In 2010 and 2012 there were significant flooding events in the Murrumbidgee with the later resulting in significant damage in Wagga and surrounding areas. The last time that the area had a similar sized event(s) was in 1974/75. These earlier events were measured by the Hydrographers at the time around their peak and a high stage rating was created. This high stage rating had been applied to all subsequent ratings and was thought to be valid as it was accepted that high stage ratings were stable and unlikely to significantly change over time.

Fast forward to 2010 and 2012 and the measurements made at comparable heights to the 74/75 floods yielded far less water for the same height.

WHY?

There had been many changes over 40 years, in not only how we now operate as Hydrographers but in the Catchment Morphology. The original gaugings were taken by current meters and from bridges, whilst the later measurements were taken using Acoustic Doppler Current Profile technology in a moving boat. There had also been significant changes in the catchments over the last 40 years. There has been revegetation of riparian areas and flood plains that had once been cleared for farming, this may have been legislatively driven for environmental benefits. There had also been many civil changes to the flow line with new bridges, flood levees, quarries and golf courses etc. that may or may not have had an impact on why there was a difference.

So the challenge is to validate if the measurements both in 1974/75 and in 2010/2012 were accurate and to identify whether the catchment changes were significant enough to alter the flood flow dynamic. Another point of interest was at what point was the change if real, as it will have a significant impact on the total flow of the Murrumbidgee over the last 40 years (≈13% in a year with a significant flow event).