3.1 Introduction
The work done by Balmforth & L e a ^ 7'^®'^^ at Sheffield suggested
that the Vortex with Peripheral Spill would be a good design for an overflow. The Main Drainage Department at Sheffield City Council agreed to look into building a full scale prototype and to help with a project to monitor the overflow to assess its performance.
A suitable site was found at Bacon Lane in the Attercliffe area of Sheffield, an old leaping weir overflow in a brick sewer which was performing very poorly, constantly spilling into the River Don
(Fig.3.1). The overflow consisted of a 915 x 610mm egg shaped brick
sewer going straight to the river. Below Bacon Lane there was a hole in the bottom of this allowing sewage to drop down into a similar sewer below, and off to a trunk sewer running parallel to the river. The hole, however, was often bridged by detritus, so that much of the flow, even during dry weather, crossed over and was discharged to the river. This was not an acceptable state of affairs, and it was necessary to replace the overflow with a better design.
The upstream catchment running into the Bacon Lane sewer has an area of 55 hectares, and drops 120m over its 2km length (Fig.3.2). The higher areas encompass housing estates and allotments, the lower areas light industry and wasteland. The catchment is predominantly drained with combined sewers, though there are a few separate systems. Permitted industrial discharges into the sewers are from an abbatoir, an alloy casting firm, a tooling firm and a concrete company. Occasionally tankers empty the effluent cleared from septic tanks into the sewer at the top of Bacon Lane. In addition discharges of large volumes of thick oil have been observed, the source of which is unknown.
3.2 The Vortex Overflow Prototype
It was decided to build a vortex overflow at the Bacon Lane site for several reasons. Other designs which might have been used, such as a
stilling pond or high sided weir would have required large rectangular chambers, the construction of which would have proved difficult, and may have required relaying some of the upstream pipes to reduce the steep gradient. The vortex, being more amenable to positioning in a circular shaft, proved an easier and cheaper option. In addition the site had the required drop between inlet and continuation sewer. The vortex needs a drop of about 1.8 times the equivalent diameter of the inlet sewer, to allow for a conical floor and the height of the
continuation chamber below (See Appendix A). It was possible to design
a new chamber to fit neatly at the position of the old overflow, with only small changes required to the existing pipes. The steepness of the incoming sewer should not hinder the performance.
Based on the work by Balmforth and Lea, a prototype was designed for this site (Fig.3.3). The model tests suggested a chamber diameter of 3.5D - 5.5D where D is the equivalent diameter of the incoming pipe. The smallest factor was chosen for the prototype, giving a chamber
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diameter of 2.7m. The chamber volume was recommended as 20D , from which the required weir level was deduced.
Having designed this, the continuation flow could then be set by the size of the orifice plate. Because of the uncertainty of the nature of
the development expected on the catchment Formula A was not
appropriate. Estimates for 6 DWF varied from 111 l/s to 271 l/s,
depending upon the amount of development of the catchment. Eventually,
after advice from Dr Balmforth, an orifice size of 225mm was chosen. It was thought that it would be relatively easy to replace the orifice plate with one of a different diameter to change the overflow setting
should it be necessary. Construction began in the Spring of 1987 and
was completed within 3 months.
The vortex chamber fits in the centre of a 4.6m diameter precast concrete segmented circular shaft which was sunk in Bacon Lane around the old overflow. The main components of the design are the overflow chamber itself, the continuation chamber below and the spill channel which takes sewage from the weir to the spill pipe.
In dry weather conditions the sewage flows round the 2.7m diameter overflow chamber and down the central 225mm orifice into the continuation chamber. From there it flows down another 915x610mm egg
shaped sewer which joins the Effingham Road trunk sewer 15m further
When the flow increases as a result of rainfall, the vortex chamber begins to fill. Eventually it discharges over a peripheral weir, into the spill channel. On the inside wall at the bottom of the spill channel is an opening into the continuation chamber, which can be sealed by an automatic penstock. The Effingham Road trunk sewer has a level sensing device on it, just downstream of its intersection with the outflow pipe from the overflow. Should the level in the trunk sewer be low enough to indicate spare capacity then the penstock will remain open, allowing any storm sewage which spills over the weir to return to the system and continue downstream. However, when the level in the trunk sewer rises above a certain threshhold, the penstock will close. The spill channel, which is bounded at the far end by some steps, will begin to fill. Eventually the level will reach the top of the steps and the sewage will begin to flow along the overflow pipe, another 915x610mm sewer, and down into the River Don. After the storm the level in the trunk sewer will reduce, and as it passes a second threshhold, the penstock will be opened, allowing sewage trapped in the channel by the steps to be released back into the continuation chamber.
This penstock and its control gear have suffered a number of mechanical and electrical problems resulting in it being closed during much of the project. This causes almost all the sewage spilling to go to the river. At the end of the storm, the sewage remaining trapped in the spill channel by the steps seeps under the penstock into the continuation chamber.
The manhole containing the overflow has been fitted with strip lights, which can be turned on from the cabinet on the surface, and safety railing, both of which considerably reduce the difficulties of working in the sewer.
The Bacon Lane Overflow is part of the Don Valley Intercepting Sewer Strategy which aims, amongst other things, to provide capacity for present and future flows in Sheffield and to minimise the pollution to the River Don from sewage.
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Vortex SSO Industry SSO(A) SSO(B) Catchment Boundary Residential