air flood model for great britain - university of oxford · • a physically based flood routing...

1 CONFIDENTIAL ©2014 AIR WORLDWIDE

AIR Flood Model for Great

Britain


- Model released in 2008 and peer-

reviewed

- Licensed by various clients

across UK and Europe

- AIR was selected as the primary

modeller for ABI in preparation for

FloodRe

AIR’s Inland Flood Model for Great Britain


Components of the AIR Inland Flood Model for

Great Britain

FINANCIAL

HAZARD

ENGINEERING

Intensity

Calculation

Exposure

Information

Damage

Estimation

Policy

Conditions

Insured Loss

Calculations

Event

Generation

Localized

Physically-

Based

Hydraulic

Model

Large

Scale

Stochastic

Hydrological

Model

Off-Floodplain

Hazard

Estimation

FINANCIAL


Hazard



Great Britain

FINANCIAL

HAZARD

ENGINEERING

Intensity

Calculation

Exposure

Information

Damage

Estimation

Policy

Conditions

Insured Loss

Calculations

Event

Generation

Localized

Physically-

Based

Hydraulic

Model

Large

Scale

Stochastic

Hydrological

Model

Off-Floodplain

Hazard

Estimation

FINANCIAL


Physically based

flood routing

module

A Large-Scale Hydrological Model Computes Runoff

and Discharge at Each Location and Time Step

Statistical-

physical

runoff

generation

module

… OR input from

weather radar

Input from simulated

rainfall event…


AIR’S Inland Flood Model Accounts for More Than

300,000 km of River Network

Over 11,000

stream links

constitute

the river

network

Over 15,000

small

catchments

are modelled

explicitly as a

part of the

runoff module


A Two-Step Approach is Used for Rainfall

Simulation

• Perturbation of annual precipitation blocks of NCAR reanalysis data creates

random rainfall fields over a large domain

Simulation

NCAR data

• Recursive downscaling is applied to disaggregate each large-scale storm and

the 6 pre-storm days to higher resolution


AIR’s Runoff Generation Module Accounts for the

Local Physiography and Antecedent Conditions

• Runoff is generated depending on

the local physiography

(15,000 unit catchments )

• The antecedent soil moisture is

conditioned on the pre-storm rainfall

accumulations

Topography

and local

climate

Geology

and soil

type

Land cover

and

urbanization

Runoff

Time


• A physically based flood routing

model uses a Muskingum-Cunge

flood routing scheme (advection-

dispersion equation)

• A double power law relationship

V = F(Q) is used to account for the

retardation effect of overbank flows and

small regulations on flood peaks

A Muskingum-Cunge Algorithm Is Used for Flood

Routing


Validation of Event Duration, Seasonality and

Annual Occurrence

0

0.05

0.1

0.15

0.2

0.25

1 2 3 4 5 6 7 8 9 1011121314

Fre

qu

en

cy

Duration, [days]

Storm duration

NCAR

Simulation

0

0.03

0.06

0.09

0.12

0.15

0.18

1 2 3 4 5 6 7 8 9 10 11 12

Fre

qu

en

cy

Month

Seasonality

NCAR

Simulation

0

0.05

0.1

0.15

0.2

0.25

1 3 5 7 9 11 13 15 17 19 21 23

Fre

qu

en

cy

Number of occurences per year

Storm occurences per year

NCAR

Simulation



Great Britain

FINANCIAL

HAZARD

ENGINEERING

Intensity

Calculation

Exposure

Information

Damage

Estimation

Policy

Conditions

Insured Loss

Calculations

Event

Generation

Localised

Physically-

Based

Hydraulic

Model

Large

Scale

Stochastic

Hydrological

Model

Off-Floodplain

Hazard

Estimation

FINANCIAL


Hydraulic Modelling is Applied to Estimate the Water

Level Corresponding to Each Peak Flow (SD Curve)

• AIR employs the widely used and well tested HEC-RAS model

• Flood water elevation mapping is done by using the GIS

extension of HEC-RAS: GEO-RAS

• Triangular Irregular Network (TIN) is used to represent the water

surface for many characteristic discharges

0 50 100 150 200 250 30052.4

52.6

52.8

53.0

53.2

53.4

53.6

53.8

54.0

Q Total (m3/s)

W.S

. E

lev (m

)

0 50 100 150 200 250 300114

116

118

120

122

124

126

128

Station (m)

Ele

va

tio

n (

m)

Legend

WS PF 20

WS PF 15

WS PF 10

WS PF 5

WS PF 1

Ground

Bank Sta

Stage-Discharge (SD)

Curve


Validation of the Hydraulic Model: Environment

Agency (EA) Flood Maps and Other Sources

EA 1000 Year Flood Map Modelled 1000 Year Flood Map

Lloyds of London


AIR’s Flood Model Accounts for Multiple Types of

Flood Defences

• More than 4000 reservoirs

and lakes are modelled

explicitly as a part of the

flood routing module

• Levees, embankments,

flooding walls and other

storage areas are modelled

physically as part of the

hydraulic model

• Thames Barrier is modelled

according to operational rules


Hydraulic Model Also Considers Potential Failure of

Flood Defences

• For each link, “standards of

protection” is derived from

detail land use and population

data

• It is being verified against

existing references

• Parameters of fragility curves

are selected from literature


Vulnerability


• Separate damage functions for detached, semi-detached, terraced

buildings and bungalows in Great Britain

• Clean-up costs including building drying costs are significant

contributors to the building damageability

• After about 0.5-1.0 metre flood depth, damageability increases less

rapidly with flood depth

Buildings and Contents Flood Vulnerability For

Residential Buildings

Figure: Residential Building and Contents Damage Functions

0.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%

0 0.5 1 1.5 2 2.5 3

Dam

age

Rat

io

Flood Depth

DetachedSemi-detachedTerraceBungalow

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

70.0%

0 0.5 1 1.5 2 2.5 3

Dam

age

Rat

io

Flood Depth

Detached

Semi-detached

Terrace

Bungalow


• Building is divided into key flood vulnerable components (structure,

fixtures and fittings, services)

• Each vulnerability component is aggregated using component cost

breakdown to determine the overall building coverage vulnerability

• Damage functions vary by occupancy, construction, and height

Component-Based Approach to Estimate Damageability

for the Commercial Line of Business

Building Structure

47%

Services29%

Fixtures

and Fittings

24%

Building Coverage Cost Breakdown for Commercial LOB

Figure: Components and overall building flood damage functions for low-rise

commercial buildings (Based on FHRC Data)


• Based on the model proposed in research by Hall et al, drying rate*

− Can increase twice by 10ºC increase in temperature

− Varies linearly with the humidity

− Increases as a square root of air-flow velocity

Vulnerability Module Accounts for Impact of

Seasonality on Losses

Moving averaged monthly conditions in

Great Britain

Building and time element loss multipliers by

month

* Hall et al. (1984). Water Movement in Porous Building Materials-VI . Evaporation and Drying in Brick and Block Materials, Building and

Environment, Vol 19, No. 1,


Off-Floodplain Losses Have Historically Accounted

for a Significant Proportion of Loss

• Many claims occur away from

rivers and streams


• Multiple variables stochastic

regression model that relates: − surface runoff and

− relative elevation of the building

location from the nearest river, is

used to estimate off-flood plain

damage estimation

• Population density is used as a

proxy for the regional drainage

conditions to modify the above-

estimated mean damage surface

• Similar to on-plain damage, off-

plain damageability varies by

occupancy, construction, height

and coverage

Off-Plain Flood Vulnerability Is Based on Surface

Runoff and Relative Elevation

Surface Runoff

Me

an D

amag

e R

atio

Relative Elevation

90%-100%80%-90%70%-80%60%-70%50%-60%40%-50%30%-40%20%-30%10%-20%0%-10%

Figure: MDR surface for off-flood

plain loss estimation for residential

LOB for a region

air flood model for great britain - university of oxford · • a physically based flood routing...

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