Thames Water Ring Main
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The Thames Water Ring Main (TWRM) - formerly known as the London Water Ring Main (LWRM) - is an approximately 80km system of mostly 2.54m (100") concrete pipelines used to transfer potable water from water treatment works (WTWs) in the Thames and River Lee catchments to distribution within London.
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[edit] Overview
The average daily flow is approximately 1000 megalitres (millions of litres), out of a total London average day demand of approximately 2600 megalitres. The system is not strictly a ring but currently comprises one major loop linking the Hampton, Walton, Ashford and Kempton WTWs clustered on the Thames upstream of Teddington Weir to central London by a southern branch via Brixton and northern branch via Kew and a separate unconnected length from Coppermills WTW to Stoke Newington. The pipeline is well below the level of most water mains, being at a depth of between 10 to 60m below ground level and approximately 10 to 30m below sealevel (as measured with respect to Ordnance Datum). It is connected to the water supply zones by some 20 shafts that extend from the ring main to ground level.
[edit] Driver
The ring main serves as a partial alternative transfer system to existing trunk mains which were suffering increasing frequencies and severities of leaks and bursts. Being some of the oldest operational pressure mains in the world (the oldest dates from 1838), the high level trunks have weakened with steady corrosion and concurrently there has been both an increased carrying requirement resulting from increasing water demand and increasing external stresses resulting from higher vehicle weights and frequencies. This has been exacerbated by limited system redundancies which has restricted preventative refurbishment. The ring main has both extended the operational life of the high level trunks by reducing the flow demands placed on them, and, by providing a high degree of redundancy, enables key trunks to be isolated and maintained.
[edit] Hydraulics
Flow through the main is not pumped but rather flows by gravity under the driving head of the service reservoirs at the source WTWs. By virtue of its depth the pipeline is under some pressure, however, the hydraulic grade line rarely exceeds ground level, and to enter supply, water must be pumped up into the distribution zones at the various pump-out shafts. In some respects, therefore, the ring main can be considered as a reservoir (albeit with negligible capacity), from which supply is drawn as required. An indication of this dynamic variation in demand is that the minimum hydraulic level does move between the Battersea and Park Lane pump-out shafts. The loop is not closed for hydraulic reasons but rather to provide the redundancy that allows any segment on the ring to be isolated and drained for maintenance without interrupting the supply to any shaft.
[edit] Construction
The ring main was constructed over two phases, the first comprising the southern leg from Ashford Common WTW to Barrow Hill Shaft and constructed from 1988 to 1991 and the second comprising the northern leg between the same locations and dating from 1991 to 1993. Each phase was in turn divided into a number of 'stages' that were separately contracted and constructed largely simultaneously. The differing contractors involved in each stage is to some degree reflected in the variation in tunnelling and tunnel construction techniques used.
[edit] Geology
The ring main lies mostly within London Clay with sections within the overlying alluvium and underlying Lambeth Group and Thanet Sand. The predominance of the London Clay lengths is by design, as being easily excavated, largely impermeable and somewhat self-supporting (for short periods), it is a near ideal tunnelling material. Where the hydraulics have required entry into the Lambeth Group and Thanet Sand, tunnelling was considerably more difficult. In particular, the Thanet Sand requires a high boring torque, is highly abrasive and, most challengingly, sufficiently permeable to contain a water table continuous with the underlying Chalk and measured at pressures up to 4 bar. An unexpected entry into the Thanet Sand while excavating near Tooting Bec Common led to the flooding of the tunnel and the temporary abandonment of a Tunnel Boring Machine (TBM). A further consideration of Thanet Sand is the presence of glauconite which oxidises on contact with air. The resulting de-oxygenated air resulted in two fatalities during the excavation of a pump-out shaft.
[edit] Future
The ring main will be extended in a series of stages over the coming decades. Work is currently underway to bridge the gap between the main loop at New River Head and the isolated segment at Stoke Newington and to extend a branch from the Brixton Shaft to Honor Oak reservoir.
[edit] Shafts
- Ashford Common - WTW
- Walton - WTW
- Hampton - WTW
- Surbiton - PS
- Hogsmill - Access
- Raynes Park - Access
- Merton - PS
- Streatham - PS
- Brixton - PS
- Battersea - PS
- Park Lane - PS
- Barrow Hill - PS
- New River Head - PS
- Stoke Newington - PS
- Coppermills - WTW
- Holland Park Avenue - PS
- Barnes - balancing storage
- Kew - PS
- Mogden - Access
- Kempton Park - Access
[edit] References
- INSTITUTION OF CIVIL ENGINEERS, 1994. Thames Water Ring Main, Supplement to Civil Engineering, Vol 102, Special Issue 2. London:ICE
