Modeling bio-physical influences on sandwave dynamics Bas Borsje, Maarten Buijsman, Giovanni Besio, Mindert de Vries, Suzanne Hulscher, Peter Herman, Herman Ridderinkhof
University of Twente, Enschede, The Netherlands Deltares, Delft, The Netherlands Delft University of Technology, Delft, The Netherlands NIOO-CEME, Yerseke, The Netherlands Radboud University Nijmegen, The Netherlands University of Genova, Italy University of California, Los Angeles, USA NIOZ, Den Burg, The Netherlands
What is underneath?
Content • • • • • • •
Introduction Objective Hypothesis Field data Model Results Conclusions
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Introduction (I) • •
Bio-geomorphology: interaction between benthos, water and sand Studying bio-geomorphodynamics offshore is important from an: • Ecological perspective (conservation of biodiversity) • Economic perspective (utilization of the seabed)
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Introduction (II) • • •
Sandwaves are present in the Marsdiep inlet Showing a spatial and a temporal variation in dimensions Result of the interaction between benthos, water and sand?
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Objectives • •
To model the wavelength and migration rate of sandwaves in the Marsdiep inlet To explore the causes for seasonal variation
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Hypothesis What causes the temporal variation in sandwave dimensions? • Variation in water temperature → kinematic viscosity → fall velocity of sediment → sediment transport • Variation in flow velocity → sediment transport • Biomass tube building worms
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Intermezzo – Parameterization (I)
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Intermezzo – Parameterization (II)
•
Lower near bed velocity → Lower ripples (direct) → Deposition of fine sediment → Lower ripples (indirect)
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Hypothesis Variation in water temperature → kinematic viscosity → fall velocity of sediment → sediment transport • Variation in flow velocity → sediment transport • Biomass tube building worms → ripple heigth → sediment transport
•
Not (Buijsman and Ridderinkhof 2008): • Estuarine circulation • Storms
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Field data • • • •
ADCP mounted under the ferry Schulpengat Velocity and waterdepth for 1999-2002 Water temperature Assumption biomass variation Lanice conchilega
wi sp su fa
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Model : Idealised bio-sand wave model • • •
Sandwaves are seen as free instabilities of the system Linear stability analysis: fastest growing mode → wavelength and migration rate (not sandwave heigth) Imposed: • Flow field (M2, M4, Z0, phase difference) • Prediction sediment transport • Interaction between benthos – fluid – sediment
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Results (1) • • • • • •
grain size water depth flow velocity M2 flow velocity M4 flow velocity Zo Phase difference
Measured:
200 m
0.4 mm 24 m 1.1 m s-1 0.11 m s-1 -0.09 m s-1 84˚
60 m y-1
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Results (I1)
• • • •
Case 2: Water temperature Case 3: Flow velocity Case 4: Biological activity Case 5: Combination
Measured:
15%
60%
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Results (III)
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Conclusions • • • •
Sandwaves in the Marsdiep inlet are the result of the complex interaction between benthos, water and sand The model is able to reproduce the wavelength and migration rate Wavelength and migration rate are dependent on different processes Importance to account for bio-geomorphological interactions offshore
Contact:
[email protected] 13-14 14-14
Model
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