Non-standard rock groynes in Poole and Christchurch Bays
Minor funds contribution 2011-2013
SCOPAC: £4,000 (£2,000 2011/12 & £2,000 2012/13)
Poole, Bournemouth, Christchurch and New Forest Councils have all built rock groynes that do not conform to the Rock Manual Guidelines.
A small tidal range and short duration of Low Water precludes:
- excavation to a design bed level
- placing of a geotextile membrane
- placing of a filter layer
particularly at the seaward end of the groynes.
Instead, a “rubble mound” groyne composed entirely of armour stone is used without conventional foundation layers. Such groynes have been built since 1986.
Bournemouth encountered problems in 2009/10 when:
- the contractor concluded that the conventional design was impossible to build;
- the consultant declined to approve the contractor’s alternative design.
In the event Bournemouth Borough Council accepted responsibility for the alternative design which was successfully completed.
Anecdotal evidence suggests that all these non-standard groynes are performing satisfactorily.
The Non-standard rock groynes study:
- surveyed the existing groynes to determine if settlement or dispersal of rock is a problem;
- investigated whether the approach taken in the Bays is appropriate for future rock groyne construction.
This topic is especially relevant to Bournemouth where the Timber Groyne Renewal Programme (TGRP) will require the removal of 51 life expired timber groynes between 2013 and 2029, and it is anticipated that the new structures will be rock groynes.