(Appendix F) for Umbrella thus becomes 7,800 feet (the similarity of the final surge dims for the two Shots is entirely coincidental). Although the postulated two-stage radial expa. resulting from the influence of the atoll reef cannot be conclusively demonstrated, Such s behavior is in better agreement with the reported observations of Project 1.3 (Reference The final radii adopted for analysis of the gamma records are essentially the same as the arrived at in Section 3.3.4 from analyses using the hypothetical surge center H (Appendis and Figures 3.136 and 3.137}. The centers of all photo-boundaries just described for the Wahoo and the Umbrella sur are considered to be coincident with that of the primary smooth boundary By. This cent called the photographic surge center X (Appendix F),is assumed to be independent of the ported surface wind speeds up to the time of the last photographically determined surge b ary (3.5 minutes for Wahoo and 6 minutes for Umbrella}. After this time it is presumed move in accordance with the official Task Force surface winds (15 knots from 090° T for and 20 knots from 050° T for Umbrella). Since two methods of determining the surge cen are employed in this report, special care should be taken to note the difference between t photographic Surge center X just described and the hypothetical surge center H, whichi simply the point defined by moving Surface zero downwind in accordance with official surt winds starting at zero time. The photographic surge center X is used exclusively throug this section; similar calculations using the hypothetical surge center H and arriving at e: tially the same conclusions are presented in Section 3.3.4. Although the difference betwe these two centers is never large, the photographic surge center X for Wahoo undergoes: somewhat abrupt change in direction of travel between 3 and 4 minutes, which probably ri in fictitious variations in the boundary plots (Appendix F) between these times. The two representations of visible surge position accompanying each gamma record ha been graphically determined for each coracle location, uSing the photo-boundaries and wi movement just described. Although these procedures are admittedly rough, actual base : movement is Sufficiently approximated to reveal some of the subtler aspects of the gamm dose rate records. For brevity these two plots are hereinafter referred to as the “trans: and the “boundary plot” (Appendix F). The transit plot consists of a plan view of the mos probable phote- and smooth boundaries at the times of their individual initial and final tra at a given Station. These transit plots are presented to indicate appropriate intercompar between the given record and other records at similar stages of transit or base surge dev ment. The boundary plot is determined by measuring the shortest distance from the give tion to the appropriate photo-boundary at 1-minute intervals. The smooth curve drawn tr these points is considered only an estimate of the actual surge position, which includes vi tions due to local irregularities in boundary, changes in surface wind speed and direction changes in the rate of base surge expansion. The sign of the plotted values indicates whe the particular boundary is radially closer to the surge center than the station (negative va or radially beyond the station (positive value). These boundary plots are used to correlai various photographically detectable features of the surge with specific portions of the gan dose rate record. The base Surge approach velocity is a vectorial combination of the radial surge expans and the local surface wind. At least two (not necessarily identical) approach velocities m considered, viz, that of the visible surge and that of the alrborne radioactive material. T visual approach velocity may be calculated for either the primary photo-boundary P, (Ar dix F) or for the outer smooth boundary B,. Since the distance of either boundary as a f tion of time is given in the boundary plots, the slope of the appropriate curve at some tir (or distance) prior to surge arrival yields the desired velocities. In most instances thes: slopes are changing rapidly, thus the approach velocities are quite Sensitive to the point: which the slope is determined. The point giving the most favorable comparison with radi logical approach velocities is one representing a distance of 500 feet from the station at t time of the first major peak in dose rate. All visible approach velocities tabulated in Tal 3.11 are determined for this point (estimated values are enclosed in parentheses). Agree 129