Fen Management Handbook - Scottish Natural Heritage

7.4.1 Water-retaining structures - bunds
Bunds are earth banks which can be constructed to impound shallow surface water
on a flat fen surface. A series of ‘contour’ bunds can be used where the fen surface
is gently sloping, as in rice paddy fields.
Bunds should be constructed of suitable locally-sourced material that is cohesive,
stable, water-retaining material. Clays and other impermeable soils are generally
the most suitable. Well compacted peat will retain water to a degree, but is usually
prone to seepage, and bunds built of peat may slump over time. Any imported
material must be compatible with the target type of fen. For example, while soils
derived from sandstones may be suitable for most sites, those from limestone may
change the base status and pH of acidic peat fens.
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Materials available from construction projects are particularly likely to
have an inappropriately high nutrient status and may contain weed
plants such as Himalayan balsam and Japanese knotweed.
Bunds are usually designed with a trapezoidal cross-section to remain stable
against subsidence and possible wave erosion. They must be set well back from
the edge of drainage channels, and the base of the bund should be keyed into
the existing substrate to reduce water seepage. Shallow gradients on the side
of the bund will help facilitate access for future vegetation management. Design
specification should allow for shrinkage or height reduction after settling, which may
be >10% on peat. Potential use of bunds as access routes across fens, particularly
for vehicles, should be taken into account in design and specification.
0,75m height
Replace vegetation
second peat fill
first peat fill
key bund into existing
peat surface
PEAT
plastic membrane
2m base width
Cross-section of a typical peat bund with plastic membrane (after: Meade, Mawby &
Hammond, 2007).
maximum water level
1m deep trench
borrow pit