coastal geomorphology and extreme water levels

Coastal geomorphology and extreme water levels

COADAPT Project

Aart Kroon, Jørn Bjarke Torp Pedersen

26 October 2010

Department of Geography & Geology

Contents

Introduction

Coastal erosion and flooding

Extreme water levels

Approach

Team at DGG-UC


Introduction

Coastal environments in Denmark

large spatial variability

Geomorphologic map(GEUS, 2009)


Introduction

Two major impacts:

1. coastal erosion might be intensified;

2. coastal flooding might occur more frequent.


Danish Coastal Authority, 2008

Coastal erosion along the West coast of Jutland

well monitoredwell known‘under control’


Coastal erosion along the inner coastlines

less monitoredless well known

First indication by aerial photographs and satellite imagery

On its way!


Westh, 2009


Coastal flooding along the coastlines

A) potential flooding areas based ondigital terrain models (contour maps of the height)

- example: ‘Klimatilpasningportal’, DCA

B) inundation areas and inundation times based ondynamic models (process-based)

- example: DHI


Coastal flooding: spatial patterns

KMS, 2010


Example DTM: Glænø, Southern Sjælland


Also activities

Example: Havvand på land (seawater at land)Cost-benefit analysis in GIS on DTM


Also activities

Example: Kystplanlæggeren (coastal planner)


Coastal flooding

DTM

+ easy to get an indication along all shores+ easy to track potential risk areas

-- no dynamics, no inundation times -- no distance from breaches included

Process model

+ precise idea of inundation process + precise information about inundation pathways

-- relatively time consuming computations-- relatively small spatial coverage


Coastal flooding

Both approaches need to be fed by extreme water level distributions

DTM

* as exceedence curves of water levels, giving an idea of return periods

Process model

* as time-series,giving an idea of the intensity and duration of the event


Summary

Why do we @ DGG need information about extreme water levels?

a) To understand erosion and accretion patterns along the shores

b) To understand the risk of flooding in coastal areas

c) To parameterize extreme events

* spatial patterns around Denmark* relative importance compared to waves, tides, …

(on its way: AKK)

d) To better understand active beach ridge systems


Extreme water levels and beach ridge systems

• Detailed observations of the height of the present day active beach ridge

• Coupling of these heights among local water level characteristics:

- High-tide level

- Extreme water level with a return period of … (SL1/100x)

Beach ridge Glænø


Extreme water levels and beach ridge systems

KMS, 2010

’Littorinahavets tid’ ca. 5-2.000 BC’Littorinahavets tid’ ca. 5-2.000 BC

’Littorinahavets tid’ ca. 5-2.000 BC’Littorinahavets tid’ ca. 5-2.000 BC

Beach ridge locations


Description of extreme water levels

Common practice (DCA)

• Long data series (years) with frequent observations (every 10 min)

• Observed distributions of water heights with special interest in the upper tail

• Statistic description of the upper tail with- Truncated Weibull distribution;- Truncated log-normal distribution.


Description of extreme water levels

Common practice (DCA)

• Exceedence curves of water levels at different locations

DCA, 2007: Extreme sea level statistics for Denmark

Esbjerg


Use of extreme water level descriptions

Two questions arise:

A) Can we use existing exceedance curves and return periods to predict flooding events under a rising sea level scenario?

B) Is the existing analysis method reliable, robust and does it describe the most important features of water levels related to flooding?


Flooding events under a rising sea level scenario

DCA, 2007: Extreme sea level statistics for Denmark

Esbjerg


25 cm sea-level rise:

Return period (yr) for what is now a 100 yr event


Flooding events under a rising sea level scenario

This will result in higher return periods, but (!!)

A) Extreme values have another distribution than the bulk of the water levels. Previous extremes are no longer extreme values and need to be described by other functions.

B) Is it a valid assumption that a rising sea level will not lead to another character of the basin? (is the rest stationary?)

C) …


Existing analysis method reliable, robust

Reliable yes,

but there is a lot of tuning:

Distribution parameters are fitted and the truncation level for the truncated distribution is fitted. Especially the latter is extremely sensible and determines the outcome!

One distribution type for all data series is preferred. Physical variable is the same and spatial comparison of outcomes is not hindered by the method.


Are the most important features included?

Water level statistics is based on very detailed high-quality long-term series

Both types of flooding analysis (DTM and inundation models) will probably need other information as well:

Water levels:

intensity, duration, frequency of occurrence, sequence

(similar to erosion due to storm events or flooding due to intense rainfall events)


Description of water level: peak-over-threshold / duration curve

Statistics of extreme water levels

using GEV (Generalized extreme value) distribution

creating water-level/duration frequency plots

Approach


1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000-5

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5

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Year

Mea

n a

nn

ual

wat

er l

evel

Esb

jerg

Hab

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r D

VR

90,

m

Yearly measured mean water level

10 years running average

Esbjerg 1890-2000


Jan0000 Nov0136 Oct0273 Sep0410 Jul0547 Jun0684 May0821 Apr0958-400

-200

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Time, 1962-2004

Wat

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vel a

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R90

, cm

-400 -300 -200 -100 0 100 200 300 400 5000

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5x 10

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Water level above DVR90, cm

Hou

rs o

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-400 -300 -200 -100 0 100 200 300 400 5000

0.2

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Water level above DVR90, cm

Cum

. fr

eq.

Esbjerg 1962-2004


Jan 1990 Jan 1991 Jan1992 Jan 1993 Jan 1994 Jan 1995-2

-1

0

1

2

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hei

gh

t (m

)

time (year) from 1990-1994

Surge observations=37

Esbjerg 1990-1994

Peak-over-threshold detection


Jan 1990 Jan 1991 Jan1992 Jan 1993 Jan 1994 Jan 1995

0.5

1

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x 104

inte

rval

bet

wee

n s

urg

es (

ho

urs

)

time (year) from 1990-1994

Esbjerg 1990-1994

Sequence analysis


Relation between intensity and duration

Probably not linear

Certainly not the same at all locations(shoreline with tides react different, often restrictive to the

duration of the high-water event)


0 2 4 6 8 10 12 14 16 183

3.2

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max

. su

rge

leve

l (m

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duration of surge (hours)

Surge observations=37

Esbjerg 1990-1994


Summary - work in progress

Mapping accretion-erosion based on aerial photographs

Helping with flood mapping procedures using KMS DTM

Detailed analysis of present day behavior in some specific areas (with master students)

Detailed analysis of beach ridge systems (with master student)

Parameterization of coastal processes (with AKK)

Detailed analysis of extreme value statistics coupled to water levels


Since last meeting

PhDAlina Kabuth started on 15-07-2010

Parameterization of coastal processes

MSc Mette Bendixen

Beach ridges and water levels in DenmarkBertram Hacke

ICZM and Danish Municipalities

PresentationsSUSCOD in SorøGeoforum in Copenhagen

coastal geomorphology and extreme water levels

Documents

inundation times

risk of flooding

coastal areasto

coastal geomorphology

potential flooding areas

inundation pathways

dca b inundation areas

extreme events