Erosion and Accretion along the Danish Coastlines

Transcription

1 Department of Geosciences and Natural Resource Management Erosion and Accretion along the Danish Coastlines Aart Kroon DANCORE SEMINAR 2014 Danish Coasts and Climate Adaptation (COADAPT) 3 December 2014, Geocenter Denmark, Copenhagen

2 Motivation COADAPT

3 Motivation Erosion COP15 Climate Conference Copenhagen What are the present-day erosion rates? (DCA, 2009)

4 Contents Erosion and accretion A. coastal erosion and accretion: spatial patterns development in COADAPT: from regional to country-wide B. impact of processes: storms and flooding events on shores vulnerability of shorelines extreme water levels and high-energy wave events shoreline alignment overwash regimes

5 Coastal erosion and accretion: spatial patterns (Photo: Per Sørensen, DCA, 12 January 2007)

6 North Sea Maarup Kirke Photo: Henning Kristensen ( ) Søndervig Photo: DCA (DCA, 2001)

7 North Sea Yearly erosion rate in m year -1 Yearly erosion rate in m year -1 (DCA, 2008)

8 COADAPT: Erosion and accretion rates in Denmark

9 COADAPT: Erosion and accretion rates in Denmark Large variability in coastal geomorphology Deglaciated landscapes (Kabuth et al., 2014)

10 Methods Data source: Topographic maps HMB/LMB Scanned paper map sheets K10 Vector dataset Survey dates Map-sheet polygon feature attributes Map sheets Orthorectified & georeferenced raster Digitize shoreline Shoreline Polyline Split by map-sheet polygons Spatial join Shoreline feature class HMB/LMB Survey dates as attributes Shoreline feature class K10 Survey dates as attributes Shoreline acquisition in ArcGIS HMB: Høje Målebordsblade, LMB: Lave Målebordsblade, K10: Kort10 (Kabuth et al., 2014)

11 Methods Shoreline change computation in ArcGIS / DSAS Digital Shoreline Analysis System (USGS) (Kabuth et al., 2014)

12 Shoreline change Denmark (Kabuth et al., 2014)

13 Coastal classification and shoreline changes SHORELINE CHANGES - ORDER OF MAGNITUDE SHORELINE CLASS BEHAVIOR Alongshore extension [m] Cross-shore range [m] Examples of shoreline-change rate (99 th percentile)[m/yr] Straightening coast Adjacent accretion and erosion m/yr, m/yr (Vigsø bight) +1.8 m/yr, -2.0 m/yr (Jammer bight) Distal accretion, longitudinal and/or latitudinal m/yr (Feddet spit) m/yr (Korevlerne barrier spits) Spit, barrier spit Proximal erosion m/yr (Krageø) Proximal stability or accretion (barrier spits) ca. +2 m/yr (Korevlerne) Pivoting spit point (mature spits) m/yr, -6.3 m/yr (Skagens Odde) Marsh Accretion ,, confined by bay extent or barrier +2.8 m/yr (Ølsemagle Revle lagoon) Marine dominated estuaries Accretion (shoreline straightening) m/yr (Tudeå) Land reclamation Accretion 10 3, confined by dam 10 3, confined by dam m/yr (Lumby Kvissel bay closure) m/yr (Amager territorial waters) Upstream accretion ,, confined by jetty length +1.5 m/yr (Hanstholm) Harbour Upstream erosion m/yr (Gilleleje) Downstream erosion m/yr (Frederikshavn) Coastal protection Upstream accretion ,, confined by groin length or breakwater position +0.5 m/yr (Lisleje) Downstream erosion m/yr (Lisleje) Beach parks Accretion 10 3 Unconfined Not computed (Kabuth et al., 2014)

14 Coastal erosion and accretion: spatial patterns Remarks First erosion-accretion map of DK with a high spatial resolution Observed patterns are in line with expectations (verified at several locations with detailed studies) Critical remarks Resolution in time is limited No distinction between natural processes and man-made interventions No direct coupling to coastal processes

15 B. Impact of processes: storms and flooding events on shores Case study barrier island (Inner Danish Waters) morphological changes vulnerability of the barrier island extreme water levels and high-energy wave events shoreline alignment and overwash regimes

16 CASE STUDY Barrier island on the southern shore of Sjælland Danish Coasts and Climate Adaptation

17 Barrier island on the southern shore ± Holsteinborg Nor Basnæs Nor Glænø Shoreline movement rate (m/yr) none Century development Coastline 1891 Coastline km Erosion of a former glacial bluff? (Holocene development) (KMS, 1989)

18 Barrier island on the southern shore Decadal development (Westh, 2012)

19 Barrier island on the southern shore Decadal development (Hansen et al., 2014)

20 Vulnerability Barrier island on the southern shore (Hansen et al., 2014)

21 Barrier island on the southern shore Risk for erosion, breaching and flooding Extreme water levels and high-energy wave events Shoreline alignment (alongshore) wave energy fluxes Onshore movement and overwash regime 1. collision, 2. overwash and 3. inundation

22 Extreme water levels and high-energy wave events Danish Coasts and Climate Adaptation

23 Waves, climate 5 years: (DMI, DHI, 2012)

24 Extreme water levels and high-energy wave events Wadden Sea / North Sea water levels: Grådyb (Kroon et al., in prep.)

25 waves Fanø Wadden Sea / North Sea waves: Fanø

26 Wave dir ( 0 ) H s (m) Duration (h) Danish Coasts and Climate Adaptation Wadden Sea / North Sea water levels and waves Peak over threshold (1.7 m) Increase duration Tide limited All events: high-energy wave conditions All events: Shore normal Max. water level (m) (Kroon et al., in prep.)

27 Extreme water levels and high-energy wave events Inner Danish Waters water levels Karresbækminde (Kroon et al., in prep.)

28 waves: Glænø

29 Wave dir ( 0 ) H s (m) Duration (h) Danish Coasts and Climate Adaptation water levels and waves Peak over threshold (0.7 m) Increase duration? Low- and moderate wave energy conditions All events: Shore oblique Max. water level (m) (Kroon et al., in prep.)

30 (Photos Sune Westh, 2012) Extreme water level event: +120 cm over DVR90, 6 January 2012 Flooding, but no breaching nor erosion Total of 30 events over the 5 year period, 12 with onshore waves

31 Shoreline alignment NORTH 15% 10% WEST 5% SOUTH EAST Hs[m] Wave transformation model (energy decay model) (Kabuth and Kroon, 2014) Wave energy fluxes at -2 m water depth

32 Shoreline alignment (Kabuth and Kroon, 2014)

33 Overwash regime G1 G2 G1 G2 G3 G4 G5 G3 G4 (Hansen et al., 2014) G5

34 Overwash regime (Stockdon et al., 2012)

35 Overwash regime (Hansen et al., 2014)

36 Overwash regime Most vulnerable site (Hansen et al., 2014)

37 Overwash regime (Hansen et al., 2014)

38 Overwash regime (Hansen et al., 2014)

39 Impact of processes: storms and flooding events on shores Concluding remarks on case study on erosion and flooding on a barrier coast Antecedent setting influences the barrier behaviour Exceedence curves for water levels are not the best indicators for erosion Wave energy fluxes can easily align the shoreline and close existing breaches Overwash regimes and robustness of the morphology (height and volumes) sufficiently indicate weak points Future

40 Thanks for your attention! Acknowledgements Alina Kabuth, Jørn Torp Pedersen, Mikkel Fruergaard Students in Physical Geography, University of Copenhagen, Denmark: Sune Westh, Martin Lauenborg Hansen, Martin Lindhardt & Mathias Lauridsen (Photo Mette Bendixen, 17 February 2011)