Living with levees: using tolerable
risk guidelines in California
Jessica Ludy, CFM. Arcadis-U.S. Inc.
Larry Roth, G.E., P.E., Arcadis-US, Inc.
Dustin Jones, P.E., Delta Stewardship Council
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Hoogwater
Guus Schooneville
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store it
convey it to the sea
let it flood
When it comes to levees: two types
Those that have been overtopped by floodwaters...
William Hammond Hall 1895
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When it comes to levees: two types
Those that have been overtopped by floodwaters...
and those that will be overtopped by floodwaters
William Hammond Hall 1895
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When it comes to levees: two types
“It should be fully understood then,
that floods will occasionally come which
must be allowed to spread…”
William Hammond Hall 1895
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Risk cannot be eliminated
California Delta
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Houston, Texas, May 2015
Well, what level of risk is tolerable?
How safe is safe enough?
Understanding tolerable (and intolerable) risk helps guide the level
of investment appropriate to reduce risk
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Outline
• Risk and tolerable risk
• Using tolerable risk in floodplain management
• Case Study: California Bay-Delta
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What is risk ?
What are the
hazards and how
likely are they to
occur?
USACE
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How will the
infrastructure
perform in the
face of these
hazards?
Who and what are in harms way?
How susceptible to harm are they?
How much harm is caused?
Risk = Probability x Consequence
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We make decisions every day based on
what level of risk is tolerable to us
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Yet, we (still) aim for the 1%.
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The 1% flood that is…
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Higher design standards
Red River Basin (US and Canada)
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Higher design standards
New York City
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Higher design standards
The Netherlands
(sorry, I had to go there)
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What guidance do we have?
1% aep – National Flood Insurance Program
(44 CFR 65.10)
• “100 year level of protection”
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What guidance do we have?
0.5% aep for California urban areas
(California State Legislature, water code 65007 (n))
• “200 year level of protection”
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What guidance do we have?
Hazard Mitigation Plan
(*MOU between FEMA and the State of California)
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What guidance do we have?
Public Law 84-99
(US Army Corps of Engineers)
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Challenges with this approach
None are safety standards
• 1/100 yr. is a levee design standard for NFIP
• Insurance and development focus
• 1/200 yr. is levee design standard
• HMP is an interim disaster rehabilitation guideline
• PL 84-99 is a disaster rehabilitation guideline
• Geometry based, focus on water surface elevations, say
very little on performance
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This approach focuses on the hazard
• Communicates that risk can be eliminated
• Lends itself toward structural measures
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Design-based standards ignore
residual risk
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1% chance called into question
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Teton Dam
Failure
Big Dig Tunnel
Collapse
Failures of critical
infrastructure
I-35W Bridge
Failure
Katrina
Flooding
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“Critical Infrastructure systems must hold
paramount the safety, health, and welfare of the
public it serves”
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•
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Exercise sound leadership
Use a systems approach
Adapt to change
Understand, manage,
and communicate risk
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How do you manage risk?
American Society of Civil Engineers 2014
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Leaders use risk informed analyses…
Tolerable Risk
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• How to apply it
• Risk-informed mapping
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• 30F-N
curves
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Eliminating risk is impossible
Tolerable Risk is the level of risk that people are
willing to live with in order to secure certain benefits.
Unacceptable
Tolerable
Broadly acceptable
Range of Tolerability
Risk cannot be justified
except under extraordinary
circumstances
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No further actions
required. Risk regarded
as insignificant
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Tolerable risk
United Kingdom
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Tolerable risk principles
(HSE 2001)
• Life safety is paramount
• Risk cannot be ignored
• Absolute safety cannot be
guaranteed
• Equity
• Efficiency
• Individual risk
• Societal risks
Goal = Reduce risk to as low as reasonably practicable (ALARP)
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Using tolerable risk guidelines enables
• Full consideration and communication of hazard and
consequence
• Consistent and transparent approach to reducing risk
• Enables systems approach
• Allocate scarce resources and focus on most urgent
risks first
• Evaluates tradeoffs
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How do you use tolerable risk to make
decisions?
• Assess, characterize, and evaluate risks
Unacceptable
Risk cannot be justified
except under extraordinary
circumstances
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Tolerable
Broadly acceptable
No further actions
required. Risk regarded
as insignificant
Frequency of (n) or More Fatalities per Year
10 -1
PROBABLY NOT GOOD
10 -2
GUIDANCE
DEFINED BY
USACE AND USBR
10 -3
10 -4
10 -5
10 -6
PROBABLY OK
10 -7
0.1
1
10
100
1,000
10,000
Number of Fatalities (n)
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F-N CURVES
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Frequency of (n) or More Fatalities per Year
10 -1
PROBABLY NOT GOOD
MERCHANT SHIPPING
10 -2
KATRINA
10 -3
FOUNDATIONS
FARM ACCIDENTS
10 -4
DAMS
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-5
10 -6
PROBABLY OK
COMMERCIAL AVIATION
10
-7
0.1
1
10
100
1,000
10,000
Number of Fatalities (n)
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F-N Plot
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Reduce risk to tolerable levels
Frequency of (n) or More Fatalities per Year
10 -1
UNACCEPTABLE RISK
10 -2
10 -3
Non-structural solutions –
e.g., improved evacuation
Structural solutions –
e.g., stronger levees
10 -4
10 -5
TOLERABLE RISK
10 -6
Goal = ALARP, as low as reasonably practicable
10 -7
0.1
1
10
100
1,000
10,000
Number of Fatalities (n)
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Life safety risk posed by levees
Netherlands tolerable risk guideline
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Applying tolerable risk guidelines in the
California Bay Delta
The Delta
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Sacramento
Suisun Marsh
San Francisco
Bay
Pacific
Ocean
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Sacramento –
San Joaquin
Delta
Stockton
California Bay Delta and Suisun Marsh
• 1,100 miles levees
• Roughly 60 islands
• Water Supply
• 25 million
• 4 million acres
• Ecosystem
• Agriculture
• 500,000 people
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Delta Plan, 2013
Delta land elevation and subsidence
Public Policy Institute of California
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Delta levees
• Wet/dry
• Project/non-project
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Delta levees
Levees built for one purpose now serve another
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Delta flood risks-hazards
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Flood (high water)
Earthquake
Condition of levees
Subsidence
Seepage
Jones Tract, “Sunny day failure” 2004
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Delta flood risks-consequence
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Delta Reform Act: levee investment
strategy to reduce risks
• Life & property
• State Interests
Coequal goals:
• Ecosystem
• Water Supply
• Delta as a Place
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Step 1: Assess flood risk in the Delta
(today and in the future)
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Step 2: Map flood risk in the Delta
AGGREGATE RISK
AGRICULTURE RISK
ECOSYSTEM RISK
WATER SUPPLY RISK
FLOOD DAMAGE
RISK (EAD)
LIFE LOSS RISK
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Set tolerable limit lines
Frequency of (n) or More Fatalities per Year
10 -1
PROBABLY NOT GOOD
10 -2
Line must be set by
policy and informed
by analysis
10 -3
10 -4
10 -5
10 -6
PROBABLY OK
10 -7
0.1
1
10
100
1,000
10,000
Number of Fatalities (n)
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Step 3: Act.
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Characterize risk
Immediate attention?
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Identify options to reduce risk
Non-structural measures
c
Structural measures
c
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Characterize risk
Are there additional areas that
could be considered for more
urgent attention?
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Identify options to reduce risk
Non-structural measures
c
Structural measures
c
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Do measures reduce risks to tolerable
levels? What are the tradeoffs?
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Effects of improved evacuation and
levee conditions on risk
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Iterative approach
CHARACTERIZE RISK
-START WITH RISKIEST
ASSESS RISK
(CONTINUOUSLY REVIEW)
IDENTIFY PROJECTS TO
REDUCE RISK TO
TOLERABLE LEVELS
ASSESS,CHARACTERIZE
RESIDUAL RISK
-ELEVATE RELATIVE
URGENCY BASED ON
ASSETS
USE EXPERT JUDGMENT
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Next steps for us
• Identify and evaluate measures to reduce risk to
tolerable levels
• Evaluate tradeoffs
• Develop a comprehensive investment strategy in
the Delta
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Why tolerable risk?
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We know the problems with design-based standards
Life safety paramount
Recognizes that absolute protection is not possible
Communicates risk clearly
Manages risk with all available options
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Why tolerable risk?
• Guides decision making
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Allocates finite resources over 1,100 miles of levees
Identify which levees pose the greatest risk
Suggests which actions to take first
Level of risk in relation to tolerable risk
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Why tolerable risk?
• Because floodplain LEADERS will use risk to
make decisions in floodplain management
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Thank you.
Our partners:
Delta Stewardship Council
Rand Corporation
Environmental Science Associates
Shannon and Wilson
Catalyst California
Contact: [email protected]
http://deltacouncil.ca.gov/delta-levees-investment-strategy
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