Case study date
2013
Size
90.00 ha
Location
Near Kings Lynn, Norfolk
Operator
Middleton Aggregates Ltd
Mineral Type
Sand and gravel
Habitat(s) Created
Reedbed, wet woodland, grazing marsh, standing open waters
Restoration / Priority Habitats
Reedbed
Wet Woodland
Partnership Working
Middleton Aggregates Ltd
Ministry of Defence
Key Issues
Planning Contraints
Flight Safeguarding
Lack of suitable infill
Public Benefits
Flood Alleviation
Background
Pentney Quarry is located in the Nar Valley, approximately 10km south east of Kings Lynn. The entire quarry covers over 90 ha of land adjacent to the River Nar. The river is over a metre above the surrounding land and is contained within levees. Although the site is recorded as being in the floodplain of the River Nar, canalisation for the Nar and flood overspill capabilities on the south side of the river have significantly reduced the risk of flooding to the quarry site. The land is predominantly peat and IDB drainage has resulted in both land fit for arable production and also land shrinkage through wind erosion.
Planning
Pentney Quarry was originally operated by Wimpey in the 1970s, with Middleton Aggregates gaining an adjacent permission in the 1980s. Initially, neither site had a restoration plan, with restoration to be agreed after extraction was completed. A consolidated planning permission for the entire site was implemented in 1996 after Middleton Aggregates took the entire site over. Most of the former workings have been restored to a series of small and large lakes surrounded by trees, and several of these now serve as active fishing lakes.
Pentney Quarry is located within a flight safeguard zone. After the loss of an RAF Tornado that hit a gull over the site, the Ministry of Defence issued a Notice To Airmen to avoid the area whenever possible. When the planning application for the extension was in preparation, the problem of bird strike forced a change of restoration from the open water and fishing pits that had gone before, to a different habitat that does not attract or support large numbers of wildfowl. Reedbed was the preferred option as, once mature, there is very little open water and the reed blocks access to the banks, making it unattractive to geese.
Prior to any excavations, a borehole archaeological survey was undertaken, whereby small amounts of material from the top and bottom of the peat layer was removed for radiometric dating, with the results being made available to Norfolk Landscape Archaeology.
Soil Handling
A soil survey was undertaken to determine agricultural land classification and the soil resources.
Considering inherent drainage limitations and wind blown erosion of the site, it is classified as Grade 3a. The restoration plan will result in the loss of this land to reedbed, although there is potential if needed in the future for drainage and conversion back to agriculture, as the soils will remain in situ. The restoration will also at the same time safeguard the soils from further drainage and wind blown erosion.
| Average thickness of mineral | = 5.07m |
| Average thickness of overburden | = 1.70m |
| Overburden volume | = 163,200m3 |
| Overburden from unworked area on southern margin (1.5m O/B x 2.44ha) | = 36,600m3 |
| Overburden brought from adjoining land to the east (2.0m O/B x 1.31ha) | = 5.07m |
| Average thickness of mineral | = 26,200m3 |
| Total overburden volume | = 226,000m3 |
The peat overburden is sub-divided into two main layers: a black amorphous peat top layer averaging 50cm in depth, and below that a set of dark reddish brown semi-fibrous layers that will be handled as one layer. The two layers will be stripped separately. The semi-fibrous lower layer will be placed in the void first, and the black amorphous peat will be replaced as the final layer.
The site is sub-divided into three phases. Prior to extraction, soils from the first will stripped and placed into temporary storage bunds, no more than 3m higher than surrounding ground level. Gravel will then be extracted, and soil will be replaced in the void as soon as practical. Stripping of soils will then progress towards the centre of the site where the gravel is thickest. At this point, stripped soils will be returned to the void immediately after stripping, reducing the need for the storage of soils above ground.
All soil striping will be by the back-actor and dumper method which has been proven as the least damaging to soils, and can even restore soil structure in certain cases.
Habitat design and Creation
Once the gravel has been extracted, there will be on average a 5.4m void. There is a lack of suitable infill to raise the entire bed level to within reach of a reedbed restoration. However, this problem has been solved by over-deepening the clay substrate underneath the gravel by approximately 2m. It will then be possible to create two retaining clay bunds within the void. The peat overburden is then placed behind the bunds.
The over-deepening of the open water will give a total water depth of approximately 8.4m in the central area. This will result in a sterile open water area that should not appeal to feeding diving ducks.
The 5.9 ha reedbed will be created on the backfilled peat either side of this deep open water void. The peat will have a ridge and furrow topography engineered into its surface. The ridges will be planted with reed seedlings, and water depths will be controlled for the first 2-3 season to allow the spread of the reed into the furrows. Reed will readily grow in water up to 1.5m in depth, so these furrows will be easily colonised. This will have the effect of creating areas of alternating wet and dry reed. The reed will eventually have a spring depth of 0.25m, which is ideally slightly too shallow for nesting bitterns, which prefer depths up to 0.5m in the spring. There will be approximately 30 small open areas (ponds) within the reedbed, but these are likely to reed up within a few years without management. The reedbed will be allowed to grade into the surrounding reed fen and wet grassland.
The restoration plan for extension at Pentney Quarry. Note that the reedbed will be wider on the south side due to the underlying gravel depth.
Calculations for Habitat Creation
|
Average depth of water below ground level (BGL)
|
1.53m (so on average the water level is 0.17m above the top level of the sand and gravel strata)
|
|
Average void depth formed by extraction is 3.74m (S&G) plus 1.7m (overburden)
|
5.44m
|
|
Minus the average depth of water BGL (1.53m)
|
3.91m
|
|
Minus assumed depth of water required for reedbeds
|
0.25m
|
|
Therefore net depth requiring backfilling
|
3.66m
|
|
Volume required for backfilling
|
215,940m3
|
|
Available overburden
|
226,000m3
|
|
Material surplus
|
10,060m3
|
Final Landuse Areas
|
Open water
|
3.7ha
|
|
Reedbed
|
5.9ha
|
|
Wet woodland / Carr scrub
|
3.75ha
|
|
Wet grassland
|
0.37ha
|
The Wider Landscape
Pentney quarry forms an island of wetland in an intensively agricultural area. There are limited opportunities at the moment to link in with other reedbeds, but future wetland creation may be possible in the Nar Valley and in the Fens. It is approximately 20km from the new reedbeds at RSPB Lakenheath Nature Reserve.
Public Benefits
The wider Pentney Quarry complex is used as fishing lakes by a local club. At the moment, the only other access is granted to the Narborough Valley Ornithological Society. There are no plans to increase public access to this site. The site does, however, provide an ecosystem service by providing flood alleviation of the River Nar.
Acknowledgement
The help and support of Middleton Aggregates Ltd is gratefully acknowledged.