Case study:Seven Hatches: Difference between revisions
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|Bank and bed modifications measure= | |Bank and bed modifications measure=Recreate gravel bar and riffles, Introducing sediment, | ||
|Floodplain / River corridor=Develop riparian forest, Revegetate riparian zones, | |Floodplain / River corridor=Develop riparian forest, Revegetate riparian zones, | ||
|Planform / Channel pattern=Channel narrowing, Introducing large woody debris, | |Planform / Channel pattern=Channel narrowing, Introducing large woody debris, | ||
Revision as of 11:16, 1 June 2017
Project overview
| Status | Complete |
|---|---|
| Project web site | http://http://webarchive.nationalarchives.gov.uk/20110303155229/http://www.streamlife.org.uk/actions/river_restoration/ |
| Themes | Fisheries, Habitat and biodiversity |
| Country | England |
| Main contact forename | Jenny |
| Main contact surname | Wheeldon |
| Main contact user ID | |
| Contact organisation | Natural England |
| Contact organisation web site | http://www.naturalengland.org.uk/ |
| Partner organisations | Environment Agency, Wiltshire Wildlife Trust, Hampshire & Isle of Wight Wildlife Trust, Wessex Water, River Restoration Centre |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
summary
The River Wylye was historically dredged at Seven hatches and is over sized compared to flow. In the top part of the site, large hatches used to control flow between channels results in slow flows. Below the hatches major dredging combined with the presence of various structures downstream means the channel has no slope, little or no flow speed in summer and heavy siltation. The object of the work is to modify the channel to a more appropriate width, shape and slope, and to re-introduce woody debris resulting in increased and varied low speed and sediment movement. The works were understaken with the parterns listed above and the local stakeholders where the local landowners, tenant farmers and the fishing clubs that owned the fishing rights.
The work was completed in November 2007 and involved:
- reducing impounding effect of sluices
- narrowing channel width in suitable locations, using hazel coppice bundles and pre-planted coir
- reinstating the historic river bed to create spawning habitat and increase physical habitat diversity
- introducing large woody debris by selectively felling/pollarding non-native trees, replanting with suitable riparian species
introduction
The Hampshire Avon catchment is on of the most bio diverse catchments in the UK, with over 180 species of aquatic plants, 37 species of fish and a wide range of aquatic invertebrates. The river has a high input from chalk-rich water from springs in the headwaters. Therefore the catchment is a Special Area of Conservation (SAC), which is part of the Natura 2000 network. As a calcareous river, it is an important habitat for several endangered species like Atlantic salmon (Salmo salar), bullhead (Cottus gobio), brook lamprey (Lampetra planeri), sea lamprey (Petromyzon marinus) and Desmoulin’s whorl snail (Vertigo moulinsiana). The vegetation community consists of a Ranunculion fluitantis /Callitricho-Batrachion community.
Location Seven Hatches is located close to the confluence of the river Wylye and the Nadder. Several impoundments are present on the site. First it is characterized by the presence of a large radial sluices replacing the original hatches that gave this location its name. Just above the sluices, the river splits into two channels, the main Wylye also known as the ‘Union’ and the ‘Butcher Stream’. Next to the sluices, the footings of a railway bridge upstream and a tractor bridge downstream are present. A small chalk stream called Law’s Ditch flows into the Wylye below the railway crossing, but it is culverted underneath the Butcher Stream for 600 meters.
Pressures and Drivers
A conservation strategy has been written for the Avon River and its tributaries in 2003. In this document, the main pressures on the ecosystem of the Avon are pollution, fisheries (both commercial and recreational) and flood defence. These pressures come from a variety of drivers, namely agriculture, industry, fisheries, recreation and flood protection. Historical dredging has damaged the river ecosystem by destruction of habitats, loss of lateral connectivity, silting up of clean gravel habitats and unnatural river flows. At location Seven Hatches, the impoundments caused by the sluices, the footing of the railway bridge and the footing of the tractor bridge have resulted in siltation due to slower flows. In combination with the historical dredging, this has lead to a loss of hard bed structures, over widening of the channel and the creation of raised floodplains. The hydrological connectivity with the floodplain has been lost and the grazing pressure of livestock has damaged riparian vegetation structures and caused erosion at some sites.
Global objectives
The global objective of the STREAM demonstration project is to restore the River Avon Special Area of Conservation (SAC) and Special Site of Scientific Interest (SSSI) to favourable conditions while addressing wider biodiversity issues outside the protected areas. Another important objective is increasing public awareness for the importance of the river and valley as natural heritage by improving public access.
Specific objectives
The location of Seven Hatches had several specific objectives:
- Modify the operation of the Seven Hatches sluices, reducing height by an average of 0.15 metres, thus increasing ecological connectivity between reaches and improving upstream habitat quality.
- Restore the historic bed level and increase the heterogeneity of bed morphology in previously dredged reaches by the reclamation and re-introduction of excavated gravel/stone bed material
- Narrow overwide channels where necessary, in order to re-establish a sinuous channel of appropriate cross-sectional area with respect to present day hydrographs
- Increase the amount of large woody debris in the channel in order to increase both the availability of this habitat type and morphological diversity of the channel
- Break out and remove the tractor bridge footings and replace with a single span bridge. To remove the impounding effect of the structure
- Enhance the availability and quality of habitat for SAC species and habitats, in particular:
- Bullhead (Cottus gobio) (increased diversity of hard bed, particularly pools during winter and riffle/fast glides during summer and increased large woody debris for, particularly, juveniles)
- Brook lamprey (Lampetra planeri) (increased availability of well sorted, fine sediment in shaded, marginal areas with large woody debris for ammocoetes and gravel/sand dominated shallows <40 cm deep for spawning adults)
- Salmon parr (Salmo salar) (increased availability of coarse substrate, with overhead cover and woody debris lodged in the channel)
- Desmoulin’s whorl snail (Vertigo moulinsiana) in the marginal zone of the channel
- The Ranunculus community as a result of increased heterogeneity in velocity and bed morphology
Measures selection
The Seven Hatches restoration site was divided in 7 reaches. No measures were planned for reach 1 and 7. In reach 2, 4 and 6, deflectors are installed to narrow the channel. The deflector in reach 2 will be based on a D-shaped outlined with wooden stakes and then filled with brushwood. In reach 4, the deflector is made with large woody debris and in reach 6, tree deflectors are constructed. Next to the deflectors, gravels are also imported in reach 4 and trees are removed. It was planned to re-grade existing banks, but this was later cancelled due to the limited machine access and minimal effect on the hydrology. Fences are used to prevent grazing of the banks by livestock. Native trees are planted to compensate for the removal of unsuitable tree species. It was planned to replace the tractor bridge, but this was later cancelled due to limited effect on the hydrology. It was planned to lower the sluices at Seven Hatches (reach 3) with an average of 0.15 meters, but this was later cancelled to protect salmon habitat upstream and fear for flooding problems in the Witton area.
Funding
The total costs of the project were £1 million pounds. 40% of this total costs was paid for by the European Union via LIFE subsidies. The rest of the costs were paid for by the state, a water company and 2 wild life trusts.
Communication
During the planning phase, the public was informed via public meetings and press releases in the newspapers and magazines. The stakeholders had to accept the concept design in the project bid and during the project there was constant contact with the stakeholders. Concerns or criticisms were investigated and the project changed when the criticisms were verified. At the end of the project a conference was held for all stakeholders and general public to show them the end results of the project in their area.
Monitoring surveys and results
Success Criteria
In the original bidding document a value for the minimal increase in spawning habitat was given, but the exact value is not known. There was also a target for increasing hydro-morphological diversity, but no exact value was given. No other clear qualitative or quantitative success criteria for other hydro-morphological, biological or flood protection targets were defined for the project.
Pre-restoration monitoring
In Seven Hatches, a detailed pre-restoration monitoring scheme was implemented. The detailed assessment method consists, namely physical biotope mapping, river corridor survey, macrophyte survey, cross-sectional mapping, depth/velocity/substrate survey, fisheries survey and repeat fixed point photography. The exact methods are described in the Monitoring Protocol.
Post-restoration monitoring
The post-restoration monitoring occurred 18 months after the project was finished. The monitoring methods were the same as the pre-restoration assessment.
Separate from the LIFE project in Seven Hatches some invertebrate monitoring was conducted before and after the measures, but the results of this survey was not reported in the project reports.
Lessons learnt
Expectations and Response
It was expected that the measures would have a positive effect on the river ecology, especially on bullhead, brook lamprey and salmonids due to the introduced hard bed substrates. A greater heterogeneity in substrates was expected which would result in re-establishment of Ranunculus and other macrophyte species. The increased hydrological continuity with the floodplain and exclusion of cattle benefits the riparian zone and typical riparian species.
Ecological Response
A total of 58 macrophyte taxa were recorded during the assessment of 5 transects. No invasive species were found and only one negative indicator Fennel pondweed was found. The key species brook water-crowfoot and hemlock water dropwort were only minimally present with less than 5% coverage. The most dominant species were Reed sweet grass and branched bur-reed with both species taking 10% of the coverage.
Compared to the pre-restoration state, the reach has less brook water-crowfoot and a strong increase in the negative indicator Fennel pondweed. A fewer number of taxa were observed and a reduced number of species were recorded as covering more than 5% of the channel. It was concluded that temporary disturbance caused by the construction of the measures could be the cause of this decrease. The installation of fences did increase marginal vegetation growth at some cross sections.
The fish survey 11 species of fish were caught in 2008 compared to 10 in 2006. There was an large increase in salmon, trout and bullhead, but a decrease in lamprey and minnow. The higher flow conditions in 2008 could be the reason why there was more bullhead and salmon, who prefer swift flowing water, and lamprey and minnow, who prefer more shallow water.
Hydro-morphological Response
The physical biotopes mostly glide and a few pools, observed in 2008 were the same as in 2006, with the exception of the created riffles. The riffles had an impact on the sediment regime, with more localized areas of sediment deposition and transport. The exclusion of livestock also decreased the input of fine sediment and an increase in sediment trapping margin vegetation.
The physical channel form was significantly changed by the measures. The depth decreased where the bed of the channel had been raised. However, the higher water level in 2008 minimized the decrease in water depth even at the riffles. Also the change in velocity was minimal due to the higher discharge in 2008 compared to 2006. There was however more coarse material and less coverage of silt.
The complete dataset and more detailed information can be found in the report ‘Physical and biological monitoring of STREAM restoration projects: Year three report’
Social-economic Response
The impact of the measures on socio-economic aspects like flood protection are not monitored and therefore unknown. The Hampshire Avon Catchment Flood Management Plan published in 2009 indicates that the area of Seven Hatches is on the border between the Salisbury area with policy 5, Areas of moderate to high flood risk where we can generally take further action to reduce flood risk, and the Upper Avon, Wylye sub area with policy 6, Areas of low to moderate flood risk where we will take action with others to store water or manage run-off in locations that provide overall flood risk reduction or environmental benefits. There are therefore still problems with flood protection that have to be addressed.
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