Bossington Test reborn, Feature 2

‘House beat’ L/H branch

Please use the following links below to discover more about the Bossington Test reborn project.

  1. ‘House beat’ – R/H branch (Bossington mill race d/s to Wallop brook confluence)
  2. ‘House beat’ L/H branch – (Upper reach to top boundary)
  3. Gravel extraction site
  4. ‘House beat’ L/H branch (bottom reach to confluence)
  5. Pipe bridge crossing – Upper ‘House Beat’
  6. ‘House beat’ Central
  7. ‘Home beat’ U/S Weir pool & Fishing hut
  8. Double ‘Home Beat’ weir structures
  9. ‘Home beat’ D/S Weir pool & Riffle
  10. Nursery side stream
  11. ‘Home beat’ to Bailey bridge
  12. Ecological monitoring
  13. Native planting of margins and in-stream macrophytes

Before weir removal – This is the uppermost extent of the project restoration site, a short distance downstream from the Horsebridge road bridge and defined by the presence of a man-made weir. (see map)

Photo 5

Photo 5 – Man-made weir installation immediately downstream of the off-take side stream (mid-left of image) which flows parallel with the R/H bank at the bottom of Bossington Mill garden.

Photo 6

Photo 6 looking downstream from the weir. Even though this reach escaped the dredger and therefore features a natural gravel bed, the channel is significantly over-wide with shallow, uniform depth and degraded Ranunculus crowns due to sub-optimal flows.

Photo 7

Photo 7 shows the same reach looking upstream. Habitat quality is degraded due to the over-wide channel and provides little interest for salmonid spawning or for angling. The river bed gravels are impacted with sediment due to low flow velocity and scour.

This upper extent of the restoration site is of interest from a topographical data-collecting point of view. Because of the proximity to road bridges and infrastructure, sites such as this were invariably left un-dredged. They provide a useful reference point for relic bed level gradients to inform downstream bed-raising where historical dredging practices have destroyed the original gravel bed.

During

Photo 8

Photo 8 – Construction of a new low-level inside meander to optimise channel width.

This over-wide, upper river section was reduced in width by approximately 40% to create an optimal depth-to-width channel profile. The new bank margin was constructed from ‘As dug’ gravel from an extraction point in the adjacent floodplain (3) and re-planted with site-won root stock from the original bank (see mid-left of shot). A light gravel-dressing of the channel bed was also undertaken to introduce an element of large flint cobbles for invertebrate feeding stations and juvenile salmonid habitat. Regarding the re-sizing of channels? – this is an undertaking best decided upon at the time of installation and a calculation that requires experience and careful consideration. Ideally this work is best undertaken during periods of low flow to ensure that energy throughput (depth and speed of flow) is sufficient to ensure that established habitat characteristics (eg.optimal Ranunculus conditions) are resistant to drought.

Over-narrowing can result in accelerated bed and bank scour and increased ‘out-of-bank’ events and in certain scenarios, bed notching and ‘headward retreat’. Conversely, under-narrowing can result in poor in-stream conditions and loss of habitat during times of average to low flows.

 

Photo 9

Photo 9 – Creation of new inside meander with translocated root stock.

Photo 10

Photo 10 – Delivery of gravel to create new riffle to optimise depth and flows in the immediate upstream reach.

Faster-than-required flow velocities were generated in this reach following the narrowing exercise. The specific purpose of this was to ‘harvest’ energy from the uppermost extent of the project gradient, for transfer and discharge throughout the downstream half of the reach (see section 4). Optimal depths and flow velocities were then established in this upper section by the installation of a substantial gravel riffle. (E)

After

Photo 11

Photo 11 Downstream view of the optimised upper ‘House’ beat with key features.

Key features:

A. Extent of narrowing works (inside meander creation with reduced bank height)

B. Existing root stock translocated to new gravel bank face.

C. Existing Tree cover on L/H bank folded back to allow scour of sediment deposits on the enhanced outside bend. Tree cover left to naturally regenerate or keeper to ‘hinge’ as required.

D. Optimised depth-to-width ratio channel with streaming flows. Bed has been lightly-dressed with ‘as dug’ gravel to create juvenile salmonid lies and invertebrate feeding stations.

E. Gravel riffle installed to optimise (attenuate) upstream flow velocities.

F. Truncated berm on inside meander is cut back to the original bank at ‘Bounce-off’ point to enhance existing marginal wetland feature. (Ammocete & invertebrate habitat)

Photo 12

Photo 13

Photos 12 & 13 – The inclusion of meso and micro habitat creation is a key component of considered chalkstream habitat restoration.

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