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IMPACT OF 2013-14 STORMS ON COASTAL GEOMORPHOLOGY IN SW ENGLAND Gerd Masselink, Plymouth University © Richard Broom Geographical Association 2015 OUTLINE • Hazard, risk & vulnerability • Winter storms of 2013/14 • Photographic overview of impacts • How to measure coastal impacts? • Regional overview of coastal impacts • Where has the sand gone? • Offshore / under water • Alongshore / around the corner • Onshore / over the top • Conclusions HAZARD, RISK & VULNERABILITY Hazards • Storms • Tide • Surge • Tsunami x Exposure • Houses • Industry • Business • Infrastructure = Coastal vulnerability Coastal risk • Erosion • Damage • Flooding • Financial loss WINTER STORMS OF 2013/14 Pressure chart for ‘Hercules’ storm 5th January 2014 Iceland US UK Violent storm winds > 60 mph, extended fetch and duration = BIG waves Largest waves are south of the depression and travel with the storm Waves generated by the Hercules storm H =8 m H =10 m H = 12 m H =14 m ‘Black Hole’ H > 15 m 2m 10 m What does a 10-m wave look like Measured and modelled wave heights 12 10 8 6 4 2 0 Dec13 Jan14 Feb14 Mar14 14 m Hercules 06/01/14 Apr14 12 m Petra 05/02/14 Comparison 2013/2014 winter with 60-year wave record 8-week running mean wave height 4 3 2 1 0 1950 1955 1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010 2015 PHOTOGRAPHIC OVERVIEW OF IMPACTS Southwest coast of England North coast W SW South coast Beach lowering, Seaton, South Cornwall 5,000 peat exposed, Hallsands, South Devon Removal of beach in Torcross, South Devon End of summer Before During DuringPetra Removal of beach in Torcross, South Devon End of summer Before After Petra During HOW TO MEASURE COASTAL IMPACTS? Beach surveys Tape measure and clinometer – does work! Real-time kinematic GPS – much better! Comparison of beach profiles before and after storm before storms after storms Accretion 160 m3 per m width before storms after storms Erosion 40 m3 per m width Airborne LiDAR surveys Difference between before and after storm is change due to storm Erosion (> 2m ) Accretion (> 2 m) COASTAL IMPACTS – REGIONAL OVERVIEW SW England and beach monitoring sites North coast W SW South coast Beach monitoring (> 30 beaches, >200 profiles) Storm impacts along coast of SW England – per profile NORTH COAST SOUTH COAST Erosion Accretion Change in sediment volume profile line (m3/m) BRISTOL CHL Beaches geographically arranged from Somerset to Dorset SW England and beach monitoring sites Bristol Channel North coast W St Ives Chesil SW SW England and beach monitoring sites Bristol Channel North coast W St Ives SW SW England and beach monitoring sites Bristol Channel North coast W St Ives Chesil SW South coast SW England and beach monitoring sites W Chesil SW South coast SW England and beach monitoring sites W Chesil SW WHERE HAS THE SAND GONE? OFFSHORE (UNDER WATER)! NORTH COAST MODEL Impact of Hercules on Perranporth North coast W South coast Perranporth beach in summer Perranporth response from LiDAR (Hercules) Erosion (> 2m ) Accretion (> 2 m) Where has the sand gone? Offshore accretion Mega-rip channel Intertidal erosion Erosion (> 2m ) Accretion (> 2 m) High tide Low tide Offshore sand model – North coast Before the winter After the winter dune Dune scarp Thin beach Rocks exposed Sand bar below low tide Storm response Bude and Westward Ho! from LiDAR Wave approach Erosion (> 2m ) Accretion (> 2 m) WHERE HAS THE SAND GONE? ALONGSHORE (AROUND THE CORNER)! SOUTH COAST MODEL Impact of Petra on Slapton Sands North coast SW South coast Gravel beaches of Start Bay, South Devon Slapton Sands Beesands North Hallsands South Hallsands Blackpool Sands Slapton response from RTK-GPS (Petra) P19 P11 P0 before storms after storms Alongshore variation in storm response Torcross Slapton Sands Sediment volume change (m3/m) Strete Gate Distance from Torcross (m) Gravel beaches of Start Bay, South Devon Blackpool Sands Slapton Sands Strete Gate Beesands North Hallsands Slapton Sands Torcross Beesands North Hallsands Angle of breaking waves during Petra on Slapton Sands 3 m waves breaking at a 30-40o angle with the shoreline generate longshore sediment transport > 10,000 m3 per day Longshore sand model – South coast rocky headland ‘new’ beach beach widening lagoon lagoon beach narrowing Storm response Carlyon and Exmouth from LiDAR Wave approach Erosion (> 2m ) Accretion (> 2 m) WHERE HAS THE SAND GONE? ONSHORE (OVER THE TOP)! GRAVEL BEACH MODEL Loe Bar post-storm 2014 Overwash fans Vegetation covered Barrier crest eroded Beach face smoothed stable Loe Bar – post-storm survey 18 Feb 2014 Accretion Erosion Accretion Accretion Accretion Erosion Onshore sand model – Gravel beaches Overwash pushes sediment onshore over the top of the barrier Sediment deposited behind the barrier will not return to the beach Also overwash on sandy beaches if dunes are absent © Mike Page CONCLUSIONS • Storm waves are related to Atlantic low pressure systems – the storm frequency, intensity and tracks will be affected by climate change • The 2013/2014 winter was the stormiest winter since 1950 • 1% exceedence Hs was exceeded by 20 storm events • 10 storms with peak Hs > 8 m • 2 storms with peak Hs > 10 m • Storm impacts show a large geographical variability • North coast = offshore sediment transport and beach erosion • South coast = longshore sediment transport and beach rotation • Beach sediment has not ‘disappeared’, but has been transported elsewhere: • offshore (under water) • alongshore (around the corner) • onshore (over the top) • Have beaches recovered? Implications for coastal vulnerability Hazards x Exposure = Coastal risk Coastal vulnerability Increase in coastal hazards due to climate change: • Sea-level rise • increased storminess (more and/or more energetic storms) Decrease in exposure to coastal hazards due to coastal management: • Coastal planning and zonation • Coastal protection • Managed realignment (managed retreat) IMPACT OF 2013-14 STORMS ON COASTAL GEOMORPHOLOGY IN SW ENGLAND Gerd Masselink, Plymouth University © Richard Broom Geographical Association 2015