Mary Tavy and Morwellham hydroelectric power stations are owned and operated by South West Water. Located in the foothills of Dartmoor, the scheme has developed over the years from its mining origins to a series of stations now supplying enough electricity for 1700 households using 4800kWh per annum. During the 1930’s hydroelectric turbines were installed at the old mining sites of Mary Tavy to take advantage of the water resource of the River Tavy.

The Devon hills have a long history of electricity generation with the towns of Okehampton and Chagford two of the first in Britain to have a mains supply of electricity in 1888 and 1891 respectively. Declining mining and weaving in the area prompted enterprising industrialists to use the opportunity to adapt the old mining water channels, known as leats, for generating electricity. Mary Tavy Power Station was therefore born with an intake on the River Tavy making use of an old mining leat to the Bennetts Reservoir before dropping 230 feet to produce 660kW.

Following the 1932 commissioning of Mary Tavy a second power station was completed in 1934. The Morwellham station takes water from the River Tavy at Abbey Weir, which is located downstream of the Mary Tavy Station. This water travels along a 200 year old canal originally built for transporting coal, wool, minerals and agricultural produce. The Morwellham station produces an additional 640kW to the national grid.

Increase in electricity demand led to the construction of a second power station at Mary Tavy in 1936. Number Two Station takes water from the River Tavy at Tavy Cleave, close to the source of the River Tavy. Water runs along another 200 year old leat to Wheal Jewell Reservoir before dropping some 560ft to drive three 650kW turbines. Total combined capacity at Mary Tavy Station is currently 2610kW.

Monitoring flow

The main objective for monitoring at Mary Tavy is to measure flow to a consistent and high quality. South West Water abstracts water from the River Tavy for use in two hydroelectric power stations. The amount of abstraction permitted is specified in abstraction licences and an Operating Agreement between the Environment Agency and South West Water. The licence and agreements specify the maximum abstraction rate at each site as well as other flow criteria such as prescribed, residual and sweetening flows.

There is a need for high quality, precision level and flow data at both intakes, to ensure that the physical flow control structures are operating in accordance with the conditions specified on the abstraction licences and in the Operating Agreement.

In March 2010, YSI Hydrodata Ltd installed two SonTek/YSI Acoustic Doppler Instruments to precisely monitor flow in the shallow leats, with flows ranging from 126m3 per hour or less to around 4432m3 per hour. The SonTek Argonaut SW (Shallow Water) is a self-contained, highly compact unit for measuring water flow, velocity and level. Typically used in canals, pipes, and natural streams, the SW is designed to account for velocity variations within the channel to calculate the flow.

The selection of the Argonaut SW for a monitoring solution at the Mary Tavy Sites was determined for the following reasons;

• 1. Measure bi-directional velocity and level acoustically

• 2. Low maintenance requirements

• 3. Less power demand

• 4. Easy installation of the hardware

• 5. Can be deployed autonomously for indefinite periods of time

• 6. Use of an instrument that requires minimal ongoing physical calibration

• 7. Most important, can measure velocity and compute flow in very low and high water conditions (inches to several feet of water)

After evaluating these factors South West Water chose the Argonaut SW allowing accurate, reliable flow measurements for reporting.

Canals, and the likes of, often experience variations in flow with little or no change in water level. In these cases, the traditional stage to discharge relationship cannot reliably be used but a combination of velocity and stage measurement can, this is often known as the velocity index method. In the case of the Mary Tavy intakes, water levels in the leats are influenced by downstream restrictions and control gates causing a variation in levels at set flow rates.

The location of the two SW instruments has been chosen carefully for maximum accuracy and to ensure the required flows are monitored to meet consent criteria. The first SW was installed in the intake leat for Mary Tavy Station 1, with the second SW in the intake leat at Abbey Weir.

Instrument selection at both sites was very important due to low stage and velocity conditions. The Argonaut SW is mounted on the bottom of the channel and uses two 45 degree slanted acoustic beams to measure bi-directional velocity, with a third central beam for precise level measurement. Velocity and level data are then combined with the channel geometry to compute flow.

The velocity-index rating method develops a Mean-Velocity and Area rating from measurable stream variables (such as stage and velocity measured by the Argonaut SW) using regression analysis. After the ratings have been developed, they are programmed back into the Argonaut SW allowing for continuous discharge calculations and output. The Mean-Velocity is the average velocity of the surveyed cross-sectional area. The Index-Velocity is a subsection of stream velocity actually measured by the Argonaut SW.

Both leats are well maintained giving confidence that ratings will not vary significantly over time. In order to calibrate the SW, flow gauging measurements were completed at varying flows then imported into SonTeks rating software, FlowPack, which was developed for SonTek by The Environmental Hydrology and Hydraulic Engineering (EHHE) program at the University of Illinois to provide a set of fast, robust and reliable discharge rating tools in accordance with ISO/USGS standards. A significant benefit to using FlowPack is that is allows quick and confident development of Velocity-Index equations using advanced analytical techniques. It also provides the user with a simple way to make rational and efficient decisions for additional data collection that will further improve measurement results. Multiple sites can be stored for easy access and updating as new calibration measurements are made.

High precision and latest generation mobile instantaneous flow measurement acoustic Doppler instruments, the FlowTracker and RiverSurveyor M9 were used to both verify and calibrate the installed continuous monitoring instrumentation. The Flowtracker is designed for wading discharge measurements and can gauge in water conditions with depths down to 2cm, making it ideal for use in small streams and canals such as the Mary Tavy intake leats.

The RiverSurveyor M9 operates on multiple acoustic frequencies allowing for reliable detailed depth, velocity and discharge data. The RiverSurveyor M9 extends to a maximum measureable depth of 80m; therefore the instrument can be used in a range of flows from low volume to extreme conditions. The RiverSurveyor M9 can be used for larger sections around the Mary Tavy Scheme, including the River Tavy, high leat levels and flood flows.

Data from the Argonaut SW and FlowTracker measurements were seamlessly imported into FlowPack, and the full analysis was promptly completed requiring only a few basic steps. The rating developed by the FlowPack software is shown along with several statistics indicating the quality of the rating. The results provide a very good correlation between mean-velocity and index velocity (r2 = 0.985 and 0.992) and a low standard error estimate of mean velocity (0.003 m/s).

Through the use of SonTek’s FlowPack software, an accurate assessment of the discharge at this site can now be obtained combining the Velocity-Index rating method. This method combined with the continuous monitoring of velocity and water level provides a level of quality assurance and ensures higher data confidence than that provided by a Stage-Discharge relationship.

Future potential

Recorded data at the intakes has enabled South West Water the opportunity to investigate and gain knowledge on how flow around the scheme is utilised. The company has said that “This information has proven valuable in increasing the Company’s understanding of the performance of the intake structures across a range of flows and, where appropriate, determining areas for improvement. The Company will be able to provide information on the operation of these intake structures to the Environment Agency”.

The Argonaut SW installations have proven a great success for the River Tavy Hydro-power System. The instruments provide additional data that can be used, along with other existing data sources, to ensure compliance with the Operating Agreement and licenses from the Environment Agency. Providing a continuous flow record for the sites, it will also assist South West Water in achieving a comprehensive flow monitoring database, supplying valuable information that can be used for future enhancements, improving operations and gaining the most efficient use of the water resource.

South West Water may install a third Argonaut SW to complete the monitoring at all three intakes.

South West Water plan to double their energy production from renewable sources by 2015 and are striving for 50% of energy needs from renewable sources by 2050. The monitoring information available from the Argonaut SW instruments supports the development of more efficient, sustainable operation of such schemes.

Isobelle Jull, Technical Support, YSI Hydrodata, Letchworth, UK

Jackie Turner, Operational Analyst, South West Water, Exeter, UK

Nick Martin, SonTek UK Product Support, YSI Hydrodata, UK

Lee Pimble, YSI/SonTek, European Support Centre, UK.

Located in Letchworth, UK, YSI Hydrodata distributes YSI and SonTek instruments and monitoring systems, providing training, installation, service, repair and calibration support services in addition to a full range of rental products.