Stream Flow Monitoring at Latrobe River Swing Bridge

Garry Leslie and Wayne Ross - Thiess Services , 22 August, 2012


In March 2010, the Department of Sustainability and Environment (DSE) funded a study to determine improvements in the reliability of surface water flow estimates utilising a horizontal acoustic doppler velocity meter (H-ADVM). Eight sites in the Gippsland region were selected, seven of these were installed at existing monitoring sites; the eighth H-ADVM was to be installed at a proposed site on the Latrobe River, downstream of the Swing Bridge. In 1965 a site was established downstream of the confluence of the Thomson and Latrobe Rivers, this was discontinued in April 1992 without having a stage flow relationship established. Conventional methods were not appropriate due to the flow conditions being classified as unsteady. Due to improvements in acoustic doppler technology the accurate calculation of flow at the Latrobe River, downstream of the Swing Bridge became a viable possibility using an H-ADVM.

The Latrobe River at D/S Swing Bridge (226027B) site was installed in a straight reach 4 km downstream of the confluence of the Latrobe and Thomson Rivers with good access for construction of infrastructure and undertaking of flow calibration measurements. A SonTek Argonaut -1.5 MHz Side Looker ADVM with a real time digital flow display was installed. The ADVM was connected to a Campbell Scientific CR800 data logger with a Modmax NextG modem for remote interrogation. The ADVM was installed on an adjustable stainless steel arm which enabled the lifting in and out of the water for easy cleaning and inspection of the unit. All other instruments were installed in an elevated housing approximately 3.00 m above natural surface level to avoid inundation. A 0-1 metre gauge set to AHD to cover the normal range of flows. A cross section of the bed profile was levelled at the location of the ADVM and later programmed into the AVDM to determine the cross-sectional area and for flow calculation.

Flow measurements were undertaken on a monthly basis to establish a velocity index to calibrate the ADVM to actual flow. Measurements were carried out using a SonTek River Cat and M9 using the stationary method. These measurements were carried out 30 m downstream of the ADVM so as not to interfere with the ADVM data collection but still close enough to establish a well-defined index relationship. In conjunction with each measurement a salinity profile was carried out to identify the presence/absence of a salt wedge. The presence of a salt wedge was only identified during continued low flows and dissipated during the subsequent higher flows.

At the end of April 2012 32 flow measurements have been carried out, and an index velocity rating established and maintained. The majority of measurements plotted within + 6% of ADVM calculated flows. However, the over bank flows during major floods which can stretch up to a couple of kilometres along either bank could not be accounted for in the total flow calculation, as the hydraulic behaviour of these flows are completely different to the one’s experienced in the main stream.