river medina restoration report
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Words: 2,421 724465A proposed scheme for the enhancement of the Blackwater cross over of the River Medina, Isle of WightIntroductionRiver rehabilitation and restoration is a relatively new concept and procedure, having emerged from the sustainable development paradigm (Wheaton, Darby and Sear, n.d cited in Darby and Sear, 2008). In recent years, river restoration has been given another, new impetus through the European Union Water Framework Direction (EU WFD) and national government targets for both water quality and biodiversity conservation (Biodiversity Action Plans (BAPs) (iwight.com, 2003). Restoration schemes, focused on ecology (Shields Jr, Copeland et al, 2003; Pretty, Harrison et al, 2003), are increasingly incorporating in more boarder requirements such as recreation and accessibility (Woolsey, Capelli et al, 2007). One of the first to be carried out in the UK was the restoration of the Rivers Skerne and Cole, Swindon (Aberg and Tapsell, n.d. cited in Boon and Raven, 2012). This scheme has been successful in improving access to the river and geomorphological processes despite increasing sediment load and phosphorous during construction (Kronvang, Svendsen et al, 1998). Yet it was considered unsuccessful in improving biodiversity (Biggs, Corfield et al, 1998) which Palmer, Bernhardt et al (2005) suggest restoration isnt always ecologically successful.A variety of different approaches have been adapted to enhance or to rehabilitate a river, depending on the level of instability in the river (Downs and Gregory, 2004; Knighton, 1998). These approaches range from wholescale morphological reconstruction used at sites such as Ewan Water (Gilvear and Bradley, 1997), the Skerne and Cole (Biggs, Corfield et al, 1998), Deep and Whitemarsh Run, Maryland (Soar and Thorne, 2001; Smith and Prestegaard, 2005) to more subtle changes including low flow sinuosity (The Medina, Shide tributary (Hector, 2013), riparian corridors (Larsen n.d cited in Petts and Calow, 1996) and artificial step-pools/riffle-pool sequences (Knighton, 1998; Downs and Gregory, 2004). All these measures have been proven to have varying degrees of success (Palmer, Bernhard et al, 2005; Radspinner, Diplas, Lightbody and Sotiropoulos, 2010). Towards the future, the fate of the UKs rivers and commitments to the EU WFD are uncertain given the BREXIT referendum. This report will discuss a proposed scheme for the enhancement of the Blackwater crossing of the Medina.Aims and objectivesAim To enhance the aesthics of the Medina cross over at Blackwater to improve local habitat and geomorphological diversity.Objectives1. To soften the appearance of pre-existing concrete structures at the cross-over to blend it into the landscape.2. To reactive the old channel course to provide a backwater shelter for fish.3. To improve bed diversity by introducing riffles.4. To reduce the channels wide through replanting bankside vegetation to encourage low flow sinuosity and increase habitat diversity.Study siteThe River Medina is the only river in the UK which flows directly north, from its source at Chale before then flowing directly north (Hector, 2013) to Cowes where it discharges into the Solent (figures 1 and 2). Its total length is 17 km, draining a 17 km2 catchment. It passes through two major towns in the centre of the island Blackwater and Newport. Historically, the river has been modified significantly. First by the Victorians who constructed a railway which ran directly between Newport and St Catherines Point, South Downs. More recently, however, this railway has been removed and during the 1960s it was channelised and straightened further to create a flood alleviation scheme. As the river flows towards the sea, from Newport to Cowes and the Solent, it is an enclosed estuary consisting of industry, a navigable channel, and a large inland harbour (Islandrivers.org, 2015).Since 2011, the river has undergone restoration and enhancement works with a view of improving ecological diversity, because the river is home to migratory Brown (Sea) trout and European Eels. The scheme was awarded 90,000 from the Sita Trust and carried out by the Newport Rivers Group, Natural Enterprise and the Island 2000 Trust and Environment Agency, to improve ecological diversity for the fish species (Hector, 2013). Divided into three phases, the scheme implemented a variety of different in-stream measures ranging from planted berms (figure 3), artificial islands and riffles (figure 4) to removing bankside trees to lighten the ambiance of the river (Hector, 2013).
Figure 1: overview of the river medina as it passes through Newport to the estuary at Cowes. Image taken from Google Maps, accessed 11.12.2016, retrieved from https://www.google.co.uk/maps/place/Newportfirstname.lastname@example.org,-1.2940365,14z/data=!4m5!3m4!1s0x48746214000d10c5:0xb93bd2652373300a!8m2!3d50.700803!4d-1.291633?hl=en&hl=enAt this point is the Shide tributary, a concrete lined channel, subject to recent work to enhance the geomorphic and ecological diversity. Also, to a lesser, extent to soften the aesthics of the river at this point.Here is the Medina as it flows towards Newport, from Blackwater. Notice how it splits into two channels. At this point, the Blackwater cross over, the railway runs directly through the middle of the old channel course.
Figure 2: a zoomed in picture of the River Medina, stretching from Blackwater to Newport which is the study area. Images taken from Google Maps, accessed 11.12.2016 retrieved from https://www.google.co.uk/maps/place/Newportemail@example.com,-1.2940365,14z/data=!4m5!3m4!1s0x48746214000d10c5:0xb93bd2652373300a!8m2!3d50.700803!4d-1.291633?hl=en&hl=en
Figure 3: the Shide tributary following construction of vegetated berms.
Figure 4: riffle close to St George's Road bridge. This is one of a variety of different in-stream measures implemented during the works.Appraisal of existing measures implemented in the River Medina restorationWhen the scheme was constructed, a variety of in-stream and bankside measures were implemented to improve the ambiance, ecology and naturalness of the river. These include artificial riffles (figure 4), vegetative and log berms and large woody debris. These measures have had some successes including the re-instatement of low flow sinuosity and the re-creation of, a more natural looking and sounding river. But some of the measures implemented have either started to fail or failed to restore longitudinal, ecological connectivity.Figure 5 shows evidence that some in-stream measures such as larch-spilling are beginning to fail. In figure 5, you can see the home owner has tried to re-enforce the failing bank line. Buchanan, Nagle, and Walter (2013) suggest failure along this bank line is related to the combined effects of high shear stress, associated with depth and velocity increases here. Thus, it may be suggested using willow spillings, without building in some form of geo-textile to secure the sediment has contributed to failure here. Another issue with this scheme is the absence of fish passages and Elver weirs which have been removed during the original works (figure 6). Lack of Elver passes may mean scheme is not ecologically successful because it has failed to improve longitudinal habitat diversity and connectivity (Lepori, Palm, Brannas and Malmqvist, 2005).Similarly, an island installed, two years, has since disappeared. This island was envisaged, by the designers, to act like a flow deflector and promote scour either side of it. It failed, in part, because of high flow events and a lack of vegetation at this site. Another possible reason it failed was there was not active post-project monitoring, which could have otherwise avoided the complete wash out of the island (Downs and Kondolf, 2002). At other sites, such as the Shide tributary have been successful in re-creating low flow sinuosity, despite the inability to retain artificially coarse bed material.
Here the bank line is beginning to failure due to recent high winter flows. Previously this had been filled with sediment to support riparian vegetation but has since been washed out. As you can see, the home owner has tried to sure the bank line up which is likely to continue to fail without immediate structural re-enforcement.
Figure 5: site 3, upstream of St Georges Road Bridge crossing. This photo shows clear evidence of bank undermining, perhaps, by high winter flows.
Figure 6: weir at site 2, St Georges Road Bridge. Towards the left-hand side, nearest the inner bank, there is a small notch. Here was where a fish pass was once installed, now it has been removed. The black circle indicates where the former fish pass once was.Figure 7: The Shide tributary, river Medina, Shide. Here you can see evidence of low flow sinuosity structures, vegetated meander bends. Towards the top of the image, you can see where there is a low-level weir-type structure which has been introduced for the purposes of catching coarse bed material and introduced some flow diversity.Velocity changes at this point, as the water flows over a rock weir. Also, the flow up and till this point is largely steady, uniform.To the left hand-side, cylix (Willow) is starting to colonise the bend. This, however, was not planted.Flow heading northZoomed in photo of the right-hand meander bend, looking from the bridge. This close-up shows evidence of scour at the head of the bend.
Figure 8: Zoomed in image of the head of the right hand vegetated berm. This structure shows evidence of scour, possibility related to secondary circulation.
Summary table of the current restoration measures in taken in the river Medina
Site along the riverMeasure takenAdvantagesDisadvantages
Site 1: River Medina, centre of NewportVegetated berms to restore low flow sinuosity and removal of a weir just upstream.Recreate low flow sinuosity, reduce fine sediment load, and improv