1 Introduction

Hydrological hazards are the most common risk phenomenon, each year generating significant societal and environmental damages. In the decade 2005–2014, almost half (192) of the average annual number of natural triggered disasters (380) had hydrological origin (mainly floods and subsidiary mass movements of hydrological origin). These disasters caused annually an average of 87.28 million victims (killed and total affected people) representing 44% of total disaster victims, and they were responsible for 22% of the total annual average cost of damages (34.55 from the total of 159.75 billion US$) (Guha-Sapir et al. 2016). The most widespread and damaging hydrological natural hazard are related to river-specific processes: flow-variation (floods, flooding, low-waters and droughts), water freezing/unfreezing and related ice jams, sediment, and channel dynamics (erosion and deposition processes). The floods have the largest share in natural disaster occurrence (almost half), and they are responsible for the biggest damages: between 2005 and 2014, the floods have caused annually 5993 fatalities and 99.6% of all disaster victims. The share of damage attributable to floods is annually more than 99% of total disaster costs (Guha-Sapir et al. 2016). In order to reduce the negative consequences of floods and related processes, over the past decades, flood mitigation has become a major concern of water policies and sustainable development strategies at different spatial scales (e.g., from European Union—EU, to national, regional, and local scale) (Zaharia and Ioana-Toroimac 2016).

This chapter focuses on Romania, a country with one of the highest flood risk exposure to floods and related damage costs in Europe (Pińskwar et al. 2012; Kundzewicz et al. 2013). It aims to highlight the main river-induced hazards and their features, based on relevant examples for different types of river-related damaging processes. After a statistical overview on the natural disasters in Romania, revealing the leading place occupied by floods, the chapter presents particularities of the specific river-born hazards as floods and flooding, low-waters and hydrological droughts, and other damaging phenomena (water freezing, sediment/channel dynamics and reservoirs’ silting), as well as significant examples of different events/hazards and references to their impacts. Finally, some considerations on hydrologic risk management are presented. The chapter offers a synthetic and actualized overview on the whole range of natural hazards related to rivers in Romania. It completes the information from previous general works on hazards published in Romania (e.g., Romanescu 2008; Grecu 2016; Sorocovschi 2016) or the information on strictly certain hydrological hazards, mainly floods (e.g., Mustăţea 2005; Romanescu 2006). Within volumes “Riscuri şi catastrofe” (Risks and catastrophes) edited by Sorocovschi (2002–2017), papers on different river-induced hazards in Romania at regional or local scale can be found (most of them on floods).

The chapter is based mainly on a synthesis of significant scientific researches and processing of statistical data from different sources (EM-DAT database, scientific bibliography, etc.).

2 Statistic Overview on Natural Hazards and Related Damages in Romania

Located in the central part of Europe, on the parallel of 45°N, Romania (area of 238,391 km2, with almost 20 million inhabitants) overlaps almost entirely (on 97% of its territory) the Danube River Basin. It has very diverse and complex geographic features that give it susceptibility to natural hazards. Its location at the jointing of several tectonic microplates favors earthquakes. The presence of the Carpathian mountains (with a maximum altitude of 2544 m), of the Black Sea coastline and of the Danube River (Fig. 9.1a), as well as a relatively dense hydrographic network (0.5 km/km2, according to Pătru et al. 2006) are specific local factors favoring the natural hazards occurence, under a transitional climate between temperate oceanic and continental, with regional differences induced by orography and external influences (oceanic in the western part, Mediterranean in the south-west and south, Baltic in the northern part, excessive continental in the east and Pontic—related to the Black Sea—in the south-east).

Fig. 9.1
figure 1

Data source for b and c: EM-DAT 2016

a Map of major relief units in Romania; b distribution of number of disasters by types (1900–2016); c damage distribution by type of top 10 disasters (1900–2016).

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The statistics (according to EM-DAT database, 2016, for the period 1900–2016) on the number of disasters by types in Romania show that out of a total of 94 events, 55% were induced by floods, 21% by extreme temperatures, 14% by earthquakes, and 7% by storms (Fig. 9.1b). The floods were responsible for more than half (57%) of the total damage costs related to the top 10 disasters in Romania (from 1900 to 2016), the earthquakes for about one-third (34%), and the droughts for 9% (Fig. 9.1c). The floods are the leader in the hierarchy of total affected people distribution by type of top 10 natural hazards in Romania. They hold 80%, followed by earthquakes (20%) (the floods occupy 9 of the 10 places in this top). Concerning the number of fatalities, the first place is occupied by earthquakes (61%), followed by floods (33%) and extreme temperatures (6%).

At European spatial scale, Romania is in the top 10 of flood disasters statistics. According to EU (2014), between 2000 and 2013, Romania was the country with the highest number of fatalities caused by floods (183, representing 19% of the total number of deaths across the 28 EU Member States) (Fig. 9.2a). With 20 flood events recorded in the above mentioned period, Romania occupies the seventh place among EU Member States. The same place is occupied if considering the total costs overall flood events, estimated at 6300 million Euros (Fig. 9.2b).

Fig. 9.2
figure 2

Data source EU 2014

Number of fatalities related to floods (a) and total costs over all flood events (b) in EU Member States (2001–2013).

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3 Floods and Fluvial Flooding

Flood (or flash flood) is a phenomenon specific to the river flow regime while flooding (or inundation) may be generated both by rivers (fluvial flooding) and other causes: heavy rains or storms (pluvial flooding), sea advancement or encroachment (coastal flooding), groundwater rising, etc. In Romania, flooding is related mainly to river overflow during the periods with large discharges or ice jam.

Generally, the mean multiannual discharges of the Romanian rivers are less than 25 m3/s, and only four rivers exceed 100 m3/s: Siret (222 m3/s), Olt (180 m3/s), Mureş (179 m3/s), and Someş (121 m3/s) (Gâştescu et al. 1983). During the most important floods, maximum discharges of major internal rivers exceed 2000–3000 m3/s (even 4000 m3/s—Siret River). The Danube River, which develops on the Romanian territory its last 1075 km (i.e., 38% of its entire length of 2780 km) before reaching the Black Sea through the Danube delta, has a mean multiannual discharge rising from 5352 m3/s at the entrance in the country to about 6450 m3/s before entering the delta, but during the largest floods, the maximum discharge exceeded 15,000 m3/s (Şerban et al. 2006; Sălăjan et al. 2011; Zaharia and Ioana-Toroimac 2013).

At the country scale, floods may occur every season, but they are more frequent in spring (30–50% of the total annual events) and summer (25–40%). In autumn and winter, it happens only 10–20%, respectively, 5–30% of the annual floods (Diaconu 1971a). The mean number of annual floods is 6–7 in the mountains and about 3 in the lowlands (Diaconu and Şerban 1994). The annual peak has mostly pluvial origins, with regional differences: between 50% in the north, and 100% in south-eastern regions. The annual floods originating from snow melting are less frequent (3–4% in the west and 0–2% in the rest of the country), while the mixed origins count for 25–30% in south and east, and for 10–15% in south-western areas (Diaconu 1971a; Diaconu and Şerban 1994).

In the small catchments, located mainly in mountainous regions, heavy rainfalls generate summer flash floods, lasting several hours. They have a high destructive potential, frequently amplified by dislocated material, turning sometimes into mud and debris flows. Some flash flood events had catastrophic consequences: the flash flood of 07/11–12/1999 from Râul Mare River’s upper watershed (Southern Carpathians) led to 13 deaths and 24 injured (Bălteanu et al. 2004); the flash flood of 06/20/2006 from Ilişua River catchment (Someşul Mare watershed, in the western side of the Eastern Carpathians) caused 13 fatalities (Şerban et al. 2010); the flash flood of 09/05/2007 on the Tecucel River (south of Moldavian Plateau) affected Tecuci town, where three inhabitants died and over 200 houses were completely destroyed (Zaharia et al. 2009); the floods of 07/22–27/2008, on many rivers in the NE Carpathians (Suceava county) caused one death and major material damages (Bostan et al. 2009).

Within middle- and large-size watersheds (thousands of km2), flood’s duration is a few days or even few weeks. The Danube River, with a watershed of more than 800,000 km2, generates slow floods, lasting up to two months (i.e., the flood of spring 2006), deriving usually from high amounts of spring precipitation overlapped to snow melt.

The largest floods and related inundations had dramatic consequences across many regions in Romania. One can mention as major inundations those occurred in: May 1912 (in Banat region), April and July 1932 (Crişuri and Mureş catchments), July 1940 (Ialomiţa watershed), July 1941 (Argeş—Vedea catchments), July 1969 (Upper Siret and Upper Prut watersheds), May 1970 (Someş—Tisa catchments), October 1972 (Jiu watershed), July 1975 (Olt, Argeş—Vedea, Ialomiţa—Buzău, Prut watersheds), August 1979 (Prut catchment), December 1995–January 1996 (Someş River watershed and Banat region), April, July, and October 2005 (almost the entire country), April–May 2006 (Danube River), July 2008 (Siret and Prut catchments), June–July 2010 (Prut, Siret, and Danube rivers), June–July 2014 (Jiu, lower Olt, and Argeş catchments), October 2016 (lower Siret catchment) (Mustăţea 2005; Şerbu et al. 2009; ANAR–INHGA 2012) (Fig. 9.3). Most of these floods are included in the Catalog of large floods in Europe in the twentieth century (Chorynski et al. 2012).

Fig. 9.3
figure 3

Data sources Mustăţea (2005), Mihailovici (2006), Oprişan (2006), Şerban et al. (2006), ANAR –INHGA (2012), Gâştescu and Ţuchiu (2012)

Largest three flood peaks recorded in Romania at the scale of hydrographical districts, and on the Danube River (the map). Data on the number of floods (exceeding the flooding stage), flooded areas and number of flooded houses (the table; reference period: 1970–2008).

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In recent years, an increase of frequency of large floods (with return period between 20 and 100 years) and their consequences has been identified in Romania. Thus, five of the nine floods included on the list of top 10 disasters by total people affected (recorded between 1900 and 2016), occurred in the period 2000–2006 (according to EM-DAT 2016). An increasing of flood damage costs was found after 2004 (Fig. 9.4).

Fig. 9.4
figure 4

Data source Rădulescu et al. (2017)

Damage costs caused by flood events in Romania (1994–2016)

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In Romania, 83% of the administrative units (communes and towns) were affected by flooding between 1969 and 2008 (ANAR–INHGA 2012). In 2010, about 100,000 people have been exposed to floods (Pińskwar et al. 2012).

Many Romanian rivers reached their historic discharges in 2005, when the highest economic losses ever caused by floods and inundations in Romania were recorded: up to 1.3 billion Euros (MMGA 2006; Rădulescu et al. 2017) (Fig. 9.4). The floods in 2005 had large spatial and temporal extension affecting most of the country during seven successive episodes all year long (Mihailovici et al. 2006a; Zaharia et al. 2006). The most affected regions were south-west (Banat region), in April, Lower Siret Valley (in July), Argeş—Vedea and Ialomiţa catchment (in September). For example, in September 2005, the liquid discharge of Vedea River (in the central part of the Romanian Plain) exceeded 14 times the monthly average of the period 1961–2006, triggered by precipitation amounts over 4 times higher than the monthly average, at Alexandria weather station (Grecu et al. 2010).

In terms of human losses, the May 1970 floods killed the most people (215), while 76 deaths were registered in 2005 (EM-DAT 2016; MMGA 2006). Significant number of fatalities (108 persons) followed the flash flood of 07/29/1991, which caused Belci dam failure (in Eastern Carpathians), as well as the flood of July 1975 (60 death) (EM-DAT 2016; Mustăţea 2005). For discharges corresponding to the hundred-year return period, the total flooding area in Romania is estimated at 1028 million ha (5.8% of country’s area), and the number of people exposed to flooding risk is about 929,000 (Oprişan 2006).

The flood of April–May 2006 can be considered historical for the Danube River. At the entry in Romania (at Baziaș gauging station), it was recorded the highest value since its establishment (in 1838), namely a maximum discharge of 15,800 m3/s on 04/15/2006 (Mihailovici et al. 2006b). Along the Lower Danube, the maximum discharges in 2006 outclassed the hundred-year return period for many gauging stations (Baciu et al. 2006; Mihailovici et al. 2006b) (Fig. 9.5). The inundations affected 154 localities, and about 16,000 people were evacuated. Nearly 2000 houses were completely destroyed, and 443 were damaged (MMDD 2008). On the Lower Danube, historical discharges were registered during the flood of July 2010: 16,220 m3/s at Galaţi and 16,240 m3/s at Isaccea (Anghel et al. 2010). According to Şerban et al. (2006), the greatest discharges on Lower Danube were recorded in 1897, namely 17,000 m3/s. In Fig. 9.6, the variation of the Danube River water level during the largest floods in the last half century (1970, 1981, 2006, and 2010) is presented.

Fig. 9.5
figure 5

Data source Baciu et al. 2006

Variability of the maximum flow in 2006 (Qmax 2006) at gauging stations situated along the Romanian Danube River, compared with the average maximal discharge (Qmax av), the average discharge (Qav), and the 100 years return period discharge (reference period: 1931–1994)

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Fig. 9.6
figure 6

Data source Anghel et al. (2010)

Variability of the water level (H) at gauging stations situated along the Romanian Danube River during the largest floods in the last half century.

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4 Low-Waters and Hydrological Droughts

Low-waters and hydrological droughts affect water resources as a consequence of precipitation deficit, but other factors, such as water freezing, permeable lithology, or water exploitation, are favoring such processes. In Romania, the low-waters and hydrological droughts are specific to summer–autumn as a consequence of precipitation deficit, and to winter, due to low amounts of precipitation and ice phenomena.

Depending on the frequency and duration of the hydrological droughts, the Romanian rivers are classified into permanent, semi-permanent (droughts only during very dry years), intermittent, or temporary (droughts every year) (Ujváry 1972). Permanent flow develops in mountainous rivers with catchments larger than 20 km2, and semi-permanent flow occurs in the Subcarpathian hills and plateaus, where hydrological drought can last approximately 30 days in watersheds smaller than 1000 km2 and 5–10 day in larger ones (Gâştescu et al. 1983). The hydrological drought reached 67 days in the Râmna River watershed in Curvature Subcarpathians (415 km2) in 1952 (Zaharia 1999). Temporary flow is the result of heavy or long rainfalls and snowmelt, and it is specific to the western part of the Romanian Plain (Oltenia), the central and north-eastern part of the Moldavian Plateau (Bârlad), Piedmont areas and South Dobrudja (Diaconu and Şerban 1994). As relatively wide watersheds affected by hydrological droughts, one can distinguish: Vedea, Teleorman, Călmăţui, Sărata (in the Romanian Plain), Başeu, Bârlad, Vaslui (in the Moldavian Plateau), Teliţa, Taiţa, Casimcea (in the Dobrudja Plateau), and Amaradia (in the Getic Plateau). Spatial distribution of rivers according to drought types and data on droughts annual frequency can be found in the Atlasul secării râurilor din România (Atlas of River’s Droughts of Romania) (IMH and IGFCOT 1974). Minimum annual mean monthly discharge with an exceeding probability of 95% (dilution flow) varies between 1 and 7 l/s.km2 in mountainous regions and less than 0.1 l/s.km2 in plateau and plain regions from south and east (Diaconu 1971b).

Between 1850 and 2008, four long periods of hydrologic deficit occurred in Romania (1858–1866, 1888–1908, 1942–1954, and 1982–1996 (Stanciu 2004); several years were excessively dry (1904, 1946, and 1990) (MADR 2008). After 2000, very dry years (2002, 2003, 2007, and 2012) alternated with years of high flows and historical floods (2005, 2008, and 2010). In the last decades, the increasing frequency of dry years and duration of hydrologic droughts have boosted the risk to associated hazards, especially in the western part of the Romanian Plain and in the Moldavian Plateau (MADR 2008).

In the case of Danube River, low-waters are specific to autumn and winter. During 1930–2010, the lowest flow recorded on the Romanian sector of Danube ranged between 990 m3/s (at Gruia 1985) and 1790 m3/s (before entering the delta, at Ceatal Izmail in 1947) (Gâştescu and Ţuchiu 2012).

The consequences of hydrological droughts, unlike those of floods, are hard to quantify, due to their complexity, higher spatial and temporal scales, as well as their relation to meteorological and pedological droughts. They affect social and economic activities, such as water supply for various demands and transportation on the Danube River. In Top 10 Natural Disasters in Romania, the complex drought of 2000 (meteorological, hydrologic, and pedologic) ranks the fourth place in terms of economic damages costs, estimated to 500 mil US$ (EM-DAT 2016).

5 Other River-Induced Hazards

Besides the extreme flows (high and low), rivers may cause negative impacts on society and environment by other damaging phenomena, such as water freezing/unfreezing, ice jams, sediment, and channel dynamics (erosion and deposition processes), as well as activation of slope geophysical processes (e.g., landslides, rockfalls, debris flow, and mudflow).

Freezing phenomena affect river flow, causing either floods and inundations, or low flow (up to river completely frost), leading to water supply deficiencies. The average total duration of river ice ranges from 20–40 days in the western part of Romania, to over 80–100 days in Northern Carpathians and Moldavian Plateau. In the winters 1953–1954 and 1963–1964, the river ice maintained for almost 150 days on rivers from eastern Transylvania and Moldavian Plateau (Gâştescu et al. 1983). Maximum duration of river ice may exceed 150 days, i.e., 151 days in winter 1986–1987, on Upper Bistriţa River, in Eastern Carpathians (Giurma and Ştefanache 2010).

On the Lower Danube, sheet ice maintains in average 15–20 days, but during the very cold winters, it can last up to 80–85 days (Miţă 1986), affecting the navigation and water supply in riparian localities.

The ice jams (called zăpoare, in Romanian) may cause severe flooding, so that they are important threats to settlements. The Bistriţa River (in the Eastern Carpathians) is representative for the high frequency of ice blocks. Their maximum duration was 84 days in the winter 2002/2003; ice blocks can reach 5–7 m height in critical areas (Giurma and Ştefanache 2010).

Sediment dynamics and reservoirs’ silting may cause more or less significant economic and environmental damages. In Romania, the annual mean specific loads of suspended sediment range from less than 0.5 t/ha/year in mountainous regions and plains to over 20–25 t/ha/year in the Curvature Carpathians, with a national average of 2.06 t/ha/year (Diaconu 1971b; Mociorniţă and Birtu 1987). Reservoir dams stored about 200 million m3 of silt during 15 years, with an annual rate of 13.4 million m3, which represents more than 1/4 (27%) of the mean annual suspended sediment load in Romania (Rădoane and Rădoane 2005; Zaharia et al. 2011). The most affected are cascaded reservoirs on Olt and Argeş rivers, which filled almost half of their total volume with sediments. On the Argeş River, five of the 13 reservoirs are more than 70% silted, and one (Ogrezeni) is completely clogged (Teodor 1999; Rădoane and Rădoane 2005). Reservoirs’ silting affects their water storage capacity and functionality, therefore generating economic losses.

Directly related to water and sediment flow is the channel dynamics, which is particularly active during floods. Erosion processes may affect the riverine area (buildings, infrastructures) causing economic damage. Sediment accumulation reduces the transport capacity of the river channel, favoring the flooding. Withal, sediment accumulation has negative consequences for fluvial navigation (e.g., on the Danube River). A recent study (Grecu et al. 2017) showed the role of floods and flooding on the river channel dynamics in Romania. As a result of large floods of 1970 and 1975, in Eastern Carpathians and Moldavian Subcarpathians, the river channels have incised by: 50 cm on Bistriţa River (at Frumosu), 80 cm on Trotuş River (at Ghimeş-Făget), 80 cm on Tazlăul Sărat River (at Lucăceşti), and 80–100 cm on Putna River (at Tulnici) (Rădoane et al. 1991). During the June–July 2010 flood, Siret River’s channel (at Lespezi and Lungoci gauging stations) had aggradations ranging from 15 to 100 cm and degradations from 65 cm to 200 cm, in different flow phases (Obreja 2012). The flash flood occurred in Ilişua River basin on June 20, 2006 caused significant changes in the morphology of the river channel: in some places, it has widened by 2–3 m and up to 10–12 m; the sediment accumulations (mud, sand, clay, and gravel) have reached 0.2–1.5 m of thickness (Purdel 2011). Floods may also determine the evolution of braided islands; as an example, on a Subcarpathian reach of the Slănic River, after the flood of 2015 with a return period of approximately 7 years, braided islands reduced their number, grew in size, and became more elongated, therefore confirming their general model of evolution after disturbances by merging of several patches (Ioana-Toroimac 2018). In lack of high-magnitude and low-frequency floods, river channels are retracting especially when human pressures diminish the water and sediment resources; for example, the analysis of the lateral dynamics of the river channels in the area located at the contact between the Curvature Subcarpathians and the Romanian Plain have shown the narrowing of the active braided channels in the period 1980–2005: the mean width of the Prahova River channel diminished by 44%, the one of Milcov River by 43%, and the one of Cricovul Dulce River by 18% (Grecu et al. 2014).

6 Management of River-Related Risks in Romania

Reducing the negative consequences of river-induced hazards has become an important concern in the national water resources and related risks management policy. As a result of the significant damages caused by these hazards in the last decade (mostly floods, inundations, and droughts), Romania adopted some strategies and legislative regulations aiming to mitigate the risks induced by such phenomena. These strategies are consistent, generally, with the European Commission’s Directives, of which, in the field of water management and associated risks, the most representative are EU Water Framework Directive (WFD) 2000/60/EC and the EU Directive 2007/60/EC on the assessment and management of flood risks (Flood Directive–FD). In Romania, the most important document regulating the flood risk management is the National Strategy for Flood Risk Management (NSFRM) for medium and long term (adopted in 2010). According to the requirements of the FD, in December 2015, Flood Risk Management Plans were elaborated for all the 11 hydrographic districts (River Basin Authorities) of the country (they were approved by the Government Decision 972/2016). These plans contribute to the achievement of the NSFRM objectives. Concerning the droughts, in 2008, the National Strategy for Drought Mitigation, Prevention and Combating Land Degradation and Desertification, on short, medium and long term was adopted. Unlike the previous documents, which emphasized the development of protection works (structural measures), the new strategies are more oriented toward non-structural measures and to develop society’s capacity to adapt to extreme hydrometeorological hazards. In connection with this, noteworthy is National Strategy for Climate Change and the related National Action Plan on Climate Change, elaborated for the first time in 2005, and updated in December 2015. In 2008, the Guide regarding the adaptation to climate change effects was adopted; it foresees, among others, measures for flood risk management, including both hard (structural) and soft (non-structural) measures (Zaharia and Toroimac 2016).

7 Conclusions

In spite of the economic advantages they have, the rivers are responsible for significant societal and environmental damages. The most widespread and damaging river-induces hazards are the floods and flooding which cause annually major economic losses and human victims worldwide. This chapter focuses on Romania, European country with high exposure to river-related hazards, with the highest number of fatalities caused by floods (183) recorded between 2000 and 2013, representing 19% of the total number of death across the 28 EU Member States (EU 2014). At European spatial scale, Romania is in the top 10 of flood disasters statistics.

The chapter presents, in a synthetic way, the particularities of the specific river-born hazards in Romania, as floods and flooding (the most common river-induced hazards), low-waters and hydrological droughts, as well as other damaging phenomena (water freezing, ice jams, sediment, and channel dynamics reservoirs’ silting), with relevant examples for the different hazards and references to their impacts.

In recent years (especially after 2004), an increase in the frequency of large floods and their consequences has been identified in Romania. Reducing the negative impact of these hazards has become an important concern in the national water resources and related risks management policy. In this sense, some strategies and action plans were adopted, of which the most relevant for flood risk mitigation are the National Strategy for Flood Risk Management for medium and long term (adopted 2010) and Flood Risk Management Plans (approved in 2016).