Mountain environments - Environmental Sedimentology - page 48

Table 2.8 Geomorphological and sedimentologic characteristics of water and sediment flows in channels (Source: Costa 1988;
reproduced from Flood Geomorphology . Baker, V.R., Kochel, R.C. & Patton, P.C. (Eds). 1988. © John Wiley, New York. This material is
used by permission of John Wiley & Sons, Inc.)

Flow Landforms and deposits Sedimentary structures Sediment characteristics*

Water flood Bars, fans, sheets,

splays; channels have
large width:depth
ratios

Horizontal or inclined
stratification to massive;
weak to strong imbrication;
cut and fill structures;
ungraded or graded
Weak horizontal
stratification to massive;
weak imbrication; thin
gravel lenses; normal and
reverse grading
No stratification; weak to
no imbrication; inverse
grading at base; normal
grading near top

Average Trask sorting coefficient
1.8 -2.7; clast-supported; normally
distributed; rounded clasts; wide
range of particle sizes

Hyperconcentrated
flow

Similar to water flood Φ graphic sorting 1.1-1.6 (poor);

clast-supported openwork structure;
predominantly coarse sand

Marginal levees,
terminal lobes,
trapezoidal to
U-shaped channel

Average Trask sorting coefficient
3.6 -12.3;

Debris flow

graphic sorting 3.0 -5.0
(very poor to extremely poor);
matrix-supported; negatively skewed;
extreme range of particle sizes; may
contain megaclasts

Φ

*Trask sorting coefficient: calculated by dividing the 75th percentile by the 25th percentile of the grain size distribution. Φ graphic sorting:
inclusive graphic standard deviation (in φ units) = [( φ 84 −φ 16)/4] + [( φ 95 −φ 5)/6.6].

2.3.1 Glacier systems and environmental change

are fundamental in controlling basin sediment
yield and may be sensitive to environmental
changes in climate and/or human disturbance.
In addition to these long-term influences glacial
processes can have short-term effects on moun-
tain sediment systems and in some cases pose
significant hazards. Three main hazards can be
identified: glacier fluctuations, glacier outburst
floods (jökulhlaups) and avalanches (Cooke
& Doornkamp 1990). Hazards related to ice
and snow are common in most glacierized high
mountain areas. Their impact on society, how-
ever, depends on the degree to which human
structures and settlements are developed in those
regions. The European Alpine countries are par-
ticularly affected by glacier hazards owing to
the combination of steep, unstable slopes and the
proximity of infrastructure and villages to the
glacial environment. Of particular importance
is the sensitivity of the glacier environment to
small changes in temperature and precipitation,
which may considerably increase the risk for
communities living near them. Outburst floods
(jökulhlaups), landslides, debris flows and debris
avalanches can destroy property and take lives.
Given the uncertainties of recent environmental

A characteristic feature of many mountainous
environments is the presence of glacier ice. At
present the distribution is restricted to the higher
mountains and polar ice sheets but in the past
was considerably more extensive. The legacy of
past glaciations still conditions sediment transfer
in most mountainous regions. This is manifest as
a direct influence in currently glacierized regions
and as an indirect control in glaciated areas where
the impact of glaciers still significantly alters
mountain sediment systems. The concept of para-
glaciation (Church & Ryder 1972) provides a
useful framework for understanding contempor-
ary mountain sediment budgets and the dis-
equilibrium that often exists between sediment
production and delivery in previously glaciated
areas (Church & Slaymaker 1989). Ballantyne
(2002) provides a comprehensive review of peri-
glacial geomorphology and the important concept
of glacially conditioned sediment availability.
He identifies six paraglacial land-systems and
recognizes the significance of these sediment
stores and sinks as sources of easily eroded sedi-
ment. Sediment stores in mountain environments