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PACIFIC SCIENCE, Vol XII, July, 1958 
posed main Hilo barrier from the active part 
of the Mauna Loa rift zone. Once a tongue 
reaches that great a distance from the vents it 
generally is well established as the principal 
flow tongue of the eruption. But the possi- 
bility of a second tongue reaching the barrier 
up slope from the first must be kept in mind, 
and, if possible, means must be provided to 
cope with It. 
BARRIERS PROPOSED FOR HlLO AREA 
Barriers Proposed in 1937. The positions of 
the barriers suggested by Jaggar (1937, 1945) 
are shown in Figures 1 and 7. The principal 
barrier was to start at the Wailuku River a 
short distance above the Pukamaui Falls 
(where the principal intake of the Hilo water 
system is located), extend 4 miles east- 
southeastward, then turn and extend 5 miles 
east-northeastward, ending about a mile south 
of the shore at Keaukaha. This proposed bar- 
rier was intended to divert southward any lava 
flows approaching Hilo along the Wailuku 
Valley or down the slope of Mauna Loa north 
of the Waiakea Homesteads. Two other shorter 
proposed barriers were located higher on the 
mountainside. One extending northwestward 
from the vicinity of Puu Ulaula, at 10,000 feet 
altitude on the northeast rift zone of Mauna 
Loa, was intended to divert westward flows 
originating on the rift zone above Puu Ulaula. 
The other, extending south-southeastward 
from near Puu Huluhulu, in the Humuula 
Saddle, was intended in effect to shift the Hu- 
muula divide farther east so that flows pooling 
in the flat area just south and west of Puu 
Huluhulu would spill westward instead of 
eastward toward Hilo. 
The plan adopted in the report of the Dis- 
trict Engineer, U. S. Engineer Department, 
closely resembled the original recommenda- 
tions by Jaggar. The barrier close to Hilo was 
to be 46,750 feet long, varying in height from 
20 to nearly 80 feet, with a flat top 5 feet wide 
and slopes of 45°. It was to be built largely of 
material available at the site. At stream cross- 
ings a cluster of concrete pipes of 48-inch di- 
ameter laid through the barrier would allow 
water to pass, but molten lava entering the 
pipes would quickly chill in them and solidify, 
plugging them. At highway crossings con- 
crete underpasses were provided, which could 
be blocked with concrete stop logs when a 
lava flow approached. For further details of 
the proposed construction the reader is re- 
ferred to the paper by Jaggar (1945), and the 
unpublished report of the District Engineer. 
For convenience of reference, some of the 
drawings of construction design are repro- 
duced in Figure 8. 
Barriers Proposed in 1930 . Following the 
1950 eruption of Mauna Loa, concern again 
increased in Hilo over the possibility of dam- 
age to the city by lava flows, and Finch and I 
undertook a restudy of the barrier proposal. 
As a result of the study, - we were more than 
ever convinced that barriers would be effec- 
tive. However, because of the growth of Hilo 
in recent years, we suggested that the position 
of the proposed barrier might be shifted south- 
westward from that previously advocated. The 
positions of both lines are shown in Figure 7. 
The new proposed barrier would consist of 
several segments. The upper and principal 
segment would extend from the Wailuku 
River at approximately 3,900 feet altitude 
east-southeastward about 12.6 miles to a point 
where the lava flow would be guided down 
slope by a natural drainage channel. Farther 
seaward other shorter barriers would direct the 
flow into forest land southeast of Hilo where 
natural topography would lead it away from 
Hilo city, harbor, and airport. The total length 
of the newly proposed barriers is approxi- 
mately 17 miles, as compared to 8.85 miles for 
that proposed in 1940. The new line extends 
south of Kaumana and the Waiakea Waena 
suburb of Hilo, which lie outside the barriers 
of the earlier scheme. It also provides more 
complete protection for the Hilo Airport area 
and the Keaukaha suburb, and protects the 
drilled wells east of the airport, which in time 
of eruption might provide the major source of 
water for the city. 
