Ancient Monuments

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Replica of the Mohne Dam, in the grounds of the Building Research Establishment, Garston

A Scheduled Monument in St Stephen, Hertfordshire

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Latitude: 51.7018 / 51°42'6"N

Longitude: -0.3745 / 0°22'28"W

OS Eastings: 512428.896376

OS Northings: 201586.286679

OS Grid: TL124015

Mapcode National: GBR H8V.84R

Mapcode Global: VHFSF.GHD8

Entry Name: Replica of the Mohne Dam, in the grounds of the Building Research Establishment, Garston

Scheduled Date: 16 October 2002

Source: Historic England

Source ID: 1020749

English Heritage Legacy ID: 32454

County: Hertfordshire

Civil Parish: St Stephen

Built-Up Area: Watford

Traditional County: Hertfordshire

Lieutenancy Area (Ceremonial County): Hertfordshire


The monument includes a replica of the Mohne Dam, built to a scale of 1:50,
located within woodland in the centre of the grounds of the Building Research
Establishment at Garston.

The replica is some 14m long and 1m high with a maximum thickness of 0.6m at
its base. The dam wall holds back a body of water forming a D-shaped pond with
a maximum width of 15m; the stream feeding the dam overflows through the
centre of the dam wall into a small drainage channel. The dam wall itself is
constructed of over 600,000 miniature mortar cubes with a concrete core;
this copies the construction technique used in the real Mohne dam which is
made of massive granite blocks with puddled clay to seal the join at its

The replica dam was constructed in December 1940 following discussions on
proposals to strike a number of dams along the River Ruhr in Germany. Dr
William Glanville the Director of the Road Research Laboratory at
Harmondsworth invited the eminent engineer Barnes Wallis to the Building
Research Station to discuss the alternatives with the then Head of Engineering
Dr Norman Davey. It was agreed that scale models would be built to determine
the best method of attack. Constructed in strict secrecy, the Garston
replica took a local workforce just seven weeks to build. Tests on the
most effective means of destruction included the detonation of ten charges
on the `wet' side of the dam at distances of between 1 and 3 feet. Other
ideas included the dropping of charges with pressure fuses at the base of
the dam and then initiating simultaneous detonation with a single large bomb.

The Garston replica was the first step in a series of extensive tests on the
best means of destroying the Ruhr dams; other model dams (none of which
survive) were built and tested by Dr Glanville's staff before final
experiments on a real dam near Rhayader in the Welsh mountains. In the event
Barnes Wallis devised his unique bouncing bomb as a means to breach the walls
of the Ruhr dams. On the 16th May 1943 19 Avro Lancasters of 617 Squadron
left RAF Scampton led by Wing Commander G P Gibson loaded with the
bouncing bombs designed by Barnes Wallis. The raid was deemed a success as one
bomb inflicted a gap some 20m high and 77m long on the Mohne dam; the Eder dam
was also breached. Although the dams were repaired fairly quickly (the Mohne
Dam took just four months), the action provided a huge boost to the morale of
the Allied war effort and is consequently world famous as the `Dambuster

The site of the monument is shown on the attached map extract.
It includes a 1 metre boundary around the archaeological features,
considered to be essential for the monument's support and preservation.

Source: Historic England

Reasons for Scheduling

The replica of the Mohne Dam in the grounds of the Building Research
Establishment at Garston is a unique survival; it is the only test dam to
survive from a small number built for the dam destruction experiments at
Garston and as such is not only of national but also international
importance. It gives testimony to the exhaustive nature of the top secret
experimental bombing trials prior to the Dambuster Raid on the Ruhr river
dams in Germany. It also illustrates the multiplicity of tasks for which
local construction companies as well as national scientific staff could be
conscripted into during the course of World War II.
The Dambuster Raid is a world famous event. The secret trials prior to the
event are, however, not so well known and the Garston replica gives graphic
testimony to this lesser known side of the story.

Source: Historic England


Books and journals
Ottaway, S, Dambuster A Life of Guy Gibson VC, (1994)
'Defence Lines Newsletter of the Defence of Britain Project' in Lilliput's Dambusters, , Vol. 7, (1997), 3
, accessed from
, accessed from, accessed from
Tyler, S, MPP Film, (1998)

Source: Historic England

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