THE CONTROL OF LAVA FLOW DURING THE 1991-1992 ERUPTION OF MT ETNA

All the actions carried out in 1992 to protect the village of Zafferana Etnea from being invaded by lava are described. An earthen barrier 234 m long and 21 m high was firstly built in January 1992 by accumulating with mechanical escavators 370,000 m3 of earth, scoriae and stones. This embankment contained the lava for about one month and was overflowed by April 9, 1992. Three additional smaller earthen barriers (lenght: 90-160 m; height: 6-12 m) were built in April to gain time while the lava front was descending towards Zafferana from the overflowed first embankment. The major effort of the 1992 operation consisted of several attempts at stopping the lava front advance by diverting the flow out from the natural and extensively tunnelled channel through a skylight near the vent. The main intervention point was located in Valle del Bove at an elevation of 2000 m, at 8 km from Zafferana, in a zone almost unaccessible from land: helicopters were hence extensively used during the whole operation. Initial interventions called for attempts at plugging a tunnel by dumping into it linked concrete blocks, hedgehogs and blasted portions of the solid levee. Each intervention caused the partial obstruction of the tunnelled channel, which determined major increases of lava overflow in Valle del Bove and the consequent halt of the most advanced fronts. However, benefits were of brief duration, at the most two weeks of respite, before new lava fronts approached again and again the outskirts of Zafferana. The final successful intervention was carried out on May 27-29. An artificial channel was dug departing from the natural one. The solid separation levee was thinned to 3 m and blasted by 7000 kg of explosives. After the explosion, 2/3 of the lava flowed spontaneously in the artificial channel and then the total diversion was obtained, the tunnel being plugged by dumping into the natural flow 230 m3 of lava boulders. As a consequence of the intervention the active natural lava front, that on May 27 was only 850 m from Zafferana, came to an halt, as did the entire flow downhill from the diversion point, bringing back the situation as it was five months earlier, a few days after the beginning of the eruption, with the new front of the diverted flow at 6-7 km from Zafferana. In June 1992, the effusion rate halved from 30 to 15 m3/s and with this reduced thrust the lava was no longer capable of covering long distances. Five months after the conclusive intervention, the diverted lava continues to flow over its initial natural field but remaining confined in the upper Valle del Bove, without any new threat to Zafferana.