Regular disasters in Viet Nam

Typhoons
Since 1954, 212 typhoons have landed on or affected Viet Nam. On average about 30 typhoons develop in the western Pacific Ocean each year, including roughly 10 generated each year in the South China Sea. Of these, on average four to six strike the coast of Viet Nam every year, between May and early December. Many years, Viet Nam is struck by 10 or more typhoons, for instance in 1964 (18 typhoons), 1973 (12), 1978 (12), and 1989 (10).
The areas most affected by typhoons are coastal provinces in the North and Centre of Viet Nam. However, typhoons in the South, though less frequent, can still cause immense damage. At the beginning of the typhoon season, in May and June, typhoons appear mostly in the North. As the typhoon season progresses, however, subsequent typhoons strike further and further south. Typhoons are most frequent in the period from June to October, and are very unpredictable. By November, the weather is cooler and this lowers the temperature of the surface of the sea. For a typhoon to develop, the temperature must exceed 26*C, so that as the weather cools, there are fewer typhoons.

Torrential rains in combination with strong winds
Seventy to eighty percent of average rainfall in Viet Nam (2500 mm a year) falls from July to September. The heaviest 12-hour rainfall recorded is 702 mm, and the largest recorded 48-hour rainfall is 1,217 mm. Torrential rains usually accompany typhoons with wind speeds of 170 km/h and higher. The intense rainfall from one of these typhoons, when added to already high water levels on rivers, can cause heavy flooding.

River floods
The annual runoffs of the 16 river basins draining Viet Nam do not vary greatly from year to year. However, there is a very large difference between the runoff in the minimum month (0.3% of the annual flow) and the runoff during the peak-flow month (up to 30%). For instance, the ratio between the average minimum and maximum monthly runoff for the Mekong River is 20, and for the Dong Nai River, 50. Peak flows, on any Vietnamese river, almost inevitably result river flooding. 

Flash floods
These are floods of short duration with a relatively high peak discharge. During storms, heavy rains can accumulate rapidly as floodwater in steep gorges, and flood waves can travel quickly causing severe, sudden destruction. Because they need only a short time to develop, they are difficult to forecast. 

Increased runoff
Inappropriate human activities -- notably deforestation and poorly planned agricultural and industrial development -- in water catchments may drastically intensify floods, particularly as a result of s. The floods are becoming higher and faster, increasing erosion and depositing silt downstream. 

Erosion and silting of river beds
Silting is an increasingly serious problem for water-control structures in Viet Nam. Artificial reservoirs are holding back sediment and inducing erosion of river banks downstream. The silting up of the reservoirs themselves is shortening their useful lives to a few decades. This can also reduce their effectiveness for retarding flood flows, thereby removing an important mechanism of flood mitigation for downstream areas. 

Slope instability, mudflows and landslides
Torrential rains in the mountains are eroding soils, causing severe landslides, and even flows of mud and stones downstream. These flows can arrive without warning, leaving little time for people to get out of the way, and often bury houses under soil and rock. 

Flooding from the sea
Typhoons are normally accompanied by storm surges, wind setup and wind waves. Strong typhonic winds generate large waves and increase the sea level at the coast even more by wind setup. By this process, the wind drives the surface waters of the sea to the coast, and raises the water level. During the last 30 years, half of the recorded typhoons caused a storm surge over 1 metre high, and 11% over 2½ metres high. On some very rare occasions, typhoons have caused surges several metres high. In some sections of the coast, the normal supply of sediment has been interrupted, and as a result, regular typhoon-generated waves are causing the shoreline to recede rapidly, allowing storm surges to penetrate further inland. 

Failures of water-retaining structures
Viet Nam has a long history of failures of river dykes inland, and of sea dykes along the coast. These have devastated villages and crops and caused loss of life and considerable damage to industry , commerce and infrastructure. 

Seawater intrusion into groundwater
Freshwater aquifers near the coast are being contaminated by seawater, partly through overpumping of groundwater, and partly because of reduced dry-season flow. This is severely disrupting agricultural enterprises and households which rely on groundwater during the dry season.
Of the above list, all except the last item deal directly with, or are associated with floods. Most of the country’s population of 71 million lives in the lowlands, which are flood-prone. Almost all the population and industrial infrastructure of Viet Nam is subject to floods.