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St Sampson Harbour wastewater catchment improvements project

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Over the next two years Guernsey Water is investing £2 million in improving the wastewater catchment around St Sampson Harbour.

This project is the biggest in the company’s recently launched its 10-year Business Plan (2016 -2025 launched in April 2016) and is a combination of stages that will deliver effective drainage to protect the island’s environment by reducing the risk of sewer flooding and pollution.

A major part of the investment is the work to improve the performance of the pumping station at St Sampson which is located on the south side of the harbour mouth, adjacent to Longue Hougue Recycling Centre.

This is a fairly complex project which has already involved a significant amount of exploratory work to evaluate the extent of the issues and work out the best solution.

Mark Walker, Capital Delivery Manager for Guernsey Water said: 

“The project will not only deliver effective drainage in this area and protect Guernsey’s environment but it will also support the growth of St Sampson and Vale by providing additional capacity for planned housing and business development.  

“It is likely to cause a little localised disruption from time to time but we will endeavour to make this as minimal as possible.”

The reasons for this major investment

Pollution

The water quality on both the north and south sides of St Sampson Harbour is sometimes an issue after very heavy rainfall when the combined sewer overflows (CSOs) activate.  This can have an impact on the quality of the nearby bathing waters.

Flooding

The area around Summerfield Road and La Hure Mare is prone to flooding during heavy rainfall which needs to be resolved.

Maintenance

The pumping station is very deep and prone to failure making access difficult so maintenance work needs to be done to upgrade this.

Development

The proposed plans to develop Leales Yard and other sites in and around St Sampson and the Vale are likely to present an even greater demand on the existing sewer and foul water system so this needs to be addressed.

Key timelines

 

Phase one

The first phase should be completed by the end of July 2016.  This involves extending and upgrading a pumped rising main to enable the station to handle greater flows (70l/s to 150 l/s) and reduce the risk of pollution from CSOs to the north and south of the harbour entrance.  At the same time Guernsey Water has investigated whether provision of a new sewer on the north side of the harbour will divert flows away from the area at risk of flooding.

As part of this first phase a tank (5m x 3m x 2.5m) will be buried in the car park along Bulwer Avenue (opposite the Les Grandes Maisons Road junction) and connected to the gravity sewer which flows to the Belle Greve Wastewater Centre (BGWwC).   This will be underground with the exception of two vent stacks with carbon filters.  Use of the car park will be suspended for four weeks (from 11 June to 13 July) during this work but it should not have an impact on road users.

This chamber is manufactured from HDPE (high density polyethylene); a material that is very resilient to the high levels of hydrogen sulphide in the sewage from the HSS catchment, which would otherwise attack and degrade a reinforced concrete chamber that would normally be fitted in these situations. The chamber will act as a storage tank.  It has been manufactured in the UK complete with reinforcement and will be installed on Saturday 18 or Monday 20 June.

 

Phase two

Phase two will involve replacing the Vale Road sewer to increase its size and storage capacity.  Investigatory work has already taken place and the replacement work will begin this coming autumn.

Phase three

The third phase will be to upgrade the St Sampson Harbour pumping station which is scheduled to take place in the spring of 2017.   This is likely to include:

  • Replacement of electrical components which are badly corroded and at the end of their design life
  • Changes to the access arrangements to make the working area more safe and secure
  • Improvements to the pumps to enable a greater range of flows to be pumped to BGWwC 

 

Phase four

The final phase will be to build the new upgraded gravity sewer around Summerfield and La Hure Mare roads which will begin next summer and will be completed by the end of 2017. This will divert flows from a very congested part of the network and will provide additional storage within the network which will reduce the levels and likelihood of spills at the CSO’s.

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