An oyster-shaped visitor’s center would be located 3.5 km along the seawall, adjacent to the turbines. Visitors would be able access the center by foot or bike, or by using the electric train that is planned to operate along the seawall’s causeway. Tidal Lagoon Power
A man-made lagoon fitted with a line of hydro turbines that can generate power as tidewaters flow past is planned for Swansea Bay, in Wales, United Kingdom.
March 4, 2014— The United Kingdom has had, and no doubt will always have, a close relationship with the sea. Concern about rising sea levels and the impact of water along coastlines is natural for any country whose land touches the sea, but this concern becomes particularly acute for island nations. But in addition to planning for the damage water can do, the United Kingdom is also invested in finding ways to use the water to its advantage. Toward that end a plan was submitted in February, after three years of development, to create an £850-million (U.S.$1.4-billion) man-made lagoon in Swansea Bay, in Wales, that would work with rising and falling tidewaters to generate 400 GWh—enough power to supply 121,000 homes. This would amount to approximately 70 percent of the Swansea Bay area’s power demand and 9 percent of the total demand in Wales.
To form the lagoon, a 9.5 km long seawall measuring between 5 and 20 m in height would be constructed between the mouths of the River Tawe and the River Neath in Swansea Bay. The U-shaped seawall would launch from the Swansea Docks on one side, extending 3.5 km out into the Bristol Channel, and return to shore at Swansea University’s new Science and Innovation Campus.
During low tide, the seawall would be 12 m above the water level, while at high tide it would remain 3.5 m above the water level, according to material on the website of Tidal Lagoon Power, the tidal power project developer. The seawall would encompass 11.5 km2 of land, and its 16 hydro turbines, located underwater at the mouth of the seawall, would work dynamically with the tides. Sluice gates located around each turbine would remain closed until the water levels in the bay and lagoon differ by approximately 4.5 m and then the gates would open to allow the water between the lagoon and the bay to equalize, powering the turbines as the water flows past. In this manner, the water level within the lagoon would be manipulated four times daily—twice in relation to high tide, and twice in relation to low tide—to generate power.
The £850-million (U.S.$1.4 billion) man-made lagoon in Swansea
Bay would be formed from a 9.5 km U-shaped seawall attached
to shore. The mouth of the seawall would house 16 hydro turbines
located underwater that would work with rising and falling tide
waters to generate power. Tidal Lagoon Power
“The Severn Estuary holds the second-highest tidal range in the world and Swansea Bay benefits from an average tidal range during spring tides of 8.5 m,” said Ton Fijen, the technical director of Tidal Lagoon Power Ltd, who wrote in response to questions posed by Civil Engineering online. “This tidal range offers significant potential for the extraction of renewable energy through the construction of tidal lagoons.” The predictability of tides and the ease of using gravity to move the water make this process a highly reliable way to generate power, according to the developer.
The seawall would be constructed from sand- and gravel-filled geosynthetic fabric tubes, stacked together to form a pyramid, and infilled with additional sand. The shape of the seawall has been designed to minimize the lagoon’s impact on the existing flow of water into the bay, and the sand and gravel that will be used to construct the wall would be dredged from the bottom of the lagoon and hydraulically pumped into the tubes to limit the introduction of foreign material into the area. Once the tubes and sand have been placed, layers of rock armor would be added on the sea and lagoon sides of the seawall, with the largest diameters of rock reserved for the wall’s exterior, where waves from the Bristol Channel would strike. Topping the wall would be a public causeway.
“The structures have been designed to allow for local conditions and weather changes as well as potential earthquakes,” Fijen said. Bund walls that will be used for protection have been designed and tested for one-in-500-year storm conditions, he added.
To gauge local support for the project, questionnaires were circulated to Swansea Bay residents. Some 2,400 were returned, and Fijen said, “The response from the local community has been overwhelmingly positive. Indeed, surveys undertaken during the consultation process show 86 percent support.”
Recreational and educational amenities that would be made possible by the tidal lagoon include an offshore visitor and education center, a national triathlon and watersports center, and various art installations.
Sluice gates located around each hydro turbine would control the
flow of water, only opening when the lagoon and bay heights
differed by approximately 4.5 m. When the gates open, water
flowing between the lagoon and the bay would equalize, powering
the turbines. Tidal Lagoon Power
The 3,500 m2, oyster-shaped visitor’s center would be located 3.5 km along the seawall, adjacent to the turbines. It would offer views of the propellers through a glass floor, as well as exhibition space, lecture halls, displays areas, and a cafe. Visitors would be able to access the center by foot or bicycle, or by using the electric train that is planned to operate along the seawall’s causeway. The center will be powered by energy from the turbines, as well as by solar panels located atop its roof.
In addition to recreational opportunities, the tidal lagoon project also seeks to reintroduce the native oyster into Swansea Bay, while the seawall will double as a reef for sea life. The developer hopes to build five full-scale tidal lagoons in U.K. waters by 2023, according to Fijen. These would be able to supply 10 percent of the United Kingdom’s domestic energy needs, according to the developer.
Depending on their location, the benefits of tidal lagoons extend beyond power generation, according to Roger A. Falconer, Ph.D., DSc(Eng), Deng, CEng, F.ASCE, a professor of water management and the director of the Hydro-Environmental Research Center at Cardiff University’s School of Engineering. While Falconer is not involved in the Swansea Bay tidal lagoon project, he is working on the Severn Barrage project, which seeks to impound water further up the Severn estuary and use it to generate power.
“Lagoons do offer huge potential to address local flooding problems,” Falconer says. While Swansea Bay does not currently have a flooding problem, he says, other areas do. For example, he says, the Somerset Levels area experienced extreme flooding this year, and a lagoon built at Bridgwater Bay—at the mouth of the Severn estuary—would have helped drain the water, he says. “You could pay the electricity company not to produce power on the rising tide, and instead of filling the lagoon from the sea water, you could drain from the land,” Falconer says.
Lagoons also offer protection from coastal erosion and rising sea levels, Falconer says, which makes the proof-of-concept program at Swansea Bay a welcome development that will serve as a pilot program.
If approved, the developer anticipates that construction of the lagoon would begin in the first half of 2015, and completion and connection of the turbine power to the national grid in 2018. As currently designed, the lagoon could generate power for 120 years, even accounting for rising sea levels, and would reach carbon neutrality after just three years of operation.
A video of the planned Swansea Tidal Lagoon project is available on the developer’s website.