Retrofitting saves money

When Yorkshire Water needed to replace the existing screening at a Combined Storm Overflow, it turned to Hydrok UK for a low-cost solution.

With 40,000 miles of water and sewerage mains to safeguard, Yorkshire Water is undergoing a major investment programme to boost efficiency while keeping prices as low as possible for customers.

The water utility is spending, on average, £750 per household to maintain and upgrade pipes and works to reduce the risk of bursts, low water pressure, discolouration incidents, sewer flooding and odour problems.

Yorkshire Water is going through the process of upgrading and replacing CSOs across the region as part of its five-year, £1.5Bn investment programme, which ends in 2010. The company is spending £90M on the CSO project. So when the Kirkgate Combined Storm Overflow (CSO) was identified as aesthetically unsatisfactory, Yorkshire Water decided to replace the existing screening. The CSO is an existing asset located beside the River Calder and adjacent to Chancery Bridge on Pontefract Road in Wakefield.

A decision was taken to replace the existing screening with 6mm 2d equipment capable of providing:

  • Inflow at 8.8 cu m/s
  • Pass forward flow at 1.5 cu m/s
  • Spill flow at 7.3 cu m/s

Because of the position of the CSO chamber, several issues were considered in order to address the solution. These included:

  • The need to retain the spill discharge siphons
  • The potential for upstream flooding
  • The chamber is located within a restricted site adjacent to the river and main Pontefract Road

After the initial modelling of possible solutions, two alternatives were identified. One entailed an upstream extension to the chamber to house the new CSO screen, offering a solution compliant with Yorkshire Water design guidelines. However, this would cause major disruption, involving major road works within one of the main arterial roads into and out of Wakefield.

The other alternative consisted of retrofitting a screening solution within the existing chamber, and which would require a relaxation on specific design criteria.

A review of the costs for each solution indicated a chamber extension solution would cost around £800,000 to £900,000, while a retrofit would be about £425,000. Yorkshire Water agreed that the retrofit solution would be acceptable, on the basis that a physical model was used to confirm the design.

Hydraulic modelling

In late 2004, the initial proposals for a retrofit solution were tabled following discussions with Hydrok UK, which promoted a screening solution based on using six No MecMex screens mounted within a steel support structure incorporating a steel weir. Over the following months, the designers within MWH and Hydrok UK continued to develop the proposal. In early spring 2005, a final screen and weir configuration was passed to the hydraulic modelling company, HydroTec, to prepare the model and carry out the hydraulic investigations to evaluate the suitability of the solution. During this time Hydrok UK continued to offer its support and input into the model design.

After confirmation from the model that the hydraulics of the proposed design would enable the CSO to function correctly, while avoiding the potential problems of upstream flooding, the order was placed with Hydrok.

In order for Hydrok to finalise its detailed design, Hydrok UK’s Martin Tolman took video footage of the model operating as well as data acquired by HydroTec in relation to how the model had been modified to achieve the solution.

When Hydrok had completed its detailed design, ensuring the screening solution would be buildable as well as structurally integral, this information was fed back to Hydrotec. This allowed the model to be adapted, accordingly, to verify that the hydraulics of the solution had not been affected.

Final approval was achieved in mid-July, enabling Hydrok UK to proceed with its procurement and fabrication programme. The project started on site in late October, with screen installation taking place in early December. The entire project was completed during January. The projected out-turn cost was around £475,000.

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