Seawater Intrusion in Semi-Closed Convergent Estuaries (Case Study of Moroccan Atlantic Estuaries): Application of Salinity Analytical Models

ABSTRACT

Salinity is an important parameter influencing the water quality of estuaries, and can constitute a serious problem to society due to the need for freshwater for industry and agriculture. Therefore, the determination of salt intrusion length in estuaries is a challenge for managers as well as scientists in this field. The managers tend to use simple and reliable tools for salinity variation. Although 2-D and 3-D numerical models are common tools for the prediction of salinity intrusion now, analytical models of salinity variation are much more efficient, and also require minimal sets of river data. In this paper, two analytical solutions, Brockway and Savenije used worldwide to assess longitudinal salinity variation in alluvial estuaries, are applied to the Moroccan Atlantic semi-closed estuaries, i.e., Sebou and Loukkos. The solutions are derived from salt convection-dispersion equations, with different assumptions for the dispersion coefficient. The estuaries' bathymetry is described by an exponential function. The performance of these two solutions was evaluated by comparing their results with field-measured salinity data. The Brockway model's salinity predictions performs well to observations especially in downstream reaches of the two estuaries (Sebou: R² = 0.95, root mean square error [RMSE] = 1.50‰, normalized root mean square error [NRMSE] = 3.45‰; Loukkos: R² = 0.95, RMSE = 1.13‰, NRMSE = 3.01‰), while the Savenije model outperformed the Brockway's model and is better to predict salt intrusion length and salinity variation along the two estuaries (Sebou: R² = 0.97, RMSE = 1.15‰, NRMSE = 2.85‰; Loukkos: R² = 0.98, RMSE = 0.95‰, NRMSE = 1.94‰). This revealed that both analytical solutions apply well to the estimation of salinity variation and the prediction of salt intrusion in these two estuaries.

KEYWORDS:

• Analytical solutions
• dispersion coefficient
• Loukkos estuary
• Moroccan convergent estuaries
• salinity intrusion
• Sebou estuary

Acknowledgements

The authors gratefully acknowledge two anonymous reviewers for their scientific suggestions and constructive comments. The authors also are indebted to Prof. W. Rockwell Geyer (Applied Ocean Physics & Engineering Dept. Woods Hole Oceanographic Institution) for numerous discussions.