Coupling pattern of estuarine and surf zone longshore currents at tidal frequencies: Case study of S. E. coast of Nigeria

Within the coastal flow-field system, the hydrodynamic coupling between the tidal channel and surf zone is among the most important, implicated in shoreline morphodynamics, river mouth bar dynamics, and therecreational quality of nearshore waters. The nature of the coupling has seldom been empirically evaluated at tidal frequencies spanning lunar cycles. This investigation is directed at filling this gap in information based on 50-day successive tidal cycle flow monitoring in an estuary–surf zone setting, S. E. coast of Nigeria. Results show a reversing flow pattern at all monitoring stations at tidal frequencies. The estuary indicated ebb-asymmetric tidal cycle residual flow velocities which at spring tide (30–38 cm/s range) act as an expanding jet relative to the flanking surf zone residual longshore current counterparts (typically ≤ 5 cm/s). The western and eastern flanking surf zones showed westward- and eastward-asymmetric tidal cycle residual flows, respectively with coastwise decreasing asymmetry reflecting the weakening impact of the estuary outflow. Coupling of surf zone – estuarine residual flow vectors indicated a higher frequency of threshold coefficient (r ≥ 0.7) at ebb than at flood stage. The observed pattern of strong estuarine residual outflow velocities and modally divergent weak surf zone flows is a favourable condition for the estuary mouth bar development. However, the eastward-skewed bar configuration fits more to the effect of eastward-directed momentum flux associated with water mass transport of the obliquely-shoaling southwesterly waves given that breaking wave-generated longshore currents in the western surf zone display a westward-asymmetry over a tidal cycle.