Infrastructure Delivery Plan (Feb 2013) - Runnymede Borough Council

Flooding and Flood Defences
Existing conditions
13.48 Fluvial flooding from the River Thames and its tributaries (the Chertsey Bourne, the
Addlestone Bourne and River Wey), are the primary sources of flooding in Runnymede.
The greatest flood risk posed to the Borough is from the River Thames, which flows
along the eastern boundary of the Borough, with the main urban areas of Egham,
Chertsey and Addlestone lying (at least in part) in areas of high flood risk. In addition,
the Basingstoke Canal and Wey Navigation discharge into the River Wey which forms
the southern boundary of the Borough.
13.49 There is a long record of flooding from rivers in the Borough, and in particular from the
River Thames. Major recorded flood events from the river occurred in 1898, 1947, 1968
and 2003, although the 2003 event was less severe than earlier events, partly due to
local drainage improvements and upstream flood control measures. A severe flood risk
warning was issued for the area in the summer of 2007, but the river did not actually
overtop its banks despite coming very close to doing so. The Environment Agency also
holds records of fluvial flooding in Runnymede for the years 1929, 1954, 1988 and
1990.
13.50 The Thames Catchment Flood Management Plan (2009) states that Runnymede is a
Borough with over 5000 properties at risk in a 1% annual probability river flood. This
makes the Borough one of the highest risk areas in the Lower Thames region. Indeed,
Runnymede has been identified as one of the top 10 local authority areas for flood risk
in England. There are no formal flood defences in the Borough, a formal defence being
classified as a structure that is specifically built for the purposes of flood defence.
Informal flood defences include structures that may act to contain flood water but were
not originally constructed for that purpose, for example the M3 and M25 motorway and
railway lines in the Borough.
13.51 There are areas of Surrey which have been broadly assessed as having “extensive”
areas in Flood Zone 3, and without significant flood infrastructure, These areas are
considered to not benefit from the minimum standard of protection. Within the Borough,
Chertsey, Byfleet and Staines upon Thames are identified as three of these areas
(Surrey Infrastructure Capacity Project Phase 1 Final Report, June 2009).
13.52 As well as fluvial flooding there are other types of flooding which put areas of the
Borough at risk, for example, ground water and surface water flooding, surcharging of
sewers and breaching of reservoirs or canals. These are the responsibility of the Local
Authority. Whilst there are reported incidences of both ground water and surface water
flooding in the Borough, there is no known history of canal or reservoir breaches in
Runnymede. The table below provides a summary of flood risks in Runnymede and
areas which have either a history of flooding or which are potentially at risk (taken from
Runnymede Strategic Flood Risk Assessment, May 2009).
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Runnymede IDP DRAFT – February 2013