Barriers Against Lava — Macdonald 
263 
especially poor because of the combination of 
these with the normal trade-wind clouds gen- 
erally present even in times of noneruption. 
For days or even weeks at a time targets in that 
area may not be visible from the air. This is 
emphasized by experience during the 1942 
eruption, when the most favorable targets 
chosen during a reconnaissance flight could 
not be seen on succeeding bombing flights, 
and the bombs had to be dropped on less 
favorable targets. 
Possibly the bombs could be placed ac- 
curately, even in dense clouds and smoke, by 
the use of infrared or radar bomb sights. Also 
it has been suggested that heavy artillery fire, 
directed by ground observers close to the 
targets, might be used instead of bombing in 
order to overcome the difficulties of poor visi- 
bility from the air. The method should be 
tried. It appears doubtful, however, whether 
the explosive charges delivered in that manner 
could be sufficiently large to produce the 
desired results. 
Still another limitation to the use of bomb- 
ing arises from the considerable length of 
time required to load planes with bombs and 
fly them to the scene of eruption, and to select 
targets. Rapidly moving flows may already 
have done their damage by the time the 
bombers arrive. Thus, for instance, the lava 
flow that destroyed part of Pahoehoe village 
on the night of June 1, 1950, could not have 
been diverted by bombing because of the very 
short time (about 3 hours) in which it reached 
the village, and because its channel walls were 
not sufficiently well established to permit 
them to be broken down by bombs. The same 
would be true of a similar rapid flow toward 
Hilo. Fortunately, a flow of equal rapidity is 
unlikely in the area near Hilo, because of the 
much gentler slopes on the Hilo side of the 
mountain and the much greater distance of 
Hilo from any likely vents. Nevertheless, 
flows too rapid to be bombed successfully 
before they reach Hilo are possible. The lava 
flow of 1859 traveled the entire distance of 
32 miles from the vents to the ocean in less 
than 8 days, over slopes averaging about the 
same as those southwest of Hilo. 
Thus bombing cannot be relied upon to 
protect Hilo from lava flows. It is a useful 
auxiliary method, and should be employed 
when possible even if lava barriers have been 
built, to help preserve the barriers in a condi- 
tion of maximum usefulness for future 
eruptions. 
EFFECTS OF LAVA FLOWS ON WALLS 
The idea of constructing walls to control 
the course of lava flows is not new. In 1881, a 
loose stone wall was hastily constructed across 
what was then the course of Alenaio Stream, 
in an attempt to prevent the lava from reach- 
ing the Waiakea mill (on the southern out- 
skirts of Hilo). The pahoehoe lava reached 
the wall, formed a pool behind it, and eventu- 
ally spilled over the wall without displacing it. 
This is an interesting illustration of the ability 
of even a loose stone wall to withstand the 
thrust of a lava flow. By chance, the flow 
stopped when the lava had progressed only a 
few feet beyond the wall. If the flow had not 
stopped, the attempt to confine the lava was 
doomed to certain failure because the wall was 
built as a dam directly across the course of the 
flow, and even though the wall confined the 
liquid lava for a short time the reservoir was 
too small to hold any great volume of lava. 
Also in 1881, a much greater project in the 
Hilo area was planned, but never executed. 
W. R. Lawrence, an engineer for the Hawaiian 
government, recommended the construction 
of an embankment along the northern side of 
Alenaio Gulch to confine the lava to the gulch 
and prevent it from spreading northward into 
the main part of Hilo. Arrangements were be- 
ing made to put 1,000 men to work on the 
project, when the flow ended and the con- 
struction became unnecessary (Baldwin, 1953: 
3). If the project had been carried out, it 
probably would have been successful. 
For many years farmers on the slopes of 
Vesuvius have built small walls in an effort to 
keep mud flows from entering their vine- 
