WATER QUALITY - Fundamentals of Hydrology - page 128

• The sewer network underneath London has not

been designed for a large modern city and
cannot cope with the strains put on it.

directly to the Thames. Bazalgette's grand sewer-
age scheme intercepted these rivers and transported
the sewage through a large pipe to east London.
This system still exists today. The actual sewerage
network is very well built and still works effec-
tively. The problem is that it is unable to
cope with the volume of waste expected to travel
through it, particularly when it rains, as it is
a combined stormwater-sewage network. The
original tributaries of the Thames, such as the
Fleet, still exist under London and any storm runoff
is channelled into them. When the volume of
stormwater and sewage is too great for the sewers
the rivers act as overspills and take the untreated
sewage directly into the Thames. This is a particu-
lar problem during summer storms when the
volume of water flowing down the Thames is low
and cannot dilute the waste effectively. To combat
this problem Thames Water Utilities (part of the
private company that treats London's sewage)
operate two boats especially designed to inject
oxygen directly into the water. These boats can
float with a body of sewage-polluted water, inject-
ing oxygen so that the dissolved oxygen level does
not reach levels that would be harmful to fish and
other aquatic creatures. To help in the tracking of
a polluted body of water there are water-quality
monitoring stations attached to bridges over the
Thames. These stations measure temperature,
dissolved oxygen concentration and electrical

At Westminster (in front of the Houses of
Parliament) the Thames is over 300 m wide; this
is confined from the width of 800 m evident
during Roman times. This great width belies a
relatively small flow of fresh water. It appears
much larger than in reality because of its use for
navigation and the tidal influence. The average
flow rate for the Thames is 53 cumecs, rising to
130 cumecs under high flows. In Table 7.1 this is
compared with rivers that flow through other
major cities. In Seoul, a similar sized capital city,
the Han River is over seven times larger than
the Thames. The effect of the small flow in the
Thames is that it does not have great flushing
power. During the summer months it may take
a body of water three months to move from west
London to the open sea. On each tide it may move
up to 14 km in total but this results in less than
a kilometre movement downstream. If this body
of water is polluted in some way then it is not
receiving much dilution or dispersion during the
long trip through London.
The second important factor is the poor state
of London's sewers. Prior to Sir Joseph Bazalgette's
sewer network of 1864 the old tributaries of
the Thames acted as sewers, taking waste water

Table 7.1 Comparison of rivers flowing through major cities

River Mean annual City on river Population in metropolitan

flow (m 3 /s) or estuary area (million)

Thames 82 London 12.0

Seine 268 Paris 9.93

Hudson 387 New York 19.3

Han 615 Seoul 10.3

Rhine 2,219 Rotterdam 1.1

Paraná/Uruguay 22,000 Buenos Aires 11.6

Source : Flow data from Global Runoff Data Centre