Characteristics, Identification, and Prevention of Landslides and Debris Flows

I. What is a Landslide?

A landslide refers to the phenomenon in which soil or rock layers on a slope move downslope as a whole or in discrete blocks, driven by factors such as river erosion, rainfall, seismic activity, and artificial slope cutting.

II. What is a debris flow?

A debris flow is a type of torrential flood that occurs in gullies or on slopes, driven by surface runoff generated by precipitation, dam breaches, or snow and ice melt, and which carries large quantities of solid materials such as mud, sand, and rock.

III. What are the characteristics of landslides and debris flows?

Landslides are characterized by “sliding” downslope, while debris flows are characterized by “flowing” along gullies; both are forms of mass movement driven by gravity, in which material moves from higher to lower elevations.

IV. How to Identify Landslides and Debris Flows?

01 Criteria for Landslide Identification

1. Topographic and geomorphic criteria: When chair-shaped, horseshoe-shaped landforms, or multi-level, irregular benches are developed on a slope, their shapes are markedly inconsistent with the surrounding slope; depressions occur in the upper part of the slope, while the toe of the slope extends further into the riverbed than the flanks; the headwaters of two gullies on the slope turn upward and then converge; these geomorphic features suggest that landslides may have occurred in these areas in the past. In addition, conspicuous cracks are present on the slope, which have recently lengthened and widened; buildings on the slope have developed cracks and tilting; and soil extrusion and frequent collapses occur at the slope toe. These geomorphic phenomena may indicate that a landslide is currently in the process of formation.

2. Stratigraphic criteria: In areas where landslides have occurred, the stratigraphy of the slid masses—whether rock or soil—is typically more disordered and the structure more loose compared with that of undisturbed slope sections.

3. Basis for groundwater analysis: Landslides can disrupt the integrity of the original slope aquifer, leading to changes in groundwater flow paths and discharge locations. When discrepancies are observed between the distribution of spring seepage zones on localized slope sections and those along the entire slope, or when a sudden increase in the number of springs occurs within a short period, or when existing springs abruptly dry up, these phenomena, in conjunction with other evidence, can be used to determine whether a landslide is currently developing.

4. Criteria for vegetation: When trees on a slope are leaning in all directions, this generally indicates that the slope has experienced a violent landslide; conversely, when the trunks of trees on the slope curve downward while the upper portions of the trunks grow vertically, this typically reflects long-term, slow creep deformation of the slope.

02 Criteria for Identifying Debris-Flow Gullies

1. According to the formation mechanism of debris flows, a certain amount of loose soil and rock is essential as the source material. Therefore, gullies and valleys with highly fractured and weathered slopes on both sides, abundant loose materials, prominent landslides and collapses, sparse vegetation, severe soil and water loss, and intense slope erosion are prone to debris-flow occurrences.

2. Topographic and geomorphic criteria: Only gullies that can collect substantial volumes of water and maintain high flow velocities are capable of accommodating and transporting large amounts of soil and rock. In the upper reaches, the gully is flanked by steep mountain slopes on three sides, with a funnel-shaped, spoon-shaped, or leaf-shaped planform; in the middle reaches, the valley narrows; and in the lower reaches, the gully mouth opens onto a broad, open terrain. Furthermore, when the elevation difference between the upper and lower parts of the gully exceeds 300 meters and the slope gradient on both sides of the gully is greater than 25 degrees, these topographic conditions are conducive to the formation of debris flows.

3. Basis for water source: Water provides the driving force for debris-flow formation. Areas prone to localized heavy rainfall, downstream of reservoirs and pond dams at risk of dam failure, and regions where seasonal snow and ice melt occur all offer conditions conducive to the rapid generation of large volumes of runoff, thereby facilitating debris-flow development. Among these, areas with frequent localized heavy rainfall exhibit the highest frequency of debris-flow occurrences.
If a gully is conducive to debris-flow formation in all three respects—source of material, topography, and water supply—it is unequivocally a debris-flow gully. However, the frequency, scale, and consistency of debris flows will vary in response to changes in these factors. Even gullies that have previously experienced debris flows remain at risk of future occurrences.

V. How to Prevent Landslides and Debris Flows?

Although landslides and debris flows are inevitable in mountainous environments, their adverse impacts can be mitigated through the implementation of proactive mitigation measures.

01 Avoiding landslides hinges on recognizing the warning signs.

Landslides are often beyond human control; however, by identifying the precursor signs that emerge before a landslide occurs—such as transverse and longitudinal cracks at the leading edge of the slope and bulging of the soil at the front margin; rapid widening and lengthening of cracks along the rear margin of the sliding mass, with new cracks continuously forming and the rear portion of the slide rapidly subsiding, accompanied by loosening and small-scale collapses or rockfalls in the surrounding rock and soil; audible frictional noises from the sliding zone due to shear displacement, along with the emergence of gas or water from cracks; the reactivation of previously blocked springs or the sudden drying up of springs and well water at the toe of the slope; abnormal panic among local wildlife; significant displacement of monitoring points on the landslide mass; the formation of drumlin-like mounds at the leading edge of the slide; and tilting or cracking of buildings, as well as the development of “drunken forests” and “saber-shaped trees”—people living near landslide-prone areas or traveling through such zones should evacuate promptly upon detecting these warning signs. In particular, if abnormal phenomena such as bulging of the soil at the slope’s leading edge or rapid widening and extension of surface cracks are observed, immediate evacuation and other protective measures must be implemented without delay.

02 When a debris flow is about to occur, run toward the sides of the slope.

Debris flows typically occur in specific debris-flow gullies; therefore, learning how to identify such gullies and staying well clear of them is essential for self-rescue. Signs include roaring sounds within the gully, a rise in the main stream’s water level, and the sudden cessation of normal flow. Abnormal behavior among flora and fauna—such as pigs, dogs, cattle, sheep, and chickens becoming agitated and unable to sleep, rats darting about erratically, and changes in plant morphology, including withering or leaning of trees—may also indicate an impending debris flow. If any of these warning signs are observed, particularly if cracks appear on the mountain slope, there is a potential risk of collapse or landslide, which, combined with prolonged or heavy rainfall, can trigger a debris flow. It is crucial to bear in mind that debris flows often erupt suddenly, leaving very little time to escape. Thus, upon hearing roaring noises in a gully or noticing a rise in the main stream’s water level accompanied by the abrupt stoppage of normal flow, one should immediately recognize the imminent threat of a debris flow and take immediate evacuation measures. During evacuation, it is important to choose the correct direction: do not run upstream or downstream along the gully’s axis, but instead head toward the slopes on either side of the gully bank—while avoiding low-lying concave areas.

VI. How to avoid flash floods and debris flows?

When it is determined that a debris flow will occur in a particular area over a specified period, emergency evacuation and protective measures must be implemented in that area, and all personnel must be forcibly relocated to safe zones.

Do not run upstream or downstream along the debris-flow channel; instead, flee to the slopes on either side of the channel, and avoid lingering in concave areas.

Construct temporary shelter shelters, ensuring that their locations avoid ditch banks and concave banks, as well as small, low-lying convex banks and the foot of steep slopes. Instead, they should be sited on gently sloping hillsides or higher terrace platforms close to the village.

VII. How to Quickly Escape When Caught in a Flash Flood Late at Night or in the Early Morning?

All residents living in areas prone to flash floods—such as gullies, canyons, and stream banks—must remain highly vigilant whenever there are prolonged, heavy downpours, especially at night. If any unusual signs are observed, they should immediately organize an evacuation, swiftly leaving the area for the nearest safe location, and make every effort to establish contact with the outside world to facilitate subsequent rescue operations. Under no circumstances should they take chances or attempt to salvage property at the expense of timely evacuation, as this could result in unnecessary loss of life and injury.

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