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Title: Calibration of Gauging Stations Using Portable Ultrasonics
Author: Water Services Scotia
Author: Environment Agency
Document Type: Monograph
Annotation: Environment Agency Project ID:EAPRJOUT_618, Representation ID: 202, Object ID: 1952
This areport .presents the results of an Environment Agency R and D Project that -was commissioned to determine the feasibility of using portable ultrasonic fIow gauges. working on. the time of flight principle to calibrate structures, particularly at ,high flows and/or non modular conditions. The research was to determine the limitations on use;accuracy and optimum configuration for gauge deployment, and arose from an increasing need for good: quality hydrometric data, both by the Agency and outside users, and the increasing availability of portable equipment -that can be moved from site to site. The report provides a background to the development :and. use of ultrasonic flow gauges, which :have themselves evolved over a number of years. The respective advantages and disadvantages of different gauge configurations are discussed,together with an introduction .to some of the theoretical uncertaintiesa that are associated with this approach to flow measurement. A comparison between the performance of twin-path and multi-path agauge configuration- is presented in the report, which shows that,whilst the twin-path system may not be as accurate as one using- more paths, the decrease in performance might not be::as large as previously. thought. In particular, it was found that deploying the two transducer paths within two identified vertical azonesa would-decrease the uncertainty in the derived flows.; The recommendations for- setting the twin-path transducer levels to .give optimum performance are: Lower path Upper path deployed at less than.0.3.of the design water depth deployed at between 0.45 and 0.7 of the design water depth During the course of the,Project three twin-path gauges were deployed as stand alone. units at a total of six different gauging stations and operated in a number of different ways.. Using. the. experience gained from these field .a studies a series of recommendations have been made regarding gauge installation and commissioning. These recommendations include .aaspects of site survey, mounting systems, power supplies, transducer selection and .:gauge parameter settings. Whilst .a .atypicala installation may take approximately 30 staff ,hours to complete, more complex or larger sites ,may take up. to 40 hours and a relatively compact site could be completed in 20 hours. The data collected from the.-gauges was used to calculate flows by three different approaches:. 1. Using the gauge as a twin-path system to calculate flows and derive rating curves; R and D.Technical Report .W189 vi 2. Operating the gauge data to derive stage-velocity relationships from which stagedischarge relationships were then calculated; 3. Operating the gauge to measure flows in real time at sites where there is no single, stable relationship between level and flow. The first of these approaches demonstrated that, with the exception of high sediment loads at the peak of some events at some of the field sites, the equipment performs well under a range of flow conditions. Using data collected at three of the field sites it is demonstrated that the gauge is able to calculate flows to within a typical uncertainty (within the recorded range) of +lO% using the manufactureras default parameter settings. This uncertainty can be reduced. to under 27% by setting one of the parameters to a revised value, and can be further reduced to +5% by optimising the parameter on site when commissioning the equipment. The typical equipment costs of operating the gauge in this manner are approximately 51,300 per site, plus up to 70 staff hours on site, plus any additional travel time and costs in excess of normal travel. If the stage and velocity data collected by a gauge are used to develop stage-velocity and stage-discharge relationships, the analysis presented in the report demonstrates that the total uncertainty in the derived flows can be reduced to between k2.5 and 25%. In addition to reducing the uncertainty in the derived flows, this approach removes any anomalies in the derived flows that may arise if one of the velocity paths should fail during a specific event. The additional cost of these benefits is assessed at almost 20 staff hours per site, aor three days work, with the equipment costs. being similar to those of the straightforward twin-path approach. An essential pre-requisite of using the ultrasonic gauge to derive stage-discharge ratings is a stable relationship between level and flow. Where this is not the case, the gauge may be used as a permanent installation to monitor flows in real time. The work presented in this report demonstrates that the gauge is sensitive enough to be able to measure river velocities under differing hydraulic conditions, and. detect the onset of non-modular flows. Operational costs are similar to those of more conventional techniques currently used by the Environment Agency such as crest tappings, with the advantage that the gauge can retrospectively installed at a site that does not have a tapping.. In addition to the three identified methods used to calculate flows at a given site, the report also presents the results of alternative approaches that were evaluated during the course of the Project. These include reflector systems, multi-level operation during an event, and non-horizontal transducer paths. In all cases it was found that whilst the gauge may work using these approaches they are less robust that the more conventional configurations. It is thus concluded that these approaches should only be adopted if site conditions dictate that they are the only viable method. R and D Technical Report W 189 vii Finally, the report provides recommendations for implementing the results of aLhe Project at both the National and: Area levei. Specific issues are identified for consideration by the National Hydrometric Group and/or Area Water.Resources and Flood Defence management ,Iteams;. and guidanke is -provided on a.selecting the appropriate approach to using the equipment. Recommendations for further -research are also -made, and ..include an assessment of- the relationship between gauge- .. performance and sediment load.
Publisher: Environment Agency
Subject Keywords: Flow measurement; Gauging; Ultrasonic gauging; Calibration
Extent: 182
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