IT, SCADA AND TELEMETRY
Restoring a
Welsh river
Trunk sewer within the Rhondda river
The Rhondda Valleys catchment is in south-east Wales, around 20 miles north-west of Cardiff. The area’s
trunk sewer network has been the subject of a detailed examination in InfoWorks CS to help identify and
propose solutions for unsatisfactory intermittent discharges (UIDs)
T
he Rhondda valleys experience substantial amounts of annual The routes walked were overlaid in the model with all of the key interest
precipitation, in the region of 2600mm – around 240 days – per areas identified, and this data was supplemented with video footage of the
year of rain. The valleys are steep-sided and the urban areas sited entire trunk sewer and photographs of all the key issues discovered.
on the valley floors are densely populated with a high proportion of hard Initially the company provided this information to Dwr Cymru Welsh
surfaces. The combination of these two factors creates very reactive Water’s operational staff, who used it to update and refine the utility’s
rivers and fast runoff into the sewer system. existing pollution strategy. This allowed the water company to target the
The sewer system is predominantly combined throughout the entire identified key polluting assets and prioritise them accordingly.
catchment. As an illustration, two of the valleys have branch connections The data provided Atkins and Dwr Cymru Welsh Water with an easily
that drain into individual trunk sewers carrying flows to the south east accessible database for the entire catchment, which has been used
before combining and then taking flows onwards 20 miles to the Cardiff recently in further work to help refine and develop the PR09 investment
wastewater treatment works. plan for the Rhondda and arrive at a strategic view of all the issues in
Atkins was commissioned by Morgan Est (on behalf of Dwr Cymru the catchment going forward. The walk-over survey itself was
Welsh Water) to investigate an ongoing pollution incident in an urbanized commended by the client and Lloyd’s Auditors as an example of best
area of the valleys. Initially it was thought that the pollution was due to practice, particularly for the health and safety measures used to
only one CSO (combined sewer overflow) within the system, but it rapidly organise and plan the survey.
became apparent that this was not the main issue within the area network. The key benefit of the survey was that it provided a much greater
Initial investigations identified that the main polluting assets in the understanding of the entire Rhondda catchment and its upstream areas,
stretch of sewer being investigated were two trunk manholes sited within which could then be fed back into the InfoWorks CS model.
the river. Then a further, unconsented CSO was discovered, which gave a
total of four polluting assets along an 800m stretch of river. Trunk sewer modeling
The Rhondda’s trunk sewer was constructed around 1920, so it is now
Initial investigations nearing the end of its design life. The sewer is predominantly of cast iron
Atkins had undertaken other UID schemes within the catchment and and around 70% of the trunk sewers – around 20km out of the entire
during site visits had found additional, previously unidentified issues in 30km length in each valley – are located in the river bed. Because the
the immediate area related to the trunk sewer in question. This led the pipe is nearing the end of its design life and has been sited in a river
company to propose undertaking a detailed walk-over survey of the along much of its length, it has been subject to river erosion. This affects
entire Rhondda catchment to proactively identify every issue that could the structural integrity of the sewer, with pier supports being washed
be found. Using the information collected during this process, Atkins away and manhole chambers being eroded.
was able to create an extremely detailed plan and a database of As a further result of immersion in the river, the trunk main is prone to
information for the entire area, which was integrated with the sewer extreme levels of infiltration and river ingress along its length. During
model to give a geospatial view. storm events it becomes hydraulically overloaded and floods, either out
20 Water & Wastewater Treatment November 2009
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