Flood Alleviation in Rivers Using Embankments and Levees - Essay Example

Flood Alleviation in Rivers Using Embankments and Levees Course Name and Code Institution Name Faculty Name Students Name Students ID Instructors Name December 31, 2008 Flooding is a common phenomenal especially in the current climate change. Either excessive rainfall or ocean flooding can cause floods. Rainfall may exceed the normal river flow and exceeds the carrying capacity of the river dumping excessive water to the neighbouring lands. An example of a levee is the New Orleans levees that do double work: keeping at bay the Lake Pontchartrain and Mississippi River on the other side. Another place that a levee has played an important role is in Netherlands. The aim of this essay is to discuss the way that levee is used to alleviate flooding. Moreover, the paper expands on the idea of designing the levee. It begins into consideration field investigation, the designing of the slope of the levee and its construction. The paper further pinpoints the causes that makes the levee to fail and brings into consideration different types of reinforcements that are used to improve the functionality of the levee. A levee is an artificial wall that is constructed as a boundary or as a means of defence. A common usage of levee is in preventing of floods into the adjacent lowland. Levee in most cases can be built for emergency or permanent earthworks. The emergency levee may be constructed due to flood emergency and the common materials that are used are the sandbags. When an additional levee is added on top of an existing levee to increase its height is called a cradge. Generally, levee is an embankment whose aim is to furnish flood protection for any seasonal high water (Petroski 2006). There are two types of levees and are named according to location and use. In the case of location, it is either termed has agricultural or urban levees. Urban levees are used to reduce flooding and providing protection for the communities. Agricultural levees are used to protect flooding from lands that are used for cultivation or agricultural purposes. Some examples of levees that are grouped in terms of use are tributary and mainline, ring, setback, sublevees and spur levees. The uses of the levees determine the name that it is referred to. Field Investigations Before the construction of the levees, field investigations are sanctioned and are carried out in two stages: preliminary and design stages. In most circumstances, the preliminary stage is not extensive because it is the foundation of general information concerning the project. Preliminary stage brings together the general geological reconnaissance, which is limited to simple soil tests and subsurface exploration. However, in the stage of design, comprehensive exploration, extensive geological reconnaissance, test pits, geophysical studies and borings are carried out. Additionally, design stage brings together various tests, which includes groundwater observations, vane share tests and field pumping tests. Slope Design and settlement Slopes are the backbone of any levees and determine the stability of the levees. The height of the levees, the adequacy of materials for embankment and the foundation conditions determines the aspect nature of analysis of the embankment design. For example, low levees, levees built of good material and a good foundation requires minimal stability analysis. Thus, the nature of the slope to be used is determined by the maintenance of the levee, type and ease of the construction, slope protection criteria and seepage. The final grade of the levee was initially accounted by the freeboard from geotechnical, hydraulic, operation, construction and maintenance uncertainties. The freeboard has been replaced by the risk-based analysis and the deterministic analysis. The risk-based analysis is used to establish nominal protection and accounts for uncertainties that are caused by hydraulic. On the other hand, the deterministic analysis is used to analyse the physical properties of the embankment material and foundation resulting in accounting of shrinkage, settlement, geological subsidence, and cracking and construction tolerances. Another important feature in the levee design is the crown width. The size of the levee crown depends on the requirements of the roadway and any future emergency requirements. However, minimum widths of 10 to 12 feet are used. Fills plays an important role in determining the steepness of levee slopes. Water content control, the type of compaction and the fill material stipulates the steepness of the levee slopes and if the foundation is strong. However, when the foundation is weak and compressible, the use of high quality fill is not logical and in such scenario, the best way is the use of semi-compacted or un-compacted fill. The semi-compacted fill is used in those areas that have fine-grained borrow soils or wet soil resulting in low sleeves with flatter levee slopes. Un-compacted fill is used in those areas that are rainy, borrow is wet and there is a high content of organic materials. The hydraulic fill is used in those areas that have pervious sands with discharging pipes. In shaping the embankment slopes, front-end loaders and rubber-tired dozers are used to move the sand. In this case, the levee requires a large footprint because of the pervious and density of the levee and the levee is susceptible to liquefaction of soil. Moreover, the hydraulic fill is susceptible on erosion due to overtopping, thus, the hydraulic fill is used in stability beams, seepage perms and pit fills. During the planning of the loading condition, there are various analyses that should be carried. Different flooding provides different ways that aids in determining the nature of slope of levee designed. The common analyses that are carried out to determine the load of the levee includes end of construction, earthquake, fully developed phreatic surface and sudden drawdown. Thus, the loadings that are caused by the four factors will determine the susceptible to failure of any construction of the levee (Easson and Hossain 2006). The sloppiness of the levee plays an important role in its stability. In the construction of the levee, there are methods that are used to improve the embankment stability. Common ways to improve stability of the embankments are flattening the embankment slopes and stability berms. The flattening plays a crucial role on shallow foundation levees since it increases its stability. Moreover, it reduces the gravitational forces and increases the resistance to sliding. The berms are more effective when compared to flattening embankment but provide the same effect. The concentrate adds additional weight at the region that is required hence, solves problems that are associated with deep-seated foundation failures and shallow foundation (Terzaghi and Brazelton1996). Levee Construction Various ways can be used to classify the constructions of the levee. One of the methods is the use of construction methods that brings into consideration compact, semi-compact and un-compact in bringing together the semi-pervious and impervious materials. The materials are classed has impervious or semi-pervious because of its maximum density at a certain water content after compaction. Pervious levee fill consists of gravel and sand that are filled by either hydraulic fill methods or using normal earthmoving equipments. Vibrators can be used in compaction in those areas that requires high degree of compaction. In the scenario that underwater placement is needed, filling can be accomplished by pervious filling dragline, end dumping and hydraulic means. After the analysis and design of the levee is accomplished, the foundation and treatment of the levee is commenced. Generally, the preparation of foundation includes clearing and grubbing with some stripping. Other steps include grubbing, clearing, disposal of products, stripping and the final preparation. Clearing is the first step, which removes any matter that is at the ground surface. Grubbing is the removal of objection matter at the foundation area of the levee. Stripping is the removal of the organic topsoil and low growing vegetation. The disposal of the debris is dumping of materials that were obtained during clearing, stripping and grubbing through environmental friendly way depending on the nature of the surrounding (American Society of Civil Engineers 1999). Trench exploration is used to analyse the nature of the region that levee will be constructed to check whether there are underground features. Other stages include dewatering of the levee foundation and the final foundation preparation. There are areas that the foundation cannot support the levee embankment due to insufficient shear strength, thus it requires certain treatment of the foundation. Foundations that requires special treatment includes sensitive clay, very soft clay, natural organic deposits, loose sands and deposited debris by man. There are ways that can be used to solve these problems which includes excavation, replacement, and displacement by end dumping (Green and Dunniclif 1993). Excavation and replacement is removal of the weak or excessively compressible foundation and backfill the excavated region with the suitable compacted material. This method is suitable in a region that the unsuitable material and the project is economically feasible. Displacement by end dumping makes foundation to displace the unsuitable material. A common way of levee construction is through stages or time intervals. This method is employed in scenarios that the foundation material cannot support the entire weight of the proposed embankment. Thus, construction through stages ensures that time is given for the foundation material to drain. Causes of Levee Failure The cases of failures of levee can be grouped into four cases: erosion, overflow, instability, embankment, and the failure by the river structures. Overflow occurs when the levee that has been constructed cannot hold a large amount of water. It may be caused by erosion due to eating of the flowing water on the sides of the levee and the banks and been deposited on the base of the levee. The deposits will greatly reduce the carrying capacity of the levee. Thus, the crown edge and landslide slope is eroded during the overflow because they are constructed by soil material. Instability of the embankment may cause the failure of the levee. Poor quality of the embankment materials, poor foundation and poor piping of the embankment causes the levee to collapse (US Army Corps of Engineers 2004). Generally, the common causes of failure in levee during flooding includes inadequate subsurface exploration, design errors, laboratory testing, improper construction, specification mistakes and poor quality of materials that were used. Moreover, levee failure may also be related to seepage and may be grouped into seepage and sliding. Thus, the seepage may be termed has under seepage and seepage through embankment which points to piping. Types of Levee Reinforcements There are special reinforcements that are used to increase the strength of the levee. Some of these reinforcements include T-walls, I-walls and aprons. The I-walls are constructed into the ground at the place where the levee is been built. The T-walls operates the same wall but the main difference between the I-wall and T-wall is that the I-wall has a thinner base compared to the wider base associated with the levee. The wider base makes the T-wall associated levee to be stronger and stable compared to the I-wall. Aprons are used to reduce the effect of erosion. Aprons are coverings that are used to cover the levees. They can either be natural e.g. pile of rocks or artificial e.g. concrete. It plays an important role when overtopping occurs. Levees play an important role in managing and maintaining the effects of floods. Before the construction of the levee, an important aspect is investigating the environment in which the levee should be constructed. Field investigation ensures that the nature of the environment is known and the types of materials that are required are put into place. Slope designing is an important aspect in determining the stability and susceptible nature of the levee that will be constructed. The nature of the slope and the height of the levee are determined at this stage. The actual construction of the levee brings into consideration many factors. Levee construction from the initial clearing of the levee location to the way that the materials that will be used will be compacted. The materials and equipments that will be used play an important role in determining the success or vulnerability of the levee. Poor quality of materials and lack of the right professional requirements is the likely cause that may make the levee to fail. There are three important types of reinforcements that can be used in ensuring that the built levee is appropriate. I-wall, apron and T-wall are the core ways that determines the backbone of the constructed levee. Each comes with unique features, which ensures that the constructed levee is successful. Bibliography American Society of Civil Engineers. 1999. Instrumentation of Embankment Dams and Levees. New York: ASCE Publications. Green, G. & Dunniclif, J. 1993. Geotechnical Instrumentation for Monitoring Field Performance. New York: Wiley-IEEE Publishers. Easson, G. & Hossain, A. 2006, Detection of Levee Slides Using Commercially Available Remotely Sensed Data. Environmental and Engineering Geoscience, 12, pp. 235-246. Petroski, H., 2006, Levees and Other Raised Ground. American Scientist, 94, pp. 7-11. Terzaghi, K., & Brazelton, P. 1996. Soil Mechanics in Engineering Practice. New York: Wiley-IEEE Publishers. US Army Corps of Engineers. 2004. Design and Constructions of Levees. New York: Lightning Sources Inc. Read More