approach velocities for these stations are, however, fairly consistent with the reported surface
wind. It seems reasonable to assume, therefore, that the primary transport mechanism at
downwind distances greater than 7,500 + 1,500 feet is the surface wind. The upwind and crosswind arrivals cannot be analyzed in this simple manner, Since the radial expansion in these
cases is being bucked to varying extents by the surface wind. Since the TOA at most upwind
and crosswind Stations is less than 1 minute, the base Surge center can be considered nearly
stationary, in which case radial expansion seems to carry the base surge to distances of 3,000
to 4,000 feet and 4,000 to 5,000 feet in the upwind and crosswind directions, respectively.
Although there are insufficient data points to be conclusive, these TOA plots suggest that the
base surge may have moved at different speeds in specific downwind directions. For Umbrella,
the slopes of the best straight lines through all stations at distances greater than 6,000 feet on
legs DL, D and DR indicate apparent speeds of 35, 23, and 17 knots, respectively. Since surface winds have been assumed to be the primary transport mechanism at these greater distances,
the suggestion of different radial speeds is apparently contradictory. This contradiction may be
resolved by postulating a nonuniform distribution of radioactivity within the visual base surge
when radial expansion effectively ceases. Since the time intervals required for these masses
to reach the stations concerned are short, this nonuniform distribution could be reflected as
apparent differences in speed along specific radii. AS previously suggested, this explanation
is at least consistent with the downwind protrusions on the isodose contours presented in Section
3.3.3. Alternatively, variable effects due to the atoll reef discussed later could result in apparent differences in speed of approach.
The rapid radial expansion of the base surge predominating at closer distances is probably
due to collapse of the central column. This transport energy is dissipated at approximately
7,500 feet downwind of surface zero and at smaller distances in the upwind and crosswind directions. Although average downwind radial velocities for this expansion have been approximated
by determining the slope of a straight line through these closer points, the treatment oversimplifies the situation, since the decrease in radial velocity with distance from surface zero is
probably not linear and since wind effects are tacitly ignored.
More reasonable estimates of base surge approach velocities can be determined both for the
visible surge from the boundary plots (Section 3.3.2) and for the airborne radioactive material
from an analysis of the rate of rise to the first gamma dose rate peak. The visual approach
velocity may be calculated for either the primary photo-boundary P, or the outer smooth boundary B,-. Since the distance of both boundaries as a function of time is given in the boundary
plots, the slope of the appropriate curve at some time (or distance) prior to surge arrival
yields the desired velocities. In most instances, these slopes are changing rapidly; thus, the
approach velocities are quite sensitive to the point at which the slope is determined. The most
informative comparison is that between the photographically and the radiologically determined
approach velocities; therefore the points are defined with respect to the time of peak dose rate
(TOP). Visual approach velocities are determined for times when Py or By are 100, 200, 300,
and 500 feet more distant than at TOP. These velocities are presented in Table 3.15 (estimated
values enclosed in parentheses). Agreement between the approach velocities determined for the
primary photo-boundary and the outer smooth boundary indicates a relatively even surge outline
in the neighborhood of the station; conversely, large discrepancies suggest lobes or irregularities.
The approach velocity for the airborne radioactive material is determined by the rate of rise
{r of r) to the first major gamma dose rate peak (hence the r-of-r velocity). To determine velocity in this manner, some shape has to be assumed for the approaching body of airborne radio-
active material.
The radiation intensity for several cloud models has been calculated as a
function of distance between the cloud source and the detector (Section 1.3.2). Assuming that
these models approximate actual surge shapes, the distance required for an increase in dose
rate from 5 to 100 percent of peak value may be obtained from these computed intensities. This
distance divided by the time required for a similar increase in recorded gamma dose rate yields
a velocity of approach dependent upon the cloud model assumed. TheSe approach velocities cal-
culated for a number of cloud models using a gammaenergy of 1.25 Mevare presented in Table
232
4;