Levee Fragility Curves and Safe Storage

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Levee Fragility Curves and Safe Storage
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Safe Storage

• Relates to the concept of levee removal for FEMA
  mapping.
• Safe storage has been considered on several
  projects including a recent FLO-2D model FIS
  Study approved by FEMA.
• Safe storage concept can be linked to fragility
  curves.

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Safe Storage Concept

• Even the levees in the poorest condition will
  store some water before failure initiates.
• Safe storage may only extend 1 or 2 ft above the
  toe of the levee but constitutes a large volume
  of water.

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Levee Safe Storage
Safe Storage ?
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Safe Storage Issues

• For the Sacramento River Basin, this volume of
  storage would significantly reduced downstream
  flood volumes and peak discharges (floodwave
  attenuation).
• When safe storage elevation is exceededlevee
  removal vs. leave breach? For what reach length?

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Levee Fragility Curves

• Fragility represents a link between a
  geotechnical risk model and predicted water
  surface elevations with a flood model.
• Implicitly defines levee failure risk for hazard
  mapping.

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Background

• Levee core and shell materials vary significantly
  especially in older levees
• Levees often failure before overtopping due to
  piping
• Breach expands to complete failure while flood
  waters are still rising

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Corps Developed Fragility Curves on the
Sacramento River

• Method
• Select curve
• Select likely failure point
• Determine distance below crest

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Levee Geotechnical Investigation

• Core and shell material
• Percent fines vs. local spoil material
• Construction methods
• Compaction vs. spoils
• Foundation condition
• Sand boils
• Field inspection, in-situ borings, lab testing

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Fragility Curve Data
Create Fragility Curves Data in BREACH.DAT
Curve ID LFP Dist. Below Crest
FS1 0.03 6.0 FS1 0.15 3.5 FS1 0.50 2.5 FS1
0.85 1.0 FS1 0.95 0.0 FS2 0.03 9.0 FS2 0.15
5.5 FS2 0.50 4.0 FS2 0.85 2.0 FS2 0.98 0.0
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Assign Levee Fragility Failure Data
Assign Levee Element, Select Curve ID, and LFP in
LEVEE.DAT
C FS3 0.5 P 3450
FS1 0.5 P 3558 FS1 0.9 P
3559 FS2 0.7 P 3669 FS3
0.5 P 3670 FS4 0.5 P 3782
FC1 0.3 P 3783 FS1 0.5
GLOBAL DATA
INDIVIDUAL LEVEE ELEMENT DATA
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Results

• FLO-2D finds the breach based on the hydraulics
  and the weak levees
• Breach can occur anywhere at any time
• FLO-2D initiates piping based on water surface
  and prescribed duration
• Pipe expands to channel breach to collapse to
  complete failure

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Benefits of FLO-2D Fragility Curves

• Combines geotechnical characteristics with flood
  routing hydraulics for a spatial assessment of
  levee failure and floodwave attenuation
• Identifies flood risk as a function of levee
  failure
• Different levee LPF will results in different
  areas of flooding

Fragility Curve Simulation
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