Lecture Outlines
Natural Disasters, 7th edition
Patrick L. Abbott
Floods
Natural Disasters, 7th edition, Chapter 14
Floods
• Rainfall varies in intensity and duration
• In small drainage basin, short-lasting maximum
floods
• In large drainage basin, maximum floods for weeks
• Events of past  forecast of future events
– Largest past event likely to be exceeded at some point
– Floods of River Arno through Florence
Side Note: A Different Kind of Killer Flood
• Unusually hot January in
Boston caused molasses in
heated tank to expand and
burst container
• 2.3 million gallons of
molasses flooded out as 9 m
high wave
• Killed 21 people and
injured 150 people
• Many trapped in molasses
after it cooled and
congealed
Figure 14.2
How Rivers and Streams Work
Longitudinal Cross Section of a Stream
• Cross-sectional plot of stream’s bottom elevation vs. distance from
source
• Profile for almost any stream is smooth, concave upward, with
steeper slope near source and flatter slope near mouth
• Base level – level below which stream can not erode
– Ocean, lake, pond
or other stream
into which stream
flows
• Profiles are similar
for all streams
because of
equilibrium
processes
Figure 14.3
How Rivers and Streams Work
The Equilibrium Stream
• Streams act to seek equilibrium, state of balance
– Change causes compensating actions to offset
• Factors:
– Discharge: rate of water flow, volume per unit of time
• Independent variable (stream can not control)
– Available sediment (load) to be moved
• Independent variable (stream can not control)
– Gradient: slope of stream bottom
• Dependent variable (stream can control)
– Channel pattern: sinuosity of path
• Dependent variable (stream can control)
How Rivers and Streams Work
Case 1 – Too Much Discharge
Too much water  stream will flow more rapidly and energetically
Response:
• Excess energy used to erode stream bottom and into banks
– Sediment produced by erosion – energy is used up carrying
sediment away
• Erosion of stream bottom results in less vertical drop  flatter
gradient  slower, less energetic water flow
Figure 14.4
How Rivers and Streams Work
Case 1 – Too Much Discharge
Too much water  stream will flow more rapidly and energetically
Response:
• Erosion into stream banks creates meandering pattern  longer
stream path, lower gradient  slower, less energetic water flow
Figure 14.6
Figure 14.7
How Rivers and Streams Work
Case 2 – Too much load
Too much sediment  stream becomes choked
Response:
• Excess sediment builds up on stream bottom
• Buildup results in increased gradient  water flows faster and
more energetically  can carry away more sediment
Figure 14.8
How Rivers and Streams Work
Case 2 – Too much load
Too much sediment 
stream becomes choked
Response:
• Channel pattern becomes
straighter  minimum
energy needed to flow
distance
• Islands of sediment form
within channel, creating
braided stream pattern
Figure 14.9
How Rivers and Streams Work
Case 2 – Too much load
Too much sediment  stream becomes choked
Response:
• Similar to stream overflowing and eroding away landslide dam
Figure 14.10
How Rivers and Streams Work
Graded Stream Theory
• Delicate equilibrium maintained by changing gradient of
stream bottom  graded stream
• Typical stream:
– Too much load, too little discharge in upstream portion 
braided pattern
– Too much discharge, less load in downstream portion 
meandering pattern
• Change in response to seasonal changes, changes in
global sea level, tectonic events
The Floodplain
• Floors of streams during floods
• Built by erosion and deposition
• Occupied during
previous floods, and
will be occupied again
in future floods
Figure 14.11
Side Note: Feedback Mechanisms
• Negative feedback: system acts to compensate for
change, restoring equilibrium
• Positive feedback: change provokes additional change,
sending system in “vicious cycle” dramatically in one
direction
– Desirable in investments accumulating interest
– Undesirable in debts accumulating interest charges
Flood Frequency
• Larger floods  longer recurrence times between each
• Analyze by plotting flood-discharge volumes vs.
recurrence interval, construct flood-frequency curve
• Flood-frequency curves
different for different streams
• Can be used to estimate return
time of given size flood
– 100-year flood used for regulatory
requirements, has 1% chance of
occurrence in any given year
– Difference between yearly
probability, cumulative probability
Figure 14.12
In Greater Depth:
Constructing Flood Frequency Curves
Impossible to know exactly
when floods will occur, but
can predict statistical
likelihood over period of time
Steps in construction:
• Record peak discharge for
each year, rank years
accordingly
Figure 14.13
• For each year’s maximum flood, calculate recurrence interval =
(N + 1) / M, where N = number of years of records, M = rank
• Plot recurrence interval vs. discharge for each year, connect
points as best-fit line
Longer records of floods  better flood frequency curves
Flood Styles
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
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
– Only two feet of moving water required to lift and carry away
average car
Figure 14.14
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
Figure 14.15
Flash Floods
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
Figure 14.17
Flash Floods
Big Thompson Canyon, Colorado, 1976
• 139 people killed, damage totaling $36 million
Figure 14.18
Flash Floods
Rapid Creek, Black Hills, South Dakota, 1972
• Pactola Dam built in 1952 to give flood protection and
water supply to Rapid City, on Rapid Creek  increased
development of floodplain
• Stationary thunderstorm poured up to 38 cm in six hours
• Canyon Lake overflowed as Canyon Lake dam broke,
flooding Rapid City
• 238 people killed, $664 million in damages
• Floodplain remains undeveloped  greenbelt
– “No one should sleep on the floodway.”
Flash Floods
Rapid Creek, Black Hills, South Dakota, 1972
Figure 14.19
Regional Floods
Inundation of area under high water for weeks
• Few deaths, extensive damage
• Large river valleys with low topography
• Widespread cyclonic systems  prolonged, heavy rains
• In U.S., about 2.5% of land is floodplain, home to
about 6.5% of population
Regional Floods
Red River of the North: unusual northward flow (spring
floods)
• Geologically young – shallow valley
• Very low gradient – slow flowing water tends to pool
• As winter snow melts, flows northward into still frozen
sections, causing floods
Regional Floods
Red River of the North:
• 1997 record floods:
– Fall 1996 rainfall four times greater than average
– Winter 1996 freezing began early, causing more ice in
soil
– Winter 1996-7 snowfalls more than three times greater
than average
– Rapid rise in temperature melted snow and ice in soil
– Floodwaters flowed slowly northward, flooding huge
areas of North and South Dakota, Minnesota and
Manitoba
Regional Floods
Mississippi River System
• Greatest inundation floods in U.S.
• Third largest river basin in world
• Drains all or part of 31 states, two Canadian provinces
• System includes almost half of major rivers in U.S.
• Average water flow in lower reaches is 18,250 m3/sec
• Water flow can increase fourfold during flood
Regional Floods
Mississippi River System
Figure 14.20