The Subterranean River

How Brick Intercepting Sewers Saved London

During the third week of June 1858, an early summer heatwave pushed shade temperatures in central London to $35^\circ\text{C}$ ($95^\circ\text{F}$) and baked the exposed low-tide mudbanks of the River Thames into a fermenting six-foot-deep crust of raw human sewage. Directly beside the river stood the newly rebuilt Palace of Westminster. Inside the committee rooms, the stench of hydrogen sulfide, methane, and ammonia rising off the river was so overpowering that curtains were soaked in chloride of lime, judges adjourned the Law Courts early, and Prime Minister Benjamin Disraeli bolted from a parliamentary session holding a handkerchief pressed to his nose.

The crisis remembered as the "Great Stink" of 1858 was the unintended result of connecting a nineteenth-century sanitary appliance to an archaic medieval drainage network. By 1850, London had swollen past 2.4 million inhabitants. For centuries, Londoners had deposited human waste into brick basement cesspools beneath their floorboards, which "night-soil men" shovelled out by lantern light, while the city's ancient covered brick streams—the Fleet, Tyburn, and Walbrook—were reserved solely for carrying street rainwater down to the Thames.

Two developments broke that separation. First, middle-class households adopted the pressurized domestic flush toilet (water closet), which discharged thirty gallons of water into basement cesspools built to hold only small volumes of solids, flooding cellars with sewage. Second, following Edwin Chadwick's sanitary reforms, the Metropolitan Commission of Sewers passed acts in 1847 and 1848 abolishing over 200,000 cesspools and ordering householders to pipe their water closets directly into the street storm drains.

Overnight, millions of gallons of raw excrement poured every day through 369 sewer outfalls straight into the central Thames. Because the Thames through London is a tidal estuary sixty miles inland from the North Sea, the sewage did not wash out to sea; twice every twenty-four hours, the incoming flood tide pushed the floating mass of waste twelve miles back upstream through the heart of the city. Meanwhile, private water companies like the Southwark and Vauxhall Waterworks drew municipal drinking water directly from those tidal reaches. During the cholera epidemics of 1848–1849 and 1853–1854, more than 24,000 Londoners died before Soho physician John Snow proved in August 1854—by mapping fatalities around the fecal-contaminated Broad Street pump and comparing households supplied by clean upstream Thames Ditton water against foul Battersea water—that cholera spread through sewage-tainted water rather than foul air.

Within eighteen days of the June 1858 heatwave driving MPs from the riverbank, Parliament passed an act granting the chief engineer of the Metropolitan Board of Works, Joseph Bazalgette, three million pounds to re-plumb the capital. Between 1859 and 1875, Bazalgette built 82 miles (132 kilometers) of brick intercepting sewers running west-to-east parallel to the Thames—three main tunnels north of the river and two south—that cut across 1,100 miles of street drains before they reached the riverbanks, carrying the city's wastewater fourteen miles downstream to covered reservoirs at Barking Creek and Crossness for release only on the outgoing ebb tide.

To keep solid waste moving along a gentle downward slope of just two feet per mile ($1:2,640$) without clogging, Bazalgette standardized a critical hydraulic geometry across London's street and branch sewers: the inverted egg-shaped brick cross-section. A combined municipal sewer must handle both roaring storm-water surges during cloudbursts and a shallow trickle of concentrated sewage at 4:00 a.m. during dry droughts. In a wide flat-bottomed or circular conduit, a small dry-weather flow spreads into a thin sheet with a large wetted perimeter $P$ relative to its cross-sectional area $A$, collapsing the hydraulic radius $R_h = \frac{A}{P}$ and reducing the bed shear stress $\tau_0$ pushing solids along the floor:

If bed shear stress $\tau_0$ drops below roughly $1.5\text{ to }2.0\text{ pascals}$ (a flow velocity below 0.75 meters per second), grit and organic solids settle out and compact into a blockage. By narrowing the bottom invert of the brick tunnel like the small end of an egg and flaring the upper vault wide, Bazalgette ensured that even the smallest dry-weather trickle remained deep and narrow, preserving a high hydraulic radius $R_h$ and a self-scouring velocity above 0.75 m/s while leaving ample overhead volume in the wide crown for storm surges.

Bazalgette excavated 3.5 million cubic yards of earth, laid 318 million Staffordshire bricks bonded with waterproof Portland cement—testing every batch for tensile strength—encased the northern low-level sewer inside the granite Victoria, Albert, and Chelsea Embankments, and installed towering Watt beam engines at Abbey Mills and Crossness to lift sewage thirty feet into the outfall reservoirs. When cholera struck England one last time in July 1866, ninety-three percent of the London deaths occurred in a single East London pocket around Old Ford where the Northern Low-Level Sewer had not yet been connected; once Bazalgette's masons bricked in that final junction, epidemic cholera never returned to London.