Bab 7 Streams and Floods

8/8/2019 Bab 7 Streams and Floods

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Streams and Floods

It is common knowledge that flooding

afflicts high-rainfall areas, like New Orleans.

But serious flooding also occurs, in desert

areas of Arizona and New Mexico. What

determines the location, timing, and frequency

of these floods? How far from a river, or how

high above it, should a house be for safety

from floodwaters? Do flood waters carve the

ground surface? Can they excavate buried fuel

storage tanks, old "sanitary landfills," or

coffins from cemeteries?

Streams

Of all geological processes, the movement

of water on the land surface probably has the

greatest direct effect on human civilization.

We depend on streams and rivers for energy,

travel, drinking water, and irrigation.

Because of humankind's fundamental need

for water, civilization began on floodplains,

the flat plains that flank a stream or river.

Worldwide, populations generally are more

dense along waterways. Most Egyptians still

live on the floodplain of the Nile River (Figure

7.1). In the United States, the New Orleans

area long has been a commercial hub because

of its location on the lower floodplain of the

Mississippi River.

Stream characteristics and their

development have been intensely studied byscientists and engineers for as long as science

has existed because of the obvious impact

flowing water has on people's lives.

Interestingly, it was not until the seventeenth

century that measurements of the volume of

rain and snow convinced investigators that

precipitation was adequate to account for the

amount of water we see in streams.

How Streams Get Started

Streams do not exist at random. They havedefinite patterns, the most common being like

the roots of a tree, where many tiny streams

flow into larger ones, converging to form a sin-

gle large river (the trunk). Thus, streams

generally form a network. This is useful

knowledge if you are lost in the woods: Walk

downslope until you reach a stream, then

follow it downstream until it flows into

another, and so on. Eventually you will reach

civilization, because people build homes nearstreams.

The origin and evolution of the stream pattern in any environment can be studiedsimply by looking at the ground around yourfeet when it rains. As the raindrops hit theground on a sloping surface, the water initiallyruns off as sheetflow, a thin layer of waterflowing downhill. Within a short distance,however, surface irregularities focus the waterinto small channels.

The location of these channels may seem

to be random, but closer examination often

reveals a pattern. Perhaps the channels follow

depressions along a path that animals have

taken for years to reach a sheltered area.

Or, perhaps they flow straight to the lowest

area. Perhaps the rocks at the surface or

under the soil are layered and tilted, so that

the water finds an easy path along their

surface. Or, possibly the rocks have been

broken into parallel fractures (joints) that

channel the water. Perhaps the tilted rocks

are a combination of strong and weak rock,

such that the weaker rock erodes faster,

creating channels for the water. The

possibilities are many.

Whatever the cause, tiny water


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channels called rills form quickly (Figure7.2). Even at this early stage, the water ispicking up a load of sediment to carry. As theymerge downslope, the channels become larger.The main channel that develops is said to

have tributaries, which are smaller feederstreams that supply water to the main stream.The region from which the streamcomplex draws water is its catchment areafor precipitation, called its drainage basin.Every bit of water that reaches the groundas rain, snow, sleet, hail, dew, or frostexceptthe portion that evaporates into the air becomes part of the water in the drainage basin. Drainage basins may have areas assmall as a tabletop in a rill system or as large

as the vast catchment of the Mississippi River(Figure 7.3).

Drainage basins differ, not only insize, but in the type of rocks exposed, soilthickness, amount of rainfall and snowfallreceived, and the slope of tributary streamswithin them. Each of these characteristicsaffects the flooding tendency in the basin.The drainage basin is a key concept indetermining flood potential (Figure 7.4).

The boundaries between the

drainage basins are called drainage divides.They are easily traced on a map by finding

the upper ends of tributaries (Figure 7.4).

Larger streams have larger drainage basins.

Drainage basins enlarge over time as the

upper end of each tributary stream erodes the

rock and soil, nibbling its way, slowly

lengthening the stream headward. This is

headward erosion. If the upper end of a

stream erodes its way long enough, it

eventually will meet another stream.Whichever of the two streams flows

more steeply will capture the drainage of the

other stream

We think of mountain streams as being

steep and tumbling, and lowland streams as

being more level and slower. This is

generally correct, as Figure 7.5 shows. Any

established stream has the longitudinal profile

shown, with a steeper slope upstream toward

its head and a much gentler slope downstream

toward its mouth.

Given millions of years, a few large

streams come Lo dominate a continent's

drainage. Usually, only one or two such master

streams develop on each continent, such as the

Amazon River in South America, the Nile and

Congo Rivers in Africa, the Mississippi River

in North America, and the Chang Jiang River

(formerly Yangtze) in eastern Asia (China).

Stages of Stream Development

Glance at aerial photos of streams, and you

will see clear differences among them. Some

stream valleys are deep and V-shaped, with the

stream occupying the entire lower portion of

the V (Figure 7.6). Other stream valleys are

very wide and flat, with the stream width being

only a very small percentage of the valley

width (Figure 7.7A). What causes such

differences? Of what significance are they to

environmental concerns?

V-shaped valleys are formed during the

early stage of a stream's evolution. This is

evident in mountainous regions, where the

stream's headwaters lie in a notch that the

stream has carved into a ridge. Stream level

may be many hundreds of feet below the level

of the ridge peaks at its sides. The rocks of the

ridge are strong enough to resist major

landsliding into the valley, at least

temporarily. However, the common presence

of boulders in such a stream indicates that

some landsliding (rockfall) is indeed occurring.

Few people live in mountain highlands, so this

mass movement is not an important

environmental concern, except to the

snowbirds who enjoy frigid weather.


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As a stream descends from the mountains,

its steep slope sharply levels out where the

mountains end and join a plain. Velocity

decreases, so the larger sediment particles

mostly gravel and some sandare deposited in

the stream bed. This increases the proportion of

fine mud particles to sand in the water. As this

happens, the stream begins to weave from side

to side, forming snakelike meanders (Figure

7.713). The meander shapes migrate

downstream as their development proceeds and

the weaving widens the valley. Previous

meander paths are commonly evident in aerial

photos (Figure 7.713).

The rocks in lowland areas are generally

sedimentary rocks, with shale being most

abundant. Shale is very thinly layered and

always fractured so it is not difficult nor or the

river to widen the valley. If the lowland materials

are loose sediment, deposited by earlier

episodes of stream development, then valley

widening will occur very rapidly, perhaps

within a few tens of years. The wide, flat

valley created by the weaving meanders is

called the floodplain (Figure 7.7A), the zone to

which floods are usually confined when the

river overflows its banks. When the river is in

flood, the floodplain is called the floodway.

Stream Discharge

The most common stream characteristicdetermined by hydrologists is streamdischarge. Discharge is the volume of waterflowing past a point each second (or minute,or hour, or day). Thus, discharge involves boththe size of a stream and how fast it flows.Discharge is calculated like this:

discharge = stream width x stream depth xstream velocity (cubic feet/ (feet)

(feet) (feet/second)second)You can see that a stream's width, length, and

velocity each control its discharge volume. This

also is shown in Figure 7.8.

Figure 7.9 shows a stream at four different

timesprior to a flood and at three stages

during a flood. Note what happens to its

discharge and depth.Stream channels differ in the amount of

frictional resistance to water flow offered by

the stream bed and banks. Resistance to flow is

large in a broad, shallow channel. It is much

less in a deep, narrow channel. Also, resistance

to flow caused by a stream's banks slows water

near the banks, creating the drag shown in

Figure 7.10. Consequently, stream velocity is

fastest at the center of a stream, and near the

surface.

Stream discharges increase downstream astributary streams add their water to the amountin the trunk stream. In very large streams likethe Mississippi River, the average discharge isalmost beyond comprehension-553,000 cubicfeet per seconda volume of more than 300 billion gallons of water per day, measuredat the river's mouth, where it dischargesinto the Gulf of Mexico.

Because discharge varies with the seasonand precipitation throughout a stream's basin, a single measurement of a stream'sdischarge means little. Discharge must bemeasured repeatedly at the same locality to provide a useful scientific picture of astream's discharge, power, and potentialdestructiveness ogists a betterunderstanding of how flow varies withtime.

In the United States, such samplinglocalities, called gauging stations, have been in operation fo more than 100 years(Figure 7.11). The U.S. Geologic a.' Survey(USGS) maintains more than 11,000gauging stations on principal streams andtheir tributaries.. Therefore, a wealth offlow data for a large number o streams isavailable from the USGS and many state


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geoiogical surveys.For any stream, a graph can show its

discharge versus water height above aselected level (called a datum). The best-fit line through the data points is called a

rating curve (Figure 7.12). Once this curve isconstructed, the only variable observeddirectly is the height of the river above thedatum, called the stage. Stage is used to predict discharge, essential in floodmanagement.

Historical Trends in Discharge. TheUnited States, atabout 220 years of age, is ayoung country compared to many others,some of which are ten times that age, ormore. Our streamflow records from gauging

stations are less than 200 years old.Consequently, no direct measurements ofstream discharge are available with which toidentify periods of drought or waterabundance for earlier times. Suchinformation is quite important, however,now that we are concerned with the possibleexistence of long-term global cycles, cyclesthat last much longer than 200 years.Without long-term data, going backhundreds or even thousands of years, it isdifficult to accurately determine long-termtrends.

How can we tell whether we are

interfering in natural processes, unless

we really know how they work? There is a

strong suspicion among scientists that manynatural processes are cyclic. For example, because

atmospheric temperature affects evaporation and

precipitation, the amount of rainfall may be cyclic.

Another example, to be considered in Chapter 17,

is global warming (the global heat balance).

Whether or not such long-term cycles exist, short-

term ones have practical consequences, so it isimportant to determine whether they exist.

Discharge measurements extending back many

years, if they existed, would provide information

useful in evaluating cyclic precipitation.

Despite our lack of stream-discharge measure-

ments for the United States beyond 200 years ago,

there is another way to approach the problem.

Stream discharge is closely related to surface

runoff: the greater the runoff, the more water in

the streams and the higher their discharge. Using

this concept, long-term stream flow records for the

Colorado River in the West have been constructed

by examining the growth rings of old trees. Annual

runoff has been correlated with the width of these

annual rings.Gauging stations have been present on the

Colorado for about 100 years, so the period from 1899

to 1963 was used as a calibration period. If rings for

times before gauging are thicker than during the

1899-1963 period, then runoff, discharge, and

precipitation were greater than today. If thinner, then

runoff, discharge, and precipitation were less than

today.

The results of this study are shown in

Figure 7.13, from which several facts are evident.

First, the period 1907-1930 contained the longest

series of high-flow years in the entire 450-year

record. Only one other period in the early1600s is comparable. Second, droughts between

1868 and 1892 and between 1564 and 1600 are of

longer duration and of greater magnitude than

for any other period during the gauged

record. Unfortunately, it was during the 1907-

1930 high-flow period that the Colorado River

water-allocation pact was completed,

apportioning this scarce water resource to Arizona

ranchers, the city of Los Angeles, and others.

What can be the meaning of a water pact

whose allocations of water were based on years of

unusually great , supply? And how are we to handle

droughts that can last for at least thirty years? Thecontinual court battles of recent years over the

distribution of Colorado River water suggest that the

answer to these questions is perpetual litigation.

Stream Velocity

Recall the longitudinal profile of streams in

Figure 7.5. Although streams all have similar

profiles, the steepness, or gradient, varies from

stream to stream. This is important, because the

main control of stream velocity is stream

gradient, which is the vertical drop of a stream

per unit distance.

For example, if a stream drops 20 feet inelevation over the distance of a mile, its gradient is

20 feet per mile. The gradient of some mountain

streams may be as great as 200 feet per mile. In

contrast, portions of the lower Mississippi have a

very gentle gradient ofonly 0.5 foot per mile.

As a stream leaves a highland area and heads

across lowlands toward the ocean, its gradient de-

creases. Does this mean that the stream's velocity


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will decrease, too? Not necessarily. There is more

water in the channel downstream, and this may

nullify the effect of decreasing stream gradient.

Stream velocity actually may increase downstream

despite the lower gradient.

As the stream moves over the bottom it causesor creates friction with the channel base and sides.

This drag creates turbulence along the water-sediment contact. Water cannot maintain a linear(laminar) flow pattern and is disrupted into achaotic pattern (Figure 7.14). Spinning eddies risefrom the stream bottom, carrying with themsediment that was deposited at an earlier time.

Because turbulence is present in all streams, theshape of the channel is continually changing,

perhaps getting deeper, perhaps getting wider, perhaps getting shallower as sediment erodedupstream is deposited at the site. Even the slowest,laziest stream experiences turbulence. Streams aredynamic systems.

Stream Load

The material carried by a stream is its stream load. Itcommonly is divided into three parts:

yBed load is the gravel-to-sand-sized coarsersediment that is too heavy to be lifted from thestream bottom. It still is moved by the water,transported by rolling or sliding. Sand-sizedgrains can be lifted from the bottom, but aretoo heavy to be carried downstream suspendedin the water, so they move in a hopping motioncalled saltation.

ySuspended load is the sediment carried moreor less continuously within the body of thewater. This sediment is so fine-grained (silt-sized and clay-sized particles) that, oncesuspended by turbulence, it is kept suspendedalmost indefinitely by ever-present eddies in thewater.

yDissolved load is the dissolved material inth e the ions that were removed from minerals androcks during weathering.

In most large streams, 80-90 percent of the load is

suspended load because clay is produced in such great

abundance by weathering processes. The bulk of

the suspended sediment in streams is eroded soil on its

way to the ocean. The amount of suspended

sediment that a stream can carry increases very

rapidly as water discharge increases and is, for

practical purposes, unlimited. In some streams, the

amount of mud is so great that the water looks like

tomato soup because of the red color of the clay.

Figure 7.15 shows suspended mud being

discharged by the Mississippi River into the Gulf of

Mexico.

Why Does It Precipitate?

Streams are fed by precipitation: rain,

snow, sleet, and hail. But why does it precipitate?

Why does the moisture in the air fall to the ground,sometimes gently and briefly, and other times in a

deluge for hours?

The answer lies in the ability of warm air to

house more moisture (water molecules) than cold

air (Figure 7.16). Recall that this is why it rains so

frequently and heavily in the tropics (Chapter 6).

Because of the more direct impact of nearly

perpendicular solar radiation at the oquator

compared to the poles (Figure 6.7), the ground at

the equator is continually warmed and thus is a

continual source of heat. Consequently, air at the

ground always is quite warm compared to the

air higher up. This warm air is less dense than coldair, and thus is buoyant, so it rises. As it rises, it

expands and cools. As the air cools, it can hold less

moisture, so the moisture is "squeezed out"

(condenses). Gravity then pulls the moisture back

to Earth's surfacein other words, it rains. This is

why the equatorial region is rainy and steamy.

Hot, buoyant air that rises and cools is only

one way to generate precipitation. A second way to

cool warm, moist air is to force it to rise over a

mass of colder air. This is what happens when

warm, moist air moving north from the Gulf of

Mexico meets cold air moving southward. This

collision along the polar front is shown in Figure7.17. The cold polar air is denser and hugs the

ground, whereas the warm air full of Gulf moisture

is less dense, and must rise over the co!d air. What

happens as the warm, wet air rises? It cools, pro-

ducing rain, just like over the equatorial region.

This phenomenon, occurring repeatedly, was

the cause of the great Midwest flood of 1993. That

year, the northward-bound warm air was stalled by

the stationary jet stream. So it rained, and it rained,

and it rained, as warm, moist air kept pushing

northward from the Gulf, a virtual river of water

vapor pushed up over colder air, condensing and

falling. This resulted in one of America's worst

floods.

A third way of forcing warm, moist air to rise isto push it up over a mountain range. This occurson the west side of the Sierra Nevada in California,where that northsouth string of mountains gets inthe way of Pacific moist air that is pushing eastward.

The air must rise to go over the mountains and, as itdoes, it cools. In rising air, the temperature dropsabout one degree Fahrenheit for every 300 foot


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increase in elevation. The rising air expands, cools,and its moisture condenses and falls. As a result,the west side of the Sierra Nevada is well-wateredand has abundant vegetation. The eastern side,

however, gets what is left, which is little. As onemoves progressively eastward across the Basin andRange Province of Nevada and Utah, each valley grows

dryer than the one before (Figure 6.10).The world's greatest annual rainfalls occur where

mountain barriers lie across the paths of moisture- bearing winds. A famous example is Cherrapunji,India, on the southern edge of the Himalaya,Earth's highest mountain range. Warm, moist airmoving northward from the Indian Ocean is forcedto rise, expand, cool, and drop its bountiful loadof moisture, with the expected result. Therecording station at Cherrapunji averages 450inches of rain a year (371/2 feet). In 1873, the totalwas a relatively low 283 inches; in 1861, theannual total was an incredible 905 inches (75 feet! ),

with 366 inches (30 feet) falling in the month ofJuly alone. For comparison, the average annual

precipitation in the United States is about 30 inches,and the highest rainfall areas in the central GulfCoast and extreme northwestern coast receive perhaps80 inches (Figure 6.4).

On our continent, the flash flood on the BigThompson River that killed 145 people in 1976 wascaused by cool air rising up a mountain slope, butit was a rise caused by the daily temperaturechange. In the late afternoon, cool afternoon

breezes flowed up the side of the Big ThompsonCanyon, cooled the warmer moist air sitting atop the

canyon slopes, and triggered the catastrophicdeluge. Such strong thunderstorms are quitecommon in this part of the Rocky Mountains andin Florida (Figure 7.18). See the section "Case Study:Big Thompson Canyon, Colorado, 1976" later inthis chapter for more detail about these storm

prone areas. Florida, however, has no mountainsand the rains there are caused in another way.

Florida demonstrates the final way to makewarm air rise, expand, cool, and rain. This occursalmost every afternoon in Florida and othersoutheastern states. Warm air near the ground isconfined by a blanket of cooler air above. But intensesunshine through the day heats the air, building

pressure. By late in the day, the air finally breaksthrough the blanket of cool air to riqe, condense to formclouds (Figure 7.19), and create thunderstorms.These rains can cause flooding because Florida'slandscape is flat as a pancake and poor drainagein some areas cannot remove the water as fast as itaccumulates.

The formula for precipitation is always the same:warm, moist air is forced to rise by some mechanism; it

expands, cools, and rain, snow, sleet, or hail falls.The only difference from place to place is themechan ism that causes the warm air to rise.

Lightning

About 2000 thunderstorms are booming overEarth at any moment. Meteorologists estimate thatthese generate about 8 million lightning strikes perday, averaging almost 100 strikes a second(Figure 7.20). Lightning, of course, is notdistributed uniformly. You may see no lightning forlong periods, or you may see many flashes withinminutes.

These 8 million daily flashes worldwidegenerate a million-million watts of electrical

power, more than the combined output of all theelectric power generators in the United States. About100 people are killed by lightning in the UnitedStates each year, about two-thirds of them inJune, July, and August, the months whenthunderstorms are most common. Three-quarters offatal lightning strikes hit men because they spendmore time outdoors than women. Golfers are

par ticularly prone to being hit.

Your chances of getting hit by lightning arevery slight: only 100 people out of the U.S.

population of about 255 million get nailed eachyear. But one very unlucky park ranger in Virginiawas struck by lightning eight times before hisnatural death (not from lightning). Lightningcaused him to lose his big toe in 1942, his eyebrowsin 1969, and his hair was set afire twice. Onlyslight burns resulted from his other unplannedelectrical connections.

Benjamin Franklin was first to establish thatlightning is electricity. In 1752, he flew history's mostfamous kite into a thunderstorm and watched thesparks jumping from a key hanging on the kite stringto the knuckles of his hand. Luckily, his kite did notreceive a direct hit by a lightning bolt; if it had, historywould remember fried Franklin. Although two-th irdsof those involved with lightning make a fullrecovery, it is probably because they were not

directly hit.Lightning occurs because of positive and

negative charges that build at the tops and bases of

clouds (Figure 7.21). The charges build by friction

between rising and falling air drafts. It is similar tothe static charge you can generate by rubbing your

feet on a rug. The upper part of the cloud develops

pc5itive,:harges, the base negative charges.

Opposite charges attract, so when the charges


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become strong enough to over power the

resistance of the atmosphere, a lightning bolt is

formed.

The atmosphere is a poor conductor of

electricity, so charges must build to at least 100

million volts before a lightning bolt can flash over.

Compare this voltage with that used in portableelectronics (3-9 volts), a car (12 volts), a table lamp

(115-120 volts), or electric range (220-240 volts).

A lightning flash typically lasts a brief fraction of

a second but generates a stunning 40 million

kilowatts of power in a channel that measures

only 0.1 inch to 4 inches in diameter. Concentrating

this extreme amount of energy into such a tiny area

superheats the air surrounding a lightning bolt to

more than 55,000 degrees Fahrenheit.

The number-one rule of lightning safety is to

stay indoors during a storm, away from metal

plumbing and wiring. Stay out of the shower and off

the phone (unless it's cordless or cellular). If you arecaught outside, don't seek shelter beneath a tree, for

trees are wonderful electrical conductors and favorite

targets for lightning, as is demonstrated by the

many farm animals that die beneath the oak they

thought was a good place to wait out the rain. Your

best protection if trapped outdoors is to lie in a low

place, for lightning seeks high points like trees,

antennas, utility poles, and church steeples.

Thunder, Son ofLightning

Thunderstorms are so called for the obviousreason that they produce thunder. But they really

should be called lightning storms, for without

lightning, there would beno noise. The enormousheat generated by a lightning bolt causes the air

near it to expand explosively, producing the intense

sound we call thunder. No lightning, no thunder.

Lightning and thunder occur at the same time. But

light energy travels at 186,000 miles per second,

whereas sound energy (which is the mechanical

shoving of air molecules) travels only 1100 feet per

second, so you see the lightning long before you

hear the thunder.

To get a rough estimate of the distance betweenyou and the lightning bolt, count the seconds from

the time you see the flash until you hear the

thunder. Every five seconds is a distance of about

5500 feet, or a little over a mile. Thunder normally

can be heard at least 10 miles from the lightning

strike, and occasionally as far as 15 miles.

Tornadoes

Thunderstorms produce not only rain, hail,

lightning, and thunder, but sometimes tornadoes

(Figure 7.22A). About 800 tornadoes are spotted in

an average year in the United States (Figure

7.22B). Canadians observe about twenty. About

one percent of thunderstorms give birth to

tornadoes, usually out of the backside of the storm.Even when using the most sophisticated

equipment, tornado occurrence is unpredictable.

They pop out of the dark clouds, extending

Earthward in a dark mass of rotating wind from 5

yards to 300 yards wide, moving along the ground

a few miles or tens of miles at 30-40 mph, and then

retreat and disappear back into the clouds. Some

tornadoes have been clocked at 60-65 mph. Some

contact the ground and then lift, only to strike

again at a distance of a mile or two. I he devas-

tating force of a tornado comes from the increasing

speed with which the air rotates as it tightens into

a funnel, in the same way a figure skateraccelerates her spin by pulling her arms to her

chest.

The word tornado is a Spanish verb that means

"to turn." In shape, a tornado resembles an

elephant's trunk, formed by winds rotating at 300

mph or more (Table 7.1). Inside, the tornado acts as

a vacuum cleaner, sucking up anything loose it

encounters. Black soil vacuumed by a tornado

gives its funnel its usually dark color. Multiple

funnels may descend from the same cloud.

Tornadic winds are strong enough to raisethe roof and collapse a substantial house, suck up

a railroad car, or open the fibers of a telephonepole and drive a straw through it. Paradoxically,the winds can carry a jar of pickles unbroken orpluck the feathers from a chicken. According to

calculations by structural engineers, a 160-mphrotating wind will produce a lifting force of over 30tons on a typical house. If the wind speed doubles tomore than 300 mph, the lifting force is 100 tons.

In the most deadly recorded tornado, 689 people died and more than 2000 were injured inthree mid-western states in 1925. In April 1991, afour-state area was devastated when more thanseventy tornadoes touched down from Texas to

Nebraska, killing at least thirty people and causingmillions in property dam age. In April 1974, the GreatPlains experienced 127 tornadoes, killing 315 andinjuring over 6000 in eleven states, with damageexceeding $600 million. The Red Cross estimatedthat 27,590 families suffered some kind of loss.

Almost three-quarters of Earth's tornadoesoccur in the continental United States, and abouta third of these happen in Texas, Oklahoma, andKansas, a swath known as "tornado alley" (Figure


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7.22B). Half of these tornadoes occur in May and June.From Kansas in June 1928 comes an

extraordinary account from a farmer named WillKeller. He saw a tornado coming and, calling to hisfamily, ran to his tornado cellar. But just beforeslamming the door, for a few seconds he took a goodlook .. .

At last the great shaggy end of the funnel hungdirectly overhead. Everything was as death. Therewas a strong gassy odor and it seemed I could not

breathe. There was a screaming, hissing soundcoming directly from the end of the funnel. I lookedup and to my astonishment I saw right up into theheart of the tornado.

There was a circular opening iii the center of thefunnel, about 50 to 100 feet in diameter, andextending upward for at least one-half mile, as bestI could judge under the circumstances. The wallsof this opening were of rotating clouds and thewhole was made brilliantly visible by constantflashes of lightning which zigzagged from side toside.

Around the lower rim of the great vortex smalltornadoes were constantly forming and

breaking away. These looked like tails as theywrithed their way around the end of the funnel.It was these that made the hissing noise. I noticedthat the direction and rotation of the great whirlwas anticlockwise [counterclockwise], but thesmall twisters rotated both wayssome one wayand some another.

It is not possible to design structures to withstandtornadoes; the winds are simply too strong, toofreakish, and too uncommon for such design to beeconomically feasible. The National WeatherService in the United States forecasts probabletornado conditions and locally tracks the formation and

progress of the storms. Most tornadoes move fromsouthwest to northeast.

A tornado watch is announced whenconditions make severe thunderstorms andtornadoes possible. However, 50-70 percent of thewarnings issued are not followed by tornadoes.Tornado watches can cover thousands of square

miles. A tornad

o warning is announced when atornado actually is sighted, and people need to takecover in a tornado cellar or basement. If you don'thave a tornado cellar or basement in your home,stay in a windowless room in the center of thehouse, such as a closet. Most tornado deaths resultfrom head injuries caused by flying and fallingdebris produced by the high winds. If you protectyour head and neck, you have a much betterchance of surviving. If you have time, turn off the

main gas and electric service to reduce the likelihoodof fire.

If you are outside, get inside. Any structure isbetter than being out in the open. If you are unableto get inside, run or drive at right angles to thetorn ad o' s path. If !here is no time to escape, lie flat inthe nearest depression or ditch. Don't stay in yourvehicle. Most tornado deaths occur in mobile homesand vehicles.

FloodsPrecipitationcaused by the mechanisms

we explored in the last sectioncan lead to arapid increase in a stream's discharge (Figure7.24). When a stream's discharge becomes so greatthat it exceeds the capacity of its channel, it overflowsits banks in a flood. Floods are the mostcommonly experienced natural hazards (Figure7.23). In the United States, rainstorms and theirresulting floods and debris flows accounted for 61

percent (337 of 531) of federally declared disastersduring 1965-1985.

Case Study: "Slow-Rise"

Flooding, U.S. Midwest, 1993

Flooding was widespread in the Midwest duringthe summer of 1993 (Figure 7.25). This was thecostliest, most devastating flood in U.S. history.Forty-eight people died and damage totaled $15 to $20

billion in Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, Wisconsin, and the Dakotas. About100,000 housing units (homes and apartment

buildings) were flooded. The failure of keyinfrastructure, including 388 wastewater facilities,spread the flood's effects far beyond the actual floodedareas. Hazardous waste was released into floodwatersfrom fifty-four sites on the federal government's most-

polluted list (see Superfund Sites in Chapter 9).

The statistics from this event are impressive:

y During the seven months from Januarythrough July, more than an average year'srainfall fell in the upper Mississippi Riverdrainage basin.

y

Ten representative weather stationsrecorded greater than normal precipitation forthe period, and eight received more than twicetheir normal rainfall. In July, three stationsreceived more than four times that month'snormal precipitation.

y Streamwater discharges of a level thatoccurs only once in ten year were recorded at154 locations. Volumes of strearnwater


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than normally occur only once in ahundred years were recorded at 45 locations.At 41 stations, new records for streamflow wereset, to the dismay of the new record holders.

What caused this calamity? Obviously, it was heweather, but the specifics are important to know. The jet

stream, a west-to-east-flowing river of air that normallyflows at high altitude over Canada during thesummer months, stalled over Iowa and the Midwest. Itformed a barrier to moisture-laden air that wasmoving northward from the Gulf of Mexico. Thatair bumped into the jet stream over Iowa anddumped its moisture almost without letup.

It rained for fifty out of fifty-five days in Iowa, andrivers rose as high as 18.5 feet above their banks (Figure7.26). Average rainfall in July is normally between2.8 inches and 3.8 inches over the state, but, in 1993,rainfall exceeded 10 inches, the highest July total in 121years of state record-keeping. The Boyer River at

Loga n in southwestern Iowa rose 15.7 feet in 24 hourson July 9.

In Van Meter, just west of Des Moines, asandbag wall built by hundreds of volunteersand National Guard troops was simply no matchfor a river that was rising nearly one foot per h our(see chapter opening photo). The floodwaterswept under the sandbags and push ed the m asi deon its way eastward into Des Moines. TheUSGS estimated that a flood of this se-verity occursonly once in every 500 years, on average. Damageexceeded $1 billion in the city (Figure 7.27).

The viewpoint of many Iowans in the summer

of 1993 was well-expressed by Mark Twain in hisclassic, Life on the Mississippi:"You can plan or not

plan and it doesn't make a hell of a lot ofdifference. What makes a difference is how muchit decides to rain." Was he correct? Can largefloods be predicted? After all, we know a lotmore about these things now than when Twainwrote those words in 1883.

Some floods can be predicted. Iowa's 1993flooding was an example of a "slow-rise" flood.Such flooding occurs where a drainage basin isvery large. Here, the rate and timing of downstreamflooding can be predicted days in advance fromupst ream in forma tion because one can see it coming.

Flash Floods

In contrast, a flash flood occurs very rapidly in

a smal drainage basin. The flood comes roaring

from a canyon in a breaking wave of water often

10-15 feet high, churning together mud, rocks, and

debris, moving boulders and trees,. destroying

buildings in its path. Flash floods happen in

semiarid mountainous regions where precipitation

occurs as intense thunderstorms and stream

gradients are high.

The Rocky Mountain region of the western

United States is a classic example of such an area.

Because of the high velocity of the water, peopledownstream may have less than an hour's warning

before the deluge hits. Consequently, there is

limited opportunity for an organized, informed

response, and emphasis turns toward saving lives,

rather than reducing property damage.

Based on the experience of many flash floods,a relationship has been determined between thenumber of hours of warning before a flash flood

and loss prevention (iFigure 7.28). No warning,of course, results in the maximum casualties anddamage. If any warning is given, the reduction inloss of life and property damage is significant.However, the usefulness of a warning decreasesfor times greater than 12-15 hours. Warningtimes greater than one or two days are of no realuse. Further, the volume of water in a flash floodis so great that most structures in the path of theflood cannot be protected, regardless of warningtime. As the graph shows, it is essentiallyimpossible to protect more than about one-third ofthem.

Cas e Study- Big Thompson Canyon,

Colo ra do, 1976. The best-studied recentexample of a devasting flash flood occurred inBig Thompson Canyon, Colorado, on July 31 /August 1, 1976. The flood and associated debris flows

claimed 139 lives and caused $50 million damage tohighways, roads, bridges, homes, and small

businesses (Figure 7.29).The brunt of the storm occurred over the

Bi g Thompson River basin between Drake and EstesPark. Rainfalls up to 12 inches were reported. AtGlen Comfort, 7.5 inches fell during 70 minutes.Peak discharge at the canyon mouth near Drake was31,200 cubic feet per second, more than four timesthe highest previous discharge recorded in eighty-eight years of flood records there. The flood crestmoved through the 7.7 mile stretch between Drake andthe Canyon mouth in about 30 minutes, an average

rate o f 15 mi les per hour, or 23 feet per second.It is easy to understand why hundreds ofresidents, campers, and tourists were caught withlittle or no warning. Carbon-14 dating of

previous flood deposits in Big Thompson Canyonindicate that a flood as large as the one in 1976 hadnot occurred for at least 1000 years and perhaps notfor 7000 years or more.


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number of years of records +

rank of discharge on list (3d hig

10 years of records + 15th highest discharge 115

Why Do Floods Occur?

Stream channels develop during times ofnormal rainfall, so they are adjusted to normalstreamflow. They can contain higher flows, up to a

point. For example, in the humid northeastern

United States, studies reveal that stream bankscan contain the highest discharge that occurswithin a 28-month period, on average. Mostfloods result when rainfalls are so great that tem-

porary storage in soil pores is insufficient to keepthe stream from rising above bank level.

Flooding is a normal, inevitable part ofany stream's life over the years. Whether or notthe flooding becomes an environmental problemdepends on how close the stream is to populationcenters. It also depends on the elevation difference

between a population center and the top of the streambank.

Floods also can be caused by snow, ice, andlandslides. In regions of heavy snowfall, such as thewestern United States, unusually high springtemperatures can rapidly melt snow, causingflooding. Snowmelt is produced faster than it can

be absorbed by saturated or frozen ground.Flooding also is common in northern climates,where river ice essentially dams the water,forming a lake behind the ice. When the ice is

breached it collapses, releasing the water in a flood.Floods from ice dams are common in mountainouswestern Canada. In warmer climates, temporarysediment dams formed by landslides replace ice

dams as a cause of floods. A classic example is the1925 slide in the Gros Ventre Valley of Wyomingand the subsequent flood in 1927 (see Chapter 5).

How Often Do Floods Occur?

Floods are natural and recurrent events. Over theyears, repeated floods build the floodplain of a river.The function of a floodplain is to provide a pathway forexcess waterwater that won't fit within the streamchannel Unfortunately, human societies all too oftenignore this floodplain function and colonize thefloodplain for its economic advantages: level groundfor construction, fertile soils for crops, ease of

access, and ready supply of water. Too often,floodinc, is viewed like an earthquake it alwayshappens to someone else, somewhere else.

Living on a floodplain is a gamble like playing direin

Las Vegas. The same rules ofchance apply: the stakes

are high, but the long-run odds are against winning.

The stakes are high because you win only if your flood

losses are less than the value gained from living in

the flood plain. The long-run odds are poor

because floods are bound to occur. The biggest

losses in built-up areas come from catastrophic

floods like the 1993 Iowa disaster or the 1976 Big

Thompson Canyon flood. Floods of this magnitude are

rare, but even a small chance for such an

occurrence is a matter of concern. The more severe a

flood, the less its likelihood of occurrence.

Calculating Flood Frequency. How large a flood

can be expected, and how often? The data needed

to calculate a flood frequency curve are obtained

from stream-gauging stations where streamflow

data are continuously recorded. The data recorded

include stream discharge, from which hydrologists

calculate flood frequencies. The method for

calculating flood frequency of a stream or river is

simple and can be used by anyone who contacts

their state's geological survey to obtain the necessary

data:

1. Obtain the highest discharge for the

stream for each year for as long as records

have been kept. If there is more than one

gauging station on the stream, choose the

station with the longest record. The

more data points you have, the more

accurate will be your estimate of flood

frequency.

2. Rank the discharges in decreasing order,

from highest to lowest.

3.To determine how often you can expect eachmagnitude of discharge to occur, apply thisformula:

Averagerecurrence =interval

For example:

Averagerecurrence == = 2.2 (one flood this sizeevery 2.2 years)interval

Table 7.2 shows the recurrence interval of floods on theDanger River based on ten years of data.

A graph can be constructed showing howfrequently any level of discharge is likely to occur. Forthis example, we used data from Table 7.2 in theequation above to generate the curve in Figure7.30. There are only ten years of records. The highestdata point is at a recurrence interval of 11 years(number of years of records + 1). Note that the


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estimate of discharge at 11 years is difficult todetermine because the data point (1700 cubic feet

per second) is an extreme value, much higher thanand, therefore, far from the other data points onthe graph.

As noted earlier, stream banks are high enough to

hold in the stream channel the highest dischargethat occurs within a 28-month period, or 2.33years. In our Danger River example, this is adischarge of 520 cubic feet per second. The

projected fifty-year flood discharge of 2500cubic feet per second is 4.8 times the river's normal"full" discharge.

Determining the highest discharge toexpect every 50 or 100 years is much more

problematic. It depends very heavily on thehighest discharge recorded during the ten-year

period. For example, if the highest discharge recordedwas 20 percent higher than the 1700 cubic feet persecond in our example, the projected discharge ofthe fifty-year flood would be 3800 cubic feet persecond. This dramatic change might mean the dif-ference between minor property damage withsmall loss of life and a major economic and

personal di saster for a community. Note that all values other than the

measured ten data points are expectations. Based onthe ten datapoints, we expecta discharge of 3500cubic feet per second every 100 yearsbut, itis quite possible that the highest discharge

between 1994 and 2094 will be only 3000 cubicfeet per second; or perhaps it will actually be 4000cubic feet per second. The small number ofdata points on which our extrapolat ion is based

ensures that there is a large degree of uncertaintyin our expectation.

In natural situations, we are always dealing in probabilities, not certainties. We use real datato make our estimates of future stream

behavior, but must not forget that ourestimates are only that stimates. The largerour data set of past stream behavior, the betterwe will be our estimate of future stream behavior.

Flood hazard (or discharge hazard)

estimates sometimes are expressed as flood

probabilities rather than as recurrence

intervals. One value is the reciprocal of the

other. (A reciprocal is 1 divided by thenumber; for example, 1/too is the reciprocal of

100.) A 100-year flood, for example, has a1/too (or 1%) chance of occurring in any given

year. A 50-year flood has a 1/5o (or 2%) chance.

The possibility of two 50-year floods occurring

in one year is small, only 1/50 X 1/50 = 1/2500, or 1

chance in 2500 (or 0.04%). But it can happen: in

Houston, three 100-year floods occurred in a

single year, 1979. The chance of that happening is

one in one million! The real world is never risk-free.

Table 7.3 summarizes flood probabilities. For

example, a flood with a ten-year return period

(column 6) has a ten percent chance of occurring in

any one yearcolumn 5), a 65 percent chance within

ten years (column 4), a 94 percent chance withintwenty-five years (column 3), and a 99.9 percent

chance of occurring within fifty years. A 100-year

flood has a 63 percent chance of happening within a

100-year period, but only a 1 percent chance in a

particular year.

A good analogy is a roulette wheel. The

number of black and red numbers is very close to

equal so that ten spins of the wheel should result i n

five black numbers and five red numbers.

Nevertheless, there will be numerous ten-spin

sequences in which the result will be six black and

four red, or seven red and three black. Occasionally,

the result may be nine and one, or rarely, ten andzero. Flood probability estimates are similar. In fifty

spins of the calendar, there should occur only one

fifty-year flood, but it is possible for two or three

of them to occur in a single year. Worse yet, which

year it will be cannot be determined in advance.

Urbanization and Flooding

The relationship among rainfall, streamdischarge, and time is of great significancewhen we think about floods. Time is moreimportant than many people realize. For example,

during a rainfall, one very important factor thatslow-, the rush of shcetflow runoff into a streamchannel is vegetation. First, the plant leaves shieldthe soil from the full impact of raindrops, so that thewater is more likely to infiltrate than to erode

thesoil. Second, plant roots and soil pores can hold alarge amount of waterat least temporarilyand thus reduce the maximum stream discharge.This creates a lag between the time of rainfalland the time of peak discharge. Put simply: ifthe Appalachian Mountains suddenly were strippedof all plants, residents would experience catastrophicflash floods.

Thus, it is no surprise that urbanizationpaving over land, building construction, strippingvegetation and soilboth increases the highestdischarge that 1ccal streams attain and decreases th elag time between the rainfall event and flooding(Figure 7.31). This increase clearly shows on ahydrograph , a plot of stream discharge over time.

Two hydrographs appear in Figure 7.32.The upper curve shows streamflow under pre-urban conditions, where plenty of vegetation and


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few impermeable surfaces exist. The lower curveshows the greater stream discharge and briefer lagtime between rainfall and peak discharge that resultfrom urbanization.

Smaller floods are more affected byurbanization tl,an are larger, less frequent floods.A 50-year or 100- vear flood is hardly affected at all

by an increase in the amount of impermeable areacaused by urbanization, because such extremefloods overwhelm the storage capacity of soil cover,no matter how extensive.

Anticipating and Controlling FloodsAdvance planning for floods or other

calamities can save both lives and property. But planning costs money, and the morecomprehensive the plan, the higher the cost. Howmuch are people who live on a river's flood-plainwilling to pay, and for what degree of protection?

Most communities that engage in flood protectionplanning use as a guide the 100-clear floodway. This isthe area along the river (floodplain and the zonesurrounding it) that is likely to be flooded once each100 years.

As you have seen, the 100-year discharge can beestimated. By using topograph ic maps (maps thatuse contour lines to show th e shape and elevationof the land surface) it is easy to see the outline of thearea that would be under water during anaverage 100-year flood. However, because ofincreasing urbanization, both the 100-year streamdischarge and the contours of the land surfacearound a stream may change over time, so the 100-year floodway may change as well.

Figure 7.33 shows the San Lorenzo River flowingthrough the town of Felton, California, south ofSan Francisco. It shows the flood-prone zonealong the river. At normal streamflow, the watersurface is at an elevation of 2405 feet, butfloodplain zoning now prevents mostconstruction below an elevation of 260-265feet. Unfortunately, a large number of homes andother structures already exist on the east side of theriver (center of map; note elevations). This is within thedesignated danger zone. Felton's citizens probably areunhappy about decreased property values andhigher home insurance premiums that haveresulted from the zoning ordinances.

Floodplain Zoning

For the reasons just described, floodplain zoning has notreceived an enthusiastic response from localgovernments. Lack of scientific personnel, lack of

money, local political pressure against unpopularrestrictions on development, and the higher cost offloodproofing or of elevating structures on stiltscombine to make difficult the passage of zoningordinances. Real estate developers, in particular,almost always are opposed to restricting the use(residential, commercial, industrial) to which theymay put their property.

Should communities pass laws restricting theuse of private property? Would it be satisfactorysimply to inform prospective builders or buyersof the danger, and allow them to do as theywish? Should the government subsidize theincreased cost of insurance for those who build indangerous areas if the town favors the developmentand no other location is suitable?

Flood Control

Several methods are used to controlfloods, none of which is without drawbacks. Theyinclude channelization, levees, and dams.

Channeliz ation of Streams.

Channelization is the name for various methodsof improving the stream's channel to increasestream discharge. The purpose is to help the streamcarry away water faster from a threatened area.Channelization may increase discharge bydredging to straighten, widen, or deepen thechannel. Within cities, stream channels often are linedwith cement to straigh ten them and keep hemfrom meandering, as streams commonly do

(Figure 7.34). Unfortunately, such majormodifications in a natural stream course not onlyare unsightly but have ripple effects both upstreamand downstream. Increasing the streamgradient at onelocation causes it to increase upstream as well, whichaccelerates erosion. Down-stream, the incidence offlooding is greatly increased because of theincreased volume of water funneled there morehastilyby the increase in stream velocity upstream. Inaddition, any change in the natural characteristicsof a stream affects the ecosystem of which it is anintegral part, usually negatively.

Building Levees along Streams. A second

method of flood control is to build artificial levees(also known as dikes), which are raised banks thatrun along the top edge of the stream channel on eachside. Streams build natural levees (Figure 7.35A) whenthey overflow their banks, because the water slowsand sediment is deposited. Natural levees usually arelow, capable of containing only the greatestdischarge that occurs during an averag e 28-m onth

period. They afford no protection against majorfloods. By constructing higher levees along the


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channel margins, the channel can hold morewater, reducing the occurrence of floods (Figure7.35B).

Artificial levee construction has been advocated by thefederal government for more than a century, but theynow are reassessing the practice. Over the years, the

U.S. Army Corps of Engineers has built numerouslevees along the Mississippi that now total morethan 2500 miles in length. Some of these levees are 30feet highThese levees have contained numerousfloods. But the great flood of 1993 breached (brokethrough) some 8001evees, creating serious flooding onland that long had been protected by them. Further,fending off a flood upstream simply creates more severeflooding downstream.

Calculations indicate that, had theagricultural levees along the Mississippi been madehigh enough to contain the 1993 floodwaters, theriver would have risen about 6 feet higher at St.Louis, putting much of that city under water. Also,the projected cost of elevating ihe levees would beseveral billion dollars. So, although the 1993floods in the midcontinent were a disaster fortowns an d farms along the Mississippi River, theCorps of Engineers does not expect to build leveeshigh enough to prevent similar floods. Instead,they are recommending government buyouts of

property in flood-prone areas and improvedflood insurance for buildings and crops.

Building Dams across Rivers. Another widelyused flood-control method is a dam; 75,000 exist in theUnited States. Hundreds of thousands more existthroughout the world. If all the water stored in

dams were released to the ocean, sea level wouldrise about 3 inches. These huge concrete or earthenstructures span a river and impound water thatcontinually pours in from upstream (Figure 7.36).Some rivers need only a single dam, whereasalong others, two or three may be necessary to managethe water volume. The Mississippi has twenty-eightlocks and dams, and forty-five major dams have

been built along its tributaries.The pinnacle of dam construction in the

United States was reached with the completion in1976 of a 34-dam complex along the TennesseeRiver and its tributaries (Figure 7.37). At the time

they were built, the sole purpose of these dams wasthe generation of hydroelectric power for aneconomica lly depressed region. Since then,however, the lakes created by these dams and thewater fesources they make available have led toextensive reforestation, tourism, and developmentof mineral resources. The dams were constructed by theTennessee Valley Authority (TVA).

Dam Problems. Most dams built since theTVA project have been multipurpose. They

generate electric power, irrigate crops, and provide recreational activities such as swimming,camping, fishing, and boating. Until about thirtyyears ago, few questioned the good brought to anarea by a dam. Since then, problems have beenrecognized, resulting in near-total abandonment ofdam building.

Among the worst problems are catastrophicdam failures. During the last few decades, severalhundred thousand people worldwide have beer, killed

by dam failures. These resulted either from inadequategeologic investigation or poor engineering duringconstruction. Recall the infamous Vaiont Damdisaster in Italy, discussed in Chapter 5.)

A 1992 national dam survey classifiedalmost one-third of this country's 75,000 dams ashazardous, 10,000 as having high hazard

potential, and another 13,500 as having significanthazard potential.

Another problem associated with dams is

inundation of large amounts of property by the lake thatforms behind the dam. This drives outhomeowners and wildlife, displaces farms, killsvegetation, and alters ecosystems. The dams alsocreate mud deposits in the reservoir, because theentering stream stops and hence drops its sedimentload. In 1941, a USGS study reported that 39 percentof existing reservoirs would be largely filled withsediment before-2000. (Figure 7.38 shows an examplefrom France.) More recent data suggest that only54 percent of American reservoirs will function formore than a century, and 21 percent will be in useless than fifty years. About one-third of the sediment in

the reservoirs may come from erodedcropland (see Chapter 3).

Just how much sediment accumulates annuallyin an "average" reservoir? Could we dredge thesediment from the reservoir and transport it to auseful place, like a construction site? What would thiscost?

Sediment yield from southeastern U.S.drainage basins has been measured, in pounds persquare mile per year:

y 110,000 from forested areas (low erosion rates)

y 1,700,000 from rangeland

y 850,000-60,000,000 from construction sites,where land is temporarily exposed in a highlydisturbed and erodible form.

Let us assume a conservative amount of sedimententering a reservoir: only 1 million pounds persquare mile per year. Dry mud weighs about 150

pounds per cubic foot. Hence, the 1 million pounds ofsediment would occupy about 6700 cubic feet,roughly the volume of a 30 x 30-foot bungalow.


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Each year, the reservoir would accumulate 6700cubic feet of mud from each square mile ofdrainage basin. The cost of dredging the mud from areservoir is about 10 to 15 cents per cubic foot, sothe yearly cost of dredging would be at least $670

per square mile of drainage basin.

When this per-square mile cost is multiplied bythe 75,000 reservoirs in the nation, it appearsthat annual dredging cost would be $50 million persquare mile of drainage basin. Drainage basinstypically encompass many hundreds or thousandsof square miles. So, the total dredging cost forAmerican reservoirs would be in the billionsannually! Further, our calculation does not includethe cost of tr ans port ing mud, which might equal thecost of dredging, depending on distance. Weconclude that the cost of removing the mud fromthe nation's reservoirs is prohibitive.

Below the dam, the water that is released afterpassing through hydroelectric turbines in the dam is freeof sediment, all of which was trapped behind thedam. As the water plunges down into the stream infront of the dam, it picks up new sediment, and theincreased erosion may extend for many milesdownstream. The increased depth of the master streamthat results from the increased erosion has effects onits tributaries as well. They also increase theirdowncutting because the elevation at which they

join the master stream has been lowered.Case Study: Egypt's Aswan Dam. Of evengreater importance in some areas downstream fromdam sites is the ecological damage and loss of nutrientsneeded for agriculture. A good example of this effect

is the delta of the Nile River in Egypt. From beforerecorded history, flooding of the delta region at themouth of the Nile was an annual event, important

because it replenished agricultural soils with freshsediment and water. But in 1968, the Aswan Damacross the Nile was completed. Its purpose was togenerate hydroelectric power and control flooding, and

both objectives were accomplished.Unfortunately, unforseen problems soon

became evident. After about thirty years, thereservoir behind the dam still is not full, nor is itexpected to rise higher. The reasons are simple:evaporation into the hot, dry desert air and

infiltration of the reservoir water into the permeablesandstone on which the dam was built.Evaporation is 50 percent greater than pre-constructioncalculations indicated. And the mud settling inthe reservoir has not sealed the permeablesandstone, as had been expected. The reducedreservoir depth has reduced power generationconsiderably.

But downstream effects of the dam are even worse.Because of the loss of sediment that used to be an annual

deposit in the delta and an increasing number ofirrigation canals that trap sediment, the delta frontis being eroded by currents along the shoreline ofthe Mediterranean Sea, reducing the land areaavailable for agriculture. Parts of the delta coastlineare receding at a rate of more than 300 feet peryear. In addition, the drifting desert sand thatencroaches on the fringes of the fertile corridor of the

Nile valley during the dry season was stabil ized by th eriver-borne mud deposited during the annualflood. Now that the flood has been eliminated,sand encroachment is much more difficult tocontrol because it overwhelms the irrigated lands atthe western edge of the Nile River valley. Further,reduced Nile flow is now inadequate to wash awaythe salts from the soil that are harmful to plant life.

Even worse, the floods that formerly deposited anew layer of nutrient-rich mud have ended. Artificialfertilizers now are needed, an expense not easily bornein a poor country. And finally, the freshwater snails thatcarry parasitic Bilharzia larvae have become much moreabundant in the Nile water used for irrigation,causing a dramatic increase in the incidence of thisdebilitating and often fatal intestinal disease. TheAswan Dam and reservoir have proven to be much lessthan the salvation event for the Egyptian people that was

prophesied in the 1960s.

Case S tudy: ValmeyerA Town wi th

Good Sense. Humans seen, slow to recognize

that we are an integral part of the natural world,not some special group apart from it. The naturalenvironment, such as a river system, containsinnninerable and often invisible interactions thathave developed over billions of years to stabilizeEarth as a place fit for the life that inhabits it.Despite our technological capabilities, we humansare a numerically trivial group compared to ants,termites, bacteria, and so on. However, we havelarger, versatile brains. We are fully capable of"thinking" our way to extinction, despite ourgo od in tentions. It is not possible to improve on Father

Nature.

One small

g

roup that has recognized the futility of0fighting catastrophes that happen repeatedly inthe same place is the people of Valmeyer, Illinois.Valmeyer is on the Mississippi River's east bankabout 25 miles south of St. Louis. The river'sfloodwaters inundated the town twice in the summer of1993, and 900 citizens saw 90 percent of their homes,offices, and public buildings destroyed (Figure 7.39). Sothey voted to construct a new Valmeyer about twomiles east, away from and higher above the river.


15/16

The Illinois State Geological Survey was asked toinvestigate and evaluate natural hazards at the proposednew site of about 500 acres.

At a town meeting in December 1993, Valmeyer'scitizens voted to ask the federal government forfunds to defray the cost of the relocation and newconstruction. Approval was granted in 1994 and 95

percent of Valmeyer's buildings and homes are beingrebuilt at the new site. Only 5 percent of the citizenschose to remain at the old townsite. The federalgovernment is moving or rebuilding about 6600structures damaged in the floods. The estimated costof the move is $9.6 million, of which the federalgovernment (the nation's taxpayers) will pay $7.2million. The remaining $2.4 million, divided among900 residents, comes to less than $3000 per person. Thecost for a family of four is about $11,000.

As a condition of the grant, the town must adoptdrainage, sediment, and erosion controls; a storm-watermanagement plan; construction guidelines; and other

mitigation ordinances to protect against activatingthe geologic hazards found by the scientists from theIllinois State Geological Survey. The citizens ofValmeyer believe it is in their best interest toadapt to their surroundings rather than continue tofight what is surely a losing battle.

Summary

Of every 100 water molecules on and near

Earth's surface, 97 are in the oceans. Most of the

remaining three lie frozen in glaciers. Although this

has been true for all of human history, it does not

mean that the water molecules do not change

location. They are always moving in the hydrologic

cycle among the oceans, atmosphere, soil, and

subsurface rocks. These movements are crucial for

human existence.

The size of streams varies with geographic

location, local topography, nature of the underlying

bedrock, stage of stream development, and the

amount of human intervention in the natural cycle.

All of these factors influence the possibility of

flooding. Although the probability of future floods

can be calculated, the estimate is based on past

occurrences and is a statistical average.

Human civilization always interferes with thenatural development of streams and their water-

carrying capacity. Urbanization, artificial

channeling, and dam construction alleviate human

problems in the short run (the term in office of

elected politicians) but are harmful in the long

run. And the long run commonly is not really that

long, only a few tens of years. It is advisable not to

live on a floodplain near an active river. As long as

people do such inadvisable things, human disasters

will continue to be more frequent than would

otherwise occur.


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Documents

Bab 7 Streams and Floods