Seagrass canopy dynamics

The seagrass-current interaction has been successfully simulated by Verduin and Backhaus (2000) by means of coupling an ocean circulation model (HAMSOM) and a Lagrangian tracer model. The model set-up was basically two-dimensional with a vertical and a horizontal coordinate. A harmonic swell wave travelling into the direction of the positive $x$-coordinate had been specified at one open boundary.

The seagrass was represented by passive Lagrangian tracers which freely followed the flow as long as they were located inside prescribed excursion limits. The movement was simply frozen when the excursion limit was reached and the flow tendency was to carry them even further out. Only in that situation, the seagrass tracers had an effect on the current speed by exerting a quadratic friction on the flow.

The basic result of Verduin and Backhaus (2000) for a location inside the seagrass meadow was, that the mean kinetic energy had a local maximum just above the upper reach of the seagrass. That was found to be in good agreement with field measurements.

It is interesting to perform the following two experiments:

A:

Now extra turbulence is produced by leaf-current friction, $\alpha=0$.

B:

All friction losses between leaves and current are converted to turbulence, $\alpha=1$.

The results for these two experiments are shown in Burchard and Bolding (2000). The sensivity to $\alpha$ seems to be small, only the profiles of averaged turbulent kinetic energy are significantly influenced. The results of Verduin and Backhaus (2000) are basically reproduced. Especially, the local maximaum of mean kinetic energy just above the upper reach of seagrass is well simulated. The only striking difference is that in our model the seagrass shows an asymmetry for the excursion which is following the residual transport caused by the waves travelling from left to right.

Data files:

seagrass.dat

height above bed in m, excursion limit in m, friction coefficient in m$^{-1}$.