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IMPACT OF STORMS ON COASTAL GEOMORPHOLOGY IN SW ENGLAND

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Presentation on theme: "IMPACT OF STORMS ON COASTAL GEOMORPHOLOGY IN SW ENGLAND"— Presentation transcript:

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

2 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

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

4 WINTER STORMS OF 2013/14

5 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

6 Waves generated by the Hercules storm
H =8 m H =10 m H = 12 m H =14 m ‘Black Hole’ H > 15 m

7 What does a 10-m wave look like

8 Measured and modelled wave heights
12 10 8 6 4 2 Dec Jan Feb Mar Apr14 Hercules 06/01/14 14 m Petra 05/02/14 12 m

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

10 PHOTOGRAPHIC OVERVIEW OF IMPACTS

11 Southwest coast of England
North coast W South coast SW

12 Beach lowering, Seaton, South Cornwall

13 5,000 peat exposed, Hallsands, South Devon

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

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

16 HOW TO MEASURE COASTAL IMPACTS?

17 Beach surveys Tape measure and clinometer – does work!
Real-time kinematic GPS – much better!

18 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

19 Airborne LiDAR surveys
Difference between before and after storm is change due to storm Erosion (> 2m ) Accretion (> 2 m)

20 COASTAL IMPACTS – REGIONAL OVERVIEW

21 SW England and beach monitoring sites
North coast W South coast SW

22 Beach monitoring (> 30 beaches, >200 profiles)

23 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

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

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

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

27 SW England and beach monitoring sites
South coast W Chesil SW

28 SW England and beach monitoring sites
Chesil W SW

29 OFFSHORE (UNDER WATER)!
WHERE HAS THE SAND GONE? OFFSHORE (UNDER WATER)! NORTH COAST MODEL

30 Impact of Hercules on Perranporth
North coast W South coast

31 Perranporth beach in summer

32 Perranporth response from LiDAR (Hercules)
Erosion (> 2m ) Accretion (> 2 m)

33 Where has the sand gone? Offshore accretion Mega-rip channel
Intertidal erosion Erosion (> 2m ) Accretion (> 2 m) High tide Low tide

34 Offshore sand model – North coast
Before the winter After the winter dune Thin beach Dune scarp Rocks exposed Sand bar below low tide

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

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

37 Impact of Petra on Slapton Sands
North coast South coast SW

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

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

40 Alongshore variation in storm response
Torcross Slapton Sands Strete Gate Sediment volume change (m3/m) Distance from Torcross (m)

41 Gravel beaches of Start Bay, South Devon
Blackpool Sands Slapton Sands Strete Gate Beesands North Hallsands Slapton Sands Torcross Beesands North Hallsands

42 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

43 Longshore sand model – South coast
rocky headland lagoon beach narrowing widening ‘new’

44 Storm response Carlyon and Exmouth from LiDAR
Wave approach Erosion (> 2m ) Accretion (> 2 m)

45 WHERE HAS THE SAND GONE? ONSHORE (OVER THE TOP)! GRAVEL BEACH MODEL

46 Loe Bar post-storm 2014 Overwash fans Vegetation covered Barrier crest
eroded Beach face smoothed stable

47 Loe Bar – post-storm survey 18 Feb 2014
Erosion Accretion Accretion Accretion Accretion Erosion

48 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

49 Also overwash on sandy beaches if dunes are absent
© Mike Page

50 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?

51 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)

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

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