Environmental Engineering Reference
In-Depth Information
upstream into London. Downstream of the
Thames Barrier, closing the Barrier can also influ-
ence water levels: a reflective wave may be set up,
which can increase downstream water levels by
several centimetres.
the flood banks downriver had been raised follow-
ing the 1953 disaster. However, through central
London where the flood defences had not been
raised, water was reported to have lapped at the
top of the riverwalls, apparently providing a graph-
ic demonstration to Members of Parliament and
perhaps ultimately influencing the eventual deci-
sion to construct the Thames Barrier and associ-
ated tidal flood defences.
Thames tidal defences
The tidal Thames has an extensive, and in some
respects, unique network of flood defences. The
central feature of these defences is the Thames
Barrier (Fig. 23.2). The Barrier is closed to prevent
high tides flowing upstream, protecting central
London from tidal flooding. The Barrier has been
designed and is maintained to provide at least a
1000-year standard of protection until the year
2030.
Downstreamof the Thames Barrier, continuous
raised defences exist on both riverbanks. For some
distance downstream of the Thames Barrier the
crest levels of these defences are defined in statute
and also ensure a current standard of protection of
1000 years. Further downriver, towards Southend,
the standard of protection is more variable, with
only the developed areas such as Canvey Island
benefiting from such a high standard of protection.
Also downstream of the Thames Barrier there
are a number of active barriers on tributaries, for
example the Barking Barrier on the River Roding
(Fig. 23.2) and the Dartford Barrier on the River
Darent.
Flooding mechanisms
On the Thames the tidal influence extends up-
stream from Southend diminishing towards the
normal tidal limit at Teddington (Fig. 23.1). Up-
streamof the normal tidal limit flooding is usually
a result of high fluvial flows from the upstream
catchment, whereas downstream of the Thames
Barrier, flooding from the Thames would be
caused by storm surges and high tides. Between
Teddington and the Thames Barrier flooding can
be caused by a combination of tidal and fluvial
sources, although storm surges provide the condi-
tions for themost extremewater levels alongmost
of the Estuary.
The Thames Barrier has an important influence
on the hydraulics of the Thames Estuary. Up-
stream of the Thames Barrier, closing the struc-
ture soon after low tide creates a large 'reservoir'
upstream. This enables fluvial flows to be stored,
whilst preventing the high tides from flowing
Fig. 23.1
Thames Estuary location plan. (See the colour version of the figure in Colour Plate section.)
