making sense of probabilistic flood forecasts decision ... · 3 decision-support methods • basic...
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Making sense of probabilistic flood forecasts –
decision support methods
Murray Dale Hydrometeorologist ([email protected])
• Probabilistic flood forecasting and why do it
• Decision-support: how those taking FIM decisions can
use probabilistic forecasts – outcomes of the
Environment Agency research project SC090032
Plan of presentation
What is probabilistic flood forecasting?
• In plain English: “What is the
chance of a flood happening?”
• Instead of saying it might flood / it
might not, a probabilistic forecast
means you can say: “There’s a
60% chance it will flood”
• And also: “…but there’s a 10%
chance it might be a really
damaging / dangerous flood”
• BUT… how do we know what to
do consistently with this
information?
Why bother?
• "Apparently a lady rang the BBC and said she
heard that there was a hurricane on the way. Well,
don't worry, if you're watching, there isn't."
Decision-support methods – project SC090032
6
Project SC090032 (2009-13)
Project outcomes
• easy-to-apply framework for practitioners
• case studies
• techniques for:
– a. Setting risk based thresholds for probabilistic flood forecasts
– b. Assessing the ‘costs’ and ‘benefits’ of relevant FIM actions
– c. Considering ‘soft’ factors in decision making
– d. Determining performance of probabilistic flood forecasts
1. Coastal surge: Colne Barrier
(Essex)
2. Coastal surge (&fluvial): Thames
Barrier, London
3. Fluvial using European Flood
Alert System model: River
Severn @ Bewdley
4. Fluvial using new Grid-to-grid
model: Cornwall (using floods of
17 November 2010 as case
event)
5. Surface Water using Extreme
Rainfall Alert system: multiple
locations
12
3
4
5
Case studies
Decisions on what FIM actions?
10 9 8 7 6 5 4 3 2 1 0.5 0
60 54 48 42 36 30 24 18 12 6 2 0
Monitoring & forecasting
Event preparation
On-site activities
Warning dissemination
Lead time (Large Catchment) in days
Lead time (Small Catchment) in hours
Deployment of staff to respond operationally to floods and/or monitor flooding in
communities
Routine & enhanced forecasting
Initiate enhanced monitoring
Flood Advisory Teleconferences
Staff Preparedness
Issue Severe Flood Warnings to public and
partners
Issue Flood Warnings to
professional partners
Issue Flood Warnings to
public
Flood awareness raising with public
Structural checks and watercourse
clearances
Deploy temporary and
demountable defences
Operate active control
structures
3 decision-support methods
• Basic method – use a probability threshold for a specific
FIM action based on judgement and local knowledge (e.g.
20%, 40%, 60%)
• Simplified method – use a probability threshold based on
the ratio of FIM action cost and the monetised benefit of that
action (a cost-benefit approach in the widest sense)
• Detailed method – establish a water level-impact
relationship for use in real-time and, in real-time, determine
whether average flood impact of the forecast water levels (if
no FIM action is taken) is greater than the FIM action cost
Detailed method – deriving average benefit
Detailed method – decision support
FIM Decision Support Tool
Date 01/05/2014 Time 12:22:42
Team XXX User XXX
Site/Community
Potential FIM action
Decision Support Method
Standard cost/benefit method Simple threshold method
Action cost £4,000 Probability threshold 0%
Forecast benefit £6,142 Forecast probability 100%
Initial recommendation Take action Initial recommendation Take action
Soft factors influencing the decision include:
1. Do you want to use this event as a practice or training event or as a PR exercise? [could change a ‘No’ into ‘Yes’]
2. Is the community at risk in danger of being desensitised (i.e. too many false alarms?) [could change a ‘Yes’ into ‘No’]
3. Is this a highly sensitive location with recent flooding? [could change a ‘No’ into ‘Yes’]
4. Have there been any missed flooding events (not forecast) at this site ? [could change a ‘No’ into ‘Yes’]
Final action decision
Justification
1 Forecast benefit comprises monetised impact of reduction in risk to life/serious injury, social impact, residential properties damage,
business/agriculture damage and infrastructure disruption.
Colne Barrier
D2. Operate active structures as necessary (e.g. close barriers)
Standard Method
Save As PDF
Load Probabilistic Forecast Result Data
Which method to pick?
• PFF forecast reliability (a good relationship between forecast
probability and observed frequency) – this includes reliability
of forecast models (hydrology & hydraulic)
• The cultural shift in forecasting & warning required
• Cost-benefit for decision making vs. the audit trail / process
Challenges
Colne Barrier events > 3.3mAOD: Forecast Surge Peak + Forecast Astronomical Peak(Forecast 1 = One tidal cycle [T + 5 to 12 hours]; Forecast 2 = Two or more tidal cycles [T + 12 to 36 hours])
2.5
2.7
2.9
3.1
3.3
3.5
3.7
0 1 2 3 4 5
Event Number
mA
OD
Actual Peak Deterministic Forecast
25/11/2007 12:15 (Ev1) 25/11/2007 12:15 (Ev1)
23/02/2008 01:30 (Ev2) 23/02/2008 01:30 (Ev2)
10/02/2009 12:30 (Ev3) 10/02/2009 12:30 (Ev3)
03/02/2010 02:45 (Ev4) 03/02/2010 02:45 (Ev4)
01/03/2010 12:30 (Ev5) 01/03/2010 12:30 (Ev5)
Upper Threshold Low er Threshold
Reliability - example
Summary
• We can’t escape uncertainty
• Objectivity in decision-making is
needed
• PFF decision-support benefits:
– An audit trail
– No such thing as a ‘false
alarm’!
– Enable better & earlier
forecasts, costs to be
saved
– Operate on all sources
(coastal, pluvial, fluvial)