Abstract

Erosion and deposition are natural phenomena in many estuaries that can cause morphological changes, leading to navigation and offshore structure problems. Many previous studies have focused on investigating the morphological changes of estuaries depending on different parameters and their combined effects with erosion and deposition processes, such as maximum flow amplitude, waves, currents, tidal flow, storms, rising sea elevation, maximum turbidity, salinity, and bed roughness height as input parameters. These studies are based on field measurements and numerical simulations by using topographic surveys, soil samples, satellite images, geological data, and bathymetric maps with the aid of ArcGIS. Some of these studies examine the effects of construction buildings such as dikes, jetties, and dams.

This paper attempts to summarize the characteristics of these natural phenomena and the parameters affecting them, in addition to the solutions to control the estuarine morphological changes.

Knowing the amount of water flow in the estuary system is important and considered the main reason for erosion, deposition, bank shifting, and sediment transport due to high velocities. Using concrete blocks to control the erosion and deposition processes was a suggestion to be applied as an effective solution under different conditions.

1 Introduction

A river estuary is a zone between the sea and the river where fresh water from the river mixes with salty seawater. This zone is influenced by various biological, chemical, and physical factors (de Pablo et al. 2022; Grasso and Le Hir 2019; McLachlan et al. 2017; Xu, Z. et al. 2020). River discharge, tidal forces, currents, salinity, maximum turbidity, suspended sediment concentration, climate, grain size distribution, bed roughness, and sea level changes all affect the morphological development of estuarine areas. River estuaries are mainly impacted by the forces of tides and currents, which influence many estuarine features. Additionally, river discharge plays a key role in water management, flood control, salt intrusion, and navigation (Cai et al. 2023), with fresh river water helping to control salinity intrusion (Cook et al. 2023). Changes in tidal and fluvial regimes can also affect sediment transport and turbidity maxima (Liu et al. 2023).

Erosion, flooding, and salinity intrusion should be experienced in estuaries, and their relationship with sea level and river discharge (Costa et al. 2023). Erosion is an important problem that occurs wildly in estuaries (Addo and Addo 2016; Elliott et al. 2007). This problem has been extensively studied to identify solutions. Estuarine morphodynamics is the main parameter that seems to be mostly affected by erosion and deposition. The hydrodynamics of flowing water through the estuary remains poorly understood, particularly in terms of controlling the erosion and deposition processes that are active in an estuary. Understanding the hydrodynamics and the factors affecting them may help understand the estuaries' morphology (Toffolon et al. 2006). Estuarine morphology was largely determined by transported residual sediment patterns (Herrling et al. 2021). The sediment deposition mechanisms qualitatively represent the consequences resulting from the development of morphology in the long term in an actual estuary (Chang et al. 2020). The morphological features of estuaries are constantly changing, so it is essential to consider the management of flow and its impact on the estuarine system. The interrelation of the estuary water flow, velocity profiles, and bed shear stress can cause changes in the estuarine regime. Experimental methods are difficult to estimate bed shear stress, especially in complex, three-dimensional flow fields (Zhang et al. 2020). The cross-section of the estuary and bed roughness variation are important parameters used to indicate bed shear stress distribution. Understanding the relationships between the morpho dynamics, sediment transport, and turbulent flow field was essential for measuring the changing roughness in coastal and fluvial areas (Hu et al. 2021).

Accurate determination of the distribution of boundary shear stress is essential for addressing a variety of problems, including erosion and deposition in an estuary (Alwan et al. 2022).

2 Literature Review

Many studies have been conducted to identify the estuarine morphodynamics and the effect of different parameters, which are carried out using field measurements and numerical simulations.

2.1 Most important parameters

The most important parameters that affect and cause estuary morphological changes are summarized in the following subsections.

Discharge and water level

Both discharge and water level at the estuary are important factors that prove to be effective parameters influencing the phenomena of erosion and deposition. It is essentially proven that high flow increases both erosion and deposition. The estuary is an active area that transports the maximum water flow from the rivers to the bay, causing erosion and accretion in hundreds of square kilometers (Ahmed et al. 2014). Estuary planform is often considered to have an ideal shape (Savenije 2015). It is described as a state of equilibrium with a constant tidal range, average flow depth, and velocity amplitude (Leuven et al. 2018). Many studies have been conducted to investigate water discharge within the estuarine zone, but it is still a complicated problem according to the estuarine topography and the great effect of tidal forces (Duc et al. 2010; Nguyen et al. 2012). Most studies focus on the effect of tides, while studying the effect of river flow rate is rare (Lee et al. 2019). Extreme floods increase water levels, velocities, and bed shear stress in inner estuarine locations and river mouths (Wang et al. 2022). Strong tides could cause an increase in suspended sediment downstream and the erosion of riverbeds. Due to the greater tidal flow, freshwater discharge is within the range of the measuring error for extreme cases during the dry season, making it unreliable (Savenije 2005). The physical and ecological properties of estuaries can be significantly impacted by river flow (Loneragan 1999). The natural fluctuation of river flows into estuaries is significantly changed by human actions, including building dams, diverting river flow, and withdrawing freshwater (Cai et al. 2014). An increase in river flow rate may cause sediment to be transported to the sea (Liu and Cai 2019), where the sedimentary dynamics are a response to fluctuations in river inflow and sea-level rise (van Maanen and Sottolichio 2018). High velocities are the main reason for the sediment erosion on one side and the deposition on the other side (Deb et al. 2017).

Concentrations of turbidity and suspended sediment

Maximum turbidity, typically in areas with high concentrations of suspended sediment (SSC), is a particularly significant parameter within the estuary zone because it affects biogeochemical cycling and has an impact on the estuary's overall environmental quality (Dyer 1988; Liu et al. 2023; van Maanen and Sottolichio, 2018). Suspended sediments have a major impact on waterways in estuarine zones (Li et al. 2020). Maximum turbidity is a complicated dynamic feature that changes according to external causes. It is represented by the migrating mass of sediment with the flow (Mitchell et al. 2017). It is caused by trapped sediment in estuaries and is considered a primary characteristic for studying the impact of floods on waterways projects (Fan et al. 2023; Mitchell et al. 2017). Tidal asymmetry is primarily responsible for this dynamic turbidity maximum development (Burchard and Baumert 1998). Maximum turbidity is indicated in the form of suspended mud that is transported during the ebbing tide. Sediment exportation occurs mostly during periods of extreme river discharge, which drives the maximum turbidity to shift downstream, leading to an increase the sediment resuspension. When the river flow rate is sufficient, the maximum turbidity of the estuary could be flushed away (Jalón-Rojas et al. 2016; Pritchard and Green 2017). Adjusting velocities to prevent suspended sediment concentration results from large sediment escape offshore and depletion of the estuarine sediment mass. Sediment depletion can cause erosion at the channel bed and destabilize the dynamic morphological equilibrium (Hoitink et al. 2017). Under sea level rise, the position, geometry, and intensity of the maximum turbidity remain mostly unchanged. Changes in river flow over time are thought to have a greater impact on the location of maximum turbidity than rising sea levels.

The ecological environment and development of an estuary coast are both significantly impacted by the changes in the quantities of sediment transportation (Huang et al. 2019), which are primarily affected by changes in bank erosion and sediment transport patterns. Estuaries in coastal areas are sensitive to dynamic factors including river discharge, tidal current, waves, and sand ridges. These circumstances cause the estuary's suspended sediment processes. The process of suspended sediment can be utilized to explain the causes of geological changes (Hwang et al. 2016).

Tides, tidal currents, waves, and wind

The estuary system has a complicated morphological change and sedimentation process due to the interference of river discharge and tidal processes that vary over time and location (Cai et al. 2014; Dalrymple and Choi 2007; Wang 2013). The driving forces are defined as climate, rivers, tides, geology, and changes in sea level (Gao et al. 2011), which have an impact on the coastline change over time. There is a significant temporal variation in the driving forces for shoreline alterations.

Along the estuary region, tidal forces are primarily responsible for managing variations in water quantity and quality as well as sediment transport (Brenon and Le Hir 1999; Islam et al. 2013; Paiva et al. 2017; Scully and Friedrichs 2007; Valencia et al. 2004). Estuary morphodynamics are often influenced by the action of tides, waves, and river flow rate, as well as the availability of sediment (Boothroyd 1985). The nutrient dynamics, benthic habitat conditions, water quantity, and navigability of the estuary will all be affected by the observed tidal fluctuation of hydrodynamics and stream flow. Tidal flow, current, and waves may lead to coastal erosion (Guo 2022).  Tidal barriers are used to restrict the movement of flowing water to the upstream direction in an estuary (Kidd et al. 2015). However, they significantly affect the morphological and hydrodynamic characteristics of the estuary (Kuang et al. 2017).

Wind, river discharge, and tide force are the main factors affecting flow circulation within the estuary (Deb et al. 2017). The transportation of the suspended sediments is mainly dependent on the wind, currents of fresh water, and tidal behavior. Additionally, foredune accretion and erosion are influenced by a variety of context controls, such as the characteristics of the storm, the wind direction and intensity concerning the shore, the tidal cycle during storms, and direct human influence on the shore through foredune and beach management (Anthony 2013).

The tidal current is the principal factor in the estuary, where riverine discharge and tide merge and cause sediment deposition (Xu, H-j et al. 2020). While near-bed crosscurrents are generated by secondary currents during ebb and flood in homogeneous estuaries, the crosscurrent direction may change in inhomogeneous estuaries. This must have a significant effect on how the estuary cross-section morphodynamically develops (Winterwerp et al. 2006).

Sediment distribution grain-size

Most estuaries around the world contain surficial sediment consisting of fine particles (Dong et al. 2020). Sediment transportation is classified as a part of hydrological engineering (Sun et al. 2022). Many research papers focus on hydrodynamics, bed transport, and dune development and migration, as well as suspended loads (Reible 2021). Water-sediment physical and chemical mechanisms were studied using mathematical modeling, laboratory experiments, and theory (Chassagne and Safar 2020; Huang et al. 2015).

The erosion and deposition phenomena related to bed load are typically associated with the nature of the seabed (Mengual et al. 2021). The existence of coarse particles in the combination causes an increase in erosion phenomena of fine particles, like fine sand and mud, in the homogeneous erosion situation. Cohesive and non-cohesive sediment types are included in the sediment particles that are transported during the sediment cycle, as shown in Figure 1. The dynamics of sediment transportation are identified as the sediment cycle, which depends on the type of soil particles. These particles are affected by different processes such as erosion and sedimentation. Numerical models could be conducted based on experiments, which provide an initial estimation of the bottom stress and bed load transportation. Water circulation, salinity, biological interaction, and sediment type all have an impact on the dynamics of sediment transportation. Cohesive sediments are transported in the water column by water currents. Saltation, rolling, and sliding affect the transportation of non-cohesive particles along the bottom. The process of aggregated cohesive sediments may create flocs. This process is known as flocculation, and it results from chemical or biological reactions. Chemical flocculation is caused by salinity, while biological flocculation is caused by bacteria and plankton which leads to the creation of large flocs known as snow flocs. Additionally, algae may have an impact on erosion as they reduce sediment resuspension.

Figure 1 Cycle of cohesive sediment deposition and resuspension. 
(Wang and Andutta 2013)

When the grain-size distribution of the suspended sediment becomes nonhomogeneous and non-cohesive particles, it is suggested that treating resuspended fluxes separately for each sediment type would be more suitable. The cohesive sediment transport model effectively accounts for the shear stress in the erosion algorithm and the deposition algorithm (Wenjin and Ruijie 2008). Calculation of critical shear velocity and estimation of erosion, erosion direction, and erosion risk were carried out by (Heise et al. 2010) by investigating the effect of the grain size and tidal currents. The velocity of the water flow and shear stress were decreased, and aggregates suspended near the bed were deposited. This deposition is expected to occur at a high rate of settling velocity. The scouring-silting variations were strongly influenced by reclamation, which affects the estuarine topography (Qu et al. 2011).

Bed roughness

In estuaries, the hydraulic characteristics near the bed influence erosion, deposition, and sediment transport (Cheng et al. 1999). Energy dissipation close to the bed causes an increase in the velocity gradient and shear stress. The grain size at the water-sediment interface, sediment transport, and hydrodynamic bed forms all contribute to the development of bed roughness elements. Bedform length and height are commonly used to assess bed roughness height and ensure safe navigation. The size and shape of bed roughness serve as indicators of sedimentological and hydrodynamic conditions (Lefebvre et al. 2022).

Salinity

The most important problem in many estuaries is salinity, which is due to the effects of extensive human activity and climate change (Kennish 2001; Kouzana and Benassi 2010; Lyu and Zhu 2019; Reyes-Merlo et al. 2013; Schettini et al. 2017; Zhao et al. 2022). It has an impact on the ecology and ecosystems of rivers and estuaries (Han et al. 2018). Estuarine areas have frequent and significant salinity variations (Andrade et al. 2022). Salinity is caused by the combination of freshwater from rivers and saltwater from oceans (Devlin and Pan 2018). Salt intrusion is affected by river discharge, channel morphology, and tidal force (Guerra-Chanis et al. 2019; Li et al. 2012; Liu et al.  2001; Zhang et al. 2010). Due to rising sea levels, salty water could flow from the ocean to the estuary and cause estuarine vegetation problems. The salinity range may go over what is ideal for their growth and even over what they can tolerate. Increased water salinity, or saltiness, could damage plants and reduce estuarine ability to protect shorelines from erosion (Smyth et al. 2024). The morphological development of estuaries is affected by the salinity structure due to its effect on the velocity distribution (Lutz et al. 1975; Winterwerp et al. 2006). The salinity structure has a significant impact on the longitudinal and transverse velocity profiles, where a very small vertical salinity gradient can have a significant impact, especially on secondary circulations. River discharge has a significant impact on saltwater intrusion (van Maanen and Sottolichio 2018).

2.2 Field observation

All estuarine problems were investigated using different methods over time to find solutions to conserve the morphology of the estuary as much as possible. The solution strategies can be grouped according to the following sections.

Survey data

The alternations between erosion and deposition were studied by (Zhu et al. 2020). This study was conducted in an estuary under the hydrological cycles, flood, and dry periods. The investigation was carried out using sets of survey charts during a specific period with the aid of GIS. It is shown that the Yangtze Estuary erosion/deposition rate is related to sediment load and river discharge. The slope of the residual water surface and the equivalent sediment transport capacity can be used to further understand the morphology-hydrology process. The river water discharge will be continuously smoothed by the cascade of major dams throughout the upper Yangtze; therefore, in the future, it is improbable to reach this level. As a result, a deposit in the Yangtze Estuary is still possible. The result of the study is a significant indicator for other estuaries that were affected by the riverine changes and exchanges of sediments, especially during increasing dam construction and climate change.

On Bali Island (Eryani 2020), the estuary is a depositional place for sediment that will eventually produce an alluvial structure. Waves, river discharge, and tidal force on the coast are factors that increase the deposition of sediment in the estuary. Within the river outlet, sediments might restrict water flow to the sea, causing backwater and flooding. Survey data, which was considered primary data, was collected at the location to understand the formation of the sediments. Secondary data for analysis was obtained from supporting data. This study investigated the characteristics of the deposition of sediment materials in the river estuary by finding the variables that cause the deposition and developing a strategy to control them. It was found that the topography, soil type, and the condition of the hydro-oceanic system are factors that cause the deposition. Jetty construction has an impact on the estuary deposition process. This method is used to normalize the river flow rate; the jetty was used for sediment control.

Bathymetric maps and ArcGIS

Bathymetric data can provide information on physical characteristics (Chakraborty et al. 2010). With the aid of ArcGIS, the sources of fluvial sediment transportation and the amounts of sediment deposition and erosion throughout the Changjiang River and the Changjiang estuary were investigated (Dai et al. 2018). The Three Gorges Dam had an impact on the river sediment-transport mechanism. The erosion phenomena in the Changjiang River area are a major source of sediment transportation. Since the Three Gorges Dam closed in 2003, the riverbed has become the primary source of sediment transportation, accounting for about half the material entering the estuary. It was found that the Changjiang River transports important amounts of water and sediment, both of which are essential to the estuary ecology. The Three Gorges Dam control in 2003 modified the Changjiang sediment transfer system. Higher sea elevation and more storms may prevent sediment movement, resulting in increased estuary erosion and an increase in seaward sediment transport. More dams are planned to be built in Upper Changjiang, which may have the unintended consequence of reducing the sediment trapped behind the dams. As a result, significant erosion in the Changjiang estuary system is likely to occur soon.

Data from different devices were utilized for morphological characterization (da Silva et al. 2017). This study was conducted to investigate the characteristics of a river estuary using hydroacoustic mapping and remote sensing during different seasons. River discharge was obtained using ADCP (Acoustic Doppler Current Profiler). The saltwater intrusion was examined using a digital hydrometer and a CTD RINKO profiler. The estuary in this study is dominated by the tide effect, a sediment importer system that is kind of mixed but vertically homogeneous and unfilled according to the dataset integration and analysis.

For four decades, the variation of the Pearl River Estuary coastlines has been investigated (Hu and Wang 2019). This study was conducted using a Landsat dataset and with the assistance of another common analytical method for calculating coastal change rates, DSAS, an ArcGIS plugin. The next processing software used in this study was MATLAB, ENVI, and ArcGIS. Visual interpretation to reclassify coasts into muddy, biological, sandy, estuarine, rocky, and artificial categories was used in this study, and then used as a criterion to check the outcome using MATLAB. After getting the results, the coasts in the estuary increased in length by 788.13 km in 1978 to 980.57 km in 2018, with land reclamation playing a significant role throughout. In 2018, because of economic expansion, artificial coastlines accounted for 67 percent of the total. Natural coasts have been deteriorating, but biological coastlines have stayed stable. DSAS gave quick and accurate results for analyzing change rates in two areas within the estuary. From an environmental standpoint, it is important to find the right proportion between coast protection and economic expansion.

Sediment samples and geological data

Some external factors and conditions affect the erosion rate, as shown by Lee et al. (2021), who estimated the external factors affecting the erosion of sediments by using sediment samples, geological data, unmanned aerial vehicles, and digital surface models, which helped to explain the estuarine morphological changes. The sedimentation and morphological variation were affected by wind speed, tidal forces, and rainfall.

Topographic survey and mud samples

Many studies were conducted to identify the estuarine morphodynamics, such as (Azhikodan and Yokoyama 2021), in which the seasonal erosion and sedimentation, as well as mud deposit characteristics in the estuarine zone with maximum turbidity, were investigated at Chikugo River Estuary, in south-western Japan. Soil samples, topographic survey, and follow-up on the magnitudes of elevation and water velocity during the ebb and tide periods were used in the investigation. It was shown that the ebb and tide forces increase the transportation of the sediments. The erosion and deposition processes were affected by the dominant river flow rate. In addition, soil deposit has a large effect on the estuary's morphological characteristics. The same researchers studied the erosion and deposition patterns in the meandering estuarine channel and its causes in the same estuary (Azhikodan and Yokoyama 2019). This study was carried out using transverse and longitudinal intensive surveys of topography conducted every 3–4 months in the upstream of the turbid estuary, monitoring salinity and turbidity with moored instruments, from 2009 to 2012. A grab sampler was used to collect sediment samples from each survey location. River discharge measurements were collected by the Japanese Ministry of Land, Infrastructure, Transport, and Tourism at a gauge station in the freshwater zone. The topographic elevation was calculated using water elevation from the Japan Water Agency as well as acoustic imaging data. In a meandering estuarine channel, the temporal and spatial dynamics of morphology with tidal variations and river flow rate variations were investigated. The results showed that for most of the year, controlling the river estuary was achieved by fortnightly fluctuations in tidal discharge, except for the rainy season when river flow predominated. The erosion and deposition processes were dominated by the semidiurnal and fortnightly tidal, as shown by sedimentation upstream during low flow intervals. During any semidiurnal tidal cycle, fine sediment moved from downstream and settled upstream. Most of the sedimentation was in the meandering channel's inner regions. The significant storm flow carried away a large quantity of deposited mud to downstream locations, resulting in a rapid rise in channel capacity. The upstream meandering channel had a seasonal cyclic pattern of erosion and deposition processes, which influenced the channel capacity. The significant changes in channel morphology are caused by sediment export due to the higher river flow rate. Depending on the sediment transportation, due to the tidal effect and river discharge, the morphological change of the river topography continues. This study proved that the most important factors causing sediment transportation were river discharge and tidal fluctuation.

2.3 Numerical simulation of the estuaries

The fast development of technology has provided the opportunity for numerical solutions to be the best for solving complex problems, especially in the absence of data (Anthony and Aagaard 2020; Panchenko et al. 2020). Investigation of morphodynamics within an estuary can be conducted by using 1D, 2D, or 3D models to examine particular estuarian problems (Guo 2022).

One-dimensional numerical model

Different software are used to investigate the morphological changes within the estuary, as seen in Sirviente et al. (2023). This study was conducted to investigate the relationship between tidal forces and bed estuary elevation variations. The relationship was analyzed using a one-dimensional numerical model along the Guadalquivir River Estuary. The numerical model was very useful in studying the main factors that promote the tidal amplification phenomenon and its relationship with the bed estuary depth. The results showed that increasing bed estuary depths intensified tidal currents near the river mouth. This phenomenon altered the saltwater transport characteristics, in addition to the fact that the bottom friction was reduced with a smaller decrease in the amplitude of the tidal wave as it propagates throughout the estuary. The changes in depth, especially in width, along the estuary are crucial for determining the magnitude of the resonant response of the tidal wave.

HEC RAS 1D software was used to investigate the erosion trend within the Meghna Estuary (Ahmed et al. 2014). This study was conducted by using satellite images and Google Earth images, in addition to the available hydraulic characteristics, such as discharge and water level data. It was shown that bank shifting was considered the main reason for erosion at the Chandpur confluence.

Two-dimensional numerical model

The TELEMAC modeling system was utilized to simulate flow and sediment dynamics to investigate depositional processes and estuarine morphology in a tide-dominated estuary (Deng et al. 2020). The results involved historical bathymetric maps. The study shows that dominant flow rates produce distinctive geomorphic characteristics for the estuary, and the sedimentation process is significantly impacted by tidal force. In Myanmar, Ahmad et al. (2018) conducted a study to investigate the relationship between bank erosion and bed material in a river estuary by using a field survey, numerical model, iRIC-Nays2DH, and data analysis with the effect of tidal current. It was found that the erosion of the bank estuary was caused by currents with strong tides. Another study in the Myanmar estuary by (Ahmed et al. 2019) investigated sediment transport, tidal currents, and channel changes by using International River Interface Cooperative iRIC-Nays2DH software. A new erosion rate equation was used to study the sediment rate in the bed channel. The results showed that the bank of the estuary shifted over a period, and the channel bed was affected and changed during the ebb periods.

Ding et al. (2013) studied erosion protection in the Touchien Estuary in Taiwan under different conditions: tides, river floods, waves, and winds. This study used numerical simulations, CCHE2D-Coast, to simulate the morphodynamic changes in the rivers and coastal zones within the estuary. Researchers simulated a typical estuarine area and divided it into six scenarios, which included solid structure installations, dikes and jetties, and smooth engineering procedures such as channel dredging and island removal. The validated numerical simulation model was used to check the morphological variations within the estuary, like deposition and erosion. The results showed that using dikes while removing the island was the best solution to prevent erosion and flooding within the estuary area.

Many solutions are suggested to control the velocity that was causing erosion and deposition. One of these solutions is to manipulate the resistance of the estuary bed by using an active structure, causing the hydraulic flow to change. This change could have desirable effects or could produce unexpected effects, e.g. Anh et al. (2021), which investigated the erosion, deposition, and hydrodynamic processes within an estuary after completing the construction of a jetty in 2009 at a river mouth. The simulation of sediment transport and hydrodynamics was carried out using models in cases with and without a jetty. It was found that these processes had been altered. The waves had no influence. Currents increased during the ebb period and decreased during the period of flood. The jetty caused an increase in the deposition process in some locations and increased the erosion process in others.

Bárcena et al. (2012) investigated water elevation and velocity by applying the river flow rate and tidal frequency to an estuary. This study was carried out using the surface analysis method in the Suances Estuary with a 2D hydraulic model to determine current velocities and water levels in several scenarios. In the innermost part of the Suances Estuary, a significant change in its hydrodynamic characteristics appeared, particularly after 1878, when a jetty was built at the river mouth for transportation purposes. The estuary has been impacted by significant industrial developments throughout the previous century. An electromagnetic flow meter was used to measure the instantaneous flows in the river, which are not affected by the tide. A portable current meter was used to measure vertical profiles of instantaneous current velocity and directions at six sites. The solution of the fluid flow regulating equations is provided by the hydrodynamic model used to estimate water elevation and velocity fields. For incompressible and unsteady turbulent flows, including the effects of the wind shear, the Earth's rotation, and bottom friction. For a short-medium-term simulation period, the suggested technique was able to estimate the estuarine-free surface and velocity while saving a significant amount of computational cost. As a result, researchers use this strategy to solve hydrodynamic problems in a specific period. It was recommended that further research should be focused on applying the best method to a real case rather than an ideal case. The studies must also consider the trade-off between the response surface methodology cost and capability against the trade-off of standard hydrodynamic modeling.

Three-dimensional numerical model

Numerical models were successfully used to investigate estuarine morphological and sediment transportation (Badru et al. 2022; Chang et al. 2020; Gao et al. 2016). These studies investigated the effect of waves, hydrodynamic, and sediment movement in different estuaries. MIKE 21 software was used in the first study to investigate the erosion process. The second study used the Coupled-Ocean-Atmosphere-Wave-Sediment Transport (COAWST) software by estimating sediment fluxes. The third study used a 3D near-shore morphodynamic model associated with different software to generate offshore boundary conditions for the morphological model. FVCOM was used for tidal simulation, and SWAN for wave propagation prediction, coupling with a 3D near-shore morphological model, WC3D. All these studies had results that proved that a large amount of sediment was carried by river flow. Furthermore, the asymmetrical tides, when combined with the dominating waves, resulted in net sediment transfer.

Zăinescu et al. (2019) studied the sedimentation process using MIKE 21/3, a coupled hydrodynamic and wave model, to investigate the effect of flood phenomena on sediment movement. It examined yearly bathymetric variations based on surveys conducted, and correlated bed variations with observed river flow rate, as well as wave height data to estimate circulation and apply bed shear forces. The model simulated current and wave dissipation during the flood period. Field data was combined with the MIKE model during storms to create a conceptual hydro-morphodynamical model. The importance of studying these two phenomena and their effect on the morphological change near the mouth was highlighted in a conceptual model. Combining currents and waves creates significant bed shear forces, which remove the material deposited during floods. However, the effect of circulation on sediment transportation was still missing. The locations of the sediment deposition were still unknown.

Measuring shear stress in the estuaries is not an easy task, and it cannot be directly measured. However, it can be measured indirectly by observing the hydraulic characteristics of the estuaries. Bed shear stress was formulated by a model based on the kinetic energy related to the wave of the water surface, which was defined as a function of orbital velocity by Stanev et al. (2009). This study focused on bed shear stress to control erosion and deposition in the North Sea. Surface wave parameters were taken from wave spectrum model simulations. Bed shear stress related to currents was simulated with the primitive 3D equation model. Significant bed shear stress and wave height due to currents were subjected to empirical orthogonal functions analysis. The results showed that local shear stress can cause sediment with horizontal distribution. In some regions, the availability of resuspended sediments on the bottom was important.

Numerical modeling with the aid of GIS

Hu et al. (2019) investigated mudflat dynamics and marsh vegetation along the downstream coast of the Yangtze Estuary. The study was carried out by monitoring various sites using a GIS platform and determined that mudflat accretion and vegetation expansion along the marsh border continued. This could be related to the interaction between sedimentary processes and vegetation establishment, especially in years when the amount of water discharged is greater. This research proved that high magnitudes of water discharge could produce sediment transport. 

Numerical modeling and satellite images

Steijn et al. (2020) suggested using concrete blocks to control shoreline locations. Numerical modeling and satellite images were used to investigate the rate of bank erosion and its causes. The bank erosion was caused by the tides from the Gulf. It was found that the erosion was related to the tidal force, due to the higher flow velocities near the bank.

Eddy correlation, log profile, inertial dissipation, and turbulent kinetic energy

Bed shear stress was investigated by Liu and Wu (2015). The investigation used four methods: eddy correlation, log profile, inertial dissipation, and turbulent kinetic energy. The results showed that shear stress is similar in the four methods. The log profile method can gain both roughness height and shear stress of the bed. The remaining methods can obtain only bed shear stress. The log profile method was estimated to be the most useful and easiest method for steady, homogeneous flow. Semidiurnal tidal accuracy in the estuary caused a strong variation in bed shear stress.

Based on the summarized literature, the conducted studies focused on investigating the morphological change of the estuaries, which was caused by erosion and deposition processes. These investigations were conducted using a numerical simulation relay on different software, topographic surveys, soil samples, bathymetric maps, and ArcGIS. Some of them examine the effect of construction buildings such as dikes, jetties, and dams.

The general structure of the causes of morphological changes in river estuaries and the method of investigation are summarized in Figure 2.

Additional literature reviews related to the problems of estuarine morphological changes are summarized in Table 1.

Figure 2 Framework of the morphological changes in the river estuaries.

Table 1 Literature review summary.

Reference	Location	Subject of the study	Method / Parameters included	Conclusion
Hu et al. 2021	Changjiang Estuary, China	Hydrodynamic characteristics of low-angle dunes when adding the tidal current effect	MATLAB software and TELEMAC-2D model Field survey data	The flow patterns were affected by the morphological characteristics when compound dunes formed in the late flood period, and the velocity profile was split into two portions. Both shear stress and roughness height in deep water were more sensitive to the average velocity, according to the association between flow resistance and flow strength. Bedload transfer generated by shear stress due to the formation of low-angle dunes was limited. Bank shifting was considered the main reason for erosion at the Chandpur confluence.
Ahmed et al. 2014	Meghna Estuary, Bangladesh	Erosion trend	Numerical simulation, HEC-RAS 1D-MODEL. Satellite images and images from Google Earth Available discharge and water level data	Bank shifting was considered the main reason for erosion at the Chandpur confluence.
van Maanen and Sottolichio 2018	Gironde Estuary, France	Sediment dynamics under the effect of sea-level rise and river inflow variations.	Numerical simulation using a SIAM 3D hydro-sedimentary model. A free surface boundary condition and the hydrostatic assumption.	Sediment export occurs mostly during periods of extreme river discharge, which drives the maximum turbidity to shift downstream, increasing the sediment resuspension. Changes in river flow over time are thought to have a greater impact on location than rising sea levels.
Islam et al. 2013	Hudson River Estuary, New York	The discharge of the stream, hydrodynamics, and the variation of sediment transport.	The dataset was collected through the River and Estuary Observatory Network using Acoustic Doppler Current Profilers The rating curve of stream discharge as a function of velocity and the height of the river stages.	The nutrient dynamics, benthic habitat conditions, water quantity, and navigability will all be affected by the observed tidal fluctuation of hydrodynamics, stream flow, and sediment transport.
Gao et al. 2011	Bohai Gulf coastlines, China	Temporal and spatial changes and the driving forces that affected the Bohai Gulf coastlines in China.	Remote sensing images to interpret the coastlines for three decades, from 1979 to 2000. Driving forces were defined as climate, rivers, tides, geology, and changes in sea level.	There was a significant temporal variation in the driving forces for shoreline alterations. In the second decade, from 1980 to 1990, the sedimentation process was a significant problem of coastline variation in the estuary; from 1990 to 2000, tidal flat reclamation for aquaculture was the main driving force; and in the last decades, port construction and aquaculture interaction were the driving forces.
Deb et al. 2017	Hooghly Estuary, India	Suspended sediments and estuarine circulation	Numerical simulation under different flow conditions. Freshwater river discharge and tidal currents were the main flow conditions	Wind, river discharge, and tide force are the main factors affecting flow circulation. High velocities were the main reason for the sediment erosion on one side and deposition on the other side. This study recommended that numerical simulation was the best way to study the sediment transported and estuarine circulation under different conditions to provide information on the dynamics of the estuary.
Xu, H-j. et al. 2020	Yangtze River Estuary, China	Numerical analysis of the sediment deposition	A two-dimensional mathematical model Tidal currents, sediment, and runoff. The installation of two dikes with long groins with a deep-water level between them.	The sediment deposition near the estuary's centerline was associated with the flow expansion in the river's expanding area, where the river reaches the sea. The residual current was the principal factor in zones where riverine discharge and tide merged. The mechanism also involves an impact that results in sediment deposition
Mengual et al. 2021	Seine Estuary, France	Relative contribution of bed and suspended load in sedimentation and its impact on morphological variations.	Morphodynamical model Tides, waves, wind, and river discharge	The erosion and deposition phenomenon connected to bedload is typically dependent on the nature of the seabed.
Winterwerp et al. 2006	Lower Scheldt Estuary, Netherlands	Velocity distribution in the Lower Scheldt Estuary.	Acoustic Doppler current profiler measurements Salinity	Salinity gradient had a large influence. Transverse and longitudinal profiles of velocity were sensitive to salinity structure.
Wenjin and Ruijie 2008	Pearl River Estuary, China	Sediment transport and water flows at Pearl River Estuary.	Numerical modeling using HSCTM-2D Active tidal current, little runoff discharge, free surface, and velocity distribution	In the erosion and deposition algorithms, the cohesive sediment transport model effectively accounts for shear stress.
Heise et al. 2010	Pearl River Estuary, China	The relationship between tidal waves and sediment grain size in the Pearl River Estuary, China.	Numerical model run by the Center for Coastal Ocean Science and Technology, Zhongshan University, Guangzhou, China. Measured data from samples and local data. Tidal generated currents and grain size.	A 90 percent risk of erosion has been estimated. Outside of the LingDingYang Channel, the number of erosion events was insignificant, considering that the amount of sediment deposited in the estuary exceeded that removed.
Qu et al. 2011	Sheyang Estuary, China	Reclamation effect in Sheyang estuary coastal area on coastal topography	High-resolution image, Landsat TM/ETM three-scene remote sensing picture A high-resolution satellite could quickly collect changing coastal landscape information.	Scouring-silting variations were strongly influenced by reclamation. They monitor and compute the moving distance of the dike line and the scouring-silting rate of the beach line using TM pictures, to react to the effect on the coastal region. The results demonstrate that the procedure is practical and effective.
Hu et al. 2019	Yangtze Estuary, China	Investigated the mudflat dynamics and marsh vegetation along the downstream coast.	A numerical method (SPSS program) monitors various sites using a GIS platform. The upstream river and sediment discharge.	Increasing water discharge over a short period could induce sediment transport to the coastal zone, resulting in mudflat accretion.
Zhu et al. 2020		Alternations between erosion and deposition, and morphological change.	Sets of surveying charts during a specific period using GIS. Daily water discharge and yearly sediment load during flood and dry periods.	Erosion and deposition rates are related to sediment load and river discharge.
Dai et al. 2018		Amount of sediment deposited and erosion, and the impact of the Three Gorges Dam.	Bathymetric maps and ArcGIS. River runoff and suspended sediment data for a specific period, as well as an estimate of erosion and sedimentation compiled by the Changjiang Water Resources Commission (CWRC).	The Three Gorges Dam control in 2003 modified Changjiang's sediment transfer system. Increasing sea elevations and more storms may prevent sediment movement, resulting in increased estuary erosion and an increase in seaward sediment transport.
Azhikodan and Yokoyama 2021	Chicago River Estuary, Japan	Seasonal erosion and sedimentation, as well as mud deposit characteristics, with a maximum turbidity	Topographic survey and mud samples. Continuous monitoring of flow velocity, water level, and turbidity during macrotidal.	Tidal forcing accelerated sediment transport. The discharge of the river was the dominating factor during the rainy season, which caused erosion and mud deposition. The mud deposit had a strong impact on estuarine morphology, with seasonal changes against external forces like tides, availability of sediments, wind, and river discharge.
Chang et al. 2020	Nakdong Estuary, Korea	Temporal and spatial variation of sediment transport and morphological change.	Numerical simulation using different models: COAWST, ROMS, WRF, SWAN, and CSTMS. Sediment transport, waves, and hydrodynamics Bed level variation and sediment fluxes.	The deposition process was the constant mean flow sediment influx because of waves and tidal forces.
Dong et al. 2020	Lingdingyang Estuary, China.	Applied parameters of erosion and sedimentation for the transportation of cohesive sediment.	Experimental method, field samples, and numerical simulation using TELEMAC-2D hydrodynamic module. UMCES-Gust Erosion Microcosm System (U-GEMS) experimental method. 12 mud samples were collected and tested in the laboratory.	The mud samples collected in the Lingdingyang Estuary were analyzed using U-GEMS, yielding an experimental average critical shear stress of 0.26 N/m2.
Eryani 2020	Ayung River Basin, Bali Island	The estuary was a depositional place for sediment that would eventually produce an alluvial structure.	Survey data, which was considered primary data, was collected at the location to understand the formation of sediment. Secondary data for analysis was obtained from supporting data. Survey data, Google Earth, soil types, and topographic information.	The topography, soil type, and the condition of the hydro-oceanic system are factors that caused the deposition. Jetty construction had an impact on the estuary's deposition process; the jetty was used for sediment control.
Hu and Wang 2019	Pearl River Estuary, China	Variation of Pearl River Estuary coastlines for four decades.	Landsat Dataset. Another common analytical method for calculating coastal change rates is DSAS, an ArcGIS plugin. ENVI 5.1, ArcGIS 10.3, MATLAB 2017a, and ArcView GIS 3.3. Visual interpretation to reclassify coasts into muddy, biological, sandy, estuarine, rocky, and artificial categories.	Because of economic expansion, artificial coastlines accounted for 67 percent of the total in 2018. Natural coasts have been deteriorating, but biological coastlines have stayed stable. From an environmental standpoint, it was important to find the right proportion between coast protection and economic expansion.
da Silva et al. 2017	Cunani River Estuary, Brazil	Hydrodynamic and morphological characteristics	Hydroacoustic mapping and remote sensing, ADCP (Acoustic Doppler Current Profiler), a hydrometer, and a CTD RINKO profiler. Data from different devices were utilized for morphological characterization. The river flow rate was obtained using ADCP, and the saltwater intrusion was examined using a digital hydrometer and a CTD RINKO profiler.	The estuary may be dominated by tidal force, a sediment-importing system that is kind of mixed but vertically homogeneous and unfilled according to the dataset's integration and analysis.
Stanev et al. 2009	North Sea, Europe	Focused on bed shear stress to control erosion and deposition.	Wave spectrum model simulations. 3D primitive equation model. Bed shear stress was formulated by a model based on the kinetic energy related to the wave of the water surface, which was defined as a function of orbital velocity.	Sediment with horizontal distribution can be demonstrated by local shear stress.
Liu and Wu 2015	Pearl River Estuary, China	Estimation of bed shear stress	Using four methods, eddy correlation, log profile, inertial dissipation, and turbulent kinetic energy. The mean water depth and the tidal range, turbulence flows, turbidity, temperature, and salinity.	Semidiurnal tidal accuracy in the estuary caused a strong variation in bed shear stress.
Gao et al. 2016	Yellow River Estuary, China	Sediment transportation	Numerical model, FVCOM, SWAN, and WC3D Initial condition for water elevation and velocity magnitude. Average magnitude of the temperature and salinity. The river flow rate was specified to represent the flood.	The sediment was distributed locally near the river mouth. The tide force, when combined with the wave effect, resulted in sediment transfer.
Ahmad et al. 2018	Sittaung River Estuary, Myanmar	Find a relationship between bank and bed material during erosion	Numerical simulations, iRIC-Nays2DH. Field survey, data collection.	Erosion of the bank estuary was caused by strong tidal currents.
Lee et al. 2021	Namdae-cheon Estuary, South Korea	External factors and conditions affecting erosion rate.	Sediment samples, geological data, unmanned aerial vehicles, and digital surface models. UAV photographs, GPS data, and elevation data.	Sedimentation and morphological changes were influenced by wind speed, tidal forces, and rainfall.
Ding et al. 2013	Touchien Estuary, Taiwan	Erosion protection	Numerical simulations; integrated coastal model. Tides, river floods, waves, and winds. Structure installations, dikes, jetties, and smooth engineering procedures such as channel dredging and island removal.	Using dikes with a removal island was the best solution for preventing erosion and flood effects in the estuary area.
Anh et al. 2021	Tam Quan River mouth, Vietnam	Erosion, sedimentation, and hydrodynamic processes after completing the construction of a jetty.	DHI software, MIKE 21 flow model FM. Sand transportation and hydrodynamic modules with and without a jetty, waves, and currents.	The jetty was caused by an increase in both the area and frequency of the deposition process, but the erosion process was aimed at increasing the frequency and narrowing the area on the north coast.
Steijn et al. 2020	Sittaung River Estuary, Myanmar	Rate of bank erosion and its causes.	Numerical modeling and satellite images. Two field campaigns provided data on tidal channel depths and flow velocities.	Erosion of the bank was associated with tidal force, and it was too small compared to it, which led to an increase in the flow velocity near the bank.
Jie et al. 2011	Changjiang River Estuary, China	Simulate sediment transport and tidal currents.	Numerical simulation using ECOMSED model. Runoff, sediment, and tidal current conditions.	The Hengsha Passage has a significant impact on silt deposition. Analyze the development of the river regime and the influencing mechanisms of hydrodynamics will be required.
Bárcena et al. 2012	Suances Estuary, Nothern Spain	Investigated water elevation and velocity magnitudes in the estuary.	Surface analysis method and a two-dimensional depth-integrated model. Incompressible and unsteady turbulent flows, including the effects of wind shear, Earth's rotation, and bottom friction.	The combined effect of tidal oscillation and river flow variation results in significant changes in current velocity and free surface, particularly in the estuary's innermost zone, where industrial activities are concentrated. Characterization of current velocity and free surface variability in this estuarine area is essential to help water in setting flood management, erosion, and water quality requirements.
Azhikodan and Yokoyama 2019	Chikugo River Estuary, Japan	Seasonal morphodynamical evolution.	Topographic surveys and sediment samples. River discharge, water level data, salinity, and turbidity monitoring.	On a seasonal basis, significant morphological variation is caused by sediment exportation by higher river flow rate during the overflow season, which returns with sediment importation caused by tidal force. The study showed that the main factors influencing sediment transport were tidal and river flow rate changes.
Zăinescu et al. 2019	Sfântu Gheorghe branch mouth of the Danube River, Romania	Effect of storms and flood phenomena on deposit and sediment removal.	Numerical simulation using the MIKE 21/3. Bathymetric variations based on surveys. Observed river freshwater and solid flows, as well as wave height data.	The importance of floods and storms in causing morphological variation was highlighted in a model. Cause of the effect of circulation on the mechanism of sediment transport was still missing. Location of the sediment deposits were still unknown.
Deng et al. 2020	Lingding Bay, South China Sea	Investigate deposit processes and estuarine morphology.	Bathymetric maps TELEMAC modeling Historical bathymetric maps and hydrographic data	In a tide-dominated estuary, lateral river-dominated outflows produce distinctive geomorphic characteristics and sedimentation processes that are significantly impacted by the tidal force.
Mitchell et al. 2017	Kaipara River Estuary, New Zealand	Field measurement to study maximum turbidity.	Field measurements High slack water and resuspension with flood and ebb tide are faster currents.	System is sensitive to material flushing downstream during high flows near the landward end of the fresh-saline water interface.
Azhikodan and Yokoyama 2018	Chikugo River Estuary, Japan	Surveyed the dynamics of sedimentation and erosion in the upper zone of the estuary, observation of the morphological change.	Field measurements Echo sounder with a high-resolution acoustic Doppler current profiler during the semidiurnal and fortnightly tidal cycles, focused on the tide-driven dynamics of SSC.	The deposition process occurred during the tidal period. For two weeks, a layer of mud was deposited on the bed, and that led to an increase in bed elevation. During the spring tides, the relationship between shear stress and SSC, affected the bed sediment dynamics.
Liu et al. 2010	Xiaoqing Estuary, China	Morphodynamic changes	Landsat images in a specific period. A second simulation of the satellite signal in the solar spectrum model was used for atmospheric correction of images for bathymetry retrieval. A basic conceptual model of mouth bar evolution Extraction of coastline and water depth utilizing remote sensing technologies was used to examine the morphodynamic model.	The evolution and development of the Xiaoqing River topography experienced an alternate process of sand spit and separating sand bar, according to a comprehensive investigation of long-term changes in the coastline and water depth near the estuary. This process contributed to the gradual shifting of the river's main channel southward.
Huang et al. 2019	Dajin Island, China	Transportation of suspended sediments	Remote Sensing Center for Natural Resources, CASI hyperspectral test flight data. EOS MODIS pictures	A low level of suspended sediment was on an Island. Furthermore, the seas around the oyster aquaculture region and inside the breakwater in the area on the west side of the island have a greater value than the surrounding waters.
Hwang et al. 2016	Han River Estuary, Korea	Monitoring processes of suspended sediments and finding the reason for the geological change.	Geostationary Ocean Color Imager and field data. Shallow water depths (40 m), a high tidal range (4–8 m), strong tidal currents (1–2 m/s), and considerable sediment input from the Han River (12.42 *106 t/yr.).	Net sediment flows were determined throughout the dry season. The net sediment flow data may be used to analyze morphological changes in the estuary. River discharge in the wet season could be explained using monthly averaged suspended sediment concentration photos. In the dry season, the Suspended Sediments Concentration of Gyeonggi-bay was greater than in the wet season. During the wet season, the Han River influences the suspended sediments concentration, although the boundary is unclear.

As a comparison with the previous literature, we can summarize the important gaps in Table 2.

Table 2 Summary of the important gaps.

No.	Gap
1	Previous studies have discussed erosion and deposition problems in estuaries around the world, highlighting the need for further research to gain a better understanding of these phenomena.
2	Each estuary has its unique characteristics; no general solution exists to control these characteristics.
3	No optimization studies were conducted at locations suffering from erosion and deposition in estuaries to obtain the required solutions that lead to a desirable change in waterways.
4	Guidelines are needed to control the protection of the estuaries.
5	More detailed and comprehensive results are needed, especially at the bed of the estuary, to understand the shear effect on erosion and deposition processes using simulation software.

3 Conclusions

This study includes a review of the literature investigating the morphological changes in estuaries caused by erosion and deposition. These investigations were carried out using different methods. Tides, currents, waves, wind, salinity, turbidity, soil grain size, and estuary bed roughness height are parameters that affect the morphological changes of the estuary. Numerical calculations were conducted using different software to simulate water flow and monitor estuary morphology. After reviewing the literature, it was found that tidal force with a maximum flow amplitude dominated and accelerated sediment transportation. Rising water levels in the sea and more storms may prevent sediment movement toward the land, resulting in increased estuary erosion and an increase in seaward sediment transport. Maximum water flow within the estuary system is important because it is considered the main reason for erosion, deposition, bank shifting, and sediment transport. Waves, currents, and tidal flow are all parameters that affect shoreline zones and cause suspended sediment transportation. The existence of coarse particles of sediments leads to an increase in erosion of mud and fine sand. Maximum turbidity is regarded as an essential parameter, while salinity has an impact on the ecology and ecosystems of estuaries due to its effect on vegetation and leads to reduced ability to protect shorelines from erosion. 

Different software was used to simulate water flow within the estuary and monitor the morphological changes. MIKE 21 and COAWST software are used to simulate sediment transportation, waves, and estuarine hydrodynamics. TELEMAC is utilized to simulate sediment dynamics. iRIC-Nays2DH software is useful for investigating sediment transport, tidal currents, and channel changes. Furthermore, to examine water elevation and velocity distribution, surface analysis is a useful method by applying river flow rate and tidal amplitude on the estuary based on numerical models. CCHE2D-Coast is used to simulate the morphodynamic process in rivers and estuarine coastal zones. Moreover, GIS is beneficial in monitoring various sites and detecting mudflat accretion and vegetation expansion. Satellite images are mostly used to identify the shoreline location and morphological changes over long periods. To measure velocity magnitudes at different locations, Acoustic Doppler current profiler measurements are helpful.

Previous literature discussed erosion and deposition problems in estuaries around the world. Precise identification of erosion and sedimentation areas requires identifying the velocity and shear stress values that provide data to develop mitigation strategies that will prevent any negative effects resulting from these two processes. These strategies may include dredging operations and the design of bank protection structures within the estuary area.

Concrete blocks may be used to control the higher velocities and shear stress that cause erosion and can be applied to control the hydrodynamics of estuaries. These blocks assist in increasing the bed resistance for erosion within the estuary area, which is affected by high velocities and shear stresses.

Some software may not be of sufficient accuracy to meet the purpose of identifying erosion and sedimentation; thus, the recommendation for future research is to use a comprehensive 3-dimensional model, such as computational fluid dynamics (CFD) software, to achieve comprehensive details and accurate results to obtain effective mitigation for long-term management.

Acknowledgments

The authors would like to thank Universiti Tenaga Nasional for their support in this research.

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