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Wallasey Offshore Breakwaters
Leasowe Bay, Wallasey, Wirral, Merseyside, UK
associated engineer
Metropolitan Borough of Wirral
date  1981, 1982
era  Modern  |  category  Breakwater  |  reference  SJ264922
ICE reference number  HEW 1430
Wallasey has the first offshore breakwater in Britain constructed for coastal defence (1981). It protects the hinterland east of Wallasey Embankment (1829) and was so effective that a second breakwater was constructed nearby a year later.
The coastline at the eastern end of the embankment was eroding, threatening the headland of Leasowe Castle and the local golf course. Records show coastal recession of 85m between 1893 and 1953, with further damage during the 1970s. The embankment itself was also vulnerable and underwent extensive repairs in 1973-81.
The Wallasey sea front is low-level natural sand dunes, easily eroded by wave action. The area also has a tidal range of about 10m, making it unsuitable for traditional timber groynes as a coastal protection measure. Numerical and physical modelling indicated that a detached breakwater would be the most cost effective option.
A 240m long rock breakwater, rounded at each end, was built 140m from the shore in 1981, together with a 50m long fish-tail rock groyne (SJ267921) connected to the shore. The breakwater has a crest width of 5m and is 6m tall with side slopes of 1 in 6 (seaward) and 1 in 3 (landward). The groyne has side slopes of 1 in 2 and a crest 3m wide. Both structures contain limestone quarried in north Wales.
Further east along the bay, the western end of Leasowe revetment was suffering erosion. To reduce scour at the seawall by deflecting the angle of wave approach, another offshore rock breakwater (SJ271925) with roundhead ends was built in 1982.
This one is 210m long and also sited 140m offshore, with a 70m long shore link to encourage retention of beach material. It has a crest width of 6m and height of 5.5m with side slopes of 1 in 5 (seaward) and 1 in 3 (landward). The tops of both breakwaters stay exposed at high tide.
Beach levels have increased in both locations and the detrimental effects of wave action along Wallasey Embankment have diminished significantly. Sand has tended to overlay the silt and mud previously exposed on the foreshore. As anticipated, a large accumulation of material connects the second breakwater to the shoreline, forming a tombolo. New dunes are forming at the east end of the site and sediment transport rates along the whole frontage have decreased.
Research: ECPK
bibliography
"Offshore breakwaters and shore evolution control" by K.W. Pilarczyk and Ryszard B. Zeidler, Taylor & Francis, n.p., January 1996
"Design of Marine Structures", in Chapter 6 of The Rock Manual, CIRIA C683, London, 2007, available at www.kennisbank-waterbouw.nl
www.cheshire-biodiversity.org.uk
www.delos.unibo.it
www.jstor.org
www.liv.ac.uk
reference sources   CEH North
Location

Wallasey Offshore Breakwaters