Trilateral Wadden Sea Mud Balance - Reflection from a scientific perspective
Piet Hoekstra
Sediment Solutions Webinar – 27th of November 2020
Topics of short reflection
• General remarks on sediment management
• Mud balance and budgets, sources and sinks: uncertainties
• Processes of supply, transport and deposition – Wadden Sea system
• Future conditions: climate change and sea level rise
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Waddenacademie | Leeuwarden 2
Sediment management
• Coastal sediment management: focus on sand
• Sand budget in coastal zone: buffer implies coastal safety (flooding, erosion); e.g. Dutch concept of
“Basis Kustlijn” (1990)
• Sand as natural resource
• Sand widely available in seabed of North Sea.
• Important contribution of mud – revise our ideas
• More integral approach for sediment management necessary – combining sand and mud and interactions (process–scale)
• Mud as “burden”or natural resource: many isolated (pilot) projects
• Cumulative effects ?
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Waddenacademie | Leeuwarden 3
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Input of Mud: long debate about the supply of mud in the North Sea and uncertainties
Transport from Dover Strait > Continental flow (60%) Previous estimates (De Kok, 2004; Fettweis et al., 2007)
Yearly residual transport: 19 Mton; large inter-annual variability of 10-25 Mton/yr
Variability due to mobilisation and supply of mud, tides, wind and waves, density patterns (outflow river Rhine)
Present estimates are lower: 10 -14 Mton/yr
Contribution of recent measurements (including remote sensing data) and modelling efforts.
3-12-2020 Leeuwarden,
Mud transport in Dutch coastal zone: coastal turbidity maximum at sea bed
van der Hout et al, 2015
Contribution of about 8 % to Continental
flow ?
Mud balance
(ongoing work)
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Sources [10
6ton/year]
Net import [10
6ton/year]
Extraction [10
6ton/year]
Processes of supply, transport and deposition
• From large patterns to small-scale processes
• Coastal continental flow versus exchange between North Sea and Wadden Sea; the connectivity of tidal basins (wind and waves)
• Mud deposition as part of morphological adjustment; focal points of deposition
• Separation of sand and mud not realistic in terms of processes: mixed sand with only 5 % of mud starts behaving as cohesive sediment
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Waddenacademie | Leeuwarden 7
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Wind-driven fluxes in tidal basins and across tidal watershed (divides)
Sassi et al, 2015. Modelling
Contribution of mud to the budget: local hotspots
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Colina Alonso et al (in prep.)
Future role of mud in times of sea-level rise
• Mud is a vital component in the process of morphological adaptation of tidal basins and estuaries
• COASTAL SQUEEZE: our coastlines are fixed and accomodation space (area to store mud) is reduced.
• Avoid seaward strategies that further promote coastal squeeze.
• Tidal basins and estuaries are complex systems and their fate not simply depends on the availability of sand or mud but also on geometry and a range of hydrodynamic processes and feedbacks
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Waddenacademie | Leeuwarden 10
Sea Level Rise and scenarios for estuaries
Leuven et al, 2019