Causes and systematics of inundations of the Krasnodar territory on the Russian Black Sea coast

The inundation situations on the Black Sea coast of the Krasnodar territory for the period from 1945 until 2013 were analysed and the main types of inundations at the coast are described. Synoptic factors of the formation of extreme precipitation and rainfall floods, features and regularities of the downstream flood wave transformation in the rivers are also studied. Therefore, assessments of seasonal and maximum flow of the Black Sea coast rivers for the period of hydrometric measurements were done. Regularities of change of the occurrence of inundations and their characteristics on the coastal terrain were analysed, for a year and on a perennial timescale. Most catastrophic and exceptional inundations arise in the summer and in early autumn. Small inundations during the remaining year reflect the seasonal distribution of river flow and floods in the Black Sea rivers. Extensive and sometimes extreme precipitation dominates the river flow regimes. The seasonal distribution of small and moderately dangerous inundations reflects, on average, a water regime of two groups of rivers of the coast – to the north and to the south of the Tuapse River. To the north of the Tuapse River, floods prevail from November until March (up to 70 % of observed floods took place in this period) as a result of precipitation and winter snowmelt during frequent thaw periods. In winter, high waters often overlap to form a multi-peak high water of 2–3 weeks’ duration. In the summer and in early autumn we observe a steady low flow. The total amount of runoff increases both in a southeast direction, and with the altitude of the river basins. Interannual variability of mean annual runoff, as well as maximum runoff, on the contrary decreases in the southern direction and with an increasing area of the river basins. The coastal high waters of the rivers of the Sochi part of the coast are typical at any time of the year, but more often floods in the cold season result from incessant rain, and thawing snow. Annually up to 25 floods have been observed. The principal reason of such distribution is the increase of extreme rainfall in the warm season. Orographic features of the coast and detailed features of rainfall only cover a small number of local river basins and a limited area. The geographical correlation of individual rainfall and subsequent floods ceases to be statistically significant for distances over 40–60 km. The annual flow cycle is mainly determined by two seasons, winter/spring and summer, with strong and weak flows, respectively; almost 71 % of all catastrophic and exceptional inundations took place in July–August (71 %) and in October–November (29 %). The characteristic features of dangerous floods are their rapid formation and propagation, a significant increase of water level (up to 5–7 m and more) and the multiple increase of water discharges in comparison with low flow period. Analysis of the interannual changes of the number of inundations at the Black Sea coast of the Krasnodar territory has shown some increase of the number of inundations in the period from the early 1970s until the early years of the twenty-first century. Quantitative assessments of risk, hazard and damage for the population and economic activities from accidental inundations in the valleys of the Black Sea coast rivers show that economic and social losses from inundations at the Black Sea Published by Copernicus Publications on behalf of the European Geosciences Union. 1290 N. Alexeevsky et al.: Causes and systematics of inundations of the Krasnodar territory coast of the Krasnodar territory are some of the highest in the Russian Federation. The basic conclusion from recent inundations is the need to consider not only the lower reaches and mouths of the Black Sea coast rivers where the main part of the social and economic development of the coast is concentrated, but also whole river basins and catchments. Further, an analysis of the efficiency of the measures applied at the coast to mitigate inundations and their after-effects is provided.


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The seasonal distribution of small and moderately dangerous inundations reflects, on average, a water regime of two groups of rivers of the coastto the north of the Tuapse River, and to the south. To the north of the Tuapse River, floods prevail from November until March (to 70 %). They result from precipitation and winter snowmelt during frequent thaw periods. High waters in the cold season of the year often overlap with each other, forming a multipeak high water with 2-3 weeks in duration. In the 5 summer and in early autumn a steady low flow is observed. The total amount of runoff increases both in a southeast direction, and with the altitude of the river basins. Inter-annual variability of mean annual runoff, as well as maximum runoff, on the contrary decreases in the southern direction and with an increasing area of a river basin. The coastal high waters of the rivers of the Sochi part are typical at any time of the year, but more often floods in the cold season result from incessant rains, and thawing snow. 10 Annually up to 25 floods are observed. The principal reason of such distribution is the increase of extreme rainfalls in the warm season.
Orographic features of the coast and detailed features of rainfall cover only a small number of local river basins and a limited area. The geographical correlation of individual rainfall and subsequent floods ceases to be statistically significant for the distances over 40-60 km 15 The annual flow cycle is mainly determined by strong winter and spring, and weak summer flows: Despite a characteristic distribution of floods and of water flow within a year, almost 71 % of all catastrophic and exceptional inundations took place in July -August (71 %) and in October -November (29 %). The characteristic features of dangerous floods are their rapid formation and propagation, a significant increase of water level (up to 5-7 m and more) and multiple increase of water discharges. 20 An appreciable increase of the number of inundations in the period from the early 1970s until the early years of the 21 th century was noted.
Quantitative assessments of risk, hazard and damage for the population and economic activities from accidental inundations in the valleys of the Black Sea coast rivers show that economic and social losses from inundations at the Black Sea coast of the Krasnodar territory are one of the highest in the Russian 25

Introduction
Coasts, valleys and river mouths are often subject to the influence of various dangerous hydrological phenomena. Of all dangerous phenomena, inundations result in the most significant economic and ecological damages, and are the greatest danger to the population. The Black Sea coast of the Krasnodar 10 territory is the most prone to this kind in Russia. In this rather small area, there were five catastrophic inundations during the last 10-20 years, which resulted in huge material damages and considerable In general, there is a certain increase in the number and intensification of the magnitude of inundations.
Which factors contribute to this impression? Is it the reaction to global and regional climate changes, the intensification of instabilities of the climate system, or the effectiveness of the existing system of forecasting inundations and flood prevention and the threat the floods pose to the settlements at the 20 coast? What could be the operative prevention measures to minimise the potential damage, and which potential means are available to strengthen this system? Unfortunately, for too many of these questions there are presently no clear answers, for several principal reasons. First, despite the known significant hydrological hazards, a complex survey of inundations at the Black Sea coast of the Russian Federation practically is presently not available. Many available 25 5

Objectives of research
The Black Sea coast of the Krasnodar territory of the Russian Federation includes the Temryuk and Tuapse administrative areas (districts), and the cities of Novorossiysk, Anapa, Gelendzhik and Sochi ( Fig. 1). These cities also are the main administrative districts by their area, proportion of the developed and unsettled territories. The total area of the coast is nearly 8015 км 2 . As a relatively narrow strip of 5 land (its average width is 23 km) the coast extends over 350 km (from the Kerch Strait to the Psou River). The land border of the Black Sea coast coincides with the watershed line between the basins of the Azov and Black Seas.
The coastal terrain is well developed. Over 1.1 million people live here constantly. About 90 % of the resident population is concentrated in a narrow strip with a width from 0.5 to 8 km and 80 % live in 10 cities and urban settlements. Big cities are Anapa, Novorossiysk, Gelendzhik, Tuapse, Sochi and Adler.
It is the largest recreational region of Russia and a fast developing cluster of various sports (in 2014 the XXIIth Olympic winter games took place in Sochi), and a large new business and cultural centre. The number of tourists per year in the region is about 7 million; most part of tourists is concentrated in the municipal districts along the seacoast . 15 This region is an important agricultural area of Russia, with a large centre of petroleum refining, production of building materials, transfer of dry and liquid goods, and transportation of natural gas and oil products. There are the important seaports the Gelezny Rog, Novorossiysk and Tuapse.
The Black Sea coast is not homogeneous by its constitution, composition and environment (Panov et al., 2012;Resources, 1969;Sergin et al., 2001). These distinctions define the specificity of the dangerous 20 natural phenomena in the coastal areas in their kind and hierarchy, and in their impact regarding their development, real or potential damages. A contrasting topographical relief and geological constitution, the irregular distribution of atmospheric precipitation create their heterogeneity. Under hydrological aspects, the Black Sea coast is an isolated basin compounded by numerous basins of small rivers. From Krasnodar territory 252 watercourses flow into the Black Sea, only 16 % have a length of more than 10 25 6 km (Hydrological Survey, 1964). Only three rivers (the Shakhe, Mzymta and Psou) have a length of more than 50 km and a drainage area above 400 км 2 (Fig. 1). The drainage density increases in the southeast directionfrom 0.3 -0.5 (and less) to 1 km/km 2 . A large water slope characterizes almost all the streams. On separate reaches, they look like mountain streams with waterfalls. The floodplain is intermittent and narrow, and usually not developed in the upper reaches and in gorges. On the seaside 5 part, numerous alluvial cones occupy the bottom of river valleys.
Total water resources of the rivers of the Black Sea coast are 7.0-7.5 km 3 /year, or about 2 % of the total river flow into the Black Sea. The total amount of runoff increases both in a southeast direction, and with the altitude of the river basins. Inter-annual variability of mean annual runoff, as well as maximum runoff, on the contrary decreases in the southern direction and with increasing area of a river basin 10 (table 1, table 2). Reductions of runoff due to economic activities are largest on the rivers of the cities of Anapa and Novorossiysk, i.e. in arid and foothill watersheds. Within these terrains, there are many ponds intercepting a part of river water, and agricultural areas demanding an artificial irrigation.
Significant water use practices take place also in the basin of the river Mzymta, mainly hydroelectric engineering, and agricultural, industrial and municipal water consumption. The majority of settlements 15 from Novorossiysk to Sochi is supplied with water, pumped from the thick alluvial depositions under the river channels.
Extensive and sometimes extreme precipitation dominates the river flow regimes. Therefore, the maximum water levels and discharges are observed in any month of a year. To the north from the Tuapse, river floods prevail from November until March (to 70 %). They result from precipitation and 20 winter snowmelt during frequent thaw periods. High waters in the cold season of the year often overlap with each other, forming a multipeak high water with 2-3 weeks in duration. In the summer and in early autumn a steady low flow is observed (Fig. 1). During this period, even rather large rivers can dry up on separate reaches for several days and even several months. Occasionally the low flow is interrupted by high waters caused by heavy rain. In total, 10 -13 floods per year happen on average. The annual flow 25 7 cycle is mainly determined by strong winter and spring, and weak summer flows: for winter and spring, we find about 82 -86 % of annual runoff, and at the Tuapse River -75 % (table 1, Fig. 1).
The coastal high waters of the rivers of the Sochi part are typical at any time of the year, but more often floods in the cold season result from incessant rains, and thawing snow. The rivers Shakhe, Sochi and Psou, basins of which have significant areas and altitudes, demonstrate a similarity of spring high water; 5 but only the Mzymta River has a well-distinguished spring-and-summer high waterfrom March until August ( Fig. 1). It is formed by the melting of the seasonal snow cover, the permanent high-mountain snow cover and of snow-patches, glaciers and rain. The duration of the low flow period is shorter compared to the rivers to the north of the Tuapse River, and in general, the river discharges are higher in this period because they are frequently interrupted by floods. Annually up to 25 floods are observed (on 10 the average [16][17][18][19][20]. Moving from the northern to the southern borders of the Sochi part of the coast (table 1) the percentage of the winter-spring season river flow decreases from 75 to 55 -65 %.
Floods are typical also for small, in fact, temporary water streams (the native name "cleft"). Their surface flow rises only in the season of rainfalls and (or) snowmelt. Absence of water in the channels of such riverbeds during the main part of the year generates the deceptive opinion about hydrological 15 safety at the end of their valleys. Therefore, adverse consequences of inundations here quite often acquire the features of catastrophic events.

Hydrological data and methods of research
Long-term observation data at 24 hydrological gauging stations of the Federal Hydrometeorology and 20 Environmental Monitoring Service (table 1) form the data basis for our research. The data are presented as averages (for days, 10-day, monthly and annual averages) water levels and water discharges, and by instantaneous maximum and minimum water discharges and levels. Secondly, numerous documentary data on inundations are collected. These data and information are part of the database «Inundations in the river mouths of the European part of Russia» described in  which is 25 8 available on the web site of the Natural Risk Assessment Laboratory NRAL of the Moscow State University (http://www.nral.org). Thirdly, we used critical marks of the height and rise of water levels which, when exceeded, lead to flooding of flood plains. They are classified as unfavourable (UP) and dangerous (DP) for the population and economic activities. They are particular to each separate reach of a channel. Fourthly, we took into account daily precipitation data at six meteorological stations (Sochi,5 Krasnaya Polyana, Tuapse, Dzhubga, Gelendzhik and Anapa) for the period 1945 -2013 along with regional criteria of dangerous precipitation. According to these criteria, rainfalls in the Tuapse and Sochi districts with an intensity of not less than 50 mm during no more than 1 hour (http://www.yugmeteo.donpac.ru/oj.jsp) are considered as heavy. Rainfalls of not less than 80 mm on the part of the coast from Anapa to the settlement Dzhubga, 100 mm for the Tuapse district and 120 mm 10 for Sochi during no more than 12 hours are considered very heavy. In mountain areas, the lower limit of very heavy rainfall values is reduced to 50 mm (Tuapse district) and 80 mm (Sochi). The collected materials and their analysis have allowed to classify, first, inundations on the Black Sea 5 coast of Krasnodar territory by their causes, and, second, to adapt for the territory under consideration and their rivers existing (in Russia) classifications of inundations by magnitude and after-effects.

Inundations and their types
Inundation in the Russian Federation is perceived as flooding by water of an area adjoining the river or 10 a water body, which leads to a material damage, loss of health of the population or to loss of human lives (Nezhihovsky, 1988). More expanded and with an ecological interpretation of this concept (Dobrovolsky, Istomina, 2006) suggest: «inundation is a temporary flooding of terrain mastered by the human for various purposes, generating negative consequences of social and economic and ecological character expressed in a material and non-material damage». On the contrary, flooding by water of not 15 mastered terrains, not accompanied by a damage, is considered as the natural hydrological process accompanying one of the standard phases of a water regime of a riverspring high water or a flood. It is not considered as inundation.
Taking the formation processes and following the new classification, stated in , at the coast, there are some generic types of inundations, of natural origin. River-flow 20 inundations dominate. At the coast they are generated by high rainfall floods (i.e. during peak discharges and without backwater effect), which sometimes are transformed into in mudflow type. More infrequent, they are induced by an intensive snowmelt in the drainage basins (including the contribution of rain), breakages of dams of ponds and of dammed or glacial lakes. The flood plains of river valleys and river mouths with alluvial cones are also subject to flooding by river waters. 25 10 Inundations of the mixed type (№1)river-flow and rainfall originare next by their occurrence. They are also called pluvial inundations. In general, rainfall inundations (a subtype of local meteorological inundations) which also are frequent at the coast are caused by heavy rainfall over the developed areas and by the "inability" of the terrain to quickly absorb or drain rainwater into surface and underground water bodies. The magnitude of rainfall inundations increases if storm drains are functioning badly, 5 therefore in the obvious and dangerous form they happen in settlements, and their frequency increases with the increase of the area of the urbanized terrains. They are also named urban inundations. During pluvial inundations, flooding is caused by river and rainfall waters, and by powerful overland streams formed by rainfall and by waters of "revived" temporary watercourses. These inundations affect not only flood plains, but also river terraces, and sides of the river valley. 10 The third type are the inundations due to storm surges and wind-wave surges, or inundations again of mixed type (№2)a flood in a river coincides with a storm surge at the coast, i.e. in conditions of a back-water effect from a sea. These inundations are possible in river mouths at the coast. They are part of a group of coastal inundations.
Other types of inundations (due to ice-jams, tsunami) represent only a potentially small danger to the 15 city of Anapa .
Inundations of one generic type differ by their characteristics (frequency, the area of flooding and the number of the river basins affected by inflow, height and duration of flooding, etc.), and also by size and structure of social and economic damage. The authors do not consider elaborating new approaches concerning such a division. Classifications already existing in Russia (Dobrovolsky and Istomina, 2006;20 Dоbroumov and Tumanovsky, 2002;Маlik, 2003;Nezhihovsky, 1988;Таratutin et al., 2011) have been used, including those used by the Ministry of Civil Defence and Emergency Situations of the Russian Federation. Accordingly, the river-flow and mixed type 1 inundations at the Black Sea coast of Krasnodar territorythe subject of this studycan be divided into small (I), moderately dangerous (II), big (III), catastrophic (IV) and exceptional (V). This classification is based on various qualitative and 25 quantitative criteria ranging from frequency, value of excess of max H over critical high-rise marks, the area of the terrain and number of the settlements (or basins) covered by the influence of inundations and their causes, or the amount of a direct material damage (as a rule at an approximate assessment) and threat for life. Among other criteria, we considered: 1) character of direct damage to industrial objects and road infrastructure, residential buildings, 2) the area and structure of flooding of the mastered terrain, 3) degree of infringement of way of life and industrial activity of people, 4) necessity of 5 evacuation of people, 5) deterioration of ecological conditions. Unfortunately, as it often happens, not for all the events comparable data are available. In general, for a separately taken river on the Black Sea coast the big inundations occur with a frequency max Q on the average of 4 -5 %, catastrophic and exceptional <2 -2.5 % respectively. Long-term observation data at 24 hydrological gauging stations of the Federal Hydrometeorology and Environmental Monitoring Service. 10

Synoptic conditions of the formation of high floods
High floods leading to river-flow inundations, and also flooding by rainfall waters and by overland streams, are formed by the availability of large volumes of water in form of abundant and steady precipitation, by storm rain and, as a special case, as a result of the destruction of waterspouts over the 15 land. Most catastrophic inundations are a consequence of mesoscale atmospheric processes arising in typical synoptic conditions that form especially powerful overcast. For the Black Sea coast of the Krasnodar territory, the formation of abnormal precipitation results from the topographical relief forcing the air upward and, hence, the process of cloud formation and precipitation.
To analyse the dynamics the atmospheric circulation one can use circulation indices. Among the most 20 well-known are the NAO-index, the Atlantic / West Russia (EA / WR-index), and the Nordic index (SCAND-index). These indices are useful in the analysis of large-scale circulation pattern using results from numerical modelling. For a detailed analysis of the synoptic situation, these indexes are often not representative. We find it more appropriate to use the classic synoptic classifications, based on a detailed description of synoptic processes of the study area. In Russia, one of the most popular is 25 12 Dzerdzeevsky's classification (Dzerdzeevsky, 1975;Kononova, 2012). It uses the concept of "elementary circulation mechanisms," and all synoptic processes combine the four basic types. 1 -zonal circulation, 2 -zonal and meridional circulation, 3 -meridional North, 4 -meridional South. All synoptic processes can be attributed to one of these groups. We use this approach in this study for the detection of the synoptic processes causing strong flooding. We therefore consider those synoptic 5 circulation pattern to cause floods in the Black Sea coast.
1. Mediterranean cyclones generated at the polar front advect abundant rainfall. The greatest recurrence and intensity of these processes is observed from October until March. However, also in summer months these processes are not infrequent. Cyclones bring to the Black Sea coast intensive precipitation with a high moisture content of wet tropical air, formed over the Mediterranean Sea. During wintertime, 10 these cyclones advect not only abundant precipitation in the form of rain and snow, but also a «heat In all above events, formation of so-called polar mesocyclones over the Black Sea waters was apparent 15 the axisymmetric vortex resembling tropical hurricanes. Since the first time this phenomenon was noticed in high latitudes, it is called «polar low». However, these cyclones are also often generated over the Mediterranean and the Black Sea. A polar low is a small, but fairly intense maritime cyclone that forms poleward of the main baroclinic zone (the polar front or other major baroclinic zone) (Rasmussen E.A., Turner J., 2003). The horizontal scale of the polar low is approximately between 100 and 1000 20 km, that is, according to the Orlansky classification, a polar low is a phenomena of subsynoptic scale (the horizontal scale synoptic processes of more than 1000 km, and the same mesoscale processes of less than 100 km) (Markowsky and Richardson, 2002). The above definition can be extended, if necessary, by specifying the dominant physical mechanism responsible for the development of the low, such as, for example a 'baroclinic polar low' or a 'convective polar low', the latter being driven 25 primarily by organized convection (Rasmussen and Turner, 2003). A striking example is the weather Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2015-335, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. and rather dry air by a cold atmospheric front. Spouts "descend" from cumulonimbus clouds. The lifetime of a waterspout is from several minutes to several tens of minutes, and they can pass a 10 significant distance. In height, these spouts can reach several hundred meters, with diameters of order of tens of meters. There are eyewitness reports that waterspouts at Novorossiysk on August 8, 2002 had a diameter of 200 m and a height over 1 km. The spouts formed in the coastal region sometimes make landfall and move over mountain ridges. As a result, the seawater involved in the circulation of a spout, falls into river basins. It is impossible to resolve this process with standard observations; therefore, 15 some experts are sceptical about a "fatal" role of waterspouts in the formation of powerful inundations (Sergin et al., 2001). Nevertheless, evidence of local residents and special research of the Krasnodar branch of Federal Hydrometeorology and Environmental Monitoring Service do not exclude such possibility. For example, after the August 8, 2002 event, the settlement Abrau-Dyurso suffered a catastrophic inundation when local residents found sea fish in their courtyards, and the river water had a 20 saltish taste. The second examplestrong downpours on June 20, 1988 (179 mm for 4 hours and 50 minutes) around Novorossiyskwas observed after the formation of a powerful waterspout over an open part of the sea and its "arrival" on the east coast of the Tsemessky Bay (Tkachenko, 2012). A part of this precipitation has probably been seawater. On the western coast of the bay, there was no rain during this day. 25 For the formation of high and dangerous floods, besides the amount of precipitation and their intensity (in the first few hours), the amount of precipitation for previous days and the degree of humidifying of the watershed is important. At strong downpours of 50-100 mm (and more) in 1-2 hours, the intensity of raising the water level in the rivers increases. times less than the observed maximum. The velocity of propagation of flood waves is also less (Fig. 3), 20 but because of the lack of reliable data this difficult to ascertain.
High floods at the Black Sea Coast rivers -due to the storm character of rainfall, large gradients of surface and rather small dimensions of the basins -are characterized by short duration, extremely fast rising and subsequent falling of the water level (Fig. 3). Floods, or their series, can last a number of days. However, the main part of the flood wave passes, as a rule, within several hoursroutinely no 25 16 longer than 0.5 -1 days. However, the part of a flood that leads to flooding lasts even less. For example, the catastrophic flood on the Tuapse River in 1991 lasted ~4.5 days, its basic part, though, passed approximately within 1 day, and flooding of the floodplain lasted less than 4.5 hours (Panov et al., 2012). Residual flooding of floodplains remains longer.
The maximum rise of level ( max H  ) in the valleys of the Black Sea Coast rivers can reach 5 -7 m and 5 even higher values (Fig. 4). The extreme water level rise is possible for catastrophic floods at parts of narrowing river valleys and channels, or upstream of bridges. The afflux component of rising water levels upstream of bridges and dams of wooden debris can be 0.5-2 m, but possibly exceeds this value. The flood wave undergoes its basic and final transformation at the lowermost reach of the river and in the river mouth, i.e. downstream of the confluence of the last large tributaries. The conditions are an 20 essential widening of the river valley, reduction of water slope, backwater effect from the sea (or from pebble and sand bars blocking the river mouth). As a result, the most dangerous flooding happens  where the human settlements are usually found with basic resort and other infrastructure objects.
The maximum depths of flooding by river waters reach more than 3 m, and by overland streams up to Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2015-335, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. first on the flood plain before arriving at the lower reaches and at the mouths of the Black Sea coastal rivers. Therefore, besides destruction, the river water unloads a thick layer (10-20 cm and more) of deposits, of debris and refuse (Fig. 7a) on the flood plain. This is another aspect of the adverse effects of inundations together with channel deformations, deterioration of water quality in rivers and adjacent seas. Deposits from floods and particularly mudflows seriously increase flood damage on developed 5 terrain and of civil constructions.
Secondly, the significant part of deposits is accumulated in channels that lead to a reduction of their water transport capacity. If a channel is not periodically dredged, its transport capacity quickly diminishes. As a result, the frequency of dangerous flooding increases. It also has happened in the settlement of Novomikhaylovskiy in 2010 and 2012, despite considerable protection measures against 10 the inundations in the settlement in the 20th century, including high and continuous dams, and a wide and improved channel. Only after the last inundation, the channel cleaning has started (Fig. 7b).
Thirdly, parts of the deposits remain in river mouths where these depositions quite often form a bar shoal which later is washed away during strong autumn-winter-spring storms. Other parts (very fine sediment particle fractions) are carried away into the sea, forming a strongly pronounced turbid plume, 15 unfavourable for recreational activities of resorts and deteriorating habitat conditions of aquatic organisms (Fig. 7c).

Temporal regularities of inundations
Despite a characteristic distribution of floods and of water flow within a year (table 1, Fig. 1), almost 71 20 % of all catastrophic and exceptional inundations took place in July -August (71 %) and in October -November (29 %). Some 52 % of large inundations happen in the summer and 26% in the September -October period. The principal reason of such distribution is the increase of extreme rainfalls in the warm season. According to meteorological records, nine rain events with a total precipitation > 100 mm/day happened in the last 50 years in November-February while in May-October such downpours were 25 19 observed, at least, 46 times, and in 85 % of cases in June-September. In March-April, there are no records of such rainfalls. Besides, heavy rains in the cold season have a longer time duration than in the warm period. This reduces the probability of the formation of dangerous high waters. The contribution from water tornadoes formed in the coastal region from June until October can be one more factor for increasing flood levels. 5 On the contrary, the seasonal distribution of small and moderately dangerous inundations reflects, on average, a water regime of two groups of rivers of the coastto the north of the Tuapse River, and to the south. It is characterized by sufficient uniformity. Some 30% of such inundations take place in winter, in the spring 12%, in the summer 28%, and in autumn 30%. The safest months with respect to inundations of all types are March (3.5 %) and especially April (1.5 %). 10 On longer time scales, we can observe a nonlinear and statistically insignificant trend of the increase of the number of inundations and, hence, of the expected damage as given in (Fig. 8a). It mainly is caused by a noticeable increase of the number of inundations in the period from the beginning of 1970s until the first years of 21th century. This positive trend can be challenged, but the objective reasons for it, nevertheless, exist. 15 First, these are the climatic changes observed in the region (Kononova, 2012;Panov et al., 2012;Sergin et al., 2001;Tkachenko, Volosuhin, 2013). The increase in water flow at a number of rivers (absolutely unequally distributed at the different rivers in this small territory), and mainly peak water discharges (especially in last quarter of 21th century) and maximum flow extremes (Fig.9) are considered as the hydrological reaction to these processes. This is true particularly, for example, for the increase of 20 anomalously high peak discharges of water, such as in 1980,1991,1997,2002,2010 (Kononova, 2012; http://atmospheric-circulation.ru/datas/), from the end of the 1950s on the increase of the duration of this type of circulation (Fig. 8b) is noted. In the early sixties, for the first time for 112 years of record (from 1899 until 2012) southern longitude processes have exceeded their average number. An 5 unprecedented growth of the duration of southern longitude processes has begun in the 1980s and only after 2000 started to drop, but they are still above the average level. That is characteristic, and the same dynamics can be found in the annual total numbers of precipitation in the region of the Krasnodar territory (Volosuhin and Tkachenko, 2013). Simultaneously, with weakening of the southern longitude processes in the 2000s, a significant increase in the frequency of northern longitude processes is noted. 10 Secondly, the growth of the number of extreme inundations can be a consequence of wide scale and not always prudent economic activity. It often includes intensive construction works at flood plains and on alluvial cones of river mouths (Fig. 10), where 100 years ago there still was no activity, and in the late eighties, with the beginning of the 1990s where in most cases only temporary constructions and kitchen gardens were established. The other reasons could be the termination (or decrease in scales) of works in 15 the Post-Soviet period of dredging channels and maintaining protective dams in good condition (Fig. 7), and unreasonable and intensive land use on watersheds. A number of scientists connect the increase in the 20th century (in comparison with the 19th century) of inundations, mudflows, rock falls and landslides to the last factor. In general, the anthropogenic contribution to inundations for the Black Sea coast is considerable, its effect constantly grows, breaking the relations between characteristics of 20 inundations both of natural climatic and hydrological factors, and enhancing differences in their interannual variability, and finally increasing the magnitude of inundations. The last example is an inundation on July 7, 2012 in Krymsk. According to the prevailing weather conditions, it would have been considerable, but has outgrown any expectations due to a combination of several anthropogenic factors. Evidence are the accumulation of the large volume of water in fish-breeding ponds and the 25 headwater upstream of the bridge, the subsequent outbreak, the destruction of trees in the river basin; Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2015-335, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Published: 19 January 2016 c Author(s) 2016. CC-BY 3.0 License. damage was caused to unlicensed residential buildings in a region of potential flooding and the untimely information of the population. Other anthropogenic factors had an influence, too.
Without a thorough discussion of this point, it will be impossible to quantify and respond to hydrological hazards, and to predict changes of inundations in the future. Nevertheless, a number of scientists at present consider (Matveeva et al., 2013) that this tendency will continue. The data of the 5 climate model ECHAM5/MPI-OM (scenario A2) highlight that during the summer season of 2046-2065 an intensive frontal region (one of the synoptic predictors of abundant precipitation) will be twice more often than in 1981-2000, and 3 times than in 1961-1980. For the winter season, we find a reverse relationship.
The second feature of the interannual dynamics of the number of inundations at the Black Sea coast of 10 the Russian Federation is their recurrence with a duration of cycles from 6 -7 to 10 -12 years (fig 9а).
The spectral analysis of the time series with the program STATISTICA10 (for five basic transformations and at different window width of the sliding average), has revealed the highest peak of the periodogram and spectral density for the period of ~8 years duration and essentially smaller in height at 7 years. Smaller peaks are found for the periods of 3.5, 5.0 and 23 and at 11-12 years. A 15 similar recurrence was found in the number of inundations in the whole North Caucasus during 1980-2013  and by V.A. Volosuhin and Ju. Ju. Tkachenko (Volosuhin, Tkachenko, 2013) in the change of the quantity of floods of category DP on the rivers of the Krasnodar territory.

Geographical features and hazard of inundations
Orographic features of the coast and features of rainfall, river-flow inundations and mixed type 1 inundations as a rule cover only a small number of local river basins and a limited area, especially in the case of rain showers of tornado-origin. Therefore, the spatial correlation of max Q for the coastal rivers is rather insignificant and quickly decreases with distance between watersheds. Within the first 50 km, the 5 correlation coefficient ( r ) still can reach significant valuesmore than 0.6 -0.7 (at a wide range of fluctuationsfrom 0 to 0.9). Within 50 -75 km r drops to 0.5 -0.6 and less, within 75 -125 km -  (Magritsky et al., 2003).
Because of the low and flat coast between Anapa and the settlement Veselovka, the shallow sea near the 25 coast and the spoon-like shape of the shore, there is a certain potential danger of storm surges and of tsunami. In addition, numerous artificial water bodies are potentially dangerous according to (Panov et al., 2012). In the Anapa area they very numerous, nearly 39 in number and with the total area of 3.5 km 2 .
In the Novorossiysk, Gelendzhik, Tuapse and Sochi municipal areas floods caused by extreme rain and Tuapse area (Fig.3); another is the fast formation and transit of floods on the rivers of these areas, because of their small dimensions, and often the mudflow character of the floods.
Most of all inundations happen in the lower reaches and the mouths of the Black Sea coastal rivers and accordingly greater damage and higher losses. Large economical activities and larger populations are concentrated there, and most of the factors of "spontaneous behaviour" of river, rainfall and seawater 5 take place here. Therefore, a stricter approach is necessary for this terrain with a higher degree of scrutiny with respect to issuing permissions for the placement of social and industrial objects in this region (land use and land planning), for the estimation of cost of their insurance and of the protective action, and for the population evacuation in case of emergency.
During catastrophic inundations, the respective damage is high without depending necessarily on the to more destructive ones) as the curve is close to an exponential function, but with higher steepness (Fig. 11). Certainly, the reliability of the relationship developed by the authors of the paper still is low; the confidence interval wide. The reasons are the small number of cases (17 values) and the low reliability of the initial data. However, similar relationships dictate essentially our understanding of the danger of those or other inundations.
In general, the economic annual risk of river-flow and mixed type 1 (river waters + rainstorm + slope The basic conclusion from recent inundations consists of the need to consider (as objects of the efforts) not only the lower reaches and mouths of the Black Sea coast rivers where the main part of the social and economic development of the coast is concentrated, but also total river basins and catchments 20 Sergin et al., 2001). This is the watershed, where its physiographic features Other engineering measures such as the increase in height of dams around the objects of importance, channel "replacement" as in a case in the lower reach of the Tuapse River, maintenance of free drainage or filtration of rainwater in inhabited terrains are also suggested. At present, the operative practice of "artificial dambreaking" of coastal barriers in the river mouth is rarely applied. Coastal barriers formed 25 by sea waves and storm surges block the river channel and do not allow the river waters to flow freely Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2015-335, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Published: 19 January 2016 c Author(s) 2016. CC-BY 3.0 License. into the sea. At the approach of the flood wave, the coastal dam, which routinely protects the mouths of the Black Sea rivers from the wind induced sea surges, is at the initial moment of the development of inundations a serious obstacle for the free discharge of river water into the sea, i.e. being one more factor to contribute to inundation.
Regulating the maximum flow by water reservoirs on the Black Sea Rivers is ineffective owing to the 5 impossibility of building in this region large regulating storage capacity. They fill up fast with sediment, including landslip and mudflow deposits. Additionally there is a high danger of destruction of the dams because of the high seismicity of the terrain. Where artificial reservoirs nevertheless are built, they are an additional factor leading to powerful nature-anthropogenic river-flow inundations.
Therefore, such hydraulic engineering structures should be constructed under multi-hazard aspects and 10 designed very reliably.
Among non-engineering approaches, it is necessary to pay attention, first, to an increase of efficacy of the preliminary forecasts of the maximum water levels and discharge, the timely warning of the population and subjects of economy about the approach of "the big water". Modernization of the hydrometeorological monitoring system for this purpose is required. The first steps in this direction have 15 already been made by the Ministry of Emergency and Civil Defence in the Krasnodar territory. Since November 2012, the computerized system of monitoring flood situations on the rivers and reservoirs (http://test.emercit.com/overall.html) in the region is in place. The installation of several rainradartracking stations for monitoring the intensity and quantity of precipitation is required. Vulnerability assessment and increasing of preparedness of local population are also important aspects among 20 measures of flood risk minimization (Zemtsov, 2014).
Secondly, a clear understanding of the reasons, features and systematics of the origin and development of inundations and their adverse hydrologic-ecological, morphological, social, and economic consequences is necessary. Here are good prospects for numerical modelling and GIS-technologies ( Fig. 12). Modelling of water and debris flow is required to estimate flooding borders, water levels, 25 28 depths and flow velocities in key areas. Such data could be the base for a detailed hazard assessment and zonation of the river valleys (Petrakov et. al., 2012).
Thirdly, restrictions (by various meansfrom administrative measures to flexible flood insurance) are necessary for the processes of developing the territory to reduce its potential flooding hazard. For this purpose, these limits (for inundations different in their dimensions) should be made known and the 5 terrains differentiated at the degree of their hydrological hazards. This will be required for solid landuse planning and planning permissions.

Conclusions
The list of the dangerous natural phenomena at the Black Sea coast of the Krasnodar territory of the 10 Russian Federation is extensive, but inundations cause the greatest damage. Such situation arises from the influence and interactions of many different factors. Among natural factors are the specific location of the area, the complex orography of the territory, the high drainage density, small basins area, and the big water slopes and weak regulating ability of river watersheds. An important role is played by the large quantity and the extremeness of rainfall, and the intense flood regimes of the rivers. Extreme 15 floods form rapidly and transit fast downhill. That leads to a fast and substantial increase of water levels, the frequent transformation of rain floods into mudflow-like streams, and the contribution of powerful storm and overland streams to additional terrain flooding. Among anthropogenic factors are the location of the main part of settlements, objects of the industry, social sphere and the resort industry, the transport infrastructure in river valleys and the mouths of the Black Sea coastal rivers. 20 By genesis at the Black Sea coast, inundations are generated by river-flow and river-flow-rainstorms (mixed type 1). They dominate in number, repeatability and damage values. At the coast and in river mouths the inundation can be caused also by storm surges, or by storm wave induced surges, and the interaction of river and the sea. We can distinguish inundations by terrain coverage (the number of involved watersheds and rivers) and intensity, and by magnitude of the damage -small, moderately 25 dangerous, large, catastrophic and exceptional inundations. The probability of their occurrence accordingly is ~20, ~10, ~4-5, ~2-2.5 and <1 % respectively.
The floods, which lead to river-flow and mixed type 1 inundations, are formed by abundant and heavy storm rainfallsat the transit of southern cyclones, cold atmospheric fronts of extensive Atlantic cyclones covering Eastern Europe, or accompany cyclones quickly arriving from the northwest. As a 5 special case, falling into categories of "cloud burst», rainfall discharges as a result of the destruction of sea-born tornadoes on land. The mountain relief has a considerable role in the formation of abundant precipitation. Catastrophic inundations are generated by an abnormal combination of synoptic processes and convective phenomena.
The characteristic features of dangerous floods are their rapid formation and propagation, a significant 10 increase of water level (up to 5-7 m and more) and multiple increase of water discharges (at times practically from values close to zero to several hundred m3/s and even >1000 m 3 /s). On the interannual scale, the increase of the number of inundations and, hence, the damage involved is implicit. It mainly is caused by an appreciable increase of the number of inundations in the period from 25 Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2015-335, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Published: 19 January 2016 c Author(s) 2016. CC-BY 3.0 License. 30 the early 1970s until the early years of the 21th century. The main reason is found in a changing climate but another influence, especially regarding the extreme inundations, are anthropogenic such as irrational and badly planned economic activities in channels, flooded terrains, and on river watersheds. The mean annual frequency of inundations and dangerous floods on the entire coast is about once in 0.3 years. The number of inundations in the region varies with a duration of cycles from 6 -7 to 10 -12 years. 5 Not all administrative areas of the coast are equally in danger and vulnerable to river inundations. The most dangerous areas are the Novorossiysk, Gelendzhik, Tuapse and Sochi municipal districts. The larger Sochi area is in danger of a high frequency of inundations, whereas in the other three areas the high danger results from higher extremes of storm rain floods. Not surprisingly, here are more cases of catastrophic inundations and loss of human lives. In general, the total annual economic and social risk 10 from river inundation can be approximately estimated at 13.3 mln. U.S. Dollars and two human lives at the Black Sea cost of the Russian Federation.
Our systematic analysis will increase awareness of the public to raise the level of safety and security of the terrain, and objects and population in the Black Sea Coast area not only by improving engineering actions, but also by the optimization of the terrain, and the increase of system effectiveness of the 15 monitoring and forecasting critical hydrometeorological situations, resulting in improved early danger warnings.  Sveltyi 1966Sveltyi -1993Sveltyi ,1996Sveltyi -1997  circulation.ru/datas/). Nat. Hazards Earth Syst. Sci. Discuss., doi:10.5194/nhess-2015-335, 2016 Manuscript under review for journal Nat. Hazards Earth Syst. Sci. Published: 19 January 2016 c Author(s) 2016. CC-BY 3.0 License. Figure 11. Empirical relationship between direct material damage (millions of US dollars) and type of inundation (river-flow and mixed type 1), or probability of max Q