Natural and anthropogenic impact at the East Frisian coast - last 1000 years and future perspectives

Transcription

1 Natural and anthropogenic impact at the East Frisian coast - last 1000 years and future perspectives Alexander Bartholomä Senckenberg Institute Past: Present: Future: Past, Present and Future - development of the barrier island system - land relamation and dike construction - Dyke maintainance, preservation of the modern Wadden Sea system - Hazard analysis and risk managemant - Intergrated Coastal Zone Managment with alternative concepts in coastal zone protection and ecosystem conservation 1

2 Past: : East Frisian coast 800 AD Without dike line (based on historical maps, by Homeier 1969) Past: : East Frisian coast 1300 AD with dikes without dikes - land reclamation - southward migration by transgression - reduction of tidal prism - tidal flats with natural salt marshes - loss of accommodation space 2

3 Past: : East Frisian coast 1800 AD with dikes without dikes - land reclamation - southward migration by transgression - reduction of tidal prism - tidal flats with natural salt marshes - loss of accommodation space Present and Future: East Frisian coast with dikes without dikes - land reclamation - southward migration by transgression - reduction of tidal prism - tidal flats with natural salt marshes - loss of accommodation space 3

4 Past to present: Sea-level rise Discontinuous sea-level rise with transgressive and regressive phases Energy gradients in the Wadden Sea 53 46' N 53 42' N coarse fine Baltrum Sea-level rise results in: - coarsening of sediments - sediment lag effects close to the dike line - Elevation of tidal flats - Wadden Sea squeeze (all below breakpoint) Langeoog ' E ' E ' E Spiekeroog mainland 0 5 km Sediment belts along the East Frisian coast back-barrier tidal flat of Spiekeroog island settling velocity [cm/s] at 5 C and S=3% island Examples of energy gradients Mando Denmark Pellworm Schleswig-Holstein Langeoog East Frisland Rottumeroog West Frisland [km] y y = = x r = r = 0.92 y y = = x r = r = 0.91 y y = = -0.08x r = r = 0.75 sample P point regression R line confidence V interval 95% prediction V interval 95% y y = = -0.05x r = r = 0.90 other regions of the Wadden Sea dike

5 Wadden Sea squeeze and sea level rise Reconstruction of the fully developed tidal system without dike constructions N Spiekeroog W E S Langeoog sand flats present dike line mixed flats mud content total sediment (in %) mud flats saltmarsh 'N mean high water level ( m above chart datum) pleistocene sands 53 34'N 'E 'E 0 0 km 5 km - lost area: more than 50 % of the entire tidal system - mud flat and salt marsh deposits landward of the present day dike line 5

6 Salt-marsh erosion Preditecd salt-marsh erosion along the coastal dike line With summer dikes or dammed retention pits Under certain wave & water-level conditions, broken-wall construction patters can prevent salt-marsh erosion Mai & Zimmermann (2002) Wittig et al. (2004) Human responses to the expected sea-level rise Risk analysis: - storm-flood risks - hydraulic boundary conditions at coastal constructions - risk assessment for local environments Risk and Coastal Zone Management: e.g. by the Coastal Protection and Sea-level Group (CPSL) Policies, Strategies, Management Projekts and Activities: COMRISK (Common Strategies to reduce the risk of storm floods in coastal lowlands) DEKLIM (German Climate Change Research Program) KRIM (Climate change and preventive risk and coastal zone management along the German North Coast) ICZM Integrated Coastal Zone Management ENCORA (Initiative for Coordination Action within European coastal research and practitioners communities) LOICZ (Land-Ocean Interactions in the Coastal Zone) 6

7 Breakpoint analysis (CPSL 2001/2005) Moderate increase in sea level : 3mm/year for large basins 6 mm/year for small basins 8.5 mm/year for salt marshes 3 6 mm/ year pioneer zone Major sea-level rise: - massive erosion of salt marshes - in sandy areas, erosion rate increases between 250% (6mm/yr) and 330% (8.5mm/yr) - fore-shore erosion, retreat of barrier islands - present dynamic equillibrium still active - Increase of duration of tidal inundation implies stronger sediment accumulation on tidal flats and salt marshes (longer settling period) - Elevation of tidal flats; and expansion of salt marshes Changes in wave and /or tide-domminated sections are not considered Main topics of coastal protection and management Sand nourishment Sea dikes Dike strenghening over the last centuries time: height / lenght / volume 1900: 4.8 m / 33 m / 82 m³ 1200: m / 5.5 m / 3 m³ 1600: 3.0 m / 19 m / 32 m³ 1955: 5.9 m / 56 m / 149 m³ 2000: 7.8 m / 105 m / 325 m³ 2100: 8.5 m / 150 m / 500 m³???? Dune management Habitat maintenance Salt-marsh management Outbanking of summer polders 7

8 Däne mark Nordsee Helgol and Niederlande Deutschland Reinundation of sommer dikes - Salt marsh accretion in protected areas (Langeoog island) accumulation rate 5 cm/month by courtesy of Freund (2006), ICBM Marine Lab. Concept of creating artificial tidal basins in the shelter of the dikes Sea Dike - controlled tidal range - slow infilling with clay - new salt marsh formation - ideal breeding conditions - new clay resource - ideal for aquaculture - tourismus mean high-tide level Land low dike tidal basin tidal flat tidal channel mean low-tide level Sluice new salt marshes Schematic drawing of an artificial tidal basin with multiple and sustainable usages, including tourism. 8

9 Subdivision of an artifical tidal basin in the shelter of the dikes by multiple usage concepts Summary and Outlook Land reclamation and dike constructions already reclaimed more than 50 % of the former fully devoloped tidal system In order to the predicted sea level rise,, to preserve the modern system and/or prevent further changes in the Wadden Sea environment individual solutions for a Integrated Coastal Zone Managment are essential: - Increase of accommodation space - salt marsh,, dune and habitat management on individual scales - dike strenghening and retreat, reinundation of summer dike areas, - outbanking summer polders, reactivation of retention pits Therefore we need: - Individual case studies for individual sites - Collaboration between dike protection and environmental protection 9