Scientists investigate manhole performance
Manholes are often defined simply as a Œmeans of entry for inspection of sewers and drains¹. Here, Brian Dumbleton reports on studies of pollutant transport through manholes and attempts to improve the efficiency of their design.
Scientists at the University of Sheffield¹s Department of Civil and
Structural Engineering are currently investigating the effects of
surcharging and transport of material in a typical average manhole aimed at
a better understanding and an improved design of this basic structure.
Using particle image velocimetry and laser induced fluorescence techniques,
the Department, in association with the Danish Hydraulic Institute, (DHI),
has elucidated the complex solute mixing process taking place in a
conventional surcharged manhole.
He added: “Existing water quality software does not fully take into account
of these effects and I am surprised that until now engineers and the water
industry in general has failed to recognise the advantages of studying how
long contaminants remain in a sewer system and the best way of Œcatching
Studies into this problem started at Sheffield four years ago and to date
have comprised in excess of 2,500 tests using a range of different discharge
pipes and manhole sizes.
“As a result we have found that a 1m diameter manhole is fifty times more
efficient for the dispersal of pollutants than a 1m length of pipe,” Dr
Using the science of computational fluid dynamics combined with software
entitled Œmouse from DHI, researchers at Sheffield claim that their work
will enable accurate predictions of when and where pollutants at a
particular location could cause a problem and with the use of on-line
sensors activate a dispersion system.
“Tank sewers and Œon-line storage tanks can only have a limited effect on
pollution incidents arising from overflows, whereas our software is designed
to assist municipalities and consultants to both design sewer systems and
reduce pollution,” said DHI senior hydraulic engineer Ole Mark.
This, it is claimed, will enable water companies and municipalities to
predict more accurately, and reduce potential pollution incidents from storm
Whilst engineers have a thorough understanding of the flow characteristics
in pipes and river systems, the effect of manholes is largely unknown,
mostly resulting from the lack of appropriate software.
According to Dr Guymer: “Evaluating alternative means of reducing the
pollutant load in sewer physical processes effecting the transport of
pollutants has already been successfully modelled. But the ability of
commercial models to accurately describe the physical characteristics of a
sewerage network related to dispersion is limited by a lack of information
to quantify the dominant mechanisms involved.” He continued: “Results to
date have indicated that the location and design of manholes has had a
significant effect on both travel time and mixing processes. Increase in
travel time resulting from a surcharged manhole results in a solute mean
velocity through the manhole which is much less than that of the upstream
Following the completion of laboratory studies and the application of a
simple numerical algorithm, the University is again planning research in
association with DHI, to utilise particle image velocimetry and fluorescence
“Storm control tanks and Œtank¹ sewers are expensive but improved flow
simulation studies in manholes will benefit wastewater treatment plants in
addition to predicting when, where and at what concentration pollutants will
arrive at a treatment works” said Ole Mark.
“Although manholes at suitable intervals are designed to facilitate
inspection rodding and jetting to keep the sewers clean, the turbulence and
mixing effects they introduce are more difficult to model than those of
pipes or channels” said Dr Guymer. Research planned for early 1999 will help
take such modelling studies further.
Meanwhile, manufacturers are continuing to challenge the traditional
approach to manhole design, which has remained largely unchanged for over 50
RMC Aggregates, for example, has recently launched the modular ŒReadyRaise.
The ReadyRaise system includes a series of concrete spacers, positioned
above a reinforced concrete base.
An additional bedding compound applied beneath the base provides an impact
absorbing base and ensures the vertical accuracy of the whole structure
eliminating undue stresses.
A collar is locked into place on the base, above which a series of spacers
dependent on the depth of the manhole are fitted.
The manhole cover is then set on a layer of bedding compound designed to
provide additional stress resistance and impact absorption.
This is designed to enable manhole covers and frames to be lowered or raised
during road reconstruction or maintenance work with minimum disruption to
the actual manhole, and is an important factor influencing its hydraulic
RMC Surfacing managing director Mike Sturgeon says: “The advantage of the
ReadyRaise system is that Œonce and for all it provides an ideal solution
for effective manhole construction, eliminating the need for return visits,
continual repair and maintenance.”
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