Environmental Engineering Reference
In-Depth Information
that created spectacular erosional and depositional land-
scape in the loess and basalt of the Columbia Plateau of
eastern Washington commonly known as the channeled
scabland about 15,000 years ago (Bretz 1923; V. R.
Baker 1981). Maximum discharge was up to 21 Mm
3
/s
(for comparison, global discharge of all rivers is 1.2
Mm
3
/s) with speeds averaging 20 m/s. Total kinetic en-
ergy of this unparalleled breach was about 400 PJ, its
power most likely between 1 TW and 2.5 TW, and its
vertical kinetic power density was up to 13.5 MW/m
2
,
ten times the impact of the fastest tornado gusts. These
enormous flows reshaped about 8000 km
2
, moving
boulders up to 10 m in diameter and creating such im-
pressive features as Columbia's Grand Coulee in a matter
of hours or days. Another enormous flood took place
some 8,450 years ago when the glacial Lake Agassiz
(containing some 163,000 km
3
and extending from
Manitoba to Quebec) emptied within less than a year
into the Hudson Bay with the maximum discharge rates
of 5-10 Mm
3
/s (Clarke et al. 2003).
Sediments carried by rivers include dissolved com-
pounds, a fine load of suspended clay, and silt particles;
and coarse bedload (gravel, stones, boulders) that re-
quires high velocities for downstream transport (Gou-
die 1984). Stream competence, the maximum movable
weight of individual bedload pieces, varies with the sixth
power of water velocity (a flow of 4 m/s can carry stones
64 times more massive than one of 2 m/s), and the
stream capacity (ability to move total bedload) goes up
with the cube of the velocity (doubled speed moves an
eight times larger load). Average shares of the three com-
ponents are difficult to estimate, but a ratio of 4:5:1 may
be a good global approximation. The best available re-
construction of the global prehuman sediment discharge
adds up to about 14 Gt/a; humans have increased this
flux through accelerated erosion by about 2.3 Gt/a but
at the same time cut the mass of the sediment reaching
the ocean by 1.4 Gt/a because of retention in reservoirs
(Syvitski et al. 2005). Sediment flux of 16.3 Gt/a would
be equal to roughly 130 PJ of lost potential energy, or
less than 0.1% of the total loss in global water runoff.
Uneven heating of the atmosphere creates distinct
pressure belts (equatorial low, subtropical highs, mid-
latitude lows) that give rise to predictable steady or semi-
permanent prevailing winds (trade winds blowing toward
the equator, prevailing westerlies in mid-latitudes) on a
planetary scale as well as to smaller rapidly moving atmo-
spheric systems (thunderstorms, tornadoes, and hurri-
canes). These cyclonic flows are irregular, short-lived
(mostly between tens of seconds and a few hours), and
often very intense (wind speed commonly in excess of
20 m/s in heavy thunderstorms, up to 130 m/s in torna-
does). Kinetic energy of common thunderstorms sweep-
ing typically 50-150 Mm
2
with winds of 15-25 m/s
during 8-15 min will be between 30 TJ and 300 TJ,
and their total power will range from 75 GW to 600
GW.
Simply prorated, their power density would be 1.5-4
kW/m
2
, but because most thunderstorms release their
vast kinetic energies aloft, the impact in the lowermost
100 m above the ground is equal to just between 30
W/m
2
and 100 W/m
2
, and the vertical surfaces have to
withstand briefly power densities up to 20 kW/m
2
,
enough to do scattered damage. The United States has
about 100,000 such events a year, only 3,000 of them
severe (Allaby 2004). Kinetic energy of hurricanes,
whose 30-50 m/s winds (maxima up to 90 m/s) can af-
fect up to 1 Tm
2
for many hours, is small when prorated
over the entire impact area (mostly just 200-500 W/
m
2
), but the eye of the cyclone produces brief energy
