - the main reason of changes in the safety of the population and economy; to prepare a modern information basis for the characteristic of probability and scales of dangerous hydrological processes; to create methods of estimation, calculation and forecasting of characteristics of the hydrological hazards at local, regional and basin level.

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