Hydro-Logic have been engaged by Dwr Cymru/Welsh Water since 2000 to operate, maintain and improve their hydrometric network. One of the projects involved the feasibility, design and installation of a new ultrasonic transit-time flow gauge on the discharge compensation flow channel from Cwmystradllyn Reservoir in North Wales.
The new gauging station was required to replace a flume which was not performing correctly due to backwater problems caused by vegetation growth downstream. This was resulting in a considerable over-estimation of the discharge. As the vegetation growth was occurring in a SSSI it was not possible to clear the downstream channel and because of this an alternative method of flow measurement was required.
To overcome these hydraulic problems a Sarasota 200 ultrasonic transit-time gauge was selected as the most cost-effective solution, taking advantage of the 4-path capability for accurate low and high-flow gauging with all the paths firing at a 60 degree angle to the direction of flow. This multi-path configuration in a concrete channel with a good straight approach channel meant that once set-up there would be minimal on-going calibration requirements and therefore the long-term cost of ownership is low.
The compensation flow measurement requirement was a key part of compliance with the water resources abstraction licence conditions associated with the neighbouring reservoir level gauging site. Both sites were part of the telemetred network of over 100 sites maintained by HL and so an Isodaq Hawk XT GSM outstation was linked to both the transit-time gauge and the reservoir level site to ensure data was captured on a daily basis to ensure compensation discharges was always maintained in compliance with the abstraction licence requirements under the Water Act 1991.