Modeling extreme beach retreat and erosion volumes. A tool for susceptibility analysis Modeling extreme beach retreat and erosion volumes. A tool for susceptibility analysis Jorge Trindade * & Ana Ramos-Pereira * [email protected] [email protected] * 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 1.Introduction Introduction 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 susceptibilityanalysis. 2.Study Studyarea area 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. Hydrodynamic Hydrodynamicreference referencevalues values 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 5 6 7 8 9 10 11 12 13 14 15 16 17 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 = 0,93 0 500 1000 1500 2000 2500 3000 3500 0 1000 2000 3000 m 3 /m m 3 /m Previsto Medido Predicted 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 -1 0 1 2 3 4 5 6 7 8 m P2 ↑ pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m a b c P1 ↑ pnmr nmm -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P3 ↑pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P4 ↑pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P5 ↑pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 40 60 80 100 120 140 m m P6 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P2 ↑ pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P3 ↑ pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P4 ↑pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P1 ↑pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P5 ↑pnmr -4 -3 -2 -1 0 1 2 3 4 5 6 7 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 -1 0 1 2 3 4 5 6 7 8 m P4 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 0 50 100 150 200 m m P5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P2 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m P3 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 m a b c P1 nmm Model Modelcalibration calibration Successive adjustment of the K (transport rate term) and Є (slope dependent term) for P1 in each beach system and on the first storm event to best fit measured results. In the worst case scenario themodelexplainsover89%ofthemeasuredprofile. Model Modelvalidation validation K and Є calibrated values where then applied to the rest of the 29 profiles in the three beach systems to access model performance and results validation. In most cases the model explains more than 90% (20 in 32 cases) of the measured changes due to storm events. The maximum relativedifferencevalue is -39,7%. SBEACH SBEACH Modeling Modeling Larson & Kraus, 1989; Wiseet al., 1996 Model Model calibration calibration K e e Є empirical empirical ajustment ajustment (P1) P1) Validation Validationof ofthe the volumetric volumetricbudget budget (P2 (P2, P3, P4, P5 e P6) , P3, P4, P5 e P6) Morphodynamic Morphodynamic reference referencevalues values Hydrodynamic Hydrodynamicreference referencevalues values coastline retreat. Winter offshore mean significant wave values reach 2.5m and waveswitha5yearrecurrenceperiodcanbehigherthan9m. 3. Methodology Methodology: (i)Morphodynamicdata – 3 yr of beach profilling(2004 – 2007); (ii)Sediment sampling in the most active sector of the beach profileandlab treatmentefor granulometricanalysis; (iii)Offshorewavedataprocessing; (iv)Stormsurgevalues determination; (v)Extreme volumetric predicted values (H s R100 ) based on SBEACHmodelling. 4.Parametrization Parametrization (i)H s ; (ii)Springtidemeanlevel; (iii)Stormsurge; (iv)Geometricpropertiesofthemeasuredbeachprofiles; (v)Sedimentpropertiesofthebeachprofiles. Morphodynamic Morphodynamicreference referencevalues values Days Profile Vol. budget (m 3 /m) SR 15.02.2006 01.03.2006 P1 -26,5 P2 -16,8 P3 -20,3 P4 -15,9 P5 +14,2 14.06.2006 28.11.2006 P1 -27,9 P2 -16,3 P3 -20,5 P4 -0,9 P5 -5,0 Days Profile Vol. budget (m 3 /m) AZ 31.01.2006 14.02.2006 P1 -19,0 P2 -22,1 P3 -25,2 P4 -18,7 P5 -16,9 P6 -47,7 28.02.2006 14.03.2006 P1 -6,7 P2 -22,6 P3 -11,7 P4 -7,2 P5 -3,8 P6 -23,6 Days Profile Vol. budget (m 3 /m) LZ 13.02.2006 02.03.2006 P1 -10,6 P2 -20,5 P3 -13,6 P4 -52,5 P5 -52,2 02.03.2006 15.03.2006 P1 -73,6 P2 -70,7 P3 -84,3 P4 -47,6 P5 -34,8 SR (P1) AZ (P1) LZ (P1) Measured pre-stormvolume (m 3 /m) 173,9 (15.02.2006) 90,6 (31.01.2006) 340,1 (13.01.2006) Measured post-stormvolume (a) (m 3 /m) 147,4 (01.03.2006) 71,6 (14.02.2006) 329,5 (02.03.2006) K (m 4 /N) 2,50*10 -7 2,50*10 -7 2,50*10 -7 Є (m 2 /s) 0,005 0,005 0,005 Predicted post-stormvolume (b) (m 3 /m) 131,4 69,9 301,6 Relative volume difference (a and b) (%) -10,9 -2,3 -8,5 Empirical Empiricalcalibration calibration 0 100 200 50 Meters 0 150 300 75 Meters 0 100 200 50 Meters S ta . 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