Barriers Against Lava — Macdonald 
273 
The lines indicated in Figure 7 for the 
course of the newly proposed barriers are in- 
tended only as suggestions of an approximate 
route. Their precise position should be de- 
termined by detailed surveys like those made 
by the U. S. Engineer Department for the 
route of the earlier proposed barrier. 
The route laid out by that department in 
1940 takes complete advantage of natural 
topography and crosses the contour lines at 
the maximum possible angle. It would protect 
the harbor and the central part of the city as 
completely as the alignments suggested in 
1950. Only if it is considered economically 
justified to protect a larger area are the posi- 
tions suggested in 1950 to be preferred. 
The lines on the map show the main barrier 
as continuous walls, as was the barrier recom- 
mended by the U. S. Engineer Department in 
1940. An alternative construction, suggested 
by Eaton (personal communication, 1956), is 
a series of short segments set en echelon to 
each other as shown in Figure 9A. This design 
would provide possible means of confining 
portions of the flow that may spill over any 
one segment of the barrier, by extending a 
lower segment to a point beyond the spill- 
over. Figure 9B illustrates the way this might 
be done. It should be noted, however, that it 
might not be possible to force the spill-over 
into the channel behind the lower barrier seg- 
ment if the space behind that segment had al- 
ready been occupied by an earlier portion of 
the flow. If the barrier is constructed in short 
echelon segments, it should be started higher 
up the Wailuku River than indicated in Figure 
7, possibly as high as 6,400 feet (about 2 miles 
east of Puu Huluhulu), to avoid building the 
segments at a greater angle to the natural di- 
rection of flow than would be a continuous 
barrier and thus actually increasing the likeli- 
hood of a spill-over. The idea warrants careful 
consideration in relation to topographic stud- 
ies of greater detail and precision than are 
possible on existing base maps. 
Wentworth (unpublished communication, 
1955) has suggested that complete reliance be 
placed on hurried construction of a barrier 
after a flow has actually started to advance on 
Hilo. Experience during the 1955 eruption 
Fig. 7. Map of the area in the vicinity of Hilo, showing the route of the barrier proposed by Jaggar in 1937 and 
surveyed by the U. S. Engineer Department in 1940; and that of the barriers proposed by Finch and Macdonald 
in 1950. The latter route is only approximate. 
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