Countess Wear Sewage Treatment Works

Climate change and urbanisation can put a great stain on the things communities take for granted, and on local environments. The Water and Waste industry is no different.

Critical infrastructure previously deemed secure can find itself at risk from flooding due to both increased run off of rain in newly urban areas, and from the impact of climate change on weather and tides.

Rising risk

In the South West of England, both factors are proving a challenge. An increase of urbanisation and the reduction of permeable surfaces that results, has increased cities’ runoff volumes. Climate Change has also effected rainfall patterns, with climatologists recording rainfall of higher intensity and shorter duration. This has made it an imperative that the region improve and protect critical infrastructure and construct more flood defences.

Against this backdrop, Pell Frischmann was appointed to carry out a preliminary investigation to determine the consequences of flooding at Countess Wear treatment works, including the impact of such an event to the local fauna and flora. The Countess Wear works are located on the River Exe estuary and are the biggest sewage treatment works in South West Water - serving a Population of around 220,000.

Environmental sensitivity

The Exe estuary is a Site of Special Scientific Interest (SSSI), Special Protection Area (SPA) and a designated Ramsar Site. It can be characterised as a mosaic of coastal habitats that supports internationally important numbers of wintering and passage water birds, including ringed plover (Charadius hiaticula) and black tailed godwit (Limosa limosa). The Estuary is also home of the largest UK flocks of avocets.

In addition, it is an area that includes populations of breeding birds and nationally important rare plants and invertebrates. As well as the recent discovery of a previously thought extinct fly!

Planning for 100 years

Pell Frischmann collaborated with the University of Exeter to prepare a hydrogeological model. The model was used to simulate the ground water response to high water levels in the event of a 1 in 100-year flood with 1 in 1 year high tide, and included an allowance for 100 years of climate change raising tides and impacting on rainfall.

The model was used to determine the extent of ground water cut off required to provide adequate protection from ground water flooding. The model was also used to assess the effect of the ground water permitted to go throw, mainly from the ground below the services.  

Using the model information and our specialist sector knowledge, Pell Frischmann evaluated options and subsequently delivered detailed civil and structural design of flood defences at the treatment works.

Outcomes

The designed solution comprised of approximately 500m metres of sheet piles, with each pile being 10.5 metres long. The sheet piles are designed to go 9 metres in the ground, to reduce ground water and overtopping. The piling works will be beginning initially with the delivery of 19 lorries worth of sheet piles.

The flood defences will provide protection for 1 in 100-year fluvial/pluvial or 1 in 200-year tidal flooding. The protection will increase the resilience of the treatment work in climate change and prevent contamination of the River Exe in such an event – in turn protecting the surrounding environment and ensuring people can continue to rely on their water and sewerage services. 

Stats

  • icon-arrow220,000 - people supported
  • icon-arrow500 - metres of sheet piles
  • icon-arrow1 in 200 - year tidal flooding
Countess Wear Outfall Uv Vel4

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