1
Introduction Understanding the effect of Posidonia oceanica on near-shore dynamics is a challenging area with devoted research basically boosted not before the late 80ies-early 90ies, but even now many processes have not been discerned. Major impacts identified are the reduction of flow speed, the modification of flow and turbulence structure, attenuation of wave energy, thus obviously local modification of sediment transport. The usage of numerical models to simulate such impacts is still at an early stage, and most available parameterizations rely on few laboratory experiments. The approaches used up to now are basically two, and both work on tuning the drag coefficient of the bottom friction: Kobayashi-type models and Nepf-type models. The first approach is based on the work of Kobayashi et al. (1993), and is focused on dynamics due to waves only, providing a friction coefficient responsible of wave dissipation based on (wave) Reynolds number. The approach has some limitations, for example the flat, constant bottom, the very high density of -rigid- canopy as well as the absence of wave breaking. The second approach is based instead only on laboratory experiments with steady flow (no waves), and it is based on the work of Nepf (1999). She assumed that the bottom friction due to seagrass meadows is function of canopy density, and in fact she identified two different regimes with drag coefficient density-dependent and density-independent. One has to wait till the year 2001 for the first publication investigating the bottom friction for both wave and flow at the same time (Teeter et al., 2001), again however, with very local meanings of the tuning of parameterizations and using fixed values for wave dissipation instead of canopy density dependent values. Very recently, few papers addressed the issue of modeling, in a pretty much realistic way, seagrass meadows impact on flow-wave-sediment dynamics in the near-shore (e.g. Chen et al., 2007). The approach used is basically Nepf-type, and it is focussed on the estimate of the bottom drag coefficient for current and, with similar formalism, for waves. The main reason for the choice of this approach is the canopy density range (Nepf results are based on relatively low density range, while Kobayashi very high). Chen et al. 2007 approach Characterising seagrass meadows, a key parameter is the fractional volume occupied by canopies (ad): where d is the shoot diameter, n is the shoot density, l is the shoot height and h is the water depth. Originally, in Nepf (1999) ad was defined without the term l/h, now introduced by Chen et al. (2007) in order to account for submersion of canopies. Following Chen et al. (2007), the formulation for the drag coefficient for currents is: where Cd is the drag coefficient used for bottom boundary layers calculation, C B is the skin friction drag (set to 0.001) –the drag exerted at the water-sediment interface-, C D is the form drag, the drag due to obstacles (seagrass leaves), expressed following originally Nepf (1999) but then modified by Chen et al. (2007) as: Cd 1.17, if 10-3 > ad > 10-2 (shoot density-independent range); Cd -0.255ln(ad), if 10-2 > ad > 10-1 (shoot density-dependent range); The value of Cd is referenced to a height of 7,5 cm (Nepf, 1999), but we need to recalculate it at the reference depth of the lowermost model level, so we used the classical logarithmic profile for this. This does not mean that the bottom boundary layer in case of seagrass meadows is logarithmic (which should be not the case), but this approximation wouldn’t be larger than uncertainties associated to the estimate of Cd. In the case of waves, Chen et al. (2007) adopted: With f the wave friction factor, f B the wave skin friction factor and the wave form drag friction factor. Implementing Posidonia oceanica effects in ROMS J Chiggiato a M Sclavo a S Carniel a , JC Warner b , RP Signell b a Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Venezia b USGS Woods Hole, MA-USA Case Study along the Western Italian coastline This implementation is being used to study the effect of Posidonia oceanica on the nearshore dynamics along the Gulf of Terracina (Western Italian Coastline, Tyrrhenian Sea). Few experiments have been carried out with a idealized model setup. We plan to run more experiments in order to have an complete picture of model sensitivity. It is presented here one experiment, using a model domain linearized compare to the actual Gulf of Terracina, with Posidonia oceanica meadow located at 20 m depth and alongshore/crosshore extension similar to the realistic position, with shoot density of 300 shoots/m 2 , shoot diameter of 0.02 m and shoot heigth of 0.6 m, which leads to ad = 3x10 -3 . The idealized case include a moderate/weak wind-wave storm coming from 235° N (SW sector), with wind 5 m/s, wave at the boundary of 2 m significant height and 6 s mean period. The bathymetry (figure 1), is a regular grid composed by 52 x 102 points along a domain of 5000 x 2500 m (therefore a resolution of 100 x 25 m) and three open boundaries (E-S-W). The Posidonia oceanica meadow is located between depths 17 and 20 m, with cross-shore extension of 250 m and alongshore of 2000 m. A simulation without Posidonia oceanica (control run) but with the very same physical setting has been carried out as well, as comparison. Final Considerations ROMS is being used to investigate the near-shore dynamics and seagrass meadows impact along the Terracina Gulf (Italy). The model has been succesfully run in a semi realistic way simulating the two most severe wave storms along the Terracina littoral. Running the model in a fully realistic way is straightforward since it is only needed to use realistic forcing (wind, tides) as well as the large scale general circulation through the boundary in order to carry out such simulations. The model has been also set up in an idealized configuration testing the role of seagrass meadows in modifying the flow-wave dynamics. The idealized configuration was due to carry out multiple runs with lower computational effort compared to the realistic implementation, however posidionia oceanica will be implemented easily also in the realistic configuration. On the other hand, the are some relevant issues that need to be addressed before fully trusting overall results when using the inclusion of posidonia oceanica in the model. • The scaling term shoot length over water depth (l/h) in the calculation of the drag coefficient Cd has been tested in the literature for emerging or weakly submerging canopies (0.75 < l/h < 1), while we are using deeply submerged canopies (l/h ~ 0.025); It may be helpful to let ROMS and SWAN exchange Z 0 ; • The estimate of the wave dissipation requires a necessary validation using local data and of course validation of results will be a necessary step in the evaluation of the model performance and goodness of the parameterizations included. d h l d n ad d h ad C C ad Cd D B 2 1 1 d h ad f f ad f B 2 1 1 Preliminary Implementation in ROMS The steps required are: 1) To build a Seagrass_Mask which is zero outside seagrass meadows and is instead function of ad inside seagrass meadows. Following steps do not apply when Seagrass_Mask is zero; 2) To estimate Cd based on fractional volume data of posidonia oceanica; 3) To convert Cd at the reference height of the lowermost ocean model level; 4) To estimate the bottom roughness Zo; 5) To use Zo in the wave-current bottom boundary layer by Madsen (1994) in ROMS in order to estimate stresses exerted on currents and wave-currents systems; 6) To use the value of the equivalent Nikoradse roughness length Kn (Kn = 30Zo) to feed the bottom friction induced dissipation in SWAN. This value, function of Z 0 is estimated before the simulation and following Chen and then held fixed. Preliminary Results The circulation depicted is broadly anticiclonic, according to the SW storm (figure 2). Note that Posidonia oceanica meadow causes a reduction in flow speed in the downstream direction. The impact is relevant on the bottom currents, where the flow speed is reduced by up to 50% along the meadow. Still some reduction is observed on surface currents, of course to a much lesser extent. One of the key parameter in this computation of the ‘volume flux reduction’, that is: Where U is the velocity downstream the meadow from the run using Posidonia oceanica and the velocity from the control run. This resulted in a reduction of some 25%, which, indeed, seems a fairly large value compared to the literature. Impact on the significant wave height (figure 3) is less evident, just a few centimetres over 1.5 m height. In particular it can be noticed that the wave height slightly increases along the meadow, probably a consequence of the wave-current interaction. In this case, a relevant parameter is the ‘wave energy flux reduction’, that is: F=ECg Where E is the wave energy density and Cg the group velocity, and F WITH , F WITHOUT is the energy flux downstream the meadow estimated respectively by the run using Posidonia oceanica or by the control run. The wave energy flux reduction obtained is 4%, consistent with the deep submersion of the canopy (0.6 m height over 20 m depth). 100 1 dz U dz U VFR CONTROL POSIDONIA 100 1 WITHOUT WITH F F WFR Towards the realistic simulation The coupled system ROMS/SWAN has been run in two different extreme event scenarios. This application has been carried out using a realistic bathymetry, obtained merging data provided by Ente Regione Lazio at two different resolutions. The resulting bathymetry is shown in figure 4. The computational grid is divided into 266 x 200 points, with variable resolution (130÷170 m along longitude, 60÷150m along latitude, with higher resolution in the actual areas covered by Posidonia oceanica) and 10 linear terrain following levels on the vertical. The two tests simulate two extreme events from SW or SE with wind 10 m/s and 5 m wave heigth (10 sec period) applied to SWAN boundary. ROMS was able to simulate with very high resolution such extreme wind-wave storm event (figure 5). This model set up will be run in hindcast mode, including the effect of Posidonia Oceanica. Results on current and wave field will be compared to ADCP data collected during BEACHMED-e May-June 2007 campaign along the Gulf of Terracina (figure 6) References Chen S. N., Sanford L. P., Koch E. W., Shi F. and North E. W. (2007) A nearshore model to investigate the effects of seagrass bed geometry on wave attenuation and suspended sediment transport. Estuaries and Coasts, 30:296-310; Kobayashi, N., Raichle A. W. and Asano T. (1993) Wave attenuation by vegetation. Journal of Waterway Port Coastal and Oceanic Engineering-ASCE 119:30-48; Nepf, H. M. (1999) Drag, turbulence and diffusion in flow through emergent vegetation. Water Resources Research, 35:479-489; Madsen, O. S. (1994) Spectral wave-current bottom boundary layer flows. Coastal Engineering 1994. Proceedings, 24th International Conference Coastal Engineering Research Council, 384-398; Teeter A. M., Johnson B. H., Berger C., Stelling G., Scheffner N. W., Garcia M. H., and Archure T. M. (2001) Hydrodynamic and sediment transport modelling with emphasis on shallow water, vegetated areas (lakes, reservoirs, estuaries and lagoon). Hydrobiologia 444:1-24. Acknowledgments This research is being carried out within the European Project BEACHMED-e in the framework of the research contract between CNR-ISMAR and Università di Roma “La Sapienza” Figure 1 Idealized Bathymetry Figure 2 Surface currents (left panel) and bottom current (right panel) from experiment with posidonia (top), without posidonia (middle), and the differences between the two (bottom) Figure 3 Significant wave heigth from experiment with posidonia (top), without posidonia (bottom) Figure 4 Realistic bathymetry of the Gulf of Terracina (Italy) Figure 5 Results from the successfully semirealisti c test with ROMS_SED stressed to simulate extreme wave storm scenario Figure 6 Significant wave heigth data collected during May 2007 by two ADCPs; one more offshore with respect to the meadow (black line) and one more inshore (red line) f

Introduction Understanding the effect of Posidonia oceanica on near-shore dynamics is a challenging area with devoted research basically boosted not before

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Page 1: Introduction Understanding the effect of Posidonia oceanica on near-shore dynamics is a challenging area with devoted research basically boosted not before

Introduction

Understanding the effect of Posidonia oceanica on near-shore dynamics is a challenging area with devoted research basically boosted not before the late 80ies-early 90ies, but even now many processes have not been discerned. Major impacts identified are the reduction of flow speed, the modification of flow and turbulence structure, attenuation of wave energy, thus obviously local modification of sediment transport. The usage of numerical models to simulate such impacts is still at an early stage, and most available parameterizations rely on few laboratory experiments.

The approaches used up to now are basically two, and both work on tuning the drag coefficient of the bottom friction: Kobayashi-type models and Nepf-type models. The first approach is based on the work of Kobayashi et al. (1993), and is focused on dynamics due to waves only, providing a friction coefficient responsible of wave dissipation based on (wave) Reynolds number. The approach has some limitations, for example the flat, constant bottom, the very high density of -rigid- canopy as well as the absence of wave breaking. The second approach is based instead only on laboratory experiments with steady flow (no waves), and it is based on the work of Nepf (1999). She assumed that the bottom friction due to seagrass meadows is function of canopy density, and in fact she identified two different regimes with drag coefficient density-dependent and density-independent.

One has to wait till the year 2001 for the first publication investigating the bottom friction for both wave and flow at the same time (Teeter et al., 2001), again however, with very local meanings of the tuning of parameterizations and using fixed values for wave dissipation instead of canopy density dependent values. Very recently, few papers addressed the issue of modeling, in a pretty much realistic way, seagrass meadows impact on flow-wave-sediment dynamics in the near-shore (e.g. Chen et al., 2007). The approach used is basically Nepf-type, and it is focussed on the estimate of the bottom drag coefficient for current and, with similar formalism, for waves. The main reason for the choice of this approach is the canopy density range (Nepf results are based on relatively low density range, while Kobayashi very high).

Chen et al. 2007 approach

Characterising seagrass meadows, a key parameter is the fractional volume occupied by canopies (ad):

where d is the shoot diameter, n is the shoot density, l is the shoot height and h is the water depth. Originally, in Nepf (1999) ad was defined without the term l/h, now introduced by Chen et al. (2007) in order to account for submersion of canopies.

Following Chen et al. (2007), the formulation for the drag coefficient for currents is:

where Cd is the drag coefficient used for bottom boundary layers calculation, CB is the skin friction drag (set to 0.001) –the drag exerted at the water-sediment interface-, CD is the form drag, the drag due to obstacles (seagrass leaves), expressed following originally Nepf (1999) but then modified by Chen et al. (2007) as:

Cd 1.17, if 10-3 > ad > 10-2 (shoot density-independent range); Cd -0.255ln(ad), if 10-2 > ad > 10-1 (shoot density-dependent range);

The value of Cd is referenced to a height of 7,5 cm (Nepf, 1999), but we need to recalculate it at the reference depth of the lowermost model level, so we used the classical logarithmic profile for this. This does not mean that the bottom boundary layer in case of seagrass meadows is logarithmic (which should be not the case), but this approximation wouldn’t be larger than uncertainties associated to the estimate of Cd.

In the case of waves, Chen et al. (2007) adopted:

With f the wave friction factor, fB the wave skin friction factor and the wave form drag friction factor.

Implementing Posidonia oceanica effects in ROMS

J Chiggiato a M Sclavo a S Carniel a, JC Warner b, RP Signell b

a Istituto di Scienze Marine, Consiglio Nazionale delle Ricerche, Venezia b USGS Woods Hole, MA-USA

Case Study along the Western Italian coastline

This implementation is being used to study the effect of Posidonia oceanica on the nearshore dynamics along the Gulf of Terracina (Western Italian Coastline, Tyrrhenian Sea). Few experiments have been carried out with a idealized model setup. We plan to run more experiments in order to have an complete picture of model sensitivity. It is presented here one experiment, using a model domain linearized compare to the actual Gulf of Terracina, with Posidonia oceanica meadow located at 20 m depth and alongshore/crosshore extension similar to the realistic position, with shoot density of 300 shoots/m2, shoot diameter of 0.02 m and shoot heigth of 0.6 m, which leads to ad = 3x10-3.The idealized case include a moderate/weak wind-wave storm coming from 235° N (SW sector), with wind 5 m/s, wave at the boundary of 2 m significant height and 6 s mean period. The bathymetry (figure 1), is a regular grid composed by 52 x 102 points along a domain of 5000 x 2500 m (therefore a resolution of 100 x 25 m) and three open boundaries (E-S-W). The Posidonia oceanica meadow is located between depths 17 and 20 m, with cross-shore extension of 250 m and alongshore of 2000 m. A simulation without Posidonia oceanica (control run) but with the very same physical setting has been carried out as well, as comparison.

Final Considerations

ROMS is being used to investigate the near-shore dynamics and seagrass meadows impact along the Terracina Gulf (Italy). The model has been succesfully run in a semi realistic way simulating the two most severe wave storms along the Terracina littoral. Running the model in a fully realistic way is straightforward since it is only needed to use realistic forcing (wind, tides) as well as the large scale general circulation through the boundary in order to carry out such simulations. The model has been also set up in an idealized configuration testing the role of seagrass meadows in modifying the flow-wave dynamics. The idealized configuration was due to carry out multiple runs with lower computational effort compared to the realistic implementation, however posidionia oceanica will be implemented easily also in the realistic configuration.

On the other hand, the are some relevant issues that need to be addressed before fully trusting overall results when using the inclusion of posidonia oceanica in the model.

• The scaling term shoot length over water depth (l/h) in the calculation of the drag coefficient Cd has been tested in the literature for emerging or weakly submerging canopies (0.75 < l/h < 1), while we are using deeply submerged canopies (l/h ~ 0.025);

• It may be helpful to let ROMS and SWAN exchange Z0;

• The estimate of the wave dissipation requires a necessary validation using local data and of course validation of results will be a necessary step in the evaluation of the model performance and goodness of the parameterizations included.

dh

ldnad

d

hadCCadCd DB 2

11

d

hadffadf B 2

11

Preliminary Implementation in ROMS

The steps required are:

1) To build a Seagrass_Mask which is zero outside seagrass meadows and is instead function of ad inside seagrass meadows. Following steps do not apply when Seagrass_Mask is zero;

2) To estimate Cd based on fractional volume data of posidonia oceanica;

3) To convert Cd at the reference height of the lowermost ocean model level;

4) To estimate the bottom roughness Zo;

5) To use Zo in the wave-current bottom boundary layer by Madsen (1994) in ROMS in order to estimate stresses exerted on currents and wave-currents systems;

6) To use the value of the equivalent Nikoradse roughness length Kn (Kn = 30Zo) to feed the bottom friction induced dissipation in SWAN. This value, function of Z0 is estimated before the simulation and following Chen and then held fixed.

Preliminary Results

The circulation depicted is broadly anticiclonic, according to the SW storm (figure 2). Note that Posidonia oceanica meadow causes a reduction in flow speed in the downstream direction. The impact is relevant on the bottom currents, where the flow speed is reduced by up to 50% along the meadow. Still some reduction is observed on surface currents, of course to a much lesser extent. One of the key parameter in this computation of the ‘volume flux reduction’, that is:

Where U is the velocity downstream the meadow from the run using Posidonia oceanica and the velocity from the control run. This resulted in a reduction of some 25%, which, indeed, seems a fairly large value compared to the literature.

Impact on the significant wave height (figure 3) is less evident, just a few centimetres over 1.5 m height. In particular it can be noticed that the wave height slightly increases along the meadow, probably a consequence of the wave-current interaction. In this case, a relevant parameter is the ‘wave energy flux reduction’, that is:

F=ECg

Where E is the wave energy density and Cg the group velocity, and FWITH, FWITHOUT is the energy flux downstream the meadow estimated respectively by the run using Posidonia oceanica or by the control run. The wave energy flux reduction obtained is 4%, consistent with the deep submersion of the canopy (0.6 m height over 20 m depth).

1001

dzU

dzUVFR

CONTROL

POSIDONIA

1001 WITHOUT

WITH

F

FWFR

Towards the realistic simulation

The coupled system ROMS/SWAN has been run in two different extreme event scenarios. This application has been carried out using a realistic bathymetry, obtained merging data provided by Ente Regione Lazio at two different resolutions. The resulting bathymetry is shown in figure 4. The computational grid is divided into 266 x 200 points, with variable resolution (130÷170 m along longitude, 60÷150m along latitude, with higher resolution in the actual areas covered by Posidonia oceanica) and 10 linear terrain following levels on the vertical. The two tests simulate two extreme events from SW or SE with wind 10 m/s and 5 m wave heigth (10 sec period) applied to SWAN boundary. ROMS was able to simulate with very high resolution such extreme wind-wave storm event (figure 5).

This model set up will be run in hindcast mode, including the effect of Posidonia Oceanica. Results on current and wave field will be compared to ADCP data collected during BEACHMED-e May-June 2007 campaign along the Gulf of Terracina (figure 6)

ReferencesChen S. N., Sanford L. P., Koch E. W., Shi F. and North E. W. (2007) A nearshore model to investigate the effects of seagrass bed geometry on wave attenuation and suspended sediment transport. Estuaries and Coasts, 30:296-310;

Kobayashi, N., Raichle A. W. and Asano T. (1993) Wave attenuation by vegetation. Journal of Waterway Port Coastal and Oceanic Engineering-ASCE 119:30-48;Nepf, H. M. (1999) Drag, turbulence and diffusion in flow through emergent vegetation. Water Resources Research, 35:479-489;

Madsen, O. S. (1994) Spectral wave-current bottom boundary layer flows. Coastal Engineering 1994. Proceedings, 24th International Conference Coastal Engineering Research Council, 384-398;Teeter A. M., Johnson B. H., Berger C., Stelling G., Scheffner N. W., Garcia M. H., and Archure T. M. (2001) Hydrodynamic and sediment transport modelling with emphasis on shallow water, vegetated areas (lakes, reservoirs, estuaries and lagoon). Hydrobiologia 444:1-24.

Acknowledgments

This research is being carried out within the European Project BEACHMED-e in the framework of the research contract between CNR-ISMAR and Università di Roma “La Sapienza”

Figure 1Idealized Bathymetry

Figure 2Surface currents (left panel) and bottom current (right panel)

from experiment with posidonia (top), without posidonia (middle), and the differences between the two (bottom)

Figure 3Significant wave heigth from experiment with posidonia (top),

without posidonia (bottom)

Figure 4Realistic bathymetry of the Gulf

of Terracina (Italy)

Figure 5Results from the

successfully semirealistic test

with ROMS_SED stressed to

simulate extreme wave storm

scenario

Figure 6Significant wave heigth data

collected during May 2007 by two ADCPs; one more offshore

with respect to the meadow (black line) and one more inshore

(red line)

f