Learning Objectives

Of all the natural disasters we study this semester, the one you are most

likely to be impacted by is flooding. Floods happen in all 50 states and

in all climates, in all kinds of landscapes, and at all times of the year.

Your goals in studying this chapter are to:

• Learn the impact flooding has on the United States each year.

• Learn what agencies monitor and issue flood warnings.

• Understand the types of floods and their causes.

• Understand river systems and how they evolve over time.

• Learn how to recognize a floodplain.

• Understand how floods are monitored and measured.

• Understand what a “100-year flood” is, and what it is not.

• Understand the pro’s and con’s of various flood mitigation measures.

• Understand how other factors affect flooding.

(NPS)

Measuring Floods

Flood stage: The depth of water at which a river flows over its banks, potentially causing property

damage.

Flood gage: An instrument that uses sensors to measure the height (called “stage”) of river water

above the river channel bottom (see diagrams). A staff gage is a big ruler used for visual check of river

stage, and is also measured from the bottom of the river channel.

Discharge: The volume of water that flows by an established point in a period of time, typically

measured in cubic feet per second (cfs). The formula is cross-sectional area of the river channel times

flow velocity (D=A*v).

Hydrograph: A chart of river discharge over time,

like the one at right.

(USGS)

(USGS)

(USGS)

A flood gage (above) and a flood staff (below).

Diagram of flood gages and a river’s cross-sectional area.

(EPA)

BYU-I text

(USGS)

The 100-Year Flood

Perhaps no subject in natural hazards is more commonly

misunderstood than the “100-year flood.” Simply put, the 100-year

flood is a statistical device used for insurance purposes – it is the

highest flood of the past 100 years, or if a long enough record is not

available, it is a statistical projection of what that flood might be. The

unfortunate name implies that such a flood happens at 100 year

intervals, but that is not the case. Natural systems do not obey simple

statistics, and floods do not keep schedules. Human history is simply

too short (especially in the relatively young United States) to have a

record of natural systems’ full range of capacities. Because a 100-year

flood is the biggest one that happened only once in the past 100 years,

statisticians would say that it has a 1% chance of happening in a given

year. More accurately, however, that probability only applies to the

past, not the future.

FEMA puts it this way: “Another example of problems encountered by

emergency managers dealing with the infrequency of major disasters

are the terms “hundred year flood” and “five hundred year flood,”

which are insurance terms indicating that there is a 1 percent or 0.2

percent chance of such a flood occurring each year. Communities

often do not understand that they can experience several “hundred year

floods” in a matter of weeks or even days and that if a property has

flooded once, it will flood again.”

So, the most important flood mitigation measure is the ability to

recognize a river’s long-term floodplain, and to keep permanent

structures out of it. In most cases, the long-term floodplain

approximates the topographic floodplain of a river.

For regional floods, however, especially in low-relief (flat) areas, flood

waters can accumulate on any flat ground if the water cannot infiltrate

or drain away fast enough. The best protection? Avoid low spots, and

designated 100-year floodplain areas should certainly be avoided.

100-year flood explained (poster)

(USGS)

Flood Mitigation

The most effective flood mitigation measure is to keep people

and things out of the reach of flood waters, but each of the

measures below is used around the world. Each mitigation

measure has its strengths and weaknesses, and each has

consequences.

Dams: By creating space where flood waters can accumulate,

dams can be an effective flood control measure. The risk

comes, of course, when the flood waters exceed the

reservoir’s capacity. Dams cause profound changes to river

systems by stopping the flow of sediment downstream,

changing the local habitats for fish and wildlife, and stopping

fish migration, and so are not built as regularly as in the past.

Levees: Levees are berms or walls built to contain a river or

other water body. Many cities and towns in the U.S. are

protected by levees, including New Orleans, St. Louis, and

Sacramento. The appeal of levees is that we can build them

wherever needed to protect property or create usable land.

The weakness is that levees can fail. Levees can also have

effects on river systems that may not be obvious at first – they

can create a bottle-neck, or narrow passage on a river that

causes the river to back-up into upstream areas, increasing the

severity of floods. This has happened repeatedly on the

Mississippi and Missouri rivers upstream from St. Louis,

Missouri (see photos at right).

(NASA)

Levees at St. Louis

In the Great Mississippi River Flood of 1993, the levees at St. Louis caused the rivers to back up and flood their topographic floodplains.

Levee breach videos

Top Left: Wall-type levees protected New Orleans neighborhoods (to the left) from the canal waters (to the right), which are connected to nearby Lake Ponchartrain. Several of these levees failed during Hurricane Katrina in 2005 when water over-topped them and eroded out their foundations. Bottom left: Levees can fail in many different ways, making correct design and construction vital. Because each location provides a unique set of challenges, no one design fits every situation, making levee design complex. Below: A failed levee is especially destructive because once through the levee, water flows freely into floodplains. (NSF)

(NSF) (NOAA)

The Gateway Arch in St. Louis, Missouri rises majestically from on top of the levee on the west bank of the Mississippi River. (Google Earth image)

(NPR St. Louis)

Channel Modification measures may include removing brush and

trees, deepening and widening the channel, and in some cases

changing the channel to a more favorable route or location. An

example of successful channel modification is the Santa Clara and

Virgin rivers in and around St. George, Utah (images at left). In the

upper photo before the floods, the Santa Clara river channel was

very narrow, and it was choked with brush and trees. After the

2005 flood, measures were taken to remove brush and trees, widen

the channel, expand the floodplain, and reinforce the banks. Golf

courses were put in the floodplain because they resist erosion and

suffer less damage in floods than most other things. In December

2010, an even bigger discharge occurred on both rivers, yet the

flood was contained in the expanded river channels, and property

damage there was much less than before. Downstream

communities in Arizona were not as fortunate because mitigation

measures had not been taken there.

1993

2013

early 1990’s

Virgin River 2010 flood videos

Rip-rap (photo at right) is a blanket of boulder-size rocks designed to

prevent erosion at strategic locations. The rocks must be large enough

that flood waters cannot move them. They are placed with care to

make them stable.

Other Factors in Floods

The Role of Urbanization

From a flood’s point of view, cities are essentially large expanses of

impermeable material like roofs, pavement, roads, and parking lots. In

contrast to rural and farm areas where a high percentage of rainfall and

snowmelt soak into the ground, very little infiltrates in developed

areas. That means that compared to the countryside, floods in urban

areas are higher and the water moves faster (see photo at lower right).

(State of California)

(FEMA)

The Role of Sedimentation A river’s sediment load must be taken into account in flood mitigation

measures. Floods can carry and deposit tremendous amounts of clay,

silt, sand, and even gravel, making both recovery and mitigation of

future flooding more difficult. The Yellow River (shown here) is called

“the Sorrow of China” because it has a very high sediment load and

floods frequently. For centuries, the Chinese have built levees along

the river, but that has only served to trap sediment between the levees

and make the river bottom grow higher with every flood, until the river

is higher than the surrounding countryside and it becomes impossible

to contain it. Despite continued flood mitigation efforts, the sediment

load of the Yellow River makes it one of the world’s worst flood

hazards.

Yellow River flood videos

The Role of Climate and Landscape

Most soils and bedrock have evolved in the climate in which we find them.

For example, in tropical climates, soluble materials like calcium and salt were

long ago flushed out of the soils, while these might be abundant in some desert

soils. As a consequence of this evolution, soils in wet climates generally have

a high capacity to absorb water, while soils in desert climates do not. Flash

floods are common in deserts because nearly all of a sudden rainfall runs on

the surface.

Caliche is a natural cement formed in desert soils when water evaporates,

leaving behind its mineral content. It can be several meters thick and as hard

as any man-made concrete, or it may be present only in thin layers. Because

caliche is impermeable, it prevents rainfall from soaking in and is a major

reason for flash-floods in cities like Las Vegas and Phoenix.

Above: a caliche boulder with cemented pebbles. Below: Heavy machinery can be required to excavate through caliche.

(USGS)

(Pima County, AZ)

Desert flash flood videos