GS ENVS 103 CH 13 Floods

Regional floods vs flash floods

regional floods cover more area while flash floods are localized

Regional floods

river overflows banks on a large scale, flooding entire regions Within large drainage basins, such as the Mississippi River, maximum floods result from widespread rains that last for many weeks, producing high waters that also last for many weeks.

Red River of the North The Red River of the North is unusual because it flows northward (figure 13.23), draining parts of South Dakota, North Dakota, and Minnesota before flowing into the Canadian province of Manitoba. Floods are likely here each spring. In 1997, nature won the round as flood levels set records throughout the region. A Presidential Disaster Proclamation was declared for all 53 counties in North Dakota. Why are floods so common along the Red River of the North? Several factors combine to create this situation:

(1) The Red River valley is geologically young—only about 9,000 years old—and the river has not carved a deep valley. (2) The gradient or slope of the riverbed is very low, averaging only an 8 cm/km (5 in/mi) drop in elevation. The gradient is only 2.4 cm/km (1.5 in/mi) just south of the Canadian border. The nearly flat river bottom causes slow-flowing water that tends to pool into a broad and shallow lake during high-water flow. (3) River flow increases as winter snow melts. The meltwater runs northward into still-frozen lengths of the river, where ice jams up and impedes water flow, causing floods.

CHANNELIZATION Humans try to control floodwaters by making channels

(1) clear of debris, (2) deeper, (3) wider, and (4) straighter. All these activities make it easier for water to flow through the channel. There are more than 60,000 km (37,000 mi) of channelized streams in the United States.

To grasp the fundamentals of how streams work, a few key variables must be understood:

1) discharge, the rate of water flow expressed as volume per unit of time; (2) available sediment, the amount of sediment waiting to be moved; (3) gradient, the slope of the stream bottom; and (4) channel pattern, the sinuosity of the stream path. Streams occupy less than 1% of the land surface but convey the rainfall runoff (discharge) from all the land and carry away loose sediment (load).

Indigenous global flood stories are documented in almost every corner of the Earth. Most flood legends have several things in common:

1. Humans survived by constructing a boat or some sort of floating vessel; 2. The floods happened quickly, and ended quickly; 3. There were warning signs that a flood was coming.

How Rivers and Streams Work A river is simply a large stream. Streams reveal much about their behavior when examined over their total length.

A cross-sectional plot of a stream's bottom elevation versus the stream's distance from its source yields a fairly consistent and revealing relationship (figure 13.2). When we exaggerate the vertical scale on the graph to emphasize the relationship, the profile of the stream bottom appears relatively smooth and concave upward, with a steeper bottom or slope (higher gradient) near the stream source and a flatter bottom (lower gradient) near the stream mouth. Figure 13.2 shows the Arkansas River, but by changing the scales of elevation and length, this longitudinal cross-section could serve for virtually any stream in the world.

The 1983 Tucson flood showed the problems that come with building protective walls in hit-and-miss fashion. When the stream reaches behind the end of a protective wall, damages are concentrated (figure 13.41).

A stream is a system with delicately triggered feedback mechanisms; it must be treated as a whole or not at all. Hit-and-miss bank protection creates problems.

flash flood

A sudden, violent flood that occurs within a few hours, or even minutes, of a storm. Within small drainage basins, brief, localized downpours can cause fast-moving but short-lasting flash floods, as occurred near Emporia in 2003 and on the Guadalupe River of central Texas in 1972 (see chapter 10).

DAMS, RESERVOIRS, AND NATURAL STORAGE AREAS Flood sizes may be lessened by storing some floodwater in reservoirs behind human-built dams or by diverting water from the river into natural storage areas with low topography.

All dams have life spans limited by the durability of their construction materials and style, and by the rate at which stream-delivered sediment fills in their reservoirs. Despite all the massive dams and extensive reservoirs that have been built, major floods still occur downstream due to overtopping (e.g., Rapid City, South Dakota) and to heavy rains that fall below the dam (e.g., Guadalupe River, Texas, and Big Thompson Canyon, Colorado). In addition, major killing floods have been unleashed by failed dams. In the United States alone, hundreds of dams have failed (table 13.4). The problems with dam integrity have not all been solved. • In 1981, the U.S. Army Corps of Engineers studied 8,639 dams and judged that 2,884 of them were unsafe.

The Concrete Approach: Los Angeles In response to flood damage, Los Angeles not only cleared, straightened, and deepened its river channels, but it also lined them with concrete to further reduce friction and speed up flow.

As long as flood volumes stay smaller than channel capacity, there are no urban floods. It should also be noted that concrete-lined channels obliterate the habitat of all riverine plants and animals (figure 13.38). The "soul" of a community is less well served by a concrete ditch than a tree-lined stream.

In 1879, the Mississippi River Commission placed a major emphasis on building levees, yet the 1882 flood broke through the levees in 284 places.

By 1926, there were 2,900 km (1,800 mi) of levees averaging 6 m (20 ft) in height. But the 1927 floods breached the levees in 225 places, inundating 50,000 km2 (19,300 mi2) and drowning 183 people.

INSURANCE Flood insurance has been available to farmers and townspeople through the National Flood Insurance Program since the 1950s, but it has not been popular.

For example, when the Red River of the North flooded Grand Forks, North Dakota, in 1997, only 946 out of more than 10,000 households were covered by flood insurance. Four months before the flood, FEMA had spent $300,000 on a media campaign warning citizens of the ominous snow-melting conditions in 1997. The FEMA ad campaign motivated only 73 Grand Forks homeowners to buy flood insurance.

PRESIDENTIAL DISASTER DECLARATIONS Under the Disaster Relief and Emergency Assistance Act, federal disaster relief is provided to states and communities if they receive a Presidential Disaster Declaration (PDD).

Declarations are made at a president's discretion, but a criterion for issuing a PDD is "a finding that the disaster is of such severity and magnitude that effective response is beyond the capabilities of the state and the affected local governments." The frequency of disastrous floods, storm surges, and rains is shown by these declarations; floods provoked 64% of the PDDs in 50 years of record (table 13.5).

SANDBAGGING When a big flood is on the way, a common response is to quickly build temporary levees using hastily filled bags of sand and mud.

During the 1993 Mississippi River flood, an estimated 26.5 million sandbags were filled and set in place. Some of the sandbag levees did lessen the damages, while others did not. But even where sandbag levees failed, a real therapeutic value was observed in the people working together for the common good (figure 13.34).

The U.S. Geological Survey maintains more than 7,000 stream-gauging stations that measure streamflow. Some of these stream gauges have been operating for more than a century.

Each stream-gauging station measures water depths, channel width, and water velocity, allowing calculation of the discharge or flow volume.

Rapid Creek, Black Hills, South Dakota, 1972 (cont) This time the lesson was learned by many. Canyon Lake Dam and many bridges were redesigned and rebuilt to prevent debris accumulations. The portion of Rapid Creek floodplain inundated in 1907 and 1972 was declared a floodway, and rebuilding was not permitted.

Even most buildings that survived the flood were moved out of the floodway. In their place lies an 8 km (5 mi) long greenway featuring a golf course, picnic areas, bike and jogging paths, recreation areas, ponds and ice-skating rinks, low-maintenance grasslands, and an area reseeded with native vegetation: in short, the floodway is now being used for activities that will not be harmed by the occasional flood. A Rapid City slogan is: "No one should sleep on the floodway."

Nonetheless, a stream's task is to move the sediment present with the water provided. How can a stream accomplish this task?

Excesses in discharge or load are managed by changing dependent variables, such as gradient and channel pattern.

FLOOD FREQUENCIES Another way of looking at flood runoff within urban areas is to see how urbanization affects the frequency of floods. Roofs and pavement increase the surface runoff of rainwater, thus causing higher stream levels in shorter times; that is, runoffs become flash floods.

Figure 13.37 shows the effects of building storm sewers (percent of area sewered) and of roofing and paving (percent impervious). For example, notice on the vertical axis that a discharge of 100 ft3/sec occurred about once every four years in the rural (unurbanized) setting but now happens about three times per year after urbanization.

(Flash Floods Cont) About 50% of these deaths are vehicle related. Not enough people appreciate what a shallow-water flood can do to a car (figures 13.16 and 13.17).

Flowing water about 0.3 m (1 ft) deep exerts about 225 kg (500 lb) of lateral force. If 0.6 m (2 ft) deep water reaches the bottom of a car, there will be a buoyant uplift of about 680 kg (1,500 lb), which helps the 450 kg (1,000 lb) lateral force push or roll the car off the road. Many automobile drivers and riders die in floodwater that is only 2 ft deep.

The lessening of gradient in a stream's lower reaches is partly due to the limitations of base level, the level below which a stream cannot erode.

For many streams, base level is the ocean, but base level for the Arkansas River is where it joins the Mississippi River. For a small stream, base level may be a lake or pond into which the stream drains.

The Great Midwestern Flood of 1993 (cont) A wet winter and spring passed into an even wetter summer of 1993 for Iowa and parts of eight other upper Midwest states, causing record flood levels on the lower Missouri and upper Mississippi Rivers (figures 13.26 and 13.27). In 1993, floodwaters occupied more than 20 million acres. The entire state of Iowa was declared a federal disaster area, as were sections of eight other states— North Dakota, South Dakota, Minnesota, Wisconsin, Illinois, Missouri, Nebraska, and Kansas. Flooding killed 48 people, completely submerged 75 towns, destroyed or damaged 50,000 homes, closed 12 commercial airports, and shut down four interstate highways (I-29, I-35, I-70, and I-635). Damages totaled about $12 billion, making this flood one of the most expensive disasters in U.S. history.

High water levels began in April and continued through August. Some towns had more than 160 consecutive days of flooding. At the end of August, the upper Mississippi River basin had endured record high floods, yet this floodwater mass did not significantly affect the lower Mississippi River basin because the input from the Ohio River was low (see figure 13.23).

The Uncoordinated Approach: San Diego The channelization style of the lower San Diego River has changed with the political winds. In the late 1940s, the U.S. Army Corps of Engineers was left alone to design a flood-control channel for the river mouth. Its channel is 245 m (800 ft) wide, has a natural bottom over which ocean tides roll in and out, and has walls of large boulders (riprap).

Hotel builders leaned on a weak city government in the late 1950s to gain permission to build on the floodplain next to the edge of the natural channel. The river channel is about 7.5 m (25 ft) wide, is naturally vegetated, and has a capacity of about 8,000 ft3/sec (figure 13.39). The 20th century's biggest flood in the San Diego River discharged 72,000 ft3/sec in January 1916. Awaiting the next major flood is a concrete-lined channel of 49,000 ft3/ sec capacity, which empties into a natural channel that will hold 8,000 ft3/sec, which in turn feeds into a 115,000 ft3/sec capacity channel (see figure 13.39). The planning process does have its flaws.

FLORENCE, ITALY, 1333 AND 1966 The people of Florence knew their largest flood; it occurred on 4 November 1333, when the River Arno inundated the city to 4.22 m (14 ft) depth. The flood is memorialized by several artistic creations.

However, 633 years later, to the day, the River Arno flowed through the city at 6.2 m (more than 20 ft) depth, causing heart-wrenching destruction and damage to ancient and priceless paintings, sculptures, tapestries, books, maps, musical and scientific instruments, and more. The 1966 events started on 3 November in the River Arno headwaters, when enough rain fell to equal one-third of the average annual rainfall. Upstream dams could not hold all the water. The river raged through villages, killing people, ripping apart buildings, and bursting open drums of oil. Some of the treasures of Florence were smashed, some were buried beneath the 500,000 tons of mud deposited by the flood, and many more were coated with a smelly, oily slime.

THE FLOODPLAIN Floodplains are the floors of streams during floods (figure 13.12).

Humans who decide to build on a floodplain are gamblers. They may win their gamble for many years, but the stream still rules the floodplain, and every so often it comes back to collect all bets.

The floodwater that escapes confinement by levees spreads out, inundating farms and buildings (figure 13.32).

In 1993, 1,083 out of 1,576 levees in the upper Mississippi River system were overtopped or damaged.

Weather Conditions for 1993 Floods Why were the 1993 floods so big? The weather pattern was remarkably similar for the big floods of 1927, 1973, and 1993. In each case, the preceding autumn and winter were wet, thus leaving the ground saturated as the new year began. In each of the years, the polar jet stream bent southward, forming huge troughs of low pressure that attracted cyclonic systems into the Mississippi River basin to drop their moisture.

In 1993, a cold air mass over Greenland and a high-pressure ridge over the northeastern United States resulted in large-scale bends in the polar jet stream (figure 13.28). The high-pressure ridge over the eastern United States produced record-breaking high temperatures. At the same time, the low-pressure trough over the Mississippi River basin brought moist air from the Gulf of Mexico into contact with cool, polar air masses, resulting in long-term heavy rainfall. The persistence of the polar jet stream pattern caused storm after storm to dump its water in the nation's heartland, producing high flood levels that went on week after week.

LEVEES. Levees have opponents and proponents. Opponents suggest that the costs of building more levees and dams may be higher than the value of the buildings they are protecting. They recommend lowering or removing levees along some farmland to allow river floods to spread out and dissipate their energies over a wide expanse of land, thus also lowering water levels in levee-protected major cities and towns.

In contrast, proponents of levees say that the increased carrying capacity of a river allows more years of high-water flow to occur without flooding. They state that flood damages would have been many billions of dollars more without the levees. The management of the 2011 floodwater in the Mississippi River is cited as an example of use of levees to minimize economic losses in a major flood.

REGIONAL FLOODS Regional floods are different from flash floods. High waters may inundate an extensive region for weeks, causing few deaths but extensive damages and severe tests of human endurance. Regional or inundation floods occur in large river valleys with low topography when prolonged, heavy rains result from widespread cyclonic systems.

In the United States, about 2.5% of the land is floodplain and home to about 6.5% of the population. Floodplains contain much valuable property that is periodically flooded.

Rising water goes through flood stages where water level is high enough to overtop the river banks/levees.

In the action stage, water begins over topping the banks. In the minor flood stage, roads, parks, and yards may be covered by water. In the moderate flood stage, building inundation occurs; roads are closed and evacuations may be necessary. In the major flood stage, buildings may be completely submerged; lives are threatened and large-scale evacuations may be necessary.

Flooding in China The Huang (Yellow) River is reputed to have killed more people than any other natural feature; perhaps that is why it became known as the "River of Sorrow." In the lower 800 km (500 mi) of its course, it flows over floodplain and coastal-plain sediments. Attempts to control river flow and protect people and property go back at least as far as the channel dredging of 2356 BCE. Levees are known to have been constructed since at least 602 BCE.

In the past 2,500 years, the Huang River has undergone 10 major channel shifts that have moved the location of its mouth as much as 1,100 km (more than 680 mi) (figures 13.29 and 13.30). In 1887, the Huang overtopped 22 m (75 ft) high banks, "discovered" lower elevations, and began flowing south to join the Yangtze River. The 1887 floods drowned people and crops, creating a one-two punch of floods and famine that were responsible for more than 1 million deaths. In 1938, the Yellow River levees were dynamited in the war with Japan, resulting in another million lives lost to flood and famine. Today, the riverbed is 20 m (65 ft) higher than the adjoining floodplain!

The outbursts from glacial meltwater lakes created the largest known floods in Earth history.

In the span of a few thousand years, the massive continental ice sheets melted in gigantic quantities, raising sea level by some 130 m (425 ft) (see figure 12.15).

FLOOD-FREQUENCY CURVES Everyone living near a stream needs to understand the frequency with which floods occur. Small floods happen every year or so.

Large floods return less often—every score of years, century, or longer. A typical analysis of flood frequency involves a plot of historic data on flood sizes versus recurrence interval (figure 13.13).

Mississippi River System The greatest inundation floods in the United States occur within the Mississippi River basin, the third largest river basin in the world; it drains all or part of 31 states and two Canadian provinces (figure 13.23).

Of the 28 biggest rivers in the United States, 11 are part of the Mississippi River system. In the lower reaches of the river, the average water flow is 18,250 m3/sec (645,000 ft3/sec); this may be increased fourfold during an inundation flood.

The Great Midwestern Flood of 1993 The summer of 1993 saw the biggest flood in 140 years of gauged measurements for the upper Mississippi River basin (table 13.3).

Notice how late in the year the 1993 peak flood occurred at St. Louis (table 13.3).

Antelope Canyon, Arizona, 1997. The plateau country of the southwestern United States is world renowned for the canyons cut into it—for example, Grand Canyon, Glen Canyon, and numerous tributary canyons leading to the Colorado River. Some of these tributary canyons are wonder-inspiring narrow clefts. can simultaneously touch both walls with your hands even though the walls rise near-vertical for 30 m (100 ft) above your head (figure 13.18).

Now imagine a localized, late afternoon thundercloud 18 km (11 mi) away—too far away for you to see or hear it. What can happen in the slot canyon? On 12 August 1997, in Antelope Canyon in Arizona, a flash flood roared down the canyon as a 3.3 m (11 ft) high wall of water that picked up 12 hikers and tumbled them along with it while helpless viewers watched from the canyon rim. The natural cathedral turned into a death trap; only one hiker survived.

Red River of the North (cont) In 1997, several variables combined to unleash record floods: (1) Fall 1996 rainfall was about four times greater than average. (2) Winter 1996 freezing temperatures began earlier than normal, thus freezing the water saturated in the soil. (3) Winter 1996-97 snowfalls were 3 to 3.5 times greater than average, bringing and storing more moisture in the region. (4) Spring 1997 began cold, including a blizzard on 4-6 April that brought 25 to 30 cm (10 to 12 in) of snow and record low temperatures, which delayed melting and draining. (5) Finally, flooding began as a rapid rise in air temperature melted snow and soil ice, sending water flowing.

On 8 April 1997, the average temperature was -13° C (9°F), but on 18 April, the average was 14°C (58°F). Snow melted, soil ice melted, and the ground everywhere was covered by overland waterflow that overwhelmed the water transporting ability of the flat-bottomed Red River. In North Dakota, the flood caused damages exceeding $1 billion; forced the evacuation of 50,000 people; destroyed potato, sugar beet, and grain crops; prevented planting of the next crop; and drowned farm animals, including 123,000 cattle. In Manitoba, the flood caused damages exceeding $815 million and forced the evacuation of 25,000 people. How common is a flood this size? The Red River flood frequency curve (see figure 13.14) indicates that the 1997 flood has a recurrence interval of almost 200 years.

Rapid Creek, Black Hills, South Dakota, 1972 At the foot of the Black Hills sits Rapid City, first settled south of Rapid Creek in 1876. The early inhabitants were wary of the Rapid Creek floodplain, and the wisdom of their caution was borne out by a large flood in 1907. In 1952, Pactola Dam was built 16 km (10 mi) upstream to provide flood protection and a reserve water supply for Rapid City. The dam eliminated most small floods, giving some people a false sense of security. The river reoccupied its floodplain with vengeance, leaving destruction totaling $664 million in 2002 dollars. Floods in the region killed 238 people, mostly in Rapid City, and destroyed 1,335 homes and 5,000 automobiles.

On Friday, 9 June 1972, southeast winds bringing moist air from the Gulf of Mexico met a cold front coming from the northwest. Under conditions similar to those at Big Thompson Canyon, the moist air turned upward to build 16 km (10 mi) high thunderclouds that remained stationary due to weak upper-level winds. Shortly after 6 p.m., heavy rain began to fall; up to 38 cm (15 in) fell in less than six hours, but most of the rain fell downstream from Pactola Dam. Rain runoff filled Canyon Lake, built on the western edge of Rapid City. The spillway at Canyon Lake Dam became plugged by automobiles and house debris, causing the lake level to rise an additional 3.6 m (12 ft). Then the dam failed at about the same time as the natural flood crest arrived, unleashing a torrent of water on Rapid City (figure 13.22).

Flood Styles Killer floods are unleashed by several phenomena: (1) A local thundercloud can form and unleash a flash flood in just a few hours. (2) Abundant rainfall lasting for days can cause regional floods that last for weeks. (3) The storm surges of tropical cyclones flood the coasts. (4) The breakup of winter ice on rivers can pile up and temporarily block the water flow, and then fail in an ice-jam flood. (5) Hot weather can cause rapid melting of snow. (6) Short-lived natural dams made by landslides, log jams, or lahars fail and unleash floods. (7) Human built dams and levees fail, causing voluminous floods.

One useful way to distinguish flood styles is by contrasting flash floods with regional floods. A typical flash flood results when rain falls intensely for hours in a small area, causing the runoff of a fast-moving, powerful flood of water that lasts only a short time. Regional floods occur when lots of rain falls over a large area for days or weeks, causing river flood levels to rise slowly and then to fall slowly.

Feedback Mechanisms Many systems display either negative or positive feedback. Negative-feedback occurs in self-regulating systems and works to maintain a system in equilibrium. In the case of a stream, when too much water pours into the channel, it triggers negative-feedback responses whereby increased erosion lowers the gradient to slow the water flow and maintain equilibrium.

Positive feedback is also known as the "vicious cycle." It occurs where one change leads to more of the same, and the whole system changes dramatically in one direction. In a human sense, positive feedback can have positive results if, for example, the system is your money in the bank; the interest you earn gathers more interest, and the system (your money) grows rapidly. But positive feedback can have negative results if, for example, you have credit-card debt and the resultant interest charges add to your debt, causing you to pay interest on earlier interest charges. Positive feedback can make you rich or poor depending on whether you choose to invest your money or incur debt.

The Hit-and-Miss Approach: Tucson In the northern reaches of the great Sonoran Desert lies Tucson, Arizona, where annual rainfall averages only 28.3 cm (11.14 in). September of 1983 was the second rainiest in Tucson history; it was capped off by the arrival of Tropical Storm Octave, which dropped 17.1 cm (6.71 in) of rain.

Rillito Creek cuts across Tucson, collecting much of the urban runoff and feeding the Santa Cruz River. On 1 October 1983, the Santa Cruz River at Congress Street was discharging 1,490 m3/sec (52,700 ft3/sec) of floodwater, causing major damage in the urban area. When the waters abated, 13 people were dead. Based on the entire flood history of Tucson, the Federal Emergency Management Agency had estimated the 100-year flood to be 30,000 ft3/sec. But the 1983 flood was 1.76 times bigger. Looking at the 20th-century flood record in Tucson shows that six of the seven largest floods occurred between 1960 and 1983. This coincides with growth in population from 265,700 in 1960 to 603,300 in 1984. All the paving and roofing that comes with urbanization may be increasing flood sizes.

Strategies to Reduce Runoff There are actions that can be taken within urbanized areas to lessen urban runoff and floods.

Roofs of buildings can be built to support soil and plants that capture rainwater, which is held for plant transpiration and later evaporation. Roads and sidewalks can be built using permeable materials, thus allowing much rainwater to soak into the ground. Storm-sewer pipes that carry away rain runoff can be diverted into human-dug ponds and basins in city parks and open spaces, where the water can soak underground.

A change in ocean circulation would in turn make changes in global climate. At 12,900 years ago, climate cooled about 5°C (9°F) in the event known as the Younger Dryas (see figure 12.16).

Scientists attribute this dramatic plunge back into colder temperatures to a gigantic meltwater flood through the St. Lawrence River into the North Atlantic Ocean

Dams have long been designed with emergency spillways to divert floodwater that exceeds the design of the dam, thus helping prevent dam failure and catastrophe.

Some levees are now being built with a similar philosophy of planned failure points. Certain levee sections are built lower, allowing extreme floods to flow out over open land or minimally developed land. This helps prevent levee failure at sections protecting cities or highly developed land.

The most famous of the ice-dam failure floods are preserved in the "channeled scablands topography in southeastern Washington. Here, the underlying Columbia River flood basalts were swept clean of overlying sediments, while river valleys were cut into the plateau, creating a maze of gigantic channels between scoured flatlands.

Some of the biggest floods burst forth from the widespread glacial Lake Missoula, which was impounded by a glacier that flowed across Clark Fork and acted as a dam (figure 13.43). Ancient wave-cut shorelines testify to a 610 m (2,000 ft) deep lake. When the ice dam failed, a water volume of 2,500 km3 (600 mi3) was released in a flood that lasted up to 11 days. For comparison, the flood volume was more than five times greater than the volume of Lake Erie. Discharge is estimated to have been greater than 13.7 million m3/ sec (484 million ft3/sec). Floods moved at velocities greater than 30 m/sec (67 mph).

A Different Kind of Killer Flood The 15th of January 1919 was unusually hot in Boston. As the temperature climbed higher, the pressure of 27 million pounds of expanding molasses was too much for its heated tank to hold at the Purity Distilling Company.

Steel bolts popped with a sound like gunfire, and the steel panels of the tank burst apart, releasing a flood of crude molasses; 2.3 million gallons flowed forth as a 4.5 m (15 ft) high brown wave. As the molasses flowed, it cooled and congealed, holding people so tightly in its sticky grasp that rescue workers spent hours freeing them. The great molasses flood of 1919 killed 21 people and injured 150. Flood threats are not always obvious.

Societal Responses to Flood Hazards People like to be near rivers. Rivers provide food and drink, business and transportation, arable land and irrigation, power, and an aesthetic environment. But being near rivers also means being subjected to floods. Human responses to flood hazards have been in two main categories: structural and nonstructural.

Structural responses include constructing dams to trap floodwater; building levees along rivers to contain floodwater inside a taller and larger channel; engineering projects designed to increase the water-carrying ability of a river channel via straightening, widening, deepening, and removing debris; and short-term actions such as sandbagging. Nonstructural responses include more accurate flood forecasting through the use of satellites and high-tech equipment, zoning and land-use policies, insurance programs, evacuation planning, and education.

When a flood crest passes downstream, stream level does not fall as rapidly as it rose.

That is because the stream was fed water by underground flow of rain that soaked into the ground and moved slowly to the stream; that is, the falling limb of the hydrograph has a gentle slope.

The first suggested action was to save Cairo, Illinois, by blasting a gap in the Birds Rock levee, thus sending floodwater out through the New Madrid flood way to submerge 530 km2 (130,000 acres) of Missouri farmland and to evacuate about 300 residents. This was the first time that three Mississippi River flood control systems were open at the same time. The system as a whole performed as designed. A huge flood was managed. Overall losses were about $4 billion, but they would have been much higher without the water diversions.

The Missourians challenged the action in the U.S. Supreme Court, but lost. A 3 km (2 mi) gap was blasted through the levee on 3 May, sending out a wall of water up to 15 feet high that washed away 2011 crop prospects and damaged about 100 Missouri homes, but Cairo, Illinois, was spared. High water continued flowing downstream in a slow motion disaster. Even though the highest water levels were a week or two away, people along the lower Mississippi River began evacuating as their anxiety rose along with the water level. On 14 May, 10-ton steel floodgates were slowly raised at the Morganza Spillway, sending Mississippi River water westward into the Atchafalaya Basin floodway. Slowly rising water flowing through the Atchafalaya River to the Gulf of Mexico affected about 25,000 people, 11,000 buildings, and 600 oil and gas wells. At the same time, the concrete control structure at the Bonnet Carre Spillway was opened, thus diverting some Mississippi River water into Lake Pontchartrain, which then moved south into the Gulf of Mexico. If the Morganza and Bonne Carre Spillways had not been opened, both Baton Rouge and New Orleans would have been flooded.

Flood frequency can also be expressed as the statistical probability of stream discharges of a given size arriving in any year or number of years (table 13.2).

The bigger the flood, the longer the return period and the smaller the probability of experiencing it in any one year. Statistically, the 100-year flood has a 1% chance of occurring any year. What is the probability that a 100-year flood will occur once in 100 years? The obvious answer (once) is wrong; from table 13.2, the probability is only 63%. No flood has a 100% chance of occurring.

We must distinguish between yearly versus cumulative probability. In cumulative probability, the longer the wait for a 100-year flood, the more likely its occurrence becomes. Nevertheless, the yearly probability of a flood is the same for any year regardless of when the last flood occurred.

The confusion that commonly arises when hearing of a "100-year flood" has led some people to stop using the term and to replace it with "1%-chance flood." A 1%-chance flood is a flood event that has a 1% chance of occurring or being exceeded in any given year.

Desert floods are different. Most of the damage is due to bank erosion, not inundation. Stream channels are cut into loose, sandy sediments, forming weak banks that crumble easily.

The critical floods here are not those with an urban-style, high-water peak of short duration (see figure 13.36), but longer-duration floods (flat-topped flood hydrograph) that have time to soak the dry stream bed and banks, thus freeing later floodwaters to concentrate their energies on erosion. Flood erosion changes channel location both by cutting new channel segments via over bank flow and by meander migration where the banks erode rapidly. Since the 1940s, some stream banks have eroded laterally more than 300 m (1,000 ft). One big storm and erosive flood can change stream-channel positions dramatically (figure 13.40).

The discharge of a huge volume of cold, low-salinity glacial meltwater could change the global circulation of deep water through the world ocean (see figure 9.29).

The discharge of a huge volume of cold, lowsalinity glacial meltwater could change the global circulation of deep water through the world ocean (see figure 9.29). The deep-ocean circulation is driven by regional differences in the heat and salinity of ocean water, and this could be changed by a huge influx of cold, fresh meltwater.

Case 2—Too Much Load If a stream is choked with sediment and has insufficient water to carry it away, this also triggers negative feedback.

The excess sediment builds up on the stream bottom, increasing the gradient and causing streamwater to flow faster and thus have more load-carrying capacity (figure 13.9).

Floodwaters moved gravelly sediments in giant ripples up to 15 m (50 ft) high with distances between their crests of 150 m (500 ft) (figure 13.45).

The failure of an ice-dammed lake can send so great a volume of water running over and eroding the land that it can change the paths of rivers. In North America, melt water floods flowed along the Mississippi River to the Gulf of Mexico, the St. Lawrence and Hudson Rivers to the North Atlantic Ocean, and the Mackenzie River to the Arctic Ocean or via the Hudson Strait to the Labrador Sea.

Big Thompson Canyon, Colorado, 1976 (cont) At 6 p.m., the Big Thompson River flowed with 137 ft3/ sec of water; at 9 p.m., the flow was 31,200 ft3/sec. Flow volume was 3.8 times greater than the estimated 100-year flood.

The flash flood killed 139 people, and 6 more were never found; 418 houses were destroyed, along with 52 businesses and more than 400 cars, and damages totaled $36 million (figures 13.20 and 13.21).

Flood-discharge volumes are plotted on the vertical axis, and the recurrence intervals are plotted on the horizontal axis in years between floods of each size.

The longer the historical record of floods in an area, the more accurately the curve can be drawn. With a flood-frequency curve, the return times of floods can be estimated. You can try this in figure 13.13: move upward from 100 years, intercept a curve, and then read to the left to obtain the expected flood size. The U.S. Federal Emergency Management Agency (FEMA) uses the 100-year flood in establishing regulatory requirements.

The scablands landscape is marked by scoured bedrock and immense former waterfalls, such as Dry Falls in Grand Coulee, Washington (figure 13.44).

The massive floods carried boulders more than 10 m (33 ft) in diameter for miles.

Common rudiments that warn of a coming flood are: the advanced construction of a boat, the storage of animals, the inclusion of family, and the release of birds to determine when the water level subsides - these link all these flood tales.

The overwhelming consistency among flood legends found all over the globe indicates that they were derived from the same origin - a record of dramatic Earth shifts.

Constructing Flood-Frequency Curves Floods are random events. It is not possible to predict just when a flood will occur or what its discharge will be. But we need to know how often a given-size flood may occur in order to intelligently develop land.

The process used is statistical. If enough runoff records exist for a river, then probable flood frequencies can be estimated. Here is a relatively simple method for constructing a flood frequency curve: Begin by determining the peak discharge for each rainfall year. Ignoring the chronologic order of the rainfall years, rank each annual flood in order, from biggest discharge (= 1), second biggest (= 2), etc., on down to the smallest. To plot a curve such as figure 13.13 for a river of interest, first calculate recurrence intervals for each year's maximum flood using the formula recurrence interval = (N + 1) / M where N = the number of years of flood records, and M = numerical rank of each year's maximum flood discharge. After calculating a recurrence interval for each year, locate that value (in years) on the horizontal axis (which usually is a logarithmic axis as in figure 13.13); then move upward until reaching the appropriate discharge value. Stop and plot a point marking the intersection of the recurrence interval and discharge values. After plotting a discharge versus recurrence interval point for each year, draw a best-fit line through your plotted points.

Figure 13.36 shows flood hydrographs from similar-size rainstorms in Brays Bayou in Houston, Texas, both before and after urbanization in the drainage basin.

The rainstorm of October 1949 mostly soaked below the surface and flowed slowly underground to feed the stream running through the bayou. Following urbanization, the rainstorm of June 1960 produced a flood hydrograph with a very different shape. This is a proverbial good news-bad news situation for city dwellers. The good news is that the urban flood lasted only 20% as long; the bad news is that it was four times higher.

Some Historic Mississippi River Floods New Orleans was founded in 1717 in the lower Mississippi River basin. It experienced its first large flood in the same year

The response to the flood in 1717 was the same as today— levees were built higher to keep the river water inside its channel.

Big Thompson Canyon, Colorado, 1976 Saturday, 31 July 1976 was the eve of the 100th birthday of Colorado statehood and the start of a three-day centennial weekend. Late-afternoon cloud building is a common phenomenon and usually concludes with upperlevel winds pushing the thundercloud eastward over the plains. On this day, the mid and upper-level winds were weak, the thunderstorm remained stationary, and a "cloudburst" ensued (figure 13.19).

The slightly tilted updraft structure allowed rain to fall profusely. From 7:30 to 8:40 p.m., rainfall was as heavy as 19 cm (7.5 in); in four hours, it equaled a typical year's total. Rain runoff from the steep, rocky slopes fed a flash flood that roared down the canyon, with an initial wall of water reaching 6 m (20 ft) high in the narrows at the eastern end of Big Thompson Canyon. The flood crest moved 25 km/hour (15 mph) through the entire canyon, which did not allow much time to spread warnings. More than 400 automobiles were on Highway 34 within the canyon. Drivers were presented with a quick choice—abandon their cars and run upslope or stay inside and try to outrace the flood. Those who abandoned their cars spent an uncomfortable night on the rain-swept canyon walls; those who stayed with their cars died. The new road signs now advise: "Climb to safety! in case of a flash flood."

The channel pattern responds by straightening to shorten the flow distance and increase the gradient.

The straighter stream still contains excess sediment, causing the water to pick its way through as a braided stream (figure 13.10).

ICE-DAM FAILURE FLOODS Some of the biggest floods known on Earth occurred during the melting of the continental ice sheets.

The sudden failure of ice dams still occurs today and has been observed and photographed—for example, at ice-dammed Strandline Lake in Alaska. When ice dams blocking the largest glacial lakes failed, stupendous floods resulted. Their passage is still recorded in lake sediments; by countryside stripped of all soil and sediment cover; by high-elevation flood gravels; by an integrated system of braided channels (a mega-braided stream); by abandoned waterfalls; by high-level erosion; and by large scale sediment deposits.

The Binational Approach: Tijuana and San Diego Rivers commonly serve as boundaries between nations. But the boundary between westernmost Mexico and the United States is drawn as a straight line, cutting through the 6,039 km2 (2,325 mi2) drainage basin of the Tijuana River: 80% in Baja California, 20% in California.

The two countries agreed on a Los Angeles-style project to cement the river channel to the sea. Mexico carried out its side of the agreement, but then the environmental ethic arose in California, and the cement-lined channel project was blocked in the United States. The result is the large concrete channel in Tijuana sends high-velocity floods charging into the farms and subdivisions of southernmost San Diego.

La Niña conditions in the Pacific Ocean commonly result in wetter weather in midwestern North America in spring and early summer.

The year 2008 was a La Niña year. Powerful cold-weather fronts moving slowly to the east and warm, moist air flowing north from the Gulf of Mexico collided over the Midwest again and again, week after week, and flood records were broken. • In Iowa, 83 of its 99 counties were declared disaster areas. • In Cedar Rapids, Iowa, the Cedar River rose to 31.12 ft, more than 11 ft higher than its previous record in 1929. •High-water levels forced the evacuation of a hospital, a prison, and thousands of homes (figure 13.1). Property damages during regional floods are high (table 13.1), and high-water levels take weeks to subside to nonflood levels.

HYDROGRAPHS A hydrograph plots the volume of water or stream depth versus time; it records the passage of water volumes flowing downstream (figure 13.35).

There is a time lag for rainwater to flow over the ground surface and reach a stream channel, but stream surface height usually rises quickly once surface runoff reaches a channel; that is, the rising limb of the hydrograph is steep.

2011 Flood Management April and May 2011 storm systems, including three with major tornado outbreaks (see Chapter 10), brought voluminous rains to the Mississippi River system, resultingin record or near-record river levels in Illinois, Missouri, Kentucky, Tennessee, Arkansas, Mississippi, and Louisiana. They needed to protect the southern region, including Baton Rouge, New Orleans, and the numerous oil refineries and chemical plants along the lower Mississippi River; this meant giving orders in regions to the north to divert massive flows of Mississippi River water into floodways and out onto historic floodplains.

These actions were designated in the 1930s to serve as relief valves whenever major river flood stages became too high, but now these floodways have farms and towns upon them. The goal of minimizing overall flood damage means deliberately causing harm and economic damage to many people so that even greater numbers of people may avoid harm and economic damage. Cost/benefit ratios must be evaluated.

FORECASTING Thanks to technologic advances, our growing knowledge of weather and floods allows better forecasts of the time and height of regional floodwaters.

These forecasts have significantly reduced the loss of life. But it is interesting to note that the twin trends of better forecasting and engineering are offset by ever-greater dollar losses during big floods. We know more, yet suffer greater damages.

ZONING AND LAND USE A standard approach to lessening flood losses is to ban building on the portion of the floodplain that will be covered by the 100-year flood.

This policy was adopted by the National Flood Insurance Program in the early 1970s, was issued as Executive Order 11988 in 1977, and has been used by the Federal Emergency Management Agency (FEMA) since 1982. Notice in table 13.2 that the 100-year flood has a 1% chance of occurring in any year, a 9.6% chance of happening each 10 years, and a 22% chance every 25 years. Although adoption of the 100-year flood standard does not prevent all structures from being flooded, it does discourage some construction at frequently flooded sites.

The U.S. Army Corps of Engineers works hard to make the Mississippi River stay put and continue flowing through the important cities of Baton Rouge (the capital of Louisiana) and New Orleans (a major U.S. port). It takes a huge commitment of time, money, and effort to control a river's course.

This switching course from the Mississippi to the Atchafalaya has not been entirely a natural one; humans have inadvertently aided the process by cutting through meanders (see figure 13.7). A particularly lengthy meander used to exist where the Mississippi and Atchafalaya rivers met. This meander so annoyed ship captains in the early 1800s that it was cut through to shorten their trips. Congress directed the Army Corps of Engineers to build flood-control structures that allow 30% of the water to flow down the Atchafalaya but to keep 70% in the Mississippi River (figure 13.25). Nature prefers the Atchafalaya; humans prefer the Mississippi. This is an ongoing battle that never can be finished.

After 1927, a project design flood was developed as a hypothetical "maximum probable" flood of the Mississippi River. Flood peak discharges were calculated along the river at about 25% greater than the 1927 flood.

To control and manage floods: (1) levees were raised higher and built in new places, (2) the river bottom was dredged to increase the volume of water that could flow in the river, (3) dams were constructed on tributary rivers to capture and control floodwater, (4) river meanders were straightened to shorten the river by 270 km (170 mi), and (5) diversions were established to funnel part of the major river flow into flood ways to utilize smaller rivers and to flood lowlands. Despite all these efforts, in the spring of 1973, the Mississippi River system flooded along 1,930 km (1,200 mi) of rivers in 10 states, again inundating 50,000 km2, reaching record flood heights at numerous sites, and staying above flood stage for 97 days at Chester, Illinois, 77 days at St. Louis, and 63 days at Memphis.

Johnstown, Pennsylvania, Flood, 1889 The deadliest dam-failure flood in U.S. history occurred on 31 May 1889. The old dam was modified, cottages were built, and the South Fork Fishing and Hunting Club opened for its wealthy owners. Everyone knew the dam was low quality; its frequent leaks were usually patched with mud and straw. The dam impounded a 3 km (2 mi) long lake that was up to 1.6 km (1 mi) wide and 18 m (60 ft) deep at the dam.

Unfortunately, it was located on the floodplain at the fork of the Little Conemaugh and Stony Creek Rivers. Beginning in 1838, the state built the South Fork Dam 23 km (14 mi) up the Little Conemaugh River from Johnstown. The dam site was later abandoned, reused, and abandoned again; then, in 1881, it was purchased privately. Then, truly bad weather struck. On 30-31 May, up to 25 cm (10 in) of rain fell in 24 hours. Johnstown was flooded up to 3 m (10 ft) deep. But a bad situation became worse. At 4:07 p.m. on 31 May, the dam failed. A flood crest up to 11 m (36 ft) high swept into town carrying debris, including the stock of a barbed-wire factory. The worst of the flood was over in 10 minutes, but more than 2,200 people were dead, which amounted to about 7% of the residents and included the elimination of 99 entire families. On 19 July 1977, another big rainstorm flooded Johnstown up to 2.4 m (8 ft) deep. No dam failed this time; nonetheless, 80 people were killed by floods in the region.

A close look at the meandering process tells us much of value (figure 13.7).

Water does not flow at even depth and power across a stream. Instead, a deeper, more powerful volume of water flows from side-to-side, eroding the river bank on one side and then on the other. This lengthens the path of the stream from left to right, decreases the gradient, and slows the water flow. • Notice that deposition of sediment occurs on the inside bend of each meander where water is shallower and less powerful. • The meandering process can proceed so far that two erosional banks can merge and create a shortcut that straightens the river locally (see center right of figure 13.7).

The Mississippi River carries immense volumes of sediment to the sea, where it is deposited within the Birdfoot delta lobe building out into the Gulf of Mexico (figure 13.24).

When a river deposits sediment on its channel floor, its channel bottom elevation grows higher.

Avulsion

When major floods occur and water over topping banks and levees flows to lower elevations outside the channel, the river may adopt a new lower-elevation course and abandon its old channel Look at the delta lobes in figure 13.24; each represents an avulsion event, a changing of the course of the Mississippi River. Today, you can stand on the Mississippi River floodplain north of New Orleans and look up at the river and see ships passing above you. This is an unstable position for a river. It leads to river-channel abandonment and establishment of a new path to the sea.

Another "too much load" situation for a stream is the presence of a lake. For example, if a landslide dams a stream, it adds excess load that the stream will attempt to carry away. The stream will gradually fill in the lake basin with its load of sediment until flow can reach the dam (figure 13.11).

When the stream is able to flow rapidly over the steep-gradient face of the dam, it does so with heightened erosive power, allowing it to carry away the landslide dam as well as the sediment fill in the lake. In a geologic sense, lakes are temporary features that streams are striving to eliminate.

THE EQUILIBRIUM STREAM Numerous factors interact to make streams seek equilibrium,

a state of balance in which a change causes compensating actions.

Individual flood-frequency curves must be constructed for each stream because each stream has its own characteristic floods. A flood-frequency curve should serve as the

basis for designing all structures built on a floodplain and determining where buildings should be located for the highest probability of safety. Planners can decide what size flood (how many years of protection) to accommodate when determining how the land is to be used.

A typical stream has too much load and too little discharge in its upstream portions, thus it maintains a

braided channel pattern there. In its downstream segments, the typical stream has too much discharge, finer sediments in its load, and less friction, thus it runs in a meandering pattern there. Streams also change their equilibrium states from one season to another and in response to global changes in sea level and to tectonic events. That most streams have the same longitudinal cross-section is a testimony to the effectiveness of their negative-feedback mechanisms.

Ancient floods cooled the Earth, and torrents of water, bigger than any flood ever humans have ever seen on Earth,

covered a swath hundreds of miles wide, with thousands of feet of churning water reshaping the planet's surface.

Case 1—Too Much Discharge (cont) The stream also responds by increasing the sinuosity of its channel pattern through meandering (figure 13.5).

meandering stream cuts into its banks, thus using some of its excess energy to erode and transport sediment.

No matter where you live—be it the tropics, the plains, or the desert

desert—floods occur. Within a human lifetime, everyone will have a flood pass near them.

When designing roads, bridges, and buildings, it is seductive to consider only the smaller floods and save large amounts of money on initial construction costs. However, these initial savings can be eaten away by

higher maintenance and repair costs. In the long run, it is commonly cheaper to build in anticipation of large floods; this can save money in the future and also eliminate much of the human suffering that occurs when homes and other buildings are flooded.

Human intervention in the meandering process has been common. Ships spending time and energy sailing long

humans excavating through meanders to shorten sailing paths. This human intervention has had unintended yet profound effects on rivers such as the Mississippi.

By similar analysis, even though a "150-year flood" may occur one year, it is still possible for another of the same size to come again in the following year or even in the same year. For example

in 1971, the Patuxent River between Baltimore and Washington, DC, had a flood that was 1.6 times bigger than its calculated 100-year flood. The next year, in 1972, the Patuxent River conveyed a flood that was 1.04 times as big as its 100-year flood. A 100-year flood, or any other size flood, is a statistically average event that occurs by chance, not at regular intervals. As the adage states: "Nature has neither a memory nor a conscience."

The word levee is derived from the French verb

lever, which means to raise. Humans commonly increase the height of natural levees built by stream overbank flow, or we construct new levees where none exist.

Notice how the meandering pattern lengthens the flow path,

lowering the stream's gradient and thereby slowing water flow (figure 13.6).

Case 1—Too Much Discharge If a stream has too much water, it will flow more rapidly and energetically. The move away from equilibrium triggers

negative-feedback responses which work to correct the imbalance: (1) Some of the excess energy is used in eroding the stream bottom (figure 13.4). (2) The sediment picked up by erosion adds to the load carried by the stream, thus consuming more of the excess energy. (3) Notice in figure 13.4 that the slope of the stream bottom is lowered by erosion, reducing the vertical drop downstream and causing slower and less energetic water flow.

How valuable are flood-frequency curves? Their reliability is directly related to the

number of years of flood records; the longer the record, the better the flood-frequency curve. In this method, the recurrence interval for the largest flood on a river is the most suspect point; it is based on a sample population of one. Statistical methods are available to help plot the upper segments of flood frequency curves for the rare, extra-large floods. Using a statistical technique, the 1997 flood plots with a recurrence interval of roughly 200 years (figure 13.14).

Why is flood insurance purchased by such small percentages of people? It is an expense, and some homeowners may be hoping that

politicians will provide federal dollars to help disaster victims. For example, the U.S. Congress passed a bill providing $6.3 billion to aid people hit by the 1993 flood in the upper Midwest.

Flash floods cover small areas and are sometimes called upstream floods, whereas regional floods

sometimes called upstream floods, whereas regional floods inundate large areas and are called downstream floods. Flash floods can be deadly, whereas regional floods cause widespread economic losses. In 2011, U.S. flash floods killed 69 people and caused damages of $1.44 billion; regional floods killed 44 people and caused $6.77 billion damages.

GRADED-STREAM THEORY All streams operate in a state of delicate equilibrium maintained by constantly changing the gradient of the

stream bottom, thus sustaining a graded stream. Every change in the system triggers compensating changes that work toward equilibrium.

The greater the discharge, the greater

the load of sediment carried. Both discharge and available sediment are independent variables—that is, the stream has no control over how much water it will receive or how much sediment is present.

It's important to remember these legends because they are telling us that the Earth changes, and when it shifts,

the oceans create great floods, mountains tumble into the sea, and many lands are buried beneath water.

FLASH FLOODS Large convective thunderstorms can build up in a matter of hours and quickly set loose the terrifying walls of water known as flash floods. Steep topography helps

thunderstorms build and then provides the rugged valleys that channelize the killer floods. Most flood-related deaths in the United States are caused by flash floods.

Levees are commonly built of soil and sediment and thus are not strong; levees can fail. As levees become water saturated, the river finds weak spots, compromising the levees by

wave attack, erosion by overtopping, failing by slumping, and undermining by piping (figure 13.31).

LEARNING OUTCOMES All around the world, when large volumes of rain fall, floods will flow. After studying this chapter, you should:

• understand that streams are equilibrium systems. • be able to explain positive versus negative-feedback. • appreciate how and why streams construct floodplains. • be familiar with flood-frequency curves. • know the differences between flash floods and regional floods. • be familiar with levees and the control of flood flow. • understand the effects of urbanization on flood peak heights and duration.