Nador lagoon is a coastal system connected to the sea through a narrow and shallow inlet; understanding its hydraulic performance is required for its design and operation. This paper investigates the hydrodynamic impacts of the whole lagoon due to tidal waves using a numerical approach. In this study we use a two-dimensional, depth-averaged hydrodynamic model based on so-called shallow water equations solved within triangular mesh by a developed efficient finite volume method. The method was calibrated and validated against observed data and applied to analyze and predict water levels, tidal currents, and wind effects within the lagoon. Two typical idealized scenarios were investigated: tide only and tide with wind forcing. The predicted sea surface elevations and current speeds have been presented during a typical tidal period and show correct physics in different scenarios.
An understanding of the physical oceanography of coastal areas provides a foundation for the study of processes such as hydrodynamics, as well as a basis for effective management of the coastal zone. Integrated water management of endangered coastal areas could be able to restore their ecosystems. Numerical models have been developed and applied to coastal areas, in order to simulate hydrodynamic and environmental processes. These models constitute an administrative tool for decision makers in order to apply the right measures to restore the endangered coastal environments.
Coastal lagoons are areas of shallow, coastal water, wholly or partially separated from the sea by sandbanks, shingle, or, less frequently, rocks. Lagoons show a wide range of geographical and ecological variations. The most important of them in Moroccan coasts is Nador lagoon.
Nador lagoon is located on eastern coast; recently, it has been the subject of many investigations on water quality, currents, flora, fauna, fishing, and aquaculture [
In the literature there are some examples of hydrodynamic estimation in coastal lagoon, among others; Brenon et al. [
The aim of this paper is the application of a developed 2D finite volume method to the Nador lagoon, based on the well-established shallow water system including bathymetric forces, Coriolis effects, friction terms, and eddy-diffusion stresses, simulating the impact of wind and tidal waves on the hydrodynamics circulation in Nador lagoon; here the flow is forced by the components of semidiurnal tidal at one real inlet. Recently, the same model has been widely used as shown, for example, in Lovato et al. [
The Nador lagoon is the second lagoon complex of northern Africa (115 km2), the broadest paralic environment of Morocco, and the only one located along the Mediterranean coast of this country. It comprises a broad area bounded to the northwest by the Beni-Ensar city, to the southeast by the village of Kariat Arekmane, and to the southwest by the northern extremity of the Bou-Areg plain (Figure
Location and bathymetry of the Nador lagoon study area.
State the relative shallowness of the lagoon in relationship to its surface area and its length; inviscid shallow water equations were used to simulate sea surface elevations, current fields due to tides, and storm surge and investigate the responsible forcing mechanisms [
Based on the simplifications described above, the primitive form of depth-integrated governing equations includes a continuity equation and momentum equation in each of the
The surface stress
In the present study, a numerical model has been used to simulate the hydrodynamics behavior of Nador lagoon. The finite volume method is used to solve governing equations (
Nador lagoon mesh used in computational model and schematization of an example cell-centered finite volume.
To simplify, the above hydrodynamic equations can be written in a matrix form as follows:
Herein
The evaluation of source terms in (
Finally, the stability criterion adopted has followed the usual in explicit finite volumes; the time step is set according to the Courant-Friedrichs-Lewy (CFL) criterion equal to 0.65.
The numerical computation has been carried out on a spatial domain that represents the lagoon of Nador through a finite volume grid which consists of 8075 triangular elements and 14042 nodes. The bathymetry of the lagoon, obtained by combining several data sets, has been interpolated onto the grid. The finite volume method allows for high flexibility with its subdivision of the numerical domain in triangles varying in form and size. It is especially suited to reproduce the geometry and the hydrodynamics of complex shallow water basins such as the Nador lagoon. The principal hydraulic forcing of the Nador lagoon is the tide and the wind. The main astronomical tidal constituents in this lagoon are semidiurnal
Using the numerical model must begin with calibration and verification by means of adjusting Manning’s roughness coefficient. This strategy was done using tidal flow at the inlet, extrapolated from harmonic analysis of measured flow in Beni-Ensar harbour, for two days (approximately four tidal cycles). One set of data (12 to 14 May 2014) was used for model calibration and another separate set (20 to 22 May 2014) for model verification. Typical predicted and observed values for sea surface elevation SSE versus time are presented in Figure
Predicted and observed SSE versus time for validation (b) and verification (a) period.
In order to achieve a stable time-periodic solution, the model was run for further 5 days, forced by
Table
Parameters of the hydrodynamic model.
Parameter | Symbol | Value |
---|---|---|
Time step |
|
|
Water density |
|
1025 Kg⋅m−3 |
Air density |
|
1.225 Kg⋅m−3 |
Wind drag coefficient |
|
|
In the first case only the tidal forcing is considered and no wind is prescribed. The numerical simulations are presented in Figure
Time sea surface level in Nador lagoon during a typical period.
To quantify the speed spreading of the waves and more behavior understanding, velocity fields and their magnitudes are presented for the hole basin in Figure
Velocity fields and magnitudes corresponding to those in Figure
Time series of computed current velocity (top) at Boukhana inlet in a typical tidal period.
From environmental viewpoint, certainly, the spatial variability in water circulation was controlled by the intricate geometry of the lagoon, which influences and modifies the current pattern, but also, this result shows a relation to the current tide structure which, during the period analyzed, was controlled by sea water. The spatial variability of water circulation has a noticeable influence in the risk assessment of water pollution in the lagoon. Thus, careful characterization of water renewal is necessary, in order to implement a methodology of risk assessment for environmental management.
In this study, the wind is imposed from the east on one hand and the west on the other hand. Its intensity is 2 m/s−1. When the tidal forcing is supplemented by the wind action, the lagoon circulation changes radically. Figure
Comparison of velocity fields and magnitudes in Nador lagoon for the wind cases.
This paper has presented the application of comprehensive hydrodynamic numerical procedure, specifically conceived for shallow water modelling, to the Nador lagoon. Through this numerical model, simulations of the effect of tide and wind on water current of the lagoon are carried out. The numerical results show correct physics in different test regimes. The influence of different winds forcing on the water circulation has also been discussed. Nevertheless, flows in such complex domains can be computed, providing correct physics without the need for generating adaptive grids or complicated reconstruction of numerical fluxes. Overall, the method shows reasonable accuracy while ensuring the required properties of the shallow water flows. Finally, much more efforts are required. The model calibration with experimental or observed data will be a challenge for future studies.
Total depth from the sea bed to the free surface (m)
Cartesian components of depth-averaged velocity (m/s)
Bed elevation above a fixed horizontal datum (m)
Acceleration due to gravity (m/s2)
Water density (kg/m3)
Air density (kg/m3)
Components of wind speed (m/s)
Bed shear stress components
Free surface shear stress components
Bed friction coefficient
Wind drag coefficient
Time (s)
Cartesian horizontal distances from origin.
The authors declare that there is no conflict of interests regarding the publication of this paper.