Coastal Dynamics 2013 193 SUSPENDED SEDIMENT MODELLING ...

Coastal Dynamics 2013

SUSPENDED SEDIMENT MODELLING IN THE BAY OF BREST IMPACTED BY THE SLIPPER LIMPET CREPIDULA FORNICATA Alexis Beudin1, Georges Chapalain1, Nicolas Guillou1

Abstract A hydro-sedimentary model of the bay of Brest that takes into account the large presence of slipper limpets Crepidula fornicata is presented. The hydrodynamic and suspended sediment transport are computed with the 2DH modules of the TELEMAC numerical modelling system. Various turbulent near-bed processes are integrated and the sedimentary assemblage is treated as a number of components of different grain sizes and erosive properties. Mean current and suspended sediment concentration are compared with field data collected above a dense slipper limpet canopy. Incorporation of the bio-roughness parameterization yields a convergence of current velocity predictions on measurements. The hydrodynamic and filtration activity effects of C. fornicata are also analyzed through the variability of the maximum suspended sediment concentration and the mud mass balance between different bay compartments. Key words: multicomponent and mixed sediment transport, biota effects, modelling, field measurements

1. Introduction For more than 40 years, the gastropod mollusk C. fornicata has proliferated in many European bays hosting oyster farming. In the bay of Brest (Brittany, France), its stock has been evaluated to 125 000 tons wet weight in the year 2000 (Guérin, 2004) disseminated over nearly a third of the seabed (Fig. 1) with local density up to 4500 individuals per square meter. Prior to theirs ecological (Chauvaud et al., 2000) and economic impacts (Blanchard, 1997), slipper limpets modify the texture of the seabed, filter near-bed suspended sediments and reject feces and pseudo-feces that predominantly accumulate locally (Ehrhold et al., 1998). Moulin et al. (2007) investigated in laboratory the impact of C. fornicata on the hydrodynamics of the benthic boundary layer. They showed that despite the increase of the apparent roughness length with C. fornicata density, the shear stress at the water-sediment interface is reduced by the presence of the shells. In addition, the exchanges of suspended particulate matter between the canopy and the overflowing water also decrease as the shell density increases. To our knowledge, no integrated study was undertaken on hydro-sedimentary impacts of C. fornicata at the scale of the bay of Brest. Moreover, only few field investigations of the turbidity are reported in the literature (Monbet and Bassoulet, 1989). The present study aims at improving knowledge on fine particles dynamics in the complex sedimentary environment of the bay of Brest with potentially strong ecological constraints by C. fornicata. Focus is placed on short-term processes ranging from a minute to a couple of weeks. A numerical approach based on the hydrodynamic module TELEMAC-2DH (Hervouet, 2007) together with the sediment transport module SISYPHE (Villaret et al., 2011) is adopted. Parameterizations of major effects produced by C. fornicata, namely macro-roughness, shear stress partition and water-sediment filtration, are implemented. Three types of results are examined. The predicted current velocity and suspended sediment concentration are first compared with available field point measurements. Suspended sediment concentration patterns at the scale of the bay are then considered. Finally, exchanges of fine particles between various geographical compartments are treated on the basis of a novel particle marking approach.

1

Laboratoire de Génie Côtier et Environnement, Centre d’Études Techniques Maritimes Et Fluviales, 155 rue Pierre Bouguer, Plouzané 29280, France. [email protected], [email protected], [email protected]

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Figure 1. Superimposed maps of seabed sediments (Le Berre, 1999) and C. fornicata covering (Guérin, 2004)

2. Model Description 2.1. General assumptions The role of stratification is ignored and a two-dimensional horizontal (2DH) well-mixed approach is adopted. The bottom boundary layer flow is assumed turbulent over a rough bed. The horizontal dispersion coefficients are predicted by Elder (1959) formula. Low suspended sediment concentrations allow for the water-sediment mixture to have a Newtonian behaviour. The sedimentary assemblage (Fig. 1) is treated as a number of components of different grain sizes and erosive properties, namely non-cohesive and cohesive. The non-cohesive particles are picked-up from the bed according to Chapalain and Thais (2000) following Celik and Rodi (1988) along with Smith and McLean (1977). Erosion of cohesive sediments is described by the well-known law of Partheniades (1965). When mud content is greater than 30 %, the seabed is assumed to behave as a cohesive mixture and Partheniades erosion law is applied. When mud content is between 30 and 70 %, the critical shear stress of the cohesive sand-mud mixture is enlarged following Mitchener and Torf (1996) laboratory and field data

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compilation. When mud largely dominates (> 70 %), the bed sediment is eroded as if it was pure mud. The vertical structure of the bed is simply represented by an active layer on top of a quasi-infinite active stratum used to refill or reduce the active layer to a predefined thickness after each bed evolution computational time step (see section 2.3). 2.2. Specific assumptions with regard to C. fornicata The C. fornicata are usually staked on one another in piles of numerous individuals. Here, we considered characteristic piles of 5 individuals (ncrep/pile) with rectangular cuboid dimensions of 3.2 cm in height (hpile), 5 cm in width (spile) and 6.7 cm in length. Above a spatial numerical density dcrep max of 118 piles/m2 corresponding to 590 ind/m2, the spacing between piles becomes smaller than their height which allows skimming flow to develop (Friedrichs, 2004). According to Raupach (1992), an increase of the roughness elements density has no further effect on bed friction. Beyond 300 piles/m2 or 1500 ind/m2, the bed sediment is entirely covered by shells and no sediment is available for erosion. Production of biodeposits is not taken into account in the present study focused on short-term processes. 2.2.1. Adaptations in the hydrodynamic module TELEMAC-2DH The bed roughness length (z0) depends either on the abiotic bed sediment median grain size according to the expression z0=d50/12, proposed by Soulsby (1997), or on C. fornicata beds according to Lettau (1969) formula given by: 2

z 0=α h pile spile

min (d crep , d crep max) ncrep/ pile

(1)

where α is an empirical parameter estimated at 0.248 from field measurements performed by Chapalain and Thouzeau (2007). 2.2.2. Adaptations in the sediment transport module SISYPHE On account of C. fornicata piles drag form, skin friction (τ0') acting on sedimentary particles resting on the bed is only a fraction of the overall bed shear stress τ0. It decreases as C. fornicata piles density increases up to 300 piles/m2. We use Raupach (1992) expression given by: τ0 '=

τ0 1+ β λ

(2)

where β is the ratio between quadratic friction coefficients calculated for bed with and without C. fornicata, and λ is a specific roughness density defined as the frontal area per unit ground area. The filtration rate per unit ground area Fr is expressed as: Fr=n crep Fr crep Ci Zref

(3)

where Frcrep is the filtration rate of one C. fornicata individual and Ci Zref is the near-bed concentration of sediment class i. According to Troadec (1992), Frcrep equals 123 milliliters per C. fornicata individual per hour. 2.3. Model set-up The model procedure is set-up on a domain covering the Iroise Sea between longitudes 5°613 W and 4°186 W and latitudes 47°645 N and 48°795 N (Fig. 2a). The computational mesh is made of 25818 nodes and 46740 triangular finite elements with edge size of 3 km offshore in the Iroise Sea to about 10 m in the Brest harbour.

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S1

a)

b)

Figure 2. Computational domain georeferenced in Lambert zone II (a) and geographical compartments (black: Far-field domain, gray: Nearby domain, red: Central bay, pink: Elorn estuary, cyan: Auberlac'h inlet, light green: Daoulas bay, khaki: Aulne estuary, brown: Poulmic cove, yellow: Fret cove, blue: Roscanvel bay) from which mud is tracked (b)

The computational time step for hydrodynamic is set to 10 s while the one for sediment transport is increased to 1 min. TELEMAC-2DH is driven in the Iroise Sea by tidal sea surface elevations (SHOM) and at the Aulne and Elorn river boundaries by yearly-averaged fresh water discharges equal respectively to 24 and 6 m3/s (Monbet and Bassoulet, 1979). In the outer domain of the Iroise Sea, the bed sediment is treated as pebbles which grain-size has been adjusted to 4.2 cm in order to fit flow velocity predictions and measurements data (SHOM) at the Strait entrance. Nearby and within the bay, the bed sediment map published by Le Berre (1999) and completed with C. fornicata covering from Guérin (2004) (Fig. 1) has been digitalized and a specific granulometry has been imposed to each sedimentary unit. Grain-size distributions have been evaluated from various available sediment samples collected and compiled by Guillou et al. (2010). The finest considered grainsize fraction (25 µm) is treated as a cohesive sediment class. Aulne and Elorn water discharges are assumed to contain a second cohesive sediment class of 6 µm with a constant concentration of 10 mg/l according to yearly-averaged solid discharge estimation of Monbet and Bassoulet (1989). The 25 µm mud fraction initially present on the bed is marked in 9 distinct arbitrary geographical compartments of the bay (Fig. 2b) in order to track its origin. The empirical erosion parameters γ0 and M introduced respectively by Smith and McLean (1977) and Partheniades (1965) are set to their default values 0.024 and 10-3 kg/m2/s. The critical shear stresses of mud and cohesive mixture (mud fraction between 30 and 70 %) are set respectively to 0.1 and 0.2 N/m2. The active layer and active stratum thicknesses are set respectively to 2.5 d50 and 100 m initially. To investigate the hydrodynamic and filtration activity effects of C. fornicata on suspended sediment transport, three numerical experiments have been conducted. Numerical experiment E1 does not take into account C. fornicata at all. Numerical experiment E2 integrates the bio-roughness parameterization (Eq. 1) and shear stress partition (Eq. 2). Numerical experiment E3 additionally turns on the filtration of suspended muds.

3. Model Applications and Results 3.1. Comparison of model results with field point measurements Here, we use measurements of flow velocity and acoustic backscatter performed at 40 cm above the bed in a 20-m-deep site located at 48°309 N and 4°388 W (see Fig. 2a) where C. fornicata density is around 1500 ind/m2 (Chapalain and Thouzeau, 2007).

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3.1.1. Mean current velocity Flood dominance asymmetry and phase predicted by the model are in reasonable agreement with field data (Fig. 3). The simulation ignoring C. fornicata hydrodynamic effects (E1) over-estimates velocity amplitude by relatively 50 % while taking into account C. fornicata as bed roughness (E2) reduces the relative error to 25 %.

Figure 3. Comparison of mean velocity (cm/s) at 40 cm above the bed at station S1 predicted by TELEMAC-2DH with bare sediment (blue line) , with C. fornicata roughness (green line) and measured (black dot markers) from 12

April to 18 April 2006 3.1.2. Total suspended sediment concentration Total suspended sediment concentration maxima calculated by the model are between 10 and 20 mg/l in accordance with the range of direct measurements of total suspended sediment concentration determined from water samples by Chapalain and Thouzeau (2007). When comparing simulations E2 and E1, the bioroughness doubles the total suspended sediment concentration at low tide and the predicted signal exhibits a semi-diurnal peak in contrast to the quarter-diurnal signal in the current shear stress and acoustic backscatter. The skin friction in the model is found not to be sufficient to trigger local erosion at ebb tide but suspended sediment is brought from upstream as we observe a bump in the predicted signal at the end of ebb tide. Between simulations E3 and E2, filtration is found to have no effect on total suspended sediment concentration.

Figure 4. Comparison of total suspended sediment concentration (mg/l) at 40 cm above the bed at station S1 predicted by SISYPHE simulation E1 (blue), simulation E2 (red), simulation E3 (green) and measured acoustically (black) on 13 April 2006 – the water depth (m) (black dash) is superimposed to appreciate tidal stage

3.2. Estimation of suspended sediment concentration in the bay The depth-averaged total suspended sediment concentration predicted in the entire bay of Brest by the full options model E3 is presented in Figure 5 for an average tide in transition from spring to neap tides (18-19 April 2006). Suspended sediment load appears more important in the Aulne, Elorn and Daoulas inlets and predominantly made of muddy particles. More suspended sediment is present in the central bay during flood than during ebb. This feature is explained both by the stronger erosive capacity of currents during flood than during ebb and by the fact that mud advected seaward by ebb currents is maintained in suspension in the cyclonic gyre developing behind pointe de l'Armorique at the following flood.

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Pen Ar Vir

Flood

High Tide

Ebb

Low Tide

Figure 5. Depth-averaged current (graphic scale 1:2000) and total suspended sediment concentration (mg/l) predicted with simulation E3 at flood, high tide, ebb and low tide of an average tide on the 18-19 April 2006

C. fornicata piles have a significant impact on suspended sediment concentration due to their hydrodynamic properties while filtration activity has generally a negligible influence except in the east coast of the Poulmic cove (Fig. 6).

a)

b)

c)

Figure 6. Maximum depth-averaged suspended mud (mg/l) predicted during an average tide with simulation E3 (a) and relative difference (%) with simulation E2 (b) and simulation E1 (c)

Around the polder of Brest and in the south-eastern part of the bay, suspended sediment concentration is enlarged by a factor of 1.5 due to the presence of bio-roughness. In the mouth of the Aulne and Elorn

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estuaries, in the Roscanvel bay and in the Fret cove, turbidity is reduced by a factor of 2 by cause of C. fornicata piles sheltering effect. The east coast of Poulmic is located downstream of Pen Ar Vir where strong erosion occurs during flood tide (Fig. 5). A dense C. fornicata canopy is located in-between. When filtration is active, less mud is available for erosion at the following ebb. 3.3. Analysis of sediment transport pathways and fate in the bay The mass of suspended mud exported from the bay of Brest through the Strait by ebb currents is estimated at 155 tons. 90 % of this amount originate from the Central bay, the rest equally comes from Sainte-Annedu-Portzic cove and Roscanvel bay. The amount imported by flood reaches only 99 tons. This yields a net efflux of 48 tons. At measurement station S1, just before low tide, 25 % of suspended mud originate from the Aulne compartment (Fig. 7). No significant influence from the adjacent Daoulas and Poulmic compartments is predicted as the associated suspended mud pathways are confined close to the coast (Fig. 8).

Figure 7. Time evolution of total suspended sediment concentration (mg/l) and its major contributions at station S1 predicted with simulation E3 during an average tide

Figure 8. Tracked suspended mud (see Fig. 2b) 100 µg/l contour predicted with simulation E3 at low tide

Estimating evolution of mud mass in each bed compartment also helps to understand deposited sediment fate. Generally, suspended mud deposits in the bay of Brest during period of transition from spring to neap tides. This prediction is in agreement with the water clarification trend pointed out during this period by Chapalain and Thouzeau (2007). The Aulne estuary, the Poulmic cove and the Auberlac'h inlet are the three more efficient mud sinks. If C. fornicata were not considered, the Elorn estuary would act with the same efficiency but as a source of mud and the other compartments would be less efficient in trapping mud by respectively 60 % in the Daoulas bay, 50 % in the Roscanvel bay, 30 % in the Poulmic Cove, 7 % in the Nearby domain, 6 % in the Central bay, and 2 % in the Aulne estuary and Auberlac'h inlet. The mud mass balance prediction in the Fret cove is not impacted by C. fornicata.

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4. Conclusion and perspectives A 2DH hydro-sedimentary model adapted from the TELEMAC numerical system was proposed and applied to investigate the short-term dynamics of fine suspended sediments in the complex environment of the bay of Brest impacted by the invasive C. fornicata. A panel of local, synoptic and tracking analysis was performed. The following main conclusions may be derived: the advection process is important at the measurement station S1 located in the south-eastern part of the bay; the incorporation of C. fornicata roughness parameterization following Lettau (1969) improves current velocity predictions; C. fornicata have a significant impact on fine suspended sediment concentrations and deposition rates mainly through theirs hydrodynamic roles. The tracking method, useful to investigate the advection of remotely suspended sediments, appears also promising for evaluating sedimentary exchanges between beds of different geographical compartments (e.g., bays, estuaries, basins, harbors). Biodeposition process due to C. fornicata is in course of integration and long-term simulations under various scenarios of tidal and hydraulic conditions are planned to evaluate the potential sink of fine sediments the invasive slipper limpets are responsible for. In a further future, the present developments are expected to be implemented in a 3D biogeochemical model of the bay of Brest.

Acknowledgments This work was undertaken as part of a PhD funded by the French Ministry of Ecology, Sustainable Development and Energy at the Institute for inland and maritime waterways (CETMEF). Bathymetry data and tidal sea surface elevation are provided by the French Hydrographic and Oceanographic Office (SHOM). Field measurements were collected with the help of divers from the European Institute of Marine Studies (IUEM). Many thanks to G. Thouzeau at the Marine Environmental Sciences Laboratory (LEMAR) for providing C. fornicata data and A. Simon at CETMEF for digitizing and reworking bed sediment map.

References Blanchard, M., 1997. Spread of the slipper limpet Crepidula fornicata (L. 1758) in Europe. Current state and consequences. Scientia Marina, 61: 109-118. Celik, I. and Rodi, W., 1988. Modelling suspended sediment transport in non-equilibrium situations, Journal of Hydraulic Engineering, 114: 1157-1191. Chapalain, G., Thais, L., 2000. Tide, turbulence and suspended sediment modeling in the eastern English Channel, Coastal Engineering, 41: 295-316. Chapalain, G. and Thouzeau, G., 2007. Rôle des structures biogènes sur l'hydrodynamisme et les flux sédimentaires dans la couche limite de fond, PNEC, 24p. Chauvaud, L., Jean, F., Ragueneau, O. and Thouzeau, G., 2000. Long-term variation of the Bay of Brest ecosystem: benthic pelagic coupling revisited, Marine Ecology Progress Series, 200: 35-48. Ehrhold, A., Blanchard, M., Auffret, J.P. and Garlan, T., 1998. The role of Crepidula proliferation in the modification of the sedimentary tidal environment in Mont-Saint-Michel Bay (The Channel, France), Comptes Rendus de l'Académie des sciences Paris, Earth and Planetary Sciences, 327: 307-311. Elder, J.H., 1959. Dispersion of marked fluid in turbulent shear flow, Fluid Mechanics, 5: 544-560. Friedrichs, H., 2004. Flow-induced effects of macrozoobenthic structures on the near-bed sediment transport, PhD thesis, Universität Rostock, 96p. Guérin, L., 2004. La crépidule en rade de Brest: un modèle biologique d'espèce introduite proliférante en réponse aux fluctuations de l'environnement, PhD thesis, Université de Bretagne Occidentale, 426 p. Guillou, N., Chapalain, G. and Leprêtre, A., 2010. Interpolation Spatiale des Distributions Granulométriques des Sédiments de Fond, CETMEF, 45p. Hervouet, J.M., 2007. Hydrodynamics of Free Surface Flows, John Wiley and sons. Le Berre, I., 1999. Mise au point de méthodes d'analyse et de représentation des interactions complexes en milieu

200


littoral, PhD thesis, Université de Bretagne Occidentale, 236 p. Lettau, H., 1969. Note on aerodynamic roughness-parameter estimation on the basis of roughness-element description, Journal of Applied Meteorology, 8:828-832. Mitchener, H. and Torfs, H., 1996. Erosion of sand/mud mixtures, Journal of Coastal Engineering, 29: 1-25. Monbet, Y. and Bassoulet, P., 1989. Bilan des connaissances océanographiques en rade de Brest, CEA/IPSN. Moulin, F., Guizien, K., Thouzeau, G., Chapalain, G., Mülleners, K. and Bourg, C., 2007. Impact of the invasive species, Crepidula fornicata, on the hydrodynamic and transport properties of the benthic boundary layer, Aquatic Living Resources, 15: 15-31. Partheniades, E., 1965. Erosion and deposition of cohesive soils, ASCE Journal of the Hydraulic Division, 91:105-139. Raupach, M.R., 1992. Drag and drag partition on rough surfaces, Boundary-Layer Meteorology, 60: 375-395. Smith, J. and McLean, S., 1977. Spatially averaged flow over a wavy surface, Journal of Geophysial Research, 82:1735-1746. Soulsby, R.L., 1997. Dynamics of Marine Sand: a Manual for Practical Applications, Telford, 249p. Troadec, S., 1992. Estimation de la consommation chez la crépidule (Crepidula fornicata), Stage IUT, IFREMER. Troadec, P. and Le Goff, R., 1997. Contrat de Baie de la rade de Brest, Communauté Urbaine de Brest. Villaret, C., Hervouet, J.M., Kopmann, R., Merkel, U. and Davies, A.G., 2011. Morphodynamic modelling using the Telemac finite-element system, Computers and Geosciences, 53: 105-113.

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