Infrastructure - River Ems
Slipper Mill Pond forms part of the estuary of the River Ems into the sea in Chichester Harbour.
The River Ems has a catchment area (see left) of some 55 square kilometres, and its upper reaches drain the South Downs. Flows in the river are typical of chalk upland streams or "Bournes". There is little or no surface water in prolonged dry periods as the downstream springs have lowered the level of the water-table. However when the underlying aquifers are full, the rain runs off and the stream system starts to function.
Under extreme conditions flows of as much as 6 cubic metres per second have been noted at the Environment Agency gauging station at Westbourne. It has been estimated that the Ems carries a suspended load input of 1,450 tonnes/km2/yr and bedload of between 268 and 541 tonnes/yr (estimated by Rendel Geotechnics and the University of Portsmouth (1996) - for more information visit the SCOPAC website).
The Emsworth Stream system has been used since medieval times to power water mills in its lower reaches, and the complex channel system is testament to this. Features affecting the flow regime into the pond include how the sluice gates are set at Constant Springs just below Lumley Mill, and the flow controls at the Queen Street. The maintenance and settings of the structures and sluices are the responsibility of the individual owners of the land.
The notes below cover features of the lower reaches of the river affecting the pond. In practice this means the section South of the A27 Emsworth Bypass.
A27 Embankment (SU 4753 1066)
The River Ems crosses the embankment in two places - in a culvert and via a syphon. A constantly running spring found during the A27 construction has been piped and surfaces just south of the railway at SU 4750 1063.
Constant Springs & Lumley Mill (SU 4752 1064)
Groundwater levels in this area are always high.
The main river flows directly to Lumley Mill whilst the Eastern branch runs to the West of Mill Lane and then to Lumley Mill. There is a side branch/overflow which is shown on OS maps to connect to the Hermitage Brook.
The main streams combine at the sluice just below Lumley Mill and then split into two. The flow in the spillway channel to the east is controlled by 3 sluices and runs down past the cottages in Lumley Road to Peter Pond. The western channel (the Environment Agency “Main River” is controlled by weirs in the garden of Constant Springs and continues south under the railway. A minor side channel from this feeds some water features.
Constant Spring Sluice in spate - March 2001
In winter and at times of high rainfall these channels run full to capacity and it is usual for the garden of Constant Springs to be waterlogged. Constant Springs house has areas which are below normal groundwater levels and often has flooding problems.
The groundwater level is said to be never more than 0.5m below ground level even in summer. The area between the house and the A27 is regularly flooded in winter. The owner of Lumley Mill is very concerned about ground water levels as they affect the foundations of his house.
Western Branch of the Ems (SU 4750 1062)
After leaving Constant Springs the main channel turns sharply westward immediately south of the railway line. The Westernmost branch culvert joins the channel where it turns South again. This branch of the river also takes runoff from the A27.
The river proceeds southwards until SU 4750 1060 where there is a weir into a bunded area - the balancing pond in South Brook Meadow to the West of Gooseberry Cottage. This was constructed by the DoT, when the A27 was upgraded, as a balancing pond area. There is a non return valve at the southern end where it exits into Peter Pond. This bunded area is waterlogged during the winter months.
Gooseberry Cottage is lower than Peter Pond and used to flood fairly frequently - bunds have now been constructed in an attempt to prevent this.
The main channel continues southwards along the western edge of the bunded area ( where the pond for the Old Flour Mill used to be) until it passes under the A259 at SU 4751 1051. After a few metres, the channel widens and divides into two. One channel (the spillway) turns eastwards via a sluice. The other channel proceeds south towards Queen Street Mill.
Culvert joins main channel
Westward turn of main channel
Main Channel - under railway
Intake structure - looking West towards the main channel
Queen Street Mill (SU 4752 1057)
The Mill has been converted into offices. There is a turbine still in place under the mill, which is now prevented from turning and bypassed because of the noise it produces when working. It is alleged that this does not restrict flows under the mill. It has been suggested that the turbine could be removed as it is of industrial archaeological interest but the suggestion was rejected by the owner on the grounds that the channel under the mill is in poor condition.
The discharge from the mill is into the sea at Dolphin Quay and is 0.75m high and 1.75m wide. A secondary spillway runs parallel to the A259 for part of the way. A culvert 0.5m high and 1.5m wide has been constructed and it emerges at SU 47552 1057.
Eastern Branch - Cottages in Lumley Lane SU 4751 106
The gardens of these cottages are flooded under exceptional circumstances. The upper tidal limit is shown on OS maps at about the middle cottage and the whole area to the West of the stream and below the Railway embankment is shown by the Environment Agency as being liable to flooding.
This has been observed at exceptional tide and weather conditions. Problems have been experienced with obstructions in the channel when there have been large flows.
Eastern Branch - Peter Pond (SU 4752 1059)
This is hydraulically continuous with Slipper Mill Pond. Hermitage Brook enters from the East side via a non-return valve and takes the discharge from the Hermitage Housing SUDS system. Brook Cottage, flanking the brook, and the adjacent stretch of Lumley Road are the lowest lying points in the area apart from Gooseberry Cottage.
Debris grating and entrance
to Queen Street Mill
Lumley cottages and gardens
A259 Bridge and connection to Slipper Mill Pond
Hermitage Brook as seen from Lumley Road
Eastern Branch - Slipper Mill Pond (SU 4753 1056)
Tidal flap gates at the seaward end allow seawater to enter when the tide is high enough. The adjacent sluice is used to lower the pond level for bank maintenance. Levels in the pond are maintained to preserve wildlife both in the Slipper and Peter ponds.
The general bank crest level is lower than the lowest point of Lumley Road. Tidal flooding of the road and elsewhere can only occur when the bank is submerged and is independent of the management regime of the gates and sluice. The A259 between Slipper Pond and Peter Pond is also affected by tidal flooding under extreme conditions.
South Wall Sluice & Gates