London is a recognised area of serious water stress as it not only has limited water resources but is also very vulnerable to drought. It is one of the driest capital cities in Europe, with available water resources per head of population similar to Israel. Generally speaking, the amount of water in the Thames, River Lea and the aquifer is enough to meet London’s
current demands, but after sustained periods of low rainfall, water has to be drawn from reservoirs to meet the demands. Londoners currently use more water than the national UK average, 161 litres per person per day, as opposed to 150 litres per person per day. The present balance of supply and demand in London is in deficit by approximately 180 million litres per day.
Climate change will affect the availability of water by reducing river flows, reducing groundwater recharge, increasing surface evaporation, and increasing the risk of broken water mains due to subsidence. In addition to the decrease of water availability, climate change will also affect water resource planning by changing the patterns of water demand. Domestic water use is expected to increase due to hotter summers, leading to an increase in garden watering and personal washing. According to estimates, outdoor water use will increase demand in the Thames region by approximately 50 million litres per day by 2025. Careful management is required to ensure that current water resources are sufficient to meet the present demand.
The majority of London’s water supply comes from two sources; from rivers such as the Thames and Lea, and from boreholes that are driven deep into the chalk aquifers. It is situated at the eastern edge of the London Basin Syncline, which is Britain’s most extensive chalk aquifer. Groundwater is an essential source of high quality water and accounts for approximately 40% of public water supply in the Thames region. However, although borehole water is of a better quality than the water from rivers and requires less treatment, the supply is limited and so river water is stored in reservoirs that surround London to the North and West. River water first passes into reservoirs where it is stored, before being transported into the Water Treatment Works where it is turned into domestic supply.
London’s reservoirs store on average 30 million cubic meters of water and are found to the North in the Lea Valley, and to the West of London. The reservoirs to the west of London are supplied by the tributaries of the Thames while the reservoirs in the Lea Valley are supplied by the River Lea and the New River, a 400 year old aqueduct. Although used primarily for water storage, reservoirs are also utilised in the first phases of the water treatment process.
London’s lost rivers
The expansion of London of the last 200 years has resulted in the loss of several open rivers which have been culverted underground or turned into canals. This has a large effect on the potential flood risk of the city, as these underground rivers cannot aid with the drainage of high rates and volumes of runoff following excessive precipitation or snow melt.
- The River Neckinger
- The River Walbrook
- The River Fleet
- The River Westbourne
- The River Tyburn
- The River Effra
Current statistics indicate that the River Thames is rising on average approximately 3mm per year. Due to the fact that a significant proportion of the city lies in the flood plain of the river and its tributaries, London is exposed to a high potential of flooding than any other urban area in the UK. 15% of London is in the floodplain, which includes 49 railway stations, 75 underground stations and 10 hospitals. London’s flood risk comes from five different sources – tidal, fluvial, surface, sewer and groundwater, and the city is prevented from flooding by a complex system of flood defences. It should also be noted that South East England is sinking due to ‘isostatic rebound’ from the last ice-age.
Major Flood Defence Systems
- The Barking Barrier
- Royal Docks Impounding Flap and the Gallions Reach Flood Gate
- The King George V Flood Gate
- The Thames Barrier.
London’s primary flood defence system was completed in 1983. It is made up of 10 different floodgates and is engineered to protect London from a tidal surge of up to 7m. The barrier will stop meeting its original design standards in 2030 due to rising sea levels. The Thames Barrier, upstream sea walls, and 32km of embankments downstream were designed to provide a 1 in 1000 year level of protection up until 2030 for London and surrounding areas. Between 1983 and 2001 the Thames Barrier was closed 62 times to protect London from tidal flooding. By 2100 it is estimated that the Thames Barrier will need to close about 200 times per year to protect London from tidal flooding. Unless further investments into flood management measures are continued, current flood protection systems will not be able to cope with the increased risk of flooding that are associated with climate change predictions.
The Thames Gateway
The threat of rising sea levels and increased probability of flooding holds a potential threat to the proposed Thames Gateway developments, sited to the east of the Thames Barrier. This is a substantial housing and commercial development which is on low lying land of the flood plain, downstream of the Thames Barrier which could be subject to increased flood risk. This area will hold 120,000 new homes, and create 250,000 new jobs by 2016, with 91% of these new homes being located in the floodplain.