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Fen Management Handbook - Scottish Natural Heritage

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and the surface to acidify, resulting in the spread of bog mosses (Sphagnum)<br />

and common cotton-grass (Grootjans & van Diggelen 1995). If this change<br />

is undesirable, acidification can be partly reversed by blocking the drains, but<br />

strong acidification can only be reversed with calcium- and/or bicarbonate-rich<br />

groundwater. This happened by accident at one site in Belgium where wetting a fen<br />

with groundwater from a canal reversed acidification (Boeye et al. 1995).<br />

Reversal of adverse hydrological change is not necessarily possible over any<br />

timescale, for technical, socio-economic and political reasons. Examples include<br />

large-scale flood alleviation schemes protecting extensive areas of rich agricultural<br />

land, and reduction of groundwater abstraction where this constitutes a significant<br />

proportion of the public water supply. In such cases it may be necessary to<br />

consider ‘artificial’ solutions to achieve favourable hydrological conditions, but take<br />

care when considering switching source of supply e.g. ground to surface water.<br />

If the cause is short-term natural climatic variability, there is very little that can or<br />

should be done.<br />

In some cases, environmental conditions may have changed since the cause(s) of<br />

unacceptable hydrological conditions were established. A common example is the<br />

increase in surface water and groundwater nutrient concentrations over the last<br />

30-50 years, following large-scale application of artificial fertilisers from the 1950s.<br />

This occurred after the extensive drainage of the fens to lower soil water levels<br />

and decrease flooding. Simply blocking the drains could now lead to flooding with<br />

nutrient-rich waters, which could do as much harm as good to fen vegetation which<br />

is highly sensitive to nutrient enrichment.<br />

7.1.6 Produce an action plan and implement<br />

The options for water management should be assessed rigorously against technical<br />

and economic feasibility i.e. will it work hydrologically and are the proposed solution<br />

affordable. Health and safety implications, operational implications and other risk<br />

factors also need to be assessed. Secondary criteria might include benefits to<br />

recreation, amenity and sustainability. For example, artificial irrigation of a formerly<br />

groundwater-fed fen using pumped groundwater distributed through a system of<br />

pipes might approximate the natural hydrological condition most closely, but is<br />

unlikely to be the most sustainable option in the long-term.<br />

Bunding and damming techniques (see below) have been developed on a number<br />

of cutover peat bogs, and the knowledge is transferable to relatively flat fens with<br />

at least one metre of intact peat. The same techniques may also be applicable to<br />

flat fens with silt or mineral soil, depending on detailed stratigraphy i.e. which layers<br />

hold water, restrict water movement, or act as conduits for flow.<br />

Large valley fens differ from many fens, in that they generally have a central<br />

watercourse flanked by seepage areas that may be sloping. Here, the management<br />

of the central watercourse becomes important in that the degree to which the<br />

river has cut down into its bed determines the hydraulic gradient across the<br />

fen, and hence the speed at which water is drawn off and lowered in the soil.<br />

While the general techniques of lowering land level, bunding and damming may<br />

be appropriate to valley fens, it is also important to consider the larger scale<br />

management of the watercourses on which the fens depend. The experience<br />

described here has been gained largely in the New Forest, one of the largest<br />

complexes of valley mires in the UK.<br />

Direct removal or reversal of the cause(s) will not always restore favourable<br />

hydrological conditions in the short-term: it depends in part on the scale of the<br />

problem. For example, it may take 10 years or more before the beneficial effects of<br />

reducing groundwater abstraction in some aquifers, such as Sherwood sandstone,<br />

149

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