PROBLEMS OF Pu TRANSPORT REVEALED IN APPLECATIONS OF THE METHODS
Dust-flux measurements at the Nevada Test Site and elsewhere have shown that
even under nearly ideal conditions, there are major difficulties in relating a
measured dust-flux to the underlying surface properties. It may be possible
to determine the physical factors causing particles to initiate motion, but in
order to be practical, these factors should also be parameterized to measurable
variables in some general way.
Shinn et al., 1976, found that upward dust-flux (suspension) apparently could
be parameterized cto "soil erodibility-index" using an empirical expression as
follows:
F=-F, (U,/U,)
z=
—
ytl
,
50
(6)
where U_ is constant, and F_ and y are hypothesized to depend on soil erodibility index, perhaps as shown in Fig. 3. Even though this hypothesis is not
adequately verified, it is important to note that (1) the soil erodibility
factors will strongly control the suspension rate of Pu host particles, and
(2) the rate of Pu suspension is proportional to wind speed raised to a power
of y + 1, which has been observed to exceed a value of 6.
This fantastically
sensitive response to wind speed is verified in experimental data of Gillette
(1976) and coworkers. Without the sensitivity of fast response, dust-flux
measurements, these observations could not have been made, and their implications for Pu-transport would have gone unnoticed,
The Nevada Test Site is unique in that the natural suspension rate due to
convective winds is very low compared to more erodibile sites in western U.S.
After several seasons of dust-flux investigations, we have concluded that the
natural desert shrublands and dry lakes are not subject to wind erosion unless
they are physically disturbed such as by foot or vehicular traffic. The
T
T
T
T
T
T
T
T
7
10
-
;
40
ary
x
Ne
5
718
x—-—_
P
30 -
Y
ys “
-
er
_)
2
= 6
“SS
>
—
I
oO
mw
desert shrublands are covered by a "desert pavement” consisting of small
pebbles weakly bound together by a surface crust.
Likewise, the dry lakebeds
are covered by a crust following each rainstorm.
Our investigations are
inconclusive about long-term dust-flux at the Nevada Test Site because the
magnitude of the flux is so low as to require newer instrumentation than we
presently have.
0
There have arisen, however, several definite questions at the Nevada Test Site
which require further research. These are as follows:
1.
What is the role of saltation flux (horizontal transport of larger soil
particles, bouncing along the surface) on the overall net loss of Pu by
vertical dust—flux?
2.
What are the magnitudes of the rate of deposition and the rate of suspension occurring simultaneously during wind events to produce a net Pu
transport?
200
pt
400
i
}
400
L
l
800
i
1000
Soil erodibility index
Fig. 3.
Relationship of the power y and the reference flux Fy, to the
erodib{lity of soil surfaces in Texas and Nevada.
177