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Debris flows are unsteady and non-uniform multiphase flows, composed of a mixture of poorly sorted sediment in water, with a highly percentage of coarse material (e.g., sand, gravel, boulders), typically greater than 50%. Usually, in this type of two-phase flows, the amount of fine materials (e.g. silts and clays) in turbulent suspension is also high. In fact, the material incorporated in this type of flows is inherently complex, varying from clay sized solids to boulders of several meters in diameter.

Several authors tried to establish debris flow classifications according to flow characteristics and rheological behavior in order to avoid ambiguous approaches to the phenomena by scientific community.

For this purpose, Tamotsu Takahashi developed one of the most practical and objective debris flow classification proposes. According to Takahashi, there are a few different types of debris flows, which could be distinguished from site-specific geological and geomorphological characteristics. Generally shall be considered three types of debris flows:

• Stony-type debris flow
• Turbulente-muddy-type debris flow
• Viscous-type debris flow

Briefly, in stony-type debris flows is typically observed a multilayer flow in which collisions between particles are the dominant mechanisms in energy dissipation. On the other hand, turbulent-muddy-type flows are dominated by turbulent mixing stresses. Finally, viscous‑type debris flows are governed by viscous stresses, resulting in a flow with a viscoplastic behavior.

Referências bibliográficas:

Takahashi, T., 2007. Debris Flow: Mechanics, Prediction and Countermeasures. Taylor & Francis.

Generally, each debris flows surge is composed by three different parts: front, body and tail. Commonly, an abrupt bore forms the head of the flow consisting mostly of boulders, followed by a gradually tapering body, and finally a more watery tail. Behind the boulder front, the stage height and number of boulders gradually decrease, and the surge is charged with pebble-sized sediments and then becomes more and more dilute until it finally appears as muddy water.

According to Takahashi, this common configuration is frequently recorded in stony-type debris flows. Although, it can be noticed on turbulent and viscous-type debris flow with large boulders transport capacity as well. In these cases, large boulders are carried randomly in the flow body.

References:

Takahashi, T., 2007. Debris Flow: Mechanics, Prediction and Countermeasures. Taylor & Francis.

Debris flows are characterized by a dense mixture of water and solid material with a huge transport capacity, even for large boulders. This capability turns debris flows into a natural phenomenon with inestimable destructive capacity. This type of flows are potentially very destructive as they cause remarkable erosion of the substrates over which they flow, thereby increasing their sediment load and further increasing their erosive capabilities. They have the ability of pick-up and transport even large and well “secured objects” (e.g. vehicles, trees trunks, parts of infrastructures, large boulders), thereby giving rise to the potential for considerable damage.

Additionally, on relatively low gradient and decreased confinement areas (usually coincident with human settlements areas), debris flow can initiate significant solid material deposition, with the consequent loss resulting therefrom. In these conditions, streams and rivers can be completely filled with sediments promoting flow overtopping and adjacent lands flooding. Usually, this occurrence significantly increases the overall damages caused by debris flows events.

In fact, there are several historical records of completely buried settlements by debris flows events which carried thousands of tons of solid material.

Debris flows prone regions are geomorphological easy to recognize on the field since they generally present a very particular spatial configuration. Mainly they are formed by a source area, followed by a stream transport channel(s) and a depositional area. In general, debris flow follows the pre-existing runoff streams. However, they can also move down through steep slopes marking their own path by substrates erosion.

Data from: (Nunes & Sayão, 2014) e (Calligaris & Zini, 2012).

Typically source areas are characterized by steep slopes (often greater than 15º) mantled by a cover of unconsolidated soils or loose sediments and spare vegetation.

Once the mass failure starts, the debris will travel downhill, in general along steep slope streams and gullies. As all viscous type flows, debris flows tend to stop upon reaching a relatively low gradient or in areas of decreased confinement such as alluvial fans at the mouth of small basins.

Debris flows deposits are characteristically poorly sorted fans commonly containing large fragments resting unsupported in a finer-grained matrix. Debris fans are usually characterized by an inverse grading and commonly show a preferred alignment of their long axes parallel to the direction of flow.

References:

Calligaris, C. & Zini, L., 2012. Debris Flow Phenomena: A Short Overview?

Nunes, A. L. & Sayão, A., 2014. Debris Flows e Técnicas de Mitigação e Convivência. 14º Congresso Nacional de Geotecnia. (in Portuguese)

Debris flows may occur whenever hillslope sediment cover becomes suddenly saturated with water and flows into a channel, or when excess water on slopes causes extensive hillside erosion and channel scour. Briefly, debris flows are initiated when the applied shear stress exceeds the yield strength of the material involved.

The main conditions required for debris flows occurrence include the availability of relevant amounts of loose material, steep slopes and sudden water inflows that may come from intense rainstorms, collapse of channel obstructions, rapid snowmelt, glacial lakes outburst floods, among others.

For the occurrence of debris flows, predisposing and triggering factors need to be present. A usual characteristic of the debris flow is their close relation with high intensity meteorological events. In fact, debris flows are typically triggered by extreme intensity rainfall occurred in a short time period, that can uplift the ground water level reaching a critical level or when the rainfall intensity exceeds the infiltration rate creating a saturated layer from the surface.