Fen Management Handbook - Scottish Natural Heritage

Case Study 3.3
Hydro(geo)logical impact assessment and
recommendations for remediation for a valley-head
mire – Cleddon Bog, Monmouthshire
Cleddon Bog SSSI is a valley-head mire approximately 15ha in extent, located
around 9 km south of Monmouth, in south-east Wales. Lowland mire habitat forms
the primary interest feature, and the surviving open mire habitat is now confined to
approximately one-third of its original presumed area and comprises a range of bog
and poor fen plant communities, mostly dominated by purple moor-grass Molinia
caerulea.
There is a comprehensive body of evidence which shows that the condition of the
mire feature at the site is deteriorating. Over the past few decades an expansion
of purple moor-grass and scrub at the expense of key mire species was noted.
A project was commissioned by CCW to identify the causes of the deteriorating
condition, and to suggest actions for remediation.
A hydrological conceptual understanding of the site was developed through a desk
study and a hydrological feature and hydrochemical survey of the site. A long-term
(1972–2004) hard-copy water level record for six shallow dipwells within the site
was digitised and analysed (see figure below).
The following influences on site wetness were identified and assessed in
detail during the study:
Quantity of water entering the site. Detailed calculations, using
historical rainfall and evapotranspiration records, were carried out to
model the fate of rainfall within the catchment to Cleddon Bog, and
thus to assess the hydrological impacts of changes in catchment
vegetation cover. The calculations showed that if conifer plantation or
broadleaf woodland was replaced by open heathland, hydrologically
effective rainfall (surface water runoff and groundwater flows) would
be increased by 77% or 55% respectively. The recent clear-felling of
the Broad Meend hillside, which overlooks and supplies water to the
most important area of open mire habitat within the site, was estimated
to have increased its contribution of hydrologically effective rainfall by
around 77%.
Central area of Cleddon
Bog, purple moor-grass
dominated groundwater
seepage slope in the
foreground, running into
topogenous mire in the
middle-ground
Diversion of surface water
inputs to site by a long
peripheral track
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