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 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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