# Hydrodynamic Modelling in a Mediterranean Coastal Lagoon--The Case of the Stagnone Lagoon, Marsala
## 4 Conclusions  
The aim of this study is to increase the knowledge about hydrodynamic circulation in Mediterranean coastal lagoons like the Stagnone, testing the predictivity capacity of a 2 DH model like MIKE21. These types of model work by averaging the vertical velocity along the depth, and, for this reason, using field vertical punctual measurement as a benchmark, the velocity's NSE cannot be expected to be high. Moreover, focusing on the recorded values of the Vx component of velocity, it is possible to underline how this component is in a range around zero, and this confirms the predominant importance of the velocity's Vy component. The sensitivity analysis conducted has been crucial to understanding the lagoon behaviour, since the presence of the vegetation induced a complex estimation of the bed friction factor; in fact, _Posidonia oceanica_ and _Cymodcea nodosa_ leaves induce a particular roughness in the lagoon bed. Moreover, in the northern area, where there is a lower ratio between vegetation height and water depth with respect to the southern area, the vegetation leaves work differently on the hydrodynamics. Several values of roughness coefficient have been tested, establishing the optimum as \(\mathrm{Ks}=20\)-\(25\,\mathrm{m}^{\ddagger}/\mathrm{s}\) for the 2006 field campaign, confirmed by the simulation of the 2007 field campaign. The rate of accordance between recorded and simulated values can be estimated by reading Table 1 in Appendix A, where RMSE and \(\mathrm{R}^{2}\) are reported. The wind friction factor has been also investigated. All the variables show a dependence on the interaction between the air and the water surface; in fact, the simulation without the wind force returns lower NSE values. A general agreement between the simulated and recorded values can be found using a constant value of wind friction factor equal to \(f=0.001\)-\(0.002\). But in the northern area, at different times, the simulated water velocities are directed towards the south, with the wind, while the recorded ones are directed towards the north, with the rising tide from the south mouth. The eddy viscosity coefficient does not appear to play a valuable role in the hydrodynamic circulation. For the different tested values, there is no appreciable effect on simulated water level and velocities. Finally, focusing on the variation in the simulated water depth time series, as well as the Vy and Vx velocity components, it is necessary to underline that the model reproduces the seiches phenomena with difficulty. In the northern areas, the hydrodynamism of the Stagnone Lagoon is poorly described by the model, and the wind effect is overestimated. Due to the presence of relative high vegetation in the northern area, the motion field is strongly dominated by tides as registered by in situ data. The presence of vegetation in the form of long leaves induces a strong damping effect that cannot be explicated by a common bed roughness coefficient. Nevertheless, the model produces more reliable values in areas closer to the south mouth, as the NSE for water depth and the Vy velocity component suggest. In the southern area, the 2 DH model looks more capable of replicating the variability in the water depth and water velocity. Moving northward, the model predictivity becomes more difficult. This implies that all the NSE values are higher when they refer to stations deployed closer to the south mouth. The Vx velocity component looks poorly replicated, in all cases returning NSE not acceptable, always with negative values. This study shows how the lagoon circulation is affected by some problems and the dependence on tide and wind regime. Finally, seiches also play a key role in the lagoon circulation, as well as more complex bed roughness effects. The Stagnone Lagoon is a vulnerable environment with morphological complexity; the circulation is dominated by external forces recorded in two different field campaigns.