THE WINTER STORMS OF 2013/14: OCEANOGRAPHY AND COASTAL IMPACTS Gerd Masselink, Plymouth University
Coastal Futures 2015 OUTLINE
• Wave conditions during 2013-2014 winter • Pictorial overview of storm impacts in SW England • Regional overview of coastal response in SW England • Coastal response on the North coast – Perranporth • Coastal response on the South coast – Slapton Sands • Conclusions WINTER STORMS OF 2013/14 Measured and modelled wave heights
12 10 8 6 4 2 0 Dec13 Jan14 Feb14 Mar14 Apr14
14 m 12 m
Hercules 06/01/14 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 Hercules W
South coast Overwash gravel ridge at Westward Ho!, North Devon Destruction of rock arch, Porthcothan, North Cornwall
Before Hercules After Hercules Removal of Whipsiderry beach, North Cornwall
Before Hercules After Hercules Destruction of sea defence, Fistral Blu bar, Newquay Beach erosion, Perranporth, North Cornwall
End of summer 2013
After Hercules Removal of Monkey House, Porthreath, North Cornwall
Before Hercules After Hercules Southwest coast of England
North coast
South coast Petra SW Coastal flooding, Porthleven, South Cornwall Coastal flooding, Looe, South Cornwall Beach lowering, Seaton, South Cornwall Hot Wok Restaurant, Plymouth, South Devon Dune erosion, Thurlstone, South Devon Removal of beach in Torcross, South Devon
EndBefore of summer
DuringDuring Petra Removal of beach in Torcross, South Devon
EndBefore of summer
AfterDuringAfter Petra Gravel on road, Slapton Sands, South Devon
Before Hercules After Hercules Damage to SW railway line, Dawlish, South Devon
Cost: £1M per day to the SW economy COASTAL IMPACTS – REGIONAL OVERVIEW SW England and PCO beach monitoring sites
North coast
W
South coast SW Beach monitoring (> 30 beaches, >200 profiles) Storm impacts along coast of SW England – per profile
BRISTOL CHL NORTH COAST SOUTH COAST
/m)
3
Accretion
Erosion Change sediment Change volume (m line in profile Beaches geographically arranged from Somerset to Dorset SW England and beach monitoring sites
Bristol Channel
North coast
W St Ives Chesil
SW South coast COASTAL IMPACTS – NORTH COAST Perranporth Impact of Hercules on Perranporth
North coast
W
South coast Perranporth beach in summer Perranporth response from LiDAR (Hercules) Wave approach
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) COASTAL IMPACTS – SOUTH COAST Slapton Sands Impact of Petra on Slapton Sands
North coast
South coast SW Gravel beaches of Start Bay, South Devon
Blackpool Slapton Sands Sands Beesands
North Hallsands
South Hallsands Slapton response from RTK-GPS (Petra)
before storms after storms P19
P11
Wave P0 approach 3
Sediment volume change (m /m) Torcross Alongshore variation in storm response approach Wave Distance from Car Car park Erosion Torcross (m) Accretion Strete gate Angle of breaking waves during Petra on Slapton Sands Longshore sand model – South coast
Before the winter After the winter rocky ‘new’ headland beach
beach widening beach narrowing
lagoon lagoon Storm response Carlyon and Exmouth from LiDAR
Wave Erosion (> 2m ) approach Accretion (> 2 m) 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 storm 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 • Coastal response is strongly related to the storm-wave direction • Beach sediment has not ‘disappeared’, but has been transported elsewhere (in nearshore bars, along the beach, around the corner) • Beach recovery during the spring and summer 2014 has been limited • Incomplete recovery leaves beaches vulnerable to further erosion • Remedial action must be informed by understanding of the processes obtained from monitoring – state-funded organisation such as the PCO play a key role LONG-TERM CONTEXT Waves and beach volume Perranporth since 2006
2013/2014 winter Waves and beach volume Slapton Sands since 2007 2013/2014 winter
P18
P5 – south end P5
P18 – north end