Brisbane River

Brisbane flood prediction map, 1933. Drawn by the Survey Office, this map was prepared by the Bureau of Industry in 1933. At the time it provided information on which areas of Brisbane were likely to be inundated under various flood scenarios: 10, 15, 20, 25 and 30 feet. These areas were shaded blue, pink, yellow, green and plum respectively. Because ‘improvements’ (dredging and channel widening) had been made in the lower river, flood heights were expected to be at least four feet lower from the Victoria Bridge downstream, however, no appreciable effects were expected upstream. The map contained a list of major floods since 1841 as well as a table estimating the probable duration of floods at different heights. The last major flood was on 7 February 1931 which rose just below 15 feet. Its approximate extent can be inferred from the areas shaded red. According to the map, 15 feet at the Port Office gauge was the ‘critical level’ where ‘no serious damage need be anticipated by a flood which does not exceed that height.’ A flood around 15 feet or below would not be treated as lightly. The cost of this map was three shillings. Collection of the Museum of Lands, Surveying and Mapping

Brisbane flood heights, 1841-96. In response to the 1893 floods, the head of the Water Supply Department, J.B. Henderson, compiled this diagram in recording the history of floods from 1841. Based on historical accounts, it was probably the first time any comprehensive and comparative analysis had been made. According to reputable accounts, Brisbane’s first recorded flood was in September 1825, but no information could be found regarding its height and it was not included. The first major flood shown on this diagram was 14 January 1841 which, by Henderson’s calculations, was at least two inches higher than the ‘Great Flood’ in early February 1893. Included on the diagram were the computed flood levels for three flood mitigation schemes recommended by the Water Supply Department: ‘A’, ‘B’, ‘C’. Scheme ‘B’, straightening the river by cutting through Kangaroo Point and New Farm, would have the highest flood mitigation impact. According to Henderson, in an 1893 flood event the water level would be reduced to about one foot over the floor of the Queen’s Hotel, the equivalent height above the Norman Wharf. Queensland votes and proceedings, vol.4, 1896