• 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.

  • Bardou 2003

    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.


    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.

     geomorf 1geomorf 2

    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.


    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.

  • Regarding to predisposing factors, the most relevant are: 


    • catchment morphometric characteristics (mostly slope) 
    • debris or unconsolidated materials availability
    • land use politics and construction activities (e.g. deforestation, earthworks involving cut and fill, drainage works) 

    The most significant triggering factor is likely to be the water presence and the respective development of transient high pore water pressures along pre-existing or potential rupture surfaces. Thus, several related events can be pointed out as triggering factors, such as:

    • high intensity rainfall conditions, usually occurred in a short time period*
    • landslides
    • volcanic eruptions
    • seismic activity
    • sudden snowmelt

    *There are a few records of debris flows triggered by long period rainfalls of weak/medium intensity as well

  • Despite of the fact debris flows may descend a stream as a single episode in minor cases, mostly they are characterized by an extremely unsteady and non-uniform pulsing flow. In fact, a typical debris flow consists of a series of waves, often superimposed in “normal” flood flow in the channel. In these situations, flow may cease altogether before and between waves. According to Takahashi, surge waves can occur in any debris flow type, although they are more common in viscous type debris flows.

    According to Suwa, et al. (2009) there are four main causes for the occurrence of pulsing behavior on debris flows:

    • sensitive response to strong rainfall intensities
    • a successive formation of landslide dams and respective failure
    • forces balance failure between shear stress and shear strength of the source slurry
    • growth of instability in the flow motion, due to steep slopes and long runout distances


    Suwa, H., Okano, K. & Kanno, T., 2009. Behavior of debris flows monitored on test slopes of Kamikamihorizawa Creek, Mount Yakedake, Japan. International Journal of Erosion Control Engineering, Volume 2, pp. 33-45.

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

  • Generally, steep mountainous regions which are mantled by a cover of unconsolidated soils and sediments are particularly susceptible to various types of water-related mass movement’s, namely debris flows. These steepland regions are often overwhelmed with high-intensity storms and extreme rainfall events, which are responsible for sudden water inflows and, on the one hand can lead to slope failures and landslides, and on the other can develop debris flow, if debris or unconsolidated materials are available. These requirements are met in many mountainous basins under different climatic conditions, turning debris flows into a widespread worldwide phenomenon.

    In fact, debris flow occurrences are recorded all over the world. Among the countries most affected by debris flows, there are few that stand out, namely Venezuela, Japan, China, Thailand, Nepal, Indonesia, Philippines, Italy, Switzerland, France, Germany, Portugal, Slovenia, Colombia, the United States of America, Peru and Australia


  • There are two different strategies for mitigating debris flows hazards: structural and non-structural mitigation measures.

    The structural mitigation measures include all procedures/civil works in order to control debris flow phenomena and generally they aim to prevent its generation, solid material runout transport and/or deposition. In general, this type of mitigation measures comprises the employment of special protection systems usually related with significant construction works.

    On the other hand, non-structural mitigation measures comprise all the measures that allow the natural hazard risk management, namely land use planning and development, information and sensibility campaigns, emergency plans, insurance programs, among others. Such mitigation measures also include the employment and development of debris flows warning systems and event forecasting through the definition of precipitation thresholds


    • interception and retention of solid material
      • retention dams
      • breakers
      • flexible barriers
      • retention basins
    • slope stabilization and erosion control techniques
    • streams erosion control and channelization
    • drainage works
    • protection structures
      • deflection walls
      • shelters
    • land use planning and development
    • warning systems:
      • event warning systems (example: videocameras, encompassing ultrasonic or radar gauges, ground vibration sensors, trip wires)
      • advance warning systems (example: rainfall thresholds based on hydrometeorological monitoring)
    • bioengineering techniques for slope stabilization
    • information and sensibility campaigns
    • emergency plans