Trowse Mill Fish Pass Improvement

Challenge

In 2015, the Environment Agency commissioned us to assess the feasibility of improving the ability of fish to migrate upstream, past a mill which defines the tidal limit of the River Yare, at Trowse Mill on the south-eastern side of Norwich.

The River Yare at this point is a low energy river with a Qmean of 2.7m3/s. The level is controlled by four tilting sluice gates at the mill which serve to maintain upstream water levels for the purposes of abstraction and ecology. It also incidentally defines the limit of tidal influence. Under high flow conditions, the gates are lowered flat to reduce upstream flood risk.

The initial brief was to confirm the feasibility of using the adjacent Trowse Newton Drain as a conduit for fish whilst reducing flood risk. This waterbody splits from the main river approximately 500m upstream of the mill and re-joins the main river approximately 500m downstream. We were subsequently commissioned to investigate the feasibility of constructing a fish passage solution beneath the mill itself.

Solution

We undertook a full appraisal of all fish passage solutions; starting from an approach of working with natural processes to deliver the most sustainable solution possible including environmental, budgetary and operational-phase resource perspectives.

This process involved data collection and consideration by our in-house specialists as well as the following specific tasks:

  • Undertaking both a hydromorphological and a fluvial geomorphological audit of the study reach that followed broad Environment Agency guidelines. This included upstream and downstream of the mill and its influence zone to understand processes outside the impoundment zone. This data was used to predict changes to channel morphology and bed sedimentology as a result of all considered fish passage options. It identified improvements to channel hydromorphology and potential instability zones linked to bed degradation and bank collapse.
  • Reviewing water level data sets from nearby client gauges to determine upstream and tidally-influenced downstream design levels.
  • Obtaining buried and over-ground services information to identify design constraints.
  • Developing a bespoke hydraulic model of the reach to enable impacts on flood risk to be appropriately and efficiently determined.
  • Specify, commission and manage ground investigation works to inform the design process.
  • Topographic survey, including entry of the culvert under the mill, classed as a confined space under the Water UK Classification & Management of Confined Space Entries guidance.

Benefits

Outline designs for a bypass channel to the south of the mill were developed by the project team. Benefits include:

  • Improved ability for fish to migrate upstream past the mill.
  • Maintenance of the extent of locally-rare pool riffle habitat and no disruption to sediment transfer processes, ensuring net hydroecological/morphological impacts are positive.
  • Both construction & operational-phase health & safety risks, and buildability issues associated with confined space entry, were designed out.
  • No negative impact for the end user on the aesthetics of the mill complex.
  • No increase in flood risk.

Want to know more?

Find out more about the work our fisheries team does in our environment section.

You can also contact Jon Whitmore for more information on this specific project.

JBA Trowse Mill fish pass     JBA Trowse Mill fish pass     JBA Trowse Mill fish pass



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