## 1 Introduction  
A considerable proportion of estuaries have intermittently open connections to the ocean, presenting the typical characteristics of Intermittently Closed and Open Lakes and Lagoons (ICOLLs) [1]. Natural and anthropogenic hazards such as climate change, growing human populations and increasing urbanisation produce many pressures on estuaries and have a negative impact on human assets and livelihoods [2, 3]. Consistent with effectively managing estuarine systems, monitoring methods have received attention due to advances in the realism of numerical models, which facilitates high-resolution simulations of such systems. This monitoring requires two types of information: (1) a numerical model that describes the hydrodynamics of the system and (2) measurements used for model simulation that incorporates a more comprehensive mechanism (i.e., a data assimilation (DA) scheme) to improve model estimates. In response to decision makers' requirements to identify the extent of estuarine problems, reliably measured data are essential to understand system characteristics via numerical modelling. Evolution in technologies has led to more extensive and accurate observational datasets, which have substantially improved the ability to examine model performance [4]. Because observations are approximations to the truth, a befitting combination of observations and theory is required to replicate the truth as much as is achievable [5]. Although [PERSON] was addressing planetary orbits, his declaration also extends to our circumstances. DA is a process through which a model of a dynamic system is improved by incorporating observations of the real system to make a suitable combination of observations and theory [6]. Measured data alone are often insufficient to provide adequate insight into the physics of the system to support high-quality decision-making in management planning, particularly with regard to complex waterbodies like estuaries. Due to cost and technical limitations, field experiments are often restricted to fixed locations and within certain periods of time. Additionally, observation errors and anomalies can result in the misunderstanding of real physical processes, highlighting the importance of merging numerical modelling with observations to provide valuable flow behaviour information. DA is an effective tool that allows us to combine mathematical models, data resulting from instrumental measurements and prior information for the analysis of complex physical phenomena [7]. Estuarine hydrodynamics are traditionally studied using Eulerian measurements using fixed-position instruments; these sparse and discrete data have also been used to estimate boundary conditions of numerical models, in addition to model calibration and validation [8]. In response to the gap in data density and continuity and in view of these constraints, Lagrangian drifters (i.e., moving sensors) have recently been introduced for estuarine applications [9; 10]. The most commonly in situ instruments used for measuring currents are the Acoustic Doppler Current Profiler (ADCP), acoustic doppler velocimeter (ADV) and conductivity-temperature-depth (CTD) profiler [11; 12], which can be deployed as a fixed-point instrument (anchored). These instruments measure only the temporal variation of the currents at a certain depth and location; thus, the small control volume of Eulerian devices, including their inability to sample the spatial structure of large domains, necessitates the application of a large number of instruments [13]. Lagrangian drifters have been extensively used in oceanic systems [14], largely because they provide deeper insights into flow dynamics as they carry multiple physical, chemical, or environmental information of the flow and are more cost-effective than Eulerian measurements [15]. Lagrangian DA substantially improves the accuracy of modelled velocity fields of large water systems and are more effective than fixed-measurements DA in improving oceanic circulation models, especially with limited instrumentation [16].