Investigating the Intertidal Zone

Collaborative working

Collaboration between buildings historians and marine scientists has the potential to generate innovative, transdisciplinary research methods that can lead to new insights into the value - and future use - of our coastal heritage as the intertidal zone experiences rapid environmental change. This was the key finding of a pilot study initiated by Antony Firth, Head of Marine & Coastal Heritage for Historic England and jointly funded by Historic England and the Marine Institute, University of Plymouth. The study brought together researchers in marine ecology and architectural heritage to investigate artificial structures of historic significance in the intertidal zone of Plymouth Sound.

While relatively short and small-scale, running only from January to April 2023, the pilot study clearly established the value of intertidal historic structures as marine habitats: more species were found on historic than contemporary structures and different species were found on historic structures than on natural rock. These findings open a discussion about the value - and use - of historic coastal structures in a future where the intertidal zone will experience changes in sea temperature, heat extremes and energy levels (from waves and swell) that will affect natural and artificial structures alike, which may also change the species and habitats they support.

Fresh insights emerge from new ways of working together based on a shared understanding of marine environments as cultural landscapes, landscapes are unique places formed through the interactions of humans and nature over time.

Plymouth Sound

A key aim of the project was to investigate the value of collaborative, interdisciplinary projects at coastal heritage sites. Plymouth Sound was chosen for three reasons: it is the site of the UK’s first National Marine Park; there is specific expertise in marine heritage and marine science close-to-hand at the University of Plymouth; and it is of national historical and environmental significance.

Plymouth Sound is a unique marine environment and a significant historic landscape that has defined a city for centuries.

It is the place where the largest naval base in Western Europe co-exists with fragile beds of sea grass. It is the place where many of the world’s most daring voyages began and where fishing boats today still leave for the daily catch. It waved farewell to the men, women and children of the Mayflower, and four hundred years later welcomed descendants of the Native Americans who enabled their survival. It began Greta Thunberg’s Atlantic adventure and gave sanctuary to survivors of the Titanic.

The pilot study focused on the 6 kilometre stretch of foreshore from Tinside to Devil’s Point; that is, from the bluff known as the Hoe to where Royal William Yard juts out into the mouth of the Tamar.

Since the construction of the 1.5 kilometre long Plymouth Breakwater, ‘The Great National Undertaking’ of 1812 – 1844, this stretch of foreshore has had its own distinct natural and human history as the breakwater calmed the waters of the inner Sound, enabling marine and human life to flourish, with amongst other things the rise of leisure sea-swimming. This hybrid maritime history can be read in the numerous little-known structures found the length of the foreshore, often layered upon each other, sometimes forgotten and left to the sea.

Identifying intertidal heritage structures

Combining the fieldwork skills and methods of historic buildings research with those of marine ecology, postgraduate students Emma Jones, Marine Ecology, and Joseph Panes, Heritage Theory and Practice, worked together at low tide identifying and recording historic structures and their biodiversity. They were able to assess how these structures compare to the adjacent natural habitat and demonstrate the distinctiveness of historic structures as habitats when compared to contemporary structures and materials.

Plymouth’s waterfront is one of England’s most significant coastal heritage sites. This is reflected in the area’s extensive protection from the Barbican Conservation Area at Sutton Harbour to the east – with its fifty-plus seventeenth to nineteenth century dwellings and commercial premises – to the Royal William Yard - John Foulston’s Royal Navy victualling yard, 1811- 29, a fortified site comprising twenty-six Grade I, II* and II structures – to the west.

The waterfront between these two areas features, from east to west, the seventeenth-century Royal Citadel, a scheduled monument; the Hoe, which is a Grade II Park and Garden with thirteen listed monuments including Smeaton’s Tower, 1759 (moved from Eddystone Rock in the 1880s) and, most famously, the site of Sir Francis Drake’s (possibly mythical) game of bowls; and Millbay Docks including Brunel’s Grade II listed Inner Basin.

However, knowledge and protection of Plymouth’s historic waterfront structures does not fully extend to the intertidal zone.

Some larger historic structures such as Tinside Lido, West Hoe Pier, and the Firestone Bay Sea Wall are listed; but most of the extant historic structures are a bricolage of small-scale interventions that are largely undocumented and unprotected. To address this gap, a full catalogue of all the assets in the survey areas has been produced including histories for each structure based on research in the Plymouth City archives. Their significance lies in their collective value as evidence of changing activities over time. Thirty-seven historic structures were surveyed, including twelve sets of steps, five piers, four swimming pools, three sea walls and thirteen other structures ranging from railings to groins, cabling, navigation beacons and mooring rings.

Reflecting the changing history of Plymouth Sound, these small-scale, everyday structures can be arranged into four categories based on their original purpose:

• Royal Navy (defence, communication etc.)
• commercial shipping
• tourism
• and recreational swimming.

Due to its proximity to the Devonport Naval Base and the Royal William Victualling Yard, many structures at Devil’s Point and Firestone Bay are linked to the Royal Navy. Starting with the Firestone Bay Sea Wall in 1827, The Admiralty built structures such as the boom defence rings, navigation beacon and the submarine cables to strengthen the defence, improve accessibility to the Devonport, and enhance communication. The Navy’s presence in this area has diminished, especially after Royal William Victualling Yard was vacated in 1992, making many of these structures redundant.

In Millbay, the structures are primarily connected to commercial and passenger shipping. Although there has been a commercial maritime presence in the area since the eighteenth century, Clyde Quay, Brunel’s Pontoon, and Trinity Pier originate from the Great Western Dock Company’s expansion of the dockyard in the 1850s. A combination of bomb damage, the collapse of ocean passenger shipping, and the preference for large container ships instead of smaller freighters led to the decline of the dockyard as a substantial commercial entity. However, the area has been subject to fervent regeneration efforts throughout the past 15 years.

In a notable example of adaptive reuse, Clyde Quay and Trinity Pier have been renovated into pedestrian boardwalks and used to host international sailing competitions.

Finally, the increasing popularity of outdoor bathing and expanding tourism industry gave rise to most of the structures on the eastern section of the Stonehouse foreshore (Firestone Bay, Tranquillity Bay, and Eastern King Point) and The Hoe. Between the 1850s and 1914, there were piecemeal additions to these areas to improve pedestrian access to the sea. The local authorities in Stonehouse and Plymouth constructed the steps at Second Beach, Tranquillity Bay, Eastern King Point, Reform Beach, and Lion’s Den to establish designated bathing areas. Additional pressure from patrons to improve the outdoor swimming facilities led to the erection of enclosed tidal pools (Reform Beach Sundeck, Shacky Pool Platform), bathing houses and diving platforms.

Likewise, as holidaymaking and leisure time became commonplace during the Victorian/Edwardian era, The Hoe evolved into a popular vacation destination. Consequently, West Hoe Pier, Watermans Slip, Banjo Groyne, and the Shacky Pool Landing Place Steps were built to provide steamer ship and yacht access, to varying degrees of success.

The Plymouth Sound foreshore underwent its most intense construction period during the 1930s. Using the designs of John Wibberley, the Plymouth City Corporation embarked on an ambitious foreshore development scheme, leading to the installation of the concrete beaches at Pebbleside and Tinside, the East Tinside Sun Terrace, and the Extension Pier. The Corporation also enlarged Shacky Pool, installed a tidal pool at Firestone Bay, and, as its centrepiece project, built Tinside Lido, one of the most celebrated outdoor pools in the country.

The 1930s proved to be the pinnacle of the foreshore’s development.

Bomb damage led to the demolition of the Promenade Pier after World War Two. As tourism declined, investment dwindled, and coastal erosion started taking effect, the artificial structures started to deteriorate. By the end of the twentieth century, many were in a desperate condition. The concrete foreshores at Pebbleside and Tinside had fragmented; Extension Pier had partially collapsed; the City Council removed the Shacky Pool Diving Stage; and, most ominously, the iconic Tinside Lido was permanently closed.

Nevertheless, efforts have been made to renovate or repurpose some structures in the twenty-first century. After a vociferous local campaign, Tinside Lido was restored and reopened in 2003 and bathers continue to use it in the summer months. Likewise, The City Council partly refurbished Extension Pier, converting it from a landing stage into an observation platform; and filled in the Reform Beach and Shacky pools, transforming them into sundecks.

The structures continue to be used by the local community today.

In the summer, people of all ages use the pools, steps, and stages along the foreshore; and Navy families gather on the platforms in Stonehouse to welcome sailors home from their tours at sea. Throughout the survey, we observed the public using the structures to relax, exercise, and honour their loved ones. These structures help make Plymouth Sound a uniquely accessible and engaging place for Plymothians and visitors.

Rapid biodiversity assessment

As well as being assessed for their heritage and contemporary value, each historic structure was subjected to a ‘rapid biodiversity assessment’. The biodiversity of adjacent natural rock and contemporary/non-historical structures was also recorded. The results of the combined survey were mapped using GIS. Eighty species were recorded overall: 70 on the natural shore and 71 on the historic structures. Sixty-one species were common to both heritage structures and natural rock, 9 were unique to natural rock and 10 unique to the historic structures.

Most of these structures are heavily disturbed, especially during summer months in peak tourist season. It is, therefore, unsurprising that they support fast-growing species such as green or brown alga, which are the first to grow on uncolonised or disturbed structures. In the natural habitats with lower disturbance, other organisms such as crustacea and molluscs have had a chance to outcompete and even graze on the fast-growing species over time, a process known as ecological succession, which may, therefore, be a driving influence on the differences in biological community recorded within the study area.

Historic structures support biodiversity

While only a snapshot, it is apparent from this rapid assessment that historic structures have the potential to support a wide range of biodiversity, although this is highly dependent on the nature of the structure itself.

This emerges from the additional data recorded in the historic building survey. Crucial to the success of the pilot, Historic England’s Level 1 recording system for historic buildings was modified at the request of our marine ecologists to include data relating to weathering of materials and structural decay.

Mapping this additional data enabled researchers to begin to understand what enables a historic structure to support biodiversity: information was gathered on the different materials encountered, recording their textures, joints and cracks, and delaminating layers. These set them apart from both contemporary structures (mostly smooth-faced concrete) and natural rock as a habitat for marine life.

The survey demonstrated that historic structures make good and sometimes species-specific eco-habitats, in part, because of historic craft practices - choice of materials, construction methods – and in part simply because they are historic; that is, the longer a structure has been exposed to the sea the more weathered it becomes and, therefore, the more attractive it is to some types of marine life.

New insights, new questions

Further study is needed to elucidate the driving influences in these differences and to understand how we can conserve both biodiversity and the historic environment. However, these initial findings contribute to a conversation about the future use of coastal structures within a combined conservation framework, where the value of a historic asset might have not only a heritage value but also an environmental benefit through attracting species diversity (see recent publication by Baxter, Coombes and Viles). While this shows the historic environment has a role to play in environmental conservation, we should also acknowledge it needs to be balanced with its cultural value and future use and enjoyment by people in coastal communities (see work by Claesson; Jarratt; Firth).

In practice, as sea-level rise brings more structures within the intertidal zone of our historic harbours and port cities, decisions about this balance may favour managed weathering over repair or restoration; decisions that will require revised assessments of significance that balance historic against environmental values. These are discussions and decisions for the near future; what is more certain is that the Plymouth Sound pilot study demonstrates that there is much to be gained from interdisciplinary research in coastal heritage and marine science.

Further information

Baxter, T,  Coombes, M and  Viles, H:  ‘Intertidal Biodiversity and Physical Habitat Complexity on Historic Masonry Walls: A Comparison with Modern Concrete Infrastructure and Natural Rocky Cliffs’, Marine Pollution Bulletin (2023) Mar 188, 114617.

Claesson, S: ‘The Value and Valuation of Maritime Cultural Heritage’, International Journal of Cultural Property, 18 (2011), 61–80.

Firth, A, The Social and Economic Benefits of Marine and Maritime Cultural Heritage: Towards Greater Accessibility and Effective Management (Honor Frost Foundation, 2015).

Firth, L,  Thompson, RC, White, FJ,  Schofield, M  , Skov, MW,   Hoggart, SPG, et al: ‘The Importance of Water-Retaining Features for Biodiversity on Artificial Intertidal Coastal Defence Structures’, Diversity & Distributions, 19 (2013), 1275–83.

Jarratt, D: ‘The Importance of Built Heritage in the English Seaside’, in Routledge Handbook of the Tourist Experience, ed. Richard Sharpley (Oxford: Routledge, 2022), 481–97.

Historic England: Understanding Historic Buildings: A Guide to Good Recording Practice (Historic England, 2016)

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