Sudden rise of Mary not a shock
THE rapid rise of the Mary River on Saturday took some by surprise but rises of a metre an hour in river height is not that unusual in this region.
River rises of more than a metre an hour have occurred in the Mary on numerous occasions. In 1955 Gympie had three floods in a matter of weeks: March 10 13.87m, March 28 21.44m and May 1, smaller at 11.20m.
The March 28 flood rose at a metre or more an hour for an extended period.
The same happened in February 1972.
The weekend's flood event also demonstrated just how lucky Gympie was in January last year when the river inundated Mary St and many other areas reaching a height of about 19.4m.
Last year the major feeder creeks to Gympie's east did not have the rainfall that occurred in the river's headwaters, thus saving the city from a much higher flood peak.
On Friday night the deluge of 300mm plus fell in the Six Mile Creek catchment and rapidly flowed towards the Mary, which was already swollen as a result of rainfall earlier in the week in its headwaters.
Flooding in the Mary is controlled by the amount of rain that falls in the headwaters area towards Conondale and Kenilworth.
This in turn is further swollen by the tributaries on both the eastern and western side of the river.
In 2011 most of the flooding was the result of rainfall in the headwaters and on the western side of the river.
The weekend's flooding was the result of rainfall in the Six Mile catchment.
If similar falls had been recorded in the Deep Creek catchment, coupled with the western creeks, the flood height would have been much higher.