Environmental Engineering Reference
In-Depth Information
for heavy damage. These rates are surpassed by the power
of avalanches. A slab of dry snow measuring 10 m
10
m
20 m, weighing about 500 t and hanging 500 m
above the lower slopes of a mountain valley contains po-
tential energy equal to nearly 2.5 GJ. Kinetic energy of
such a falling mass, assuming average speed of 30 m/s,
would be 225 MJ, resulting in vertical power densities
around 110 kW/m
2
, a rate directly comparable to the
vertical wind density of tornadoes. Not surprisingly,
the effects of these two sudden energy releases are devas-
tatingly similar.
Once the precipitation reaches elevated ground, the
gradual release of its potential energy is usually a subdued
affair. For example, meltwater from 1 m of snow accu-
mulated during winter in a high mountain valley, when
dropping on the average 1 km to reach the main valley
channel within one month, would reduce its potential
energy by just 380 mW/m
2
. Total potential energy of
2 m of precipitation in maritime mountains (3 km above
sea level) would be just short of 60 MJ/m
2
a year
(1.9 W/m
2
). The importance of this potential energy
is much greater than the power densities can indicate.
This quiet and unrelenting flux imparts a common direc-
tion to many ecosystem functions (D. H. Miller 1981).
Moreover, since it wears away rocks and moves the prod-
ucts of weathering, falling water is a key agent of geo-
morphic processes.
Surface runoff returns the precipitated water rather
rapidly: average residence times of fresh water range
from just two weeks in river channels and weeks to
months in soil to years in lakes and swamps. Annual river
runoff ranges typically between 200 mm/a and 300
mm/a, but it is as high as 800 mm/a for the Amazonian
South America and as low as 25 mm/a for Australia. Not
surprisingly, the Amazon alone carries about 16% of the
planet's river water, and the world's five most volumi-
nous streams (Amazon, Ganges-Brahmaputra, Congo,
Orinoco, and Yangtze) carry 27% of all river runoff (Shi-
klomanov 2003). Estimates of global surface runoff range
from 33,500 km
3
to 47,000 km
3
a year. The most likely
average, excluding the Antarctic ice flow but including
the runoff to continental interiors, is 44,500 km
3
(Shi-
klomanov 2003). Assuming the mean continental eleva-
tion of 840 m (Ridley 1979), the potential energy of
this flow is about 367 EJ. Waterfalls are the sites of the
highest concentrated releases of potential energy of flow-
ing water (Czaya 1981). Maxima are 16.25 GW for the
Inga Falls on the Congo; 5.2 GW for the Sete Quedas
on the Paran´; and 4.9 GW for the spectacular 72-m-tall
Iguass´ ; Niagara's potential is 3.4 GW.
Stream flows are indispensable for transporting huge
loads of sediments, accreting nutrient-rich alluvial plains
that became the core areas of all high cultures, and
also providing humankind's first practical inanimate
source of mechanical energy (see section 8.2) that has
been exploited since the 1880s for generation of inex-
pensive electricity (see section 9.2). Stream velocities are
unevenly distributed throughout a channel, but the flows
are typically around 0.5 m/s in wide lowland rivers and
between 2 m/s and 3 m/s in floods; none exceed
9 m/s. Cross-sectional kinetic power density of a flood-
ing stream can be up to about 13 kW/m
2
, less than
in many thunderstorms and 1 OM below tornado or
avalanche impacts, but considerably longer durations of
typical floods and their ability to weaken the foundation
soils have much greater impacts than the power density
alone would indicate (threshold for structural damage is
@18 kW/m
2
).
The biggest known catastrophic flood was not caused
by a stream but by the discharge of glacial Lake Missoula
