Environmental Engineering Reference
In-Depth Information
releases as intense as 0.5-1 MW/m
2
of vertical surface:
few structures can remain undamaged under
such
onslaught.
The impact of tornadoes is frequently even worse.
Analyses of 10,826 U.S. tornadoes indicate the mean
width of their path to be 126.7 m, average length 9.6
km, speed 60 m/s, and duration 160 s (Schaeffer, Kelly,
and Abbey 1980). Total energy in the average 100-m-tall
tornado funnel is about 275 GJ, its power is 1.7 GW,
and its power density is about 1.4 kW/m
2
. Wind cross-
sectional densities average about 135 kW/m
2
, and maxi-
mum wind gusts release up to 1.35 MW/m
2
against the
structures. The largest tornadoes (classified as ''incredi-
ble'') will not surpass this highest localized impact, but
their total energies in the lowest 100 m may be up to
150 TJ, with power up to about 100 GW, about 60
times more powerful than the average event, and their
destruction path can be as wide as 1.5-2 km and extend
over more than 150 km.
Kinetic energies of water and wind are the most wide-
spread agents of geomorphic change. Some of them can
produce major changes in a matter of hours (flooding
streams, catastrophic rains), but most of the erosional
and sedimentation processes proceed at very slow rates
and are revealed on timescales of 10
3
-10
6
years; new
methods of measuring cosmogenic nuclides (
10
Be)
in exposed rocks (Bierman and Nichols 2004) can be
used to quantify this gradual denudation. And in some
instances erosion rates are, counterintuitively, unrelated
to precipitation. Burbank et al. (2003) found no measur-
able difference in erosion rates across the Greater Hima-
layas despite a fivefold range of precipitation (tectonic
uplift is the key factor affecting erosion). Denudation
rates are often expressed in Bubnoff units, with 1 B
ΒΌ
1
mm/1000 years or 1 m/Ma, an equivalent of 1 mm/a
2.7 Typical ranges of annual denudation rates. Plotted from
data in Saunders and Young (1983), Selby (1985), Clayton
(1997), and W. W. Hay (1998).
or 1 m
3
/km
2
a. Figure 2.7 summarizes the rates of all
important slow geomorphic processes.
Considerable ranges for most of them are not sur-
prising: erosion rates are very difficult to quantify, and
differences in relief, effective precipitation (the net after
subtracting evapotranspiration), and ground cover com-
bine to give rates ranging over 3 OM. Perhaps the high-
est recorded rate, 19 kB, was from a volcano in New
Guinea (Ollier and Brown 1971); erosion in the Hima-
layas can be sustained at nearly 10 kB; and the Indus
river incises through the bedrock at extremely high rates
of 2-12 mm/year, or 2-12 kB (Burbank et al. 1996).
The British Isles are being denuded at rates ranging
mostly between 20 B and 100 B. In many lowlands the
