Flood Notes

Properties of Streams

• Gradient - change in elevation divided by length of stream
  • Decreases downstream
• 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?