The embanking of the tidal Thames is the process by which the lower River Thames was transformed by human intervention from a broad and shallow waterway winding through malarious marshlands 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 only a small part of this process.
Beginning in Roman Londinium, embanking was pursued more vigorously in the Middle Ages. It was mostly achieved by farmers reclaiming marshland and building protective embankments, and frontagers pushing out into the stream to get more riverfront property. Currently, over 200 miles of walls line the river's banks from Teddington down to its mouth in the North Sea defending a tidal flood plain where 1.25 million people work and live. Some London streets originated as tracks running along the wall but today are not even in sight of the river.
Since the Thames has a large tidal amplitude, early modern thinkers did not believe local people could have embanked it, so the works were attributed to "the Romans". The current explanation is that tides were small at first, requiring only modest embankments and 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 also made it navigable by larger ships that could travel further inland which led to London growing to be the world's largest port.
Before and after the walls The natural river At the foundation of Roman Londinium, , the Thames was a river flowing through marshland, possibly five times wider than today. At some point it became infested with malarial mosquitoes.
The original site of Londinium may have been chosen for being the first place where someone travelling up the river would have encountered a broad tract of dry land. This area, mainly gravel, came down to the water on the north bank. The south bank was a wide expanse of marshes and mudflats, largely covered at high tide when the river rose through its tidal range of just over . The first embankation took place during the Roman occupation, when a quayside was built in the London Bridge area (see ).
Below, the river flowed to the sea through broad marshes, rarely touching firm ground. This continued to be the case until the Victorian era, although by that time the river had been restrained at high tides by earthen banks. Much of today's urban London is built on reclaimed marshland.
For millennia humans made use of the ecology of the marshes. They served both as a location for grazing, fishing and fowling, and as a source of raw materials. Reeds and rushes were harvested for thatching and flooring and salt was collected in a seasonal industry on the estuary's outer mudflats.
The flood plain of the Thames was intersected by many small, winding creeks, some of which may have been navigable. The remains of a Roman ship were found on the contemporary site of Guy's Hospital.
Sir Joseph Bazalgette, describing the Low Level Sewer of south London, said it
Victorian historians incorrectly theorised that the Roman-era Thames practically had no banks, but instead spread out into a vast lagoon at high tide, and argued for a specious etymology of "London" as Llyndin ("lake fort"). The central bed of the Thames has not significantly changed position since it first became estuarine, and more recent research suggests that in Londinium the tide was not large, and at one time non-existent (see ).
Today
In London the Thames flows through an alluvial plain, a geological formation which is wide. This flood plain is less than above mean sea level, in places less than . in size, it contains 500,000 properties at risk from flooding, plus 400 schools, 16 hospitals, 8 power stations, numerous industrial estates, London City Airport, dozens of surface and subterranean railway stations, and the central part of the London Underground network. An estimated 1.25 million people live and work in the tidal flood plain. of artificial barriers along the sides of the river form part of the flood defence network, in combination with nine major barriers which include the Thames Barrier at Woolwich, dozens of industrial floodgates, and hundreds of privately-owned minor moveable structures.
These artificial walls along its length make the modern river virtually a canal and laterally restrain it at high tide. It is wide where it passes through central London. The continual processes of embanking and raising of its banks have increased its tidal range, which at London Bridge is , and the constriction of the river and subsequent hydrodynamic scouring have deepened it.
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.
Where a tributary such as the River Lea meets the Thames it is necessary to cope with tidal water escaping laterally, traditionally managed by building walls alongside it. The walls must be carried high enough upstream to meet the rising land. In recent times some tributaries, such as Barking Creek and Dartford Creek, have been given barriers against exceptional tides.
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 DugdaleFor the reliability of Dugdale as a source see : Underestimated by historians, "it deserves some credit as a carefully-researched piece with some genuinely innovative features". amassed a collection of legal records (1662) 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 (1843) 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 (1086). 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.Samuel Smiles was one of the first to write biographies of engineers. In his opinion the walls were built by immigrant "Belgian tribes" who were reclaiming marshland. "The work was carried on from one generation to another, as necessity required, until the Thames was confined within its present limits". Sir George Biddell Airy the Astronomer Royal too was sceptical of the Roman theory.
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 (1885) F.C.J. Spurrell"The quiet man of Kent", F.C.J. Spurrell was an amateur geologist and archaeologist whose papers are still cited. He had the means to devote himself full time to the investigation of the Thames Estuary. "A diffident and somewhat cautious researcher, his publications are regrettably few and terse in tone, striving as he did always to present the 'bare facts' of his investigations": . 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 ("tidal clay") must have formed when the spring tides deposited sediment (a process still observable in his day at certain river margins, called Saltings). 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 ("the yew is intolerant of water and cannot live in salt").
From these data, and from sightings of fossils like estuarine shells and diatoms, Spurrell proposedIn the first paragraph of his paper. 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. "[M]ost modern authorities would agree with Spurrell (1889) that the alluvial plain was an area of marsh dissected by creeks, and that the tidal limit was further seaward than today". (The sinking of the land with respect to sea level is best described as relative, however, since it is a complex of several factors: see The advancing head of tide, below.)
The origins of the walls
As a specimenChosen because the archaeological evidence was best preserved. of his maps, Spurrell published the one of Higham, Kent shown here. It had a network of walls enclosing reclaimed marshland. The highest walls were the current tidal defences; progressively lower walls lay inland and were (he said) the old defences. The Thames at high spring tides continued to deposit mud on the Saltings but of course not on the landside areas; these were now lower. This was an instance of land reclamation by successive 'inning' of marshland (see below).
Large amounts of Romano-British pottery could be found in the Kent marshes in Spurrell's dayProbably because there had been pottery factories there in the Roman era, to take advantage of the abundant clay: ("I have seen over a hundred unbroken pots at one time, and such immense quantities of broken fragments, that the new embankment of the railway there was in places made of them".) But the levels of human occupation were feet below the present-day marsh:
Towards the end of his life he wrote: