Report No FR0204



June 1991



  1. To assess the effectiveness of a sewerage rehabilitation scheme, designed with the aid of UPM interim water quality planning procedures, in achieving the designated river quality performance target.
  2. To monitor and evaluate the performance of the interim water quality planning procedures.

These objectives were to be achieved by the intensive monitoring of a sewer and river system, before and after the implementation of a sewerage rehabilitation scheme involving the construction of a CSO with on-line storage.


The traditional approach to the design of CSO has been based upon sewerage orientated hydraulic criteria alone. This approach takes no account of the impact on the receiving environment. A new approach, developed under the UPM programme, aims to provide tools to assist engineers and water quality planners to design CSOs to take account of the environmental impact of spills and enable river quality objectives to be met. An interim water quality planning procedure has been devised which incorporates the use of SRM-Il, TSR and WASSP together with a river impact procedure called CARP. The effectiveness of this approach needs to be investigated by evaluating the performance of a sewerage rehabilitation scheme which involved their application at the design stage.


The results of the field monitoring of the CSO and river showed that:

  1. There was no deterioration in river quality downstream of the cso. The river quality improved upstream. Long term river biology is severely impacted by frequent pollution incidents originating upstream, which may mask the effects of the CSO. Proposed intermittent pollution criteria were only exceeded for two storm events.
  2. Observed mean storm sewage concentrations were considerably less than those derived from standard SRM-Il concentrations tables.
  3. The WASSP model for the new scheme overpredicted spill volume and frequency when compared with monitored data.
  4. The effects of the SRM-II and WASSP overpredictions were reflected in the performance of CARP. However, the CARP based scheme has been successful in terms of river impact performance.


  1. Further lower intensity monitoring should be carried out at Littleborough to assess long term river quality changes and tank performance in response to temporal rainfall variability. The "verified" WASSP model should be reassessed.
  2. CARP should continue to be used with improved SRM-Il procedures where large capital investment is involved, until the more robust modelling techniques (MOSQITO, MIKE 11) have been evaluated.
  3. The reliability of WASSP CSO spill volume predictions should be investigated and improved verification procedures identified.
  4. Post-construction performance evaluation and monitoring should be carried out at more sites.


Section 1 outlines the background to the development of the UPM procedures and the sewerage rehabilitation design at Littleborough. The field site and data collection programme are discussed in Section 2. Data collation and analysis are considered in Section 3. Results, conclusions and recommendations are presented in Sections 4, 5 and 6 respectively. The Appendices contain a summary of the data collection and analysis.

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