Dynamics of water exchange and salt flux in the Macuse Estuary, central Mozambique, southern Africa

Abstract

Studies of water-particle flow dynamics in shallow estuarine systems show that tidal currents control water exchange, salt flux and residence time. We used the 3D Estuary, Lake and Coastal Ocean Model (ELCOM) to estimate the dynamics of tidal currents, salt flux and residence time in the Macuse Estuary, central Mozambique. The model was calibrated using data acquired from field-data measurements obtained in 2014 with an acoustic Doppler current profiler and conductivity-temperature-depth (CTD) casts and a tide-gauge. The water flow dynamics that were tracked indicated that tidal currents of 100 cm s^–1 were dissipated by friction caused by bathymetric morphology during both the spring and neap tides. The water dissipation was translated into a phase-difference delay of ~15 min between the catchment zone and the outlet during flood and ebb tides. The study showed that tidal currents and river discharges controlled the salt flux (2.07 × 105 g kg^–1 m3 s^–1) and the variation in residence time from hours to 40 days. The water ages were mostly driven by U-velocity tidal currents, bathymetric gradients and seasonal river discharges of ~500 m3 s^–1. River discharges seasonally affected salinity changes between 15 and 30, and changes in the concentration of suspended sediments of ~300 mg l^–1. In addition, it was shown that ELCOM was able to track the water-particle dynamics well, indicating the model’s suitability. These results contribute to our understanding of the effect of water-exchange dynamics on residence time and salt flux in shallow coastal inlet systems. Keywords: bathymetry, bottom morphology, coastal inlet, ELCOM, hydrodynamics, residence time, water quality