Modeling extreme beach retreat and erosion volumes. A tool for susceptibility analysisModeling extreme beach retreat and erosion volumes. A tool for susceptibility analysisJorge Trindade* & Ana Ramos-Pereira *

jorgetrd@univ-ab.pt anarp@campus.ul.pt

* Centre of Geographical Studies (IGOT/UL)Edifício da Faculdade de Letras, Universidade de Lisboa

Alameda da Universidade, 1600-214 Lisboa, Portugal

European Geosciences Union

General Assembly 2012

Vienna | Austria | 22 – 27 April 2012

slif.info.ul.pt

11.. IntroductionIntroduction

Beaches are among the most dynamic systems in the coastal

zone. This is due to the great variability in the main triggering

factors that contribute to morphological change. Dramatic coast

line retreat can occur in a short period of time due to episodic

extreme wave events endangering people and property and

therefore defining the local susceptibility to erosion. This

research aims to determinate beach recession and volume

erosion due to sediment loss during extreme wave events in non

artificialized beaches of the Portuguese west coast, for

susceptibility analysis.

22.. StudyStudy areaarea

The central west coast of Portugal is a wave dominated high

energetic coastal environment. Storm frequency and magnitude

are very important features on the definition of the annual local

sediment budget and on the anthropogenic elements exposure

to the direct action of waves through momentary or permanent

coastline retreat.

HydrodynamicHydrodynamic referencereference valuesvalues

5

6

7

8

9

10

11

12

13

14

s

Tz

Início → 16.02.2006 (Sn)

Direcção modal = NW

0

1

2

3

4

5

6

7

0 20 40 60 80 100

m

horas

Hs (0m - zh)

Mr (0m - nmm)6,28

5

6

7

8

9

10

11

s

Início → 23.10.2006 (Le)

Direcção modal = W

0

1

2

3

4

5

6

7

8

0 20 40

m

horas

7,34

56789

1011121314151617

s

Início → 13.02.2006 (Sn)

Direcção modal = WNW

0

1

2

3

4

5

6

0 10 20 30

m

horas

5,34

5

6

7

8

9

10

11

s

Início → 03.03.2006 (Sn)

Direcção modal = WNW

0

1

2

3

4

5

0 20 40 60 80

m

horas

4,29

5

6

7

8

9

10

11

12

13

14

s

Início → 16.02.2006 (Sn)

Direcção modal = NW

0

1

2

3

4

5

6

7

0 20 40 60 80 100

m

horas

6,28

5

6

7

8

9

10

11

s

Início → 03.03.2006 (Sn)

Direcção modal = WNW

0

1

2

3

4

5

0 20 40 60 80

m

horas

4,29

0m (msl) 0m recession [Dissipative profile] 3m recession [Dissipative profile]

R² = 0,930

500

1000

1500

2000

2500

3000

3500

0 1000 2000 3000

m3/

m

m3/m

Pr

ev

isto

Medido

Pr

ed

icte

d

Measured

-3

-2

-1

0

1

2

3

4

5

6

7

8

90 110 130 150 170 190 210

m

m

P5

pnmr↑

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P4

pnmr↑

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P3

↑ pnmr-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P2

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

a

b

c

P1

↑ pnmr

nmm

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P1

↑ pnmr

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P2

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P3

pnmr↑

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P4

pnmr↑

-3

-2

-1

0

1

2

3

4

5

6

7

8

90 110 130 150 170 190 210

m

m

P5

↑ pnmr

-4

-3-2

-10

12

34

56

78

m

P2

↑ pnmr

-4

-3-2

-10

12

34

56

78

m

a

b

c

P1

↑ pnmr

nmm

-4

-3-2

-10

12

34

56

78

m

P3

↑ pnmr

-4-3

-2-1

01

23

45

67

8

m

P4

↑ pnmr

-4-3

-2-1

01

23

45

67

8

m

P5

↑ pnmr

-4-3

-2-1

01

23

45

67

8

40 60 80 100 120 140

m

m

P6

-4

-3-2

-101

23

45

67

8

m

P2

↑ pnmr

-4

-3-2

-10

12

34

56

78

m

P3

↑ pnmr

-4-3

-2-1

01

23

45

67

8

m

P4

↑ pnmr

-4

-3-2

-10

12

34

56

78

m

P1

↑ pnmr

-4

-3-2

-10

12

34

56

78

m

P5

↑ pnmr

-4-3

-2-1

01

23

45

67

8

40 60 80 100 120 140

m

m

P6

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P2

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P4

-3

-2

-1

0

1

2

3

4

5

6

7

8

0 50 100 150

m

m

P5

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P1

-3

-2

-1

0

1

2

3

4

5

6

7

8

m

P3

-4-3

-2-10

123

45

678

m

P4

-4-3

-2-10

12

345

67

8

0 50 100 150 200

m

m

P5

-4

-3-2

-101

23

456

78

m

P2

-4-3-2

-10

123

45

678

m

P3-4

-3-2-1

01

234

56

78

m

a

b

c

P1

nmm

ModelModel calibrationcalibrationSuccessive adjustment of the K (transport rate term) and Є (slope dependent term) for P1 in eachbeach system and on the first storm event to best fit measured results. In the worst case scenariothe model explains over 89% of the measured profile.

ModelModel validationvalidationK and Є calibrated values where then applied to the rest of the 29 profiles in the three beachsystems to access model performance and results validation. In most cases the model explainsmore than 90% (20 in 32 cases) of the measured changes due to storm events. The maximumrelative difference value is -39,7%.

SBEACHSBEACH

ModelingModelingLarson & Kraus, 1989;

Wise et al., 1996

ModelModelcalibrationcalibration

KK e e ЄЄ empiricalempirical ajustmentajustment

((P1)P1)

ValidationValidation ofof thethevolumetricvolumetric budgetbudget(P2(P2, P3, P4, P5 e P6), P3, P4, P5 e P6)

Morphodynamic Morphodynamic referencereference valuesvalues

HydrodynamicHydrodynamic referencereference valuesvalues

coastline retreat.

Winter offshore mean significant wave values reach 2.5m and

waves with a 5 year recurrence period can be higher than 9m.

33.. MethodologyMethodology::

(i)Morphodynamic data – 3 yr of beach profilling (2004 – 2007);

(ii)Sediment sampling in the most active sector of the beach

profile and lab treatmente for granulometric analysis;

(iii)Offshore wave data processing;

(iv)Storm surge values determination;

(v)Extreme volumetric predicted values (HsR100) based on

SBEACH modelling.

44.. ParametrizationParametrization(i) Hs ;(ii) Spring tide mean level;(iii) Storm surge;(iv) Geometric properties of the measured beach profiles;(v) Sediment properties of the beach profiles.

MorphodynamicMorphodynamic referencereference valuesvalues

Days ProfileVol . budget

(m3/m)

SR

15.02.2006����

01.03.2006

P1 -26,5P2 -16,8P3 -20,3P4 -15,9P5 +14,2

14.06.2006����

28.11.2006

P1 -27,9P2 -16,3P3 -20,5P4 -0,9P5 -5,0

Days ProfileVol . budget

(m3/m)

AZ

31.01.2006����

14.02.2006

P1 -19,0P2 -22,1P3 -25,2P4 -18,7P5 -16,9P6 -47,7

28.02.2006����

14.03.2006

P1 -6,7P2 -22,6P3 -11,7P4 -7,2P5 -3,8P6 -23,6

Days ProfileVol . budget

(m3/m)

LZ

13.02.2006����

02.03.2006

P1 -10,6P2 -20,5P3 -13,6P4 -52,5P5 -52,2

02.03.2006����

15.03.2006

P1 -73,6P2 -70,7P3 -84,3P4 -47,6P5 -34,8

SR (P1) AZ (P1) LZ (P1)Measured pre-storm volume (m3/m) 173,9 (15.02.2006) 90,6 (31.01.2006) 340,1 (13.01.2006)

Measured post-storm volume (a) (m3/m) 147,4 (01.03.2006) 71,6 (14.02.2006) 329,5 (02.03.2006)

K (m4/N) 2,50*10-7 2,50*10-7 2,50*10-7

Є (m2/s) 0,005 0,005 0,005

Predicted post-storm volume (b) (m3/m) 131,4 69,9 301,6

Relative volume difference (a and b) (%) -10,9 -2,3 -8,5

EmpiricalEmpirical calibrationcalibration

0 100 20050 Meters 0 150 30075 Meters 0 100 20050 Meters

Sta. Rita beach Azul beach

Foz do Lizandrobeach

0m (msl)

System boundary

0m recession [Dissipative profile]

0m recession [Reflective profile]

3m recession [Dissipative profile]

3m recession [Reflective profile]

References:BLOTT, S. & PYE, K. (2006) – Particle size distribution analysis of sand-sized particles by laser diffraction: an experimental investigation of instrument sensitivity and the effects of particle shape. Sedimentology, 53, p.671 – 685;COSTA, M. (1994) – Agitação marítima na costa portuguesa. Anais do Instituto Hidrográfico, 13, p.35 – 40;GAMA, C.; DIAS, J.; FERREIRA, Ó. & TABORDA, R. (1994) – Analysis of storm surge in Portugal, between June 1986 and May 1988. Proccedings of Littoral 94, Lisboa, p.26 – 29p.;LARSON, M. & KRAUS, N. (1998) – Numerical model for simulating storm induced beach change. Report 5: Representation of nonerodible(hard) Bottoms. Technical Report CERC-89-9, U.S. Army Corps of Engineers, Vicksburg, U.S.A., 47p.;LARSON, M.; KRAUS, N. & BYRNES, M. (1990) – Numerical model for simulating storm induced beach change. Report 2: Numerical formulation and model tests. Technical Report CERC-89-9, U.S. Army Corps of Engineers, Vicksburg, U.S.A., 120p.;OLIVEIRA PIRES, H.; PESSANHA, L. E. (1982) – Características direccionais da agitação marítima na região de Sines. Instituto Nacional de Meteorologia e Geofísica, 12p + fig.;

ROSATI, J.; WISE, R.; KRAUS, N. & LARSON, M. (1993) – Numerical model for simulating storm induced beach change. Report 3: User’s Manual. Technical Report CERC-93-2, U.S. Army Corps of Engineers, Vicksburg, U.S.A., 64p.;STOCKDON, H.; HOLMAN, R.; HOWD, P. & SALLENGER JR., A. (2006) – Empirical parameterization of setup, swash, and runp. Coastal Engineering, 53, p.573 – 588;TABORDA, R. & DIAS, J. A. (1992) – Análise da sobreelevação do mar de origem meteorológica durante os temporais de Fevereiro/Março de 1978 e Dezembro de 1981. Geonovas, 1, p.89 – 97;Trindade, J.; Pereira, A. R.; Neves, M.; Borges, B. & Paixão, R., 2007. Monitorização da morfodinâmica de praias com dGPS e Estação Total. Comparação de resultados. Publicações da Associação Portuguesa de Geomorfólogos Vol. V, p. 293 - 301WISE, R.; SMITH, S. & LARSON, M. (1996) – SBEACH: Numerical model for simulating storm-induced beach change. Report 4: Cross-shore transport under random waves and model validation with SUPERTANK and field data. Technical ReportCERC-89-9, US Army Corps of Engineers, Vicksburg, 260p.;