Fen Management Handbook - Scottish Natural Heritage

uilt dams can only be formed in ditches no larger than 70cm wide and 60cm
deep, whilst with an excavator drain width can extend to a maximum of 150cm with
a depth of 120cm depending on the peat composition. Above these dimensions
the peat can become unstable. The peat used for damming must be saturated, as
once the peat dried it shrinks and looses the ability to retain water and will not form
a watertight dam. Therefore, do not use the original ditch spoil and only use the
darker peat taken from the bottom and wet sides of the ditch.
7.5 Managing flowing water
In some parts of the UK the management of relatively small watercourses is an
integral part of managing fens. The New Forest is a particularly good example, and
similarly the Surrey and Dorset heaths. Here, stream beds and associated drains
were lowered as part of wider drainage schemes, for forestry for example, which
has led to bed levels eroding further upstream into more sensitive areas of fen
and mire, thus lowering the water table and drying out the fens. The techniques
described below operate on a “landscape level” to arrest and reverse these
processes.
7.5.1 Drain filling using heather bales
Heather bales have been used successfully on many sites as a robust and cost
effective method of drain filling to halt and prevent erosion cutting back into a fen
or mire system. Heather bales can be used to support the leading edge of peat at
points of headward erosion, and to halt erosion by conveying water over the bales
and on downstream.
The heather is cut locally, for example as part of heathland management, packed
into bales approximately 75 cm x 50cm x 50cm, and held in place by chestnut
stakes. The bales gradually infill with sediment and become impermeable. To avoid
subsidence and degradation, the water table needs to be fairly constant throughout
the year so that the bales remain submerged. Impermeable dams of spoil or clay
created at intervals along the drainage channel will help support the water level over
the bales. Spreading remaining spoil over the surface of the bales once installed
can accelerate the establishment of mire and soakway plants and provide some
additional support.
In the New Forest, a maximum of 12,000–14,000 bales can be produced
in a winter. The limiting factor is their durability during storage; the bales
need to be used within a year of being produced to avoid degradation.
Further information is available in the Introduction to the New Forest Wetlands Project
Drain infilling and dam
construction using heather
bales
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