ㄽᩥࡢせ᪨㢟┠Investigating tidal evolution and tide–river runoff interactions in a two-inlet lagoon
and multi-channel estuary㸦2 ࡘࡢධཱྀࢆᣢࡘࣛࢢ࣮ࣥ࡜࣐ࣝࢳࢳࣕࢿࣝἙཱྀ࡟࠾ࡅࡿ₻ộ㐍໬࠾ࡼࡧ₻ộ࡜Ἑᕝὶฟࡢ┦஫స⏝࡟㛵ࡍࡿㄪᰝ◊✲㸧
FARUQ KHADAMI

Understanding hydrodynamics induced by an interaction between tide and river flow isessential to continuously improve the management of delta and estuarine systems. The
investigation of the interaction between tides and river flow could also enhance our understandingof the alteration process of the environment in tidal rivers. This research investigates thespatiotemporal tidal dynamics in various setting topography, aiming to understand the evolutionof tidal waves in single-channel, multi-channel and multi-inlet lagoons. The work was divided into
two main projects that worked in two site locations: the Segara Anakan Lagoon and the Ota RiverEstuary.
The first work aims to investigate the tidal dynamics in Segara Anakan Lagoon, which canbe the main factor that leads to sedimentation patterns. The Segara Anakan Lagoon (SAL) is a
two-inlet lagoon with a high sedimentation rate in the western part. The two-dimensionalhydrodynamic model was used to investigate the tidal dynamics. Spatial computation of tidal
duration asymmetry, peak discharge asymmetry, and slack water asymmetry are used to identify
the tidal dynamics characteristics of the SAL. The model results show that tidal wave propagatesto the lagoon from both inlets and amasses in the central part of the lagoon. Analysis of the tidal
asymmetry shows that the tidal duration and slack water asymmetry are ebb dominance in both
inlets, while the peak discharge is ebb dominance in the western lagoon and flood dominance inthe eastern lagoon. However, the tidal duration, peak discharge, and slack water asymmetry show
flood dominance in amassing areas. The convergence of tidal waves in the central part of thelagoon results in abrupt changes in the tidal range and near-zero discharge in the amassing area.
Moreover, the tidal asymmetry pattern might have implications for the sedimentation pattern inSAL.
The second work aims to investigate the tidal evolution in a multi-channel estuary, in thiscase, the Ota River Estuary. The Ota River estuary has a straight channel, the Ota diversion channel(ODC), and a branching channel. This study examines tidal distortion, tidal asymmetry, andsubtidal water elevation variations in two topographical settings, straight and branching channels,under low-flow conditions. The ratio of the amplitudes of the quarter-diurnal and semidiurnal tidal
species shows that the tidal distortions in the straight and branching channels are similar. The
skewness of the tidal duration shows a tendency to be flood-dominant in both topographicalsettings. However, the increase in tidal asymmetry on the branching channel is steeper than thaton the straight channel. In addition, changes in bathymetry and channel width also contribute tothe steep increase in tidal asymmetry in branching channels. Moreover, the variation in the subtidal
water elevation analysed using the decomposition of the subtidal friction term shows that thevariation is more influenced by the interaction of the tides and tidal asymmetry in the junction
area. In contrast, the variation is more affected by the subtidal flow in the straight channel.
Moreover, the interaction between tides and river runoff in the straight ODC shows that the mixing
was controlled by neap–spring tidal oscillation; high discharges significantly influence mixing
during the neap tide. Furthermore, the results of the wavelet transform indicates that during thespring, tidal straining increases and induces intense mixing, while during the neap, river discharge
has a more significant influence on circulation. The interactions between tides and river runoff
influence the appearance of a convergence zone and estuarine turbidity maxima that are locatedapproximately 4.8 km from the river mouth.
The tidal evolution mechanism in a branching channel is examined using a barotropichydrodynamic model in an idealized junction domain, inspired by the Ota River Estuary junction.
Even though the model was simplified, it successfully reproduced the increase in nonlinearity at
the junction apex. A sensitivity analysis of tidal nonlinearity to the width of the upstream channel
at the junction was performed by varying the upstream channel width from the same width as thebranch channel width to three times the branch channel width. The relationship between theupstream channel width at the diversion point and tidal nonlinearity was not linear, whereas tidal
nonlinearity was maximized when the width was twice the branch channel width. The convergenceof the tides in the narrow junction induces an increase of M4 tidal constituent that raises the tidal
nonlinearity, while in the case of a wider channel, which the wide of the upstream channel is morethan twice of the branches channel, the river runoff dampens the M2 and M4 tidal constituents thatlead to decrease tidal nonlinearity.