According to experts, factors such as terrain, slope, geological conditions, and the effects of climate change, particularly heavy rainfall following storms, are the primary causes of flash floods and landslides in mountainous provinces.
More than a week after Typhoon Yagi and its aftermath, heavy rains led to significant landslides and flooding in several northern provinces, resulting in severe damage: 292 people dead, 38 missing, and 1,908 injured. Lao Cai province was the hardest hit, with 118 fatalities and 50 individuals unaccounted for. Specifically, the landslide and flash flood in Nủ Village, Bảo Yên District, on the morning of September 10, flattened 33 houses, leading to the deaths of 52 people and leaving 14 missing. Several provinces in the Northwest, such as Yên Bái and Cao Bằng, also faced severe flash floods and landslides.
PGS.TS Trần Lê Lựu, coordinator of the Master’s Program in Technology, Water Reuse, and Management at the Vietnam-Germany University, identified several causes of flash floods, primarily linked to heavy rainfall and water accumulation on hills with weak geological foundations following prolonged rains. When saturated, the soil structure on deforested mountains becomes loose and disintegrates, triggering landslides and flash floods.
According to PGS Lựu, various factors determine the danger level of flash floods, including steep terrain, weak geological foundations, fault lines, and a lack of vegetation and large trees to stabilize the soil. However, the most dangerous factor remains the heavy rainfall in mountainous areas, which cannot drain quickly enough, leading to accumulation, overflow, and flash floods. This phenomenon is similar to the bursting of a water reservoir and the soil on a hilltop.
“Flash floods in high mountainous areas with steep slopes are often more severe due to the significant volume of soil and rocks that slide down, along with the large amounts of accumulated water,” stated Mr. Lựu. When the gravitational force of the water on a slope exceeds the resisting forces, landslides occur. Consequently, these processes usually unfold rapidly and unexpectedly, leaving residents unprepared to respond or evacuate.
The mechanism of flash flood formation and movement. (Photo: NVCC).
ThS Thái Bá Ngọc, an environmental geology expert from the National University of Ho Chi Minh City, stated that flash floods and landslides are both types of geological disasters with exogenous origins.
Flash floods typically occur in steep terrains, such as at the foot of mountains or in valleys. This dangerous phenomenon is also present in areas with low vegetation cover, leading to unstable soil surfaces. Flash floods happen when both loosely bound soil and rocks on the flow path and fast-moving water currents are present simultaneously.
Scene of the flash flood in Nủ Village, Phúc Khánh Commune, Bảo Yên District, Lao Cai Province, on September 12. (Photo: Ngọc Thành).
On the other hand, landslides result from natural geological shocks, caused by weathering, changes in soil moisture, or shifts in the protective structure at the base of slopes. These natural shocks disrupt the cohesion of soil and rocks on steep hillsides and mountains in areas with weak geological conditions, high porosity, sandy/rocky soils, and sparse forests. They can also occur in high mountainous areas with steep slopes where construction projects are inadequately researched concerning geological factors. For example, the process of cultivating rice in terraced fields can lead to deep water infiltration, making the soil loose; prolonged heat can crack the soil, and heavy rains can create sliding paths that cause rapid landslides of soil and rocks, resulting in large sections of hillside sliding several kilometers.
According to Mr. Ngọc, rainfall is often the primary trigger for geological disasters. After prolonged rainfall, soil and rocks on mountain slopes may slide down into streams, accumulating and forming natural dams, creating lakes on the mountains. This leads to prolonged water saturation of the soil and rocks at the bottom and walls of the lake. Continued rainfall results in increasing water accumulation, causing dam breaches and creating floods with a mixture of water, mud, rocks, and vegetation that sweep away all obstacles in their path. When encountering flatter terrain, the flow widens, reducing speed, and sediment will accumulate, covering the entire area.
However, he noted that although heavy rainfall triggers landslides, whether or not they occur depends on the terrain, slope, and geological conditions in different locations. This also explains why provinces like Lao Cai, Yên Bái, and Cao Bằng experience frequent flash floods and landslides.
ThS Ngọc indicated that, geologically, most of the mountainous areas in the North are composed of highly weathered ancient rocks. The weathered layer, which can reach depths of 15 to 30 meters, often contains clay minerals (especially montmorillonite) that exhibit significant changes in characteristics, particularly swelling greatly when wet, which determines the susceptibility to deformation and disintegration of this type of soil.
Particularly, during a prolonged heatwave in the North (from April to July), the soil structure has been significantly compromised. Subsequently, the extended rainy periods in August and early September, influenced by Typhoon No. 3, caused the weakened soil structure to become easily saturated and flow like mud. “The slopes in mountainous areas are naturally stable, but under adverse conditions, soil strength diminishes, leading to collapses that bury everything at the foot of the slope,” he said. When a high slope collapses, the large volume of soil can cause serious consequences. This is also the reason for the flash floods in Nủ Village.
According to ThS Ngọc, depending on the length of the travel path and the slope of the terrain, the destructive power of flash floods can be greater. When the flow of a flash flood encounters large obstacles such as dikes or dams, the enormous volume of water will rebound, combining with the incoming torrent, creating conditions for whirlpools with tremendous speeds that can submerge all objects.
Additionally, experts assess that the transition from El Niño to La Niña has complicated the weather, along with the effects of climate change, causing La Niña to exhibit abnormal characteristics, leading to more severe and prolonged heavy rains, floods, and landslides. Furthermore, in rivers or steep slopes affected by human activities such as deforestation and slope cutting, geological disasters are more likely to occur, Mr. Ngọc noted.
Flash floods in Nủ Village flatten everything. (Photo: Ngọc Thành).
Mr. Thái Bá Ngọc added that preventing geological disasters such as flash floods and landslides is challenging, as the terrain in these areas is very vast with steep hills, making it impossible to build enough structures to respond adequately.
Therefore, in mountainous regions, it is necessary to delineate and identify areas at high risk for flash floods and landslides. In the geological sector, local authorities must focus on identifying key areas within a province where risks are high to monitor and warn residents. It is essential to study, analyze, and determine the spatial distribution of various landforms, classify slopes, and identify geomorphological processes to understand the primary causes of each type of disaster, thereby forming a basis for predicting areas with potential disaster risks.
Mr. Ngọc proposed that in the future, a comprehensive monitoring system should be established to cover various points for warning against flash floods and landslides, as well as constructing dam systems to prevent mud and rock flows and auxiliary structures. To implement monitoring solutions, feasibility studies should be conducted, including detailed surveys of geological structure, hydrogeological conditions, and determining whether active faults exist, and modeling to forecast areas at high risk of landslides. For projects in areas with high landslide risks, drainage channels should be constructed along water flows from the base of mountains to roads. Additionally, multiple terraces should be created along the slopes for appropriate design.
In the long term, PGS Trần Lê Lựu suggested that authorities need to improve housing planning in mountainous and valley areas. Houses should not be built close to mountain slopes to prevent the risk of flash floods and landslides. It is necessary to plant forests to cover bare hills. Install rock cages and wire mesh on slopes to prevent landslides and soil erosion. Before each rainy season, relevant agencies should conduct surveys and assess the potential for flash floods in their respective areas and prepare plans for drills and evacuation when necessary.
Regarding the flash floods in Nủ Village (Lao Cai), PGS Lựu stated that after relocating residents, authorities need to assess the potential for continued landslides in this area to devise suitable land-use plans. He emphasized the need for environmental protection programs, forecasting, and drill plans to help residents respond quickly to potential natural disasters.
