The flow measurements in tidal estuaries/coastal waters are abstruse, subjected to rapid variation in direction and amplitude due to its inherent unsteady nature as a consequence of tidal oscillation associated with uneven bathymetric anomalies, depth-variations, mean flow direction, tidal current speed, density gradient affected by salt wedge intrusion, temperature gradient and suspended particles. Thus, a highly complicated phenomenon is being coped with.The acquisition of continuous measurements of flow field is a high-priority issue for water resource management, an environmental, social, and economic asset. However, it not easily to obtain flow fields with conventional measurement methods under unsteady condition. Although modelling for flows and transport phenomena is becoming increasingly easy due to advancements in computing technology and continues to advance rapidly, even sophisticated 3D numerical models encounter difficulties in reproducing the complexity and diversity of mixing and sedimentary processes.Acoustic tomography for flow measurements is novel/cutting-edge method. Although the method enables us to continuously collect instantaneous flow fields (inverse analysis), it is difficult to measure them with a high spatial resolution. A combination of both the acoustic tomography data and numerical model make it possible to obtain reliable and high-resolution flow fields (data assimilation).