Barriers to Protect Hilo from Lava Flows 1 
Gordon A. Macdonald 2 
The city of Hilo, on the island of Hawaii, 
lies on the flank of one of the world’s most 
active volcanoes, Mauna Loa. For more than 
a century the danger of destruction of the city 
by lava flows has been recognized. Old docu- 
ments recount the apprehension with which 
Hilo residents watched the advance of the 
lava flows of 1852 and 1855. In 1881 concern 
was even greater, as the flow front crawled 
within a mile of the shore of Hilo Bay. Early 
in his studies of Hawaiian volcanoes, the late 
Dr. Thomas A. Jaggar recognized the threat 
to Hilo, and for many years the safety of the 
city and methods by which it might be in- 
sured were among his principal concerns 
(Jaggar, 1931, 1949). 
In 1937, following preliminary studies by 
the staff of the Hawaiian Volcano Observa- 
tory, Jaggar proposed the building of a bar- 
rier, or barriers, on the lower slopes of Mauna 
Loa to deflect lava flows from Hilo harbor and 
its immediate vicinity. Such a barrier would 
consist essentially of a great wall stretching 
diagonally across the slope. The purpose of 
the barrier would not be to hold back the 
flow, like a dam, but to turn the flow and di- 
rect it away from the vital area. In 1938 a study 
of the project was begun by the U. S. Engi- 
neer Department (now U. S. Army, Corps of 
Engineers) . A route and design for the barrier 
were chosen, and the entire proposal was sub- 
jected to careful study. It was found in the 
estimate of the Engineer Department to be 
entirely feasible. The official report, in January 
1940, stated: "The District Engineer believes 
it is possible to protect the harbor and city by 
a properly located and constructed barrier.’’ 
The construction of the barrier was not carried 
1 Publication authorized by the Director, U. S. 
Geological Survey. Manuscript received Feb. 8, 1957. 
2 Present address, Department of Geology and Geo- 
physics, University of Hawaii. Contribution No. 6, 
Hawaii Institute of Geophysics. 
out because it was considered not to be a 
justified function of the War Department 
(Jaggar, 1945: 340-341). 
It is the purpose of this paper to review the 
need of protection for Hilo, and the methods 
by which it might be accomplished. When I 
first heard of the proposal to protect Hilo from 
lava by means of walls to deflect the flows, I 
was very doubtful whether the method could 
be successful. However, the study of active 
flows during 7 eruptions and of many older 
flows, in the course of 17 years of experience 
with Hawaiian volcanoes, has convinced me 
that such walls have an excellent probability 
of succeeding. Attempts to build diversion 
barriers during the 1955 eruption of Kilauea 
have not weakened that conviction, though 
they have shown that the walls must be care- 
fully planned, and properly placed and 
constructed. 
Whether barriers are likely to be needed, 
and whether successful barriers can be built, 
are questions properly falling within the field 
of the volcanologist. Answers to only those 
questions are attempted herein. The question 
of whether a barrier should be built involves 
complex considerations of relative values of 
the area to be protected, income to be ex- 
pected from the area, effects of loss of the 
area upon the economy of surrounding areas, 
effects of displacement of population as a 
result of loss of the area and influence on ad- 
jacent areas, cost of construction of the bar- 
rier plus interest on the cost, the ability of the 
community (either locally or at large) to pay 
this cost, and no doubt other factors. There 
are also the legal questions arising from diver- 
sion of lava onto land that otherwise might 
not have been covered during that eruption. 
These questions fall outside the province of 
the volcanologist and must be decided by 
economists, sociologists, and lawyers. 
258 
