Fen Management Handbook - Scottish Natural Heritage

It is likely that groundwater discharge from the sand and gravels reduces
significantly during the drier summer months of each year, although this has not
been proved by observation or measurement.
Groundwater also discharges vertically upwards into the site, probably in a diffuse
fashion that depends on the permeability and extent of the silt and clay. It is difficult
to comment on the volumetric significance of these flows with any certainty.
The figure below shows soil water levels along a transect at 90 degrees to the
stream for March and August 1991. During the spring, soil water levels were
universally between 0.3 and 0.5 mbgl, along the transects. During the summer
however, soil water levels fall by a larger amount close to the river; at distance (50
m and more) from the stream the soil water levels fell by 0.1 to 0.3 m, whereas
in the dipwells closer to the stream (0 – 50 m) the levels fell by up to 1.1 m. This
behaviour is assumed to be caused by lowered in-stream water levels resulting from
the straightening, deepening and regular clearance of the channel.
Since the soil water level regime has been established to be unfavourable for the
fen habitats within the site, options for hydrological remediation have been explored
during the development of a Water Level Management Plan for the site. The
following options have been identified:
– Do nothing: current site management practices would be continued.
– Complete restoration of the river channel: the aim would be to restore the
natural hydrological functioning of the site, with the assumption that higher
channel water levels would support soil water levels during summer periods in
the vicinity of the river channel, thus restoring a more favourable hydrological
regime to the site.
– Construct a parallel channel to maintain site soil water levels: Gilman
(1990) suggested diverting part of the riverflow from immediately below the road
bridge in the centre of the site, into a shallow, higher level channel parallel to the
main channel. The higher water levels maintained in this parallel channel would
support soil water levels in adjacent areas during dry periods.
– Level control structures in the Western Cleddau channel: water levels within
the main river channel would be raised by installation of level control structures
with the raised water levels supporting soil water levels in the adjacent fen.
53