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The Impact of Flood on Ecosystem and Effective Flood Protection Strategies
Flood is a natural hazard which causes diseases, loss to life and damages public systems and properties. Due to the dramatic effects of flood on people and infrastructure, the effects of flood are often viewed as negative. However this is not always true. Groundwater recharge, increased fish production, creation of wild life habitat, recharging wetlands, creation of flood plains, rejuvenating soil fertility are the benefits of flooding. However the benefits provided by floods of different magnitude to ecosystem services are still unclear.
The one-third of natural disaster is flood and it affects more people than any other type of disaster. Based on magnitude, frequency, duration and volume floods are characterized. These characteristics determine the effect flood on ecosystem and people. In order to create unique habitat and to support biological productivity and biodiversity, rivers need flood. Seasonal floods are not only beneficial for rivers but also influence biotic composition, nutrient transport and sediment distribution. However unpredictable floods destroy aquatic organisms. The specific effects of flood on ecosystem and its services are not clear. However the importance of flood for the protection of ecosystem functions is greatly recognised.
Drinking water, soil formation, primary production and areas for recreation or tourism are the services provided by the aquatic ecosystems. Small floods are beneficial to aquatic ecosystems. However extreme floods cause loss to these services.
Rivers exists in the depressions of earth surfaces. Hence to which anthropogenic changes and impacts are easily accumulate. As result river ecosystems are highly exposed to anthropogenic stress. The dynamics of river ecosystem depend up on the matter supplied from adjacent ecosystems. The flood formation process controls the distribution of mineral organic matter, nutrients, pollutants and genetic informations (ie such as sees, eggs of aquatic organisms etc) to the flood plain. The flood formation process depends up on climate, catchment geomorphology and its use, plant cover and character of river network.
When the land/water ecotone diversity reduces, the distribution of allochtonous organic matter undergoes changes. This adversely affects river productivity and biodiversity. It also adversely affects the self purification process of river. Extreme floods which cause damage to humans are often beneficial to natural populations. Floods are essential elements of natural cycle. Periodic flooding stimulates the production of aquatic macro invertebrates and result in increase of reproduction of fish species which use flood plain for spawning. Extreme floods reset the aquatic organisms and communities living in the flood plain.
Due to the diversities of riverine fishes, the effects of flooding on them are complex. Seasonal floods coordinate natural systems by giving environmental cues for migration and spawning. According to fish species, life stage and recovery period the effects of flooding on them are different. Juvenile fishes suffer heavy losses during extreme floods in high gradient rivers. However the effects of flooding on adult population are depend upon geomorphic changes. In high gradient rivers, the timing and duration of flood and environmental state control the impacts of flooding on fishes. Flooding increases the number and diversity of aquatic habitats available to fishes and it also intensifies the natural processes in the food web. Due to flood, the availability of terrestrial nutrients increases and as result there will be a rapid growth of zooplanktons and macro invertebrates. These organisms are consumed by higher predators and which in turn increases the production in food chain. According to Maher (1994), the diversity of fishes and their spawning are high in flood plain. Flood brings positive effects on fishes living in large low gradient rivers. However it has negative impacts on fishes living in large high gradient rivers. The mortality rates of benthic fishes living in large high gradient rivers are high during flood. However the benthic fishes living in large low gradient rivers survive floods.
The factors of clear water such as pH, oxygen and nutrient concentrations influence the primary production. Initially flood inhibit primary production in the water, however after flood the ecosystem starts process on mobilized nutrients. Small seasonal floods not only contribute nutrients to the aquatic ecosystems but also stimulate primary production. Increased primary production then supports the aquatic food web. However large floods either excessively stimulate primary production or change the composition of primary producer communities. Pearl et al (2016) reported that during flooding there will be an increase in phosphorous (P) and nitrogen (N) concentration and which in turn lead to increased primary production. The parameters such as nutrient supply, light penetration and flushing rates control primary production in the water after flood. The primary production brings benefits to ecosystem only up to certain extent, there after Harmful Algal Bloom happens (HABs).
Due to the over bank flow of river during floods, the soil erosion rate and sediment deposition rate between river and flood plain undergoes changes. Flood causes river bank collapse as well as landslide which poses threat to people living in hilly terrain. However the positive aspect of flood is that it improves soil formation process and recharges soil fertility. The 70% of soil erosion occurred during extreme flooding will be re-deposited in catchment area itself. This is an important process in maintaining coastal forests and wetlands. The positive effects of flooding are depend upon land use practices and bad land use practices may lead to catastrophic flooding. Compared to extreme flood, small flood influences soil formation process only to a small extent.
Flooding plays a key role for recharging underground water resources. Groundwater recharge depends upon duration of flood and flood plain land use pattern. When large area inundated more and more groundwater recharges take place. As a result flood mitigation strategies adversely affect groundwater recharging process. However, if groundwater level comes above the surface of the earth it may lead to extreme flooding.
Extreme flood causes the outbreak of weather related infectious disease. Flood not only damages sanitation systems but also lowers the quality of water treatment. As a result of flood, industrial wastes, agricultural wastes and sewages mix with drinking water. The two common diseases occur after floods are diarrhea and gastrointestinal disease. According to Kazama et al (2012) small flood causes wide spread occurrence of gastrointestinal disease than large flood. Joshi et al (2011) reported that sporadic flooding causes wide spread occurrence childhood diarrhea than seasonal flood. Since the diseases occur due to seasonal flood are predictable, they can be prevented by taking enough preparations. It has been reported that due to large flood cholera outbreak occurs in many regions. It has been discovered that the leptospirosis disease widely occurs after flood in the municipalities situated in the flood plains of Southeast Asian and South American countries. After extreme flood there is a great possibility for the spread of Malaria disease, because the stagnant water acts as breeding ground for mosquitoes. Boyce et al (2016) showed that Malaria spreading rate is 30% higher in areas surrounding recently flooded river. Due to the activities in the flood plain after two weeks of flooding, the outbreak of cryptosporidium disease reported among the children of Germany. After understanding how flood adversely affects human health, it is possible to prevent the spread of diseases by taking enough precautionary measures such as vaccination.
Flood causes the contaminants such as human waste, animal waste and herbicide and pathogens to easily mix with drinking water. Flood causes metals stored in the sediments to enter into the drinking water. If its concentration goes beyond permissible limit it adversely affects immunity system, liver, kidney and nervous system. Flood adversely affects food sources such as fishes, live stocks and crops. It increases water availability for agriculture and soil regeneration. As a result of flooding, fish habitats and food sources for fish also increase. Small flood is beneficial to native fishes. However extreme flood destroys crops, drown live stocks and reduces the fish density.
Flooding has a negative impact on tourism industry and recreation activities such as fishing, boating and swimming etc.
Man made flood control structures can prevent flood only up to a certain point and they have many environmental effects. Flood control structures cause the separation of main channel from flood plain, dessication and destruction of wetlands, alters hydrologic regime, reduce the geomorphic influence of flood and hide natural cues. It adversely affects the fishes depend upon flood plain for spawning and foraging. Due to the construction of flood control structures water flow reduces and it adversely affects aquatic organisms which depend on flowing water for spawning. It also reduces the sediment load reaching downstream areas. As a result sandbars and gravel bars, necessary for spawning, will not form. When a dam constructs, water reaching flood plain reduces and as result the nature of flood plain changes from sink to source of pollutants.
Flood control measures can be classified in to two (1) structural and (2) non-structural. Dams, dikes, diversions etc are the structural flood control measures. Watershed management, laws and regulation, zoning, efficient flood forecast-warning etc are the non-structural flood control measures
By studying past rain fall data, moisture content and snow cover, it is possible to forecast flood events effectively through mathematical modelling. By reducing soil erosion and energy of water flow vegetations can reduce the damages caused by flood. The geomorphic effects of flooding depend on the plant cover in that area. It is possible to protect the land and soil by preventing erosion and sediment transport through terracing and afforestation. Through infiltration enhancement and increasing water storage in the catchment area, it is possible to control flood. It is possible to reduce the intensity of damages caused by flood through flood risk mapping and by shifting people from flooded areas to safe places. The frequency of flood events can be understood by Frequency analysis. Vulnerability analysis will reveal the after effects of flooding. The damages caused by flooding can be understood by micro and macro scale flood loss estimation.
Flood control structures constructed decades ago may lead to pollution and river congestion. Uncontrolled urbanisation in the flood plain areas increases the intensity of this danger. It is possible to reduce the intensity of this danger by carrying out proper flood risk assessment. It is an illusion that the flood protection systems will give absolute protection from flood. However by combining structural and non-structural flood protection measures, it is possible to prevent the flood effectively.
References
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2, Zalewski M (2006). Flood pulses and river ecosystem robustness. Frontiers in Flood Research. IAHS Publi.305
3, Hickey, J T and Salas, J D (1995). Environmental Effects Of Extreme Floods. U.S.- Italy Research Workshop on the Hydrometeorology, Impacts, and Management of Extreme Floods Perugia (Italy)
4, Shah, M A R; Rahman, A and Chowdhury S H (2015). Challenges for achieving sustainable flood risk management. Journal of Flood Risk Management.
5, Kundzewicz, Z W (2002). Non-structural Flood Protection and Sustainability. Water International, Volume 27, Number 1, Pages 3–13