harbours, fairways and mud

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Thijs van Kessel

19 March 2021

Interactions between

harbours, fairways and mud dynamics in the Wadden Sea

N a a m A

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Harbours and fairways

5

• depth (harbour itself + fairway)

• fixation of natural channel dynamics

• sheltering

• sedimentation (SSC )

• dredging and dispersion (SSC )

• risk of return flows

• harbours don’t produce or consume mud, but store it temporarily

• + and – don’t necessarily cancel each other

• (ship and harbour) size matters

Port Den Helder

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Size matters

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Mud dynamics in the Wadden Sea

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• natural concentration gradients, SSC higher in Wadden Sea than in North Sea

• large variations: tidal, neap-spring, summer-winter, calm or storm period

• sediment balance: net import

• large gross fluxes, small net fluxes

• maintenance dredging small compared to gros fluxes, but large compared to net fluxes

add pictures on spatial sediment distribution, temporal dynamics, mud balance

Hommersom (2010)

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Mud balance (from report + presentation Ana last time)

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Interactions

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1. Presence harbour, jetties 2. Deeper channel

3. Sedimentation

4. Dredging and release

5. Storage on land (extraction)

1 2 3

4

5

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Generic picture

2 1

3 4

5

2. Deeper channel

• sand/mud sedimentation

• more mud towards harbour

• fixation

3. Mud sedimentation within basins 4. Dredging and release

Size- and location-specific impacts

S

M

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Effect quantification

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• Sedimentation, maintenance dredging and release

• Channel deepening

• Mud extraction

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Sedimentation, dredging and release

during maintenance dredging in between maintenance dredging time-average effect

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Effect of deepening and extraction

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effect of deepening 1985 - 2005 difference between releasing and extracting SSC change (g/l)

Van Maren et al. (2015)

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Implications for harbour management (short term)

• There are pluses and minuses, so there is something to manage (location-specific)

• Optimisation of sediment release in space and time

• Resilience indicator for human impact on sediment budget:

𝑅 = ^𝐵

𝐵+𝑁

• with B = dredging volume, N = natural sedimentation volume. R → 0 = safe; R → 1 = danger

Options + -

Release nearby Cheap

Possible for low R

High SSC Return flows Release far away Low SSC

Less return flows, dredging

Expensive,

High SSC elsewhere

Extraction Less dredging Expensive

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Implications for harbour management (long term)

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• How large should ships and harbours be?

• Can we afford deeper channels and more dredging?

• Initially: small investments, big rewards → strong incentive for growth

• Ultimately: larger investments, smaller or even negative rewards

• Problem: reward = sum of cost and benefit, but cost and benefit at different parties

− cost: general tax payer, nature

− benefit: shipping company, local harbour community

• What is the optimal size and draught, didn’t we overshoot?

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Questions for ‘your’ port

Voettekst van de presentatie

• Impact of maintenance dredging on concentration and mud balance?

• Impact of capital dredging (deeper navigation channel) on concentration and mud balance?

• What is the resilience indicator R?

• How large are the return flows?

• Is there room for improvement of the maintenance dredging strategy?

• Is there still room for growth in size and depth?

Same framework, different location and size-specific answers

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