Chapter 13: Floods

Regional floods occur when ________.

Prolonged, heavy rains result from widespread cyclonic systems

How does the meandering process occur?

*Water flows from side-to-side in a stream, eroding its banks. *Erosion occurs on one side of the stream and net deposition on the other.

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

As the delta lobe builds seaward, the river's length increases, and sediment is deposited within the river channel. When a river deposits sediment on its channel floor, its channel-bottom elevation grows higher. Over time, the channel bottom may build to an elevation higher than its adjoining floodplain.

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

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. (8) Atmospheric rivers deliver huge volumes of rainfall.

Numerous factors interact to make streams seek equilibrium, a state of balance in which a change causes compensating actions.

(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 (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).

Why are floods so common along the Red River of the North?

(1) The Red River valley is 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 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.

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) (5) diversions were established to funnel part of the major river flow into floodways to utilize smaller rivers and to flood lowlands

What factors came together to cause the Red River Flood in the spring of 1997?

*A rapid rise in temperature *Heavy rainfall the previous fall *Heavy snowfall the previous winter

Major flooding of the Mississippi River occurred in 1927, 1973, and 1993. What weather conditions contributed to all three flood events?

*A southward bend in the polar jet stream *Repeated wet storms over a long period of time *Saturated ground from wet autumn and winter seasons

According to graded-stream theory, which of the following are true?

*All streams operate in a state of delicate equilibrium by constantly changing the grade of the stream bottom. *Streams are more likely to have braided sections in their upstream portions than meandering sections.

Big Thompson Canyon, Colorado, 1976

*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 *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."

Which of the following are ways in which a stream will typically respond to increases in discharge?

*Erosion will increase along its bottom. *The sediment load will increase.

Which of the following can cause flooding?

*Failure of an ice dam *Heavy rain over a short period of time *Failure of levees *Heavy rain over a long period of time

Flash floods vs. Regional floods

*Flash floods cover small areas and are sometimes called upstream floods. *Flash floods can be deadly *Regional floods inundate large areas and are called downstream floods. *Regional floods cause widespread economic losses.

How do flash floods and regional floods differ?

*Flash floods occur in smaller sized drainage basins. *Regional floods are caused by longer rain events.

Which of the following are true about floodplains?

*Floodplains become the floor of streams during floods. *They are built by both deposition and erosion.

Which of the following accurately describe the cross-sectional profile of a typical stream when the graph of the profile is vertically exaggerated?

*It is steeper at or near the source of the stream. *It has a lower gradient near the mouth of the stream.

What factors contributed to the deadly 1972 flood of the Rapid River?

*Most rain fell downstream of a dam built to protect Rapid City. *Extremely heavy rain over a short period of time *Failure of the Canyon Lake dam in Rapid City *Development on the flood plain in Rapid City

Regional floods differ from flash floods in that they ______.

*Occur over a wider area *Can last for several weeks *Occur in large river valleys

After a study of flood potential in New Orleans was conducted in response to the 1927 flood, new flood control measures were put in place, including _____.

*Raised levees and new levees *Dredging of the river bottom *Construction of floodways *Construction of dams

According to the green flood frequency curve in this figure, ________.

*Small floods occur more frequently. *Larger floods have a longer recurrence interval. *Small floods happen every year or so.

Which of these factors led to the large scale of the Big Thompson Canyon flash flood in 1976?

*Steep rocky slopes that directed runoff into the canyon *A cloudburst from a thunderstorm *19 cm of rain over four hours

Which of the following are true about floods in Florence, Italy?

*The flood of November 1333 was the biggest flood on record for 633 years. *Floods in Florence leave behind thick deposits of mud and other debris.

Rapid Creek, Black Hills, South Dakota, 1972

*The moist air turned upward to build 16 km (10 mi) high thunderclouds that remained stationary due to weak upper-level winds *up to 38 cm (15 in) fell in less than six hours, but most of the rain fell downstream from Pactola Dam. *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). *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.

Mississippi River System

*The third largest river basin in the world; it drains all or part of 31 states and two Canadian provinces. *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.

Moderate Flood Stage

-Building inundation occurs, roads are closed and evacuations may be necessary

Major Flood Stage

-Buildings may be completely submerged; lives are threatened and large-scale evacuations may be necessary

Minor Flood Stage

-Roads, parks, and yards may be covered by water

A 66-year flood occurs on the Flushing River in 2014. What is the probability of a 66-year flood occurring on the same river in 2015?

1.5%

According to the flood recurrence interval equation, a flood that occurs after 90 years of record-keeping, and is the seventh-largest ever recorded has a recurrence interval of _______ years

13

The flooding that occurred in the upper Mississippi River basin in _______ (year) was caused by winter, spring, and summer all being wetter than usual; this flood was the biggest in 150 years in the basin.

1993

When designing roads, buildings, and bridges in a floodplain, engineers consider the statistical probability of a flood of a given size happening in a given year. A 50-year flood has a ____ probability of occurring in a given year.

2%

According to the flood frequency curve for the Navasota River, a flood event with a discharge of 60,000 cubic feet per second has a recurrence interval of approximately ______ years.

20

According to the flood recurrence interval equation, a flood that occurs in 2014, after 149 years of record keeping, and is the third largest ever recorded has a recurrence interval of _______ years.

50

Antelope Canyon, Arizona, 1997

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.

Which of the following is true about the largest floods in an area?

A future flood will likely exceed the largest flood already recorded in an area.

HYDROGRAPHS

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

Flash Flood Definition

A local and sudden flood of relatively great volume and high velocity that lasts for a short time following a few hours of intense rainfall. Flash floods cause many deaths.

Levees

A natural or human-built embankment along the sides of a stream channel

How do streams respond to an increase in sediment load?

A negative feedback is triggered as sediment is deposited on the stream bottom, thereby increasing the gradient and velocity.

The Flood Control Act of 1928

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

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 FEMA since 1982.

Flood-Frequency Curve

A typical analysis of flood frequency involves a plot of historic data on flood sizes versus recurrence interval

Match the letters to the correct descriptions to explain how a stream breaches a natural dam.

A- Landslide creates natural dam. B-Stream fills lake basin with sediment. C-Water flows over dam, eroding it.

Dams

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)

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

Avulsion

An abrupt change in the course of a stream and the adoption of a new channel.

Graded Stream

An equilibrium stream with an evenly sloping bottom adjusted to efficiently handle water flow (discharge) and sediment (load) transport.

What factors led to the Great Midwestern Flood of 1993?

An extremely wet winter followed by wet spring and summer

Braided Stream

An overloaded stream so full of sediment that water flow is forced to divide and recombine in a braided pattern.

Case 2- Too Much Load (2)

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. 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.

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 13.38 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.

A stream's cross-sectional profile flattens out as it approaches ______ because it cannot erode below it.

Base level

The Mississippi River has long "wanted" to abandon its current course and flow down the Atchafalaya River channel to the ocean. What cities would it no longer flow through if the U.S. Army Corps of Engineers allowed it to do so?

Baton Rouge New Orleans

Sediment carried by the Mississippi River is deposited at its mouth, forming a large _________.

Delta

Desert Floods

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 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.

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.

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.

The __________ of Rapid Creek was declared a floodway following the 1972 deadly flood event and was subsequently redeveloped as a greenway with buildings prohibited.

Floodplain

______ become the floor of streams outside of the channels when streams overflow their banks.

Floodplains

Floodplains

Floodplains are the floors of streams during floods. Streams build floodplains by erosion and deposition, and streams reserve the right to reoccupy their floodplains whenever they see fit. Humans who decide to build on a floodplain are gamblers.

Steep topography

Helps thunderstorms build and then provides the rugged valleys that channelize the killer floods.

Managed Retreat

Houses, commercial buildings, and towns are moved out of flood-prone areas in a process called managed retreat. After the 1993 Mississippi River flood, the U.S. Congress authorized the Federal Emergency Management Agency (FEMA) to encourage retreat and to buy vulnerable properties. By 2017 more than 38,000 properties in 44 states were acquired by FEMA; some were moved, many were destroyed, and open space was created. Entire towns have relocated—including Valmeyer in lllinois, Pattonsburg in Missouri, and Soldiers Grove in Wisconsin.

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.

Regional Floods

Huge, long-lasting floods in large river valleys with low topography resulting from prolonged heavy rains over an extensive region. They cause heavy economic losses.

The flood hydrograph in figure 13.36 is typical of rural, unurbanized areas. But what happens in an urban setting?

Humans cover much of the ground with houses and other buildings, pave the ground for streets and parking lots, and build storm-sewer systems to take rainwater runoff directly to streams. Covering the ground with an impervious seal prevents rainwater from soaking into the ground and causes rainwater to flow rapidly across the surface, thus reaching the stream ever more quickly.

CHANNELIZATION

Humans try to control floodwaters by making channels (1) clear of debris, (2) deeper, (3) wider, and (4) straighter. A typical channelization project involves clearing the channel of trees, debris, and large boulders to reduce channel roughness. Then channel capacity is increased by digging it deeper, wider, and 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.

Case 2—Too Much Load (1)

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 stream water to flow faster and thus have more load-carrying capacity. 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.

THE BIGGEST FLOODS

In almost every part of the world, tales are told of ancient deluges far greater than any seen in modern times. In India, it is said that Vishnu, the god of protection, used one of his 10 lives to save Mother Earth from a great flood. In China, they celebrate Yu the Great, who helped protect the people from the overwhelming floods of the Yellow River. American Indian origin mythologies begin with an Earth completely flooded. In Babylonia, clay tablets record the Gilgamesh Epic telling of the great flood for which Utnapishtim built an ark that sailed for seven days and seven nights and saved his kin and cattle. The Genesis book of the Bible describes how Noah built an ark to save his family and pairs of all the animals from 40 days and 40 nights of rain that covered the world with water. These sagas recount events that occurred around 6000 bce to 1000 bce. Were floods larger in those times? Or were these rare events with very long recurrence intervals—the "1,000-year" inundation floods?

Yearly vs Cumulative Probability

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

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.

Structural responses

Include Constructing dams to trap floodwater; Building levees along rivers to contain flood-water 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

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.

As stream discharge increases, sediment load ______.

Increases

Increased erosion that occurs on the outside of a bend in a river and deposition on the inside of a bend has the net effect of ______.

Increasing a stream's meandering process

A stream can't change how much sediment is present or the amount of water it will receive. In this way, discharge and load are ______ variables.

Independent

The 1997 Red River flood was considered a 200-year flood. Why is it now only considered to be a 65-year flood?

Large floods in 2007 and 2010 changed the flood recurrence curve.

Levees (2)

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. The floodwater that escapes confinement by levees spreads out, inundating farms and buildings.

How much of the available sediment is moved by a stream as ______ is determined, in part, by the ______ (slope of the stream bottom) and the rate of the flow.

Load; Gradient

Atmospheric Rivers

Long and relatively narrow "rivers" of water-vapor-rich air that flow horizontally from the tropics into midlatitude sites such as western North America and Europe.

The Hit-and-Miss Approach: Tucson (2)

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. Computations of 100-year flood size based only on the post-1960 flows yields an estimate of 2,700 m3/sec (96,000 ft3/sec); this tripling of expectations accounts for the effects of urbanization on runoff.

What is the difference between negative and positive feedback? Negative feedback....______.

Moves a system toward an equilibrium, whereas a positive feedback drives continued or runaway change

Negative Feedback

Occurs in equilibrium systems where one change triggers another change that tends to negate the initial change and restores equilibrium.

Positive Feedback is also known as the "vicious cycle."

Occurs in nonequilibrium systems where one change triggers more changes in the same direction.

Pineapple Express (Type of Atmospheric River)

One type of atmospheric river has gained fame as the Pineapple Express, bringing tropical moisture from the Hawaiian Islands to the west coast of North America *It has been estimated that 30 to 50% of annual precipitation on the west coast occurs due to atmospheric river events.

To plot a curve for a river of interest, first, calculate recurrence intervals for each year's maximum flood using the formula

RI= (N+1)/M N= The number of years of flood records M= Numerical rank of each year's maximum flood discharge.

The Hit-and-Miss Approach: Tucson

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. Was this size flood really that rare an event?

Floodways

River channels and adjacent lowlands reserved to handle flood water diverted from a larger river.

Strategies to Reduce Runoff

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 to deliver their water into human-dug ponds and basins in city parks and open spaces, where the water can soak underground.

A study by Robert Criss and Everett Shock

Shows that peak floodwater heights in the past 150 years have increased 2 to 4 m (6 to 13 ft) for the same water volume in upper Mississippi River sections with levees and engineered channels. Meanwhile, there has been no increase in flood heights per water volume on the mostly nonengineered upper Missouri River. Where human engineering has deepened and narrowed rivers, flood crests are commonly higher than in nonengineered sections.

________ canyons such as the one shown here, can be extremely hazardous because they are prone to flash flooding.

Slot

During the 2011 flood of the Mississippi River, the Morganza and Bonne Carre ______ were opened to divert water out of the river channel, thus protecting the cities of New Orleans and Baton Rouge.

Spillways

A ______ during a tropical cyclone can cause flooding of coastal lowlands.

Storm surge

Which of the following accurately describes the diversity of the profiles (longitudinal cross-sections) of streams around the world?

Streams around the world, in all types of settings, have similar stream profiles.

Actions that alter the natural river system, such as building dams or straightening channels, are considered to be ______ to flood hazards.

Structural Responses

What is the best way to predict flooding in an area?

Study the history of flooding in that area.

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.

How common is a flood this size? The Red River flood-frequency curve indicates that the 1997 flood has a recurrence interval of almost 200 years.

The 1997 flood discharge set a record, but records are made to be broken. The 1997 record fell in 2009, and the flood-frequency curve was recalculated.

The 20th century's biggest flood in the San Diego River

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 (figure 13.40). The planning process does have its flaws.

This is an unstable position for a river. It leads to river-channel abandonment and the establishment of a new path to the sea.

The Mississippi River has long "wanted" to abandon its course (avulse) and flow down the shorter and steeper path to the ocean offered by the Atchafalaya River.

Available sediment

The amount of earth material waiting to be moved by a stream.

Recurrence Interval

The average time interval between floods or earthquakes of a given size.

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). The channel will handle a flow of 115,000 ft3/sec, which is estimated to recur about every 400 years (figure 13.40).

Meandering

The curves, bends, loops, and turns in the course of an underloaded stream that shifts its bank erosion from side to side of its channel.

Ice-Dam Failure Floods (3)

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 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. 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.

Following the 1972 Rapid City flood, _______.

The floodplain was declared a floodway and rebuilding was not allowed

Ice-Dam Failure Floods (2)

The immense floods flowed southwestward across the Columbia River plateau, eroding channels and stripping the overlying sediments off an area of 7,250 km2 (2,800 mi2) to create the channeled scablands topography. The scablands landscape is marked by scoured bedrock and immense former waterfalls, such as Dry Falls in Grand Coulee, Washington (figure 13.45). The massive floods carried boulders more than 10 m (33 ft) in diameter for miles. 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 13.46).

Base Level

The level below which a stream cannot erode; usually sea level.

Load

The loose sediment and dissolved chemicals being carried by the stream

Sinuosity

The quality of curving in and out. *Used to describe the channel pattern.

Discharge

The rate of water flow expressed as volume per unit of time.

In 1717, New Orleans was founded in the lower Mississippi River basin. The first large flood arrived in the same year...

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

Gradient

The slope of a stream channel bottom; change in elevation divided by distance.

How do streams breach natural dams?

The stream deposits sediment in the basin created behind the dam until it is filled. It will then flow over the dam, eroding it.

The Binational Approach: Tijuana and San Diego

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 met warm, moist air flowing north from the Gulf of Mexico. These air masses collided over the Midwest week after week, and flood records were broken.

Regional or Inundation

These floods occur in large river valleys with low topography when prolonged, heavy rains result from widespread cyclonic systems.

The cross-sectional profile of the river shown in the figure is ______.

Typical of rivers all around the world

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 from 1961 through 2010.

Action Stage

Water begins overtopping the banks

A close look at the meandering process tells us much of value

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

Flood Stages

Water level at which a river overflows its banks or levees.

Why does the very shallow gradient of the riverbed of the Red River increase the likelihood of flooding?

Water tends to pool, rather than flow away.

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

Ice-Dam Failure Floods- Some of the biggest floods known on Earth occurred during the melting of the continental ice sheets.

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 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.

Flash Floods

Within small drainage basins, brief, localized downpours of rain can cause fast-moving but short-lasting flash floods, and they are deadly. A nearby thunderstorm can quickly send a killer flash flood downstream onto unsuspecting people enjoying life under a clear sky.

Which river, sometimes called the "River of Sorrow," is believed to have killed more people than any other natural feature?

Yellow River, China