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