Bank protection and flood defense
River Mersey and its tributaries have a history of flooding and high tidal, with the first incident of flooding being recorded in 1767. Warrington town is ranked as the tenth based on the number of properties that are at risk of flooding. Reports from Environment Agency indicated that the most recent floods to occur in the location were in February 1990, and October and November 2000(Environment Agency, 2011). Embankment has been constructed in the area, however, due to climate change that leads to alteration of weather patterns, floodwall needs to be constructed in order to support embankment. Floodwalls and embankment have been used since time immemorial to control floods and protect properties. Despite the Environment Agency and Defra advocating for new strategies and alternative methods for mitigating and reducing floods, embankment and floodwalls remains the most appropriate structure. In England and Wales, maintenance of major embankments and floodwalls is done by Environment Agency. However, there are floodwalls and embankment constructed along the watercourse owned by private entities.
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It is important to understand that flood defense structures needs to be maintained and managed continuously in order to make sure that the communities in low-lying areas and their properties are protected. In extreme condition, embankment and floodwalls may be damaged or overtopped by the floodwater, thus requiring to be constructed under strict guidelines and designs in order to minimize the chance of being damaged or overtopped. Under certain circumstance, embankment and floodwalls may not be a viable option, thus alternative means of controlling floods needs to be applied(Morris, & Smith, 2004). These means include using advance flood warning, providing communities with flood protective measures and constructing houses and properties that more resilient to flood. This paper analysis in detail the problems affecting River Mersey and its distributor in relation to flood risks and how it can be mitigated using embankment and floodwalls. Specifically, the paper discuses different design styles applicable in the construction of embankment and floodwall that helps to reduce and mitigate flood in Warrington town, particularly along the River Mersey.
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Embankment is normally constructed using earth, but clay core may be increase strength as well as reduce seepage. In the other hand, floodwall may be constructed using a combination of different types of materials which include sheet-piling, concrete, masonry and bricks.
Construction of floodwall and embankment
There are several factors that determine the option of constructing embankment and floodwall. These factors are analyzed in detail in the table below.
|Environment||Ideal for a rural setting, but can be used in an urban environment if space permits||Ideal for urban situations where the defense can be designed to blend into the local infrastructure.|
|Under-seepage||Embankments may require a cutoff against seepage on permeable soils, but the longer seepage path often makes this less of an issues than with a wall||Walls are likely to require a cutoff against seepage when constructed on permeable soil|
|Space||Takes up a lot of space. For example, embankment which is 2.5 meters high requires footprint foundation of not less than 15 meters wide.||Appropriate in locations where space for the defense is limited.|
|Asset management||Generally requires periodic maintenance and inspection, including grass cutting, control of unwanted vegetation, repair of damage by cattle and dealing with infestation by burrowing animals||It requires minimal maintenance, but the design should address the need for inspection of critical elements to ensure continued functionality|
|Cost||Cost mainly depends on the source of fill material. Use of locally obtained material can significantly reduce costs and the overall environmental impact||Depends on the material used. For example, where cladding is involve it increases the cost. Access for construction and foundation conditions|
|Foundation||Embankment and floodwalls can both be complicated by the presences of permeable or weak foundation|
Main components of floodwall
Floodwalls have four main components; cutoff, reinforced concrete, cladding which is optional depending with the location of construction and caping. Cutoff is included in order to increase stability, thus reducing the chance of floodwall sliding(Defra and Environment Agency, 2007). Also, cutoff helps to minimize seepage under the wall. As a result, cutoff should be constructed deeper than the foundation as shown in the figure below.
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Main components of embankment
As indicated in the table above, embankment requires wider space for constructing footprint as compared to floodwall, therefore, it is important to consider this factor in the initial stages of construction. Embankment have two components; crest and clay core. The inclusion of clay core is meant to reduce cases of seepage through the embankment. It is normally constructed underneath the foundation of the embankment. Crest level of embankment is significantly important component that needs to be considered in three levels of embankment constructions such as design, construction and regular maintenance. Crest level determines the quality of defense against the severity of the flood that is foreseen to occur. Generally, the design of the crest level is determined by the return period of the flood. It is then that freeboard is addedto typical give the final crest level that needs to be constructed. The guidelines for assessing and designing freeboard is provided by Environment Agency(Environment Agency, 2011). Freeboard is described as the allowance margin to take care of uncertainties such as water levels, long-term corrosion of the embankment, erection of tolerances and wave consequence.
Considering the present conditions of embankment along the banks and tributaries of River Mersey, it will be appropriate to construct a sheet pile walls in order to improve flood defense in the area. Studies have shown that sheet pile walls serves as an effective means of enhancing flood defense due to the improved water-tightness. First, sheet pile act as cutoff. When natural deposition of soil occurs in the interlocks, water-tightness is enhanced. Additionally, the issues of water seepage may be eliminated by using operational sealing system in the sheet pile interlocks. Furthermore, sheet pile cutoff does not only minimize the leakage, but also enhances overall embankment stability(Defra and Environment Agency, 2007). This is achieved due to intersection slip circles present in the sheet pile. The intersection stabilizes the crest levels as well as inner shoulder. The outer shoulder is also enhanced by further lowering the seepage line. Sheet pile also permits for natural flow of water through creation of weep holes in the design stages. Regeneration of natural flow helps in supporting aquatic life as well as preserving the rivers banks. Also, natural flows helps to minimize the chances of outer shoulder drying up and creating cracks in the case of clay soil.
Since the materials for constructing sheet pile walls include the use of steel which is rust resistant, plant roots and animals cannot burrow through the sheet. This helps in eliminated backward erosion taking place outside the walls caused by preferential flow pathways, a condition described as piping(Morris, & Smith, 2004). By introducing sheet pile walls to help improve flood defense in River Mersey and its tributaries, it will support the creation of large design flood without taking up extra land. This is achieved by increasing the height of sheet pile above the crest level as well as driving it deeper. The modifications requires simple designing to fit the required changes. Also, sheet piles walls have the capacity to hold increased pressure of water due to bending strength. Increasing water pressure is distributed along the body of the embankment.
Advantages of using sheet pile to enhance effectiveness of embankment
- Sheet piles walls are cost effective and requires less time to construct. Considering that sheet piles that are used to construct walls is manufacture in factories under controlled conditions, it increases the speed of manufacturing as well as installation thus reducing the time. Also, in the incidences of unexpected flood, sheet piles walls may serve as the suitable option since it can be erected within a very short of preparation.
- With the improved technology, high frequency vibratory pile may be used in the areas known to experience earth vibration. This technology allows for pressing of sheet pile through the body of embankment without causing any disturbance to the embankment.
- Sheet pile permits for regulated flow of water from the inner shoulders to the outer shoulders of the embankment. When the flow of water is not needed in the outer shoulder, the interlocks of sheet pile can be sealed. In the other hand, when the soil in the outer shoulder dries up, weep holes can be created to slowly pass the water and the outer side damp.