Tangiwai Railway Bridge Guarded By Flood Alarm

Press, Volume XCIV, Issue 28157, 21 December 1956, Page 14

Tangiwai Railway Bridge Guarded By Flood Alarm

A flood warning device guarding the Tangiwai railway bridge, scene of the flood disaster on Christmas Eve three years ago, is now in use by the Railways Department. The warning device comprises a series of electrodes, mounted on a 20ft high concrete pylon in the riverbed, and connected by cable to audible ai d visual alarms in Waiouru station and to an audible alarm in the train, control office at Ohakune. The pylon is eight miles upstream from the Tangiwai bridge, where the Auckland-Wellington Main Trunk railway crosses the Wangaehu river. At least half an hour’s warning is given before any flood waters can reach the bridge. On the downstream side of the pylon, at 12-inch intervals from top to bottom, pairs of insulators, capped with lead domes as electrodes, are sunk into the pylon wall. As each group of electrodes becomes covered by flood waters, electrical circuits ire completed and warning lights are lit at Waiouru station. At appropriate levels, a warning bell is also sounded. Provision is made for constant testing of the equipment, and any failure of the electrical circuits results in an alarm being sounded. Many stringent requirements had to be satisfied before the final selection of a warning device was made. It was necessary to ensure that the system would not be affected by the weather, and that it would not be

vulnerable to stray rifle bullets or ' action by vandals. It also had to be designed so that it would not be out : out of action by boulders, ice, mud, volcanic ash, or sand Because of these requirements, various types of mechanical warning , devices had to be discarded. None of : the usual kinds of warning device was found to satisfy all the needs in . this particular instance. The design of the system emerged from the combination of factors as i one which would normally give an ; “all safe” indication and be capable , of transmitting several different types ; of information about river conditions, i The system incorporates alarms which will indicate any failure, gradj ual rising of the river waters, a prei determined height of wave-front, and I a dangerous flood height. : To guard against power failures, i the system is battery-fed, and so [ arranged that it will remain in oper- ! ation for more than 24 hours after : any failure of the power supply to i the battery-charging equipment.