Flood Resilience of Critical Assets
Based on work with UK Water Utilities
April 2012
Alec Yeowell
Yeowella@halcrow.com
Halcrow is a CH2M HILL company
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Contents
1. What is Flood Resilience?
2. Why is it important?
3. How can it be assessed?
4. What could be done about it?
5. Should we do anything about it?
6. How sure can we be?
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What is resilience?
• Flood resilience – this is where structures are designed to reduce
the consequences of flooding to a property and facilitate recovery
from the effects of flooding sooner than conventional buildings
e.g. use of water resistant materials to floors, walls and fixtures with the
setting of services such as electric controls and cables within the property
being at a higher level; raising of the finished floor levels within the
building
• Flood resistance – this is the method of construction that can
prevent entry of water or minimize the amount of water that may
enter a building or construction site e.g. bunds and walls; storage of
exceedence flow
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Why is it important
Flood level
experienced on
site in 2007
Flood level
experienced on
site in 2007
Risk:
- Low probability
- High consequence: significant and long duration loss of critical service to
customers
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Why is it important
Environment Agency Assessment of River flow return periods experienced in 2007
Yorkshire Water experienced significant flooding at several water and wastewater treatment
sites. Severntrent Water customers experienced service disruptions
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The Ofwat planning framework
• Stage 1 Risk screening
• Stage 2 Risk analysis
• Stage 3 Intervention analysis
1.
2.
3.
4.
5.
6.
7.
8.
6
Assess the annual probability and associated
depth of flooding for the asset of concern
Determine the damage that will be caused
(engineer and operator experience)
Determine the loss of service that will be caused
(consider inter and intra- dependencies
Consider factors for change e.g. climate change
Monetise the costs and the service impacts
Determine risk-costs over whole life of the asset
(this constitutes the ‘without mitigation scenario’)
Consider the potential for intervention, not
forgetting carbon costs (do nothing, monitor,
enhance etc..)
Model costs and benefits of options and select
optimum
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Stage 1 Risk screening
Hazard Assessment
Consequence Assessment
•
Coarse screening
•
•
EA mapping boundaries
Measures of consequences of
asset failure
•
Fluvial, pluvial coastal
•
Population served
•
GIS proximity
•
Capacity and power rating
Risk scoring
•
Hazard
•
Consequence
Workshop review
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•
Sense checking
•
Site history
•
Operator knowledge
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Stage 1 Hazard screening
Flood Zone 3: Land assessed,
ignoring the presence of
flood defences, as having a
1% (1in100) or greater
annual probability of fluvial
flooding or a 0.5% or greater
annual probability of tidal
flooding.
•
Coarse screening
•
EA mapping boundaries
•
Fluvial, pluvial, coastal
•
GIS proximity
Short section of
formal flood
defence.
Flood Zone 2: land assessed,
ignoring the presence of flood
defences, as having between a 1%
(1in100) and 0.1% (1in1000) annual
probability of fluvial flooding or
between a 0.5% and 0.1% annual
probability of tidal flooding in any
year.
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Stage 1 Consequence screening
Factors considered
Stage 2 selection
• local topographical factors
• Sites known to flood in 2007
• operational issues
• sites deemed at high risk in
screening
• vulnerabilities and resilience
• knowledge of the site
• interdependencies with other
assets
• sites that site managers
considered to be problematic,
based on historical events
• historical event information
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Stage 1 Risk screening
1.3 Assets at risk?
• Data from 1.1 and 1.2
• Spreadsheet analysis tool
results
• Operator knowledge of
sites
• Probability and
consequence
information
combined
1.3.1 Combine measures of hazard likelihood and
asset failure consequence to produce overall risk
score
No
Asset "at risk"
Yes
Input to Stage 2 analysis
Exclude assets from further
analysis
• Workshop
activities to
validate
• Selection of
assets for further
study
1.3.2 Plan detailed risk assessment activities and develop
action plan for Stage 2 analysis
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Stage 1 Risk screening
• Probability and
consequence
information
combined – but
qualitative
• Workshop activities
to validate
• Selection of assets
for further study
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Stage 2 Risk assessment
Hazard Assessment
Vulnerability Assessment
Consequence Assessment
• Quantitative
• Site survey
•
Impacts
• NFCDD levels
• Critical levels
•
Lost service
• Historic flood
data
• Building thresholds
•
Costs of failure
Define Consequences
Do nothing/Carry on
as you are
Define Consequences
Option/Intervention 1
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Using river model data to estimate probability
Existing flood wall:
1in 340 year (D5)
Sludge works
thresholds: 1in
290 year (D4)
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Site surveys to establish critical levels
• Thresholds at doorways
• Location of water sensitive equipment
(level where inundation will cause
damage)
• Tops of tanks
• Top of existing flood defences and
geographical features of the site
• A general “mid-point” for the site,
indicative of significant failure potential
• Levels for individual
assets/processes/equipment
• Historical flood levels
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Stage 2 Risk analysis
1. Risk profile for the
asset to be
generated by
resilience tool
2. Intervention costs
and benefits
required for CBA
assessment
2.4 Risk analysis
INPUTS
• Flood outlines & depths
• Detailed understanding of
flood impact at each site for
each source
• Understanding of asset
reliability
• Appreciation of
"interdependencies"
PROCESS STEP
OUTPUTS
2.4.1 Whole life risk analysis
(monetise, annualise and produce whole life risk profile for
each site)
No
1) Quantified,
monetised
estimates of risk
2) Estimate of whole
life cost of
ownership (based
on monetised and
annualised risk
values)
Risk analysis
quantification
sufficient?
Yes
Update site
reports
1.3.2 Plan detailed risk assessment activities
and develop action plan for Stage 2 analysis
3.1.1 Technical selection of interventions and risk
mitigation measures
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Stage 3 Methodology for
Resilience Analysis
4. Example CBA and resilience analysis for
fluvial and pluvial hazards

Vulnerability assessment based on
appraisal of critical processes and
equipment
Failure modes and effects type
process used to estimate damage
costs and service losses
Worst case flood level including climate change
13.50
Worst case flood level
13.00
Existing resilience level (from defence wall)
12.50
12.00
11.50
Level mAOD

Treatment works example
11.00
10.50
10.00
9.50
9.00
8.50
8.00


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CBA to understand future risk costs
and service losses (these can be
monetised using available damage
cost and WTP data)
CBA to compare costs and benefits
of options for meeting specific
level of protection (say 1/1000
level)
March 2012
5yr
10yr
25yr
50yr
75yr
Return period
100yr
Existing flood wall:
1in 340 year (D5)
Sludge works
thresholds: 1in
290 year (D4)
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200yr
1000yr
Stage 3 Intervention analysis
Assessing the costs and benefits:
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Stage 3 Intervention analysis
What type of interventions?
• Resistant interventions
• Resilient interventions
• Build a big wall or embankment
• Flood warning alarms
• Raise equipment
• Response plans
• Seal buildings (doors and vents)
• Demountable flood defences
• Usually more expensive, risk averse and
precautionary
• Keep spares
• Usually less expensive and less certain
outcomes
• Should we value keeping our options open?
•
Interventions can be grouped by:
•
Resistant
•
Reliability
•
Redundancy
•
Response and Recovery
Cabinet Office: Keeping the Country Running: Natural Hazards and Infrastructure
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Stage 3 Intervention analysis
Assessing the costs and benefits with the Resilience
Spreadsheet tools:
Existing flood wall:
1in 340 year (D5)
Sludge works
thresholds: 1in
290 year (D4)
• Assets can have 2 critical levels
• Each level can be assigned damage costs,
costs of failure and duration of lost service
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Stage 3 Intervention analysis
Assessing the costs and benefits:
Probability vs Damage Curves for each scenario
£450,000
Area under curves
describes the
probable annual
flood damage
£400,000
£350,000
Do nothing
Option 1
Option 2
P0
P1
P2
P3
P4
P5
P6
P7
P8
P9
P10
P11
P12
P13
P14
P15
P16
P17
P18
P19
P20
Max
Damage Costs
£300,000
£250,000
£200,000
£150,000
£100,000
£50,000
•
Probable annual damage
•
Do nothing damage from
1in10 year return period
•
Option 1 provides
additional level of
protection 1in200 year
return period
•
Option 2 provides
additional resilience to
account for uncertainty
but will cost more
Less
frequent
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0.1000
0.0900
0.0800
0.0700
0.0600
0.0500
0.0400
0.0300
0.0200
0.0100
0.0000
£0
Annual Exceedance Probability
More
frequent
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Stage 3 Intervention analysis
Assessing the costs and benefits:
Options annual costs and benefits (present value £)
Option 1
Option 2
20,000
Probable annual damage
•
Option 1 provides additional
level of protection 1in200
years
•
Option 2 provides level
additional level of
protection to 1in200 years
bit costs more
•
Direct damage costs
•
Costs of failure (COF) e.g.
responding to customer
complaints
•
Customers willingness to pay
for improved resilience?
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
0
•
-20,000
PV £
-40,000
-60,000
-80,000
-100,000
-120,000
-140,000
Time (years)
NPC
Option 1
Option 2
60,000
40,000
20,000
NPV £
-20,000
-40,000
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
0
-60,000
-80,000
-100,000
-120,000
-140,000
Time (years)
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Stage 3 Intervention analysis
Uncertainties:
• Flood levels defined for each probability
• Direct damage costs
• Costs of failure (COF) e.g. responding to customer
complaints – how many? Levels of compensation
• Customers willingness to pay for improved resilience? –
May be available from market surveys
• Final out turn costs of intervention
• Financial and social discounting rates
• Climate change impact on storm frequency and hydrology
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Stage 3 Intervention analysis
Opt_1_NPV
• Option 1
NPV (£1,000)
• Provide 1in200 level of resilience
(excluding any freeboard
allowance) by raising equipment
40
• Cost £50,000 fairly certain
-160
0
2
4
6
8
10
12
14
16
18
20
22
24
26
28
30
32
34
36
38
40
22
24
26
28
30
32
34
36
38
40
Year
• Provide 1in200 level of resilience
(including freeboard allowance of
extra 300mm) by providing flood
wall
Freeboard is an additional allowance
applied to flood defences to account for
uncertainty
-60
-110
• Option 2
Opt2_NPV
40
NPV (£1,000)
• Cost £150,000 fairly uncertain
-10
-10
-60
-110
-160
0
2
4
6
8
10
12
14
16
18
20
Year
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Issues and learning
1. Risk assessment processes work
2. CBA: some interventions can be cost beneficial using only direct cost avoidance
3. CBA: existing customer surveys may not capture appropriate customer preference
measures
4. Environment Agency data sets are valuable (river models)
5. Site survey approach is required for detailed assessment: all sites are different
6. Climate change allowances may need to be included – but uncertain – needs
further work
7. Freeboard allowance may be used for uncertainty - but may not always be cost
beneficial – should be based on uncertainty
8. Adaptive asset management solutions may be preferable where there is
significant uncertainty about the future
9. How do you value the benefits of adaptive asset management solutions and
flexible options?
10. Resilience is not just about flooding – Ofwat guidance is being updated and a
UKWIR project is about to start which will significantly broaden our
understanding
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Broader Resilience themes
• Flood risk resilience
• Asset deterioration
• SEMD (Security and Emergency Plans)
• Supply and demand management
• Climate change
• System interdependency
• Assets owned by others
• Insurance
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