Hampshire Avon - Hale
Hampshire Avon - Hale
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 Hale is located in the Lower Avon area, just upstream of the town of Fordingbridge.
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 the location of Hale, the river is fast flowing with little flow variability within the channel. Historical land drainage works by dredging has made the channel too wide and deep with deposited spoil on the right and left banks. The hydrological connection with the floodplain was lost. The floodplains were used as grazing areas for cattle and sheep, slowing the development of a vegetation community which could facilitate a stable narrowing of the channel. In addition, the submerged macrophyte diversity is poor with a few dominating species.
Due to the dredging, the river bed is dominated by poorly sorted fine gravel and sand, a substrate with very limited spawning opportunities and fry habitat for salmonid species. Since there was a lack of riparian shrubs and trees, there was very little large woody debris present in the stream and little shading.
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.
The restoration works at location Hale had several specific objectives:
- Re-energise the reach by providing variations in flow characteristics and channel dimensions (improve channel structure)
- Increase both the heterogeneity of bed morphology and margins in previously dredged reaches (improve substrate structure)
- Increase the availability of suitable spawning habitat for salmon, bullhead and brook lamprey
- Increase the amount of woody debris in the channel in order to increase both the availability of this habitat type and morphological diversity of the channel
- Enhance the availability and quality of habitat for, 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
In the bidding documents for the permits, there was a criteria value of spawning habitats. There was also the criterion to increase morphological diversity, but this was not quantified by a value
Location Hale was divided in 7 reaches. No measures were planned for reach 1 and reach 7. At two locations (Reach 2 and 4) 30-40 meter spawning riffles were created using existing and imported gravels. The existing gravels are used to create a stable crest. Gravels are placed on top and below the crest of the riffle to provide a suitable depth for spawning.
In two reaches (Reach 4 and 6) upstream current deflectors were constructed to create a varying flow and to narrow the channel. These deflectors are made of large tree limbs set at angles between 30° and 60° and are 12-15 meters in length. The tree limbs are secured to prevent washing away.
Woody debris is added at all reaches (except reach 1 and 7). In reach 2, native trees were planted to serve as future source of woody debris. Temporary fencing is placed to protect the trees from graving by livestock. In the other reaches, trees were coppiced or pollarded to get woody debris in the stream. In reach 5, the woody debris was used to install a tree kicker.
At location Hale, a rapid assessment survey was done during pre- and post-monitoring. The rapid assessment survey consists of a physical biotope survey where biotopes were mapped and a river corridor survey to assess vegetation. The assessment is based on expert judgment, although the method has possibilities to add qualitative and quantitative results of analysis undertaken for particular elements. The pre-monitoring effort was to record the hydro-morphological and biological conditions of the site before any measures where taken.
The post-monitoring occurred two years later using the same monitoring techniques. The results are compared to see if the restoration measures had any effect on the site.
Expectations and Response
The positive results of the restoration measures mostly involve the expected increase of flow variability by the reshaping of the channel. The created riffles were expected to promote local sediment transport and flow variability which results in a change of substrate composition of the river bed from sand/silt dominated to gravel/pebbles dominated. Growth of Ranunculus and other aquatic plants should be increased on the riffles. The riffles also provide a suitable spawning habitat for salmon (S. salar) which would have positive results on the population size of this species.
The planted trees would increase habitat for terrestrial invertebrates and nesting birds. Large woody debris deflectors at reach 6 would promote geomorphological processes like sedimentation of silt. Brook lamprey (L. planeri) should benefit from the increased habitat opportunities created by the woody debris. The improvement of the vegetation should develop habitat suitable for Desmoulin’s whorl snail (V. moulinsiana).
Fish, macro-invertebrates and phytoplankton were not surveyed, so the effect of the restoration measures on these biological quality indicators is not known. Barble has been seen to be spawning on the riffles, but this was not an official part of the monitoring process.
The vegetation structure and species diversity appears to be similar to the pre-monitoring. Water crowfoot (Ranunculus penicillatus) occurs intermittently along the channel. No other macrophytes are found in the post-monitoring. In the pre-montoring, horned pondweed (Zannichellia palustris) was present in the river.
The riparian buffer zone varies from narrow to non-existent in case of grasslands which support sheep and cattle grazing. Upstream broadleaved woodland is present on the left bank with sycamore (Acer pseudoplatanus) and ash (Fraxinus excelsior) with occasional willow (Salix sp.) and English oak (Quercus robur). On the left bank, reeds like reed sweet grass (Glyceria maxima), common reed (Phragmites australis) and reed canary grass (Phalaris arundinacea) are present with occasional hawthorn (Crataegus monogyna) and bramble (Rubus fruticosus agg.). Downstream, both banks have similar vegetation. Dominant species are reed sweet grass (G. maxima) and common nettle (Urtica dioica) with other species like yellow flag iris (Iris pseudacorus), reed canary grass (P. arundinacea), bramble (R. fruticosa), water mint (Mentha aquatic), great willow herb (Epilobium hirsutum), branched bur-reed (Sparganium erectum) and occasional alders (Alnus glutinosa).
The dominant physical biotope at the Hale site was glide. At the start of the last meander bend, a deep pool was observed. Upwelling was observed at two locations. It should be noted that the discharge and water levels were high and this could have obscured localised diversity created by the restoration measures. The two areas of upwelling can be related to the large riffle upstream of the first upwelling area and the woody debris deflectors within the second upwelling area.
Due to the high water level during the post-monitoring, the influence of the restoration measures on the patterns of sediment deposition in the channel bed could not be assessed. Bank erosion at meander bends is reduced due to increased marginal vegetation and local exclusion of livestock by through fencing.
The impact of the measures on socio-economic aspects like flood protection are not monitored and therefore unknown. There was speculation that the increase in marginal vegetation cover is related to the sustained higher water levels in 2007 and 2008 at the time of the survey. The Hampshire Avon Catchment Flood Management Plan published in 2009 indicates that the area of Hale has sufficient flood risk management, but could be improved to cope with future climate change.
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.
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.
Name: Natural England enquiry service
Organization Name: Natural England
Phone-Number: 0044 845 600 3078
Physical and biological monitoring of STREAM restoration projects: Year three report
Post works restoration assessment of the Stream restoration project sites at Hale / Seven Hatches
- Narrow water courses
- Initiate natural channel dynamics to promote natural regeneration
- Recreate gravel bar and riffles
- Introduce large wood
- Adjust land use to develop riparian vegetation
- Revegetate riparian zones