This was built to hold a freshwater reservoir, and was part of a 1970s
feasibility study for an ambitious scheme which never happened.
"1972: a feasibility study commissioned by the Government to build a
barrage across half of The Wash to capture the freshwater from the
four main rivers, to improve navigation through sea locks, to provide
recreational facilities and an area of land for a power station, etc.
was undertaken. This led to the circular trial bank/bund being built
to the east of Sutton Bridge and the Nene. The purpose of which was to
act as reservoirs but the report concluded it would be too costly. "
The reservoir structure is sometimes called the "outer trial bank",
presumably to distinguish it from the smaller artificial island/trial
bank nearer land. The outer bank is mentioned in this discussion of
the inshore island:
"Our competition to find the purpose of the Island is now closed. The
winning entry came from the 1976 report on the feasibility study of
The Wash Water Storage Scheme. This suggested the bank was used as an
inshore trial bank. The bank was built in 1972. It measures 60m in
length and is 12m high with side slopes of 1 in 4 gradient. The
construction of this island led to a second larger island being built
4.5 miles out to sea. [Not sure where they're measuring from!] That
island contains a lake of one hectare. The full Wash Water Storage
Scheme proposed to create large fresh water reservoirs by banking off
large sections of the Wash between the outfalls of the Nene and the
Great Ouse. The conclusions of the feasibility study were largely that
although such a scheme was technically achievable, there wasn't
generally such a demand for water as to warrant the large costs
involved. However the report didn't rule out such a scheme becoming
necessary sometime in the future."
See both artificial banks at once on this map:
Nowadays, according to English Nature, "The Outer Trial Bank, an
artificial island, is an important site for breeding seabirds."
There isn't any more detail online - just brief references adding confirmation:
"The twin redundant lighthouse towers could then be seen up the Nene
Outfall. The large round trial bank came visible, out to the east of
us. This was an experimental water storage reservoir. "
". . . fresh water reservoir storage proposals in the mid-1970s, some
testing results of which still remain."
The MP for Norwich North speaking in 1992:
"I would like to mention the Wash barrage. Recent correspondence in
the local press has asked why, 20 years ago, we did not grasp the
opportunity to build that barrage. Many of our problems of water
supply and flood control in Fenland would not be costing us so much
now if the requisite investment had been put into the Wash barrage
some years ago."
Picture of "Trial Bank"
Central Water Planning Unit (1976)
The Wash water storage scheme. Report of the feasibility study.
Full title: The Wash water storage scheme
Corporate author: Department of the Environment; Central Water
Planning Unit; Steering Committee
Chairman: Pemberton, F.
Other features: Feasibility Study. Report, appendices
Thanks for the interesting question.
Best wishes - Leli
"Trial Bank" "The Wash"