Amersham November 2020 Affinity Water

The River Misbourne was confined to a heavily shaded former mill leat, disconnected from its floodplain and lacking flow diversity. The aim of this River Restoration project was the reconnection of the River Misbourne to its former paleo channel in the centre of the floodplain and restoration of the existing river channel for a total length of 1.14 km.The chalk stream is currently failing to meet Good Ecological Status (GES) under the Water Framework Directive (WFD). Affinity Water (AW) commissioned this project, as part of a drive to reinstate chalk stream characteristics within the River Misbourne system, with a particular focus on fish, invertebrate and flow regimes.

Habitat

River and wetland

Activity

River re-alignment and restoration

Statistic

1.14km of river improved

Project requirements

The problem

This was our first project delivered under Affinity Water’s AMP 7 River Restoration Framework as Principal Contractor.  We worked alongside the designer Cbec and Affinity Water’s own specialist team to transform this section of the River Misbourne in terms of natural form and consequential recovery of a rich biodiversity.

Project objectives

  • Creation of a new re-meandered channel through a grazing field, to reintroduce natural process and re-connect the River Misbourne to the flood plain.
  • Installation of large wood at the embankment breach at the upstream extent, followed by backfilling of the redundant channel with material from excavations.
  • Reprofiling of the long section by adding additional gravels to improve flow velocities within the new re-meandered channel and existing channel.
  • Construct two cattle crossings with four water gates.
  • Stock fencing installed along both banks to prevent livestock poaching within the riparian corridor and the associated increase in fine sediment input.
  • Backwater creation using the existing redundant mill leat channel.
  • Installation of sixteen brash and ten gravel berms within existing mill leat to create a more sinuous planform and improve flow velocity and variability.
  • Construction of wetland area.
  • Construction of a 300m earth flood bund.
  • Maintain public rights of way
  • Appropriately manage flood risk locally and any knock on impacts
  • Successful stakeholder interactions
Our approach

The solution

Site-won soil from the excavation of the paleo channel was utilised to fill the old mill leat. Works started in November and wheeled dumpers were quickly swapped to low ground pressure, tracked dumpers to ensure that we remained on programme in wet conditions whilst managing H&S risks.

A phased approach to excavation of the new channel enabled us work off-line to the live flow and continue excavations in the dry, even though the River Misbourne was in flood.

A high-pressure gas main and sewage pipe crossed the new channel and required precast concrete protection. This ground strengthening and protection created the best locations for crossing with plant once the new channel was operational to allow the filling in of the former channel.

Formalised cattle crossings were required over the new channel.  Five Rivers designed and installed an innovative design with tensioned wires and oak droppers, they provided permanent demarcation that does not impact flood risk

 

Project outcomes

The results

Upon partial completion of the scheme we experienced exceptionally high rainfall and subsequent high flows, which meant we needed to return the following year to finish some elements.  This gave us the opportunity to work with Cbec to evaluate the success of features already installed and make modifications to kick start natural processes even further.

 

The finished scheme is very successful, and has transformed this straightened channel into a meandering, biodiverse, connected floodplain.

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Testimonial
Our River Restoration programme is fundamental to delivering enhancements to the internationally important chalk rivers across our supply area. The positive collaborative attitude and quality of outputs shown by FiveRivers in delivering this challenging project was excellent throughout, and I look forward to working with them in delivering further benefits for the environment.
David Watts, Programme Delivery Manager, Affinity Water

Our services

Services implemented.

Martin Whitfield - Commercial Director at Five Rivers

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