Hydrology - water in the Fen

The hydrology of the Fen is complex and not completely understood. 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. There is a north-western channel, known as St Michael’s Brook, drawn from the Mill Stream which flows south from Winchester. It flows under the Garnier Road bridge, then a short distance through Zone 1 before it enters the garden of Prior’s Barton House. It then emerges into Zone 2 and follows the western edge before crossing the Fen between Zones 2 and 3 to join the eastern channel.

The eastern channel, known as the Floodstoc, 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. It then exits the Fen via a culvert under the Lockburn. The flows are probably augmented by groundwater from the west especially near Clausentum Road. 

The volume of water is controlled by the adjustable weir in the Mill Stream which maintains the water level north of Garnier Road and therefore the flow of water over the non-adjustable concrete lintel and into St Michael’s brook.

The 1870 map shows historic inflows to and on the Fen when it was managed as a water meadow, 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. Remnants of these are still visible.

The excavations on the Fen in 2017 created a pond in Zone 4 and exposed part of the old course of St Michael’s Brook which lies parallel to Clausentum Road. This has been left as a ditch and together with the pond is always wet due to groundwater flow. The work also lowered the soil level and created a series of pools which connect with the Floodstoc near the aqueduct. The connecting channel ensures that the water levels in these areas are level, and was successful in maintaining a healthy wetland area during the last hot dry summer. There is also water flow into the NE corner through a pipe installed in October 2018, as well as water flow to and fro across the peaty banks which maintain equilibrium with the ground water table.

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