Flood Notes
Properties of
Streams
- Gradient - change in elevation
divided by length of stream
- Velocity -
- fastest in center, near surface,
outside of curves
- slower on edges, near bottom,
on inside of curves
- Discharge - volume of water per
unit time
- Discharge varies with width,
depth and velocity
- Discharge increases downstream,
varies over the year
- Sediment load
- Size depends on velocity
- Volume depends on discharge
- Graded stream
- Gradient, velocity, discharge
and sediment load interact to create equilibrium conditions
- Concave-up profile
Stream Features
- V-shaped channel - in high gradient
- Downward erosion, landsliding
on walls of canyon
- Lower gradient:
- Meanders: due to erosion on
the outside of curves, deposition on the inside
- Floodplain: flat area
around river that river overflows into
- oxbow: lake that was once
a meander
- cutoff: when river cuts
across the neck of a meander
- Natural levee: high area
near river formed when river overflows and drops sediment
- Braided stream: fluctuating
discharge and sediment load (deserts, glaciers)
Factors that affect flooding
- Rainfall -
- Hawaii, spring 2006.Parts
of Oahu had 92” of rain in March, On Kuaui, earthen dam failed,
7 dead
- Snowmelt
- 1986, American River.“Pineapple
Express” melts all snowpack in one week
- 1986 started off as a dry
year. By February, Sacramento was 50% behind average rainfall.
- 1985 had been a dry year,
and the Bureau of Reclamation barely met their water contracts to farmers
in the Central Valley
- USBR retains more water in
Folsom (above 70%) because of fear of drought
- Pineapple Express strikes
- 10” of rain in a week
- Water enters Folsom Lake at
200,000+ cfs
- Levees below rated only to
115,000 cfs
- USBR chooses to push levee
capacity to 135,000 cfs
- Despite boils on both sides,
levees on American hold until water crests
- Infiltration - degree to which
water soaks in
- Topography - steeper drains faster
- Vegetation - tends to decrease
runoff by softening raindrop impact, increases infiltration
What do people do to increase
risk from flooding?
- Decrease infiltration
- Pave watershed: more water
runs off, water moves through system more quickly
- Remove vegetation: claer-cut
logging, mining
- Channelization: paving streams
- Decreases infiltration,
moves water through faster
- Channelization does prevent
erosion on bank
- Alter floodplain
- Decrease channel volume
- Decrease floodplain capacity
- Alter flow patterns
- During 1986 floods, water
treatment plant diverted flow against opposite bank, causing massive
erosion of levee
- Construct flood control projects
- increases population and development in flood plains
Flood risk strategies
- Manage development
- Flood zoning
- Flood insurance - federally
funded
- Mandatory if you have
a federally-guaranteed mortgage
- “voluntary”
otherwise (but your lender will probably require it)
- Why develop floodplains?
- Cost: undeveloped, flat
land is cheaper to develop
- Population pressure
- Who benefits?
- Developers, local
government, local economy
- Who incurs risk?
- Homeowner, federal
government
- Build flood control structures
- Levees - must have an impermeable
barrier beneath (clay)
- Can add an impermeable barrier
within
- Flood control levees are typically
built right next to river
- Setback levee includes a bigger
portion of the floodplain
- Greater capacity for floodwaters
- Relieves erosion on levee
- Lower height means less
energy when breached
- Bypass: area designed to flood
- Yolo-Sutter Bypass system
built in 1917
- Can divert almost 1 million
acre-feet of water from Sacramento River
- Weirs at Fremont and Sacramento;
drains back into river at south end
- Bypass also serves as wetlands,
wildlife refuge
- Dams
- Local dams: Folsom, Oroville,
Shasta
- Multipurpose dams
- Flood control
- Water supply
- Hydroelectricity
- Recreation
- Conflicting uses
- Types of dams
- Arch dams - use shape
of dam to transmit stress to canyon walls
- Use less materials
to build
- Fail catastrophically
- Require very stable
site - no faults, no landslides, no unstable rock
- Vaiont, Italy
- 1963: landslide
triggered by changes in reservoir level
- Water overtopped
dam, killed 1029 downstream
- Dam survived
the disaster
- Gravity dams: Use sheer
weight to resist force of water
- Can be earth- or
rock-filled, or can be made of concrete
- Require impermeable
site; concrete dams more forgiving then earth-filled
- Historic Dam disasters
- Johnstown (Pennsylvania,
1889) flood:the perfect storm of bad dams
- Old, poorly built
earthen dam - originally for canal boats, abandoned when RR
built
- Bought, then poorly
maintained as resort lake
- Downstream channel
narrowed as city expanded
- 2200 dead
- St. Francis Dam, Los
Angeles
- Part of Walter Mulholland’s
grand plan for bringing water from Owens Valley to LA
- Spectacularly bad
siting for dam
- Built across
San Francisquito Fault
- One end of dam
built on ancient landslide
- Spectacularly bad
dam design
- Primitive gravity
dam without sufficient bracing
- Height raised
twice during construction without widening the dam (to increase
capacity of reservoir)
- Both ends of dam
failed
- 450 dead, 900 buildings
destroyed
- Teton Dam 1976
- Poor siting of earth-filled
dam
- Porous rock on one
abutment, fractured rock on the other
- Took 30 minutes
to fail
- 11 dead, $5 billion
damage
- Despite these failures,
most dams are safe
- Dams usually more controversial
for environmental and economic reasons
- Destruction of riparian
habitat, fish issues
- Who pays? Federal?
State? Local?
Sacramento Flooding
- Critical Issues:
- Inadequate flood protection
on both the American and the Sacramento
- American: Folsom to
downtown
- Sacramento: Pocket Areanorth
to Yuba City
- What is the flood risk?
- Only about 80 years
of record
- May not be representative
years
- Seems to be increase
in large storms over time
- Record too short
to give accurate picture
- Maybe climate
is changing - trying to predict a moving target
- Use size of drainage
basin to predict maximum possible flow
- Still a very large
range of possible flows
- American River
- Current estimate
of 100-yr flood: about 200,000 cfs
- Current estimate
of 200-yr flood: about 250,00-300,000 cfs
- Current protection:
115,000 cfs
- About 85 year protection
- Increasing development in
areas with inadequate levees
- SAFCA currently trying
to get all Sacto. levees up to 100 yr protection
- What determines 100 yr.
floodplain?
- Flow
- Ability of levees
to withstand flow
- Sacto. levees not able
to withstand 100 yr. flow
- Recent discovery of
dip in levee in Pocket Area
- Seepage of levee in
Natomas area
- Choosing the most appropriate
solution
- Improved levees
- Necessary, but not
sufficient
- Must be done to
get to 100 year protection
- Current levee
configuration cannot handle 200 year flow
- Currently levee
projects underway on Sacramento; American repaired over last
decad
- Raising Folsom
- Options:
- Add more outlets
on Folsom
- Problem -
current outflows can deliver more water than downstream
levees can handle
- Lower water level
in Folsom
- Requires very
accurate weather prediction
- Has immediate
economic repercussions (lower water sales)
- Raise Folsom (and
associated levees) 7 feet
- Auburn Dam
- Proposed dam:
- Concrete gravity
dam
- 700 feet
- Impound 2.3 million
acre-feet
- Minimum cost $3
billion
- Issues at Auburn
- Seismic safety
- Auburn Dam initially
planned as thin-arch dam
- Construction on
foundation began in 1960’s; halted by Vietnam War
- Problems arose
anchoring foundation in highly fractured metamorphic rock
- Project developed
special techniques to inject grout into fractured rock
- Construction on
foundation was halted in 1975
- Oroville Dam
completed. Upon filling of reservoir, earthquake, M6.1
- Opened questions
on possibility of earthquakes at Auburn site
- Dam redesigned
as concrete gravity dam
- USBR says dam
could take 9 inches of displacement along fault and not
fail
- USGS says that
worst-case scenario is greater than 9 inches of motion
- Environmental
- Loss of riparian
habitat
- Endangering fish
- Loss of recreational
use (rafting, kayaking)
- Cost
- May turn out to
be greater than $3 billion
- US govt. has declined
to fund
- Who pays?
- State cannot
raise funds
- Seeking local
funding
- Continuing
to seek US funds - depends on re-election of John Doolittle
- Delay
- Dam first proposed
in 1928
- Dam construction
delayed for 40 years
- Is it better to
have some flood protection right now, or better flood protection
later on?