Project Mosselwad is funded by the Wadden Fund
MUSSEL HUMMOCKS AFFECT FLOW PATTERNS AND FOOD UPTAKE IN
INTERTIDAL MUSSEL BEDS.
Fieldsite
Jasper Donker, Maarten van der Vegt and Piet Hoekstra
Department of Physical Geography, Utrecht University
@ j.j.a.donker@uu.nl
1. Problem definition Observations suggest that relief is largest in deeper lying, current dom- inated mussel beds. Relief develops when sedimentation occurs in ir- regularly covered mussel beds. High relief is expected to influence flow patterns and thereby transport of food. Variations in food availa- bility can affect the long term stability of the mussel bed.
1. How does an elevated mussel patch (hummock) influence flow pat- terns, turbulence and vertical mixing?
2. How do hummocks influence food availability and uptake?
3. Field observations
4. Model Results 2. Methods
(a.) Flat patch (b.) Hummock (reference)
(c.) Elevated band (d.) Checkerboard Hummocks
How do spatial patterns influence food uptake?
5. Conclusions
• Mussel hummocks influence flow by accelerating flow over the hummock and routing flow around the hummock.
• Geometry influences acceleration and routing, routing increases for more elongated and rougher hummocks.
• Wide hummocks are the most beneficial for high food uptake.
How are flow patterns affected by hummocks?
What are the effects of hummock geometry on these flow patterns?
6. References & Acknowledgements
Shellfish reefs are able to stabilize sediment and at- tenuate wave forcing (Borsje et al., 2011; Donker et al., 2013). Opportunities for mussel bed restoration in the Dutch Wadden Sea are explored. For this purpose we need a better understanding of the processes that influencemussel bed stability. For coastal protection purposes the type of mussel bed will be important as some beds remain flat while others develop eleva-
tions.
- What is the effect of mussel bed relief on flow patterns and food availability?
The Netherlands Dutch
Wadden Sea
For water level 0.6 m, flow from the left.
- Large velocity increase:
over hummock (+50%) in channel (+25%)
- Wake behind hummock(-25%) - Reduced velocities in front of hummock (-10%)
- Highest velocities on front and wake side of hummock
1b. Flow is partly accelerated over the hummock and partly routed around the hummock.
Sensitivity analysis with standard case:
- Hummock length 8m - Hummock width 2m
- Roughness height of 0.05 m on the hummock - Roughness height of 0.02 m on the sandy shoal Increasing hummock length (Fig 7a)
- Gradual change from flow acceleration to flow routing
Increasing hummock width (Fig 7b)
- Flow area decreases, all velocities increase Increasing surface roughness (Fig 7c)
- Reduces flow acceleration and increases routing
Fig 6: Effects of changes in
hummock length (a.), width (b.) and surface roughness (c.) on the ratio between the input velocity at the upstream boundary and the velocity over the hummock and in the channel respectively.
Fig 5: Spatial distribution of depth averaged flow velocities for rising tide (0.6 m).
Top of the hummock is outlined in black.
Field observations
- 4 Week experiment around small hummock (~7x3m) (location see Fig 1.) - Local flow & turbulence conditions
- ADV (32 Hz), 2 above mussel hummock 1 in channel
Model study
Flow patterns studied using SWASH (zijlema, et. al., 2011) - Non-hydrostatic model
- Idealized mussel hummock
- Boundary conditions based on field observations - Prescribed flow on left hand boundary
- 10 m sponge layer at right hand boundary to damp reflections - Periodic boundary conditions north and south
Food uptake is studied using coupled model (based on Simpson et al., 2007) - Advection-diffusion model with explicit food uptake (in 3D)
What are typical flow velocities around a mussel hummock?
How is vertical mixing influenced?
Observations of flow velocity (Fig 5) reveal:
- Low water: flow over hummock is larger
- High water: velocities are similar (hummock sensor is located closer to bed) - Large increase in channel velocity when hummock emerges
Fig 3: Observations of flow velocity on top of the hummock and in the adjacent channel
- Turbulent Kinetic Energy (TKE) is increased over hummock
- Vertical mixing is increased
Hummock length (m)10 20 30 40
0.5 1 1.5 2
Hummock length (m)
scaled velocity
(a.)
0.02 0.04 0.06 0.08 0.1 0.9
1 1.1 1.2 1.3
Roughness height (m)
scaled velocity
(c.)
2 4 6 8
0.8 1 1.2 1.4 1.6
Hummock width (m)
scaled velocity
(b.)
scaled velocity
10 20 30 40
2
1.5
1
0.5
(a.)
10 20 30 40
0.5 1 1.5 2
Hummock length (m)
scaled velocity
(a.)
0.02 0.04 0.06 0.08 0.1 0.9
1 1.1 1.2 1.3
Roughness height (m)
scaled velocity
(c.)
2 4 6 8
0.8 1 1.2 1.4 1.6
Hummock width (m)
scaled velocity
(b.)
scaled velocity
1.6 1.4
1 0.8
1.2
2 4 8
Hummock width (m) (b.)
10 20 30 40
0.5 1 1.5 2
Hummock length (m)
scaled velocity
(a.)
0.02 0.04 0.06 0.08 0.1 0.9
1 1.1 1.2 1.3
Roughness height (m)
scaled velocity
(c.)
2 4 6 8
0.8 1 1.2 1.4 1.6
Hummock width (m)
scaled velocity
(b.)
66
0.04 0.06 0.12
Roughness height (m)0.08 0.1
scaled velocity
1.3 1.2
1 0.9
1.1
(c.) distance (m)
distance (m)
10 20 30 40
4 12
8
0.3
0.25
0.2
0.15 velocity (m/s)
Field site
Fig 2: The model bathymetry with mussel hummock used in the model study.
Hummock, 6 x 4 m base, 0.4 m high, full domain 100 x 15 m.
distance (m)
distance (m)
0 10 20 30 40 50 60 70 80 90 100 15
10
0
5 height (m)
40
0
sponge layer
0 0.1 0.2 0.3 0.4 0.5
10−4 10−3 10−2
velocity (ms−1) TKE m2 s−2
Channel Hummock
Hummock Channel
0 0.1 0.2 0.3 0.4 0.5 10-4
10-3 10-2
Velocity (m/s)
TKE m s-2-2
Fig 4: Observed TKE over the hummock and in the channel as a function of flow velocity.
Change in food uptake with re- spect to hummock (Fig7b).
Flat patch -3%
Elevated band
+15% (over same area) Checkerboard Hummocks First hummock +0%
Second hummock +3.6%
General trend:
More flow routing -> decreased food availability
More flow acceleration -> in- creased food availability
0 0.1 0.2 0.3 0.4
velocity (ms−1 )
12:00 13:00 14:00 15:00 16:00 17:00 18:00 19:00 20:00 21:00 22:000 0.5 1 1.5
Water level (m)
On Top
Water level Channel
day time
Borsje B.W., et. al., (2011), How ecological engineering cvan serve in coastal protection. Ecological Engineering 37, p113-122 Donker J.J.A., et. al., (2013),Wave focing over an intertidal mussel bed, Journal of Sea Research 82, p54-66
Simpson J. H., et. al., (2007), The interaction of tidal advection, diffusion and mussel filtration in a tidal channel, Journal of Marine Systems 68, p556-568
Zijlema M., et. al., (2011), SWASH: An operational public domain code for simulating wave fields and rapidly varied flows in coastal waters, Coastal Engineering 58(10), p992–1012 Techincal support: Bas van Dam, Marcel van Maarseveen, Henk Markies, Chris Roosendaal and Reinier Schrijvershof
1a. Flow velocities and mixing are increased over hummock.
Grid repeats in y-direction
Flow direction
Low water Emerging
hummock
Fig 7: Tested mussel bed types: (a.) a mussel patch with no elevation; (b.) a single hummock with a height of 0.4 m; (c.) a mussel band with a height of 0.4 m; (d.) two hummocks of 0.4 m height in a checkerboard formation.
