Extreme Flood Event Measurement Technique Utilising Measurement of Maximum Velocity

Rebekah Webb - Ventia , 21 December, 2018

Flood events represent a valuable opportunity to collect stream discharge measurements which may then be used to calibrate hydraulic models. As hydraulic models are relied upon to predict flood levels, it is imperative that the data collected during floods is accurate. Unfortunately, undertaking discharge measurements during a flood event may be a difficult and dangerous task with significant risks posed to the hydrographer and the instruments used in this activity.

During the Victorian floods in 2010, 2011 and 2012, analysis of gaugings conducted at some sites conducted with both traditional mechanical current meters and acoustic Doppler current profilers (ADCP) indicated that both instruments failed to produce reasonable results. In some cases, the ADCP failed to detect the bottom of the river to measure the depth, or the velocity was not accurately determined due to heavy sediment load or excessive cavitation. Additionally, during these events there were times when the measuring instrument was swept away.

As velocity and water height in a river vary drastically with time during a flood event, obtaining an accurate discharge measurement using traditional techniques may not be possible, even at sites where it is safe to deploy instruments. Therefore, a new measurement technique which allows a quick sampling of the velocity may be more appropriate in such flow events. Such a method would also provide greater personal safety for both the hydrographer and the instruments by reducing their exposure time.

Ventia has developed a method based on the site-specific relationship between mean velocity, maximum measured velocity and hydraulic depth. The method was derived from the historical discharge measurements taken at each site. It is envisaged that this measurement technique will be utilised at sites during extreme flood events, when it is unsafe to either the hydrographer or the instruments to allow a long exposure time. This is most likely at heights that have yet to be measured at the stations and the technique is designed to allow the collection of data even under these trying conditions.