Handling flow information

Hydro-Logic's Rod Hawnt and Alec Willis on flow data management

Increasing environmental regulation is causing many water companies to review the quality of their open-channel flow monitoring networks.

Improvements in the provision of flow information are required to satisfy legislation for Abstraction Licences under the Water Act 1991 and discharge consents under the Environment Agency's (EA) flow monitoring policy to comply with the Urban Wastewater Treatment Directive.

This article describes an on-going project for Dwr Cymru Welsh Water, aimed at improving monitoring systems for a telemetred hydrometric network across Wales. The terms of the contract are framed so as to encourage the timely delivering of quality controlled data to a defined standard. This is feasible because the contractor takes management responsibility for all aspects of the data collection and information management process, including operation and maintenance of the monitoring infrastructure. This necessitated the implementation of a dedicated telemetry data acquisition system closely integrated with a time-series information management system, because of the special requirements for open-channel flow data processing and reporting.

Comparisons are drawn with the EA's flow monitoring policy now being implemented by water companies at WwTWs through the MCERTS self-auditing process. In each case the solutions provided are markedly different, due to several technical and historical factors, but the overall objective is the same - delivering high quality processed flow information to a desired standard.

Hydro-Logic was initially involved with Dwr Cymru Welsh Water on a project to rehabilitate part of a telemetred network of reservoir level and flow gauges across the north and south-eastern operational areas, where an obsolete radio-based network needed replacing. The new network was established to monitor water resource licence compliance including the need to release compensation water to downstream watercourses. Following the success of the initial project, Dwr Cymru Welsh Water decided to let a contract for the long-term operation and development of the network, including a monthly maintenance programme. The performance of this contract, won by Hydro-Logic, is itself monitored via a system of key performance indicators (KPIs) which tracks the performance of both the network and the contractor.

Responsibility for all network monitoring and maintenance operations is assumed by the contractor, with the client having full access to data captured via telemetry. The major elements of the contract include:

  • daily data acquisition from telemetry recorder outstations,
  • routine monthly maintenance of outstations including flow gauges,
  • ad-hoc call-out maintenance to address instrument failures or data anomalies,
  • on-site validation of recorder readings using gauge board and other manual checks,
  • monthly delivery of processed quality controlled records with a data report,
  • annual survey and maintenance to check gauge and instrument calibration,
  • KPI analysis indicates performance on measurement quality, record continuity and the timeliness of data delivery and reporting.

The procedures are standard practice for open-channel hydrometry networks around the world. Such practices fit well with the traditional low-maintenance 'fit-and-forget' culture for other telemetred flow monitoring systems. Corporate SCADA and operational database systems are often insufficiently customised for detailed quality control and reporting of open-channel flow data. This was recognised by Dwr Cymru Welsh Water and the reason why a dedicated telemetry data acquisition and information systems, based on the HydroLog4 system, was procured for the project. The system procured has an open design allowing data exchange with existing corporate systems so resource information can be shared by other managers and planners.

The environment for operating flow and level monitoring instrumentation in Wales is always challenging and often extremely hostile. Telemetry monitoring with alarms is a major advantage for efficient management of this process. Reservoirs and intakes in Wales are often in upland and sometimes mountainous areas where reliable telemetry has proved difficult to establish at reasonable cost. Potential problems are numerous:

  • sources and reservoirs are often remote from works and accessible only via 4-wheel drive vehicles, raising lone-worker safety issues,
  • public telephone and electricity services are poor due to geography,
  • battery-powered instruments are needed where mains electricity is unavailable,
  • vandalism is a major problem, and solar-panels or 'shiny' locks are a particular target,
  • instruments need to be protected from driving winter rains and sub-zero temperatures particularly in north Wales,
  • low-flow measurement weirs are prone to debris accumulation, siltation of stilling wells and algae growth,
  • flood events, hydraulics and instrument drift can all degrade measurement performance and reduce data quality.
Achieving Reliability

Reliable monitoring could only be achieved under such conditions through a regular monthly maintenance programme, which also involved data collection from backup recorders. As part of this project over 60 level and flow sites were surveyed and audited. Recommendations were made for improving monitoring infrastructure at intakes, compensation and residual flow gauges, including the installation of battery-powered telemetry recorders with level sensors.

The existing corporate SCADA system could not be extended at reasonable cost, and so a dedicated telemetry system was specified to support a choice of communications services. Outstations and measurement sensors were required to operate unattended using battery power for two-three months. The Isodaq range of outstations met the requirement with a choice of GSM cellular, PSTN or unlicensed low-power radio modems.

Nearly 50 sites have so far been rehabilitated and GSM-based outstations have been commissioned at nearly 90% of these sites. PSTN-based telephone lines were available at most of the remainder, with radio-path surveys indicating that only one unlicensed radio-based system was viable at reasonable cost without repeater stations. Radio may form part of the telemetry solution at other sites, but installations are awaiting the outcome of a trial at one site with a new battery-powered satellite modem which utilises the 48 low-earth-orbit satellites in the Iridium network. If successful this new satellite service will offer a much lower cost-of-ownership solution than traditional satellite data collection platforms. Both GSM and satellite outstations are equipped with low-powered modems, but special power-management techniques are also required to extend battery life between maintenance visits.

Instrument housings utilised at sites range from existing brick huts to bollard style enclosures made of a flame-retarded polyethylene material, and designed to look like street furniture. The advantage of the bollard is that telemetry instruments, batteries and even GSM aerials are all housed out of sight from inquisitive vandals, who generally do not target them. At one site a determined group were frustrated by the robustness of the installation and it remained intact after repeated attacks.

For compensation flow monitoring the emphasis is on the low-flow measurement capability to ensure flow never falls below a minimum flow threshold. For a well-designed, sharp-edged weir plate with rectangular or V notch profile, uncertainty of flow measurement of better than +/-5% is achievable with good practice. This means the level sensors must provide instantaneous measurement accuracy ideally to within +/-2.0mm to a resolution of 1.0mm. This is achieved with either float-driven shaft-encoders or high-accuracy depth pressure sensors deployed within stilling wells or pipes so as to damp down naturally occurring noise oscillations in level and flow. It is almost impossible, however, to maintain consistency of measurement accuracy without a regular maintenance programme. Typically this involves:

  • cleaning debris and algae from weir crests and gauge boards,
  • taking check measurements using gauge boards or dips,
  • making adjustments for sensor drift or offsets,
  • occasional current-meter gaugings and sensor calibration checks,
  • pumping silt from stilling-wells at regular intervals,
  • clearing silt or boulders from weir approach channels,
  • annual check levelling of measurement datums.

Dwr Cymru Welsh Water specified a routine monthly maintenance programme, which also enabled backup data to be recovered from loggers and batteries to be changed if necessary. The field visits form a key part of the quality control process, which continues using office-based information systems including graphical or tabular display and editing of logged data. Where necessary calibration drift error, offsets and spurious data points are corrected. Level data is converted to flow records using weir rating equations. Ratings are checked and adjusted if necessary, together with associated gauge zero and measurement datum offsets, on at least an annual basis. These data processing requirements, plus monthly and year-book style data tabulation for routine reporting, are standard features of the HydroLog4 flow and time-series information system used in this contract.

The process is typical of standard hydrometric data management practice. In contrast, until recently the emphasis at WwTWs has been on a minimum cost, low-maintenance regime for operational control purposes, with little attention paid to flow data quality. Instrumentation has been characterised by mains-powered flowmeters utilising non-contact ultrasonic 'look-down' level sensors. The use of stilling wells has been abandoned due to the problems of siltation and bio-fouling of inlet pipes. Many ultrasonic level sensors are robust devices mounted directly over un-stilled channels at secure sites, with little need to provide further protection from vandalism. These devices produce quite noisy, temperature and foam affected measurements, typically to an accuracy of 3-5mm dependent on range. SCADA telemetry captures flow data, which is generally pre-processed by the flowmeter instrument to a daily record, but few systems provide specific quality control facilities to help the routine verification of flow measurement accuracy. Monthly maintenance programmes are an unaffordable luxury and so many problems due to poor calibration, instrument drift or sudden offset errors are often missed.

The EA's new flow monitoring policy for discharge consents is now setting challenging new standards for water companies, including a requirement to monitor total daily discharges to watercourse with +/-8% overall measurement uncertainty. The new MCERTS auditing regime is part of this process, addressing routine maintenance and calibration procedures as well the suitability of the overall system design. The current AMP3 investment programme will certainly enable water companies to satisfy the regulator and meet environmental obligations. But there is also a realisation that by improving the reliability and accuracy of monitoring systems, operational efficiency can also be monitored with potential cost-savings in future through further process optimisation.

Contracting out

The Dwr Cymru Welsh Water contract has demonstrated a strong case for considering dedicated systems for flow monitoring applications, when operational SCADA and database systems and associated maintenance procedures cannot be adapted at reasonable cost. The MCERTS auditing regime is influencing improvements in infrastructure at WwTWs. Yet many companies have still to address the problems of how to manage the enormous volume of flow data to be collected and how to verify the quality of the information derived. For one company at least, a major water resource monitoring network is being managed under contract by specialists who are providing flow information derived from quality-controlled data. The client derives major benefits by the contractor taking over data-ownership responsibilities and the resulting increased certainty that the quality of flow information is maintained in the long-term.



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