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DEBRIS FLOWS

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Detailed studies of torrential gullies capable of generating debris flows, with tridimensional simulations (RAMMS::Debrisflow), obtaining flow values as height, velocities and impact pressures, in order to accurately define the best situation of the protection, as well as their features.

What is it?

 

Debris flow is a rapid movement of water and detritus, predominantly of coarse fractions such as sand, gravel and blocks. These flows have little cohesion and usually are channeled down to the depressions. These flows in the phase of adaptation often generate fans.


There have been various definitions of Debris Flow; Johnson (1970) defined them as a mass movement induced by gravity with a dynamic performance between landslides and flash floods, and with the mechanical characteristics that differentiate Debris flows from the other processes. Varnes (1978) defined them as a mass movement of granular solids, water and air, which moves like a viscous flow. Johnson & Rodine (1984) added that clay is also present, but only in minor amounts.

RAMMS::Debrisflow 2D simulation

Morphology

 

Debris flows have different morphologies but, in general, their length is much greater than its width and its ratio becomes 10:1 or even more than 50:1 (Van Steijn, 1989). In a Debris Flow there are three areas with different morphology: the source area, the channel area (track path) and the accumulation zone. The source area is usually short and steep, where there is abundant soil or where the bedrock is high weathered. The slope of this area is always high and generally varies between 25º and 40º, values ​​which most authors agree.

Debris Flows tend to flow through existing gullies, but also can move downhill without confinement and overcome obstacles in almost any direction. At the top of the channel cross section is typical elliptical shaped or V and their background is free of deposits, while the lower part of the channel is occupied by deposits corresponding to the flow tail.


The speed of Debris Flows covers a wide range of values ​​from 0.5 m/s to 20 m/s, depending on the material that forms the flow (particle size, concentration and classification of solid grains) and propagation characteristics of the area (topography, confinement, roughness), among other factors.


Deposits formed bu Debris Flows are often very chaotic, with no internal structure defined. Morphologically these deposits we can distinguish three zones (Moya, J 2001-2002):

     -The source area: defined as the area where the movement originates
     -The channel: parallel to the stream; it's the track path of the flow
     -The accumulation zone: morphology of these areas is like a fan



You can find the source area of ​​a Debris Flow in two very different points (Hürlimann 2001):


     -Inside a channel network hydraulic basin: the origin of this Debris Flow is a flood due to erosion by water, an increase of sediment content until the flood becomes a stream with a high concentration of sediments.
    
-Outside the catchment channels: these flows changes stem from a sliding surface deposits located hillside, which can reach the stream to continue on its route.

RAMMS:Debrisflow 3D simulation

Material characteristics

 

The mixture of solids and water that form a Debris Flows moves as a single homogeneous medium. The theoretical composition of the sediment is between 60% and 90% of the total and the rest is water. The usual densities are usually between 1.8 and 2,6 gr/cm3.


This material shows a wide range of grain sizes ranging from clay to coarse gravel and blocks. The main area where Debris Flows can trigger, is associated with deposits (glacial sediments, and col·luvions regolith). Truly, the vast majority of debris flows are initially landslides. When slippage occurs, the mixture of sediment and water becomes a flow, and during the deposition it becomes a nearly rigid body.


The solids concentration varies between 25% and 86% (by volume), and between 35% and 90% by weight. The water content is generally comprised between 10% and 30% by weight, although it can be higher. In general, only a small fraction of the material is thin, particularly silt and clay (approximately between 10% and 20%); within this percentage, the clay can be non-existent without exceeding, usually 5% (Costa, 1984). This wide range grading allows Debris Flows to contain high concentrations of solids and thus, high densities.

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