Tidal Power: A Question of Scale?
Whilst the remarkably well preserved site of Nendrum Monastery on the western shores of Strangford Lough in Northern Ireland may feature the earliest known example of a tidal mill in the world – and, by extension, effectively the starting point for the process of technological evolution that has resulted in today’s tidal energy devices like barrages, lagoons and tidal stream turbines – it suffers from an issue encountered frequently in archaeology: namely that of incomplete evidence and the need to interpret what survives, with the likelihood that any interpretation will be fraught with inaccuracy and conjecture.
This may be familiar to viewers of Channel Four’s Time Team (now sadly also confined to the depths of the past) where, almost on a weekly basis it seemed, entire settlements were reconstructed in astounding detail seemingly on the basis of little more than a handful of pottery shards, the occasional pit, and perhaps the odd wall or two. Admittedly it is in fact possible to say a great deal about a site even if the items listed above comprise the sum total of all features and objects uncovered; not least the fact that a structure existed, its likely whereabouts and possible form and function, and – using the pottery – a likely date for its occupation. The pottery could even hint at possible trade links with faraway places. In order to present this assemblage and any resulting interpretations to a lay audience, however, some form of visual reconstruction is usually necessary and this is where imaginations begin to play a more prominent role, as demonstrated by the often spectacular 3D (re)creations of roundhouses, Roman villas and other assorted ancient monuments, through which Tony and the team could stroll at their leisure.
It is quite likely that many of these efforts are reasonably close to the truth, and even if they fall short of the mark, they do at least succeed in providing entertainment for some, and even inspiration for others (myself for one). Thus, the numerous reconstruction illustrations encountered in the museum at Nendrum Monastery, including a rather impressive physical model of the whole site, served to provide valuable insights into what the location may have been like throughout the duration of its occupation. This included the tide mill, which, despite its sophistication was also a wonderfully simple way of extracting usable energy from the regular rise and fall of the water in the Lough. Such mills may well also have existed around the shores of the Severn Estuary throughout the latter half of the 1st millennium AD; although, as yet, no examples have been identified.
What did exist in more recent times, however, were far larger and more complex structures, such as those at Berkeley and Westbury-on-Severn, both in Gloucestershire. These were certainly in operation from the 18th century, and possibly earlier. Whilst the basic mode of operation differed little from the early medieval tide mills of Northern Ireland, including Nendrum, involving the impounding of water at high tide within a pond, and its subsequent release through waterwheels as the tide ebbed, they also made full use of more modern forms of milling technology, such as vertical wheels, and gearing mechanisms. The latter could enable multiple millstones to be operated by only one or two wheels, whilst simultaneously providing the necessary power to hoist sacks of grain into the upper storeys of the building.
In the case of the example at Berkeley much of the machinery remains in situ within the building (so I have been informed), now derelict following the demise of the last commercial enterprise there in 2004. This may, however, have little to do with tidal power as the mill was converted during the 20th century, first to steam power and then to electricity, whilst the millpond and tailrace have since silted up. In order to fully understand how such facilities operated, therefore, and, in turn, their significance within the context of local communities for whom the tidal cycle of the Severn may have functioned as a focus of livelihood and identity, it seemed necessary to see a tide mill as it might have existed a century or more ago.
There are presently only five restored tidal mills in the British Isles. This may seem like a good number for those who are unfamiliar with such features, as I was only two years ago; however, considering that more than 700 mills were once in existence around the Atlantic coasts of Europe, including many in the British Isles, the remaining examples can hardly be seen as representative. Nonetheless, those that do survive have been restored with care and attention to detail, enabling at least two of them to function once again as they may have done during their working lives. The closest mill to Bristol, where I am currently based, is that at Eling near Southampton which, until it closed for refurbishment earlier this year, produced its own flour on an almost daily basis. Beyond that, the other options were either an expedition to the coast of Suffolk and the working mill at Woodbridge or the slightly more accessible example at Carew Castle in Pembrokeshire. From an archaeologist’s perspective the choice wasn’t difficult to make, and without further delay I headed west.
The castle at Carew has stood on its promontory overlooking the nearby Carew River, a tidal arm of the Cleddau Ddu, since the beginning of the 12th century when the Norman rulers of England sought to extend their influence into Wales; however, a defensive settlement has been shown to have existed there from the Iron Age. The nearby Celtic cross, one of the finest in Wales, may even hint at the location’s status as a royal centre for the Welsh Kingdom of Deheubarth prior to the arrival of the invaders from the east. It is also likely that a mill existed nearby to supply the castle during the medieval period; however, documentary records for a tide mill date from the 16th century with the present structure being of 19th century construction.
The building comprises four floors, in addition to the under storey which houses the two vertical waterwheels, and functioned primarily as a corn mill, grinding grain into flour. After a relatively long period of use (longer than most modern power stations at least), the mill ceased operation in 1937 until its restoration in the 1970s. Initially the machinery was put back to use for demonstration purposes; however, now it stands dormant – clean, tidy, well-organised, but too fragile to resume operation. Recent feasibility studies have investigated the potential for breathing new life back into the now arthritic cogs, wheels and gears, in addition to the possibility of installing a modern turbine for generating electricity but, much like the great majority of tidal energy proposals, it remains little more than a report rather than any determined action.
Perhaps the most striking thing about the mill at Carew is the causeway linking the mill building on one side of the river channel with the far bank and housing both the wheels beneath the building, and the sluice gates which allow water to enter during the flood tide. In effect this is a barrage. A very small barrage, at least in contrast to those proposed for the Severn, but a barrage nonetheless, and probably not much smaller than the Annapolis Royal tidal barrage in Nova Scotia. It comprises a solid wall built laterally across the width of a river, thereby effectively cutting off an arm of the waterway from the “natural” operation of the tides. The tides do still affect the millpond created by this structure, but they now rise and fall at the whim of the mill owner or operator (presently the Pembrokeshire Coast National Park Authority). Essentially this could be seen as merely a small-scale version of what might have come to pass on the Severn if a barrage had ever been built. And on that basis, would it have been such a bad thing? At Carew during my visit it was the mill pond that provided the most conspicuous habitat for wading birds, including shell duck and heron, sustained no doubt by the schools of fish that could be seen swimming in the relatively clear waters. The flow of the incoming tide was also discernible, whilst the scent of the salty waters still pervaded the air.
But then, the Carew River has no tidal bore, which would be entirely eradicated by a Severn Barrage, and its populations of fish are almost certainly less substantial and diverse than the much larger and more complex Severn. Five hundred years ago the mill pond causeway may have been relatively expensive and could have taken months to build, but that contrasts starkly with the billions of pounds and close to a decade required for the Cardiff-Weston barrage proposals of recent years. In many respects the issue of tidal power is very much a question of scale. Small-scale developments, in terms of size of the buildings and structures, the geographic space they occupy and influence, and the time they take to build have generally been more popular and successful; as demonstrated by the many hundreds of tide mills, the few successful examples of tidal barrages and the current trend towards investment in small-scale tidal turbines and tidal lagoons. Meanwhile, despite the unwavering faith of some its advocates, the comparatively massive Severn barrage continues to flounder. A large-scale fish in a relatively small sea.
 W.E. Minchinton, ‘Early Tide Mills: Some Problems,’ Technology and Culture, 20:4 (1979), 777 – 786.
 For more on the cross, tide mill and castle see: J.R. Kenyon, ‘Carew Cross, Castle and Mill,’ Archaeological Journal, 167 (2010), 29 – 33.