Inside the tidal Pools
During the summer of 2024, a comprehensive survey was undertaken by Dr A Hall, Marine Biologist to evaluate the effectiveness of the ecological interventions (pre-cast tide pools, rock munched tide pools, and the seed rock). The report concluded that the provision of water retaining features and increased surface heterogeneity in both the concrete and granite rockpools allowed for a diverse range of rockpool species to colonise the revetment. The combination of enhancement techniques has resulted in a diverse community, and we highly recommend these techniques for use on future schemes.
Below are some of the species observed
Species observed in the Granite Artificial Rockpools:
Environmental Improvements at the Southsea Coastal Scheme
a) purse spongeSycon ciliatum
b) prawnPalaemon sp
c) peacock wormSabella pavonina
d) chitonLepidochitona cinerea
e) perwinklesLittorina saxatalis
f) corkwing wrasseSymphodus melops
a) wood pigeonColumba palumbus
b) green algaeBryopsis plumosa
c) sea spiderPycnogonid sp
d) top shellSteromphala umblilicalis
e) green shore crabCarcinus maenas
f) shannyeLipophrys pholis
The Southsea Coastal Scheme in Portsmouth is the UK’s largest local authority led coastal defence project. Spanning 4.5km, the scheme reduces the risk of coastal flooding and erosion for over 10,000 homes and 700 businesses. Split into six sub-frontages, the construction is being delivered in phases.
Sub-frontage 4 construction was completed in May 2024, inclusive of a range of environmental improvements, such as the installation of 45 precast tidepools, 55 rock-munched granite rockpools and pre-colonised seed rock to encourage and enhance the marine biodiversity of the new coastal structure. Funding for these initiatives was obtained from the Water Environment Improvement Fund and Local Levy.
Frontage 1
Long Curtain
Moat
Frontage 2
Clarence Pier
Frontage 3
Southsea Common
Frontage 4
Southsea Castle
PORTSMOUTH
Frontage 5
Pyramids to Speakers Corner
Frontage 6
Speakers Corner to Eastney
Southsea Castle
The existing coastal defences at Sub-frontage 4 (Southsea Castle) were reaching the end of their serviceable life and were in need of upgrading to meet the required standard of protection for the next 100 years. The primary defences consisted of a sloped stone block revetment with a sheet pile and rock toe at the seaward end of the structure. Part of the primary defence area is designated as a Solent Wader and Brent Goose Strategy Site. The area is the only site in Hampshire where Purple Sandpipers regularly feed and roost during the overwintering period.
In order to minimise any impact on the Purple Sandpipers and to improve the biodiversity of the revetment, a Greening the Grey package of measures was developed. The aim of these measures was to speed up colonisation of the revetment, to quickly recreate and improve the feeding resource for the Purple Sandpipers and to also improve the biodiversity of the revetments both now and into the future.
These included:
• Placement of ‘seed rock’ to help speed up colonisation of the new rock armour revetment.
• Placement of 45 pre-cast tidepools and 55 rockmunched granite pools to increase the biological diversity of the new revetment.
• The contractor also retained areas for the sole use of the Purple Sandpipers to both roost and feed during the construction works
• A minimum protected area of 0.25ha was made available throughout the overwintering period, ensuring the Purple Sandpipers were able to roost at all states of tide.
• An ornithological watching brief was in place throughout construction during the overwintering period to measure, monitor and if necessary, change practices to ensure the protection of the Purple Sandpipers.
We have been continuing with the monitoring of the Purple Sandpipers’ usage of the site, and we are thrilled to announce that they have returned in the same numbers as prior to construction, which is a testament to the effectiveness of the Greening the Grey initiative.
Techniques
Seed Rock
This project trialled the reuse of the existing colonised rock as ‘seed rock’. The rock that was present as part of the old structure was moved to the toe of the new revetment rather than being buried as previously planned. By placing this at the toe of the new rock armour revetment, this has given the opportunity to accelerate recolonisation to an area that would otherwise remain as an ecological desert for a considerable number of years.
A review of published literature and discussions with various academic experts was undertaken, and this indicated that colonisation and recovery of seaweeds on fully disturbed / bare substrates is likely to be prolonged over 10-15 years. On artificial structures there have been very few long-term monitoring studies to evaluating the succession of ecological communities. Where populations of seaweeds remain, then re-growth and recolonisation and therefore habitat recovery is likely to be within three years.
The technique was developed with skilled operatives to move the rock with minimal handling to prevent damage to the marine fauna and flora on the rocks. Rock was moved from its original position on the old revetment directly to the new position, again to prevent damage or desiccation of the species. The placement of the rock was in the same orientation, although it was lower in the tidal frame, species are able to survive at this level. The colonisation of the new revetment seemed to be rapid; however, statistical analysis was not conclusive and further monitoring will be carried out in future years to determine the success of this trial.
Tide Pools
Rock armour installation Southsea Castle
Purple Sandpipers on Rock Pools at Southsea
Rock Munched Pools
Placement of sand coloured seeded rock Credit: Boskalis
Rock Munched Pools
Pre-cast Pools
