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FLOODS TYPES AND THEIR CAUSES

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Chapter No. 01

By 𝗠𝗢𝗛𝗗 𝗦𝗔𝗗𝗔𝗤

𝗜𝗡𝗧𝗥𝗢𝗗𝗨𝗖𝗧𝗜𝗢𝗡 𝗔𝗕𝗢𝗨𝗧 𝗙𝗟𝗢𝗢𝗗

A flood is an overflow of water that submerges land that is usually dry.[1] In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrology and are of significant concern in agriculture, civil engineering and public health. Human changes to the environment often increase the intensity and frequency of flooding, for example land use changes such as deforestation and removal of wetlands, changes in waterway course or flood controls such as with levees, and larger environmental issues such as climate change and sea level rise. In particular climate change's increased rainfall and extreme weather events increases the severity of other causes for flooding, resulting in more intense floods and increased flood risk.

Flooding may occur as an overflow of water from water bodies, such as a river, lake, or ocean, in which the water overtops or breaks levees, resulting in some of that water escaping its usual boundaries or it may occur due to an accumulation of rainwater on saturated ground in an areal flood. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these changes in size are unlikely to be considered significant unless they flood property or drown domestic animals.

Floods can also occur in rivers when the flow rate exceeds the capacity of the river channel, particularly at bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural flood plains of rivers. While riverine flood damage can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked by rivers because the land is usually flat and fertile and because rivers provide easy travel and access to commerce and industry. Flooding can lead to secondary consequences in addition to damage to property, such as long-term displacement of residents and creating increased spread of waterborne diseases and vector-bourne disesases transmitted by mosquitos.

𝗧𝗬𝗣𝗘𝗦 𝗢𝗙 𝗙𝗟𝗢𝗢𝗗

Fluvial floods (river floods)

infographic fluvial floods

A fluvial, or river flood, occurs when the water level in a river, lake or stream rises and overflows onto the surrounding banks, shores and neighboring land. The water level rise could be due to excessive rain or snowmelt.

The damage from a river flood can be widespread as the overflow affects smaller rivers downstream, which can cause dams and dikes to break and swamp nearby areas..

To determine the probability of river flooding, models consider past precipitation, forecasted precipitation, current river levels, and well as soil and terrain conditions.

The severity of a river flood is determined by the duration and intensity (volume) of rainfall in the catchment area of the river. Other factors include soil water saturation due to previous rainfall, and the terrain surrounding the river system. In flatter areas, floodwater tends to rise more slowly and be shallower, and it often remains for days. In hilly or mountainous areas, floods can occur within minutes after a heavy rain, drain very quickly, and cause damage due to debris flow.

To determine the probability of river flooding, models consider past precipitation, forecasted precipitation, current river levels, and well as soil and terrain conditions.

Pluvial floods (flash floods and surface water)

infographic pluvial floods

A pluvial flood occurs when an extreme rainfall event creates a flood independent of an overflowing water body. A common misconception about flood is that you must be located near a body of water to be at risk. Yet pluvial flooding can happen in any location, urban or rural; even in areas with no water bodies in the vicinity. There are two common types of pluvial flooding:

Surface water floods occur when an urban drainage system is overwhelmed and water flows out into streets and nearby structures. It occurs gradually, which provides people time to move to safe locations, and the level of water is usually shallow (rarely more than 1 meter deep). It creates no immediate threat to lives but may cause significant economic damage.

Flash floods are characterized by an intense, high velocity torrent of water triggered by torrential rain falling within a short amount of time within the vicinity or on nearby elevated terrain. They can also occur via sudden release of water from an upstream levee or a dam. Flash floods are very dangerous and destructive not only because of the force of the water, but also the hurtling debris that is often swept up in the flow.

Coastal flood (storm surge)

infographic coastal flood

Coastal flooding is the inundation of land areas along the coast by seawater. Common causes of coastal flooding are intense windstorm events occurring at the same time as high tide (storm surge), and tsunamis.

Storm surge is created when high winds from a windstorm force water onshore — this is the leading cause of coastal flooding and often the greatest threat associated with a windstorm. The effects increase depending on the tide - windstorms that occur during high tide result in devastating storm surge floods. In this type of flood, water overwhelms low-lying land and often causes devastating loss of life and property.

The severity of a coastal flood is determined by several other factors, including the strength, size, speed, and direction of the windstorm. The onshore and offshore topography also plays an important role. To determine the probability and magnitude of a storm surge, coastal flood models consider this information in addition to data from historical storms that have affected the area.

𝗖𝗔𝗨𝗦𝗘𝗦 𝗢𝗙 𝗙𝗟𝗢𝗢𝗗

Flood is usually a result of natural causes. It may also be caused by man-made factors. It causes huge damage to life and property. There are many different causes leading to flooding. Some prominent among them include:

Massive Rainfall

Drainage systems and the effective infrastructure design aid during heavy rains. They help the drainage of excess water into reservoirs in an easy way. But in cases of heavy rainfall, the systems stop working. Thus flood is caused.

Overflowing of the Rivers

The people living along the river always have a risk of life from the overflowing of the Rivers. To prevent such a situation, a string of dams are built. However, if these dams are not managed properly, they may cause flooding and huge damage.

Collapsed Dams

In the event of huge rainfall, the dams built begin to collapse. Thus, causing the flood situation to become even critical for the people living around.

At the time of the high melting of snow due to heavy precipitation and other factors, the situation of flooding arises. Adopting sustainable measures for heavy precipitation can help in dealing with the flooding situation.

Deforestation

The cutting of trees in a reckless manner i.e. deforestation is also a major cause of man-made flooding. Trees prevent soil erosion and also the loss of crops. The vegetation is also enriched as a result of more and more trees. This also blocks the massive flow of rain, thus preventing flooding.

Climate change

The climatic changes caused due to human practices also add to the risk of flooding. Human beings cut trees in a large number, thus affecting the process of photosynthesis. Thus increased level of carbon-di-oxide in the atmosphere cause changes in climate posing threats of natural disasters like floods etc.

Emission of Greenhouse Gases

The burning of fossil fuels, the industrial influences, the pollution all is depleting the level of the ozone layer and increasing the level of greenhouse gases, becoming a major cause of man-made flooding.

Other Factors

The broken supply lines cause the outflow of water but lead to less damage. Also, there is water flow from the washing machines. Furthermore, overflow from dishwashers worsens the situation. Also, the lack of proper sewage systems adds to the destruction of this natural disaster.

Thus, a flood can be caused both due to natural causes as well as it can be a human-made flood.

Flood causes a huge loss of life and property. Waterborne diseases spread as a result of Floods causing health problems. Moreover, the destruction of roadways and infrastructure facilities, the disturbance of ecosystems, improper sewage systems all demand serious efforts of adopting sustainable measures.

Taking steps such as afforestation, decreased the emission of harmful gases into the atmosphere could help. Also, enrichment of vegetation, fewer deeds causing pollution and treatment of sewage could be useful ways to combat the situation.

𝗖𝗢𝗡𝗦𝗘𝗤𝗨𝗘𝗡𝗖𝗘𝗦 𝗢𝗙 𝗙𝗟𝗢𝗢𝗗𝗦

Floods impact on both individuals and communities, and have social, economic, and environmental consequences. The consequences of floods, both negative and positive, vary greatly depending on the location and extent of flooding, and the vulnerability and value of the natural and constructed environments they affect.

The consequences of floods, both negative and positive, vary greatly depending on their location, duration, depth and speed, as well as the vulnerability and value of the affected natural and constructed environments. Floods impact both individuals and communities, and have social, economic, and environmental consequences .

Floods have large social consequences for communities and individuals

As most people are well aware, the immediate impacts of flooding include loss of human life, damage to property, destruction of crops, loss of livestock, and deterioration of health conditions owing to waterborne diseases. As communication links and infrastructure such as power plants, roads and bridges are damaged and disrupted, some economic activities may come to a standstill, people are forced to leave their homes and normal life is disrupted.

Similarly, disruption to industry can lead to loss of livelihoods. Damage to infrastructure also causes long-term impacts, such as disruptions to supplies of clean water, wastewater treatment, electricity, transport, communication, education and health care. Loss of livelihoods, reduction in purchasing power and loss of land value in the floodplains can leave communities economically vulnerable.

Floods can also traumatise victims and their families for long periods of time. The loss of loved ones has deep impacts, especially on children. Displacement from one's home, loss of property and disruption to business and social affairs can cause continuing stress. For some people the psychological impacts can be long lasting.

In Australia floods are the most expensive natural disasters

In Australia, floods are the most expensive type of natural disaster with direct costs estimated over the period 1967-2005 averaging at $377 million per year (calculated in 2008 Australian dollars).

Until recently, the most costly year for floods in Australia was 1974, when floods affecting New South Wales, Victoria and Queensland resulted in a total cost of $2.9 billion. The Queensland Government estimates costs for the 2011 floods will exceed this figure for Queensland alone; with the damage to local government infrastructure estimated at $2 billion, and the total damage to public infrastructure across the state at between $5 and $6 billion.

Types of loss from floods. Modified from Disaster Loss Assessment Guidelines

Can the lost item be bought and sold for dollars? Direct loss:

Loss from contact with flood water

Indirect loss:

No contact – loss as a consequence of flood water

Yes – monetary (tangible) e.g. Buildings and contents, vehicles, livestock, crops, infrastructure e.g. Disruption to transport, loss of value added in commerce and business interruption, legal costs associated with lawsuits

No – non-monetary (intangible) e.g. Lives and injuries, loss of memorabilia, damage to cultural or heritage sites, ecological damage e.g. Stress and anxiety, disruption to living , loss of community, loss of cultural and environmental sites, ecosystem resource loss

Flooding in key agricultural production areas can lead to widespread damage to crops and fencing and loss of livestock. Crop losses through rain damage, waterlogged soils, and delays in harvesting are further intensified by transport problems due to flooded roads and damaged infrastructure. The flow-on effects of reduced agricultural production can often impact well outside the production area as food prices increase due to shortages in supply. On the other hand, flood events can result in long-term benefits to agricultural production by recharging water resource storages, especially in drier, inland areas, and by rejuvenating soil fertility by silt deposition.

Damage to public infrastructure affects a far greater proportion of the population than those whose homes or businesses are directly inundated by the flood. In particular, flood damage to roads, rail networks and key transport hubs, such as shipping ports, can have significant impacts on regional and national economies.

Short-term downturns in regional tourism are often experienced after a flooding event. While the impact on tourism infrastructure and the time needed to return to full operating capacity may be minimal, images of flood affected areas often lead to cancellations in bookings and a significant reduction in tourist numbers.

Flooding of urban areas can result in significant damage to private property, including homes and businesses. Losses occur due to damage to both the structure and contents of buildings. Insurance of the structure and its contents against flooding can reduce the impacts of floods on individuals or companies.

Floods have significant consequences for the environment

In many natural systems, floods play an important role in maintaining key ecosystem functions and biodiversity. They link the river with the land surrounding it, recharge groundwater systems, fill wetlands, increase the connectivity between aquatic habitats, and move both sediment and nutrients around the landscape, and into the marine environment. For many species, floods trigger breeding events, migration, and dispersal. These natural systems are resilient to the effects of all but the largest floods.

The environmental benefits of flooding can also help the economy through things such as increased fish production, recharge of groundwater resources, and maintenance of recreational environments.

Areas that have been highly modified by human activity tend to suffer more deleterious effects from flooding. Floods tend to further degrade already degraded systems. Removal of vegetation in and around rivers, increased channel size, dams, levee bank and catchment clearing all work to degrade the hill-slopes, rivers and floodplains, and increase the erosion and transfer of both sediment and nutrients.

While cycling of sediments and nutrients is essential to a healthy system, too much sediment and nutrient entering a waterway has negative impacts on downstream water quality. Other negative effects include loss of habitat, dispersal of weed species, the release of pollutants, lower fish production, loss of wetlands function, and loss of recreational areas.

Many of our coastal resources, including fish and other forms of marine production, are dependent on the nutrients supplied from the land during floods. The negative effects of floodwaters on coastal marine environments are mainly due to the introduction of excess sediment and nutrients, and pollutants such as chemicals, heavy metals and debris. These can degrade aquatic habitats, lower water quality, reduce coastal production, and contaminate coastal food resources.

𝗜𝗠𝗣𝗔𝗖𝗧 𝗢𝗙 𝗙𝗟𝗢𝗢𝗗𝗦 𝗢𝗡 𝗧𝗛𝗘 𝗘𝗡𝗩𝗜𝗥𝗢𝗡𝗠𝗘𝗡𝗧 𝗔𝗡𝗗 𝗛𝗨𝗠𝗔𝗡𝗦

Environmental Pollution

Floods are devastating to the environment and ecosystems in the affected region. They have a direct impact on humans, social life, the economy as well as the ecosystem. Floods have negative and positive consequences and vary based on area, depth, time and water speed. Here are the collective impacts of floods on the environment.

EROSION AND SEDIMENTATION

High-speed flood water causes riverbank erosion. Developed and urban areas become the most affected in this case. Sedimentation clogs rivers and reduces the storage capacity for wetlands and dams. Flood water carries a vast quantity of sediments leaving behind deposits after the water recedes. If the impact is extreme, floods reduce water quality, affect industrial use and the supply of water to humans.

DESTRUCTION OF PROPERTY

Floods lead to the destruction of property in developed areas where humans reside. That could be in urban areas and plains. The supply of essential services like drinking water, electricity, and transport become interrupted. In many properties, floods clog different households with water, and in worse conditions, floods sweep them away. If your items in the house are affected by floods, you can check royaldesign.com for better deals to replace the worn-out items. That is because you are likely to need to replace household items after the devastating consequences of flooding.

DISPERSION OF DEBRIS AND POLLUTANTS

You will find debris, nutrients, plastics, and different pollutants in floodwater. Contaminants such as pesticides, bacteria, and chemical reagents diffuse, quickly reducing water quality. The most common debris found includes stones, trees, and pieces of destroyed property that end up affecting the natural habitats of marine species that reside in rivers and seas.

INJURIES AND DEATH

Floods lead to injuries and death among humans and livestock. The number of those affected depends on the magnitude of the flood, and the total population living in those regions determine fatalities. Highly populated areas are likely to report more cases than low populated areas. Slow-rising riverbank floods allow people to relocate to safe areas and minimises the number of casualties. However, flash floods are likely to take people by surprise, increasing the number of fatalities.

𝗙𝗟𝗢𝗢𝗗 𝗦𝗔𝗙𝗘𝗧𝗬 𝗣𝗟𝗔𝗡𝗡𝗜𝗡𝗚

Aftermath of flooding in Colorado, 2013

Flood rescue in Nangarhar, Afghanistan in 2010, accompanied by the Afghan Air Force and USAF air advisors

In the United States, the National Weather Service gives out the advice "Turn Around, Don't Drown" for floods; that is, it recommends that people get out of the area of a flood, rather than trying to cross it. At the most basic level, the best defense against floods is to seek higher ground for high-value uses while balancing the foreseeable risks with the benefits of occupying flood hazard zones.:22–23 Critical community-safety facilities, such as hospitals, emergency-operations centers, and police, fire, and rescue services, should be built in areas least at risk of flooding. Structures, such as bridges, that must unavoidably be in flood hazard areas should be designed to withstand flooding. Areas most at risk for flooding could be put to valuable uses that could be abandoned temporarily as people retreat to safer areas when a flood is imminent.

Planning for flood safety involves many aspects of analysis and engineering, including:

observation of previous and present flood heights and inundated areas,

statistical, hydrologic, and hydraulic model analyses,

mapping inundated areas and flood heights for future flood scenarios,

long-term land use planning and regulation,

engineering design and construction of structures to control or withstand flooding,

intermediate-term monitoring, forecasting, and emergency-response planning, and

short-term monitoring, warning, and response operations.

Each topic presents distinct yet related questions with varying scope and scale in time, space, and the people involved. Attempts to understand and manage the mechanisms at work in floodplains have been made for at least six millennia.

In the United States, the Association of State Floodplain Managers works to promote education, policies, and activities that mitigate current and future losses, costs, and human suffering caused by flooding and to protect the natural and beneficial functions of floodplains – all without causing adverse impacts. A portfolio of best practice examples for disaster mitigation in the United States is available from the Federal Emergency Management Agency.

Control

Main article: Flood control

In many countries around the world, waterways prone to floods are often carefully managed. Defenses such as detention basins, levees,[34] bunds, reservoirs, and weirs are used to prevent waterways from overflowing their banks. When these defenses fail, emergency measures such as sandbags or portable inflatable tubes are often used to try to stem flooding. Coastal flooding has been addressed in portions of Europe and the Americas with coastal defenses, such as sea walls, beach nourishment, and barrier islands.

In the riparian zone near rivers and streams, erosion control measures can be taken to try to slow down or reverse the natural forces that cause many waterways to meander over long periods of time. Flood controls, such as dams, can be built and maintained over time to try to reduce the occurrence and severity of floods as well. In the United States, the U.S. Army Corps of Engineers maintains a network of such flood control dams.

In areas prone to urban flooding, one solution is the repair and expansion of man-made sewer systems and stormwater infrastructure. Another strategy is to reduce impervious surfaces in streets, parking lots and buildings through natural drainage channels, porous paving, and wetlands (collectively known as green infrastructure or sustainable urban drainage systems (SUDS)). Areas identified as flood-prone can be converted into parks and playgrounds that can tolerate occasional flooding. Ordinances can be adopted to require developers to retain stormwater on site and require buildings to be elevated, protected by floodwalls and levees, or designed to withstand temporary inundation. Property owners can also invest in solutions themselves, such as re-landscaping their property to take the flow of water away from their building and installing rain barrels, sump pumps, and check valves.

In some areas, the presence of certain species (such as beavers) can be beneficial for flood control reasons. Beavers build and maintain beaver dams which will reduce the height of flood waves moving down the river (during periods of heavy rains), and will reduce or eliminate damage to human structures,at the cost of minor flooding near the dams (often on farmland). Besides this, they also boost wildlife populations and filter pollutants (manure, fertilisers, slurry).UK environment minister Rebecca Pow stated that in the future the beavers could be considered a "public good" and landowners would be paid to have them on their land.

𝗔𝗡𝗔𝗟𝗬𝗦𝗜𝗦 𝗢𝗙 𝗙𝗟𝗢𝗢𝗗 𝗜𝗡𝗙𝗢𝗥𝗠𝗔𝗧𝗜𝗢𝗡

A series of annual maximum flow rates in a stream reach can be analyzed statistically to estimate the 100-year flood and floods of other recurrence intervals there. Similar estimates from many sites in a hydrologically similar region can be related to measurable characteristics of each drainage basin to allow indirect estimation of flood recurrence intervals for stream reaches without sufficient data for direct analysis.

Physical process models of channel reaches are generally well understood and will calculate the depth and area of inundation for given channel conditions and a specified flow rate, such as for use in floodplain mapping and flood insurance. Conversely, given the observed inundation area of a recent flood and the channel conditions, a model can calculate the flow rate. Applied to various potential channel configurations and flow rates, a reach model can contribute to selecting an optimum design for a modified channel. Various reach models are available as of 2015, either 1D models (flood levels measured in the channel) or 2D models (variable flood depths measured across the extent of a floodplain). HEC-RAS,[39] the Hydraulic Engineering Center model, is among the most popular software, if only because it is available free of charge. Other models such as TUFLOW[40] combine 1D and 2D components to derive flood depths across both river channels and the entire floodplain.

Physical process models of complete drainage basins are even more complex. Although many processes are well understood at a point or for a small area, others are poorly understood at all scales, and process interactions under normal or extreme climatic conditions may be unknown. Basin models typically combine land-surface process components (to estimate how much rainfall or snowmelt reaches a channel) with a series of reach models. For example, a basin model can calculate the runoff hydrograph that might result from a 100-year storm, although the recurrence interval of a storm is rarely equal to that of the associated flood. Basin models are commonly used in flood forecasting and warning, as well as in analysis of the effects of land use change and climate change.

Flood forecasting

Main articles: Flood forecasting and flood warning

Anticipating floods before they occur allows for precautions to be taken and people to be warned so that they can be prepared in advance for flooding conditions. For example, farmers can remove animals from low-lying areas and utility services can put in place emergency provisions to re-route services if needed. Emergency services can also make provisions to have enough resources available ahead of time to respond to emergencies as they occur. People can evacuate areas to be flooded.

In order to make the most accurate flood forecasts for waterways, it is best to have a long time-series of historical data that relates stream flows to measured past rainfall events. Coupling this historical information with real-time knowledge about volumetric capacity in catchment areas, such as spare capacity in reservoirs, ground-water levels, and the degree of saturation of area aquifers is also needed in order to make the most accurate flood forecasts.

Radar estimates of rainfall and general weather forecasting techniques are also important components of good flood forecasting. In areas where good quality data is available, the intensity and height of a flood can be predicted with fairly good accuracy and plenty of lead time. The output of a flood forecast is typically a maximum expected water level and the likely time of its arrival at key locations along a waterway, and it also may allow for the computation of the likely statistical return period of a flood. In many developed countries, urban areas at risk of flooding are protected against a 100-year flood – that is a flood that has a probability of around 63% of occurring in any 100-year period of time.

According to the U.S. National Weather Service (NWS) Northeast River Forecast Center (RFC) in Taunton, Massachusetts, a rule of thumb for flood forecasting in urban areas is that it takes at least 1 inch (25 mm) of rainfall in around an hour's time in order to start significant ponding of water on impermeable surfaces. Many NWS RFCs routinely issue Flash Flood Guidance and Headwater Guidance, which indicate the general amount of rainfall that would need to fall in a short period of time in order to cause flash flooding or flooding on larger water basins.

In the United States, an integrated approach to real-time hydrologic computer modelling utilizes observed data from the U.S. Geological Survey (USGS), various cooperative observing networks, various automated weather sensors, the NOAA National Operational Hydrologic Remote Sensing Center (NOHRSC), various hydroelectric companies, etc. combined with quantitative precipitation forecasts (QPF) of expected rainfall and/or snow melt to generate daily or as-needed hydrologic forecasts.The NWS also cooperates with Environment Canada on hydrologic forecasts that affect both the US and Canada, like in the area of the Saint Lawrence Seaway.

The Global Flood Monitoring System, "GFMS", a computer tool which maps flood conditions worldwide, is available online. Users anywhere in the world can use GFMS to determine when floods may occur in their area. GFMS uses precipitation data from NASA's Earth observing satellites and the Global Precipitation Measurement satellite, "GPM". Rainfall data from GPM is combined with a land surface model that incorporates vegetation cover, soil type, and terrain to determine how much water is soaking into the ground, and how much water is flowing into streamflow.

Users can view statistics for rainfall, streamflow, water depth, and flooding every 3 hours, at each 12-kilometer gridpoint on a global map. Forecasts for these parameters are 5 days into the future. Users can zoom in to see inundation maps (areas estimated to be covered with water) in 1-kilometer resolution.

𝗦𝗢𝗖𝗜𝗘𝗧𝗬 𝗔𝗡𝗗 𝗖𝗨𝗟𝗧𝗨𝗥𝗘

Myths and religion

Flood myths (great, civilization-destroying floods) are widespread in many cultures.

Flood events in the form of divine retribution have also been described in religious texts. As a prime example, the Genesis flood narrative plays a prominent role in Judaism, Christianity and Islam.

𝗘𝗧𝗬𝗠𝗢𝗟𝗢𝗚𝗬

The word "flood" comes from the Old English flod, a word common to Germanic languages (compare German Flut, Dutch vloed from the same root as is seen in flow, float; also compare with Latin fluctus, flumen).