Case study:River Nar Castle Acre Common WEG project
Project overview
| Status | Complete |
|---|---|
| Project web site | |
| Themes | Environmental flows and water resources, Fisheries, Habitat and biodiversity, Hydromorphology, Land use management - agriculture, Monitoring, Water quality |
| Country | England |
| Main contact forename | Charles |
| Main contact surname | Rangeley-Wilson |
| Main contact user ID | User:Charles Rangeley-Wilson |
| Contact organisation | River Nar Restoration Group |
| Contact organisation web site | |
| Partner organisations | |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
The primary driver here was the restoration of hydrological connectivity and process-driven self-restoration to a 2km section of the river that had historically been diverted to the edge of the floodplain and perched to build up a head of water to drive a mill. This type of channel diversion and perching is common on chalk streams, and has the effect of divorcing the river from the floodplain, and of lowering the gradient over long reaches. This creates a sediment retention issue, especially now that the mills are not worked, which has historically, in turn, precipitated dredging / management works which have tended to exacerbate the problems. Diffuse agricultural pollution becomes locked within these sorts of channels, leading to nutrient spiralling – from the sediment to the water-column and back again – each time flows or other activities disturb the bed material. But at no point can the river flush, escape its banks or place this nutrient loading on the floodplain. The nutrient enrichment and channel morphology skews the habitat in favour of a more limited and pollution-tolerant community of invertebrates and plants. In the pre-existing channel there was a notable absence of ranunculus. The overall aim, then, was to restore the channel to the centre of the floodplain and thus to restore the true gradient, natural channel dimensions (which will allow high flows to escape the banks and flood marginal habitats) and a natural meandering planform and pool-riffle sequence, enlivened by large woody debris together with floodplain wetland features (Stage Zero) over parts of the project site: with the overall aim of restoring hydrological connectivity between the river and supporting floodplain and of increasing biodiversity and favourable habitat for rheophilic species of fish, plants and inverts. Planning involved a full topo survey to establish the best corridor for the new channel, assessment of historic maps and nearby relic and existing natural meanders to form baseline dimensions and the careful plotting of a course that navigated the new channel along a line that would find gravel at the correct depth for the projected bed level (so that gravel was neither too shallow, not too deep along the chosen course) and which also incorporated relic sections of the original channel and various ditch networks subsumed into the project as wetland features.
Monitoring surveys and results
We have established a five-year monitoring programme in collaboration with the Norfolk Rivers Trust running from 2020 to 2025 including five monitoring sites (see attached docs). Site 1a and 1b are side by side reaches of the pre-existing, dredged, perched channel, and new hydrologically connected and dynamic channel; Site 2 is a section of the pre-existing channel which connects phases 1 and 2 of the project in which no changes were made save a significant release of gradient when the new channel was joined downstream; Site 3 is the upper Phase 1 reach of the newly constructed channel; and Site 4 is in the Stage Zero flooded woodland. The NRT will be monitoring for changes in biodiversity and abundance amongst the plant, invertebrate and fish communities.
Lessons learnt
As described, the diverted, perched and impounded pre-existing channel form at this project site (typical of mile after mile of English chalk stream) disables natural fluvial and ecological process, by
a) greatly reducing gradient (and therefore flow velocity) and morphological heterogeneity and
b) by divorcing the stream from the floodplain. Natural processes are a function of natural channel shape and gradient and consist of a two-way relationship between the physical form of the river and the ecological engineering that form enables.
A simple example would be how a restoration of gradient will restore ranunculus, which provides habitat for the blackfly larvae which scrub the water of diatoms: thus a change to the physical shape of the river can improve the water quality along it. In consultation with NE we took the decision to introduce nothing to the site except the changes made to the physical form, allowing the form to shape the habitat and later the plant and animal occupants of that habitat to shape the form. The in-stream plants established within one year of Phase 1 included extensive beds of starwort, berula and ranunculus, but rare marginal plants too, including bog-bean. It is notable too (at the time of writing April 2022) that benthic algae is more or less absent from the substrate of the new channel but prevalent in the side, spring-fed channel, suggesting the new channel will favour rheophillic invertebrates as well as plants. An unexpected impact has been the way in which the Konik ponies interact with the meander planform to improve the biodiversity of the plant communities: they graze (and poach) the point-bars on the inside of the meanders, but not the banks above the undercuts on the outside of the bends (see pictures in accompanying folder). Another impact of note is the way in which a stream returned to its natural level in the floodplain creates a scour line at the margin between the gravel floor of the river bed and the clay-peat banks above. On the outsides of bends this scour line becomes a significant undercut, as the stream nibbles away at the motile gravel and sand but not the cohesive upper layer. These undercuts provide fantastic refuge habitat for larger fish. Early results of NRT monitoring suggest a very healthy fish community, with all the size classes from juvenile 0+ and 1+ to large 5+, whereas in the pre-exiting channel the size range was more restricted to 2+ and 3+. This suggests that the fish have responded well to the increased heterogeneity of habitat in the new channel. Interestingly, the Stage Zero flooded woodland is already populated with bullheads and some juvenile trout. Also of note is how in the Stage Zero area the diversion of flow from the perched leat to the base of the floodplain has resulted in a matrix of flooded channels and backwaters: the river has cleared it’s own pathway along a course from which it was diverted centuries ago.
Image gallery
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Other case studies in this subcatchment: Castle Acre Rehabilitation Project, Nar SSSI project, Narborough Rehabilitation Project, River Nar Restoration Project, River Nar, Mileham River Restoration Project, West Lexham Rehabilitation Project
Site
Project background
Cost for project phases
Reasons for river restoration
Measures
MonitoringHydromorphological quality elements
Biological quality elements
Physico-chemical quality elements
Any other monitoring, e.g. social, economic
Monitoring documents
Additional documents and videos
Additional links and references
Supplementary InformationEdit Supplementary Information
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