Integrated Catchment Quality Modelling Project

The integrated catchment modelling studies were designed to include the main river systems of the Calder, Mersey, Micker Brook, Douglas, Irwell, Darwen, Weaver, Irk, Medlock, and Glaze river catchments. The study area covered about 3,900 km² and included 1,500 km of watercourses. The aim of the study was to assess the compliance of the river systems within these catchments against water quality standards (i.e. UPM standards and WFD chemical standards) and to identify, with supporting information on existing river condition, potential locations where intermittent discharge may be contributing to failing watercourses.


We provided sewer modelling services to simulate urban discharges to the river systems within the catchment. This involved updating and upgrading over 100 discrete urban drainage network models comprising;

  • 1,781 Combined Sewer Overflow outfalls,
  • 97 Wastewater Treatment Works final effluent outfalls,
  • 2,645 Surface Water outfalls.

We developed a process, in conjunction with the client UU and the client’s framework consultant MWH, for upgrading the models from just hydraulic to water quality standard.  This involved, for each model;

  • Disconnection of CSO spill pipes from SW networks to allow    clearer identification of high pollutant loading sources,
  • Conversion of all fixed pumps to screw pumps to avoid state change instabilities in the water quality simulation parameters,
  • Addition of tank drain down mechanisms where these were missing to allow continuous simulation (rather than discrete design storms),
  • Removal of all ‘dry’ pipes as these too invoke instability in the water quality calculations,
  • Addition of population and trade flow water quality parameters and calibration to historic WwTW values,


We ran continuous simulations for 10 years of historic rainfall for each sewer model and collated the spill results for each of the 4,523 outfalls.  Just the logistics of running such a large number of models for such a long length of time was challenging.  There were also model stability issues to either resolve or accommodate in the results preparation.

In a follow on stage we developed outline solutions at key assets.  A large part of this work was to identify opportunities for strategic scale sustainable drainage systems (SUDS).