River Test at Bossington

River Test at Bossington Estate

In collaboration with the Environment Agency (Romsey), Bossington Estate continues an ambitious programme of habitat and chalk stream restoration on the River Test, with this, its second major project focusing on reach-scale issues along the 400 metre double bank length of ‘Old Stews’ beat.

Old Stews 14 July 2011 - 179

These ‘before’ images showing the middle section of ‘Old Stews’ were taken on 14th July 2011 at the height of the trout fishing season when the river should have been at its most productive in terms of weed growth and fly life. 

14 July'11 - Diatoms on Ranunculus 176

The wholesale dying of Ranunculus weed beds coupled with prolific diatomous blooms, blanket sedimentation and laminar flows were some of the key issues we were asked to resolve.

Project Brief:

Our brief  was to establish the causes and symptoms of habitat degradation via an in-depth channel & floodplain survey and provide a detailed design and construction plan to address the key issues. Old Stews is one of the most sought after and famous trout fishing beats on the middle Test so expectations both from the owners and the rods were high as we embarked on major surgery to overcome the ravages of historic channel dredging.

It was visually obvious from the outset, that ‘Old Stews’ had been the subject of land drainage ‘improvements’ as witnessed by classic symptoms of slow flows, heavy siltation and dying beds of Water Crowfoot. Channel Engineering works are believed to have taken place during WWII and again in the 1950’s for land drainage improvements. (Interestingly the estate discovered in its archives a post-war black and white cine film of drag-line dredging on the estates water which we are hoping to make available on this site). We found the watercourse to be straightened, over-wide and over deep with laminar flows and blanket sedimentation throughout. In the middle and lower reaches especially, the ‘cliffing-faced’  fishing bank was perched  on average 1 metre above summer water level and at its deepest we probed through the soft sediment to find the hard river bed between 12′ and 15′ (3.5 to 4.5 metres) below summer water level. This was a portrait-shaped cross section in extremis and undoubtedly the most severely dredged river channel we have encountered to date.

By way of providing a diagnosis and prescriptive remedy the surveyed gradient of Old Stews was of particular interest.

Res for web use - Old Stews Offtake 46jpg

The river Test is dendritic and braided at this location downstream of Stockbridge and the channel supplying Old Stews is itself a man-made, perched channel impounded by sluices. Old Stews is the watercourse which starts from the footbridge top left of the image.

The water entering Old Stews cascades into the channel via a sluice and spillway at an elevation significantly higher than its confluence with the ‘natural’ Thalweg at the downstream end of Old Station beat.

web ready - Old Stews offtake 30

 

Our detailed topographic survey and channel assessment identified three distinct sub-reaches, each with its own physical characteristics.

  1. Upper (Uniformly shallow, straightened channel, 30% over-wide, inorganic sandy deposits, good weed growth, steep gradient)  The upper third of the project featured a shallow, significantly over-wide section of river flowing down an unusually steep gradient. The channel was dominated by Water Crow Foot (Ranunculus), Starwort (Callitriche) and Water Parsnip (Berula). All of these plants were growing in  ‘dunes’ of sand over-lying the natural river bed gravels.
  2. Middle (ca. 20% over-wide, straightened channel, pot holes in river bed up to 1m deep, laminar flows, nominal weed growth, heavy sedimentation) Dredging operations to lower the river bed appear to have commenced here. The middle reach was nominally over-wide, over-deepened and straightened as a result of historic channel engineering. Organic and inorganic sediments were uniformly blanketed over the bed and the ability of weeds to grow was compromised.
  3. Downstream end (Optimal channel width, straightened channel, portrait-shaped depth-to-width ratio, Channel depth varied  between 3.5 & 4.5 metres, laminar flows). The downstream third of the project was the most severely degraded and featured a portrait-shaped channel section with the deepest recorded depth of 15′ down to hard bed. One third of this was deposited sediment.

 

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Before Construction (Upstream Reach): Over wide and dredged channel.

Read more about the history of Cain Bio-Engineering and its introduction to river restoration projects, across an impressive array of the UK’s most prestigious chalk streams and their tributaries.