Cornish dimension

Hi-tech modelling technology is improving the plans for Truro's sewerage system - and ensuring that investment is well spent.

PELL FRISCHMANN’S Catchment Management team has worked with South West Water (SWW) for 18 years. For 13 of those years, it has been improving the performance of the Truro wastewater network. In partnership with SWW and its framework contractors the company has been evaluating, designing and delivering a programme of improvements in the city and the surrounding area with a value of £5.5M.

Truro is the centre for administration, commerce and tourism for Cornwall. In addition to Truro, the catchment also contains the parishes of Feock, Playing Place, Threemilestone, Gloweth and Calenick on the outskirts. The Truro catchment drains to three rivers, the Allen, Kenwyn and Tinney, which combine to form the tidal River Truro.

The main legislation protecting the tidal river quality is the Shellfish Waters Directive, due to the oyster beds 7km downstream from the city. SWW is responsible for 131km of public sewers, which are predominately combined.

Some Truro properties have historically suffered sewage flooding and the city suffered the effects of fluvial and tidal flooding. This has led to some investment by SWW and the Environment Agency (EA). SWW is committed to protecting properties from the risk of sewer flooding as part of its AMP4 Capital Investment Programme. The EA has also constructed tide protection gates on the Truro River at Newham to protect the city from tidal surges. The Local Development Framework has identified 3,500 new homes along with significant commercial growth to meet the needs of Truro. Over the next 30 years the growth will put extra pressure on the sewerage system and it is imperative that SWW understands where investment will need to be focused to maintain compliance with directives and regulatory standards.

To this end, Pell Frischmann compiled the latest Drainage Area Study (DAS) and Drainage Area Plan (DAP) of the catchment to identify current and future investment needs.

Pell Frischmann used Wallingford Software’s Info Works CS product as a platform for modelling Truro’s sewerage system. The model has evolved to incorporate increasing detail in the catchment.

For the AMP3 National Environment Programme (NEP), the macro model of the whole catchment containing the agglomerated CSOs was verified for use as the principal design tool for improving the frequency of CSO discharges to the required standard for shellfisheries, namely the maximum agglomerated spill frequency of ten significant spills per year.

Tide level

Since 2005, Pell Frischmann has been involved in the evaluation and design of flood alleviation schemes to remove DG5 flooding within the catchment. To understand the interaction with the highways’ drainage, tide level and combined sewers the level of detail included in the model has increased significantly.

The gullies in the city centre have been surveyed and added to the model to study the surface water network’s interaction with the combined sewers and identify the sources of floodwater. To ensure that flooding solutions are robust during a 1 in 30 year return period event, appropriate return period tide levels have been calculated using the Defra joint probability method.

In the past two years, Pell Frischmann has invested in the InfoWorks CS 2D module, which in conjunction with LiDAR data has allowed overland flood routes to be represented accurately by the model. The below ground 1D and above ground 2D model has enabled Pell Frischmann to identify sources and mechanisms of flooding. The floodwater in at least one part of the city centre was found to be arriving via overland flood routes from upstream areas and without the benefit of the 2D module this source of flooding may never have been identified.

Earlier in 2009 the Pell Frischmann team, in partnership with BAM Nuttall and SWW, was awarded the Pure Award for Environment for its work to resolve the flooding surrounding Old Bridge Street Car Park in central Truro.

By applying its knowledge to the area the team developed a solution that saved SWW £900,000 over the conventional tank solution.

Carbon footprint

The CSOs in the area were reviewed with the result of relocating an existing CSO to improve the hydraulics of the outfall, without increasing the agglomerated CSO spills. The scheme also reduced the carbon footprint of the project in relation to the capital cost and in the operational costs by removing the need to pump the tank contents back into the sewerage system after each storm event.

This has also reduced health and safety risks from the solution since man entry of a tank for cleaning and pump maintenance/replacement has been removed. The most significant improvement for the Truro residents will be felt during a storm event greater than a 1 in 30-year return period. The conventional tank would have filled during such an event, however the solution provides enhanced flooding protection above a 1 in 30 year event.

As part of the PR09 process, Pell Frischmann has carried out flood risk assessments at key wastewater infrastructure assets such as sewage treatment works and sewage pumping stations in the area, and at more than 500 sites throughout Devon and Cornwall.

This work has identified where capital investment is needed to protect such assets from flooding as well as developing over 60 potential new flood remediation schemes.

The WFD will be the most substantial piece of EC water legislation to date. It requires all inland and coastal waters to reach good status by 2015.

It will do this by establishing a river basin structure, within which environmental objectives will be set, including ecological target for surface waters.

To understand the sewerage and river flooding better, it would be beneficial to construct a hydraulic and quality model of the Rivers Kenwyn and Allen to analyse the impact of the tide on river levels through the city centre during times of heavy rainfall.

This model could be amalgamated with the verified model of SWW’s public sewerage system to enable assessment of the inter-relationship between the river and sewerage system. And it could then be used to assess the performance of CSO overflows with respect to water quality in the Truro River to allow a qualitative approach to CSO discharge management.

This is becoming important to meet the requirements of the WFD and should demonstrate that SWW is meeting its sewerage management obligations. This will need to be jointly funded by the stakeholders responsible for the urban drainage assets within the city.

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