Project: Mottisfont Oakley Beat River Restoration

Client: The National Trust | Environment Agency

The Mottisfont Oakley Stream is a modified channel forming part of the Middle Test SSSI Unit 89. This unit was assessed as Unfavourable – no change. The assessment cited the following as reasons for unfavourable status;

  • The unit did not comply with river profile or river plan-form targets
  • The unit did not comply with either the river bank vegetation or the riparian zone targets

The restoration of the Mottisfont Oakley Beat was driven by the recommendations of the Test and Itchen Restoration Strategy undertaken by Atkins in 2012. The aim of the recommendations was to rectify bed gradient of the long-river profile by bed raising and removal of impounding features, and to improve the channels habitat features.

Mottisfont Oakley Beat River RestorationAquascience Ltd Oakley Beat River Restoration

The project was further informed by bed level surveys and water level calculations undertaken by Southampton University who used cutting edge GPS hydrological data gathering craft.

The project brief proposed the reasons for failure and to move the river reach towards favourable condition.

This project was comprised of the following components;

  • Importation of 1600 tonnes
  • Removal of a 10meter wide impounding weir. This weir was comprised of concrete bags which spanned the channel
  • Re grading of river bank 55m (on true left-hand bank) to allow for better angler access to the river
  • Reduction of a 40m gravel bar invert level
  • Placement 1600 tonnes of imported gravel to raise bed levels in accordance with the detail supplied by Southampton University

The Oakley Beat forms part of the Middle River Test SSSI Unit 89. This is a sensitive chalk stream channel and adequate safeguards need to be taken during all stages of the project to minimise the risks to the environment.

The Works

Weir removal and re-profiling adjacent river bed level - Completed by largely hand from within the channel by water safety trained Aquascience staff.

Re-grading of river bank (LHB) - Scraping by a 14t excavator initially tracked across the existing bed raising, onto the true left bank and moved downstream to the weir. Generated material will be spread out on the western downward slope adjacent to the work area.

Reduction of a large gravel bar invert - Where the gravel bar from a previous contract is too high, 350mm of the angular gravels are to be scraped back by a 14t excavator working from within the river. The generated material from this scrape will be used to extend the bar downstream.

Introduction of bed gravel - Placement of the gravels was carried out using a 30t long reach excavator moved to specific stationary positions on large bog mats, minimising disturbance to the embankment. Finishing and tidying of the new gravel bar was be completed by the 14t excavator working within the channel.

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