CE 385 D Water Resources Planning and Management

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CE 385 D Water Resources Planning and Management

• Flood Management - 1
• Daene C. McKinney

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Floods

• Floods affect the lives of more than 65 million
  people per year
• More than any other type of disaster, including
  war, drought and famine
• In East and Southeast Asia, during the monsoon
  season, rivers swell to over 10 times the dry
  season flow
• About 13 (of 45,000) of all large dams in the
  world in more than 75 countries have a flood
  management function

USGS - top www.ci.austin.tx.us - bottom
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Hydrologic Cycle
Precipitation, P(t)
Runoff, streamflow, Q(t)
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Flood Damage

• Injuries and loss of life
• Social disruption
• Income loss
• Emergency costs
• Physical damage
• Structures, utilities, autos, crops, etc.
• Lost value of public agency services
• Police fire protection, hospitals, etc.
• Tax loss
• Property and sales

www.ci.austin.tx.us
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Streamflow Hydrograph
Basin Lag
Centroid of Precipitation
Peak
Time of Rise
Recession Limb
Discharge, Q
Rising Limb
Baseflow Recession
Inflection Point
Baseflow Recession
Baseflow
Time
Beginning of Direct Runoff
End of Direct Runoff
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Storm Runoff

• Rainfall Divided
• Direct runoff (Pe)
• Initial loss (before DRO, Ia)
• Continuing loss (after DRO, Fa)

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Shoal Creek Flood - 1981
Precipitation
Streamflow
www.ci.austin.tx.us
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Stream Gauging

• Q VA
• Estimate
• Cross-sectional area
• Average velocity
• Subdivide cross-section
• Determine "average" flow for each subdivision
• Sum for total flow

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Stage - Discharge Curve

• Stage (height) and discharge (flow rate)

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Extreme Events Return Period

• Extreme events
• Random variable (Q) Realization (q) Threshold
  qT
• Extreme event
• if Q qT
• Recurrence interval
• t Time between occurrences of Q qT
• Return Period
• T Et Average recurrence interval

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Guadalupe River near Victoria
Exceeded 16 times, 16 recurrence intervals in 69
years
Exceedance probability
Return Period
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Flow Exceedance Distribution

• Q is RV Annual Maximum Flow
• qT is flow with return period of T years
• Flow exceedance probability
• Exceedance Distribution

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Events Considered in Design

• Return periods (T)
• 1 100 years (Minor structures)
• Highway culverts bridges, Farm structures,
  urban drainage, air fields, small dams (w/o LOL)
• 100 1000 years (Intermediate structures)
• Major levees, intermediate dams
• 500 100,000 years (Major structures)
• Large dams, intermediate small dams (w LOL)
• Probable Maximum Precipitation (PMP)
• Probable Maximum Flood (PMF)

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Flood Damage

• Event damage
• Damage from flood events (e.g., 10-, 50-,
  100-year events)
• Used for emergency planning
• Expected annual damage
• Average annual damage for events that could occur
  in any year
• Used for project B/C analyses

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US Federal Flood Programs

• Two agencies
• US Army Corps of Engineers (USACE)
• Focused on reducing flood damage through
  implementation of various protection works
• Federal Emergency Management Agency (FEMA)
• Focused on flood insurance as a means for partial
  recovery of losses for property owners
• Floodplains flooded by the 100-year flood are
  subject to
• land-use management provisions (no development in
  the floodway, properties must be elevated, etc.)
  and
• flood insurance is mandatory for properties
  located within this zone if communities are to
  remain eligible for certain disaster relief
  programs.

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Flood Damage Reduction(a US Corps of Engineers
Perspective)

• Identify a plan that will reduce flood-damage and
  contribute to national economic development (NED)
  and is consistent with environmental protection
• Benefits
• Locational (BL) Increase in income from
  additional floodplain development
• Intensification (BI) Increase in income from
  existing floodplain activities
• Inundation reduction (BIR) Plan-related
  reduction in physical economic damage, income
  loss and emergency costs
• Costs Total implementation costs OMR costs (C)

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Inundation Reduction

• Economic damages With and Without plan
• Expected Annual Flood Damage
• Risk of various magnitudes of flood damage each
  year
• Weight damage by probability of event occurring

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Flood-Damage Reduction Measures
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Effect of Flood Management Measures
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Planning Study

• Which measures, Where to locate, What size, How
  to operate
• Formulate ? Evaluate ? Compare various
  alternative plans
• Reconnaissance phase
• Find at least one plan that
• Has positive Net Benefits
• Satisfies environmental constraints
• Is acceptable to local stakeholders
• Estimate flood damages Without plan
• Feasibility phase
• Refine and search the set of feasible plans
• Detailed studies of channel capacity, structural
  configurations, etc.
• Evaluate economic objective, environmental
  compliance, etc.
• Design phase

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Computing Expected Annual Damage
stage-discharge
flow-probability
stage-damage

• Compute
• Damage exceedance distribution
• Probability that Flood Damage (FD) is specified
  level (fdT)
• Expected Annual Flood Damage

damage-probability
Expected Annual Damage
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Computation of Expected Annual Damage

• Construct basic relationships for without-plan
  situation
• Flow exceedance distribution
• Stage-discharge curve
• Stage-damage curve
• Damage exceedance distribution
• Compute the area beneath the damage-exceedance
  distribution (expected annual flood damage) for
  each location and sum to obtain the total
  expected annual flood damage
• Repeat step (1) for each alternative flood plain
  management plan under investigation
• Repeat step (2)
• Subtract results of step (4) (with plan) for each
  plan from without-plan results. The differences
  will be expected annual flood damage reduction
  for each plan

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Expected Annual Flood Damage
Stage-discharge curve
Stage-damage curve
Flow exceedance distribution
Damage exceedance distribution
Calculating Expected Annual Flood Damage
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Benefits of EFD Reduction

• Expected Annual Flood Damage reduction
• Difference between EFD with and without
  protection

Calculating Expected Flood Damage Reduction
Benefits
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Floodplain Protected by a Levee

• Probability of overtopping or geo-structural
  failure
• Need stage-discharge relationships in the channel
  and on the floodplain
• Flood stage in the floodplain protected by a
  levee is a function of
• Flow in the stream or river channel,
• Crosssectional area of the channel between the
  levees on either side,
• Channel slope and roughness,
• Levee height.
• If floodwaters enter the floodplain
• Water level in the floodplain depends on the
  topological characteristics of the floodplain

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Levees

• Probability of levee failure function of
• Levee height
• Distribution of flows
• Probability of geostructural failure

• Probability of levee failure
• 15 probable non-failure point, PNP
• 85 probable failure point, PFP

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Example
Inundated 130 businesses and 732 residences,
second-story flooding, eight lives lost.

• Urban basin.
• Floods have caused significant damage
• Flow is measured at a USGS gauge nearby
• communities in the basin have been flooded
  periodically
• Increased development in the upper portion of the
  basin promises to worsen the flood problem, as
  urbanization increases the volume and peak
  discharge

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Example

• Flood problem analyzed to identify opportunities
  for damage reduction
• Set of damage reduction alternatives formulated
• Evaluate each alternative in terms of economic
  performance
• Display the results so that alternatives can be
  compared
• Identify and recommend a superior plan from
  amongst the alternatives
• The standard for damage-reduction benefit
  computation is the without-project condition.
  Expected annual damage should be computed
• For the computation, discharge-frequency,
  stage-discharge, and stage-damage relationships
  were developed following standard procedures

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Discharge - Probability Function

• The existing, without-project discharge-frequency
  relationship was developed from the sample of
  historical annual maximum discharge observed at
  the USGS gauge

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Stage - Discharge Function

• The present, without project stage-damage
  relationship at the USGS gauge index point was
  developed from water-surface profiles computed
  with a computer program

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Stage - Damage Function

• Developed with the following procedure
• Categorize structures in the basin
• Define an average-case stage-damage relationships
  for categories
• Add emergency costs

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Flood Damage Exceedance Frequency
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EAD Integration Procedure
Damage ()

• Area between each pair of points is found by
  Integration.

Area added as last step in integration
Area under curve is expected annual damage
First exceedance value should be at zero damage
Last exceedance frequency
Exceedance Probability
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Expected Annual Flood Damage
Trapezoid Rule
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Uncertainty

• In flood damage-reduction planning, uncertainties
  include
• Future hydrologic events streamflow and rainfall
• choice of distribution and values of parameters
• Simplified models of complex hydraulic phenomena
• geometric data, misalignment of structure,
  material variability, and slope and roughness
  factors
• Relationship between depth and inundation damage
• structure values and locations, how the public
  will respond to a flood
• Structural and geotechnical performance when
  subjected to floods

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Introducing Uncertainty

• Assign probability density functions to
  evaluation functions
• At any location an orthogonal slice would yield
  the PDF of uncertainty
• EAD and benefits determined in the same way as
  before, however, a Monte Carlo sampling is used
  to sample from the functions to produce
  independent probability damage functions that
  are integrated to compute EAD
• Monte Carlo sampling is repeated (replicates)
  until stable expected values are computed.

Darryl W. Davis, Risk Analysis in Flood Damage
Reduction Studies The Corps Experience, World
Water Congress 2003 118, 306 (2003)