Debris Flows - Environmental Geomechanics - page 2

As a first approximation, debris flows can be treated as one-phase flows, and
their flow properties can be studied using classical rheological methods. The study
of debris flows is a very exciting albeit immature science, made up of disparate
elements borrowed from geomorphology, geology, hydrology, soil mechanics and
fluid mechanics. The purpose of this chapter is to provide an introduction to the
physical aspects of debris flows, with specific attention directed to their rheological
features. Despite attempts to provide a coherent view on the topic, coverage is
incomplete and the reader is referred to a series of papers and topics. A few topics are
particularly commendable [BRU 84, COU 97, JOH 84, ZIM 97]. Some review papers
provide interesting overviews, introducing newcomers to the field to the main concepts
[ANC 07, CHE 87, COU 96b, IVE 97, IVE 05, RIC 99, TAK 81].

1.2. Typology of torrential flows

1.2.1. Watershed as a complex physical system

The notion of a torrent refers to a steep stream, typically in a mountainous context
[MON 97, WOH 00]. According to a few authors, a stream can be referred to as a
torrent as soon as its mean slope exceeds 6% [BER 27]. For bed slopes ranging from
1 to 6%, it is called a torrential river . For bed slopes lower than 1%, it can be merely
called a river . In addition to the slope, the sediment supply is generally considered
as another key ingredient in torrential watersheds. Depending on the nature of the
soil and relief, slopes can provide a large quantity of poorly sorted solid materials
to torrents. Supplied materials have sizes ranging typically from 1 mto10m.The

situation is very different from the one encountered for streams on a plain, where
bed material is much finer and ordered (typically 1

μ

m to 10 cm) since it generally
results from transport that occurred during previous floods. Finally, one of the chief
ingredients of torrential watersheds is water. Owing to the small dimensions of
torrential watersheds (typically from 0.1 to 100 km 2 ) and the steep slopes, floods are
sudden, short and violent. The flood regime differs significantly from plain floods,
which are characterized by slower kinetics and smoother variations with time. Figure
1.1 shows a typical watershed. The upper part is generally degraded and submitted
to erosion to a more or less large extent. It supplies water and sediment to the floods.
Below this basin, the torrent enters a gorge, sometimes with very abrupt flanks
depending on the nature of the soil. Then the torrent discharges onto the alluvial
fan. The slope transition between the gorge and the alluvial provides interesting
information on bed equilibrium. Generally, a watershed with an abundant supply
of sediment and intense bed load transport in the past is characterized by a smooth
transition from channel to fan.

μ

For plain rivers, sediment transport results from the action of water: water entrains
materials either by pushing them along the bed (bed load transport) or by keeping
them in suspension as a result of turbulence (suspension). In a torrential context, as