Pinkhill Meadows
Details from Manual of River Restoration Techniques: Floodplain wetland Mosaic - Pinkhill Meadow
Pinkhill Meadow is located in a 4 hectare meander of the River
Thames at Farmoor Reservoir. In a detailed landscape assessment
of the reservoir site in 1988 the meadow was identified for its
potential for wetland creation. Few areas of wetland had survived
agricultural improvement in this part of the Upper Thames Valley.
Approximately 1.5 hectares of the meadow still had a valuable
relic meadow flora including species such as Adders Tongue,
Great Burnett and Pepper-Saxifrage.
The aim of the scheme was to complement the river and the
reservoir habitats by restoring the floodplain wetland within
the meadow which had been largely disturbed during
reservoir construction.
A key objective was to restore habitat for breeding waders
and wildfowl, notably redshank, and to create a place where
people could experience a wide variety of wetland wildlife at
close quarters and enjoy a more "natural" floodplain landscape.
The project was a collaboration between the landowners
( Thames Water) and the National Rivers Authority (NRA),
advised by Pond Action.
A concept plan for the site was prepared from the landscape
appraisal and developed into the detailed design, incorporating
the project objectives with the site constraints.
The mosaic of over 40 ponds and pools was designed to maximise
the topographical, and hydrological diversity of the site. This
included specific creation of individual waterbodies with a
wide range of maximum depths and permeability, and low
angle, undulating drawdown zone areas to encourage wetland
plant diversity.
Two phases of excavation were undertaken, the first in June
and July 1990 and the second over the winter period in 1991/2.
By phasing the works it was possible to better understand the
detail needed for the more complex works in the second phase.
Excavation was based on detailed landscape design drawings
provded by the NRA landscape architect, and firmly led by close
project management and continuous on-site supervision from
key members of the project team. In this way the inexperienced
machine operator was able to achieve the very subtle variations
in topography in relation to water levels. The 20,000m³ of
excavated material was carefully graded into a low hill near the
adjacent Pinkhill Lock, but outside of the floodplain. This was
then planted with trees and shrubs and sown with a wildflower
seed mix.
In phase 1 four waterbodies were created; the main pond, wader
scrape and two reedbed pools. In phase 2 the existing waterbodies
were extended, added to and re-profiled to create areas of
shallow water, wet meadow, mudflat and temporary pool habitat.
Observations of the phase 1 works provided valuable detail
for the improvements undertaken in phase 2. Observations of
actual as opposed to design water levels in the pools were
used to refine the new excavation levels, marginal areas and
undulating contours of the wet meadow. The location of the
mudflats was also based on the usage of the various areas of
the site by different bird species.
Key features created:
Deep water
The main pond is up to 2.5m deep and covers an area of just
under 0.5ha and was excavated down into the gravel aquifer.
The size and depth increases the diversity of habitats and
isolates the several islands reducing the likelihood of predation
of bird nests. The depth also ensures open water and from
a management viewpoint it also restricts the complete
colonisation by marginal wetland plants.
Shallow-water areas and edges
These areas were designed to between 0.3m below and
0.1m above normal water levels. As the main pond level will
fluctuate by about 0.3m, reflecting groundwater levels,
these areas are important to retain shallow slopes at the
water's edge.
Temporary ponds and pools
The site also includes small temporary pools, some isolated
and some bordering the larger waterbodies. Sizes vary from
a few square metres to the larger two semi-permanent ponds
(approximately 100m²). These transient ponds are designed to
dry out in drought years (two or three times since excavation)
and provide a habitat with low fish predation, benefiting
many aquatic invertebrates and some amphibians.
Wader scrape
A 400mm maximum depth shallow pool was formed within
the alluvium overlaying the gravel aquifer and the water
level controlled by means of a connecting pipe to the main
pond. This feature provides extensive muddy margins for
feeding waders and Teal, particularly during autumn migration.
Gravel islands and margins
Created over an area of 0.1ha, these provide nesting habitat
for Little Ringed Plover and Common Tern. The gravel was
carefully selected from a local source to ensure a good size
distribution, important for some nesting birds. Selectively
placed cobbles and boulders also provide some cover.
Mudflats and islands
These were created by excavating into the alluvium. Gentle
slopes of 1:20 minimum provide feeding and nesting habitats
for wading wildfowl, but also created a more open habitat
suitable for some marginal wetland plants.
Undulating wet grassland
Small variations in topography were engineered to create an
undulating meadow with water levels close to the surface
(between 0.1m and 0.2m above normal water level). This
marshy/tussocky Rush and Sedge dominated area was
designed to provide feeding and nesting areas for waders,
particularly Redshank and Snipe.
Reedbeds
Two linear reedbeds, totalling over 250m in length, were
excavated along the eastern edge of the site. Shallow trenches
were dug and planted with pot grown Common Reed. These
serve as a boundary to human disturbance from adjacent
footpaths and provide valuable nesting and foraging habitat
for wetland birds such as Reed Warbler and Water Rail.
Scrub
A double row of mixed shrubby Willow (incorporating some
Hawthorn, Blackthorn and Dog Rose) forms a 4m wide hedge
at the eastern edge of the site linking adjacent areas of reed,
meadow, hedges and woodland. As with the reedbed the
presence of the hedge also helps to mitigate the disturbance
of the nearby footpath, as well as sustaining a rich insect
population and providing over-wintering cover for frogs,
toads and newts. The scrubby character is to be retained by
staggered coppicing on an annual basis ensuring a permanent
screen is maintained.
Continuous post-project appraisal was carried out on this site
for the first 5 years after construction and the results showed
that this small wetland creation scheme quickly acquired an
extremely rich wildlife community.
In these 5 years over 20% (over 60 species) of all Britain's wetland
and aquatic plant species colonised the site. In the main pond
alone the plant community was one of the richest recorded in
ponds in the county. Similarly 22% (over 150 species) of Britain's
macroinvertebrate species were recorded on the site, including
12 breeding species of dragonfly.
Breeding wader densities have been very high, in one year up
to 100 pairs/km² equaling that of grazing marsh and other
important British wader habitats. In 1993 and 1994 two pairs
of Little Ringed Plover bred, representing 15% of Oxfordshire's
breeding population. Unfortunately the site was too small
to sustain such densities and the plovers have not returned
since 2000.
As a result of the commitment of the partners and the continued
appraisal of the site's development some minor and major
modifications have been funded in every second or third year
between 1992 and 2002.
These have included: managing gravel islands; scraping new mudflats;
creating new pools; doubling the size of the main reedbed;
annual coppicing and thinning of the willow scrub.
A key reason for the huge success of Pinkhill, in terms of its
pond creation, is the combination of three critical factors for
creating biologically high quality sites:
good water quality; high degree of landscape connectivity to other wetlands;
complex mosaic design.
Original Information Providers:
Richard Hellier
Ponds Conservation Trust
2013 Update
The Pinkhill Meadows project has been extremely successful in
demonstrating that high biodiversity clean water ponds can be
created. The concepts at Pinkhill have been applied extensively
in the Million Ponds Project and in UK biodiversity policy.
Four monitoring pools have been regularly surveyed since works
were completed. These have indicated that rich macrophyte,
aquatic macroinvertebrate and wetland bird assemblages have
been created, with rapid colonisation following project works.
Following colonisation, the site is now in the top 10% of pond
sites in the UK for aquatic macro invertebrates, supporting
approximately 20% of all UK wetland plant and macroinvertebrate
species.
Removal of Bulrush ( Typha latifolia ) occurred in the 5 years
following the works to prevent initial domination by this species.
Invasive species, in particular New Zealand Pygmyweed (Crassula helmsii ),
are a recent concern. Management of the
site has changed over time - sometimes left abandoned for a
time and, at others, focussed on specifically. Costs for this have
been modest with a lot of the work carried out by volunteers.
The reintroduction of grazing in 2008 has been welcomed as a
management technique to open up marginal areas and prevent
further encroachment of reeds. Maintaining grazing is identified
as a priority.