About a mile upstream of the south Devon town of Totnes, the tree-lined tranquillity of the river Dart is interrupted by a weir. Water cascades over this concrete barrier and after heavy rain there is a spectacular display of power with swirling whirlpools and foamy white water. Slate-grey herons and sparkling white egrets stand sentinel by the weir and the occasional grey seal lurks below, waiting to feast on fish that linger too long. There has been a weir at this bend in the Dart since the 16th century, built originally to harness the power of the river; the present rather bland construction dates largely from the 20th century.
The weir is a downwards-sloping concrete barrier that interrupts the flow of the river so that a large pool of water accumulates upstream, isolated from the tidal downstream water about three metres below. This pool of water is a store of potential energy that was used in the past to drive several water mills in the town a mile away. A channel, the leat, ran from the pool all the way in to Totnes and providing the leat stayed above the level of the river it contained the energy to drive a water wheel. Only one mill building now survives: the Town Mill dating from the 17th century but with 19th century additions. This was used as a water mill until 1945 and currently houses the Tourist Information Centre. The leat is still intact and can be viewed along much of its path, through an industrial estate, under the main railway line and passing near the front of Morrisons superstore. The leat is celebrated in the name of the town’s large medical centre, the Leatside Surgery.
Over the past year, a neat, stone-clad, turf-roofed building has materialised by the side of the weir. This is the turbine house of the new Totnes Hydro which once again harnesses the power of the Dart. On the downstream side of the building are two tube-like structures roughly aligned with the descending surface of the weir, each tube containing an Archimedes Screw. Water from the pool behind the weir passes under the turbine building to enter the tubes, pressing on the blades of each Archimedes Screw causing them to turn and driving the turbines. The Archimedes Screws can be viewed from the downstream side and I find them mesmerising – turning steadily, water splashing, feeling almost alive – as they transform the potential energy of the water in to kinetic energy and subsequently electrical energy.
When there is a good head of water, the turbines generate about 250 kW of power. Output will depend on flow in the river (higher power after heavy rain) and the head across the weir (typically about 3m but reduced by spring tides). Generation may cease altogether for about two weeks in a dry summer when water flow in the Dart is low. Currently, the electricity generated is powering the local comprehensive school and an aluminium foundry on the nearby industrial estate and any surplus enters the grid. To put this in to perspective the overall energy produced is enough to power the equivalent of about 300 homes. In time, the hydro will also provide electricity for the new ATMOS project. This is a community-led development of homes and businesses on the former Dairy Crest site in Totnes.
The river Dart is an important route for migrating fish and the weir already contained a fish pass to help sea trout and salmon overcome the barrier. The pass was, however, in poor condition so that fish were having difficulty moving up the weir leading to losses to hungry herons and seals. The new Hydro project includes renovating the existing fish pass and building an additional modern fish pass alongside the turbine building. These should help migrating fish so that, in time, the piscine population on the Dart increases; new fish counters have also been installed to help monitor traffic.
So far so good, but if equipment is installed to capture the energy of the river it is bound to alter the flow over and around the weir. You can see this clearly on the upstream side of the turbine house where water in the pool flows towards the new building to enter the Archimedes Screws, eventually discharging in to the river below. Although water flow through the turbines is carefully regulated by sluices to make sure that the weir does not dry out, less water now flows across the weir than before. This redistribution of water has remodelled islands in the river downstream and night fishermen have had to relearn safety on the river. We should not forget, however, that when the weir was first built and water was directed down the leat to power the Totnes mills some 500 years ago, water flow in the river must have been changed to a much greater extent. There is also the question of noise. The new Archimedes Screw turbines do emit noise as they turn and there is some splashing of water. The turbine building is insulated and the current level of noise from the new installation is no more than I can remember coming from the weir on a full flood.
The Weir Hydro project was developed by the owners of the weir, Dart Renewables, working closely with the Totnes Renewable Energy Society (TRESOC). TRESOC was set up by local residents to enable the community to develop renewable energy and to retain control of the resources. On a practical level TRESOC aims to supply local homes and businesses with “local” energy. If everything works to plan, the Totnes Weir Hydro should generate 1.35 GWh of electricity each year, saving 550 tonnes of carbon dioxide. The majority of this electricity will be used to power local enterprises.
Disclaimer: I am a member of TRESOC and have invested in some of their projects.