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.


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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

them.”

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

Guymer claims.

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

overflows.

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

inlet pipe.”

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

technology.

“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

years.

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

performance.

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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