Floods are a major concern in many parts of the world due to the many losses experienced in time of floods. Therefore, flood management techniques are very important in areas that are prone to frequent flooding. Flood management techniques are structures designed with the aim of containing floodwater in order to control floodplains. The designing of flood control techniques entails estimating the levels of water that causes floods and then measuring the required height that will create a working flood defense. Flood defenses are strong barriers that prevent water from flooding floodplains (Woods & Woods 2007, p.5).
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Floods generally occur when flowing surface water spills over the restraining banks into dry land. Floods are a natural occurrence in almost all river systems. Areas that are prone to flooding include those that are located downstream of dams and low lying areas. Flooding causes vast losses which include human and animal life, soil erosion, damages of properties, destruction of vegetation and other environmental damages. Also, areas that are affected by floods could also be prone to droughts. This can even cause further loss of human and animal lives. Flood waters are usually polluted with harmful bacteria resulting from sewage. This means that people affected by the floods are at greater risks of getting infective diseases (Proverbs, et al., 2011, p. 221).
However, floods also have positive impacts on ecosystems. One benefit from flooding includes more fresh water for domestic and irrigation usage. Other benefits include large deposits of minerals and nutrients into affected areas. Apart from these, floods also help to improve the condition of water ecosystems. However, they can be considered to be the most damaging compared to volcanic eruptions and earthquakes due to their frequencies. Therefore, strict measures must be in place to prevent the massive losses occurred during floods (Gruntfest & Handmer, 2001, p.12).
Methods used to control floods
Techniques that are applied in controlling floods involve the adaptation of the river environment and areas which are 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 areas that lie below the flood elevation and entirely on the floodway and river channel. The majority of techniques applied on floodplains lie far from the river but they are designed to reduce flood damage.
Levee/Floodwall around structures
This technique involves the construction of a levee/floodwall around structures that are located in floodplains. Levees can either be permanent or temporary. Construction of the levee usually requires the use of strong, artificial or natural material that can endure pressure from the floods (Hyndman & Hyndman, 2010, p.356). The principle of using levees and other barriers is to raise the height of structures located in floodplains which floodwater must then climb in order cause flooding. These structures are there to offer protection to structures. However, they put other structures at a high risk of flooding due to the then increased water retention in the floodplains. Furthermore, serious damage to protected structures can then occur when the levees are unable to hold back floodwater. This is because the pressure that the floods hit the structure is extremely high compared to when there is no barrier (Green, 2004, p.36).
The use of levees, dykes and floodwalls has a negative impact on the natural river processes (Harmancioglu, 1994, p.42). Ideally, developed water spills should form a natural channel which provides a way for the floods to flow. Therefore, levees would reduce the ability of the floodplains to process floodwaters. The inability of the floodwaters to flow freely within the floodplains would interfere with fish in their aquatic habitat. The river would respond by creating other channels in order to allow excess water to flow. The developed channels clear the backwater as well as reduce habitat complications (Harmancioglu, 1994, p.43).
Off-stream detention pond
The main aim of creating detention ponds in floodplains is to collect the floodwaters once the river has reached its maximum capacity. Detention ponds are constructed away from the floodway but must be within the floodplain catchment. When directing water into the detention ponds it will have an impact on reducing the amount of water a river carries. This will have a negative impact on natural river processes due to the reduction of water flowing downstream. Reduced amount of downstream flow could then cause a reduction in sediment transport, which may lead to the rising of the ground level of the channel 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 expected. This may lead to an increased development of river channels (Ali, 2002, p8.3).
Detention ponds also create new surroundings for the breeding and maturing of fish. However, the construction of detention channels must be done in a way that creates a connection between the pond and the mainstream so that the rivers natural environment can be maintained. Severe damage to the environment can occur when the detention ponds break down leading to flooding in the floodplains.
Techniques applied on floodway
The floodway is made of river banks and the active channel. In general, the floodway forms the part of land that is immediately next to the river. This piece of land allows flood waters to pass without raising the floods depth 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 consists of cutting the riverbank backwards to produce a gentler slope (Masoudian, 2009, p.14). It may involve replanting or resurfacing the bare bank using materials afterwards. Reducing the bank slope will have an impact on the increasing flood transportation at the channel level due to the increased bank width. This usually happens because reducing the bank slope increases the surface area of the bank channel, which then increases the volume of bank flow. However, if 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 then decreases the rates of soil and bank erosion. Vegetation along river banks may also trap sediments within flows, which may then lead to a buildup of banks, increasing the effectiveness of banks in controlling floods (Masoudian, 2009, p.16).
The fact that reducing the bank slopes then also reduces erosion along the river banks has a negative impact on natural river processes. Without erosion on the river banks there will be a reduced amount of sediments, a reduced level of wood debris and a reduced amount of channel migration. Therefore, the erosion occurring naturally along river banks is vital in maintaining an active balance within river systems. Aquatic habitats will also be affected; reducing bank slopes has a negative impact of clearing areas where fish may hibernate during the day in order to be safe from predators. Juvenile fish usually hide in undercut river banks making it an essential component of an aquatic habitat (Masoudian, 2009, p.17).
Reinforcing riverbanks
This technique involves adding supportive material to riverbanks in order to increase their stability and in resisting flood flows. The most commonly used reinforcement method involves planting natural vegetation as it acts as a stabilizer and increases the ability of riverbanks to control floods. Planting vegetation on riverbanks can be done through hydro seeding, which involves various methods to add a mixture of water, fertilizer and seeds into riverbanks. The planted seeds will then grow and form a vast network of root systems. The root system helps in holding the soil together, which then 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).
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Planting mature plants has the advantage of providing immediate protection against floods through flow reduction. The other method involves the use of plant mats, which are either natural or synthetic materials implanted with plant seeds and fertilizers. The mats are then spread on the riverbanks, and then the watering process follows to allow the seeds to germinate and support the following growth. Apart from irrigating the plant mats, continued instabilities 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 formation of strong riverbanks that can withstand pressure from flowing water. The other impact is the prevention of enhanced 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 increases 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 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. Like plants, gabions are meant to strengthen river banks, which boost’s the river banks ability to resist pressure from flowing water. However, for gabions to be effective plants should be added to them. Gabions also deteriorate in time, which means that replacements are required when they stop functioning properly. The use of gabions reduces the natural erosion that occurs along riverbanks. This then reduces the amount of sediments delivered to downstream habitats. In addition, flows that are deflected by the gabions may create new river channels (Mascarenhas, 2011, p.82).
Gabions also lead to an increase in water velocity, which has the disadvantage of reducing the amount of backwater that is essential for the survival of fish and other aquatic life. Well-designed gabions may serve as a reliable source for spawning gravel as well as habitats for 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 traps requires the proper assessment of sediment load within a river in times of flooding. Maintenance of sediment traps require continued mining of sediments after every serious flood event. Sediment mining reduces the amount of sediment deposits in the river channel, which in turn increases the channel volume as well as the flood transportation. Removing sediments could only have a short term impact of improved flood transportation because of the 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 the channel morphology. Any changes in the 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 then have further impacts on the gradient both upstream and downstream. An increase in the gradient in one location of the river may lead to the formation of a “v point” or otherwise known as a “nick point”. This leads to increased erosion in the channel, which extends to a point where the gradient is stable or where there is bedrock resistant to erosion (Raudkivi, 1993, p.35).
The removal of sediments affects aquatic habitats in several ways. When fine sediments are removed, there is a creation of a spawning habitat for a variety of fish species. On the other hand, removal of spawning gravel reduces the level of spawning habitats (Raudkivi, 1993, p.41).
Flow realignment
This technique of flood control involves the digging of new and deeper channels on the river bed but with a different position compared to the existing channels. When flow realignment is done to increase flood transportation, there is a reduction in frequency and severity of floods. Flow realignment creates different structures for the river flow. This interferes with natural river processes both upstream and downstream. The most significant impact of flow realignment is obviously on aquatic habitats. The use of heavy equipment to make flow realignments creates disturbances on the river bed; this alters existing aquatic habitats. Therefore, usage of flow realignment techniques needs thorough assessment of potential impacts on the aquatic ecosystem (Fleming, 2002, p.47).
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