Oualidia lagoon, Morocco: an estuary without a river

Abstract

This study describes some physical oceanography features of an estuary maintained by fresh groundwater inflow. The mesoscale variability of currents, temperature and salinity are documented from moored current meter observations of 12 days duration at three stations. Current speeds exceeded 0.7m.s^–1 during spring tides and decrease to 0.1m.s^–1 during neap tides. Current directions followed the major channel axes, reversing by 180 degrees between the flood and the ebb tides. Harmonic analysis indicated that tidal currents were mainly semi-diurnal, with a predominance of the M2 harmonic, but included M4 and M6 sub-harmonics, possibly resulting from non-linear interactions of the flow with the shallow bottom and channel curvatures. Close to the lagoon entrance the water is of marine character but, further upstream, salinity decreased significantly. In the absence of river runoff, this decrease must result from some yet ndisclosed underground freshwater inflow. The water renewal time estimated using the tidal prism method was at least two tidal cycles. Using the fresh-water fraction method, the groundwater inflow was estimated to be 2.3m³.s^–1 at most. Coupled 2D hydrodynamic and advection-dispersion models (MIKE21) were then used to verify some renewal times. The tidally averaged renewal time for the whole lagoon was found to be 7 days, while the local renewal time at the upstream end of the lagoon was 25 days. Finally, an inverse modelling approach using MIKE21 was used to estimate the groundwater inflow required to produce the observed salinities. That inflow was found to be 0.25m³.s^–1 for the period under consideration, showing that the tidal prism method gave flows that were far too high.

African Journal of Aquatic Science 2005, 30(1): 1–10