The calculation of estuarine flushing times in convergent estuaries using fresh-water fraction method

ABSTRACT

The flushing time in an estuary is important for water quality analysis, and it is one of the major transport time scales used in estuaries to quantify the hydrodynamic processes and for water resources management strategies. The purpose of this paper is to investigate the influence of tidal amplitude, river discharge, and salinity stratification on the spatially varying flushing time of the two selected Moroccan Atlantic semi-closed estuaries, i.e. Bouregreg and Loukkos. A freshwater fraction method (FFM) was used to calculate flushing time for both estuaries using a spatially varying freshwater fraction. The results showed that the flushing time varies from 2.7 to 6.3 days for a river discharge ranges from 17 to 22 m³s⁻¹ during spring tide, and from 3.4 to 11.2 days for a river discharge ranges from 17 to 31 m³s⁻¹ during neap tide in Bouregreg estuary, and varies from 10 to 13.6 days for a river discharge ranges from 3.47 to 5.12 m³s⁻¹ during spring tide, and from 9.11 to 13.1 days for a river discharge ranges from 3.47 to 5.48 m³s⁻¹ at neap tide in Loukkos estuary. In addition, the flushing time near the mouth for both estuaries is shorter during spring tide than during neap tide because of the larger amplitude of the spring tide. At the estuary mouth (e.g. Bouregreg estuary), the potential energy anomaly of the water column is negligible (<4 J m⁻³).

KEYWORDS:

• Moroccan estuaries
• flushing time
• freshwater fraction method (FFM)
• potential energy anomaly
• spring–neap tides

Acknowledgements

The authors gratefully acknowledge two anonymous reviewers for their scientific suggestions and constructive comments. Also, the authors would like to express their gratitude to: Mr, Amine ES-Saidi and some colleagues during field observation; for their availability and help.

Disclosure statement

No potential conflict of interest was reported by the author(s).