Overview of Flood Studies in the World and Vietnam

CHAPTER 1. OVERVIEW OF STUDIES ON FLOODING IN THE WORLD AND IN VIETNAM

1.1. Some concepts of inundation and flooding

Flooding is the phenomenon of water infiltration caused by heavy rain on the surface of a low-lying area [ 20].

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Flooding is the phenomenon of inundation caused by water from a river overflowing into a normally dry area, due to large floods, due to the failure of flood control works or due to rising water in coastal estuaries [17], [18], [ 20].

Flooding is a general concept that includes both inundation and flooding.

Man-made floods are floods caused by humans due to improper reservoir operation or accidents at reservoirs built by humans.

Early season floods are floods that occur at the beginning of the flood season, when the humidity of the basin is low, the water level of the reservoirs is very low, and the loss of rain is very large. Therefore, despite heavy rain, the floods are usually not large. In the Ca River basin, early season floods often occur from July to August.

Mid-season floods , also known as main-season floods , are floods that occur in the middle of the flood season, when the humidity of the basin has increased significantly and the reservoirs have been filled with water to a certain level. Heavy rains occurring during this period cause major floods. In the Ca River basin, mid-season floods usually occur from September to October.

Late-season floods are floods that occur at the end of the flood season, when the humidity of the basin exceeds the highest level of the year, and the reservoirs are filled with water. However, the rain during this period is usually not heavy, so it rarely causes particularly large floods, but sometimes it is still capable of causing large floods. In the Ca River basin, late-season floods often occur in November.

Heavy rain is a phenomenon of rain with a total rainfall of over 50 mm in 24 hours, including rain with a total rainfall of over 50 mm to 100 mm in 24 hours.

is heavy rain, rain with a total rainfall of over 100 mm in 24 hours is very heavy rain [47]. Normally, rains with a total rainfall of about 220 - 250 mm in 3 days will cause flooding in the downstream of the Ca River.

1.2. Overview of flooding abroad and in Vietnam

1.2.1. Flood situation abroad

Natural disasters often occur almost everywhere in the world and in Vietnam, and are increasing in scale, frequency and intensity. Natural disasters and their impacts on the economy, society and environment are increasingly alarming worldwide. One of the natural disasters that greatly affects human life is flooding. Especially in recent decades, when climate change has shown clear signs, the impact of flooding has become increasingly severe.

The prevalence of natural disasters of hydrometeorological origin, especially floods. Some examples of this problem have occurred in the world: Thailand, China, the Netherlands, the United States, some African and Oceanian countries.

In Thailand, there have been eight major floods in the past 100 years (1938, 1983, 1995, 2010, 2011, 2013, 2014, 2017). In 2011, historic floods caused severe flooding in the Chao Phraya River basin as well as the Mekong River, starting in late July and continuing for more than two months. Flooding affected about 6 million hectares of land, more than 300,000 hectares of agricultural land, in 58 provinces, from Chiang Mai in the North to areas of the capital Bangkok near the branches of the Chao Phraya River. This was described as "the worst flood in terms of water volume and number of people affected". Seven large industrial zones were flooded up to 3 meters deep and lasted for about 40 days [23].

In China, where there are many large river systems, there have been many terrible floods in history, causing huge disasters. The flood of 1870 was the largest flood on the Yangtze River, in Yichang with a basin area of 1,010,000 km 2 , the peak flood flow was determined.

110,000 m 3 /s and caused great devastation in Shanghai [93]; In August 1931, China experienced heavy rains and caused a historic flood, causing the deadliest natural disaster in Chinese history and inundating 180,000 km 2 [83]; From June to September 1954, the Yangtze River flood caused catastrophic flooding, mainly in Hubei Province. The flood level reached a historic high of 44.67 m in Jingzhou, Hubei and 29.73 m in Wuhan [93]; In 2020, heavy rains and floods occurred again in the Yangtze River basin, many Chinese cities along the river issued red alerts due to rising floodwaters, causing severe landslides, inundating and destroying many roads and crops. Since June, the water level of more than 400 rivers in this country has risen to alarming levels, even the water level of 33 rivers has broken the record of the historic flood in 1998. The cause is believed to be the Mei-yu cloud band that stretches from Tibet through southern and central China to southern Japan from late spring to summer combined with the impact of climate change. Heavy rain in China occurs along the Mei-yu cloud band. This flood lasted for more than 1 month [22].

On the Yellow River , in 1887, a flood caused a dike to break, resulting in a major flood. Due to the low-lying plains nearby, the flood spread rapidly across northern China, covering an area of about 130,000 km 2 , blocking agricultural settlements and commercial centers. After the flood, two million people were left homeless [92].

In Europe , some major floods were listed by Rudolf Brázdil et al. [102] as follows:

In Western Europe (Belgium, Denmark, the Netherlands, Ireland and the United Kingdom) historic floods were recorded between September and October 1763, causing flooding in Ireland, England and Scotland, due to heavy rainstorms; January 1809 saw major flooding across much of England, Wales and the Netherlands, and glacial melt floods were also recorded in Central Europe, notably in Bohemia on the Vltava and Elbe rivers, in Slovakia on the

River Danube. In the UK, high levels of flooding were recorded on a number of rivers: the Thames, Severn, Trent, Exe and Eden, which span England.

In Central Europe : In July 1342, a widespread flood occurred, considered the most severe in Central Europe in a thousand years, caused by heavy rains lasting eight days. Major floods occurred in the river basins: Mainz, Neckar, Werra, Fulda, Elbe, Danube and the Rhine to Mainz. The peak flood module could reach 160-180 l/skm 2 on the main stream, leading to widespread erosion. Floods in February and March 1784: due to the winter of 1783/1784 that lasted over much of Europe. In February 1784, a warm southerly wind led to a sudden thaw across Europe, as well as on glaciers. In addition, there were heavy snowfalls and heavy rains, causing catastrophic floods across Europe. In Central Europe, flooding affected large areas of Germany, Austria, the Czech Republic and Slovakia, Belgium, the Netherlands, Luxembourg and northern France. The flooding caused heavy damage to property (destroying many bridges, factories and houses) and people. It was the largest winter flood on record.

In the Mediterranean : The catastrophic flood of November 1617 was one of the largest on record in the coastal areas of Valencia and Catalonia. In Northern Europe: A major landslide occurred in September 1345, blocking the river Gaula at Sør-Trøndelag in Norway and a lake quickly formed. When the blockage was cleared, a massive flood moved downstream towards the fjord downstream; In December 1743, a large area of western Norway was flooded due to melting snow and ice in the mountainous part of Norway. Water reached 2.5 m above the church wall, about 500 farms were damaged, mainly due to avalanches and landslides; In July 1789, Norway's most devastating flood, the Ofsen or Storofsen, occurred in a large area of eastern Norway, killing at least 79 people and causing extensive damage to more than 1,500 farms.

The cause was that in the winter of 1788/1789 there was a lot of snow, and in May 1789 the snow began to melt and heavy rains occurred due to storms.

In the Americas , in 1844, a record flood occurred on the Mississippi River in St. Louis. After the flood, Congress in 1849 passed the Marsh Act, which provided land grants for the construction of larger levees; In 1927, the most destructive flood in U.S. history, began when heavy rains fell on the Mississippi in the summer of 1926, and by September, the Mississippi's tributaries in Kansas and Iowa swelled. On New Year's Day 1927, the Cumberland River crest in Nashville reached a record 55 feet (17 m). The levees failed in 145 places and flooded 20,000 square miles (70,000 km2 ) , with some areas reaching depths of 33 feet (10 m); In 1965, a flood hit towns along the upper Mississippi, including Wabasha, Minn. This was the largest flood on the river in history. Among the worst affected was the town of Clinton, Iowa, which was flooded to 9 m; the severe flood of 1983, the second largest in the lower Mississippi basin in 56 years, at Red River Landing in Louisiana, which was flooded for 115 days; In 1993, floodwaters rose along the Mississippi and Missouri rivers and their tributaries in the Midwest, flooding an area of 80,000 km 2 and was the worst disaster to hit the United States in 66 years in terms of flood level, area flooded, number of people displaced, property damage, and record river volume [90].

In Africa, from 1997 - 2017 [27], major floods occurred in the years: 1997, 2000, 2001, 2010, 2015, 2017 (Figure 1.1 ).

The 1997 heavy rains in Ethiopia, combined with exceptionally heavy rains in southern Somalia - much more than the seasonal average - led to catastrophic flooding. Water levels

Figure 1.1: Some major floods occurred in Africa from 1997-2017 [ 27]

rising in the Shabelle marshes, creating a

Flooding covered more than 12 km2 [ 19]. In March 2019, floods also killed 732 people in South Africa (417 in Mozambique, 259 in Zimbabwe and 56 in Malawi) and caused major flooding [21].

In Australia, the statistics of floods from large to small are as follows: In June 1852 , Gundagai, New South Wales (NSW) experienced a particularly large flood on the Murrumbidgee River, which washed away most of the town of Gundagai, leaving only three houses. The town was later rebuilt on higher ground; From December 2010 to January 2011, floods occurred in Brisbane and South East Queensland (QLD), affecting more than 200,000 people across the state. In Brisbane alone, floods peaked at 4.46 m on January 13, inundating more than 28,000 homes and leaving 100,000 people without power; In December 1916 at Clermont and Peak Downs, QLD, a cyclone swept across the coast along the Whitsunday Pass, bringing heavy rain to Clermont, Sapphire, submerging the low-lying Clermont, so the town was later rebuilt on higher ground; in late November 1934 , 140 mm of rain fell in a 48-hour period in Melbourne, 350 mm in South Gippsland, causing landslides and flooding many roads; in February 1893 at Ipswich, QLD, commonly known as the Black February Floods, heavy rains were caused by tropical cyclone Bundinyong. The Crohamhurst weather station recorded 914 mm of rain and another measuring point recorded nearly 889 mm in Brisbane; In February 1927 , a tropical cyclone struck north of Cairns, causing heavy rainfall across Queensland, as far as Toowoomba. Torrential rain from 9 to 17 February caused major flooding in the area; in April 1929 , North Tasmania, the northern part of Tasmania, was prone to heavy rainfall in short periods, with up to 500 mm of rain falling over three days. The floodwaters carved a path through the area, destroying everything in their path, including vehicles, buildings and railway tracks; in February 1955 , the Hunter Valley, NSW, largely

River systems in the state were flooded. The Hunter Valley floods have become iconic in the Australian psyche for the dramatic nature of flood damage and rescue. About 15,000 people were evacuated from around these two towns; January–April 1974 in Brisbane, QLD, after a particularly wet year, the city of Brisbane was inundated when Tropical Cyclone Wanda made landfall north of the city on 25 January 1974. On 29 January, the Brisbane area recorded 900 mm of rain (314 mm in 24 hours); August 1986 , 327.6 mm of rain in 24 hours caused flooding on the Hawkesbury and Georges Rivers, NSW. Torrential rain flooded roads forcing many motorists to abandon their cars. Bus services were severely disrupted in the city and trains were stopped due to flooded tunnels.[26]

In short , floods are a type of natural disaster that occurs all over the world and causes huge losses in terms of human life and property. The scale of floods depends on the scale of the river basin (basin area, river length, etc.) and climatic conditions. Floods caused by melting ice are quite small in scale (Norway, in 1789, flooded 2.5m). In large river basins, such as the Mississippi River in 1927, flooded a large area of up to 70,000 km 2 , with some places flooded to a depth of 10 m. In large basins and due to the effects of ice creating natural dams that then break, floods are extremely large (Kuray River, Russia, peak flood flow up to 18 million m 3 /s [88]).

Therefore, flooding is an issue that scientists around the world have been, are and will be particularly concerned about.

1.2.2. Flood situation in Vietnam

Vietnam is a country that is regularly affected by natural disasters, especially natural disasters of hydro-meteorological origin such as: storms, tropical depressions, floods, flash floods, heat waves, severe cold, frostbite... Natural disasters have, are and will cause damage to both people and property, occurring frequently and widely throughout the country, because Vietnam is one of the countries with a geographical location that is "vulnerable". Moreover, in the context of climate change,

With the rapid development of economy, society and exploitation of irrigation and hydroelectric works... the negative impacts of natural disasters on the economy and society are increasingly serious.

According to [41], in the past 1,000 years, there have been 188 floods that broke the Red River dike, causing serious flooding in the Red River Delta, especially in the years: 1945, 1968, 1969, 1971, 1978. In the 19th century alone, there were 26 dike breaks, the most notable of which was the flood in 1893. In the 20th century, there were 20 dike breaks, especially the floods in 1945 and 1971. The flood in the Red River Delta in 1971 was an especially large flood. This is considered the largest flood in the past 250 years in Northern Vietnam. From August 12 to 21, 1971, a weather complex including the tropical convergence zone, the western hot trough, the Pacific high pressure and the circulation of a storm hit southern China, causing heavy to very heavy rains across northern Vietnam. The average rainfall measured in the Red River basin was 255 mm; the Thai Binh River basin was 247 mm. There were places with heavy rains, such as in Sin Ho 454 mm, Lao Cai 386 mm, Tan Cuong 678 mm. The average rainfall in the Northern Delta was about 200 mm. The downstream areas of the Red River and Thai Binh River experienced a historic flood, considered the largest since 1902 to 1999. On August 20, the flood peak in Hanoi was 14.13 m, exceeding the alarm level 3 by 2.63 m, and remaining above the alarm level 3 for 8 days. On the same day, in Pha Lai, the flood level reached 7.21 m, exceeding the level 3 alarm by 1.71 m, and the time it remained above the level 3 alarm was 12 days. The flood on the Day River system peaked at level 3. In Lao Cai province, on March 19, 1971, the flood level was recorded at 86.85 m. On August 19, flood water broke the dykes in the downstream of Lo River, Da River and the left bank of the Red River in Vinh Tuong district, and the Thanh Tri dyke on the right bank of the Red River. On the 20th, Lam Thao dyke broke; the next day, Vinh Lai dyke and Cao Xa dyke broke. Many other dyke sections were also broken with similar lengths.

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