River Wensum Restoration Strategy Swanton Morley Restoration Scheme Reach 14a

Transcription

1 River Wensum Restoration Strategy Swanton Morley Restoration Scheme Reach 14a At a glance River restoration benefits: Improved planform, channel cross-section, flow variation and sediment process. Improved connectivity between river and floodplain. Driver for restoration: Addressing the physical modifications of the river to meet the requirements of the Water Framework Directive; restoring the River Wensum Special Area of Conservation (SAC) to favourable condition; and contributing to the chalk rivers Biodiversity Action Plan. Area restored: 0.88 km of River Wensum from downstream of Swanton Morley weirs to Castle Farm. National Grid Reference TG to TG Environmental impact assessment: No statutory EIA required. As best practice an Environmental Action Plan has been produced. Project Partners: Natural England and the Water Management Alliance. Construction period: June - September Site detail The Swanton Morley scheme (Unit 51, Reach 14a from the feasibility studies) extends 0.88 km downstream of Swanton Morley weirs (refer to scheme plan). The river channel was too wide and deep due to past dredging activities, and was disconnected from the floodplain by large spoil embankments located on either side of the river. The reach also lacked tree cover and in-channel woody debris. The feasibility studies identified the need for improvement to the main channel and for reconnection of existing side channels. This should help improve the ecology of both the Special Area of Conservation (SAC) and Site of Special Scientific Interest (SSSI). The detailed design, undertaken by our consultants Atkins, identified a suite of restoration measures to re-establish natural form and functionality to the river. These included the installation of gravel glides, pools, lateral shelves (berms) and woody debris, together with selective tree planting to create riparian shade, and the partial removal of spoil banks to improve floodplain connectivity. The changes to the river channel have been designed to ensure that under all flow conditions the finest sediment is kept on the move, but under bank full conditions the coarser gravels, which provide the best habitat, remain in place.

2 A small meander loop, bypassed as part of a land drainage scheme in the 1950s, has been reinstated by plugging the existing straight channel so that flows are diverted along the loop. The meander had become silted up and required de-silting to expose the natural hard bed consisting of pockets of gravel and chalk. The channel sinuosity within the meander has been further increased by the use of woody debris. One of the missing features along this reach is tree cover. The only exception is around the meander loop, which has been untouched for many years, and mature willow and alder line the old course of the river. The majority of these have been retained, but in order to enable access for machinery selective coppicing has taken place. All this material has been re-used as woody debris, and to create a brush wood berm in an area where plant is unable to safely access. The lack of tree cover is likely to impact severely on the fish within the reach and may contribute to high predation rates as well as affecting recruitment. Selective planting of native species of tree and shrub has taken place on the open stretches of channel to complement the other restoration features. Another aspect of the scheme has been the creation of fish refuge areas by re-connecting floodplain drainage ditches to the river. These will provide valuable shelter for fish and other wildlife during high flow events. As a result of past drainage activities large spoil embankments existed either side of the river. These prevent the majority of high flows from inundating the floodplain, as would naturally occur, and force flow downstream carrying with it large amounts of sediment. As part of the scheme we have removed sections of embankment to encourage floodplain wetting during periods of high flow. This should have the combined benefits of increasing floodplain biodiversity and providing a sink for suspended sediment, as well as reducing flood risk to people and property in the downstream catchment. The level at which water will spill on to the floodplain has been set so that there is a low probability of summer flooding and therefore minimal impact on the agricultural use of grazing marshes. Having a flow gauging station immediately upstream of the scheme has provided valuable data for the restoration design process, but has also imposed a restraint in that any physical changes to the river must not impact on the accuracy of the gauging station. Situated within the floodplain is an Internal Drainage Board main drain and the floodplain connection works have needed to ensure that the drain does not become surcharged with flood water which could have adverse impacts on downstream landowners. This has been overcome by working with the existing topography and using spoil to locally raise sections of the floodplain to create an isolated area which will wet up during high flows. The other important element of floodplain connection has been to provide a route for water to drain back to the river once river levels have receded.

3 Environmental initiatives The scheme has been designed to minimise the use of synthetic and imported materials. Flow deflectors formed from large woody debris (LWD) have been sourced from trees located on site, whilst taking care to minimise removal of any trees located directly on the river bank as these provide the greatest ecological benefit. The gravel required to create the glides and plug has been sourced from a local quarry to minimise the carbon impact associated with transportation. Synthetic materials have been discounted from the design entirely. No waste material has been taken from the site, thus reducing the potential effects on local traffic as well as reducing the carbon impact of the scheme. Traditional restoration schemes have tended to take place between late August through to March to avoid the main fish spawning and breeding bird seasons. This is the first time works have commenced earlier in the year and this has necessitated a carefully planned and stringent programme of ecological mitigation measures. Environmental benefits The restoration scheme aims to increase habitat and flow diversity which will benefit a number of environmental receptors including: Geomorphology naturalisation of river form and improved floodplain connectivity; Fish and aquatic invertebrates; Aquatic plants (water crowfoot plant communities); Water vole; Otter. In so doing, the restoration scheme will contribute to the SSSI and SAC achieving favourable ecological condition, contribute to attainment of Good Ecological Potential under the Water Frameworks Directive, and improve the local landscape character.

4 Series of before and after photographs taken in June 2012 (left hand side) and October 2012 (right hand side). Top picture shows filled berm (Bf2); middle picture woody debris and glide (RT1 and Glide G2); and bottom picture filled berm in foreground (Bf5).

5 Series of before and after photographs taken in June 2012 (left hand side) and October 2012 (right hand side). Top picture shows glide (G5); middle picture shows plug and entrance to meander loop; and bottom picture reinstated meander loop.

6 Series of before and after photographs taken in June 2012 (left hand side) and October 2012 (right hand side). Top picture shows reinstated meander loop; middle picture shows refuge area created in drain; and bottom picture the floodplain reconnection works opposite Castle farm.

7