Dynamic preservation of the coastline
Content Introduction The Dutch coastal defence by dynamic processes Data collection Management of the sand resources
Growing importance of marine sand and gravel resources Annually between 47 and 59 billion tonnes of solid material is extracted worldwide. Sand and gravel forms with 68% to 85% the greatest part of it. Marine extraction is increasing due to the exhausting of the resources on land. In the European seas yearly c. 94. million m 3 is extracted.
Applications of marine sand and gravel Coastal extensions and artificial islands Landfill Beach nourishment Burial of pipelines Industrial use for concrete and mortar sand
The Dutch lowlands Some figures about The Netherlands: NL 55% of the country is laying below sea level 60 % (10.2 million people) live below sea level, locally as low as -7 m. 65% of the national gross product is from this area Below mean sea level 55%: of its territory 60%: of its population 65%: of its national gross product Hansje Brinkers The earliest early warning system
Keeping the country dry for more than 1000 years Subsidence
Yearly measuring of the coastline Basal Coast Line, since 1990 No further retreat of the entire 400 km coastline excepted High tide Check points Low tide Yearly survey to detect coastal retreat
Coastal defence methods Coastal nourishment program: At the beach In the foreshore At the beach and foreshore Aim: Dynamic preservation of the coastline Maintaining the overall sediment availability in the coastal system
Sand engine In 2011 21.5 million m 3 of marine sand was dredged and dumped at the foreshore Expected to be transported along the coast in northwards direction in 20 years The behaviour of the sand body is monitoring by jetski s and video camera s
Protection of a coastal protection dam 2014-2015 The nourishment is still in progress, ready in 2015 To protect a 5 km long dam in 2014 a project started to supply 36 million m 3 sand in the foreshore. Against the dama dune ridge will be formed Existing situation new situation
Beach nourishment amounts along the Dutch coast Yellow beach nourishment, blue foreshore nourishment, red line percentage of erosion of the basal coastline
New land by interupting longshore transport by extension of a harbor dam Sedimentation over 5 km, about 7 million m 3 and has been collected behind the dam. Reach of equilibrium expected in 2025 Former coastline
Methods for detecting extractable sand resources
Acoustic methods Multibeam Side scan sonar for obstacles, like wrecks Multibeam for morphology Vertical acoustic profiling with for detecting thicknesses of the sand resources and clay layers
Sea bed sampling Boxcorer Undisturbed sample 0-50 cm Vibrocorer Undisturbed Sample 0-6 m Disturbed sample Van Veen grab 0-20 cm
Management of marine sand resources
2D mapping of sand extraction areas Presentation of the D50 of the sand fraction and the silt fraction <63 micron of the upper 2 metres of the sea bed in a sand extraction area Extraction permitted at -20 m isobath or at 20 km from the coastline
Search for pollutants Each sand extraction area has to be investigated for the presence of contaminants. The following elements have to analysed: Dry and organic matter Fraction <2 micron Cadmium Chrome Copper Mercury Lead Nickel Zinc Mineral oil Polycyclic Aromatic Hydrocarbons PCB s Organochloric pesticides Tributylene Of each 6 cores in an area a mixed sample of each 0.50 cm depth has to be analysed
Management of marine sand resources To assure that on the long term the sand budget will sufficient to maintain the coastline, a tool has to be developed to provided the authorities with adequate information. Management of offshore sand resources requires both knowledge on the quantity and quality of sand and a tool to visualize this information. Traditional 2D mapping -> good result, but time consuming i.e. costly New method: A 3D model which can be used in a decision support system
Available data Zone of interest Informations of boreholes Particle size analyses High resolution acoustic information Extraction permitted beyond the 20 m isobath or at 20 km from the coastline
Soil sample locations Pattern of soil sample locations in the zone of interest
Data Layer model Pattern of interpreted high resolution acoustic information in the zone of interest
Method 3D kriging using spatial correlation structures Interpolation of soil classes, silt % and shell % Voxels 250 250 0.5 m 1. Fine sand (63-105 µm) 2. Medium sand (105-210 µm) 3. Coarse sand (210-420 µm) 4. Very coarse sand(420-2000) 5. Sand unknown 6. Clay/loam 7. Peat 8. Shells Soil classification 1. Silt 0-2% 2. Silt 2-4% 3. Silt 4-10% 4. Silt >10% 1. no shells 2. shells 0-1% 3. shells 1-10% 4. shells 10-30% 5. shells unknown Silt fraction classification Shell percentage
Data layer model based on available information Total depth below sea bed 12 m Fine sand Medium fine Medium coarse Coarse Sand grain size unknown Clay Peat Shells
Extractability criteria Fine-grained layers: critical thickness and type of sediment sand extractable fine grained or clay sand non extractable
Different scenarios Extractability criteria Geology? Scenario Cri*cal thickness Lithology of fine- grained layers A no - B1 1,0 metre On clay/peat layers B2 0.5 metre On clay/peat layers C 1,0 metre On silt classes 3 & 4, clay/loam/peat)
Sand thicknesses with different scenarios Thickness to 12 m below sea bed without critical thickness of unextractable clay, silt or peat layers Thickness of the extractable sand resources with critical thickness of 0,5 m upon an unextractable layer
Sand volumes with different scenarios
Extractable sand with pipelines and cables routes No dredging within 500 m at each side of the cable or pipeline Short term, no extraction in the pipeline routes and electric cable routes from windmill parks Long term, lifetime pipelines and electric cables c. 30 years. After removal sand resources become available.
Conclusions Small scaled foreshore sand nourishment appeared to be more effective on the short term than large scale foreshore nourishment because of the long term process involved. A flexible decision-support system with a powerful and easy-to-understand visualization component is more useful than traditional mapping results. The flexibility of the system, with possibilities for refinement and for the addition of non-geological expertise, is its main strength.
Thank you for your attention!