PG. 50FLOODS CHAPTER 12Part of the Hydrologic Cycle - the circulation of Earth's

water supply

Streams are defined as running water confined to achannel 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. 501. Three things affect velocityA. Gradient - the slope of a stream channel

sinuosity=channel length/straight line lengthMOST STREAMS FOLLOW THIS RULE1.) lower gradient=downstream section =higher sinuosity= meandering

2.) higher gradient = upstream section=lower sinuosity= straighter

BOARD and ROPE demoB. Channel Characteristics - reduce friction

a. shape - semicircle ideal; least amount of water in contact with channel

b. roughness - smooth less frictionc. size - reduce ratio of WETTED perimeter to x-section.

PG. 501. 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 velocityQ 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”

Pg. 512.Streams seek equilibrium (give and take) –

a state of balance between deposition (put) and erosion (take).

Stream equilibrium depends on 1.) discharge2.) 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 erodingthe 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 dischargeThe river tries to reduce this increase of discharge/velocity HOW WILL IT SLOW ITSELF DOWN?

1.) excess energy erodes the stream bottomND 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 CONTINUEDADD 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 CONTINUEDADD 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. 13.62.) straightening the channel increases slope=take away sedimentsBOARD AND ROPE DEMO

Pg. 513. 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 1River 2

Pg. 514. 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

BedLoad

River Flow Suspended Load

Pg. 515. Control on erosion and stream valley development

A. BASE LEVEL-

a. Ultimate base level - ocean

b. Temporary base level - lake, mainstream 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. 52B.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

Pg. 52C.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. 526. Stream valleys and characteristic features

A. Narrow stream valleys-tend to be young and/or farther upstreama. V-shaped, steep valley wallsb. waterfalls and rapids commonc. relatively straight channeld. far from (above) base level, erode downwards

lots of channelization

WHY???????

Pg. 52B. 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. 53Why 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 basinb) sudden snow/ice melt c) landslides-damming

Basically, too much water for the channel that was created under “normal” conditions.

Pg. 53

TYPES OF FLOODS

1.FLASH FLOODS - very rapid, usually small drainage basin

occur in semi arid areas most often

canyons most dangerous

Pg. 53

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. 542) Big Thompson Canyon, CO 1976popular recreation area outside of Denver

Photo by W. R. Hansen, Courtesy US Geological Service

a) 139 deadb) $36 - 50 million in damagec) 7.5 “ rain in 4 hours ND pg. 382 fig. 13.15d) 20’ high wall of water e) New Road Signs - climb to safety in case

of flash flood!

Pg. 542. REGIONAL OR SLOW-RISE FLOODS

High water may cover an area for weekscause 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 13.22PHOTO 1 PHOTO 2

Pg. 54-551717 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. 55HOW 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 stationa) highest discharge each year plot against time

Pg. 55 continuedHOW 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. 13.11ND pg. 379 Table 13.2

Pg. 55URBANIZATION 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 damageHYDROGRAPHND pg. 392 fig. 13.283.Dams - do not provide flood control because they

are usually too full. They can fail.

Pg. 56CASE STUDY1) Tijuana and San Diego -

a) Tijuana channels and San Diego doesn’t – water spreads out in S.D.= floodingWHY?? ND pg. 394 Fig. 13.31

Pg. 562) Tucson - not consistent with building protective walls

no walls and stream widens & erodesND 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.

QUIZ1.) What feature is this feature?

2.) The cross section of the rivers below is thesame, what is this cross sectional area?

a.) 24m2 b.) 12 m2 c.) 8m2 d.) 6m2

3m

8m

3m

24m

1m

a.) b.)1m

4m

6m

4m

c.)

QUIZ

3.) Which channel will produce the most friction due to wetted perimeter?a.)a b.)b c.) c

3m

8m

3m

24m

1m

a.) b.)1m

4m

6m

4m

c.)

QUIZ4.) Which stream is probably the slowest due to friction?

5.) Which stream represents a highly channelized stream?

a. b. c.

QUIZ6.) 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.) What are the 3 main things that control velocitya.) meanders, sediment load, gradientb.) hydraulic action, channel characteristics, sediment loadc.) discharge, abrasion, bed loadd.) gradient, channel characteristics, discharge

QUIZ8.) Which of the following represents a river high abovebase level?

9.) Which of the following would be eroding more?

10.) Depositing more?

OCEAN

a.

b.OCEAN

c.

OCEAN

11.) Flash floods are best described asa.) fast and covering large areab.) slow and covering large areac.) fast covering small aread.) slow covering small area

12.) Big Thompson Canyon is best described asa.) fast and covering large areab.) slow and covering large areac.) fast covering small aread.) slow covering small area

12.) What is the annual probability of a 500 yearflood occurring?

a.) .02%b.) .2%c.) 2%d.) 20%