Flow Measurement for Hydro: Catchment characteristics

For all hydro schemes, knowledge of the variation in flow past the intake over a year is a critical factor in developing an efficient, cost effective design.

In the most simple example the Environment Agency (EA) will set a minimum flow that must remain in the river. Any flow above this minimum level can be abstracted. The exact terms of the flow that can be abstracted (net flow) is set out in an abstraction licence which must be obtained from the EA before works commence.

The net flow increases during and after rainfall events and falls during dry spells. Developing the most profitable scheme depends on accurately predicting this variation, averaged over 10 years or more. It is only with the knowledge of this flow variation that the optimum design can be determined. The relationship between rainfall, temperature, humidity and run-off is dependent on a particular catchment's characteristics. For example, rock fissures or bogs can act as reservoir storage, smoothing the river flow and making it less 'flashy'. Conversely, man-made drainage systems can increase the flash. Although predictive software is available and used by Heidra, it cannot replace actual site data.

By using concurrent satellite rainfall radar measurements over a year, long term EA guaging and rainfall station data, on-site flow and rainfall measurements, together with EA approved predictive flow software (Low Flows), Heidra has developed leading edge capabilities in predicting the above variation. The work was part funded by Dartmoor National Park and performed in partnership with Devon Association for Reneable Energy.

Heidra has developed a temporary weir construction to allow the flow to be monitored using a simple ultrasonic sensor which is protected against the interference of stream debris. The final weir design has been tested successfully through one of Dartmoor's wettest winters.

The location of weir is placed as near to the final intake point as feasible, which is determined by Low Flows plus Heidra's in-house optimisation software.

 

The data logging system Heidra uses is capable of being left unattended for 6 months. For such situations we recommend the use of a digital camera to monitor the weir and inclusion of a remote dial-in capability to both check the correct operation of the system and download pictures. Reduced cost options are supplied by Heidra for situations when a local person can make weekly visits to check operation.