The brief
To design & build an ark site to support White-clawed crayfish Austropotamobius pallipes in Hampshire, following feasibility analysis of two possible locations.
The endangered, White-clawed crayfish is the only species of crayfish native to the UK. It has suffered a dramatic decline since the 1970's & 1980’s, following the introduction of non-native crayfish, and the disease ‘crayfish plague’ carried by crayfish species from North America. The establishment of new populations at isolated ‘ark sites’ is one measure being implemented across the UK to secure the long-term survival of White-clawed crayfish.
The solution
The project was delivered across four phases spanning three years:
Phase 1: Feasibility study.
Phase 2: Design & permissions.
Phase 3: Construction.
Phase 4: Establishment.
Phase 1: Feasibility.
A combination of site survey & desk-based research fed into the production of a feasibility report critically assessing the suitability of two proposed sites and making a clear recommendation on which location to locate the project. This included:
• Topographic & photographic survey
• Chemical & physical soil analysis
• Infiltration testing
• Water budget calculations
• Analysis of landscape context & nearby designations
• Existing service identification
• Proposed outline design description & budget projections
• Construction & maintenance implications
Phase 2: Design & Permissions.
Further investigation work identified by the feasibility study was undertaken, including jetting & camera inspection to ascertain the current state of existing rainwater drainage network (pipes & chambers) that could feed the pond. Production of a full design package was developed collaboratively with the multi-organisation project team, as well as updates to the build budget.
Planning and highways pre-applications were undertaken, before obtaining full planning permission. This included supporting information such as drawings and block plans, design & access statement, Construction Environment Management Plan, tree survey, traffic management plan, landscape & maintenance specification. Preliminary Ecological Appraisal, protected species surveys and BNG calculation was undertaken by Arcadian Ecology.
A key part of this stage of the process was identifying the most suitable pond lining option for this particular site.
Phase 3: Construction.
• Tree cutting works were carried out to remove the risk of surrounding ash die-back affected trees shedding limbs or falling into the pond and damaging the liner, along with removal of the shrub & scrub layer within the site itself.
• The bulk of the site work involved the creation of a 0.25 hectare water-tight pond using a bentonite liner between layers of puddle clay.
• Islands and shelves were created to provide depth variation and various crayfish habitat features were incorporated including rock rolls, loose aggregate and submerged deadwood generated by the ash die back work.
• The surface water drainage system was repaired and replaced as required to provide a clean rainwater supply.
• A lined filtration channel was created to bind & filter out any pollutants from the surface water run-off before this water enters the pond, using a combination of limestone and clay aggregates and vegetation.
• Soft landscape included marginal and oxygenating plants, seeding, terrestrial plug planting and bare root re-stocking using oak and hazel in the adjacent woodland edges where ash trees had been thinned.
• Fencing and gates were installed to prevent livestock access.
Morton : Pattison acted as the Principal Contractor and Principal Designer, managing all aspects of CDM pre-start and on site.
Phase 4: Establishment.
Morton:Pattison maintained the soft landscaping during the first year to assist with its establishment, submitting regular maintenance reports and carrying out replacement of failed woody species during the first winter.
The outcome
The renovation work to the existing dilapidated pipework has doubled the catchment area, allowing the pond to be topped up naturally during rainfall events, minimising water level fluctuations even during the subsequent hot summer.
While the site establishes the chemical characteristics of the pond will be monitored before captive-reared crayfish are released. Meanwhile, the site is already benefitting a whole host of other species due to the particular assemblage of habitat features created with White-clawed crayfish in mind.
The project was delivered through the Watercress and Winterbournes Landscape Partnership Scheme funded by the National Lottery Heritage Fund. Match-funding also being provided by the Test and Itchen Catchment Partnership’s Ecological Resilience Fund (supported by Southern Water), North Wessex Downs National Landscape, Hampshire & Isle of Wight Wildlife Trust, and Test Valley Borough Council. And of course, the private landowner that has generously allowed the project to take place on his farm.
