Mousehole essential sewer upgrades

We have been working very closely with the Harbour Office, Environment Agency and Cornwall Council over the last several months to understand and investigate reported defects on our sewers in Mousehole Harbour.

As part of our planned survey work in the area we successfully undertook a dye test on Thursday 28 May 2020. The dye test confirmed that there are some defects on our sewer pipe, to the south end of the harbour.

In light of this, we developed a plan to replace the defected section of sewer and have applied for the necessary licence from the relevant Marine Management Organisation [MMO] as we are working in the foreshore to undertake this work.  We are currently awaiting feedback from the MMO regarding the licence application and are hoping to receive the necessary licence by the end of this month (November 2020). We will update the details of our planned work in the Programme of works below.

Update - Friday 19 February 2021

Our contractors have now replaced the defective 300mm and an additional 20 metres of 225mm pipe. This week we have carried out further dye tests which have confirmed that there are no leaks beyond the repaired sections. Next week will be removing the pumps from the wet well, changing a manhole cover in the carpark and starting to demobilise from site.

We have also been able to assist the school with a new water connection for the Community and School garden. We are proud to help fund this worthwhile community project and it is hoped the supply will be installed as soon as the ground conditions permit.

Programme of work


Date Activity
Monday 3 August 2020

Undertaking a non-intrusive survey to help facilitate permits required to undertake works in the harbour.

We will also be conducting further planning works to facilitate the repair the of the tidal flap valve on the storm overflow outfall from the pumping station. This outfall does not go into the harbour.

Monday 17 August 2020 - For up to two weeks

Work to replace the manhole covers will begin.

During this time we will also be conducting trial holes on the beach around some of our assets.

Tuesday 1 September 2020 - for up to three days Undertaking cleaning and further investigations on our outfall pipe.
w/c Monday 30 November 2020

Installing a temporary pump in the sewage pumping station which will be commissioned W/C Monday 14 December.

Damaged outfall flap valve to be removed and a new inline non-return valve to be installed in the chamber.
w/c Monday 7 December 2020 – for approximately two weeks

Installing a new vent pipe from the manhole chamber with the large concrete surround in the harbour, across the beach and up the slipway to the next manhole chamber.

This will prevent airlocks within the network during periods of high flow and allow the online storage tank to operate more efficiently.

Please note: there will be limited access on the slipway during these works which will be carried out at periods of low water.

Monday 4 January 2021

Sewer replacement work to begin. Please see Q&A question ‘How will this be done?’ below for more information.

w/c Monday 15 February 2021

Our contractors have replaced the defective 300mm and an additional 20 metres of 225mm pipe. This week we have carried out further dye tests which have confirmed that there are no leaks beyond the repaired sections.

w/c Monday 22 February 2021

We will be removing the pumps from the wet well, changing a manhole cover in the car park and starting to demobilise from site.


Last updated on: 14/12/2020

Q: What investigations have taken place so far?

A: We have undertaken CCTV surveys of our gravity sewer, combined sewer overflow (CSO) and visual inspection of our pump station. We have also completed a dye test on the gravity sewer within the harbour.

Q: What have you found?

A: Our investigations have shown that there are defects on our gravity sewer which will require further work.

Q: What further work will take place, and when?

A: We are looking to replace the defected section of sewer and having been granted the necessary licence from the Marine Management Organisation (MMO) in November 2020, we are looking to undertake this work in January 2021.

We have been liaising with the Environment Agency throughout the works and the planned approach going forwards.

We will update the details of any planned work in the Programme of works above.

Q: What will your working hours be?

A: As the work to replace the defected sewer will be tidal and weather dependant, there may be the need for some work to be undertaken outside of normal working hours and on weekends. However, we will always look to complete any work with the minimal possible disruption.

Q: How will this be done?

A: We will undertake the work using a construction method called an online sewer replacement. This means blocking the existing sewer pipe at strategic manhole chambers and temporarily overpumping the defective section of sewer, which will require some modifications to the existing manhole chambers. This will allow the defective pipe to be broken out and replaced with a new plastic pipe surrounded by concrete for protection.

Q: Will the work affect my wastewater services?

A: There will be no impact on customers’ wastewater services throughout the works.

Q: Will the work be noisy?

A: In order to divert sewer flows so the defected pipe can be repaired, we will be required to put in place overpumping 24/7 during the works. Unfortunately, this is essential to ensure there is no affect on customers’ wastewater services.  Although some of the equipment needed to complete this work may be noisy we will be using specific noise-reducing equipment to ensure disturbance is kept to a minimum.

Q: Is it safe to swim in the Harbour?

A: Mousehole Harbour is a working harbour and not a designated bathing water. The water quality is not routinely tested or assessed and the water quality of the stream(s), the surface water drain(s) discharging into the harbour and the harbour itself can vary widely.

Water quality can be poor, particularly after wet weather, and can contain harmful bacteria and other pollutants, especially after rainfall.

Avoid splashing the stream and harbour water in your mouth and wash your hand with clean water before eating or drinking.

The Harbour Commissioners will be placing public advisory signage around the harbour to ensure users of the harbour are aware of the above.

Q: Where can I find more information?

A: Regular updates and further information will be available on this page and you will be able to see at the top of the Q&A when it was last updated. If you have any further questions or concerns, you can contact our Services Helpline on 0344 346 2020.