Hearing
Abnormal roaring sound from the mountain
Debris Flow Characteristics
- High flow velocity, high sediment concentration, strong scouring force, and high impact force.
- The surface of the debris flow moves faster, while the movement speed underneath is slower.
- The flow velocity of a debris flow is influenced by the particle size of the contained sediment, the ratio of solid particles to mud and water, and the gradient of the stream channel. The velocity of a gravel-type debris flow is approximately 3–10 m/s, while a mudflow-type debris flow ranges from about 2–20 m/s.
- The front of the debris flow is raised with a large volume, typically concentrated with large boulders, while the subsequent flow is often a mudflow.
- Within a debris flow, larger stones are generally distributed towards the top, and smaller stones towards the bottom.
- Because the stream valley outlet typically has a gentle slope and a greater width, which effectively reduces the debris flow's velocity, debris flows often form an alluvial fan (or fan-shaped deposit) in this area.
Causes of Debris Flow
- Sufficient Water: Adequate water acts as an effective lubricant, lowering the friction between sediment particles within the debris flow and facilitating the movement of solid materials.
- Abundant Sediment Deposit: A sufficient supply of loose soil and sand provides the necessary solid material for the debris flow.
- Adequate Slope: A sufficiently steep slope provides the gravitational energy needed for the debris flow to overcome the friction of the sediment and continue its downward movement.
What the Video Tells Us
The essential prerequisites for a debris flow include an abundant accumulation of upstream sediment, suitable topography, sufficient slope, and ample moisture. On geologically unstable slopes, large and small rocks, sand, and soil weathered from the mountainside accumulate in valleys or at the foot of slopes due to landslides, mass movements (or landslips), and rockfalls. If these accumulated materials on a steep slope are exposed to heavy rainfall, and the resulting water cannot drain effectively, the mixture of soil, sediment, and water will flow rapidly downhill under gravity, forming a destructive『debris flow』 with far-reaching consequences.
Signs of an Impending Debris Flow
Observe the Warning Signs of a Debris Flow with Your Five Senses
Sight
Stream water is abnormally turbid
Sight
Streamflow decreases abruptly
Smell
Odor of humus/decomposed soil
Touch/Vibration
Feeling ground vibration
| Time of Sign Occurrence | Sign/Indicator | Cause | |||
|---|---|---|---|---|---|
| Hours Before | One Hour Before | Minutes Before | Debris Flow Occurs | ||
| 1. Nearby landslide or debris flow occurs (Sight) | Indicates that the surrounding slopes and geology are already unstable | ||||
| 2. Sudden increase in torrent flow (Sight) | Heavy rainfall may be occurring upstream | ||||
| 3. Abnormal roaring sound from the mountain (Hearing) | Landslide or debris flow may have already occurred upstream | ||||
| 4. Abrupt decrease in streamflow (Sight) | The upstream torrent may have been blocked by collapsed sediment | ||||
| 5. Floating driftwood in the stream (Sight) | Landslide or bank erosion may have occurred upstream | ||||
| 6. Abnormally turbid stream water (Sight) | Landslide or bank erosion may have occurred upstream | ||||
| 7. Sound of rocks grinding in the stream (Hearing) | Caused by increased streamflow | ||||
| 8. Odor of humus/decomposed soil (Smell) | Uprooted trees or odor from the decomposed soil layer upstream | ||||
| 9. Sound of cracking trees (Hearing) | Sound of trees being struck and broken by an upstream debris flow | ||||
| 10. Abnormal animal behavior (Sight) | Animals have keen senses and may be reacting to abnormal natural phenomena that humans cannot yet perceive | ||||
| 11. Feeling ground vibration (Touch/Vibration) | Vibration caused by the rolling motion of the debris flow | ||||
| 12. Upstream sound of "Go" (Hearing) and a flash of fire or lightning-like light | Phenomenon caused by the impact of large boulders during the debris flow | ||||
| Hours Before | One Hour Before | Minutes Before | Debris Flow Occurs |
Legend: Will Occur High Likelihood Possible Likelihood |
|
Distinction Between Debris Flows and Ordinary Sediment-Laden Water Flow
| Characteristic | Debris Flow | Sediment-Laden Water Flow |
|---|---|---|
| Transport Mechanism | Bulk transport | Individual particle transport |
| Driving Force | Primarily the sediment mass | Primarily the water body |
| Boulder Distribution | Large boulders concentrated at the front | No distinct distribution pattern |
| Particle Composition | Wide range of particle sizes, from clay to large boulders/cobbles | More uniform particle size composition |
| Deposition Characteristics | Massive deposition, with coarse particles accumulating at the front | Deposits along the flow direction, exhibiting sorting, with finer particles distributed at the front |
| Sediment Concentration | Volumetric concentration approx. 0.28 or higher | Low volumetric concentration |
| Stream Bed Scour Pattern | Intense scouring of the streambed and both banks, vegetation uprooted | Exhibits general and localized scouring, vegetation remains present |
| Fluid Viscosity | High | Low |
| Movement Pattern | Tends to flow in a straight line, does not flow around obstacles | Depends on circumstances, often flows around obstacles |
Debris Flow Classification Methods
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