Barriers to Protect Hilo from Lava Flows'

Barriers to Protect Hilo from Lava Flows' GORDON A. MACDONALD2 THE CITY OF HILa, on the island of Hawaii, out because it was considered not to be a lies on the flank of one of the world 's most justified function of the War Department active volcanoes, Mauna Loa. For more than (Jaggar, 1945: 340-341). a century the danger of destruction ofthe city It is the purpose of this paper to review the by lava flows has been recognized. Old docu need of protection for Hilo, and the methods ments recount the apprehension with which by which it might be accomplished. When I Hilo residents watched the advance of the first heard ofthe proposal to protect Hilo from lava flows of 1852 and 1855. In 1881 concern lava by means of walls to deflect the flows, I was even greater, as the flow front crawled was very doubtful whether the method could within a mile of the shore of Hilo Bay. Early be successful. However, the study of active in his studies of Hawaiian volcanoes, the late flows during 7 eruptions and of many older Dr. Thomas A. Jaggar recognized the threat flows, in the course of 17 years of experience to Hilo, and for many years the safety of the with Hawaiian volcanoes, has convinced me city and methods by which it might be in that such walls have an excellent probability sured were among his principal concerns of succeeding. Attempts to build diversion (Jaggar , 1931, 1949). barriers during the 1955 eruption of Kilauea In 1937, following preliminary studies by have not weakened that conviction, though the staff of the Hawaiian Volcano Observa they have shown that the walls must be care tory, Jaggar proposed the building of a bar fully planned, and properly placed and rier, or barriers, on the lower slopes ofMauna constructed. Loa to deflect lava flows from Hilo harbor and Whether barriers are likely to be needed, its immediate vicinity. Such a barrier would and whether successful barriers can be built, consist essentially of a great wall stretching are questions properly falling within the field diagonally across the slope. The purpose of of the volcanologist. Answers to only those the barrier would not be to hold back the questions are attempted herein. The question flow, like a dam, but to turn the flow and di of whether a barrier should be built involves rect it away from the vital area. In 1938 a study complex considerations of relative values of of the project was begun by the U. S. Engi the area to be protected, income to be ex neer Department (now U . S. Army, Corps of pected from the area, effects of loss of the Engineers). A route and design for the barrier area upon the economy of surro unding areas, were chosen, and the entire proposal was sub effects of displacement of population as a jected to careful study. It was found in the result of loss of the area and influence on ad estimate of the Engineer Department to be jacent areas, cost of construction of the bar entirely feasible. The official report, in January rier plus interest on the cost, the ability of the 1940, stated: " The District Engineer believes community (either locally or at large) to pay it is possible to protect the harbor and city by this cost, and no doubt other factors . There a properly located and constructed barrier. " are also the legal questions arising from diver The construction ofthe barrier was not carried sion of lava onto land that otherwise might 1 Publication authorized by the Direcror, U. S. not have been covered during that eruption. Geological Survey. M anuscripr received Feb. 8, 1957. These questions fall outside the province of 2 Presenr address, Deparrmenr of Geology and Geo physics, University of Hawaii. Conrribution N o. 6, the volcanologist and must be decided by H awaii In srirure of Geophysics. economists, sociologists, and lawyers. 258 Barriers Against Lava- MACDONALD 259 ACKNOWLEDGMENTS ity in coming centuries will differ appreciably from that of the last. Since the 1955 eruption ofKilauea the mat The vents of flank eruptions of Mauna Loa ter of lava barriers for the protection of Hilo are concentrated along two zones of fractur has been the subject of frequent discussion ing, known as rift zones, that extend re with Jerry P. Eaton, geophysicist of the Ha spectively east-northeastward and southwest waiian Volcano Observatory, U. S. Geological ward from the summit of the mountain. The Survey. Earlier, it had been discussed re northeast rift zone averages about a mile in peatedly with the late Thomas A. Jaggar, width, and trends almost directly toward Hilo. Ruy H. Finch (my predecessor as director of It is marked at the surface by innumerable the Observatory), and Chester K . Wentworth. fissures in the ground, and lines of cinder and Although minor differences of opinion exist, spatter cones built at the sites of eruptions. I believe we agree on essential points. The three small cinder cones known as the Curtis Kamai, engineer for the Territorial Halai Hills , within the city of Hilo itself, ap Highway Department, through the courtesy pear to lie on the prolongation of the north of that department, worked closely with us east rift zone; but fortunately the portion of during the 1955 eruption and paid special the rift zone below an altitude of 6,000 feet attention to the behavior of flows in relation has been inactive for many hundreds of years. to barriers. Discussions with him during and Eruption along the northeast rift zone has since the eruptionaregratefullyacknowledged. built a broad, rounded ridge trending toward Doak C. Cox, geologist for the Hawaiian Hilo. The north slope of this ridge intersects Sugar Planters ' Association Experiment Sta the south slope of Mauna Kea , producing a tion , has contributed valuable discussion of broad valley through which the Wailuku River the manuscript. and its tributaries flow eastward into Hilo Bay (Fig. 1). Because ofthis topographic configu NE ED FOR PROTECTION ration, all lava flows erupted from the northern Hilo Bay lies at the junction ofthe slope of part of the rift zone below approximately Mauna Loa volcano with that of Mauna Kea 11,500 feet altitude are directed toward Hilo to the north (Fig . 1) . Most ofthe city ofHilo, within a belt about 6 miles wide. Whether or south of the Wailuku River, is built on geo not they reach Hilo depends largely on the logically recent lava flows from Mauna Loa. volume of lava released during the eruption, The very existence ofHilo Bay is the result of and whether it is concentrated into a single these flows, which constitute all of the broad Row or spread as several flows over the upper promontory that exten ds eastward to Leleiwi slope of the mountain. Point. These flows cannot now be dated in It is the restriction of Rows approaching years, but probably most of them are less than Hilo to this relatively narrow (6 miles wide) 2,000 years old. belt just south of the Wailuku River that Since about 1820, when our real knowledge makes feasible the protection of the city by of Hawaiian volcanoes begins , Mauna Loa diversion barriers. has been among the most active volcanoes in Flows issuing at points on the northeast the world . During that period it has erupted rift zone above 12,000 feet altitude probably on an average once every 3.6 years, and the will turn westward in the Humuula Saddle (as total lava poured out has been more than 4 did the flow of 1843), and consequently do billion cubic yards. Nothing in the geological not constitute a threat to Hilo. record indicates that this degree of activity is Since 1850 there have been 6 major erup abnormal in the history of the volcano , nor is tions in the northeast rift zone, producing 8 there reason to expect that the degree ofactiv- major lava flows with an aggregate volume of 260 PACIFIC SCIENCE, Vol. XII, July, 1958 156° 20'--+----4 ""-:-------+------20° EXPLANATI ON _____ BARRIER PROPOSED IN 1937 - --- BARRIER PROPOSED IN 1950 m,tttt'i'!?iViJD LAVA FLOW WITH DATE 0 19 - --+-- - --- -""l':i 10 19 ° Contou r inte rva l 10 0 0 feel. 156° FIG. 1. Map of the island of Hawaii, showing the locatio n of the city of Hilo, and of barriers proposed to protect it from lava Rows originating on the northeast rift zone of Mauna Loa. more than 1,000,000,000 cubic yards. Ofthese, from a vent at 9,200 feet altitude and ad 7 Rows have advanced toward Hila, and in vanced northeastward 16 miles, coming to a 1881 lava actually invaded part of the present halt 12 miles from the shore of Hila Bay. As city. The volume of the 1881 Row toward compared with these Rows from the northeast Hila was approximately 250,000,000 cubic rift zone, the 1859 Rowan the northwest yards. In 1942, a Row with a volume of ap slope of the mountain and the 1950 Rows proximately 100,000,000 cubic yards started from the southwest rift zone each had a vol- Barriers Against Lava- MACDONALD 261 umeofapproximately 600,000,000 cubic yards. with the mouth of the Wailuku River is ap This latter volume is more than twice that of proximately 45,000,000 cubic yards, and in the 1881 flow, and 6 times that of the flow of the same area the central channel below a 1942. The distance from the source of the depth of5 fathoms has a volume ofonly about 1859 flow to the point where it entered the 3,000 ,000 cubic yards .Part of any flow enter ocean is 32 miles.

Load more