Torrents and mudflows

Torrents

In the Austrian Forest Act 1975 torrents are defined as follows: “A torrent is a permanently or temporarily flowing waterway which, as a consequence of its rapid, only short-term swelling, removes solid material from its catchment area or its bed to a dangerous extent, carries this along and deposits it within or outside its bed or carries it to another stretch of water.” (Forest Act 1975, section 99 para.1) 

The main characteristics of torrents in the Alpine area are their high gradients and the differing run-offs which, due to snow-melting or heavy thunderstorms, vary strongly within short periods of time. Less often, torrents exist in hilly flatlands. There they occur on steep hillsides and typically do not overcome great differences in height but often have larger catchment areas than mountain streams have.

The catchment area of a torrent comprises the area of precipitation that the torrent and its inlets drain as well as the area where the torrent deposits its material. There are about 12,000 torrent catchment areas in Austria. All catchment areas of a Federal Province are listed in an ordinance of the competent Provincial Governor.

A torrent can be roughly divided into three sections:

1. the upper catchment (upper reaches), the precipitation collection area,
2. the middle reaches, the area of bedload transport, and
3. the debris cone in the lower reaches, the zone of bedload deposition, which is often densely populated.

Among the most important hazards in torrential catchment areas are

• floods,
• mudflow as well as
• the mass transport of stones, gravel, mud (“bedload”), and wood (“woody debris”).

Floods develop as a result of extreme precipitation, which, in torrent catchment areas, leads to rapid increase in run-off and consequently to floods in the valley. Depending on the geology of the catchment areas a flood event may, by the force of the water flowing away, erode great amounts of solid material (”erosion”), transport it through the torrent as bedload, and deposit it on the debris cone. In steep torrents, extreme bedload transport events may result in the formation of mudflows.

Mudflows

When the quantity of the bedload picked up and transported by the water is so big that the mass which is moved constitutes a mixture of earth, debris of different sizes, often huge rock fragments or wood, this is called a mudflow. Mudflows have a particularly high destructive force. They pose a threat even to persons inside buildings.

Preconditions for the development of a mudflow include a sufficient quantity of water which accumulates either due to heavy precipitation or as a result of backwater, the existence of sufficiently large masses of material as well as a situation enabling those masses to start moving. This depends on the steepness of the embankments, the presence of seepage waters, the grain size of the bedload, the quality and characteristics of the vegetation cover and, eventually, on the extent of looseness of the material.

Protection against torrents and mudflow

The modern system of torrent, avalanche and erosion control measures comprises active and passive measures with permanent or temporary effects.

In torrential catchment areas dams are built to stabilise the beds of torrents against depth erosion and to retain sediments and woody debris; furthermore, dams to “dose” flood discharge and sediment transport as well as to “break” the force of landslides are used.

In densely settled areas it may sometimes be necessary to develop the discharge section (regulation) or stabilise banks, whereas in the open land natural inundation and sedimentation areas form part of control concepts.