River is freshwater providing natural resources for food, recreation, energy, transportation, irrigation and drinking. However, the quality and quantity of river water have been limited due to climate change and other pollutions from sewage and food waste or farm effluent in the whole world. Moreover, other river issues which results from hydraulic principles of river have occurred frequently since the ancient civilizations in many countries, such as erosion and floods. This can not only cause destruction of life including fauna, flora and human, but also damage property like roads and bridges especially in urban areas and destroy farmlands in rural areas. Therefore, various types of techniques and practical schemes have been discovered and utilized in managing and restoring river in order to create sustainable (complex) ecosystems and protect human health and other vegetations. Take the United Kingdom for example, the largest river-River Severn, which is approximately 354 kilometres length from source that locate in the Plynlimon Mountains in West Wales to the sea (Witts, 2011), flows through several countries such as Shropshire and also has a large number of tributary rivers, which can be illustrated in Figure 1. BBC News (2011) states that it has various types of pollution and needs major restoration to comply with new European Union Legislation. However, the attractiveness of river restoration in recent years is normally following the damage of the point source of water pollution. Thus, diffuse pollution of the proposal river will be focused in the upper stretch of River Severn at Caersws and the related restoration will be introduced in the following sections.
The main objective of this project is to critical analyze the potential pollution or issues of a stretch of River Severn at Caersws and to discuss the effects of restoration on local ecosystems and economy. Moreover, the several types of restoration schemes and techniques will be analysed including stream bank stabilization, enhancing riparian buffers by adding trees and natural grasses and other additional techniques. In additional, the changed hydraulics will also be explained and analyzed before restoration and afterward in this paper.
Figure 1. Map of River Severn
image map of the river Severn
(Source: Mandy Barrow, 2011a)
2. Description of River Severn at Caersws
2.1 Site Description of the River
Caersws is a small village in the upper valley of the River Severn, which can be illustrated in Figure 2 and Figure 3 (National Grid Reference SO 033 917). At Caersws the River Severn has a catchment area of 375 km2 and bankfull discharge is approximately 70 m3/sec (Williams and Rhodes, 1982). The main rock type of the proposal area is resistant rock and the bed is formed in coarse alluvial deposits (gravel and cobbles) having a mean grain size of 40mm (Haslam 1978; Williams and Rhodes, 1982). According to measurement in Figure 3, the average width of the proposal stretch of river below Caersws is around 20 metres and the length is approximately 4.2 kilometres. The aimed stretch of the river is surrounded by a large scale of farmland and green land with some trees.
Figure 2. Map of the Location of River Severn at Caersws
(Source: Mandy Barrow, 2011b)
Figure 3. The Google Map of the River Meander near the Victorian Workhouse in Caersws
make-Meanders-near victorian workhouse in Caersus-google map.jpg
Note: Red star shows the location of Caersws; Blue lines are boundaries of restoration; Red arrow line is the direction of the flow
(Source: Google Map, 2011)
2.2 Why Restore River?
It also can be seen that many meanders have been formed around Caersws as the river is flowing across low land where the water flows slower than other areas and the main erosion tends to be horizontal (sideways) which is different from the vertical erosion in mountains. As a consequence, the river has cut through a loop and an ox-bow lake has been formed in a long time period, which shows in Figure 4. As can be seen in the figure, a large number of sands are formed and the old river bed has charged, which results in the loss of local ecosystems and vegetations. For example, the uprooted trees and branches and sediments have been eroded by the force of water especially when the river is fall. To some extent, it will affect biodiversity of species, blockage of river which also related to floods and water turbidity. Furthermore, fertilizer, feaces of sheep and other chemicals from farming can be washed out into river, which can affect water quality whether in local area or downstream.
On the other hand, there are some instability problems of channelization in the stretch of river as showed in Diagram 1, which results in expensive maintenance for controlling sediment and erosion in a long term. As for the principles of river management, natural development of river is more sustainable rather than control; thus, a less expensive and sustainable scheme with monitoring programme can be accepted for river restoration to achieve effective transport sediment and avoid erosion if the net value of aggradations or degradation of the river will reach nearly zero (Soar and Thorne, 2001). To sum up, it is better to restore the river bank and manage surrounding areas in sustainable methods in order to avoid potential issues of river and enhance the biodiversity of local ecosystems.
Figure 4. An Example of Ox-bow Lake
(Source: Mandy Barrow, 2011c)
Diagram 1. The Current Description of the River Severn at Caersws
(Source: Hand Drawing by Chai, 2011)
2.3 Analysis of formation of Ox-bow Lake and Hydraulics of the River
As hydraulic theory states that energy may be constant along the streamline for an ideal fluid (i.e. viscous of frictional effects are insignificant), which means the large number of energy can be created when the water dash on the river bank as the velocity of flow is zero (Chadwick, Morfett and Borthwick, 2004). Thus, hydraulic action can break down the rocks, remove and drag rocks from the bed and banks of the river by the force of the running water, especially in the extreme rainfall period because of unstable flows of great water force. Moreover, in this large stream the plants become more wispy and are confined to the sides. Based on these theories, Ox-bow Lake has been formed as the speed of river flows is faster on the outside of meander causing erosion and is slower on the inside of the meander resulting in deposition (Diagram 2). The part of the meander has been cut off and changed to Ox-bow Lake, such as point A in Diagram 1. Although other areas of the river are not formed Ox-bow Lake at moment, they have eroded and deposited of the sites, which means the change of river channel and potential pollutions may be found in the future. This is also a reason why we need to restore the river bank and improve local ecosystem as well.
Diagram 2. The Formation of Ox-bow Lake and Erosion of Meander
(Source: Mandy Barrow, 2011d; Geograph High, 2011)
3. Critical Analysis of River Restoration
Downs (2001) mentioned that river restoration is an important activity of river management in Europe to enhance aquatic and riparian habitat, improve water quality and quantity, and facilitate human use. The River Severn at Caersws, which is a good case in point, needs to be restored because of erosion and deposition of the site. The main objective of restoration engineer includes:
i) Dredging sediment in the river bank and improving the aesthetic quality of stream;
ii) Protecting the bank-lines from erosion;
iii) Maintaining the current level of river bank and flow regime;
iv) Enhancing diversity of vegetation and sustainable of local ecosystem.
The variety of schemes and techniques can be used in different point of the river in Diagram 2 to minimize aggradation or degradation and mitigate tendency of the meandering river, including regular meander path of similar meander bends, rock vortex weirs at meander crossings, riprap (stone) around bendways, and various vegetation around river bank. In the following section, the detailed planning of restoration will be explained (Diagram 3 & 4) and the impacts of river restoration on economy, local ecosystem and visual aspect will also be analysed.
3.1 The Restoration Plan and Hydraulic Analysis
It is not necessary to change the main shape and structure of the river; however, it is possible to restore the river bank at the similar conditions in different parts such as meander bends and width. According to Diagram 1, the area A, B, and C should be focused on in the restoration as the major erosion and deposition are occurring in these areas. During the first stage, the general mechanical dredging can be used to remove damaged trees and other blockages in the river and remove hard rock and rocky materials from deposition sites to erosion sites in A, B, and C to create similar river width as possible as we can. The flow picture shows the potential mechanics used during dredging process in the development.
Diagram 3. The Plan of River Restoration in A, B, and C
(Source: Hand Drawing by Chai, 2011)
Diagram 4. The Plan of River Restoration in D and E
(Source: Hand Drawing by Chai, 2011)
Secondary, a large number of soil that is suitable for grass and othe vegetation growth should be filled in the derdged site achieving the same level of surrounding fields. Then, the derdged rock should be binded together with large size of rock using meshes, which are like ‘Gabions’ and placed and filled on the sites of cut banks as rock-lined embankments showed in Figure 5. Before restoration a large number of warer force washed out soil and sand from the river bank and then form the steep slop with Î± (Diagram 5). However, the slop with Î² (Diagram 5) will become more gently after resotration, which result in that the force of running water can be mitigated and reject bank erosion and lateral shift of planform of the river. Additionallly, Chadwick, Morfett and Borthwick (2004) stated that “spiral vortex around the outside of the bend causes an asymmetrical flow pattern which tends to erode material from the region near the outside of the bend and deposit it near the inside” (Figure 6). Thus, stone riffles (vortex weirs) will be placed at meander crossings in the river to maintain the grade and to decrease meander wavelength showed in Figure 6. Consequently, both Rip-rap and weirs may not only change the wavelength of flow and the bed slop, but also maintain the stability of channel cross sections. To some extent, the landform and flow regime of the river may not be changed in a long term peroid after restoration, such as 50 years later.
Figure 5. The Rip Rap of the River Restoration
(Source:Google Picture, 2011)
Diagram 5. The Bed Slop Before and After Restoration
(Source: Hand Drawing by Chai, 2011)
Figure 6. Current Pattern of the River
(Source: Chadwick, Morfett and Borthwick, 2004 Page 529)
Moreover, willow spiling and faggots should be ultilised to control erosion in D and E and also to stabilise wide and shallow point bars as they are generally suitable for light erosion and may not withstand stronger forces (Environment Agency, 2010). However, the hyrdaulic will also be changed after restoration and the principle of the hydraulic change is similar to the change of that from strech A to C. According to Figure 7, the wood stick can break up the water force and wave prior to scouring of the river bank and damage the force of backwater. Faggots are normaally installed underwater to limite degradation and spiling can be applied on steep and vertical banks (Figure 7); in addition, the materials of both techniques can be sourced locally or imported to site. Take some practices in the UK for example, willow faggots and spiling have bee successfully applied sperately in the River Cam and River Skerne.
Figure 7. Schematic Diagram of a Bank Protected Using Willow Faggots and Spiling
(Source: Environment Agency, 2010).
Furthermore, coir fibre products (Aquatic Planting), which support the growth and development of plants that physically secure the bank or shoreline (Ponds UK, 2011a), have been provided in some tree-lined areas to stabilise eroding banks. It means that the rolls can sperate and break the water force directly to the river bank in order to protect trees and further problems of the river such as blockage. Because the river is located in the low farm land and resistant rock area, verious rooted plants and other species can be established whether formed as rolls or not, including water-lilies, Canadian waterweed and water starwort, potamogeton natans, mosses, sparganium erectum, and Rorippa nasturtium-aquaticum agg (Haslam and Wolseley, 1981; Haslam, 1978; Reader’s Digest, 1985). The coir fibre rolls are 3 metres long and normally six plants per metre (18 plants per roll) (Ponds UK, 2011a), which is illustrated in Figure 8. At the last, fences which was damaged by water force might be replaced arround the river bank in order to keep animals (sources of pollution) far away from water body.
Figure 8. The Photo of Coir Fibre Rolls
(Source: Ponds UK, 2011a)
In conclusion, verious types of techniques have been ultilised for river restoration to control erosion and deposition and improve biodiversity, such as derdging, rip-rap, vortex weirs, willow spiling and faggots, coir fibre rolls and fencing. Dredging which is the first stage of the restoration, can maintain the similar width of river bank in most parts in order to keep velocity of river flow. Rip-rap and vortex weirs can change the slop of the river bed, the length of the water waves and stability of channel cross sections to reject bank erosion and lateral shift of planform. Other options are sustaianble methods not only to cove large areas of erosion quickly but also to install aquatic vegetation where none currently exists. As a result, the hydraulics of the river after restoration will be changed and also become more stable and sutiable for renatural river.
3.2 Economy Analysis of Project
IEPA (1998) stated that mechanical dredging is less expensive than hydraulic dredging in small project and usually cost from £5 to £20 per cubic yard including disposal. In the project, dredging is mainly utilized to move rock from the site to other site and it is hard to calculate the actual volume of dredging because of missing data; thus, the potential total cost of dredging cannot be assumed. Although there is no available cost of transporting rock from other regions, the cost effective and cheapest rock will be accepted. Ponds UK (2011b) mention that gabions (Rip-rap) are economic and well established tools for the construction of river walls – revetments for river restoration project.
As can be seen in Diagram 4, about 680 metres willow spiling will be placed in the river bank. It may cost £115 per metre which is cheaper than other complex and mixed spiling (River Restoration Centre, 2011a). According to calculation, the total cost of spiling is around £78,200. Moreover, 600 metres faggots, which are formed from local materials, will be installed in D (Diagram 4). Therfore, it is not easy to calculate the particular cost during restoration; on the other hand, the cost might be cheaper than spiling and also can be limited and saved in the operation phase. The cost of coir fibre rolls is approximately £130 per metre (River Restoration Centre, 2011b), and 720 metres of rolls will be planted in the tree-lined bank. Thus, the total cost of the rolls is £93,600 which is more expensive than other methods.
In summary, coir fibre rolls, willow spiling and derdging are more expensive in all techniques, but the cost Rip-rap and willow faggots is sitting in the middle level and others such as fences and weirs are the cost effective options for restoration project.
3.3 The Impacts of Restoration on Local Ecosystem
It is truely that every development or restraotion of river has negative and possitive impacts on environmental and social aspects whether in short or long terms. Restoration may take place in the sourrounding fields and can disturbance some normal activivties of animals and farmers, which may give occurison to pulic attention especially focused on the adverse impacts such as using private land during planning and construction processes. Therefore, the feedback of local farmers and related organizations should be collected before designing river restroation. During re-construction phase, mechancial derdging can cause adverse impacts on aquatic ecosystem as potential toxic chemicals and other contamination may be released from the bottom sediments into water body. Furthermore, it also increase water turbidity in short time period; to some extent, the change of water turbidity can affect aquatic species metabolism and interfere with spawning and water quality for ultilization especially in downstream regions (New World Encyclopedia, 2008).
Although restroation has some adverse impacts on aquatic ecosystem and water turbidity, it is not means that restoration has no possitive impacts on local environment and water body. On one hand, restoration of the River Severn in Caersws aims to control erosion and deposition to recreate more sustainable, stable and natural river in a long term. On the other hand, it also has some benefits for local ecosystem; for example, willow spiling and faggots and coir fiber rolls can bring biological, ecological and engineering concepts together by using natural materials and growing vegetation to stabilise the river structure and flow regime and maintain natural ecosystem in the site. Moreover, a number of native wetland plants used in these techniques can make a varied physcial adaptated conditions for invertebrates, fish and wildlife in the river. It means the biodiversity of local vegetation and habitats will increase after restoration and water quality will also be improved as potential chemical pollution from frams and erosion ot the river bank can be controlled, which results in safety ultilisation of water for fauna, flora, and human. In a word, river restoration may control erosion and deposition and improve the hydraulics of river and water quality and also create a susatianble and natural river with great visual view in long terms (Diagram 3 & 4).
The clean-up of the River Severn at Caersws illustrated why and how the river restored as natural source of water and how affect local ecosystem in the site. The more likely cause of restoration is the river erosion and deposition with continuous wavelength and water force, which will affect water turbidity and pollution in the local site and downstream. Moreover, the goals of restoration projects are often ecological in nature combined with analysis of hydrology and geomorphology of the channel. The variety types of techniques have been introduced during restoration processes, such as mechanical dredging, Rip-rap, vortex weirs, willow spiling and faggots, coir fibre rolls and fences. In a short term, restoration of the River Severn has some negative impacts on local aquatic ecosystem and water quality; for example, dredging can bring potential toxic condition and turbid water for aquatic species. However, the restoration is a sustainable activity of river management in Caersws to enhance aquatic and riparian habitat, improve water quality and hydraulics of flow regime, and control erosion and deposition of the river bank especially in long term periods. The width of river bank, wavelength, water force of damaging river bed and other flow regime will be changed after restoration programme. In conclusion, the restoration of River Severn in Caersws within suggested techniques is an effective method to redevelop the river become stable and natural in terms of social, economic and environmental aspects.