Transcript Slide 1

PG. 50
FLOODS CHAPTER 12
Part of the Hydrologic Cycle - the circulation of Earth's
water supply
Streams are defined as running water confined to a
channel that moves due to gravity
Streams are erosional (take stuff away) and
depositional(leave stuff behind) agents
Variables that control a stream's ability to erode and
transport material depends on
VELOCITY (including discharge and gradient)
available sediment and channel sinuosity.
PG. 50
1. Three things affect velocity
A. Gradient - the slope of a stream channel
sinuosity=channel length/straight line length
MOST STREAMS FOLLOW THIS RULE
1.) lower gradient=downstream section =higher
sinuosity= meandering
2.) higher gradient = upstream section=lower
sinuosity= straighter
BOARD and ROPE demo
B. Channel Characteristics - reduce friction
a. shape - semicircle ideal; least amount of
water in contact with channel
b. roughness - smooth less friction
c. size - reduce ratio of WETTED perimeter to
x-section.
PG. 50
1. Three things affect velocity:continued
C. Discharge - volume of water that flows past a point in
a given unit of time (cfs)
a. measured in CFS (cubic feet per second)
b. AV=Q
A is cross-section area
(width x depth)
V is velocity
Q is discharge
The river is a flowing “loaf” of bread and the cross section
is a “slice” of bread. The discharge is simply adding up all
the slices and wha la you get the total volume/dischage
“the loaf”
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2.Streams seek equilibrium (give and take) –
a state of balance between
deposition (put) and erosion (take).
Stream equilibrium depends on
1.) discharge
2.) available sediment that can be moved
3.) gradient
4.) sinuosity of the stream channel
For every change in one or any of the above the stream will
respond and attempt to return to equilibrium by doing
almost the opposite
ADD Pg. 51
NEGATIVE FEEDBACK: a change will elicit or cause an
opposite change to return BACK to the previous state.
POSITIVE FEEDBACK: a change causes further
changes.
EXAMPLES
Does negative mean bad? always, never, sometimes?
Is positive mean good? always, never, sometimes?
Pg. 51
Increase discharge causes an increase in velocity
and thus energy the stream has.
This increases EROSION of the stream bottom or
increases SINUOSITY of the channel by eroding
the stream banks.
Increase load causes sediment deposition on stream bottom
creating a braided stream (lots of sand bars).
Pg. 51 ADD THIS SO LET’S THINK ABOUT
HOW A RIVER WILL RESPOND TO CHANGES:
THOUGHT EXPERIMENTS
CASE #1: Too much discharge
The river tries to reduce this increase of
discharge/velocity HOW WILL IT SLOW ITSELF DOWN?
1.) excess energy erodes the stream bottom
ND pg. 374 fig. 13.4
2.) excess sediments will also use up excess energy
3.) the vertical drop will decrease-river is trying to get
closer to base level which slows down river.
ND pg. 374 fig. 13.4
Pg. 51 CONTINUED
ADD THIS SO LET’S THINK ABOUT
HOW A RIVER WILL RESPOND TO CHANGES:
THOUGHT EXPERIMENTS
CASE #1: Too much discharge.
The river tries to reduce this increase of
discharge/velocity HOW WILL IT SLOW ITSELF DOWN?
4.) A river may also slow itself down by eroding the banks
AND therefore increasing it’s path-increasing sinuosity.
ND pg. 376 Fig. 13.6
BOARD DEMO with rope.
Pg. 51 CONTINUED
ADD THIS SO LET’S THINK ABOUT
HOW A RIVER WILL RESPOND TO CHANGES:
THOUGHT EXPERIMENTS
CASE #2: Too much load. The river gets choked with
sediments. This causes deposition/build up of sediments.
The river tries to increase the velocity to help increase
erosion/taking away the excess sediments. HOW WILL IT
SPEED ITSELF UP?
1.) The build up of sediments increases slope=increase in
velocity ND pg. 378 fig. 13.7 and fig. 13.6
2.) straightening the channel increases slope=take away
sediments
BOARD AND ROPE DEMO
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3. Streams erode (removal of material) by:
A.) Hydraulic action - generally unconsolidated material
Push and pull of the moving of water.
B.) Abrasion – “sand blasting” and “sand papering” with
transported material (load) bedrock channel eroded as
load (sediments) scrapes along.
ADD these are both controlled by….
Discharge HOW?
Sediment load HOW?
Which river is eroding more? WHY?
River 1
River 2
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4. Streams transport material
A. suspended load- Suspension – “floaties”
B.Bed Load - move along bottom, too large to be
suspended – rolled and bounced-Logan River boulders
River Flow
Suspended
Load
Bed
Load
Pg. 51
5. Control on erosion and stream valley development
A. BASE LEVEL-
a. Ultimate base level - ocean
b. Temporary base level - lake, main
stream to tributary
ADD c. When a river reaches base level there is
no more “down” to drive the river and the river
is at equilibrium
BOARD DEMO and Board sketches
Pg. 52
B.Anything that changes base level will affect
the erosional ability of the stream
a. Earthquake/faults causing uplift land will cause….?
b. drop sea level will cause….?
c. remove temporary base level will cause…?
Let’s check it out
Remember how a river wants to reach equilibrium?
BOARD DEMO for each
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C.Any change in gradient will change erosional ability
of stream. WHY??? This diagram again.
a. Channelization -river cutting deeper into the ground
and moving debris out of channel-increases velocity
If we increase slope…?
If we decrease slope?
If we lift up an area of land? (lower base level)
If we lower an area of land? (lift up or increase base level)
BOARD DEMO
Pg. 52
6. Stream valleys and characteristic features
A. Narrow stream valleys-tend to be young and/or
farther upstream
a. V-shaped, steep valley walls
b. waterfalls and rapids common
c. relatively straight channel
d. far from (above) base level, erode downwards
lots of channelization
WHY???????
Pg. 52
B. Wide stream valleys -tend to be old and/or farther
down stream
a. wide valleys
b. floodplains, meanders
Flood plain photo -Why would people live here?
c. very sinuous channel (meanders)
d. close to base level, erode laterally
(meanders)-Not much channelization
e.) ADD where on a meander does erosion take place?
WHY?
WHY??????
Pg. 53
Why do floods occur?
1.Streams develop channels during times of normal
rainfall/normal streamflow
2. Floods occur when:
a) precipitation is above normal in drainage basin
b) sudden snow/ice melt
c) landslides-damming
Basically, too much water for the channel that was created
under “normal” conditions.
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TYPES OF FLOODS
1.FLASH FLOODS - very rapid, usually small drainage
basin
occur in semi arid areas most often
canyons most dangerous
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FLASH FLOOD CASE STUDY
1) Antelope Canyon, AZ 1997 (Photo: NWS-Flagstaff)
a) slot canyon - up to 100’ deep and few feet
wide
b) rain in area 11 miles away
c) 11’ high wall of water moved down canyon
d) 11 dead - one survivor (guide only one to
survive. Hmmmm… makes ya wonder)
Pg. 54
2) Big Thompson Canyon, CO 1976
popular recreation area outside of Denver
Photo by W. R. Hansen, Courtesy US Geological Service
a)
b)
c)
d)
e)
139 dead
$36 - 50 million in damage
7.5 “ rain in 4 hours ND pg. 382 fig. 13.15
20’ high wall of water
New Road Signs - climb to safety in case
of flash flood!
Pg. 54
2. REGIONAL OR SLOW-RISE FLOODS
High water may cover an area for weeks
cause extensive damage
Occur in large river valleys.
Pg.54
REGIONAL FLOOD CASE STUDY
1) Mississippi River 1993
a) Most expensive and deadly ever - $15-20 Billion
48 dead
b) Why did it happen?
1. weather - Jan -July >ave. rainfall in drainage basin
2. Dikes and Levees help make flood BIG
3. How do levees fail? ND pg. 387 fig. 13.21 and 13.22
PHOTO 1 PHOTO 2
Pg. 54-55
1717 New Orleans build levees 1879 major emphasis on
levee building
1927 more levees and dams
Despite all these efforts the river still floods.
When you contain the water in dikes/levees you are
creating potential for a HUGE flood.
Levee systems give a false sense of security and lead to
increased development in flood-prone areas.
Some say remove levees - others argue they help.
Pg. 55
HOW OFTEN DO FLOODS OCCUR?
1. Calculate flood frequency = how large and how often
Need a flood frequency curve ND pg. 379 Fig. 13.12
Data on discharge from stream from gauging station
a) highest discharge each year plot against time
Pg. 55 continued
HOW OFTEN DO FLOODS OCCUR?
b) curve constructed = gives discharge expected
for 50 and 100 year floods. AND others too.
c) there is a 1% chance any given year that you may
have a 100 year flood - does not mean happens
every 100 years EXACTLY.
Recurrence interval = certain sized flood happens every
100 years ON AVERAGE AND BASED ON THE DATA
WE HAVE COLLECTED MORE TIME=MORE
DATA=BETTER PREDICTIVE ABILITY
Annual Probability =reciprocal of recurrence interval
EXAMPLES AND ND pg. 378 Fig. 13.11
ND pg. 379 Table 13.2
Pg. 55
URBANIZATION AND FLOODING
RECALL WHAT INLFUENCES RIVER VELOCITY
1.Floods rise faster & higher - ground paved,
causes less ground infiltration
2.HUMAN INDUCED channelization - clear of debris,
wider, deeper, straighter, and smoother
Net result: water velocity and discharge increased so can
erode more causing more damage
HYDROGRAPH
ND pg. 392 fig. 13.28
3.Dams - do not provide flood control because they
are usually too full. They can fail.
Pg. 56
CASE STUDY
1) Tijuana and San Diego a) Tijuana channels and San Diego doesn’t –
water spreads out in S.D.= flooding
WHY?? ND pg. 394 Fig. 13.31
Pg. 56
2) Tucson - not consistent with building protective walls
no walls and stream widens & erodes
ND pg. 396 fig. 13.35
Statement by
Metropolitan Tucson Convention and Visitor Bureau
The 100-year flood has come and gone, so, by all rights,
Tucsonans should enjoy another century of great
southwestern weather.
QUIZ
1.) What feature is this feature?
2.) The cross section of the rivers below is the
same, what is this cross sectional area?
a.) 24m2
b.) 12 m2 c.) 8m2
d.) 6m2
a.)
3m
3m
b.)
1m
1m
24m
8m
c.)
4m
4m
6m
QUIZ
3.) Which channel will produce the most friction due to
wetted perimeter?
a.)a b.)b c.) c
a.)
3m
3m
b.)
1m
1m
24m
8m
c.)
4m
4m
6m
QUIZ
4.) Which stream is probably the slowest due to friction?
5.) Which stream represents a highly channelized stream?
a.
b.
c.
QUIZ
6.) What is the main thing that influences a streams
ability to transport and erode?
A)sediment size B) meander pattern C) velocity
D) hydraulic action E) drainage patterns
7.)
a.)
b.)
c.)
d.)
What are the 3 main things that control velocity
meanders, sediment load, gradient
hydraulic action, channel characteristics, sediment load
discharge, abrasion, bed load
gradient, channel characteristics, discharge
QUIZ
8.) Which of the following represents a river high above
base level?
9.) Which of the following would be eroding more?
10.) Depositing more?
a.
OCEAN
b.
OCEAN
c.
OCEAN
11.) Flash floods are best described as
a.) fast and covering large area
b.) slow and covering large area
c.) fast covering small area
d.) slow covering small area
12.) Big
a.) fast
b.) slow
c.) fast
d.) slow
Thompson Canyon is best described as
and covering large area
and covering large area
covering small area
covering small area
12.) What is the annual probability of a 500 year
flood occurring?
a.) .02%
b.) .2%
c.) 2%
d.) 20%