Alluvium / Debris flows

Detailed studies of torrents and channels capable of generating drag currents (alluvium), with 3D simulations, following the methodology proposed by the Cartographic and Geological Institute of Catalonia for the preparation of the technical guide for the zoning of the danger of torrential flows.

Detailed studies of alluvium by applying the steps established during the Technical Assistance for the preparation of a technical guide for the zoning of the danger of torrential flows (debris flow) in Catalonia, commissioned by the Cartographic and Geological Institute of Catalonia (ICGC). )

Basically, in a ravine two geodynamic processes are present, these are the floods (in English “water floods”) and the alluviums (in English “debris flows”). The definition of both processes and their descriptions allow individualizing the hazard generated by both geodynamic processes.

Strong storms often cause significant damage to gullies and their ejection cones, basically due to hydraulic processes represented by floods and alluvium. Floods are flows basically formed by water, while alluviums are formed by a mixture of water and solids (silt, gravel and blocks) with a behavior similar to liquid concrete.

Between both processes, floods and floods, there is a transition of different processes. Each of these processes can be differentiated from the rest according to its water content. While floods are basically made up of water with a small proportion of sediment, alluvium flows through the torrent as an intact mass of water and solids (silt, gravel and blocks).

Both processes can become jointly within the same storm episode. However, the presence of only one process is also possible depending on the meteorological conditions, morphology of the basin, geotechnical characteristics of the solid deposits susceptible to mobilization. Even the same flood can become a flood due to the incorporation of solid material along the course of the torrent.

The following table gives an overview of the most commonly used terms for the different types of streams in various European languages.



hydraulic behavior

GivesSince the amount of suspended sediment (solids) is a fundamental factor in hydraulic behavior, a basic knowledge of hydraulics is essential to define and differentiate floods and alluviums.

A flow of water can incorporate sediment by erosion of the stream bed due to the forces acting between the water and the stream sediment. Hydraulic formulas define that the erosion capacity is a function of the flow speed and the size of the solids that make up the sediment. The effectiveness of erosion increases as the flow velocity increases and with the increase in solid material that forms the alluvium. On the other hand, erosion is reduced depending on the size of the solids that are part of the torrent channel. Bearing in mind that the erosive capacity of the flow increases when the transported sediment content is higher, an alluvium always presents a greater erosive capacity than a flood, both having the same speed. In multiple gullies it has been observed that alluvium can carry rocky blocks with diameters greater than 1 meter.

A flood is a turbulent flow with a sediment content less than 40% with densities between 1 and 1.3 gr/cm3. Meanwhile, alluvium is a mass of solids and water that moves as if in a homogeneous medium with a viscous hydraulic behavior, similar to liquid concrete, its sediment content is between 60% and 90% with densities between 1 '8 and 2.6 gr/cm3.

A second difference between floods and alluviums, apart from the compositional one, is their dynamics. While a flood presents more or less progressive increases in its flow, a flood presents a rapid and abrupt change in it. Within a ravine, before the arrival of a flood, there is usually a minimum flow that increases suddenly when it arrives. The alluviums usually reach heights greater than one meter at their front or beginning, which will depend on the volume and flow.

Triggering mechanisms

While a flood occurs whenever the weather conditions allow it, floods present more complicated trigger mechanisms.

Basically, there are two mechanisms for the initiation of floods:

Beginning inside the channel due to channel erosion:

It consists of the incorporation of sediment during a flood. Erosion by water produces an increase in sediment content until the flood becomes a hyperconcentrated flow and finally a debris flow.

Start outside the channel due to slope slippage:

They are alluviums that have their origin in a superficial detachment of the deposits located in the middle of the slope, which reach the torrent. If water passes through the torrent, part of it can be incorporated into the detachment, changing its rheological behavior and finally becoming alluvium that flows through the ravine.

Morphological and sedimentary characteristics of the deposits

The deposits formed by the floods usually present flat morphologies and fans of great extension. The sediment usually presents a stratification and normally a classification of the clasts.

On the other hand, the deposits of the drag currents usually present fans of greater slope with lobes and lateral dikes parallel to the torrent (lévées). Deposits are often very chaotic with no defined internal structure. During the deposition of the sediments transported by the drag current, there is no separation between the solid and liquid components, so that all the material is restrained as an intact mass similar to that of liquid concrete.

Often after a rip current event, both types of deposits (flood and rip current) are observed in the jet stream. In the upper zone of the ejection cone, the drag current with its compact and chaotic structure is initially deposited. Subsequently, hydraulic processes (hyperconcentrated flows or floods) can erode and transport the finer material from the deposit of the drag current until it is deposited in a more distal part of the cone. The result is a compact and thick drag current deposit in the highest sector of the cone, while in front of it there is a wide, less thick deposit of fluvial origin.

Other observations

After an episode of exceptional rainfall, from the simple observation of the deposits and damage it is difficult to distinguish the different processes of flooding and drag currents within a torrent and the same ejection cone. However, both processes can occur in the same episode of exceptional rainfall, so the erosive dynamics of one of them can erase the deposits previously generated by the other.

However, the danger generated by both processes is very different. Two factors are those that determine this different danger and consequently the potential damage, these are:

• The maximum flow that is reached in the front of a drag current is superior to that of a flood.
• The blocks that can be transported by drag currents are of greater volume compared to those of a flood.

It is estimated that the maximum flow of a drag current can be more than ten times greater than that of floods. The increase in maximum flow at the front of a drag current can cause significant damage. Therefore, sizing calculations based exclusively on hydraulic concepts are useless to determine a design flow.

Another aspect to take into account is the transport of large blocks within the mass of drag currents. While the damage during floods is caused only by water and small sediment content, the material carried by drag currents (rock blocks, sediment, trees, etc.) can cause major damage to urban and civil structures. .