Feasibility of a Lava-diverting Barrier at Hilo, Hawaii 1 
C K. Wentworth, H. A. Powers, and J. P. Eaton 2 
The SUBJECT of the value and possibility of 
protecting Hilo Harbor and vicinity from de- 
vastation by a lava flow from Mauna Loa is 
again being given thoughtful consideration by 
the residents of Hawaii. Those who must weigh 
the pros and cons of this matter need informa- 
tion, part of which can best be appraised by 
geologists and engineers. From the geologists’ 
appraisal should come answers to questions such 
as the following: How often might protection 
from a lava flow be needed? Is it physically 
possible to divert a lava flow with a man-made 
structure? What are the necessary dimensions 
of such a structure? Of what should it be built? 
What is its expected useful life? 
Various references to cost have been made. 
Some say that a barrier is justified, regardless of 
cost; others hold to a strict accounting of sup- 
posed risk against cost, amortization, and other 
factors. These opposed views are widely sep- 
arated. Many risks could be reduced by astro- 
nomical spending, but such spending may be 
beyond reasonable relation to contemporary life 
or even to capacity of the community to pay. 
Though opinions may differ greatly, the crite- 
rion of economic justification cannot be ignored 
altogether. 
Much has been written on the subject of a 
lava barrier for Hilo. The latest and most com- 
prehensive review and discussion is by Gordon 
A. Macdonald ( 1958). His greatest emphasis is 
laid on the matter of a barrier system to be con- 
structed across the slope above Hilo to divert 
the course of an approaching lava flow. He con- 
cludes that a system of barriers can divert the 
course of a lava flow. 
The conclusions reached in this report differ 
in this matter from those expressed by Mac- 
donald because different evaluations are made 
of the same few facts available for appraisal. 
1 Publication authorized by the Director, U. S. Geo- 
logical Survey. Manuscript received October 24, I960. 
2 U. S. Geological Survey, Hawaiian Volcano Ob- 
servatory, Hawaii National Park, Hawaii. 
Among the most important of these different 
evaluations, this report concludes that the mini- 
mum condition for the successful functioning of 
a diversionary system is the construction of a 
channelway adequate to conduct the lava flow 
along the chosen route behind the barrier sys- 
tem. An adequate channel may exceed 2 mi. in 
width with rock excavation in excess of a 400-ft. 
depth along the upslope margin, even with a 
barrier 60 ft. high along the downslope margin. 
Facts needed to design the channel system and 
to appraise the amount of funds that can pru- 
dently be invested in it are imponderable — facts 
such as the volume of flow to be expected and 
the probable frequency of hazard. In the face of 
such imponderables, a downslope diversionary 
system is unrealistic; it would seem prudent to 
rely on, and plan for, defensive actions that can 
be taken during an eruption, such as causing 
distributary flows at or near the vent. 
FORECASTING ACTIVITY 
The waxing and waning of volcanic activity 
shown in the geologic history of Mauna Loa 
makes it impossible to give a dependable predic- 
tion of the probable hazard to Hilo from lava 
flows. The possible hazards cover a great range: 
Hilo might be obliterated by another eruption 
from the same vicinity as the prehistoric erup- 
tions that formed the Halai Hills (see Fig. 1); 
or it is possible that no future lava flow will 
ever reach Hilo. Since Hawaiian oral history be- 
gan, perhaps about A.D. 1100, only one lava 
flow from Mauna Loa, that of 1881, reached the 
vicinity of Hilo. 
It is natural to predict future events on the 
premise that events of the best-known past will 
be repeated; in this instance, the history of 
Mauna Loa’s activity since 1843. How disas- 
trously wrong such a prediction can be was em- 
phasized by the eruption in I960 of the Kilauea 
lava flow in Puna. After the devastating flow in 
1955, no further outbreak in that region was to 
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