Towards time and space evolving extreme wind fields

Joana van Nieuwkoop, Sofia Caires and Jacco Groeneweg

IAHR 2015

Acknowledgements:

Netherlands National Water Authority, Rijkswaterstaat

Outline

• Introduction

• Hydraulic boundary conditions

• Lifting method

• Validation method

• Results

• Conclusions and recommendations

30 June 2015

Introduction - motivation

• According to the Dutch Water Act

(“Waterwet, 2009”) the strength of the

Dutch primary water defences must be

checked with a certain periodicity for

the required level of protection of 300

to 100,000 year loads.

• The assessment is carried out using

the: Hydraulic Boundary Conditions

(HBC).

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Introduction - motivation

• The current computations of the HBC

rely mostly on the statistical

distribution of the basic variables at

(solely) the peak of the storms.

• Depending on the failure mechanism

under consideration, the combination

of the values of the basic variables at

the peak of the storm may lead to

lower failure probabilities than the

probabilities based on combinations at

other instants around the storm peak.

• To improve the accuracy of the HBC

time evolving hydraulic loads along the

water defences are needed.

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30 June 2015

Introduction - motivation

• To produce a set of time evolving hydraulic loads along the water

defences the numerical models will in the future use time and space

varying wind fields.

• For this purpose, wind fields need to be lifted. Two leading experts

in extreme value theory, Prof. Laurens de Haan and Prof. Richard L.

Smith were consulted for advice on how to approach the problem,

i.e. on how to model extreme time and space evolving multivariate

extremes.

• Both experts have recommended the use of max-stable method.

Hydr.

model

Wave

model

Introduction – max-stable method

The max-stable method involves:

1. EVA of time series at each

location

2. Augmentation of the local EVA

fits by the empirical distribution

3. Selection of ‘storm’ periods

4. Transformation of the time

series to unit GPD marginal

distributions

5. Uplifting of the transformed

‘storm’ data

6. Inversion of the marginal

transformations: back to wind

speed!

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Wind speed

[m/s]

time

threshold level

Lifting methods - choices

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• Reference location (S0)

• Peak wind velocity level (L0) at ref.

loc. (S0) depending on the desired

return period

• Number of storms (n)

• Storm period (period before and after

the storm)

example

Lake

IJssel

(NL)

U10 [m

/s]

example

North Sea

U10 [m

/s]

Wind speed

[m/s]

time

L0

Lifting methods - result

In space and time varying wind fields

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Question: How real are these fields???? Validation of lifted fields necessary

Example time series at reference location:

original lifted

Introduction – objective

Validate the ability of the lifted fields to reproduce extreme hydraulic

conditions for a case study area

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• Area: Lake IJssel (NL)

• Wind data 1979 – 2013 from

HARMONIE model (KNMI)

• Using the hydrodynamic model WAQUA

and the spectral wave model SWAN

• Four different return periods have been

studied: 1/100, 1/1000, 1/4000 and

1/10000 year

Validation Method

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35-year Harmonie

wind hindcast

35-year hindcast

(surges, waves,

loads …)

lifted wind fields

of 30 storms with

1/100 year return period

3 days before and after

storm peak

extreme loads for 1/100

return period

Hydrodynamic

Models

mean values of

30 storms

Hydrodynamic

models

Full period

4 lifting methods

30 storm periods

1 reference location

Fixed threshold (99.5%)

Type 1 tail

Etreme value

Analysis

for every location and

parameter

Best local fit

example return period 1/100 year

Considered variables

• Wave load (∝ 𝐻𝑠0.5𝑇)

• Wave power (∝ 𝐻𝑠2𝑇)

• Storminess (mean of the significant wave height from 23h before

until 6h after the peak)

• Significant wave height

• Mean wave period

• Still water level

• Wind speed

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Considered output locations

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Results

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Full line: point estimates

Dashed line: 95% conf. interval

hindcast lifted

Results

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Wave load

comparison

Markers show mean

results at the locations

along the eastern

banks of Lake IJssel

Conclusions

• In terms of hydraulic loads, the return value estimates from the four

lifting methods are rather close to each other. Moreover, the

differences between the estimates of the different lifting methods

are in general lower than the differences between them and the

estimates from the hindcast data;

• Possible causes for the differences between the estimates from

the lifting methods and from the hindcast are fundamental

differences between what the estimates based in one and the

other type of data represent.

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Recommendations

• Further study is necessary to test the pragmatic choices that have

been made in lifting the wind fields;

• Study the effect of the number of storms and the reference

location.

30 June 2015