Study

Restoration of Denny Bog Valley Mire in the New Forest, Hampshire, England

Summary

The 90 or so valley mires in the New Forest in southern England are one of the most important series of lowland valley mires in Western Europe. However, many have been damaged by artificial drainage. Since 1997, Forest Enterprise has undertaken a programme of mire restoration within the New Forest EU-LIFE partnership.

Site: In 1998, Denny Bog was selected for restoration. The bog was partly drained in 1969/70 (about 25% of the mire damaged). Away from the main drainage area the mire remained intact but was slowy degrading due to peat-slumping and peat erosion (40 m from the end of the drain lost) which threatened the remaining mire.

Phase 1: In September 1998, steps were taken to halt headward erosion along a length of eroded drainage channel (c. 45 m long x 1-2 m wide x 0.75-1.5 m deep). Four hundred heather Calluna vulgaris bales (each c. 75 cm x 50 cm x 50 cm held together with synthetic baler twine) were packed into 35 m of the channel, to the height of the surrounding surface. At approximately 5 m intervals a further line of bales was secured by driving 1.5 m-2 m long x 3 cm diameter oak Quercus stakes into the substrate. Finally a dam, supported by chestnut Castanea stakes was installed at the drain end to hold the bales in place.

Phase 2: The bog is crossed by a bridge/causeway and in spring 1999, for safety reasons, had to be replaced. To aid bog restoration, the bridge was constructed as a partial dam, boarded with wooden planks and infilled with gravel. Where channels had previously existed, 30 cm and 45 cm culverts were installed and covered with gravel. The bridge still allowed water through three main channels, and during raised water levels also through seepage culverts.

Phase 3: Phase 3 sought to block about 130 m of collapsed drain with slumped peat, and deeply incised stretches of drainage channel. A 4 m-long spillway was constructed using heather bales laid two to three deep. To provide extra stability, a row of chestnut stakes, 60 cm apart was driven into the substrate. The remaining area of slumped peat was infilled with bales for a further 8 m and another row of stakes driven in. The remaining 80 m drain was infilled and, a mire pool (c.20 m long x 5 m wide) was created to provide habitat for Odonata and aquatic plants. Infilling involved packing heather-bale plugs (approx. 2.5 m long) at about 10 m intervals just below bank height. Trees and scrub that had grown on the bank spoil were cut down and dumped in the channel. Before infilling, any vegetation growing in the drain was removed and later used to cover heather bales. The end point of restoration was finished with heather bales held in place by chestnut stakes and a 3 m-long stepped spillway apron. Spoil was spread and levelled in the drain and capped with heather turfs. Excess material (including vegetated sods) was dumped or levelled as a surface dressing on bales below the bridge.

Costs: The estimated area restored was 19 ha at a cost of £10,420 (about £548/ha). The cost breakdown is outlined below:

Hymac low ground-pressure dumper and supervision = £5,140
Labour and Forest Enterprise machinery (inc. two 'New Deal' subsidised staff for some of work) = £3,600 (add approx. £2,000 if subsidised staff not available).
Heather bales (£1.78 to £2.00 each) and wooden stakes = £1,680

Two years after completion it is too early to say if the restoration has been completely successful but early indications were positive. Installation of heather bales into the drain resulted in a dramatic reduction of water flow, a reduction in peat erosion and trapping of peaty sediment in the water. The bales installed in September 1998 were initially on the upper surface, very dry. Since water-levels have been raised and the bales become fully saturated, vegetation colonisation has progressed. Bog pondweed Potamogeton polygonifolius has been the most rapid coloniser of very wet areas, with sharp-flowered rush Juncus acutiformis, purple moor-grass Mollinia caerulea and marsh St John's-wort Hypericum elodes (all plants typical of mire plant communities).

Another good outcome has been the capacity of the bales to withstand high-waterflows without breaking up. They also provide a solid surface, which has been important in gaining the support of the Forest's commoners who were concerned that livestock, particularly ponies, would get trapped in restoration areas.

Conclusion: Given the results to date it is considered that the methods employed provide a cost-effective, sustainable and long-term solution to restoring New Forest valley mires.


Note: If using or referring to this published study, please quote the original paper.

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