FLOODING - Judith Currycurry.eas.gatech.edu/.../Chapter8/Ency_Atmos/Flooding.pdf · 2005-07-15 · Floods produce damage through the immense ... Flooding is typically coupled to water

FLOODING

C ADoswell III, University of Oklahoma, Norman, OK,USA

Copyright 2003 Elsevier Science Ltd. All Rights Reserved.

Introduction

Flooding is arguably theweather-related hazard that ismost widespread around the globe. It can occurvirtually anywhere. A flood is defined as wateroverflowing onto land that usually is dry. Flooding isoften thought of as a result of heavy rainfall, but floodscan arise in a number of ways that are not directlyrelated to ongoing weather events. Thus, a completedescription of flooding must include processes thatmay have little or nothing to do with meteorologicalevents. Nevertheless, it is clear that in some ultimatesense, the water that is involved in flooding has fallenas precipitation at some time, perhaps long ago. Theorigins of flooding, therefore, ultimately lie in atmos-pheric processes creating precipitation, no matterwhat specific event causes the flooding.

Floods produce damage through the immensepower of moving water and through the depositionof dirt and debris when floodwaters finally recede.People who have not experienced a flood may havelittle or no appreciation for the dangers of movingwater. The energy of that moving water goes up as thesquare of its speed; when the speed doubles, the energyassociated with it increases by a factor of four.Flooding is typically coupled to water moving fasterthan normal, in part because of the weight of anincreased amount of water upstream, leading to anincrease in the pressure gradient that drives the flow. Inmost cases, the damage potential of the flood ismagnified by the debris that the waters carry: trees,vehicles, boulders, buildings, etc. When the watersmove fast enough, they can sweep away all beforethem, leaving behind scenes of terrible destruction(Figure 1).

The effect of the water itself can be devastating onstructures and on the objects within them: books,furniture, photographs, electronic equipment, and soon can be damaged simply by being immersed inwater,even if they are not directly damaged by the watermovement. Moreover, floodwaters typically containsuspended silt and potentially toxic microorganismsand dissolved chemicals. This means that floodsusually compromise drinkingwater supplies, resultingin short-term shortages of potable water, with theadditional long-term costs in restoring drinking waterservice to the residents of a flooded area. The mud and

debris left behind when floodwaters recede can becostly to clean up and also represent a health hazard,especially when there are decomposing bodies ofdrowned wild and domestic animals in the debris. Insome situations, floods drive wild animals (includinginvertebrates of all sorts) from their normal habitatsand into human habitations near and withinthe flooded areas, which can create various pro-blems, especially when the animals are venomous oraggressive.

Although flooding has some large negative impactson humans, it is also part of the natural processesshaping the Earth. Floodplains along rivers andstreams are among the most fertile regions known.Most of the so-called ‘cradles of civilization’ arewithin floodplains for this very reason (e.g., the NileRiver, the Tigris–Euphrates River, among others).Hence, humans have been affected by flooding bothpositively and negatively since before historical times,whenever they find themselves in the path of thesenatural events.

Figure 1 Damage resulting from the 1977 Johnstown, Pennsyl-

vania, flash flood event. (r The Johnstown Tribune-Democrat,

used by permission.)

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Floods as a Direct Resultof Precipitation

When the waters of a flood arise directly fromprecipitation, atmospheric processes can be identifiedas directly responsible for the event. That is, rainfallsoccur that are well beyond the average values for theaffected area. It is only when those rainfalls exceed theaverage that land which is usually dry can be affected;that is, a flood occurs. Thus, the rainfall amountsneeded for floods cannot be defined in absolute terms.A precipitation event that causes a flood in onelocation might be well within the bounds of what istypical for another location. Generally speaking, thethreshold for flood-producing rainfalls increases as theannual average rainfall for a region increases.

Flash Floods

Flash floods are defined as those flood eventswhere therise in water is either during or within a few hours ofthe rainfall that produces the rise. Therefore, flashfloods occur within small catchments, where theresponse time of the drainage basin is short. Manyhydrological factors have relevance to the occurrenceof a flash flood: terrain gradients, soil type, vegetativecover, human habitation, antecedent rainfall, and soon. In steep, rocky terrain or within heavily urbanizedregions, even a relatively small amount of rainfall cantrigger flash flooding. These hydrological factorsdetermine the response of the catchment to theprecipitation event. Thus, a flash flood is clearly theresult of the concatenation of bothmeteorological andhydrological circumstances.

Most flash floods associated with rainfall areproduced by thunderstorms; that is, deep, moistconvection. A single thunderstorm cell is unlikely toproduce enough rainfall to cause a flash flood, so thetypical flash flood is the result of several thunder-stormsmoving successively over the same area, knownas ‘training’ thunderstorms (Figure 2), because itresembles the passage of cars in a freight train. Asuccession of thunderstorms results when new thun-derstorms pass repeatedly over the same place whilethe overall system of thunderstorms is very nearlystationary. The infamous Johnstown, Pennsylvaniaflash flood of 19–20 July 1977was produced by such asystem. Thunderstorms forming in north-westernPennsylvania moved south-eastward, only to bereplaced by newly formed thunderstorms, a processthat went on for several hours. The result wastorrential rainfall concentrated near Johnstown, withamounts exceeding 400mm. The ensuing flood wasresponsible for 77 fatalities and $550 million (in 1999dollars).

Occasionally, flash floods are created in conditionsthat are not favorable for thunderstorms but whichstill produce heavy rainfalls. This can occur whenmoist air is forced upward overmountains by thewindflow, called orographic precipitation. When the airforced upward is very moist, the rainfall can be quiteheavy. The steep, rocky terrain also promotes rapidrunoff of the rainfall. Flooding along theWestCoast ofthe USA or in the European Alps is often of this type;that is, not involving thunderstorms.

A characteristic of flash floods is the localized natureof the heaviest rainfall. As shown in Figure 3, the mostintense rainfall is typically confined to a relativelysmall area. When large amounts of this localizedprecipitation fall within a small drainage basin, flashfloods can occur. Sometimes, the location where flashflood damage occurs may actually receive little or norainfall. That is, the rainfall that causes the problemcan occur upstream of threatened areas. This separa-tion between the rainfall and the flood can causeconfusion because itmay not even be raining in an areafor which flash flood warnings are issued. Anotherfactor in the impact of flash floods is that theprecipitation causing the event often falls during thenight, when it can be difficult to get warnings tosleeping residents. The central part of the USA is wellknown for its heavy thunderstorm-produced rainsduring nighttime hours. Worldwide, thunderstormsare most common during the day, but on the centralplains of the USA (and in a few other places around theworld), the unique geography of the region favorsnocturnal thunderstorms. This setting promotes astrong flow of moisture northward from the Gulf ofMexico, called a low-level jet stream, during thewarmmonths of the year. Moisture carried by the low-leveljet stream helps to maintain thunderstorm systemsthat often begin during daytime hours on the higherterrain to the east of the RockyMountains. Because ofthe low-level jet stream, such storms can persist wellinto the nighttime hours, often forming clustersof thunderstorms known as mesoscale convectivesystems (Figure 4).

It is the rapidity of the event that makes flash floodsso damaging and dangerous. Flash floods involverapidly rising, fast-moving waters that can do im-mense damage; the suddenness of the onset of the floodcan result in people being caught unawares. Mostfatalities result from drowning, with perhaps sometraumatic injuries from being carried along in thedebris-laden waters and being swept into standingobjects. The potential for loss of human life with flashfloods is high. Debris carried in flash floods can formtemporary ‘debris dams’ that typically fail as watersback upbehind them. Failure of these debris dams thenresults in a ‘wall of water’surging downstream.Debris

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Figure2 Schematic of the ‘training’ effect. (A)At this time, thereare fournumbered thunderstormcells in variousstagesof development.

Cell I is mature, with both updrafts and downdrafts, and heavy rain is about to commence at point X. Cells II, III, and IV are still developing,

and have only updrafts. Cell II has precipitation forming aloft. The hatched contours are radar reflectivity, in standard units of dBZ,which is

related to the rainfall rate. (B) About 15min later, Cell I’s updraft is dissipated, and it is now dominated by downdraft. Heavy rain continues

at X while Cell II is maturing and developing a downdraft. Cells III, IV, and now V are still immature. (C) About 15 more minutes have

elapsed. Cell I’s rainfall is continuing but it is now nearly dissipated, while Cell II is entering late maturity. It is still raining at X but now the

rainfall is fromCell II, and heavy rain fromCell II is descending from aloft. NowCell III is developing its first precipitation aloft. Cell IV andV

are still immature. (Adapted from Figure 7 in Doswell CA III, Brooks HE and Maddox RA (1996) Flash flood forecasting: An ingredients-

based methodology. Weather Forecasting 11: 560–581.)

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dam failure events can happen repeatedly during thecourse of the flash flood. Not all flash floods arecharacterized by a ‘wall of water’ but all of them (bydefinition) involve rapidly rising floodwaters.

Because urbanized areas promote runoff of rainfall,rather than permitting most of the rain to be absorbedinto the ground, flash flooding is more likely in citiesthan in rural areas surrounding a city. It takes muchless rainfall in a city to create a flash flood situationthan in a rural area of comparable size.

Flash floods continue to be a major contributor toloss of life, in spite of improved precipitation fore-casting. Some noteworthy examples include events inthe Big Thompson Canyon in Colorado (1976 – 144fatalities) and near the town of Biescas in the SpanishPyrenees (1996 – 86 fatalities).

Tropical cyclones often create devastating flashfloods as a result of torrential rainfalls. In late Octoberof 1998, Hurricane Mitch caused more than 9000fatalities (the exact number is not known), mostly inNicaragua and Honduras, in Central America, from

flash floods and landslides associated with its rainfall.It was the worst weather disaster in terms of casualtiesin the Western Hemisphere during the twentiethcentury.

River Floods

River floods, in contrast to flash floods, typicallyunfold over days, or evenmonths. This is because theyoccur in large basins involving ‘main stem’ rivers likethe Missouri, or the Nile, and are usually the result ofmany individual rainfall episodes spread out overmany days. In fact, within a river flood event, severalflash flood events can occur. Again, hydrologicalfactors often contribute to a river flood, but riverfloods are not so sensitive to them as are flash floods.Whereas individual thunderstorm systems can causeflash floods, river floods are usually the result of astagnant synoptic-scale weather pattern. Localizedheavy rainfall events occurmany times during a periodof days or even months, each contributing its share of

Figure 3 Observed total precipitation (mm) during the Johnstown, Pennsylvania (JST, located by an asterisk) flash flood event. For

reference, Pittsburgh, Pennsylvania (PIT, located by the plus sign) is also shown. (Adapted from Figure 14a in Hoxit LR, Maddox RA,

Chappell CF, Zuckerberg FL,Mogil HM, Jones I,GreeneDR,SaffleREandScofieldRA (1987)Meteorological Analysis of the Johnstown,

Pennsylvania, Flash Flood, 19–20 July 1977. NOAA Technical Report ERL 401-APCL 43, NTIS Accession No. PB297412.) NOAA,

National Oceanographic and Atmospheric Administration.

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rainfall to the tributaries, which then discharge intothe main stem of a river. The river rises gradually inresponse to all the input rainfall. The river floodpotential of a situation can be increased by concurrentsnow melt and other factors besides rainfall.

The major flooding event during June and July of1993 was the result of a weather pattern (Figure 5A)that produced a storm track across the upper Mid-western USA. Abnormally low heights of the pressuresurfaces (associated with cool temperatures) over thenorthern Plains produced a pattern in which travelingweather disturbances intensified in the Midwest aftercrossing the RockyMountains. This pattern aloft alsoproduced an anomalously strong poleward flow oflow-level moisture from the Gulf of Mexico into theMidwest. Mesoscale convective systems developedalmost every evening during the early summer, typi-cally persisting through the night. These passedrepeatedly over nearly the same areas, resulting inwidespread significant rainfalls (Figure 5B) for the

period over the lower Missouri and upper Mississippibasins. In addition to these factors, considerablerainfall over the region had fallen during the previousseveral months, providing a hydrological setting thatfavored runoff of the precipitation. This event pro-duced disastrous flooding that persisted for manyweeks.

Owing to the long time scale of the rising waters,river floods pose a lower risk of fatalities; people havemore time to take proper actions. Of course, somecasualties result from waiting until it has become toolate to respond to the threat. Levee anddam failures, aswell as intentional rapid release of impounded watersto prevent the catastrophic failure of the flood controlstructures, can produce rapidly rising water situationsembedded within a river flood, and these also cancontribute to loss of life.

Because of the large scale of river floods, the damagefigures may be enormous; easily into the billions ofdollars. Crop losses are a major factor in the costs of

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Figure4 False-color enhanced infrared satellite imageof amesoscale convective system,with the light red colors indicating the coldest

(therefore the highest) clouds. Note that this image is from 17 August 2000 at 0345, local time, which corresponds to 0845UTC.

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river floods, whenever large tracts of prime agricul-tural land along floodplains are inundated. Levees areoften used to protect populated areas, so the failure ofthose levees can generate major property losses. The

damage and dislocations along the Upper Mississippiand Lower Missouri basins during the summer floodsof 1993, during which several levees were breached,illustrate the huge impact such events can have.

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Floods Arising from NonprecipitationEvents

Apart from floods resulting directly from rainfall,there are many ways in which precipitation can causefloods, perhaps long after it has fallen. When flowingwater is impounded by the construction of dams, thereis some risk that the dams will fail. Johnstown,Pennsylvania, was inundated by a dam failure duringa rainfall event in 1889, for example. Such rapidreleases of stored water can be cataclysmic, manifest-ing themselves as an enormous ‘wall of water’ chokedwith debris.

Flood can also arise through themelting of snowfall.In situationswhere the precedingwinter’s snowpack isdeep, a sudden change to warm temperatures in thespring can result in abnormally rapid melting andrunoff of the snowmelt. The devastating flood createdin Grand Forks, North Dakota, in April of 1997 is anexample. Occasionally, warm rain falls directly ontothe melting snow, exacerbating such situations byspeeding the melting process and adding more liquidwater.

Deposits of snow and ice on volcanic peaks canmeltrapidly during eruptions. The resulting runoff, oftenturned into a thick slurry by the inclusion of volcanicash, roars down themountainside and is called a lahar.A tragic example occurred with the Nevado del Ruizvolcano in Colombia on 13 November 1985, whichkilled more than 23 000 people, mostly in the town ofArmero. Another occurred in Iceland during 1996 onthe Vatnajokull glacier, with no fatalities owing to itsremote location. Lahars can continue occasionally foryears after an eruption, when heavy rains fall onto ashdeposited by the volcano.

During the winter and late spring, when ice canbuild up on rivers in cold climates, the breakup of theice can create ice dams on the river. The ice dams causethe waters to back up, sometimes flooding the landupstream of the ice dam. Then, the breakup of the icedam can result in a flash flood wave that surgesdownstream of the ice dam’s position.

Other flood situations can develop along the shoresof the world’s oceans and even with large freshwaterlakes. Tsunamis, typically caused by underwaterearthquakes and landslides, can flood the shorelineswith hugewaves that break on the shallowwaters nearthe shore. Storms of all sorts, including tropicalcyclones, can drive the waters before the winds intostorm surges that inundate shore areas when thestorms are near the land. Large lakes can experienceflooding on their shores due to seiches, which aresurges of water (usually oscillatory) within enclosedbodies of water. Seiches can be caused by earthquakesor by atmospheric processes.

Societal Impacts and Their Mitigation

The results of floods on society worldwide aresubstantial. Flooding is responsible for many drown-ing fatalities in tropical cyclones, either from stormsurges or from freshwater rain-induced flash floods.Flash floods and river floods typically produce morefatalities every year than either tornadoes or hurri-canes in the USA. In many parts of the world, floodfatalities are associated with the most significantweather-related disasters. Flood damage cost in theUSA is now on the order of several billion dollarsannually, and this figure continues to rise.

Many people now live and play in flood-proneareas: for example, within floodplains of rivers andtheir tributaries, as well as along coastlines that arevulnerable to storm-caused flooding from tsunamis,tropical cyclones, and nontropical storms. Develop-ment of flood-prone areas for habitation and recrea-tion has been increasing, with a correspondingincrease in the risks to life and property. The 1993Upper Mississippi and Lower Missouri River floodsprovided a grim reminder of the risks of buildingpermanent structures within floodplains, even whenflood-control measures have been taken.

In the case of flash floods, it is difficult to takemeasures to protect property, owing to the rapiditywithwhich the event happens.However, prevention offlash flood casualties is possible, provided warningscan be issued and acted upon properly in a timelyfashion. Considerable attention has been paid toincreasing public awareness of the dangers of drivinginto rapidly rising floodwaters, for instance, as a resultof recent experiences with flash floods. Unfortunately,situations can still arise where warnings are not issuedin time. People living and engaging in recreationalactivities in places prone to flash floods need to be alertduring heavy rainfalls and be prepared to seek safetyeven when they do not receive timely warnings.

For river floods and other relatively slow-developingsituations (suchas rising snowmelt or iceactionevents),itmaybepossible to reduce thepropertydamage aswellby removing the contents of structures. Obviously, anystructures (and their contents) built in flood-proneareasare permanently at risk; the onlyway to guarantee theirprotection from floods is to move them out of thoseareas. Prevention of fatalities in river flood events is amatter of heeding the warnings of danger and movingresidents out of the danger areas before the number ofoptions is reducedby the risingwaters andby the failureof levees or other flood-prevention structures.

Forecasting the details of flooding events is animportant part ofmitigation. Knowing precisely whenand where a flood will occur would no doubt behelpful, but it is also important to be able to anticipate

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the magnitude of the flood. An example of this is thetragedy of the 1997 Grand Forks, North Dakota case,where the river level was only a few feet higher thanthat forecast. Those few feet, however, had a largeimpact, because the flood-control operations werebased on the lower forecast value.When the river roseabove that level, the flood-control measures failedcatastrophically. In reality, such a forecast can never bea precise statement; uncertainty is implicitly a part ofevery forecast, a point that perhaps needs greateremphasis in the future.

Flooding, by its very nature, is usually a result ofboth meteorological and hydrologic processes; thecharacter of a flood is determined both by the detailedbehavior of the precipitation and by the nature ofsituation in which the event is likely to occur (soilconditions, amount of antecedent rainfall, and so on).It is not likely that precisely detailed forecasts offlooding events will ever be possible, although it iscertainly well within our capability to anticipate thepossibility of most flood events. The challenge forreducing the social impacts of floods is how best tomake use of the uncertain meteorological and hydro-logical forecasts that are within practical means. Thechallenge is to make effective use of whatever fore-casting capability we have, even as we seek to improvethat capability.

Effects of Human Activities onFlooding

In addition to the risks to lives and property thatpeople take by moving into flood-prone areas, devel-opment for human use often involves clearing land ofits native vegetation and altering the characteristics ofthe ground cover. Vegetation works together with thesoil to store rainfall, sowhen that vegetation is cleared,rainfall runoff can increase substantially. Rather thanbeing absorbed by the soil and its natural vegetation,in areas where that vegetation has been cleared (eitherfor construction or for agriculture), heavy rainfall ismore likely to run off and pour into streams and rivers,increasing the potential threat from flash floods andriver floods. Construction of roads and buildings alsoacts to increase runoff, and leads to an increasinglikelihood of localized urban flooding. Such construc-tion dramatically increases the fraction of the rainfallthat runs off, regardless of antecedent rainfall. Hu-man-caused fires can also produce at least temporaryincreases in the runoff potential in the headwaterregions of streams and rivers. It is evident that humanactivities are increasing the potential for floods aroundthe world.

Again recalling theMississippi River floods of 1993as an example, the issue of flood control through levees

and other structures was dramatically recalled topublic attention. The value of structural methods forflood control (levees, flood control dams, breakwaters,etc.) remains controversial, but the 1993 floods madeit apparent that structures such as levees can bebreached duringmajor flooding episodes, even thoughthey may be able to contain lesser events. Structuralfailures create rapidly rising waters (flash floods)artificially within a river flood event, increasing thehazards to human life as well as destroying property.The decision about when and where to take structuralapproaches will continue to be a challenge.

Finally, the use of flood-prone areas for humanactivities puts lives and property at risk, although themajor flood events may be separated by many years.The long time between events can lead to complacencyand subsequent disasters. The choices associated withland use are a continuing challenge, now and in thefuture. When humans live and play in ways that putthem in the path of potential floodwaters, majorsocietal impacts are inevitable.

See also

Air–Sea Interaction: Momentum, Heat and Vapor Flux-es. Convective Storms: Convective Initiation; Overview.Hurricanes. Hydrology: Modeling and Prediction; Over-view. Mesoscale Meteorology: Cloud and PrecipitationBands;MesoscaleConvectiveSystems.PalmerDroughtIndex. Predictability and Chaos. Radar: PrecipitationRadar.SatelliteRemoteSensing:Precipitation.SevereStorms. Weather Prediction: Severe Weather Fore-casting.

Further Reading

Agnone JC (ed.) (1995) Raging Forces: Earth in Upheaval.National Geographic Society.

Barry JM (1997)RisingTide: TheGreatMississippi Flood of1927 andHow It Changed America. Simon and Schuster.

Cluckie ID and Collier CG (eds) (1991) HydrologicalApplications of Weather Radar. Ellis Horwood.

Dingman L (1994) Physical Hydrology. Upper Saddle River,NJ: Prentice-Hall.

Hill CE (ed.) (1986) Nature on the Rampage: Our ViolentEarth. National Geographic Society.

Lorenz EN (1993) The Essence of Chaos. Seattle: Universityof Washington Press.

Ludlam F (1980) Clouds and Storms. University Park, PA:Pennsylvania State University Press.

PruppacherHR andKlett JD (1997)Microphysics of CloudsandPrecipitation, 2nd edn.Dordrecht:KluwerAcademicPress.

Ray P (ed.) (1986)Mesoscale Meteorology and Forecasting.Boston, MA: American Meteorological Society.

Sarewitz D, Pielke RA Jr and Byerly R (eds) (2000)Prediction: Decision-Making and the Future of Nature.Washington, DC: Island Press.

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