Fen Management Handbook - Scottish Natural Heritage

and the surface to acidify, resulting in the spread of bog mosses (Sphagnum)
and common cotton-grass (Grootjans & van Diggelen 1995). If this change
is undesirable, acidification can be partly reversed by blocking the drains, but
strong acidification can only be reversed with calcium- and/or bicarbonate-rich
groundwater. This happened by accident at one site in Belgium where wetting a fen
with groundwater from a canal reversed acidification (Boeye et al. 1995).
Reversal of adverse hydrological change is not necessarily possible over any
timescale, for technical, socio-economic and political reasons. Examples include
large-scale flood alleviation schemes protecting extensive areas of rich agricultural
land, and reduction of groundwater abstraction where this constitutes a significant
proportion of the public water supply. In such cases it may be necessary to
consider ‘artificial’ solutions to achieve favourable hydrological conditions, but take
care when considering switching source of supply e.g. ground to surface water.
If the cause is short-term natural climatic variability, there is very little that can or
should be done.
In some cases, environmental conditions may have changed since the cause(s) of
unacceptable hydrological conditions were established. A common example is the
increase in surface water and groundwater nutrient concentrations over the last
30-50 years, following large-scale application of artificial fertilisers from the 1950s.
This occurred after the extensive drainage of the fens to lower soil water levels
and decrease flooding. Simply blocking the drains could now lead to flooding with
nutrient-rich waters, which could do as much harm as good to fen vegetation which
is highly sensitive to nutrient enrichment.
7.1.6 Produce an action plan and implement
The options for water management should be assessed rigorously against technical
and economic feasibility i.e. will it work hydrologically and are the proposed solution
affordable. Health and safety implications, operational implications and other risk
factors also need to be assessed. Secondary criteria might include benefits to
recreation, amenity and sustainability. For example, artificial irrigation of a formerly
groundwater-fed fen using pumped groundwater distributed through a system of
pipes might approximate the natural hydrological condition most closely, but is
unlikely to be the most sustainable option in the long-term.
Bunding and damming techniques (see below) have been developed on a number
of cutover peat bogs, and the knowledge is transferable to relatively flat fens with
at least one metre of intact peat. The same techniques may also be applicable to
flat fens with silt or mineral soil, depending on detailed stratigraphy i.e. which layers
hold water, restrict water movement, or act as conduits for flow.
Large valley fens differ from many fens, in that they generally have a central
watercourse flanked by seepage areas that may be sloping. Here, the management
of the central watercourse becomes important in that the degree to which the
river has cut down into its bed determines the hydraulic gradient across the
fen, and hence the speed at which water is drawn off and lowered in the soil.
While the general techniques of lowering land level, bunding and damming may
be appropriate to valley fens, it is also important to consider the larger scale
management of the watercourses on which the fens depend. The experience
described here has been gained largely in the New Forest, one of the largest
complexes of valley mires in the UK.
Direct removal or reversal of the cause(s) will not always restore favourable
hydrological conditions in the short-term: it depends in part on the scale of the
problem. For example, it may take 10 years or more before the beneficial effects of
reducing groundwater abstraction in some aquifers, such as Sherwood sandstone,
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