Can Horizontal Acoustic Doppler Current Profilers (H-ADCP) Replace the Traditional Gauging Station?

Daniel Wagenaar - NT Department of Natural Resources, Environment, The Arts and Sport , 22 October, 2010

If we define a gauging station as an instrumentation platform mounted above the highest observed flood levels for the measuring and recording of river heights and consequential determining a stage / discharge relationship, the latest technology provides the opportunity to redefine what a gauging station could look like.

A recent exploration to find a suitable site for a new gauging station on the Daly River in the NT has forced us to look for alternative methods to building traditional gauging
stations. Within the reach identified for the new installation several suitable natural controls were located however each had highly unstable badly eroded banks, a 20 m stage range and wide flood plains above top of bank that were known to get regularly inundated. The site requirements were to accurately measure dry season base flow for accurate modelling of highly transmissive groundwater to surface water interaction whilst at the same time measuring flood stage and flow data. From a rating perspective the area has the added challenge of mobile sand slugs that move downstream at an undetermined rate, potentially affecting the accuracy of traditional rating practices.

The distinction needs to be made between boat mounted ADCPs for one off gauging purposes and in situ permanently mounted horizontal acoustic doppler’s (H-ADCPs) for the collection of continuous (time series) velocity and stage data for the determination of continuous flow. Whilst boat mounted ADCPs are used to help validate the velocity index required developing real time flow, it is the H-ADCP that is replicating a gauging station in this example. An H-ADCP offers the following potential advantages over a traditional gauging station.

  • H-ADCPs can be mounted on minimal infrastructure, minimising the impact and maintenance from flood flows.
  • Underwater battery packs eradicate the need for external (cabled) power systems.
  • Velocity can be directly measured.
  • Cost is significantly reduced with only 1 instrument required to determine stage, velocity and flow.
  • The installation is reasonably mobile.
  • Construction easily undertaken by Hydrographic staff.
  • Need for costly structures built by contractors negated. (Save $50k).
  • Installation unobtrusive and simplifies environmental and cultural permit requirements.

With these aims in mind a complete project was designed with a number of distinct steps to confirm the suitability of H-ADCP technology. The primary steps are as follows.

  1. Design, install and trial stand alone H-ADCP installations at sites but independent of established stage discharge relationships and external stage monitoring gauging station).
  2. Design, install and trial stand alone H-ADCP linked to gauging station for the supply of external power and transfer of data via existing comms to Hydro HQ.
  3. Design install and trial stand alone H-ADCP for completely independent location and develop time series flow data.
  4. Design install and trial stand alone H-ADCP with acoustic modem for transfer of data via air/water interface to a bank mounted modem for subsequent transfer of data to Hydro HQ.

This presentation concentrates on scenario 1-3.

The integration of the Horizontal Acoustic Doppler Current Profiler in the existing Hydrographic operations required the drawing up of a comprehensive Project Management Plan. The project plan highlighted that the integration process will be dependent on a deployment strategy, research, infrastructure development and the supplying of training. It was found that during the deployment process clearly defined monitoring site objectives assisted in the selection of type of instruments and data processing methodology. The researching of the acoustic technology was started originally as a single installation and has been expanded to 10 operational gauging stations.

The influences of hydraulic and site conditions on the field measurements were noticeable during the deployments at the different gauging stations. This confirmed that the configuration requirements and instrument operations are site specific. Further site specific information that is required in the flow calculation process is the velocity index, which is calculated from the velocity measured by the instrument and a series of gaugings throughout the stage range. This is required in conjunction with the cross-sectional area to calculate the flow at the gauging site.

The development of an instrument mounting for precise alignment was essential due to the fine instrument tolerances. Further development is still required on the mounting mechanism for easy and safe access to the instrument. The nature of our monitoring sites is such that the distance between the instrument and shelter exceeds the maximum cable length of the instrument, which prompted the development of a battery pack and extension to an external power supply. Further developments that have been identified from the project plan are SDI12 communications, telemetry data and Hydstra, which will form part of the final stages of the project. The documentation and software that is involved in the entire process have been streamlined into a single procedural document with only a select number of software required for the operations.

This presentation will concentrate on the practical elements of the project, with reference to the parameters that need particular attention for successful data collection. Advantages and limitations in use of the technology. And in deed whether an H-ADCP can replace a traditional gauging station.