Flood

• 1. Introduction

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  A flash flood once killed eight people who had gathered on a Sunday afternoon at a popular waterfall in a narrow canyon. There had been no observed rainfall at the site. In just one minute, the flow rate climbed from about 50 to 1500 cubic feet per second. Two larger floods struck the same spot within a week, but the waterfall happened to be empty on those days. This is a flood: an overflow of water that submerges land that is usually dry. The word reaches back to Old English floyd, a relative of flow and float, carrying the old meaning of a deluge, a tide, an overflowing of land by water. Floods are among the oldest hazards humans have faced, and among the oldest we have tried to predict. How does water move from a quiet riverbed to a force that demolishes bridges and houses? Why do some of the deadliest floods leave fertile soil behind them? And how do engineers put a number on a danger that arrives, sometimes, with no warning at all?

• 2. Many Shapes Of Water

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  Areal flooding begins on flat or low-lying ground when rain or snowmelt arrives faster than the land can absorb or shed it. The excess simply accumulates in place, sometimes to hazardous depths, because the speed of overland flow depends on the slope of the surface. Frozen ground, rock, concrete, paving, and roofs all slow infiltration to a trickle or stop it entirely. River flooding occurs in every kind of channel, from the smallest ephemeral streams in humid zones to the world's largest rivers. When the flow rate exceeds the capacity of the channel, especially at bends or meanders, water escapes its banks. In the Red River Valley of the North, spanning Minnesota, North Dakota, and Manitoba, the land is so flat that a hybrid form known locally as overland flooding appears. This former glacial lakebed, created by Lake Agassiz, drops only 236 feet over a length of 550 miles, an average slope of about 5 inches per mile. A fast spring snowmelt can push water out of a tributary and send it running overland for miles, suddenly and unpredictably, to rejoin the river far downstream or reach an entirely different streambed. Coastal flooding takes yet another form, as storm surges combine with high tides and large waves to overtop defenses. A storm surge is an additional rise of water generated by a storm, over and above the predicted astronomical tides. In arid zones, flash floods race down normally-dry channels called arroyos in the southwestern United States, where the leading edge of the flood advances slowly as it wets the sandy bed, until later and higher flows overwhelm it.

• 3. When Humans Open The Gates

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  Intentional flooding deliberately covers land that would otherwise stay dry, serving agricultural, military, or river-management purposes. Farmers flood paddy fields to grow semi-aquatic rice in many countries, a practice that is itself a form of hydraulic engineering. River managers sometimes sacrifice less valuable land to spare more valuable land, diverting flood waters at flood stage. The Dutch built this idea into their landscape with the overlaten, literally let-overs, intentionally lowered segments in riparian levees. The Beerse Overlaat sat in the left levee of the Meuse between the villages of Gassel and Linden in North Brabant. Military inundation turns water into an obstacle meant to slow an enemy, used for both offense and defense. The Dutch Republic and its successor states relied on controlled inundations such as the two Hollandic Water Lines, the Stelling van Amsterdam, the Frisian Water Line, the IJssel Line, the Peel-Raam Line, and the Grebbe Line. To count as controlled, an inundation must allow civilians a timely evacuation, remain reversible, and minimize ecological harm. Uncontrolled inundations carried no such care, as in the second Siege of Leiden during the Eighty Years War, the flooding of the Yser plain in the First World War, and the Inundation of Walcheren and the Inundation of the Wieringermeer in the Second World War.

• 4. What Makes Water Rise

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  About 30 percent of all precipitation becomes runoff, and that share can climb when melting snow adds to it. During rain, some water is held in ponds or soil, some is taken up by grass and vegetation, some evaporates, and the rest travels over the land. A flood begins when ponds, lakes, riverbeds, soil, and vegetation can no longer absorb it all. Human activity sharpens this. Draining wetlands removes natural storage, and paved surfaces absorb nothing, so water runs off in quantities the channels cannot carry. The fraction of incoming rain that promptly reaches a drainage channel ranges enormously, observed from nil for light rain on dry, level ground to as high as 170 percent for warm rain falling on accumulated snow. The single most important upslope factor in flood magnitude is the land area of the watershed upstream. For watersheds smaller than about 30 square miles, rainfall intensity comes second; for larger ones, the main channel slope takes that place. Time of concentration measures how long runoff needs to travel from the most distant point of the drainage area to the channel controlling the flood. That critical duration might be only a few minutes for a parking lot drain, while cumulative rainfall over several days matters for a river basin. Downstream, the ocean or a coastal bar often sets the final limit, and tidal fluctuations, tsunamis, and storm surges all shift the elevation of large bodies of water. The geometry of the flow channel governs the rest, controlled by depth, speed of flow, and the sediment carried within it.

• 5. The Deadliest Waters

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  The 1931 China floods stand at the top of the record for loss of life, with a death toll estimated between 2.5 and 3.7 million people. The list of the world's deadliest floods, counting only events with at least 100,000 deaths, is dominated by China and by the Yellow River. The 1887 Yellow River flood killed between 900,000 and 2 million; the 1938 Yellow River flood between 500,000 and 700,000. The 1975 Banqiao Dam failure, a result of Typhoon Nina, killed roughly 231,000, with about 86,000 dying from flooding and another 145,000 from subsequent disease. The 2004 Indian Ocean tsunami took 230,000 lives in Indonesia. Earlier centuries appear too, including St. Felix's flood, a storm surge that struck the Netherlands in 1530 and killed more than 100,000. The economic toll runs in parallel. Floods caused losses exceeding 1.5 trillion US dollars to agriculture between 1991 and 2023, the single most destructive hazard type for global agriculture. In Bangladesh in 2007, a flood destroyed more than one million houses, and in the United States floods cause over 7 billion dollars in damage every year. The harm reaches into ordinary homes as well, where industry experts in the United States estimate that wet basements can lower property values by 10 to 25 percent. According to FEMA, almost 40 percent of small businesses never reopen after a flooding disaster.

• 6. Sickness In The Aftermath

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  Drowning is the usual cause of death directly tied to floods, where the water is deep and the currents strong. Other deaths follow from dehydration, heat stroke, heart attack, and any illness needing medical supplies that cannot be delivered. Injuries gather around the event in three waves, before, during, and after, and they strike not only those caught in the water but rescue teams and people delivering supplies. Many of the worst injuries come during rescue attempts after the water has risen. Communicable diseases multiply as pathogens and bacteria travel through the water. Waterborne illnesses such as cholera, hepatitis A, hepatitis E, and diarrheal diseases spread when clean water supplies are contaminated and sanitation breaks down. Floods often knock out power transmission and sometimes generation, which can disable drinking water treatment and sewage disposal, raising the risk of typhoid, giardia, cryptosporidium, and cholera. Chronically wet houses grow indoor mold, and research suggests an increase of 30 to 50 percent in adverse respiratory outcomes from dampness and mold exposure for those living in coastal and wetland areas. Still water left behind invites vector-borne diseases including malaria, dengue, West Nile, and yellow fever. Among the most treated long-term illnesses is depression, born of the losses and tragedy that arrive with the flood.

• 7. The Gifts Of Overflow

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  Periodic flooding was essential to the well-being of ancient peoples along the Tigris-Euphrates Rivers, the Nile, the Indus, the Ganges, and the Yellow River. Smaller and more frequent floods recharge ground water, make soil more fertile, and add nutrients to certain soils. In arid and semi-arid regions, where precipitation falls unevenly through the year, flood waters supply much needed water and kill pests in the farmland. Freshwater floods help maintain ecosystems in river corridors and keep floodplain biodiversity alive. Nutrients spread into lakes and rivers can raise biomass and improve fisheries for a few years. An inundated floodplain can make an ideal spawning ground for some fish, with few predators and richer food, and species such as the weather fish use floods to reach new habitats. Bird populations may gain from the boost in food production, and the viability of hydropower, a renewable energy source, runs higher in flood-prone regions.

• 8. Reading The Water

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  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. In many developed countries, urban areas at risk are protected against a 100-year flood, an event with a probability of around 63 percent of occurring in any given 100-year period. Defenses such as detention basins, levees, bunds, reservoirs, and weirs keep waterways within their banks, and when these fail, sandbags or portable inflatable tubes try to hold the line. In the United States, the National Weather Service offers the advice Turn Around, Don't Drown, urging people to leave a flooded area rather than cross it. The same service notes a rule of thumb from its Northeast River Forecast Center in Taunton, Massachusetts, that it takes at least 1 inch of rainfall in about an hour to start significant ponding on impermeable surfaces. Computer models do the heavy forecasting. HEC-RAS, the Hydraulic Engineering Center model, is among the most popular partly because it is free of charge, while others such as TUFLOW combine 1D and 2D components to map flood depths across river channels and entire floodplains. The Global Flood Monitoring System, GFMS, maps flood conditions worldwide and is available online to anyone. It draws precipitation data from NASA's Earth observing satellites and the Global Precipitation Measurement satellite, GPM, combining rainfall with a land surface model that accounts for vegetation cover, soil type, and terrain. Users can view rainfall, streamflow, water depth, and flooding every 3 hours at each 12-kilometer gridpoint, with forecasts running 5 days ahead and inundation maps available down to 1-kilometer resolution. Attempts to understand and manage the mechanisms at work in floodplains have been made for at least six millennia, and the effort continues at every grid point on the map.