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FLOODS

Large Rivers of the U.S.

Factors Governing Flooding

• Input exceeds output will cause a flood  Too much water entering a stream

system • Factors:

 Excessive rainfall  Snowmelt off in mountains  Severe storms  Hazardous blockage of stream channel

- Trees - Rock avalanches

Factors Governing Flooding

Several causes: • Local thunderstorm  flash (upstream) flood lasting

few hours, building and ending quickly • Rainfall over days  regional (downstream) floods

lasting weeks, building and dissipating slowly • Storm surge of hurricane flooding coastal areas • Broken ice on rivers can dam up, block water flow  fail

in ice-jam flood • Short-lived natural dams (landslide, log jam, lahar) fail

in flood • Human-built levees or dams fail in flood

Flood Characteristics

• Velocity, height, and discharge of a stream increase during a flood  Stage - the elevation of the water

• Flood stage = stream exceeds the bank height

 Crest – maximum stage is reached  Upstream flood –occurs in a small, localized,

upper part of a basin  Downstream flood – occurs in a larger, lower

part of a drainage basin  Flash flood – type of upstream flood

characterized by a rapid rise of stream stage

Flash Floods often occur in rivers with steep gradients (up stream channels)

Down stream floods occur in low gradient channels

Localized Upstream

Floods

Downstream Flood

Great Flood on the Mississippi River-1993

Missouri River-1993

Flash Floods

• Thunderstorms can release heavy rainfall, creating flash floods in steep topography

• Flash floods cause most flood-related deaths  50% of flood-related deaths are vehicle-related  Most of these occur at night

Flash Floods Antelope Canyon, Arizona, 1997 • Narrow slot canyons of tributaries to Colorado River • Thunderstorm releasing rain to form flash flood may

occur too far away to hear or see • 12 hikers killed by flash flood in 1997 Big Thompson Canyon, Colorado, 1976 • Centennial celebrations brought thousands to canyon • Stationary thunderstorm over area dumped 19 cm of rain

in four hours • Runoff created flash flood up to 6 m high, 25 km/hr • 139 people killed, damage totaling $36 million

Flash Flood

Flash Floods

2-feet of water can lift and move a car

Damage from a Flash Flood

Rating Curve

Plot of Stage over Discharge at a cross-section at a river.

Used as a predictor.

Generally a linear graph.

S ta

ge Flood stage = 75 ft. Discharge after a storm is predicted to be 800 cfs. Will flooding occur?

• Hydrograph – a plot of stream discharge at a point over time  Useful tool to monitor stream behavior

remotely • Creating the Hydrograph - plot discharge or

stage on the vertical axis; plot time on the horizontal axis

HYDROGRAPH

Typical Hydrograph

Using Stream Hydrographs To Show Peak Flow

Discharge over Time

Seasonally

Review-Drainage Basin shape. A more circular watershed will have a greater peak flow at an earlier time when compared to a more rectangular watershed, because it takes a longer time for a rectangular basin to drain.

Hydrograph

Peak Flows Up-and-down Stream

Urbanization Increasing Peak Flow

Urbanization removes areas for rain to infiltrate into the ground and increases the rate of run-off into streams and rivers.

Rural vs Urban Areas In a Watershed

Removal of wetlands increases the amount of water that flows into rivers. Wetlands act as a buffer for flood water. When these environments are removed or filled in, the excess water has to go somewhere, because the hydrology of the region or locality has not changed.

Peak flow before removal of wetland.

Peak flow after removal of wetland.

Flood Frequency is the probability in years of a specific flood event returning.

For example: A 100-year flood is the flood stage that has a 1% chance of occurring in any one year. A 50-year flood is the flood stage that has 2% chance of occurring in any one year. A 10-year Flood is the flood stage that has a 10% chance of occurring in any one year.

A 100 year flood may occur in any one year; it may not. It may occur 3 times in any one year. it might occur once in three years. it is a probability; a prediction.

The greater the flood the longer the return interval-or-the lower the probability that it will return in any one year.

Disastrous flood once in 100 years

Major flood once in 50 years

Overtop the banks once in 10 years

Bankfull or higher twice a year

Streamflow about once a month

Flood frequency analysis is accomplished using semilog graphical analysis

one cycle

two cycle

A rit

hm et

ic a

xi s

Log axis

The log axis can never begin at zero.

10 20 30 40 50 60 70 80 90100

Notice that the log cycle jumps to an increment of 10

2-cycle semilog graph

32 …………………

This is a 4-cycle semilog graph

One cycle

Two cycle

Four cycle

Three cycle

10 100 10000100020 30 40 etc… 200 300400 etc… 2000 3000

At each new cycle the number line jumps up an increment of 10, that is to the log of base 10

1 2 3 4 etc…

A 6-cycle semilog graph. Again, at each new cycle there is a jump to another base of 10

Begin labeling your numbers at the wide end (see next slide)

RIGHT WAY TO LABEL THE LOG AXIS

10 100 10001

WRONG WAY TO LABEL

10 100 10001

3-cycle semilog graph

Remember, that the log axis can never begin at zero, and each new log cycle jumps to another base of 10

Flood-Frequency Curve

• Useful tool to evaluate frequency of flood events

• Long-term records are very important to use for flood frequency curves – few long term records exist!

• Curve is constructed by plotting discharge as a function of recurrence interval

• A statistical tool only – probability information is possible

• R = (N+1)/M  R = recurrence interval  N = number of years  M = ranking of flood discharge (magnitude)

Data are first ranked = M, before plotted

N = 25 R = N+1 M

Once ranked, discharge is plotted with the corresponding R (years).

Once data plotting is complete, the best fit slope is then constructed.

Each recurrence graph is site or river specific.

For these data R = 26 M

y-axis x-axis

Discharge is plotted on the linear axis

The recurrence interval is plotted on the log axis

Example of a Discharge-Frequency Curve

m = rank

Notice that we can use the semilog axis to find two sets of information:

1) The recurrence of a flood stage in years

2) And the probability

The probability is simply the inverse of the recurrence interval:

Probability = 1/RI x 100

Example: probability for a 50 year flood is…1/50 = 0.02 x 100 = 2%

We can use the semilog axis to find two sets of information:

1) The recurrence of a flood stage in years

2) And the probability

Notice that on this graph, these plotted data provide information up to a 50-year flood…

…to predict a greater flood, like a 100-year flood, the slope has to be extrapolated (extended)

The Pedernales River will have a more severe 100-year flood when compared to the Navasota River.

(That is, it will have a higher flood stage for the 100-year flood)

Comparing Recurrence Intervals For Two Different Rivers

Fig. 6.01 p.133

Three rivers, A, B and C. Each river will have a different 100-year flood, 50-year flood, 10-year flood, etc…

Each set of data for each river is river specific!

Flood Probabilities: a 100-year flood has a 1% chance of occurring in any one year; a 50-year flood has a 2% chance of occurring in any one year, etc…

More Probabilities This you now know: Probability = 1/RI

The probability of a yearly flood NOT occurring in any one year is equal to:

For example, what is the probability that a 100-year flood WILL NOT occur in any one year?

For example, what is the probability that a 37-year flood WILL NOT occur in any one year?

More Probabilities This you now know: Probability = 1/RI The probability of a certain size flood (RI) occurring within x years

For example, what is the probability that a 100-year flood will occur in 10 years?

Again

Example 2: what is the probability that a 25-year flood will occur in 50 years?

The probability of a certain size flood (RI) occurring within x years

And Again

Example 2: what is the probability that a 100-year flood will occur in 100 years?

The probability of a certain size flood (RI) occurring with x years

Human Causes: Floodplain Development

• Reasons for floodplain occupation  Ignorance of flood hazards  Inexpensive land and often extremely beautiful

• Effects of development on flood plain  Asphalt and concrete - reduce infiltration  Buildings - replace water volume, raises stream height  Filling in floodplain land - reduces volume  Storm drains - rapid delivery of storm water to streams

causing increase in stream height  Vegetation loss - farm lands and urban areas remove

natural vegetation and expose the soil - Streams can “silt up” - Silt reduces a streams capacity to rapidly carry water

away

Building On A Floodplain

Human Causes: Dams • Dam construction to create reservoirs gives sense of

protection from floods, but dams do not control floods • Life spans of dams are limited by construction

materials, construction style, rate at which sediment fills reservoir (life span ≤ 200 years)

• Major floods occur downstream due to  Overtopping  Heavy rainfall below dam  Dam failure

• 1981 study of dam safety by Army Corps of Engineers:  2,884 of 8,639 dams unsafe

Flood Control Dams: Erosional Problems

Farm drainage often increases river flooding.

A. Before the installation of drainage, some surface runoff was tapped in fields and could slowly infiltrate into the ground.

B. The drainage system transfers water rapidly to the stream and increases the likelihood of flooding.

Human Causes: Land Drainage

Insurance

• Flood insurance available from National Flood Insurance Program since 1950s, rarely purchased  Of 10,000 flooded households in Grand Forks, North

Dakota in 1997, only 946 had flood insurance  $300,000 media campaign by FEMA  73

households bought flood insurance  U.S. Congress comes to rescue – 1993 flood victims

received $6.3 billion bill providing aid

Flood Hazard Reduction Strategies

• Retention Pond, trap some of the surface water runoff

• Diversion Channel, comes into play as stream stage rises, and redirects some of the water flow into other safe places

• Channelization, various modifications of the stream channel itself to increase the velocity of water flow, the volume of the channel, or both

• Levees, raised banks along a stream channel

Use of a retention pond to reduce a flood hazard

Before a retention pond

Retention Pond

Storm drains increase the speed and volume of drainage to rivers

Levees

Can be a solutionCan be a problem

Sandbagging • Temporary levees of bags of sand and mud

 Estimated about 26.5 million sandbags used in 1993 floods

 Lessened damage in some places, but not in others  Therapeutic value

Forecasting • Forecasts of height and timing of regional floodwaters

have significantly reduced loss of life • Does not offset ever-greater damages, losses

Floodplain Regulation: Restrictive Zoning

Simply put, prevent construction where common floods occur. (But this is where land is cheap.)

The Biggest Floods: Ancient Tales of Deluge

• Tales of ancient floods are part of many cultures.

• Are these floods larger than those today, or 1,000-year floods?

Ice-Dam Failure Floods

• Biggest floods occurred during melting of continental ice sheets  lakes behind ice dams that failed suddenly

• Evidence of flood from Lake Missoula in Washington after melting of ice dam:

 Lake sediments  Land stripped of soil and sediment cover  High-elevation of flood gravels  Abandoned waterfalls; plunge pools  Large-scale sediment deposits  Mega ripples

2,500 km3 (600 miles3) of water was released in a flood that lasted 11 days; with a discharge of 13.7 million m3/sec (484 million ft3/sec); and a velocity of 30 m/sec (67 m/hr).

Channeled Scablands

Lake Missoula flood waters flowed from the top of this photo to the bottom

Great waterfalls over these cliffs

Plunge pools formed where waterfalls eroded big holes in the rock

House

Missoula flood waters flowed toward the top of this photo. The mega ripples are 15 m (50 ft) high; with a wavelength of 150 m (500 ft).