Embanking of the tidal Thames
The embanking of the tidal Thames is the historical process by which the lower River Thames, at one time a shallow waterway winding through malarious marshlands, and perhaps five times broader than today, has been transformed by human intervention into a deep, narrow tidal canal flowing between solid artificial walls, and restrained by these at high tide. The Victorian civil engineering works in central London, usually called "the Embankment", are just a small part of the process.
Beginning in Roman Londinium, it was pursued more vigorously in the Middle Ages. Mostly it was achieved by farmers reclaiming marshland and building protective embankments; also, in London, frontagers pushing out into the stream to get more riverfront property. Today, over 200 miles of walls line the river's banks from Teddington down to its mouth in the North Sea; they defend a tidal flood plain where 1.25 million people work and live. Much of present-day London is recovered marshland: considerable parts lie below high water mark. Some London streets originated as tracks running along the wall and yet today, are not even in sight of the river.
Since the Thames has a large tidal amplitude, early modern thinkers could not believe local people were capable of building mighty embankments beside it; hence the works were attributed to "the Romans". The current explanation is that tides were small at first, requiring modest embankments only; as the sea has gradually invaded the Thames valley, the embankments have been raised to match in easy stages.
Land reclamation in the Thames had political consequences. It has been argued that it made for independent farmers, contributing to the decay of the feudal system. Other consequences were said to be two clauses in Magna Carta, and one of the declared causes of the English Civil War. The deepening of the Thames made it navigable by larger ships that could travel further inland: an unforeseen result was the growth of the world's largest port.
The Thames before the walls ''versus'' the river today
The natural Thames
The natural Thames at the foundation of Roman Londinium was a river flowing through marshland. Possibly it was five times wider than today. At some point it became infested with malarial mosquitos.The original site of London may have been chosen because, going up the river, it was the first place where a broad tract of dry land—chiefly gravel—came down to the stream on the north bank; the modern City of London is built on that gravel. To the south it was very different, explained Gustav Milne:
Yet the tidal range was only just over 1 m.
Below, the river flowed to the sea through broad marshes, touching firm ground at just a few points. According to James A. Galloway
Thus much of today's urban London is built on reclaimed marshland.
Hilda Ormsby—one of the first to write a modern geographical textbook on London—visualised the scene:
where, probably, it was shallow enough to be forded.
Some of those creeks may have been navigable. Of only three Roman ships found in London, one was dug up on the site of Guy's Hospital.
Sir Joseph Bazalgette, describing the Low Level Sewer of south London, said it
Victorian historians had a theory that the Roman-era Thames practically had no banks, but instead spread out into a vast lagoon at high tide. They used it to argue for the etymology "London" ← Llyndin. This is now discounted. Further, the central bed of the Thames was in much the same position as it is today. "Notwithstanding that the waterway of the Thames is very irregular, it is clear that it has kept its present line of flow the same, within narrow limits, since it first became estuarine".
More recent research suggests that in Londinium the tide was not large, and at one time non-existent. .
During the Roman occupation the first embankation took place: building a quayside in the London Bridge area.
Today
The tidal Thames today is virtually a canal—in central London, about 250 metres wide—flowing between solid artificial walls, and laterally restrained by these at high tide. For instance, the Victorian engineer James Walker reported that, if the walls were removed Or as explained by Flaxman Spurrell, "It is evident that all the shore land of the Thames below the level of 15 feet O.D. would be continually subject to the wash of the tides of the present day if unembanked".It seems the continual processes of embanking and bank-raising have greatly increased the tidal amplitude, which at London Bridge is now 6.6 m, and the constriction and scouring have deepened the river. In a paper warning against planning complacency Sarah Lavery and Bill Donovan of the Environment Agency said:
Those are just the walls within the purview of that report, for they do not stop at Sheerness and Shoeburyness.
The Thames Estuary has a defended tidal floodplain of 35,000 hectares with 500,000 properties at risk from flooding. "Other assets within this flood-plain include 400 schools, 16 hospitals, eight power stations, dozens of industrial estates, the city airport, 30 mainline railway stations and 38 underground and Docklands Light Railway stations, with this including most of the central part of the underground network".
In London the Thames flows through an alluvial plain, a geological formation which is two or three miles wide. This plain, the river's natural flood plain, is everywhere less than above mean sea level—sometimes less than 5 feet.
The walls also affect London's water table:
Hence in boreholes, groundwater levels in Battersea Park were observed to fluctuate with the tide; but just across the river in Chelsea, hardly at all. By 1971 nearly 7,000 tons of water had to be pumped from the Circle Line daily to maintain track drainage between West Kensington and Temple stations.
Tributaries
Where a tributary e.g. the River Lea meets the Thames it is necessary to cope with tidal water escaping laterally. This was done, traditionally, by providing the tributary with its own walls. The tributary walls must be carried high enough upstream to meet the rising land. In recent times some tributaries have been given barriers against exceptional tides e.g. Barking Creek and Dartford Creek.Who built the walls?
Early speculations
The Thames walls puzzled historians for centuries. Early modern thinkers knew the Thames walls must be old, but could not account for their origins.The antiquarian Sir William Dugdale amassed a collection of legal records from which it was evident there were embankments along the Thames from the Middle Ages at least:
The scientist and architect Sir Christopher Wren thought the walls were built to restrain wind-blown sand dunes, and attributed them to the Romans, for similar reasons. The influential Victorian engineer James Walker—who was himself to lay down the lines of the Thames Embankment in central London—thought the same, adding
Walter Besant was intrigued by the mystery. He noticed several small chapels in unpeopled places along the northern wall: he surmised they had been devoted to pray for its preservation.
One of the first writers to reject the "Roman" theory was Robert Peirce Cruden. In his History of Gravesend he pointed out that the Roman authorities had no incentive to build embankments on such a scale, nor were any embanked marshlands mentioned in Domesday Book. He concluded that the Thames embankments between London and Gravesend were commenced early in the 12th century by religious houses for the purpose of reclaiming marshland "by easily executed approaches", and were completed in the 13th. Some others including the Astronomer Royal were sceptical too.
How the builders coped with 20-foot tides was not mentioned.
Flaxman Spurrell's explanation
In his Early Sites and Embankments on the Margins of the Thames Estuary F.C.J. Spurrell described his fascination with the topic:-Spurrell came to realise that large tides in the Thames are a relatively recent phenomenon. In the Middle Ages they were much smaller, where they existed at all. They would have posed no insuperable obstacle to land reclamation by farmers and other local people. It was those people who built the walls. As tides gradually increased over the centuries—which happened because the land was sinking—people easily raised them to match.
Sinking land, rising tide
Spurrell had visited the excavations for the Royal Albert Dock, the Port of Tilbury and Crossness Pumping Station, and in each place he saw—7 to 9 feet below the surface—traces of human habitation, including Roman-era pottery. This level was on top of a layer of peat, and was covered by a layer of mud. There were multiple layers of mud and peat. Spurrell thought the mud layers must have formed when the spring tides deposited sediment. But peat formation must have required a long period of freedom from the tide in order for the vegetation to grow, especially since Estuary peat was often associated with the roots of yew trees.From these data, and from sightings of fossils like estuarine shells and diatoms, Spurrell proposed that the Thames was a freshwater river originally, but had been invaded by the sea, owing to subsidence of the land.But, he said, the process was not uniform, because sometimes the land subsidence paused—maybe reversed—as was indicated by the peat layers. He thought the tidal limit in Roman times was further down the river. The river was much shallower than today. It was "not too much to suggest that the tidal water, such as now reaches London, might then have been full five-and-twenty miles away". As a later commentator explained, "The Romans did not build the embankments, not because they could not, but because they had no need to".
Spurrell's theory has been described as a "startling suggestion" since shown to be probably correct. "ost modern authorities would agree with Spurrell that the alluvial plain was an area of marsh dissected by creeks, and that the tidal limit was further seaward than today".