5 reasons why flash floods and landslides devastated the northern mountainous region

According to experts, terrain, slope, geology, and the effects of climate change with heavy rains after storms are the causes of flash floods and landslides in mountainous provinces.

More than a week after Typhoon Yagi and its circulation caused floods and landslides, many northern localities suffered heavy damage, including 292 deaths, 38 missing and 1,908 injured. Lao Cai was the most severely damaged province, with 118 deaths and 50 missing. The landslide and flash flood in Lang Nu, Bao Yen district on the morning of September 10 alone flattened 33 roofs, killing 52 people and leaving 14 missing. Some northwestern mountainous provinces have also become "hot spots" suffering from serious flash floods and landslides, such as Yen Bai and Cao Bang.

Assoc. Prof. Dr. Tran Le Luu, coordinator of the Master's Program in Water Technology, Reuse and Management (Vietnam-Germany University), said that there are many causes of flash floods , but mainly due to heavy rain and water accumulation on the tops of mountains with weak geological foundations after heavy and long rains. When absorbing a lot of water, the bonds in the soil on bare hills become loose and fall apart, causing landslides and flash floods.

According to Associate Professor Luu, there are many factors that determine the danger level of flash floods such as: steep terrain, weak geological foundation, faults or lack of large vegetation and forest trees to hold the soil. However, the most dangerous is still the heavy rainfall on the mountain that cannot drain down in time, accumulates, breaks out and causes flash floods. This phenomenon is similar to the bursting of a water and soil reservoir on the top of a hill.

"Flash floods in mountainous areas and steep slopes are often more severe due to the large amount of landslides and the large amount of accumulated water ," according to Mr. Luu. When the large gravity of water acting on a slope exceeds the resistance, a landslide will occur. Therefore, the process will often develop quickly and unexpectedly, leaving people unable to prepare to respond and evacuate.

Mechanism of formation and movement of flash floods. (Photo: NVCC).

Master Thai Ba Ngoc, an expert in environmental geology at Ho Chi Minh City National University, said that flash floods and landslides are both types of geological disasters of exogenous origin.

Flash floods often occur in steep terrain such as foothills or valleys. This dangerous phenomenon also occurs in terrain with low vegetation density, leading to unstable soil surfaces. Flash floods occur when both factors are present at the same time, including loose, poorly bonded soil and rock in the flow path and the appearance of a flow with a speed large enough to carry it.

Scene of flash flood in Lang Nu village, Phuc Khanh commune, Bao Yen district, Lao Cai province, September 12. (Photo: Ngoc Thanh).

Landslides are the result of natural geological tremors, weathering, or changes in soil moisture, displacement of the protective structure of the foot of the slope. The result of natural tremors is the loss of the connection between soil and rock on hillsides and mountains in areas with steep terrain, weak geology with large voids, sandy/rocky soil and sparse forests. It can also be due to high mountainous terrain, steep slopes, and construction without studying geological factors. For example, in the process of producing one-crop rice in terraced fields, water seeps deep into the ground, making the soil loose, when water flows into the fields, it makes the soil soft. Prolonged heat causes cracks in the ground, when there is heavy rain, it will create slides causing landslides or landslides, rocks slide quickly from the slope, the slope down causing the entire hill to slide for kilometers.

According to Mr. Ngoc, rainfall is often the initial cause of geological disasters. After a long period of rain, rocks and soil on the mountainside slide down to the stream bed, accumulate, forming a natural dam, forming a lake on the mountain, causing rocks and soil at the bottom and walls of the lake to be soaked for a long time. When it continues to rain for a long time, the amount of water accumulated increases, causing the dam to break, creating a flood with a mixture of water, mud, rocks and trees flowing rapidly, destroying all obstacles in the path of the flood. When encountering flatter terrain, the channel widens, the flow slows down, and rocks and soil materials accumulate and cover the entire area.

However, he said that although the trigger for landslides is heavy rain, whether or not landslides occur depends on the nature of the terrain, slope, and geology in different places. This also explains why provinces such as Lao Cai, Yen Bai, and Cao Bang suffered flash floods and landslides.

MSc. Ngoc said that, geologically, most of the area of ​​the Northern mountainous region is made up of ancient rocks that have been strongly weathered. The weathered crust like soil reaches a depth of 15 - 30 m. This crust often contains clay minerals (especially montmorillonite) that change their properties very strongly, especially swelling greatly when there is water, determining the easy deformation and disintegration properties of this type of soil.

In particular, when the North suffered a prolonged heat wave (from April to July), the soil structure was significantly damaged. Next, the continuous long rains in August and early September due to the impact of storm No. 3 caused the already weakened soil structure to easily become saturated and mushy like mud when exposed to water. "The slopes in mountainous areas are naturally stable, but when exposed to unfavorable conditions, the soil's durability decreases and collapses, burying everything at the foot of the slope ," he said. When the slope is high, a large volume of soil collapses, causing serious consequences. This is also the cause of flash floods in Lang Nu.

According to Master Ngoc, depending on the length of the path and the slope of the terrain it slides through, the level of destruction caused by flash floods will be greater. When the flow of flash floods encounters large obstacles such as dikes and dams of constructions, the huge volume of water will bounce back, combined with the volume of water that continues to flow rapidly, creating situations such as whirlpools appearing at terrifying speeds that submerge all objects.

In addition, experts assess that the transition from El Nino to La Nina makes the weather complicated, along with the impact of climate change , making La Nina have abnormal factors, causing stronger impacts, causing heavy rain, floods, and landslides to occur seriously and for a long time. In addition, in rivers or steep slopes and slopes affected by human activities such as deforestation and cutting slopes, there is a common tendency for geological disasters to occur, Mr. Ngoc said.

Flash floods in Lang Nu village flattened everything. (Photo: Ngoc Thanh).

Mr. Thai Ba Ngoc added that preventing geological disasters such as flash floods and landslides is a difficult problem, because the terrain of these areas is very large with steep hills and mountains, and it is impossible to build enough structures to respond.

Accordingly, in mountainous areas, it is necessary to delimit and identify areas with high risks of flash floods and landslides. In the geological sector, localities must identify the focus in a province, where there are high risks to monitor and warn people. It is necessary to study, analyze and determine the spatial distribution of terrain types, classify slopes and geomorphological processes to clarify the main causes of each type of disaster, thereby providing a basis for zoning to forecast the potential for causing disasters.

Mr. Ngoc proposed that in the coming time, it is necessary to build a monitoring system that covers all points to warn of flash floods and landslides as well as a system of mud and rock dams and auxiliary works. To implement monitoring solutions, it is necessary to conduct pre-feasibility studies such as carefully surveying the geological structure, hydrogeology, and construction characteristics, clarifying whether there are active faults or not, and modeling and forecasting areas at high risk of landslides. For works in areas at high risk of landslides, it is necessary to cut drainage ditches on water flows from the foot of the mountain to the traffic road. At the same time, create many steps along the slopes - road slopes for appropriate design.

In the long term, Associate Professor Tran Le Luu said that the authorities need to do a good job of planning houses on mountain slopes and valleys. Do not build houses close to mountain slopes to prevent the risk of flash floods and landslides. It is necessary to plant forests to cover bare hills. Install stone cages and iron nets on mountain slopes to prevent landslides and soil erosion. Before each rainy and stormy season, specialized agencies need to conduct surveys and assess the possibility of flash floods in the area where they are living and have plans to prepare for drills and evacuation when necessary.

Regarding the flash flood in Lang Nu (Lao Cai), Associate Professor Luu said that after relocating the people, the authorities need to assess the possibility of further landslides in this area to come up with a suitable land use plan. He said that there should be environmental protection programs, forecasts, and drill plans to help people respond quickly to natural disasters that may occur soon.