Hydrology - water in the Fen
River Itchen water comes from the chalk aquifer which supplies both its
source springs and the groundwater base flow. This results in a relatively
constant flow and temperature, with cool, clear water, rich in minerals
and dissolved oxygen, which supports a diverse and abundant ecosystem.
The Fen has two main inflow channels. A western channel, drawn from the
Lockburn/West Barton mill leat, which flows in under Prior's Barton Bridge,
at the north-west corner. This follows the western edge before crossing
the Fen between Zones 2 and 3 to join the eastern channel. The eastern
channel flows along the eastern edge of the Fen and appears to gain most
of its water from leaks in the bank of the parallel raised Lockburn, including
at the site of old sluices. It then exits the Fen via a culvert under the
Lockburn Aqueduct. Two other water sources may contribute. Firstly, an
old header or drain from the former Winchester College Water Meadows, which
previously supplied the east brook, although there is little evidence now
of any flow under Garnier road. Secondly, the flows are probably augmented
by groundwater from the west especially near Clausentum Road.
The volume of water is controlled by the adjustable weirs in the West
Barton Mill Race which maintain the water level north of Garnier Road and
therefore the flow of water over the non-adjustable concrete lintel on
the mill leat at the north west corner and into the small channel feeding
Lockburn Stream. This has a rotating weir adjustment, and is next to the
north side of the Garnier Road bridge.
The 1870 map shows historic inflows to and on the Fen, including a raised
header running all along the western edge enabling water to reach even
the southernmost zone. The 1895 map suggests this was achieved with an
additional aqueduct, sluices and hatches.
However the system worked in the past, the CFCG hopes to use the current
hydrology assets in the most efficient way to meet the conservation objectives
and support the four different habitat zones. The aim is to clear the blocked
brooks and some ditches to restore water flow and quality, to flush out
silt and to rehydrate the southern fen. It would be possible to restore
water flow to Zone 3 (open fen/ marsh) by clearing the western ditch and
any surviving water meadow drains, and renewing the pipe (crushed by a
sycamore) connecting it to the western/transverse stream. Zone 4 (aquatic
woodland) could be rewetted through pipes back-filling from the eastern
stream or by a new sluice on the lower Lockburn.
Whatever water is directed into the Fen returns to the Itchen via the
culvert under the Lockburn Aqueduct so there is no loss of water to the
Itchen by providing enough water to the Fen. The Fen sits about 1m higher
than the adjacent floodplain and the risk of flooding is limited to wetting
halfway across, approximately 1 metre below Clausentum Road level, once
per 100 years. The wide river basin and meadows provide reliable drainage
of excess river flow.
Above left - Fen inflow - Prior's Barton Bridge
Above right - Fen outflow - from under the Lockburn Aqueduct