Trilateral Wadden Sea Mud Balance - Reflection

(1)

Trilateral Wadden Sea Mud Balance - Reflection from a scientific perspective

Piet Hoekstra

Sediment Solutions Webinar – 27th of November 2020

(2)

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

3-12-2020

Waddenacademie | Leeuwarden 2

(3)

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 ?

3-12-2020

(4)

3-12-2020

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.

(5)

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 ?

(6)

Mud balance

(ongoing work)

6

Sources [10

⁶

ton/year]

Net import [10

⁶

ton/year]

Extraction [10

⁶

ton/year]

(7)

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

3-12-2020

(8)

3-12-2020

Wind-driven fluxes in tidal basins and across tidal watershed (divides)

Sassi et al, 2015. Modelling

(9)

Contribution of mud to the budget: local hotspots

9

Colina Alonso et al (in prep.)

(10)

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

3-12-2020

(11)

Sea Level Rise and scenarios for estuaries

Leuven et al, 2019