IMPACT OF 2013-14 STORMS ON COASTAL GEOMORPHOLOGY IN SW ENGLAND Gerd Masselink, Plymouth University Geographical Association 2015 © Richard Broom

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

x = HAZARD, RISK & VULNERABILITY Hazards Storms Tide Surge Tsunami Exposure Houses Industry Business Infrastructure Coastal risk Erosion Damage Flooding Financial loss x = Coastal vulnerability

WINTER STORMS OF 2013/14

Pressure chart for ‘Hercules’ storm 5th January 2014 Iceland UK US 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

What does a 10-m wave look like

Measured and modelled wave heights 12 10 8 6 4 2 Dec13 Jan14 Feb14 Mar14 Apr14 Hercules 06/01/14 14 m Petra 05/02/14 12 m

Comparison 2013/2014 winter with 60-year wave record 8-week running mean wave height 4 3 2 1 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 South coast SW

Beach lowering, Seaton, South Cornwall

5,000 peat exposed, Hallsands, South Devon

Removal of beach in Torcross, South Devon End of summer Before During Petra During

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 South coast SW

Beach monitoring (> 30 beaches, >200 profiles)

Storm impacts along coast of SW England – per profile Beaches geographically arranged from Somerset to Dorset Change in sediment volume profile line (m3/m) BRISTOL CHL NORTH COAST SOUTH COAST Accretion Erosion

SW England and beach monitoring sites Bristol Channel North coast W St Ives Chesil SW

SW England and beach monitoring sites North coast Bristol Channel W St Ives SW

SW England and beach monitoring sites St Ives Bristol Channel North coast W Chesil SW South coast

SW England and beach monitoring sites South coast W Chesil SW

SW England and beach monitoring sites Chesil W SW

OFFSHORE (UNDER WATER)! 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 Thin beach Dune scarp Rocks exposed Sand bar below low tide

Storm response Bude and Westward Ho! from LiDAR Wave approach Erosion (> 2m ) Accretion (> 2 m)

ALONGSHORE (AROUND THE CORNER)! WHERE HAS THE SAND GONE? ALONGSHORE (AROUND THE CORNER)! SOUTH COAST MODEL

Impact of Petra on Slapton Sands North coast South coast SW

Gravel beaches of Start Bay, South Devon South Hallsands North Hallsands Beesands Slapton Sands Blackpool Sands

Slapton response from RTK-GPS (Petra) before storms after storms P0 P11 P19

Alongshore variation in storm response Torcross Slapton Sands Strete Gate Sediment volume change (m3/m) 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 lagoon beach narrowing widening ‘new’

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 Erosion Accretion 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 x = Hazards 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 Geographical Association 2015 © Richard Broom