River Engineering: Appraising the Options for River Bank Protection - Coursework Example

River Engineering: Appraising the Options for River Bank Protection by Lecture’s and Number Engineers have gone to great extents to design many products to offer solutions to problems affecting people in their daily products. In the recent times, and following increased soil and river bank erosion, there has been the development of mechanisms that can help people take care of river banks without investing much of finances or degrading the environment. The designed products and mechanism of controlling river bank erosion however come with benefits and challenges that one would want to look much at in the analysis of their effectiveness. Introduction River bank erosion is a common happening today as it has been witnessed in many shorelines where soil is constantly washed up leading to soil erosion. Given that the work of engineers is to develop products to offer solutions to problems facing the society, there has been the development of products and techniques that can actually strengthen the banks and mitigate soil erosion. In this paper, the discussion will be on the various ways of protecting river bank erosion and the benefits and/or challenges experienced when using them. The focus will thus be on the natural, vertical, and stone methods of stabilizing the banks. Natural methods refer to the various methods of protecting the slope while the vertical methods refer to the development of structures extending to the channels of the stream to help by redirecting the flow of water. The engineering concept here is that as soon as the water has been redirected, the hydraulic forces at the boundary of the channel will be reduced to a level that cannot cause erosion. 1. Natural Bank Protection Methods Natural methods of protecting the slope fall to three major categories including self-adjusting armours, rigid armours, and flexible mattresses. The natural methods operate by “armouring” the surface of the bank making it hard for soil erosion to occur (WETLANDS ENGINEERING & HAYES, 2004). The natural method is also widely known to address problems related to overbank drainage. The major requirement with this method is geotechnical stability to aid in the proper placement of the designed armour. This in many cases is known to have caused the replacement of adjacent structures, increased environmental damage and also comes in at relatively higher costs. It should also be noted that filling material will be used in the alignment of the existing channel. Self-Adjusting Armour of Stone This method calls for the placement of stones to four (4) major configurations including windrow, “riprap blanket”, trench fill, and longitudinal stone toe. 1.1 Riprap Blanket: This is a designed blanket of stones that form a “crowd” resistant to movement. The stones are shaped in a way that they have sharp edges at their points of intersection. Their edges are rounded with “cobbles” where the cobbles will be the one to be eroded by the flow of the channel (WETLANDS ENGINEERING & HAYES, 2004). With the limited movement, there is increased stability in the riprap revetment. The benefit of riprap protection is that any flood occurring during the service line will not be able to damage beyond the set nominal maintenance. The challenge is that the technology involved is high-tech calling for highly skilled engineers. It is also an expensive method. Below is a diagrammatical representation. 1.2 Trench fill: A trench fill has been developed from the creation of massive stone toe from standard stone armour. The trench fill will mainly be constructed in a trench close to the bank of the river in expectation that the river will “eat out” to the revetment, placing the stone above the slope of the bank (CLARK, & RICHARDS, 2002). The benefit from this method is that other materials other than stone may be used making it cost effective. The challenge is that it requires high expertise which may not be available to most people. Constant re-design and monitoring in this method also poses a threat. Belo is a diagrammatical representation 1.3 Windrow: A windrow revetment is basically a rock that has been placed landward on the floodplain surface and almost touching the line of the bank at a set location, beyond which additional erosion will not be allowed. The benefit with the use of a windrow is that it is easy to design and implement. It is also cheap to design a windrow. The challenge is that it has not been effective in handling strong water movements and stands the chance of being destroyed. 1.4 Longitudinal Stone Toe: This method is an improvement of window revetment. The method operates by placing the stone at the bed of the stream as opposed to placing it at the top bank. The protection criterion is that, following the stabilization of the bank’s toe, there will be failure at the upper bank and the process to continue until there is simultaneous attainment of a stable slope and a stable bank. The stability prevents erosion and may be boosted through the development of vegetation along the river bank. The method is cheap and requires fewer resources. It is also easy to develop and maintain. On the other hand, the method may disturb other structures in the river bank. The method may also lead to environmental damage. 2. Vertical Bank Protection Methods Vertical methods will basically provide a platform where structures will be extended to the stream channels with the aim of redirecting the flow in a way that the hydraulic forces are dramatically reduced to levels that cannot cause erosion. In other words, the vertical bank protection methods can be said to be more of “protective” than “curative”. Below are the various vertical methods. 2.1 Gabions: Gabions are made by the filling rectangular wired boxes with small stones. The operational concept is that, as water goes through the small stone, the speed is reduced and thus the hydraulic forces. With reduced hydraulic forces, there will be minimal erosion bank (CLARK, & RICHARDS, 2002). In high corrosion areas, PVC together with heavy galvanized steel will be used to make the gabions to prevent corrosion. The advantage with the use of gabions is that they have shown performances and there is no need for bank preparation. The major challenge however is that they are expensive to construct. Gabions are also labour intensive in that they need constant renovations and maintenance. 2.2 Mattress Gabions: These are gabions shaped into shallow forming “broad baskets”. A continuous blanket of protection is formed when the sides are tied side by side. Mattress gabions are usually placed on riverbank slopes that are smoothly graded, sometimes allowing the growth of vegetation. The advantage with this method is that it can be used over large areas along the bank. The challenge however comes in the construction as they are expensive to construct and maintain. Constructing mattress gabions in situations of high water inflows has also proved to be a great challenge that the engineering team may have to think about. 2.3 Articulated Concrete Mattresses: This is an erosion protection method mainly used in large rivers such as Mississippi. The mattresses are formed through the joining of concrete blocks using cables and steel rods. The advantage with the use of this erosion control method is that the mattresses are durable, strong and flexible. In addition, the installation of the mattresses takes short time and they also have a good service record. When properly installed, the riverbank can be fully covered. The disadvantage however is that when installed, there is usually a small opening left that permits the removal of bank material. The method has also shown great difficulties installing in banks with sharp corners. The method is also said to require an expensive plant for placement. This is shown diagrammatically here below. 2.4 Gravity Walls: These are special kind of retaining walls relying on their weight to stand. The walls are known to support “wedge” of soil allowing the calculation of the soil friction angle. The resultant effect is that the wall’s setback will increase reducing the sliding of the wedge bank (CLARK, & RICHARDS, 2002). The reduced sliding of the will end up reducing the pressure on the retaining wall and thus the wall will be able to withstand the hydraulic pressure emanating from the water movement. In operation, the gravity walls will tend to resist passive and active forces building pressure on incoming water forces. The benefit from this method is that it is a strong erosion control method that can be used in areas with high erosions. The challenge is that this is a highly technical method that requires heavy capital investments and high qualified labour. In addition, design and installation of gravity walls is considered and expensive and tedious venture. 2.5 Cantilever Walls: These are walls mainly used in situations where the amount of soils to be retained is relatively large. The engineering design team will in this case come up with counterfort wall that will boost stability. The counterfort walls will thus be made out of reinforced concrete with the use of an anchor in increasing stability. The retaining walls will thus require more careful construction. The benefit with the cantilever walls is that they are usually economical for 25 ft. and below bank (CLARK, & RICHARDS, 2002). These walls are also flexible in the sense that they can formed in the site or in the factory. The challenge however is that they require constant renovations and maintenance. In addition, their durability is also in question. 2.6 Sheet –Piling Walls: These types of walls are mainly used in tight spaces with soft soil. In the making of these walls, vinyl, wood, or sheet may be used and then driven to the ground for a part of up to two-thirds, leaving a third of the sheet’s area on the ground. When forced to the ground, the implementation team will also need a variety of earth anchors that they will mix with soil and then tied to the wall, in this case the bank of the river (INTERNATIONAL CONFERENCE ON INDUSTRIAL ENGINEERING AND ENGINEERING MANAGEMENT, & DOU, 2013). The walls are usually firm enough to withstand hydraulic pressures from the water splashes and movements. The benefit of this soil erosion control method is that it is highly effective in controlling soil erosion. Also the piling walls can also be easily installed with the use of minimal resources. The challenge however that is the use of this method is usually limited to the height. The higher the height, the reduced will be the flexibility of the piling walls and hence reduced efficiency. 3. Stone (Rigid Armour) Typically rigid armours operate through the development of materials that are resistant to erosion. With such materials, conforming to river bank irregularities becomes a challenge as there is minimal or no flexibility. The structures are usually placed in liquid form that later hardens. 3.1 Asphalt: This is a unique liquid material that, when distributed along the river bank tend to mix with high sand contents providing permeability that help in reducing hydrostatic pressure. The benefit is that they can stand high water velocities and are also known to have low hydraulic roughness. The challenge is that they tend to become less permeable with time. Also they tend to lose thickness from weathering. 3.2 Concrete: The method involves the placing of concrete in the conventional manner that ends up forming a fine aggregate concrete. The concrete should be strategically mounted to withstand any hydrostatic pressure. The advantage is that it is able to withstand huge hydrostatic pressure. Concretes are also immune to damage and vandalism. The challenge however is that bad weather conditions will cause delayed construction. Needed provisions may also raise the cost of the project. 3.3 Soil Cement: Compared to riprap, asphalt, and concrete soil cement is the cheapest method and is known to withstand relatively higher pressures. This comes from the mixture of soil and cement forming a solid firm structure that traps the hydraulic pressure from the speedy water. The advantage with this method is that it is cheap to acquire the needed materials. However, the challenge is that it is less durable compared with structures made from asphalt or concrete. Conclusion In conclusion it is clear that there are many types of riverbank protection. As part of stabilization measures, including river control structures and riprap. For better results the riverbank protection comes out as an important part of river stabilization in the protection of life and property. Reference List CLARK, M. J., & RICHARDS, K. J. (2002). Supporting complex decisions for sustainable river management in England and Wales. Aquatic Conservation: Marine and Freshwater Ecosystems. 12, 471-483. INTERNATIONAL CONFERENCE ON INDUSTRIAL ENGINEERING AND ENGINEERING MANAGEMENT, QI, E., SHEN, J., & DOU, R. (2013). The 19th International Conference on Industrial Engineering and Engineering Management assistive technology of industrial engineering. Berlin, Springer. http://dx.doi.org/10.1007/978-3-642-38391-5. WETLANDS ENGINEERING & RIVER RESTORATION CONFERENCE, & HAYES, D. F. (2004). Wetlands 2001 wetlands engineering and river restoration. Reston, Va, American Society of Civil Engineers. http://ascelibrary.aip.org/dbt/dbt.jsp?KEY=ASCECP&Volume=110&Issue=40581. Read More