Case study:Twineham Weir Removal & In-channel Enhancements
Project overview
| Status | Complete |
|---|---|
| Project web site | http://www.oart.org.uk/our-work/projects/restoration-projects/twineham-project |
| Themes | Environmental flows and water resources, Fisheries, Habitat and biodiversity |
| Country | England |
| Main contact forename | Peter |
| Main contact surname | King |
| Main contact user ID | User:Oartpk |
| Contact organisation | Ouse & Adur Rivers Trust |
| Contact organisation web site | http://www.oart.org.uk |
| Partner organisations | Environment Agency, Rampion Offshore Wind Ltd, Sussex Piscatorial Society |
| Parent multi-site project | |
| This is a parent project encompassing the following projects |
No |
Project summary
In 2014 the Ouse & Adur Rivers Trust (OART) and the Environment Agency worked in partnership to remove four weirs on the River Adur and the Herrings stream in West Sussex opening up 4.8 km's of unrestricted river. The upper Adur has been dredged and straightened over many years and has been heavily impounded through this section until the completion of this project. The removal of these obstructions has restored river continuity, instigated a more natural dynamic flow regime and removed barriers to fish passage. With restricted levels of funding to undertake these works a novel approach was undertaken which saw the landowner employed by OART as principle contractor, saving over £160,000 and enabling work to be undertaken during suitable weather windows without the risk of increased costs due to inclement weather.
Following the removal of these weirs and the associated drop in water levels (of approximately 6' in places) a period of 6 months was left to allow natural processes to begin and allow an programme of in-channel enhancements to be developed which was focused on actual requirements rather than those which would be assumed necessary prior to the drop in levels. Throughout this process consultation was undertaken with the two affected angling clubs to ensure their requirements were considered within the framework of the enhancement programme. It was clear from an early stage that the main constraint on anglers as a result of this project was reduced water levels limiting access to the river.
Following this initial success the local Environment Agency team negotiated a £15k contribution from Rampion Offshore Wind LTD, a joint venture between EON and the green investment bank, to further enhance the upper Adur through the sections impacted by the weir removals on the Herrings Stream and River Adur. A programme of enhancements was developed in consultation with the Wild Trout Trust, angling clubs and the landowner which has seen a dozen features created by the landowner in a project managed by OART, building on the relationships developed during the weir removals.
A number of volunteers, including members of the angling club, assisted in the creation of the in-channel features as well as providing essential manpower to plant a trees along the river bank and surrounding floodplain.
- Five backwaters have been dug to provide shelter for small fish in times of flood and warm conditions in the shallow backwaters will provide an ideal nursery for fish, invertebrates and amphibians.
- Six berms have been installed along a 1km straight section of river which has narrowed the channel, sped up flows and increased morphological diversity which will in turn improve the ecology.
- 10 tonnes of gravel will be placed in two sections of river to improve spawning opportunities for fish, habitat for invertebrates replacing gravel lost over the centuries due to dredging of the river.
- 800 trees have been planted to provide shading, bank stabilisation and additional wildlife benefits such as habitat linkage.
These works have not only opened 4.8km of unrestricted river and undertaken a series of enhancements through 2.8km of this area but has shown the multitude of benefits which can be achieved through working in partnership with business, stakeholder groups and landowners.
Monitoring surveys and results
Fish surveys were undertaken pre-works with further surveys planned for 2016. In addition we are liaising with the angling clubs who have kept catch records at this site for a number of years. As predicted the immediate effect is the loss of species such as Carp which were prevalent at the site prior to removing the impounding structures and we are seeing these being replaced with species more associated with flowing river systems. At this stage these are being reported as being very small which was expected as species composition through this area changes. The backwater habitats were immediately effective with over 500 fry counted in a single refuge within days of it being completed. During the winter flooding of 2015/16 a number of larger fish were observed to be utilising the still water these refuges create in high flows.
The landowner takes a wide interest in his land and regularly undertakes walkovers to record species presence with at least two kingfishers seen returning to the river since the completion of the work, a species not recorded at Twineham for many years.
Fixed point photography is being used to monitor change in bank profiles and the rate of vegetation establishment.
Lessons learnt
The main lesson learnt from this project is that it is not always necessary to use large contractors to undertake these types of works with the landowner successfully completing this project with a large cost saving and in a third of the time quoted by framework contractors.
The main concern of the angling club was in river access and the creation of berms which slope into the water not only constrict the channel and create flow diversity but also provide year round access to the water irrespective of the water levels.