Maximize Acoustic Doppler Accuracy during Extreme Events with Hydrographic Principles

Daniel Wagenaar - Xylem Water Solutions Australia , 29 June, 2020

The accuracy of acoustic Doppler measurements is directly related to site selection, measurement principles and user operation. Site selection requirements for the application of acoustic Doppler instruments are based on a number of measurement site and hydraulic requirements. The requirements are very similar to what a Hydrologist or Hydrographer will use in the selection of a monitoring site in either natural or artificial channels.

Acoustic Doppler principles and standard operating procedures are essential during the data collection and post-processing process. The application of principles and well documented processes will ensure consistent and quality data sets that are measurable.  Most organisations have well documented standard operational procedures that align with International Standards to ensure that the application of the methodology is consistent.

Obtaining the required knowledge and skill-set to perform, interpret and analyse acoustic Doppler measurements requires considerable exposure time to field application and data review processes. Acoustic Doppler instruments, depending on the sensors used, supply a large amount of information for the operator to interpret and analyse in real-time and during post-processing. 

The application of acoustic Doppler instruments in flow measurement process requires sufficient knowledge on acoustic Doppler principles, hydraulic theory, channel and catchment characteristics, especially during extreme events. The information collected during flow measurements shown in Figure 1 needs to be evaluated in real-time and assessed against acoustic Doppler measurement principles, measurement site and hydraulic conditions.

The measurement site and hydraulic conditions at the measurement section should be assessed before and during the measurement. It is imperative that the operator has a good understanding of possible impacts on acoustic Doppler measurements and how it could affect the overall measurement accuracy.

The review and analysis of acoustic Doppler data during post-processing can be complex, especially if external references such as compass, Global Positioning System (GPS) and conductivity, temperature and depth (CTD) data are used. Multiple tier review was developed to assist operators with analysing and processing of acoustic Doppler data collected. The first tier consists of an “Initial Review” of the various references collected and associated data from external sensors shown in Figure 3. The “Initial Review” should be performed in the field to determine if the measurement quality and if additional data is required.

The second tier consists of “Detailed Review” and should be performed in the office. The “Detailed Review” process breakdown the evaluation to an individual sample. Extrapolation and other processing techniques are performed during this stage to determine a final quality score.

The multiple tier review process outcome is based on the operator’s knowledge and understanding off all the facets discussed. Automated review processes such as QRev from United States Geological Survey (USGS) is highly recommended, especially during complex flow measurements that occur during extreme events. The automated review process also gives a consistent approach in how the data is analysed and processed between operators and different hydrographic offices.