Home Transit
of Venus Sewer
History in Leeds Sundials in Leeds William
Gascoigne John Feild About
X. THE END OF MUNICIPAL OWNERSHIP
In 1974 the one and a quarter century period of municipal
ownership of the city's sewerage came to an end. Simultaneously with the
implementation of Local Government Reorganisation, which created the Leeds
Metropolitan District, the Sewerage, Sewage Treatment and Water Supply
functions were handed over, with all related assets, to the newly formed
Yorkshire Water Authority. The Water Act, 1973, provided for the Leeds City
Council to maintain the sewerage system and design new works on the Water
Authority's behalf i.e. as an Agent of the YWA. This gave City Councillors some
sort of influence on sewerage policy, but the strategic priorities were set by
the YWA Board, on which Leeds Councillors had only indirect representation. In
the early 1980's the Conservative Government completely reorganised the ruling
bodies of the Regional Water Authorities and, removing all local authority
representation, thus completed the severance of sewerage from municipal
control.
With Local Government Reorganisation, Leeds City Council's
administrative boundary was significantly extended: many outlying villages,
Rural and Urban Districts were absorbed. These Districts usually had their own
sewage treatment works. There were now 28 sewage treatment works within the
Leeds District, many of which were in a poor state of repair. Partly to
overcome the problem of disrepair and partly to economise on running costs, the
new Authority started a long term process of centralising sewage treatment by
abandonning the outlying treatment works and pumping their sewage into branches
of the former Leeds City sewers, whence it could gravitate to Knostrop.
First to go as a result of this trend was the Stourton Sewage
Works of the former Rothwell UDC. In its place was built in 1977/78 an
automatically controlled sewage pumping station.
Much of the major sewer construction in Leeds since 1974 has been
located in the areas which prior to Reorganisation were independent of the
City: Horsforth UD, Otley UD, Wetherby RD, Garforth UD, Pudsey UD, Rothwell UD
and Morley MB. This reflected the facts that most of the visible sewer problems
in the former Leeds CB had been solved and many of the outlying areas had
piecemeal, inadequate or decaying sewer systems.
New techniques of inspection however began to uncover fresh
problems, before they became visible on the ground or from manholes. The City's
engineers embarked on a vast programme of closed circuit TV inspection of sewer
conditions. Miniature TV cameras, travelling along even the smallest sewers,
could highlight potential trouble before it progressed too far: partial
sewer collapses, damage due to tree roots, leakage and excessive silting could
be dealt with at an early stage.
Developments like this have steadily led to a marked change of
emphasis in the work of drainage engineers throughout the country. 'Sewer
Rehabilitation' - or renovation and repair - is now a cornerstone of
sewerage policy.
New advances in hydraulic theory have also played their part in
the new approach. These have resulted in a much better appreciation of the way
our sewers behave under extreme flow conditions. Engineers have used computer
models for many years for the assessment of the worst flows likely to be faced
by sewerage networks. Until recently however, these models were not
sophisticated enough to take account of the effects of 'surcharging' in sewers.
Sewer pipes were consequently designed with a specified probability of flowing
'just-full'.
For example, the YWA had a policy of designing trunk sewers so
that they would probably flow 'just-full' approximately once every two years.
No-one was able to say what margin of safety against flooding was implied by
this criterion. Obviously, on the rarer occasions flows would be greater still
and would surcharge the sewers so much that the sewage would build up in the
manholes and escape over-ground. But how rare an event would this be?
In the mid-1980's, this problem had been overcome. Computer models
are now being used in Leeds, as elsewhere, which can accurately predict the way
in which water levels build up in manholes. The YWA was able to jettison the
outmoded 'once in two year pipe full' criterion as the sole design objective.
The new requirement became that sewers should be designed so that water levels
are only likely to rise up manholes to ground level with a frequency of once in
50 years.
As a further result of technical progress, flows predicted by
computers can be directly checked using solid-state electronic flow monitors.
These monitors use advanced techniques to measure the flow rate in the sewers
where they are installed. A reading is taken every 2 minutes or so and recorded
on memory chips in an associated data logger. By these means, engineers can
obtain a complete picture of the pattern of flows arising from any given
rainstorm. The minute-by-minute progress of rainstorms too is being measured similarly
with equipment above ground.
In Leeds these new techniques have been deployed most effectively
in examining the effect of coal mining subsidence on sewers in Rothwell and,
more recently, in developing the Leeds/Aire Treatment Strategy.
The Leeds/Aire Treatment Strategy really marked the culmination of
the Yorkshire Water Authority's attempts to centralise sewage treatment for
Leeds. Under this plan virtually all of the treatment works, except the central
one at Knostrop, would be demolished. The sewage flows would either be pumped
or discharge under gravity to Knostrop.