Fen Management Handbook - Scottish Natural Heritage

is simply built on top of it. Scouring around the top corners of a dam can be avoided
by installing an overflow, such as a V notch (uncontrollable) or a pipe with an
adjustable elbow (where water height can be varied).
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7.8.2
A
Peat or impermeable material
Gravels or permeable strata
Key dams well into the substrate below (A) otherwise water will by-pass the dam (B).
B
Dam
Dams and bunds must have overflow points capable of taking peak flows so that
the soft structure of the bund or the surrounds of the dam do not become eroded.
This can be determined by trial and error but the errors can be expensive, and may
have implications for neighbouring land. Rainfall in Cumbria that flooded Carlisle in
2005 destroyed kilometres of bunds created on Wedholme Flow as part of the bog
restoration programme because the capacity of the overflows was exceeded, as
they became increasingly blocked by floating debris.
Corrugated interlocking plastic piling can be driven into peat with a maul (with
suitable protection on the top edge of the plastic). This can be used in narrow to
moderate sized drains, but a size is reached at which corrugated steel is necessary
to withstand the pressure of water without bowing. A wooden beam bolted across
the top of the plastic piling reduces bowing, or alternatively a length of steel angle
on corrugated steel. Steel piling must be driven in with suitable machinery. Timber
planks, particularly elm boards, work well on ditches with relatively high water flow.
The theory is that when wet, the timber swells, and so eliminating leaks. However, if
the water level falls, the timber shrinks and any debris which becomes lodged in the
gaps between the boards prevents re-sealing when the water level rises, allowing
seepage through the dam. Further advice on plastic piling is available in Installing
Plastic Piling Dams.
7.4.3 Peat plugs
Blocking narrow drains in peat is effectively done by creating a peat plug cut from
nearby with an excavator bucket. The weight of the bucket is used to squash the
plug into the drain and create a seal. Peat plugs are more cost effective than other
dams particularly where the peat thickness reduces to between 50 and 75cm,
often with a rock substrate below. In these circumstances rigid dam materials do
not provide a watertight seal. However peat dams can only be successfully built on
peat with a shallow slope. As the peat dam does not have a spillway, if the gradient
is too steep the water flows cause erosion over the top of the dam resulting in
failure. Suitable low ground pressure excavators are now more widely available
for this work but for small scale operations peat dams are created by hand. Hand