Flood

Calle Arquitecto Morell de Alicante (Spain) during the floods of 1997.

Great flooding in Alginet, 1957

A flood is the occupation by water of areas that are usually free of it, due to the overflow of rivers, torrents or boulevards, due to torrential rains, melting ice, due to rising tides due to above the usual level, due to tsunamis, hurricanes, among others.

Fluvial floods are natural processes that have occurred periodically and have been the cause of the formation of plains in river valleys, fertile lands, meadows and riverbanks, where agriculture has normally been developed.

In coastal areas, the ravages of the sea have served to model the coasts and create marshy areas such as lagoons and lagoons that, after their human occupation, have become vulnerable areas.

Flooding can occur as an overflow of water from bodies of water, such as a river, lake, or ocean, in which water overflows or breaks levees causing some of that water to escape its usual limits, or it can occur due to an accumulation of rainwater in saturated soil in a shallow flood. While the size of a lake or other body of water will vary with seasonal changes in precipitation and snow melt, these size changes are unlikely to be considered significant unless they flood property or drown domestic animals..

Floods can also occur in rivers when the flow exceeds the capacity of the riverbed particularly in bends or meanders in the waterway. Floods often cause damage to homes and businesses if they are in the natural floodplains of rivers. Although damage from river flooding can be eliminated by moving away from rivers and other bodies of water, people have traditionally lived and worked along rivers because the land is generally flat and fertile and because rivers make travel and access easier. to commerce and industry. Flooding can have secondary consequences in addition to property damage, such as long-term displacement of residents and the creation of further spread of waterborne diseases and mosquitoes.

Causes

Local or regional causes

• In the Mediterranean area there is the phenomenon of the cold drop, produced by the differential warming between the lands and the waters at the end of the summer and early autumn (usually during the month of October), when the temperature on the lands begins to be quite cold while the seas are still at a fairly high temperature. This difference is noted especially in the eastern coasts of Spain and in other Mediterranean regions. The emblematic case of this synoptic situation is represented by the Great Lane of Valencia in October 1957.

• In East Asia, the main cause of river growth is torrential rains caused by the monsoon, often associated with typhoons. They are presented in summer and affect large areas including the Gulf of Bengala, the most precipitated area of the globe.

• The hurricanes are a Caribbean version of the typhoons, which temporarily roast the region of the Gulf of Mexico and the Antilles causing floods by the waves, up to eight meters, associated with the strong winds, and by the intense rains motivated by the sharp decline of barometric pressure. Tropical storms also often cause very strong rain.

• In temperate countries, sudden rises of temperature can cause growth in the rivers due to the rapid melting of snows, this is especially given in spring, when the ice is greater, or after strong snowfalls in unusual peaks, that after the cold wave melts, causing floods. In areas of sub-arid or arid climate itself, light or immediate floods are often caused by very intense rains for a very short period of time.

• The tsunamis or tsunamis as a possible cause of flooding, as the submarine earthquake causes a series of waves that translate into giant waves of devastating effect on the affected coasts. These catastrophes are usually given in the Pacific and Indian area, with greater seismic activity.

• Floods are not alien to land occupation. The flow of rivers is usually very variable over the years. Indeed, hydrology establishes for rivers a range of maximum flows associated with the return time. Generally the local populations, when they have long been settled in the place, have knowledge of the areas occupied by the avenues of the river or ravine, and thus respect the space of this and its channel, avoiding the occupation of it and its flood zone to avoid the flooding of its populated centers.

Causes related to fluvial dynamics

There is a fairly close correspondence in most rivers, in regards to the gauging record in a river basin and the rainfall records obtained in that basin. However, some ideas related to the comparison that can be established between rainfall and flow must be taken into account:

• The river regime will be much more irregular in the basins with dry climates. This means that, if we compare the river regime of the Miño river (in a region with rainy climate) with that of the Jucar, whose basin has a much more dry climate, the growths and floods in the case of the Jucar will always be much more violent but, instead, the flow of the Miño will be much more stable (regular or constant, without large fluctuations) and of course with a higher flow rate of 2 km.
• On the contrary, in the rainy climate regions, the river regime will show less ups and a relatively abundant and less "applied" flow to the rainy fluctuations.
• The regularity of the flow is greater in the rivers of basin very extensive than in those with a low extension basin, although there may be exceptions in the case of very humid climate in low surface basins.
• The river regime will follow the pluviometric, with a certain gap in which multiple factors will be involved (watershed extension, relief and slope, vegetation, etc.).

Flood Defenses

Construction of a coastal defense in Colombia.

Flood in Tel Aviv, Israel, 1969

Since the beginning of the Neolithic, when sedentarization began and, therefore, occupation of flat coastal areas or in river valleys, man has faced the challenge of coping with floods. In Egypt and Mesopotamia, important river defenses such as dikes, canals to divert water and improvement of channels in urban environments have already been built. Hydraulic works were also developed in Greece and Rome, both to obtain water for consumption and to avoid the risks associated with settlements in vulnerable environments. In China, the construction of large motes in the rivers was already done in the 12th century, so that an attempt was made to deal with monsoon floods. Also in Spain and northern Italy, since the Middle Ages, the construction of motes and reservoirs to regulate rivers stands out.

Currently, defenses against flooding are very advanced in developed countries. The prevention systems are based on dikes, motes, metallic barriers, regulating reservoirs and improvement of the drainage capacity of river beds. Alert systems for dangerous situations are also highly developed through weather forecasting, observation of river gauging that determines a hydrological alert, and tidal wave detection systems.

Covered by a dike gap (New Orleans floods, USA, 2005 caused by Hurricane Katrina).

The defense against marine flooding caused by the tides is highly developed in the Netherlands where a network of dikes regulates both internal and external waters. Also Venice and London have similar defenses. Regulating reservoirs are very numerous in regions with a Mediterranean climate such as California and southern Europe and serve to store water in times of drought and contain river floods.

Other actions have been aimed at removing the danger from the cities by diverting the river channel, in turn providing it with greater drainage capacity, as in Valencia or Seville. The channeling of rivers, such as the Rhine or the Segura, are larger-scale works that have led to a comprehensive plan for the entire basin (increased drainage capacity, specific diversions, reduction of meanders, construction and expansion of reservoirs, etc..). Some of these actions have been controversial due to their adverse effects, such as the elimination of meanders in the Rhine, which has favored a faster flood wave and therefore its greater virulence.

Flood risk signal in Austell, United States.

Legislation has come a long way by prohibiting construction in areas that are likely to be flooded with a return period of up to 100 years. The extensive cartography has made it possible to know which are the risk areas for their subsequent action on the ground. The reforestation of large areas in the upper and middle basins of the rivers also contributes to minimizing the effect of heavy rains and therefore subsequent flooding. However, risk areas remain, basically urbanized before the protective laws, some of them of high historical-artistic value such as Florence, which already suffered a great flood in 1966.

In developing countries, both prevention, alert and subsequent action systems are less developed, as has been seen in the successive typhoons that have devastated Bangladesh or in the tsunami that has devastated various coasts of Southeast Asia. Even so, international cooperation is favoring actions that lead to greater security for the population in these risk areas.

Effects

Primary effects

The primary effects of flooding include loss of life and damage to buildings and other structures, including bridges, sewer systems, roads, and canals.

Floods also frequently damage power transmission networks and sometimes power generation plants, which then has side effects caused by loss of power. This includes loss of potable water supply and treatment, which may result in loss of potable water or serious water contamination. It can also cause the loss of sewage disposal facilities. The lack of potable water combined with human sewage in floodwaters increases the risk of waterborne diseases, which can include typhoid, giardia, cryptosporidium, cholera, and many other diseases, depending on the location of the flood.

"This happened in 2000, when hundreds of people in Mozambique fled to refugee camps after the Limpopo River flooded their homes. They soon fell ill and died of cholera, which is transmitted by unsanitary conditions, and malaria, transmitted by mosquitoes that thrived on the banks of swollen rivers".

Damage to roads and transportation infrastructure can make it difficult to mobilize help for those affected or provide emergency medical treatment.

Flood waters often inundate agricultural land, rendering it unviable and preventing crops from being planted or harvested, which can lead to food shortages for both humans and farm animals. Entire crops in a country can be lost in extreme flood circumstances. Some tree species may not survive prolonged flooding of their root systems.

Side Effects and Long-Term Effects

Economic hardship due to a temporary decline in tourism, rebuilding costs, or food shortages causing price increases is a common side effect of severe flooding. The impact on those affected can cause psychological damage to those affected, particularly when deaths, serious injuries and loss of property occur.

Urban flooding can cause chronically damp homes, leading to indoor mold growth and resulting in adverse health effects, particularly respiratory symptoms. Urban flooding also has significant economic implications for affected neighborhoods. In the United States, industry experts estimate that wet basements can reduce home values by 10 to 25 percent and are cited among the top reasons not to buy a home. According to the Federal Agency for Housing With the United States Emergency Management Agency (FEMA), almost 40 percent of small businesses never reopen after a flood. In the United States, insurance is available against flood damage for both homes and businesses..

Floods can also have great destructive power. When the water flows, it has the ability to knock down all kinds of buildings and objects, such as bridges, structures, houses, trees, cars. For example, in Bangladesh in 2007, a flood was responsible for the destruction of more than one million houses. And annually in the United States, flooding causes more than $7 billion in damage.

Significant prehistoric floods

In prehistory there were great floods in some areas, as witnessed by the geological remains. Thus, the formation of closed seas such as the Mediterranean or the Black Sea are due to tectonic movements and climatic changes that flooded these vast areas. The end of the ice age had decisive consequences throughout the globe with the formation of new lakes and seas in areas not previously occupied by the sea.

Maximum river flows and floods

Flood mapping

Because of the damage caused by floods throughout history, many countries have developed strategies and methodologies to build flood risk maps. These maps show flooding in relation to the potential impacts it can have on people, property, and economic activities. According to the guide established by IDEAM, there are several types of flood maps.

• Inundation Susceptibility Map
• flood event map
• Map of flood threat
• Inundation Threat Zoning Map
• Flood Vulnerability Map
• flood risk map
• Emergency flood map

In addition, there are free tools that allow you to build flood maps using radar images, such as Google Earth Engine, which allows you to perform various online analyses. This platform was used to map the floods that occurred in August 2017 in Houston, Texas (United States) caused by Hurricane Harvey.

Analysis of flood information

A series of annual peak flows in a stream reach can be statistically analyzed to estimate 100-year floods and floods for 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 inundation depth and area for given channel conditions and a specified flow rate, such as for use in floodplain mapping and insurance against floods. Conversely, given the observed flood zone of a recent flood and channel conditions, a model can calculate the flow rate. Applied to various potential channel configurations and flows, a reach model can help select an optimal 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 along the extent of a floodplain). HEC-RAS, the Hydraulic Engineering Center model, is one of the most popular software, if only because it is freely available. Other models, such as TUFLOW, combine 1D and 2D components to derive flood depths along river channels and across the entire floodplain.

The physical process models of entire drainage basins are even more complex. Although many processes are well understood at one point or for a small area, others are not well understood at all scales, and the interactions of processes under normal or extreme climatic conditions may be unknown. Basin models typically combine land surface process components (to estimate how much rain 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, even though the recurrence interval of a storm is rarely the same as that of the associated flood. Basin models are commonly used in flood prediction and warning, as well as in analyzing the effects of land use change and climate change.

Flood Forecast

Anticipating floods before they occur allows precautions to be taken and people warned so they can be prepared in advance for flood conditions. For example, farmers can remove animals from low-lying areas and utilities can implement emergency provisions to redirect services if necessary. Emergency services can also take steps to have sufficient resources available in advance to respond to emergencies as they occur. People can evacuate areas that will be flooded.

To make the most accurate flood forecasts for waterways, it is best to have a long historical data series that relates stream flows to measured past rainfall events. Combine this historical information with real-time knowledge on the volumetric capacity in the catchment areas, such as the reserve capacity in the reservoirs, the groundwater levels and the degree of saturation of the aquifers in the area, is also necessary to make the flooding more accurate. forecasts.

Radar estimates of precipitation 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 a long lead time. The result of a flood forecast is typically an expected maximum water level and the probable time of its arrival at key locations along a waterway, and may also allow calculation of the statistical probable 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 about a 63% chance of occurring in any 100-year time period.

According to the US National Weather Service (NWS) Northeast River Forecast Center (RFC) in Taunton, Massachusetts, a general rule of thumb for forecasting flooding in urban areas is that at least 25mm is needed of rain in about an hour. time to initiate significant pooling of water on impervious 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 to cause flash flooding or flooding of larger water basins.

In the United States, an integrated approach to real-time computer hydrologic modeling uses observed data from the United States Geological Survey (USGS), various cooperative observing networks, various automated weather sensors, remote sensing National Operational Hydrological Center (NOHRSC), various hydroelectric companies, etc., combined with Quantitative Precipitation Forecasts (QPF) of expected rain and/or snowmelt to generate daily or as-needed hydrological forecasts. The NWS also cooperates with Environment Canada on hydrological forecasts affecting both the United States and Canada, such as in the St. Lawrence Seaway area.

The Global Flood Monitoring System, "GFMS," a computer tool that maps flood conditions around the world, is available online. Users anywhere in the world can use GFMS to determine when flooding may occur in their area. GFMS uses precipitation data from NASA Earth observation satellites and the Global Satellite measurement of precipitation, 'GPM'. GPM precipitation data is combined with a land surface model that incorporates vegetation cover, soil type, and terrain to determine how much water is being absorbed by the soil and how much water is flowing into the stream flow.

Users can view statistics for rainfall, flow, water depth, and flooding every 3 hours, at each 12-kilometer grid point on a global map. The forecasts for these parameters are 5 days into the future. Users can zoom in to view flood maps (areas estimated to be covered by water) at 1 kilometer resolution.