River Irwell Feasibility Study: Pollution and Water Framework Directive Impacts of Small Streams

Challenge

Catchment Walkover Surveys on Singleton Brook, Slack Brook and Unity Brook, which are small tributary streams within the River Irwell Catchment, in Salford Greater Manchester were undertaken by commission of The Environment Agency.

The Irwell Catchment is one of the 55% of UK rivers that fails to reach the standards required under the Water Framework Directive (WFD). The main reasons for the River Irwell not achieving good status/potential are a result of diffuse pollution from contaminated land, transport and other urban sources and point source pollution from intermittent discharges.

Irwell tributaries are also suspected to have similar issues, with numerous small, but equally polluted watercourses contributing to an overall decline in the Irwell catchment. These tributaries can provide valuable breeding, feeding and refuge habitat for fish when in good condition, and can therefore contribute to the recovery of the main river by becoming important ecological corridors. In addition to diffuse pollution, many small channels have been significantly modified. Unlike the main river, action can often be taken on tributaries through smaller scale interventions that have multiple benefits for the wider catchment.

The main objective of the project was to identify the extent and causes of diffuse pollution sources in each tributary catchment, and to assess the geomorphological condition of each watercourse, in order to identify options to improve the WFD status of the Irwell catchment.

Solution

Study methodology utilised the standardised Catchment Walkover Survey (CWS) approach developed by the Environment Agency, focusing specifically on rural point and diffuse source pollution – something we used previously this winter on a project in the Calder catchment. A two-step approach was used, comprising an initial dry-weather and subsequent wet-weather survey. Each pollution occurrence was graded from 1 – 4, depending on severity and were categorised depending on the nature of the pollution source.

During the initial dry weather survey 81 pollution sources were identified in total across all three tributaries. Frequently recorded issues included:

  • Culverts
  • Invasive non-native species such as Japanese Knotweed, Himalayan Balsam and Giant Hogweed
  • Bank erosion
  • Fly tipping.

A second wet weather survey identified additional pollution sources including road run-off, over ground run-off and field drainage inputs.

A geomorphological walkover survey was also completed to establish the baseline geomorphological conditions of each watercourse. The site walkover involved mapping key morphological conditions including:

  • Sediment sources and depositional zones
  • Bed sedimentology
  • Bank instability/erosion
  • Identifying floodplain character
  • Biotopes – flow and habitat characteristics
  • Any forms of channel modification or bank protection.

Benefit

The results of the surveys will be used to formulate options, including in-channel river restoration techniques and non-native invasive species treatment programmes to improve the existing WFD status of the Irwell tributaries. The options will be presented and discussed with stakeholders at the Irwell Catchment Partnership meeting in Spring 2018.

Our catchment and river restoration team understand that each project is different and so use a fully integrated, flexible approach to tailor the solution to both the site and stakeholders involved.

Want to know more?

Email Kimberley Jennings at JBA Consulting or email Nick Mercer at the Environment Agency for more information on this project.

Find out more about our work on our Catchment and River Restoration web page too. Other work in the Calder area includes a bank scour for Wakefield Road on the A6026 following the 2015 Boxing day floods, and over 300 T98 inspections for Calderdale Council’s Flood Risk Asset Register.



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