Various Techniques Used for Flood Control - Essay Example

? Various techniques used for flood control and their impact on the environment Introduction Floods are of paramount concern in many parts of the world due to the numerous losses incurred in time of floods. Therefore, flood management techniques are of the essence in areas prone to frequent flooding. Flood management techniques refer to structures designed with the sole aim of containing floodwaters in order to control flooding in plain land. Traditionally, designing of flood control techniques entailed estimating the levels of water that cause floods and then calculating the required elevation that will create a flood defense. The flood defenses were strong barriers that prevented water from flooding into the plain land (Woods & Woods 2007, p.5). Floods occur when flowing surface water spills over the confining banks into the dry land. Floods are a natural phenomenon, which occur in almost all river systems. Areas prone to flooding include those located downstream of dams and the low lying regions. Flooding causes immense losses, which include loss of human and animal life, soil erosion, damage on properties, destruction of vegetation and many environmental damages. In addition, areas affected by floods are highly susceptible to famines and prolonged droughts. This further causes loss of human and animal life due to starvation. Floodwaters are usually contaminated with harmful microorganisms derived from raw sewage. This puts people affected by the floods at greater risks of getting infectious diseases (Proverbs, et al., 2011, p. 221). Floods may contribute to some positive impacts on the ecosystem. One of the benefits of flooding includes offering fresh water for domestic use and irrigation. The other benefit includes massive deposition of minerals and nutrients into the affected areas. Apart from these benefits, floods also help in improving the condition of aquatic ecosystems. However, they can be regarded as the most damaging compared to volcanic eruptions and earthquakes. Therefore, stringent measures must be undertaken to prevent the massive losses incurred during floods. This paper discusses various techniques used in controlling floods and their environmental impacts (Gruntfest & Handmer, 2001, p.12). Methods used to control floods Techniques applied in controlling floods entail the modification of the river environment and areas located close to the river. Flood control techniques can be applied on the river channel, floodway or on the floodplain (Ghosh, 1997, p.55). Techniques applied in floodplains Floodplains are those regions that lie below the flood elevation and exclusively on the floodway and river channel. Majority of techniques applied on the floodplains lie far from the river, but are designed to reduce damage from floods. Levee around structures This technique entails the construction of a levee/floodwall around structures located in floodplains. Levees can either be permanent or temporally. Construction of the levee requires the use of strong, natural or artificial material that can withstand pressure from the floods (Hyndman & Hyndman, 2010, p.356). The essence of using levees and other barriers is to raise the height in structures located in floodplains which floodwater must rise to in order cause flooding. These structures offer protection to structures but put other structures into a high risk of flooding due to increased water retention in the floodplains. In addition, serious damage to protected structures can arise when the levees are unable to hold back the floods. This is because the pressure at which the floods hit the structure is extremely high compared to when there is no barrier (Green, 2004, p.36). The use of levees, floodwalls, and dykes has a negative impact on natural river processes (Harmancioglu, 1994, p.42). Ideally, water spills emerging from a river should form a natural channel which provides a way for the floods to flow. Therefore, levees reduce the ability of the floodplains to process floodwaters. In addition, the inability of the floodwaters to flow freely within the floodplains interferes with the fish habitat. The river will respond by creating other channels in order to allow for the flow of excess water. The developed channels clear the backwater as well as reduce habitat complexity (Harmancioglu, 1994, p.43). Off-stream detention pond The main aim of constructing detention ponds in floodplains is to collect floodwaters once the river is full to the maximum. Detention ponds are constructed away from the floodway but must be within the floodplains. Directing water into the detention ponds has an impact of reducing the amount of water a river carries. This has a negative impact on natural river processes due to the reduction of the amount of water flowing downstream. Reduced amount of downstream flow causes a reduction in sediment transport, which may lead to channel aggradation, and deposition of fine sediments. In addition, allowing water to flow into a detention pond may cause more water to flow into the pond than anticipated. This may lead to an increased formation of river channels (Ali, 2002, p8.3). Detention ponds create new grounds for breeding and maturation of fish. However, construction of detention channels must be done in a way that creates an interconnection between the pond and the mainstream in order to maintain the river’s natural environment. Severe damage on the environment can arise when the detention ponds break down leading to flooding in the floodplains. Techniques applied on floodway The floodway is made of the river banks and the active channel. In general, the floodway forms part of land that is adjacent to the river. This portion of land allows flood waters to pass without raising the depth of floods upstream. One characteristic of floodways is the presence of small banks either due to cuts made by previous floods or natural levees due to deposits from previous floodwater (Mambretti, 2011, p.66). Reducing the bank slope This technique entails cutting the riverbank backwards to produce a gentler slope (Masoudian, 2009, p.14). This technique may involve replanting or resurfacing the bare bank using sour materials. Reducing the bank slope has an impact of increasing flood conveyance at the channel level due to increased bank full width. This happens because reducing bank slope increases the surface area of the bank channel, which increases the volume of bank flow. In situations where the slope reduction is done through planting vegetation, it is likely that the bank stability will be increased. This has an effect of creating a natural containment, which reduces the velocity of water. A reduction in the velocity of water decreases the erosive force associated with water, which reduces soul erosion. Vegetation along riverbanks may trap sediments within flows, which may lead to a buildup of banks, increasing the effectiveness of the banks in controlling floods (Masoudian, 2009, p.16). The fact that the reduction of bank slope reduces erosion along river banks has a negative impact on natural river processes. Without erosion on the river banks, there will be a reduction in the delivery of sediments, reduced level of woody debris, and reduced channel migration. Therefore, erosions occurring naturally along river banks are vital in maintaining a dynamic balance within river systems. Considering fish habitats, reducing bank slope has a negative impact of clearing regions where fish hibernate during the day in order to be safe from predators. Juvenile fish usually hides in undercut river banks making it an essential component of fish habitat (Masoudian, 2009, p.17). Reinforcing riverbanks This technique involves adding supportive material to riverbanks in order to increase their stability in resisting flood flows. The most widely used reinforcement method involves the planting of natural vegetation to act as a stabilizer and increase the ability of riverbanks to control floods. Introducing vegetation on riverbanks can be done through hydro seeding, which involves the use of various methods to add a mixture of water, fertilizer, and seeds into riverbanks. The planted seeds later grow and form a vast network of the root system. The root system helps in holding the soil together, which strengthens the riverbanks. The other method used to introduce plant material on riverbanks is hand planting. In this technique, mature plants are inserted into riverbanks to continue with their growth (Stokes, et al., 2007, p.50). Planting mature plants has the advantage of providing an immediate protection against floods through flow reduction. The other method involves the use of plant mats, which are either natural or synthetic materials embedded with plant seeds and fertilizers. The mats are then spread on the riverbanks, and watering process follows later to allow the seeds to germinate and support subsequent growth. Apart from irrigating the plant mats, continued fluctuations in the river level can help in germination and supporting growth (Beek, et al., 2008, p.33). The main impact of introducing plants along river banks is the creation of strong riverbanks that can withstand pressure from flowing water. The other impact is the prevention of accelerated channel migration. Floods that may occur along banks with sufficient plant material may be less severe compared to those occurring in areas without plant stabilizers. The use of plant stabilizers provides a long-lasting solution to control of floods. This technique also offers an environmental friendly method of flood control, which is easy to maintain. Continued increase of vegetation along riverbanks increases channel roughness and reduces the water velocity. The use of plants also comes with the advantage of providing food for the diverse aquatic life in the protected rivers. Fish may also find a natural habitat in the vast root system generated by plants (Beek, et al., 2008, p.34). Gabions Gabions are constructed using wire mesh baskets that are filled with stones of two-six inches. Just like the use of plants, gabions are meant to strengthen the riverbanks, which in turn boosts the ability of the riverbanks to resist pressure from flowing water. However, for gabions to be effective, plants should be added to the gabions. Gabions also deteriorate with time, which requires replacement once they stop functioning properly. The use of gabions reduces the natural erosion process that occurs along riverbanks. This reduces the amount of sediment delivered to downstream habitat. In addition, flows that are deflected by the gabions may create new river channels (Mascarenhas, 2011, p.82). Gabions can also lead to an increase in the velocity of water, which has a disadvantage of reducing the amount of backwater essential for the survival of fish. Well-designed gabions may serve as a reliable source for spawning gravel. In addition, gabions create a suitable habitat for fish and other aquatic life. Techniques applied along the river channel Sediment trap/mining This technique involves excavating or dredging a depression on the riverbed. Construction of sediment requires proper assessment of sediment load within a liver in times of flooding. Maintenance sediment trap requires continued mining of sediment after every serious flood event. Sediment mining reduces the amount of sediment deposit in the river channel, which in turn increases the channel volume as well as the flood conveyance. Removing sediment may have a short term impact of improved flood conveyance because of continued deposition of sediments downstream. Therefore, continued removal of sediments is necessary to prevent incidences of flooding (Mascarenhas, 2011, p.105). Removal of sediments within the river channel affects natural river processes. One effect of sediment mining is the change of channel morphology. Changes in channel morphology affect the way the water flow and the river bed interact with each other. Removing sediments from the river channel changes the channel gradient. These changes have further impacts on the gradient upstream and downstream. An increase in the gradient in one location of the river may lead to the formation of a “nick point”. This leads to an increased erosion in the channel, which extends to a point where the gradient is stable or where there is bedrock that is resistant to erosion (Raudkivi, 1993, p.35). Removal of sediments affects fish habitat in several ways. When fine sediments are removed, there is a creation of spawning habitat for a variety of fish species. On the other hand, removal of spawning gravel reduces the level of spawning habitat (Raudkivi, 1993, p.41). Flow realignment This technique of flood control entails the digging of new, deeper channels on the river bed but with a different alignment compared to the existing channels. When flow realignment is done to increase flood conveyance, there is a reduction in frequency and severity of floods. Flow realignment creates alternative structures for the river flow. This interferes with natural river processes both in the upstream and downstream. The most significant impact of flow realignment is evident on fish habitat. The use of heavy equipment to make flow realignments creates disturbances on the river bed; this alters existing fish habitat. Therefore, application of flow realignment technique needs thorough assessment of potential impacts on the aquatic ecosystem (Fleming, 2002, p.47). References Ali, L. (2002). An Integrated Approach for the Improvement of Flood Control and Drainage Schemes in the Coastal Belt of Bangladesh. New York: Taylor & Francis,77-84. Beek, R., Nicoll, B., & Achim, A. (2008). Slope Stability and Erosion Control: Ecotechnological Solutions. London: Springer, 30-35. Fleming, G. (2002). Flood Risk Management: Learning to Live with Rivers. Chicago: Thomas Telford, 41-52. Gruntfest, E., & Handmer, J. (2001). Coping with Flash Floods. London: Springer, 3-15. Ghosh, S. N. (1997). Flood Control and Drainage Engineering: Second Edition. New York: Taylor & Francis, 40-57. Green, M. (2004). Rivers in Action. New York: Black Rabbit Books, 30-36. Harmancioglu, N. B. (1994). Coping with Floods. New York: Springer, 35-45. Stokes, A., Spanos, I., Norris, J., & Cammeraat, E. (2007). 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