15. River bed improvement upstream of Tewinbury Farm

Theme: Water Quality, Habitats and Wildlife


  • That water quality, temperature and river bed quality are such that they support chalk stream ecology

Project description

This stretch of river suffers from heavy silt deposition caused by reduced flows, dredging in the 70s, and the ponding effect of a weir. The aim is to restore the river to a silt-free, gravel bedded, shallow, fast flowing stream, with riffles and pools capable of supporting water crowfoot and breeding trout.

A partial survey has shown silt depth varies from 0m to 0.8m. This survey will be completed and in order to calculate the approximate overall quantity of gravel to be required.

Silt displacement will proceed downstream in ten 20m stages. This will ensure displaced silt is not allowed to settle over previously restored sections. Gravel will be deposited and distributed by hand, to avoid damage to the banks and bank-side wildlife.  Riffle and pool formations will be incorporated.

There will be intervals of at least two months between successive, early, upstream stages. This will allow the released silt to dissipate downstream more readily. It will also facilitate monitoring of the effects on the nearby SSSI and on downstream wildlife – in particular water voles.

To minimise the effects of silt release downstream, the work will be conducted during times of above average flow, typically winter and spring, to ensure displaced silt is transported out of the river and not allowed to settle downstream.

Surveys of fish and invertebrates will be carried out at selected points along the river at the outset and after the project.

Grid reference: TL26401404
Project lead: Friends of the Mimram
Project partners: Herts & Middlesex Wildlife Trust, Landowner
Funding source: To be confirmed
Project start: Winter 2013
Contact for more information: This email address is being protected from spambots. You need JavaScript enabled to view it.
River bed improvement upstream of Tewinbury Farm

Design by LTD Design Consultants and build by Garganey Consulting.