The trench effect is a combination of circumstances that can rush a

fire Fire is the rapid oxidation of a fuel in the exothermic chemical process of combustion, releasing heat, light, and various reaction Product (chemistry), products. Flames, the most visible portion of the fire, are produced in the combustion re ...

up an inclined surface. It depends on two well-understood but separate ideas: the

Coandă effect The Coandă effect ( or ) is the tendency of a jet (fluid), fluid jet to stay attached to a surface of any form. ''Merriam-Webster'' describes it as "the tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved s ...

from

fluid dynamics In physics, physical chemistry and engineering, fluid dynamics is a subdiscipline of fluid mechanics that describes the flow of fluids – liquids and gases. It has several subdisciplines, including (the study of air and other gases in motion ...

and the

flashover A flashover is the near-simultaneous ignition of most of the directly exposed combustible material in an enclosed area. When certain organic materials are heated, they undergo thermal decomposition and release flammable gases. Flashover occurs wh ...

concept from fire dynamics: * The Coandă effect is the tendency of a fast stream of gases to bend towards, and adhere to, nearby surfaces. The stream's static pressure tends to decrease, which creates a pressure difference between the surface and areas far from it. This bends the stream towards the surface and tends to keep it attached to that surface. * Flashover is a sudden widespread fire, which occurs when most surfaces in a space are heated until they emit flammable gases hot enough to auto-ignite. Before flashover, flammable gases may be emitted but are too cool to ignite. The trench effect occurs when a fire burns beside a steeply inclined surface. The flames lie down along the surface, demonstrating the Coandă effect. The flames heat the material farther up: these emit gases that autoignite in a flashover event. The flames from these areas are themselves subject to the Coandă effect and blow a jet of flame up to the end of the inclined surface. This jet continues until the fuel depletes.

Background

The trench effect became known because of the scientific investigation of the

King's Cross fire The King's Cross fire occurred in 1987 at King's Cross St Pancras tube station in London, England, causing 31 fatalities. It began under a wooden escalator before spreading into the ticket hall in a flashover. The fire began at approximately ...

, which occurred on the

London Underground The London Underground (also known simply as the Underground or as the Tube) is a rapid transit system serving Greater London and some parts of the adjacent home counties of Buckinghamshire, Essex and Hertfordshire in England. The Undergro ...

in 1987. The fire started on an escalator (containing wood and years of built-up grease) between the

Piccadilly line The Piccadilly line is a Deep level underground, deep-level London Underground line running between the west and the north of London. It has two western branches which split at Acton Town tube station, Acton Town and serves 53 stations. The li ...

platforms and the ticket hall at

King's Cross St Pancras tube station King's Cross St Pancras (; also known as King's Cross & St Pancras International) is a London Underground station on Euston Road in the London Borough of Camden, Central London, Central London. It serves and main line stations in List of stat ...

. Many eyewitnesses indicated that early on, the fire in the escalator was of manageable size: officers from the Fire Brigade and British Transport Police indicated that the fire appeared no larger than a large cardboard box burning. Those present were surprised when it suddenly threw a sustained jet of flame into the ticket hall. The Coandă effect alone could not explain the sudden blast that blew some survivors off their feet. The relatively inconspicuous flames associated with the Coandă effect, hugging the space on and under the inclined trench of the escalator steps, led many travellers to believe they were not in immediate danger. One survivor reported approaching the escalator and noting thick black smoke that suddenly blew up. The main loss of life came from the sudden flashover, rather than the precipitating flames of the 'Coandă effect' fire itself. The people in the ticket hall at the top of the escalators, whilst avoiding the visible flames, were caught in the path of the flashover. In the early stages of the fire, most of the flames were lying down in the escalator trench: only a few visibly protruded above the

balustrade A baluster () is an upright support, often a vertical moulded shaft, square, or lathe-turned form found in stairways, parapets, and other architectural features. In furniture construction it is known as a spindle. Common materials used in its ...

. The sides of the trench further enhanced the effect by preventing heat loss by radiation to the side, insulating and concentrating the heat along the narrow trench. Emergency services were not prepared for a major fire, due to the lack of visible flames. When the treads of the escalator flashed over, the fire grew dramatically and ignited most of the ticket hall. The

Health and Safety Executive The Health and Safety Executive (HSE) is a British public body responsible for the encouragement, regulation and enforcement of workplace health, safety and welfare. It has additionally adopted a research role into occupational risks in Great B ...

's Health and Safety Laboratory in

Buxton Buxton is a spa town in the High Peak, Derbyshire, Borough of High Peak, Derbyshire, in the East Midlands region of England. It is England's highest market town, sited at some above sea level.Alston, Cumbria also claims this, but lacks a regu ...

lit fires in -scale and -scale models of the escalator and ticket hall, which demonstrated that the trench effect was the main cause of the intensity and rapid spread of the King's Cross fire. The sudden flashover may be attributed to the

wood gas Wood gas is a fuel gas that can be used for furnaces, stoves, and vehicles. During the production process, biomass or related carbon-containing materials are gasified within the oxygen-limited environment of a wood gas generator to produce a c ...

(mainly

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes ...

) emitted from the

pyrolysis Pyrolysis is a process involving the Bond cleavage, separation of covalent bonds in organic matter by thermal decomposition within an Chemically inert, inert environment without oxygen. Etymology The word ''pyrolysis'' is coined from the Gree ...

of the wooden escalator itself. When the concentration of gas reaches a critical value, the

lower flammable limit The lower flammability limit (LFL), usually expressed in volume per cent, is the lower end of the concentration range over which a flammable mixture of gas or vapour in air can be ignited at a given temperature and pressure. The flammability range ...

, the gas suddenly catches fire in the presence of a flame, resulting in an

explosion An explosion is a rapid expansion in volume of a given amount of matter associated with an extreme outward release of energy, usually with the generation of high temperatures and release of high-pressure gases. Explosions may also be generated ...

. An episode of the television series ''

Forensic Files ''Forensic Files'', originally known as ''Medical Detectives'', is an American documentary television program that reveals how forensic science is used to solve violent crimes, mysterious accidents, and outbreaks of illness. The show was origin ...

'' entitled "Flashover" detailed the King's Cross fire, along with the computer modelling and other analyses which discovered the trench effect. An episode of the television series ''

Seconds from Disaster ''Seconds from Disaster'' is a US/UK-produced documentary television programme that investigates historically relevant man-made and natural disasters from the 20th and early 21st centuries. Each episode aims to explain a single incident by anal ...

'' entitled "London's Subway Inferno" detailed both the King's Cross Fire based on eyewitness accounts and the investigation leading up to the discovery of the trench effect. The trench effect has also been cited as the cause of rapid spread of

wildfire A wildfire, forest fire, or a bushfire is an unplanned and uncontrolled fire in an area of Combustibility and flammability, combustible vegetation. Depending on the type of vegetation present, a wildfire may be more specifically identified as a ...

s up hills, which has led on several notorious occasions to the deaths of multiple firefighters.

Background

See also

Further reading