undertaking national flood risk assessment in england...

November 18th, 2015

Undertaking national flood risk assessment in England, United Kingdom Jonathan Simm

Presentation to AWRA conference, Denver CO

© HR Wallingford 2014

Overview

UK background

Overview of method and description of model components (including comparison with FEMA RiskMAP approach)

Model outputs, uses and case study applications

November 18th, 2015 England national flood risk analysis


UK background


© HR Wallingford 2014 November 18th, 2015 England national flood risk analysis

Where does England fit in conceptually

USA Great Britain

England only

3,806,000 miles2

89,000 miles2

50,000 miles2

100% 2.3% 1.3%

USA Great Britain

England only

95,500 miles

19,500 miles.

5,600 miles

100% 20% 6%

Coastline length

Land area Colorado land area ≈ 104,000 miles2

Population Colorado ≈ 5.5 million Population England ≈ 53.0 million i.e. 10 x population in 50% land area!


England: no water supply problems (yet)

Present day 2030 projection


But it does have a problem with flooding

% of development in flood plain % of land in flood plain


November 18th, 2015

UK has a long history of flooding, especially over the last 20 years, with major flood events in 1998 – 2000 – 2002 – 2005 – 2007 – 2012 – 2013/14

England national flood risk analysis

Background


NOAA 20th Century meteorological Reanalysis http://www.esrl.noaa.gov/psd/data/20thC_Rean/

Example storm track – 3rd January 2014



November 18th, 2015

UK has a long history of flooding, especially over the last 20 years, with major flood events in 1998 – 2000 – 2002 – 2005 – 2007 – 2012 – 2013/14


Background

In 2002, UK government moved from flood defence towards flood risk management: To manage flood risk there is a need to quantify it.


NaFRA 2004

NaFRA 2005

NaFRA 2006

NaFRA 2007

NaFRA 2008

NaFRA 2011

SoN


National Flood Risk Analysis (NaFRA)

Method/Software development

National model run


Flood risk analysis underpins Environment Agency’s public flood maps

November 18th, 2015

Requirement under EU Floods Directive to produce flood risk maps and make them available to public

For this purpose, information is expressed in high, medium and low flood risk bands


© HR Wallingford 2014 November 18th, 2015

Analysis supports UK planning and policy

Environment Agency’s Long Term Investment Strategy informing 2 national comprehensive spending reviews

Government contingency planning and advice to Association of British Insurers

Futures work: Foresight Future Flooding analysis

2004. First UK Climate Change Risk

Assessment Ongoing work for Adaptation Committee

on Climate Change

Local authority planning depts England national flood risk analysis


Overview

Overview of method and description of

model components (including comparison with FEMA RiskMAP

approach)



UK risk analysis approach (addresses both probability components)

𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟

= 𝑃𝑃𝑟𝑟𝐹𝐹𝑃𝑃𝑃𝑃𝑃𝑃𝑟𝑟𝐹𝐹𝑟𝑟𝑃𝑃𝑃𝑃 𝐿𝐿𝐹𝐹𝑃𝑃𝐹𝐹 × 𝑃𝑃𝑟𝑟𝐹𝐹𝑃𝑃𝑃𝑃𝑃𝑃𝑟𝑟𝐹𝐹𝑟𝑟𝑃𝑃𝑃𝑃 (𝐵𝐵𝑟𝑟𝐵𝐵𝑃𝑃𝐵𝐵𝐵𝑃𝑃𝑎𝑎𝐹𝐹𝐹𝐹𝑟𝑟

𝐹𝐹𝑜𝑜𝐵𝐵𝑟𝑟𝑃𝑃𝐹𝐹𝑜𝑜𝑜𝑜𝑟𝑟𝑎𝑎𝑜𝑜) × 𝐶𝐶𝐹𝐹𝑎𝑎𝑟𝑟𝐵𝐵𝐶𝐶𝐶𝐶𝐵𝐵𝑎𝑎𝐵𝐵𝐵𝐵 $


NFIP omits


Sources: Extreme value analysis

November 18th, 2015

Return period (years)

Wat

er le

vel (

m)

Data requirements: River flows / levels; coastal water levels and wave conditions Note: All data and models are ‘owned’ by the Environment Agency (EA) and publicly available. However, EA analysis outcomes are rarely appealed

Insurance private impacts land-use.


NFIP focus: return periods of 100 yr (plus 10, 50 and 500yr) UK approach: 40 return periods between 1 and 1000yrs


Sources: 2015 upgraded estimate of coastal forcing


1) Multivariate extreme value analysis

2) SWAN wave model emulator

3) Surfzone model

4) BAYONET overtopping model


0.00

0.10

0.20

0.30

0.40

0.50

0.60

0.70

0.80

0.90

1.00

-2 -1.5 -1 -0.5 0 0.5 1 1.5 2 2.5

X(m) Freeboard

Pro

babi

lity

Water level load

Prob

abilit

y of

failu

re

Breach simulation

Pathways: Assessment of levee failure probability

Not required under FEMA Structural-based Inundation Procedure (SIP)

UK approach uses fragility curves to assess



Numerical Integration

Structure-specificFault tree

Limit state equation

Failure mode 1


Failure mode 2


Failure mode 3


Failure mode ..n

Structure-specific parameters, probability

distribution functions and ranges

Structure-specific fragility curve

Developing fragility curves: the RELIABLE tool

November 18th, 2015

Tool available at www.floodsite.net


http://www.floodsite.net/


Site specific fragility curve



Reliability of levees – generic fragility curves

Over 60 generic levee types

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

Overflow head (Water level - crest level)

P (b

reac

hing

¦feeb

oard

)

Condition grade1Condition grade2Condition grade 3Condition grade4Condition grade 5


Generic curves provide an economic approach to implementing BW12/NAS


Breach simulation

November 18th, 2015

Breach simulation defines breach dimensions: levee doesn’t ‘vanish’ if failure occurs (Applicable to LAMP)



Breach tools developed by HR Wallingford (now working with DSIG and USACE ERDC)

Erodible top

layer Resistant

top layer



All levees but with variable influence


LAMP is applied only to non-accredited levees, whereas UK process is applied to all levees using a whole system approach

NFIP process involves a binary (in/out) influence of levees.UK process is ‘graduated’


Pathways: Flood inundation analysis

November 18th, 2015

Analysis includes: (1) 40 return periods between 1

and 1000 years return period. (2) For defended flood plains for

each return period, a Monte Carlo analysis is required with multiple realisations of system state (levee segments failed or not failed)

(3) Analysis also covers each return period for undefended floodplains


Current FEMA process: independent reach-specific inundation

UK process: integrated across all reaches


Data requirements: Digital terrain model

Range of hydrodynamic models: Volume based Rapid Flood Spreading

Model (RFSM) Diffusion wave (Dynamic RFSM) Hybrid models (local acceleration with

sub-grid topography, RFSM EDA) Full Shallow Water Equations e.g.

InfoWorks RS & CS, TELEMAC, Tuflow, etc

For repeated analyses in Monte Carlo simulations, models must run very fast November 18th, 2015 England national flood risk analysis

Pathways: Flood inundation


Depth Damage Curve

-1.00-0.75-0.50-0.250.000.250.500.751.001.251.501.752.002.252.502.753.00

0 250 500 750 1000 1250 1500

Damage £/m2

Dept

h Metr

esHighSusceptibilityBand

LowSusceptibilityBand

IndicativeSusceptibility

Depth/Damage ($) relationships

Receptors: Consequences

Once water depths are calculated the approach is similar to HAZUS Data requirements: Property types and locations;



Depth Damage Curve

-1.00-0.75-0.50-0.250.000.250.500.751.001.251.501.752.002.252.502.753.00

0 250 500 750 1000 1250 1500

Damage £/m2

Dept

h Metr

esHighSusceptibilityBand

LowSusceptibilityBand

IndicativeSusceptibility

Depth/Damage ($) relationships

Receptors: Consequences

Data requirements: Property types and locations;


Both UK and FEMA (HAZUS) approaches are similar. (More individual calculations in UK case)


Model outputs, and case study applications



Likelihood of flooding

November 18th, 2015 Page 29 England national flood risk analysis


EAD (£K)

Risk



Risk attribution to levee segments supports risk mitigation by structural interventions



Levees only one solution: All changes and interventions can be expressed in probability-consequence space

(Adapted from Sayers et al, 2003)

Managing flood risk



(Evans, et al, 2008)

Managing changing flood risk system



Likelihood of flooding

November 18th, 2015 Page 34 England national flood risk analysis


2100 – High Emission Scenario

Likelihood of flooding: sea level rise to 2100

17th July 2014 CliMathNet Extremes Meeting


Summary and conclusions

UK and US face similar flooding problems and analysis methods use the same

underlying principles

The ability to communicate risk and potential solutions to decision-makers is critical

Full system flood risk analysis enables spatial variation in risk to be evaluated,

including residual risk attributable to levees

A wide range of flood risk management actions/decisions can be supported by the

UK style of risk modelling presented


November 18th, 2015

Undertaking national flood risk assessment in England, United Kingdom Jonathan Simm

Presentation to AWRA conference, Denver CO

undertaking national flood risk assessment in england...

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