Bath's overflowing problem
Wessex Water's Phil Brown details major CSO improvements in BathWessex Water's flagship project during this AMP3 regulatory period is to be carried out within the World Heritage City of Bath and the surrounding green belt countryside within the Cotswold Area of Outstanding Natural Beauty.
The River Avon runs along the bottom of a valley, through the city of Bath. Historically, combined drainage for the city's residents was via culverts and pipes discharging directly to the river. In the early 1900s, the then Bath Corporation engaged WH Radford, a Nottingham-based consulting engineer, to improve the drainage provision for the city. Radford's solution included the widescale provision of riverside intercepting sewers, associated local sewerage, a terminal pumping station located on the then outskirts of the city, and a WwTW located approximately 8km west of Bath, just outside the village of Saltford. The construction of riverside interceptors also included the provision of simple overflows, consisting of either simple high-level orifices or single weir outfalls to the river. These overflows, totalling over 100, relieve the sewerage system during times of heavy storm flows and reduce the risk of sewer flooding to low-lying properties in the bottom of the valley. These improvements were finished in 1914, at a cost of £253,000.
As time has progressed, the city of Bath has expanded and consequently impermeable area and personal water use have increased. With no significant improvement in the sewerage system capacity and greater volumes of flow entering the sewerage system, an increase in frequency and volume of storm sewage spill to the river is now a problem for 21st century engineers to resolve. For a number of overflows, sewage related debris is carried with the spill into the river environment and can be seen caught in riverbank flora and fauna leading to an aesthetic pollution problem. These overflows were defined under AMP2 guidelines as being unsatisfactory.
Consequently Wessex Water started its Bath Pollution Prevention project in 1995 to reduce the incidence of sewage related debris and reduce average spill frequency of unsatisfactory overflows. The project consisted of five phases at separate locations throughout the city. Proposals for phase one to four involved the provision of local, large underground storm attenuation tanks and overflow structures with improved solids separation performance.
Phase five involved local weir raising and the provision of screens at specific overflows. Phases one and five were completed between 1996-1999. However phases two and three proposals met with strong public opposition. Wessex Water became aware that if further funding was secured under the AMP3 regulation period, a catchment-wide solution could be promoted with an improved performance to that required under AMP2. Consequently phases two to four of the Bath Pollution prevention project were deferred and a new project, the Bath Combined Sewer Overflow (CSO), was included in the company's final determination for the AMP3 period.Feasibility Stage
In August 2000, Wessex Water issued a feasibility study specification to three consortia of contractors and consultants. The study required the consortia to present proposals based on providing a large-diameter storage tunnel, new terminal pumping station and pumped main to Saltford WwTW, provision of local storm attenuation tanks, refurbishment of the existing terminal pumping station and provision of a new pumped main, or an alternative option to meet the performance specification.
The proposals submitted by the consortia were assessed by Wessex Water against engineering, geotechnical, hydrogeological, environmental impacts, cost, programme, capital risk, and operational risk.
As a result of this evaluation, a Costain-led consortium was appointed as the preferred partner during the summer of 2001 to allow further proposal development for their storm attenuation tank and pumping station proposal. Development of this proposal continued until April 2002 when the board of Wessex Water approved the £28M Bath CSO project consisting of the following elements:
- relief to an existing section of riverside interceptor sewer by means of 350m of 1.5m-diameter micro-tunnel beneath the River Avon and prime development land in Bath,
- construction of a new combined pumping station at the existing pumping station site, passing forward 1,050 l/s to Saltford WwTW,
- construction of three separate, below ground storm attenuation tanks (7.5-15m-diameter and up to 15m deep) at separate locations in the west of the city,
- construction of 7.6km of 700mm internal diameter pumping main in heavily urban and rural environments to operate in conjunction with the existing pumped main,
- construction of two partially buried storm tanks (45m-diameter and up to 7m deep) at Saltford WwTW.
Due to the sensitivity of working in a world heritage setting, Wessex Water agreed to submit the project for consideration under the Town and Country Planning Act 1990. This required that a formal environmental impact assessment (EIA) be carried out, and the planning application be accompanied by an environmental statement (ES). A scoping opinion was sought from the local authority and it was agreed the EIA be targeted on the following areas:
- rivers and watercourses - proposals directly affect the River Avon and minor tributary watercourses with a significant length of the pumping main being constructed within the floodplain. Significant temporary works, in the form of haul roads and bridge crossings are required to facilitate construction of the permanent works,
- hydrogeology - Bath is renowned for its thermal springs and Roman baths. It was imperative the project proposals posed no risk to these significant tourist attractions. It was on this point the option to progress the large-diameter tunnel was taken beyond the feasibility study stage of the project,
- recreation and amenity - the city of Bath and surrounding areas are a major tourist attraction and welcomes visitors from around the world to view its Roman heritage and Georgian architecture,
- landscape and visual amenity - it was critical Wessex Water's proposals did not detract from the aesthetic beauty of the city and the surrounding countryside, some of which is located within the Cotswold Area of Outstanding Natural Beauty,
- archaeological and cultural heritage - Bath is rich in both and it was essential neither were subject to the long-term effects of the proposals,
- ecology and nature conservation - an assessment of the impacts of the temporary and permanent works on plant communities, hedgerows, badgers, bats, dormice, otters, water vole, reptiles, amphibians, kingfisher, other birds and dragonflies were carried out through field observations and liaison with interested local and national groups. The rural River Avon corridor was found to support a wide range of habitats and protected species,
- geological consideration - the project is to be carried out in the vicinity of two earth heritage sites of special scientific interest (SSSI) with mammoth fossils and gravels from the Pleistocene period having been recorded in the only known exposure of these features along the River Avon,
- agriculture and soils - an assessment was required of the short and long-term impacts on agricultural holdings and lands,
- contaminated land - the outskirts of Bath has an industrial heritage and areas close to the river are known to have been raised to reduce the impact of river flooding. The city of Bath was also subject to bombing raids during the Second World War.
- traffic impacts - traffic congestion in the city, with it relatively narrow Georgian street layout, is of considerable concern to the population. Although less than 5% of the project is carried out in public highway, construction of the works will need to be supported by deliveries and removals from the working sites,
- odour - concern expressed about the development of the sewerage system incorporating storage tanks leading to odour complaints,
- air quality - concern over the increased pollution due to the construction activity.
The EIA, including the relevant surveys and ES was started in June 2002 and completed for a planning application made in November 2002. The project was successfully granted planning consent, subject to condition by the local authority in January 2003.Current Status
The project team is currently working with Costain and its team of consultants and sub-consultants to finalise the commercial arrangement for the construction stage of the work, the construction mobilisation, and implementing early season environmental works.
Construction work is anticipated to commence this month and last for 21 months, completing in January 2005 to allow Wessex Water to meet its regulatory obligation to deliver a fully operational Bath CSO project by March 2005.