Asymmetrical tides cause an uneven sediment redistribution within a system. The faster water level rise during the flood tide, compared to the slower decline during ebb tide, activates a tidal pumping process that transports additional sediment into the waterway. When the duration of the ebb tide exceeds that of the flood tide, there is less resuspension during ebb. Sediment primarily settles during slack tide, gradually filling the riverbed. One of the most significant consequences of this process is the decline in navigability of the waterway. An example of such an asymmetrically tide-dominated delta can be found in the southwest of Bangladesh, specifically in the Mongla-Ghasiakhali waterway. This area experiences high sediment concentrations and substantial sedimentation volumes. To address the sedimentation, a conceptual design has been developed, introducing locks at Mongla and Ghasiakhali.
The amount of sedimentation within the lock chamber and channel is strongly influenced by the frequency of lock gate operations. The primary water exchange processes during lock operation include the filling and emptying of the chamber, density currents, and boat movements. The schematized box method is employed to model the water and sediment movements resulting from these water exchange processes.
A tidal cycle consists of multiple operational cycles and, depending on the operational scheme that is used, possibly a resting period. Two operational schemes were simulated: a two way traffic scheme with 12 out of 24 operating hours and a fully operational scheme. Sedimentation in the chamber and channel can be minimised by adopting the two way traffic scheme with a flushing cycle after every 12-hour operating period. Under these conditions, the sediment layer thickness is 2.65∗10^-1 m in the chamber and 2.19∗10^-3 m in the channel after one month. When the growth of the sediment layer thickness is extrapolated to represent one year of operation, the annual sediment layer thickness growth is 3.22 m in the chamber and 2.67 ∗ 10^-2 m in the channel.
After analysing the behaviour of sediment, the optimum choices for the structural and maintenance design of the lock system are evaluated. These choices are based on the functional requirements, with a focus on sediment characteristics. The most suitable lock gates, considering sediment dynamics, are mitre gates, vertical lift gates, and radial (or tainter) gates. These gates offer low to moderate opening/closing times and are effective at pushing sediment away from the lock system. The preferred filling and emptying system for locks dealing with high sediment concentrations, is openings in lock gates.
The total required dredging volume is approximately 35 times smaller than what would be required in the absence of the lock construction. The minimum frequency of dredging the chamber is once per year, whereas the channel can be dredged once every 37 years. The dredging of the chamber must be carried out during the lock’s non-operating hours, while the channel dredging can take place throughout the day. The introduction of two navigation locks will reduce the total maintenance costs for the Mongla-Ghasiakhali waterway.

The Ems Dollart estuary is facing high levels of turbidity, which hinders the growth of algae and impacts the entire ecosystem. In recent years, turbidity has increased here with 0.5 to 3% per year between 1990 and 2010, corresponding to a doubling of suspended sediment concentration (SSC) within 24 years for the upper bound of 3%. This trend is mainly attributed to a decrease in the amount of sediment sinks. The Ems-Dollart 2050 program aims to address this issue by exploring various solutions, aiming to extract one million tons of dry matter from the estuary per year. This study investigates the potential of creating additional sediment sinks by scaling up managed realignment polders. The aim of the research is:

To investigate the important factors of influence for sediment capturing efficiency in a managed realignment polder, to make the mud extraction from a turbid estuary as efficient as possible while creating new natural and agricultural value.

Using the Delft3D FM numerical model, the study focuses on two key factors that influence sediment capturing, namely the effects of the surface area and the effect of the tidal prism (i.e. the volume of water that flows into an area during a tidal cycle), with the goal of making sediment extraction from the turbid estuary as efficient as possible. The study finds that the general concept of using a managed realignment polder to extract sediment is viable. However, the calibration process using Polder Breebaart revealed that the model struggled to accurately replicate some of the accumulation patterns in the low-dynamic system in this polder. While the model seems more appropriate for the project area, significant uncertainty remains due to the sensitivity of the results to sediment properties and calibration parameters.

The study's results suggest that a smaller tidal range can result in a more uniform sediment distribution across the area. This can be achieved by implementing a submerged weir behind the open entrance to the area. An even distribution of sediment could enhance the natural values in the intertidal area, which is a key goal of the Ems-Dollart 2050 program. In addition, wind-generated waves appear to have a major influence on the distribution of sediment over the area. Due to the extra shear stresses, waves provide more resuspension, which transports the sediment further.

Furthermore, the study observes a significant accumulation of sediment in a side-basin near the entrance. The high sediment accumulation in this area, compared to adjacent areas without accumulation, may be attributed to local low energetic conditions. This finding has potential implications for raising agricultural land using transitional polders.

In conclusion, this study provides insights into the effectiveness of using managed realignment as a method for extracting sediment from a turbid estuary. While it highlights some important aspects and influences, it also underscores the need for further research to confirm the findings and reduce the uncertainty in the results. Nevertheless, the potential benefits for both natural and agricultural values make this an intriguing avenue for future exploration.

The La Plata Basin is the second largest drainage basin in South America. The Paraná River, the basin’s main river, discharges into the Río de la Plata estuary in Argentina. In the Lower Paraná Delta, near the deltaic front, a complex system of mostly natural and some man-made channels redistribute a part of the river flow. Like the river and the estuary, the channel network has been subject to sediment deposition, limiting the navigability needed for the small-scale transport of goods, leading to hindrance to tourism and other economic activities in the Lower Delta. Compared to the river system on the large scale, not much is known about the characteristics and the morphodynamic behaviour of the channel network. The objective of this research is to set a first step in providing insight on the relevant processes and human activities that influence morphology changes in the Lower Delta channel network.

Making use of existing data, a literature study and data obtained during several field surveys, various hypotheses were formulated. Hypotheses included the potential influence of land use change in the catchment and climate change related trends in precipitation patterns, climatic variability associated with El Niño Southern Oscillation, the effect of artificially deepening one of the two main branches of the Paraná River and the excavation of artificial channels in the Lower Delta in the early twentieth century. The effects of said processes and human activities are tested with a scenario-based hydrodynamic modelling study, making use of Delft3D Flexible Mesh with sediment transport and morphology modules.

Our analysis provided various insights in the behaviour of the Lower Delta channel network, and the influence of human activities and natural processes on this behaviour. A notable finding is that artificial deepening of the Paraná de Las Palmas by dredging its thalweg since 1990 has lead to a significant reduction in river flow entering channels supplied by the river branch and increased sedimentation rates in these channels. In addition, insight was gained on the spatial distribution of depositional and erosive patterns instigated by discharge and sediment input changes.

This research has provided a first, global insight into the relevant processes governing channel morphodynamics. Some of the findings can serve as input for future dredging strategies, as results imply that the current strategy has lead to limitations to the navigability of an intensively navigated part of the channel network. Furthermore, the outcome of this research can serve as a starting point for research on one of the specific topics covered by the hypotheses, including but not limited to the influence of storm surges, the effect of a potential spatial heterogeneity in soil composition and the influence of tidal divides on morphological features in the channel network.

There is a demand by Rijkswaterstaat for CO2-neutral sustainable methods and technologies to maintain the Dutch coastline. The Zandwindmolen is a fixed CO2-neutral dredge, transport and nourishment concept that nourishes sediment continuously. The purpose of using a Zandwindmolen is to provide the entire surrounding area where the Zandwindmolen nourishes with sediment and allowing it to grow with the (accelerated) sea level rise. With the traditional nourishment method, it is often desired that the sand remains at the vulnerable site for as long as possible. Whereas, in a nourishment with the Zandwindmolen, it is often desirable for the sand to move to other nearby parts of the coast as quickly as possible.
Between 2011 and 2020, 27.5 million m3 of sand has been nourished at the Ameland Inlet. Therefore, it might be considered as an erosion hotspot. Because of the fixed nourishment method of the Zandwindmolen and the continuous sand shortage at the Ameland Inlet, there is a match. However, the usefulness of a Zandwindmolen depends on the extent to which the nourished sand volume is dispersed by natural processes in the short and long term.
In this study, both the short- and long-term effect of a continuous point nourishment in a tidal channel is investigated for four sediment fractions (100, 200, 300 and 400 μm). The goal is that the sediment is dispersed by nature in order to allow the entire Ameland Inlet (adjacent coastlines, coastal foundation, outer delta and Wadden Sea basin) to grow along with sea level rise.
For optimal dispersion, the nourishment must be carried out by means of a spreader pontoon with a sediment mixture concentration of 1-2%. By doing so, a mixing plume is created in which sediment settles according to their individual settling velocity. This makes the mixing plume prone to the tidal ebb and flood current and can therefore be steered towards either the North- or Wadden Sea.
In the short term (time scale: instantaneous to a few weeks), it appears that the dispersion of sediment is mainly determined by the initial sedimentation process of sediment and by re-suspension of settling sediment particles. The dispersion by natural sediment processes is minimal. In the longer term (timescale: half a year) the sediment spreads further after it has settled to the seabed. Based on half a year, the total dispersion is mainly determined by the short term dispersion.
It is found that a continuous nourishment with the Zandwindmolen is expected to ensure that the intertidal areas in the eastern Wadden Sea can continue to grow naturally with (accelerated) sea level rise. Moreover, the natural distribution of the nourished sand will, through sediment connectivity, compensate for the erosion (autonomous and due to sea level rise) of the inlet (especially the outer delta) and is also expected to contribute to keeping the North Sea coast of Terschelling and Ameland safe. Therefore, the Zandwindmolen could be a useful new sustainable nourishment method in a tidal inlet.

A managed realignment is the landward relocation of a flood defence to re-establish tidal exchange on formerly reclaimed land. In a managed realignment, the newly formed intertidal area acts together with the realigned dike as a nature-based flood defence system. In this thesis, the focus is on the managed realignment of Perkpolder, a former polder located in the south of Zeeland (The Netherlands). This realignment serves as nature compensation for the dredging activities in the Western Scheldt. The realigned area contains an intertidal flat which facilitates ecological services.

A combination of different data sources (sediment samples, flow velocities, turbidity measurements and bathymetric data), together with a hydrodynamic model of the Western Scheldt and a morphodynamic model of Perkpolder are used to interpret the hydrodynamic and morphodynamic response of Perkpolder after the managed realignment.

Bathymetric data composed of LIDAR, multibeam and single-beam measurements shows three main developments in Perkpolder: (1) erosion of the frontal entrance area, the inlet and the seaward side of the creeks, (2) infilling of the creeks landward of the first bifurcation and (3) sedimentation on the intertidal area and even more sedimentation of the pond. By comparing the temporal evolution of the bed levels of the frontal entrance area with measured concentrations at the inlet of Perkpolder, it can be concluded that the frontal entrance area acted as a finite source of sediment that eroded quickly in the first years and increased sedimentation within Perkpolder during these years.

New realignment projects can profit from the research at Perkpolder. Bed shear stresses of the initial proposed layout of Perkpolder gives a clear indication if the creeks are setup correctly. Furthermore, an estimate of the initial sedimentation can be based on the accretion rate of other intertidal areas within the estuary. Since realigned areas are often more sheltered and have a lower elevation, accretion rates may be higher. Moreover, not only the forcing of the estuary, but at the same time the availability of different sediment sources and the initial bathymetry shape the morphodynamic response of the realignment site. Therefore, it is recommended to measure the bathymetry for a sufficient long period (e.g. 10 years after the realignment) to get grip on the mechanisms shaping the morphodynamics at these areas.

The lower Ems river suffers from hyper-turbid conditions. These conditions are caused by channel deepening over the last 50 years, resulting in an asymmetrical tidal wave. This asymmetry has resulted in an increased
import of sediment into the Ems river. Recently some effort has gone into determining whether or not the local tidal barrier (Emssperrwerk) can be used to alter the asymmetry of the tidal waves propagating through the Ems estuary. Previous modelling studies, using state-of-the-art models, have been able to reproduce prototype tests involving the same tidal barrier but very little effort has thus far gone into determining what the optimal barrier control regime is and what the restriction imposed on the barrier operation mean for the overall
effectiveness of the barrier. The operation of the tidal barrier was schematised and described using two factors: closure percentage and closure duration. The effectiveness of a broad spectrum of combinations determining the tidal barrier operation is modelled using a simple one-dimensional model which takes only into account the (simplified) hydrodynamics in the lower Ems river. The results of the model indicate that only in part of the Lower Ems the tide can be adjusted to become symmetrical or slightly ebb-dominant. This requires a closure duration longer than approximately 240 minutes. The length of this stretch of river is increase further by increasing the closure percentage. Similarly, increasing the closure percentage also leads to a decrease in the tidal volume and the amplitude of the M4 component. For this reason, the closure percentage should be kept as low as possible, as long as the barrier operation is able to reduce the sediment import into the Lower Ems. The required length of the stretch of the Lower Ems with a symmetric tide is estimated to be around 12 km. Applying restrictions to the barrier operations showed that shorter barrier
operations should be preferred, since short operations allow a larger number of
tidal waves to be altered, increasing the effectiveness in a realistic setting. Based on these considerations an optimal barrier operation consists of a closure duration of approximately 300 minutes and a closure percentage of approximately 60%. The results showed that the method used for evaluating the barrier operation is unable to distinguish between water levels signals consisting of altered and unaltered waves. The effectiveness of the barrier operation is
still estimated by considering the whole water level signal. The impact of the barrier operation in the model interfered with the seaward model boundary. This error increases in magnitude with increasing closure percentages. Making the results for those combinations unreliable. In order to make more reliable conclusions about the effect of the tidal barrier operation, subsequent models should encompass a larger domain and include a more detailed schematisation of the channel geometry, river discharge and the tidal barrier itself.

To improve the socioeconomic situation in the Republic of Nicaragua, the government has decided to strengthen the country’s transport infrastructure by building a port on the Caribbean coast. The future port will be in Bluefields Bay, which can be characterised as a lagoon-shaped estuary. The inner basin is connected to the sea by two tidal inlets, located north and south of a barrier island. This study assesses the hydrodynamics and morphodynamics of the area; in addition, it analyses the influence of the port structures on these natural processes. A qualitative analysis of the hydrodynamics and sediment transport in the area is conducted, and sedimentation rates in the navigation channel and the erosion rates of the coast are calculated. The area is analysed with the help of a conceptual model that is supported by the process-based model Delft3D. Large parts of the area consist of shallow depths of around 1.5 m below mean sea level (MSL). The bed of these shallow parts consists mainly of mud. In those areas with a depth of more than 5 m below MSL, which are the river channel and the tidal channels at the inlets, the flow velocity increases, and a more substantial fraction of sand can be observed. In the data analysis, distinct wet and dry seasons can be observed; these seasons change the hydrodynamic influences over the course of the year. The river discharge increases by a factor of 10 during the wet season. In addition, the river delivers large quantities of mud, particularly during the wet period. When the tidal flow enters the lagoon, it influences sediment transport. As the dominant hydrodynamics change throughout the year, the sediment fluxes change with them. Beyond the hydrodynamics within the lagoon, the adjacent coast is under the influence of a swell wave climate. As the waves mainly come from the east-northeast at a height of 1 m, a moderate southward longshore transport is initiated year-round. The Delft3D model is used to analyse the hydrodynamics and sediment transport of the area in more detail. The model is used to analyse the flow magnitudes and directions. This approach allows the dominant hydrodynamic components to be identified. The most relevant hydrodynamic components are considered in the Delft3D model. These include tide, river discharge, waves, and wind. The 2D mode is sufficient to model these hydraulic components. Because of the hydrodynamic climate changes that occur throughout the year due to changes in the meteorological climate, the simulation period is one representative year. The model is calibrated and validated by means of phenomenological calibration and expert judgement, as field data is scarce.

The People's Republic of China experienced large socio-economic development over the last decades, which was mainly concentrated in coastal regions. Jiangsu Province is among these regions, which shows large socio-economic differences between north and south. The Chinese Central Government has allocated Binhai County as a new coastal development zone to lessen inequality. New infrastructure (e.g. Binhai Port) was constructed and industrialisation increased in order to boost local development. Binhai County is located at the Abandoned Yellow River Delta. In 1855, the Yellow River shifted its course which cut off sediment supply to the delta. Major land retreat was stopped by the construction of sea dikes from the 1930's. Ongoing erosion is undermining the present sea defence and endangers the hinterland. The erosion along the delta has engaged the disappearance of mudflat area. Furthermore, large development in southern Jiangsu has led to constraints in usable land for which large mudflat areas were reclaimed. Multiple research pointed the negative effects of reclamation activities on ecology (e.g. decline of shorebird populations). After recognition of the importance of mudflats by the Chinese Central Government, the national policy was set to protect and restore mudflats along the Chinese coast. The erosive features of the Abandoned Yellow River Delta (AYRD) needs counteracting measures. Within this research it is aimed to integrate a multi-purpose design in which engineering and ecology are mutually enhanced. Following the Building with Nature guidelines multiple innovative solutions can be presented. Within this research it is aimed to protect the delta by implementation of islands in front of the coast. Furthermore, it is aimed to enhance mudflat growth which adds ecological value to the delta area. It is expected that by implementation of islands a reduced energetic environment is created in which fine sediment can settle. The Jiangsu Regional Model (JRM) was made available for this research, which is based on Delft3D software. An outcrop has been made at the area of interest which was refined (3:1) and further referred to as the Abandoned Yellow River Delta Model (AYRDM). The hydrodynamical forcing was completed by the addition of waves. The AYRDM was set-up with three sediment fractions from which one added cohesive fraction. The production simulation was set-up for ten morphological years by using a morphological factor of 50. The better obtained understanding of the governing processes optimised island configuration design. Fifteen configurations were designed and subdivided over two series: the Basic- and Exploratory series of eight- and seven configurations respectively. The Basic series was set-up with a systematic approach in order to represent the concept of barrier islands. The Exploratory series comprehend designs based on the entrapment of sediment by using sediment transport patterns. The definition of a mudflat was set from -0.88 m to all above, relative to mean sea level in order to measure mudflat growth. The findings within the reference production simulation (without the implementation of islands) showed a flood dominant environment in which flood direction is south. Hence, south- and offshore directed residual sediment transport were observed along the AYRD. The outgoing sediment flux from the AYRDM increased by 2.2E7 ton/year when waves were included. The addition of waves increased the average grain size by 26 µm at siltation areas due to sediment stirring and cross-shore transport. The effect of island configurations showed most favourable sediment entrapment when elongated islands and small inlet widths were applied. The shallows induce larger friction on tidal movement for which current velocities and herewith sediment transport capacity is reduced over distance. Hence, depositional areas were found to be radially deflected to the south. Secondary flow, due to eddy formation, showed significant streaming induced sediment transport behind the islands ends. Small bed slopes induce larger distances between islands and mainland China, which holds for relatively larger mudflat areas. Wave penetration was reduced significantly when islands were implemented, which created a less energetic environment. The largest increase of mudflat relative to the reference simulation is 59.8 km2 after ten morphological years by implementing 16 km2 of island area. The findings within this study are representative for the Abandoned Yellow River Delta area and show the potential of islands in order to protect the delta and create additional ecological/recreational benefits alongside.

As Chinese economy booms during recent decades, river management attains more investment, and is required to develop more rapidly for flood defense as well as facilitating navigation, which brings a lot of changes and threat to most of aquatics, especially those endangered species. Yangtze finless porpoise is one of those endangered species, of which the habitat is faced up with severe threats from hydraulic engineering. Hechangzhou reach, situating in the lower Yangtze river and 250kmupstream from the Yangtze river estuary(Shanghai), is one of the important navigation channels in Yangtze river and is also one of the main nature re serves for Yangtze finless porpoise. In order to adjust the discharge distribution of the two branches in Hechangzhou reach, Chinese Department of Water Resources conducted a project to construct 2 submerged sills in the north branch of Hechangzhou reach.Acknowledging the importance of both navigation development and protection of endangered Yangtze finless porpoise, investigating the impact of two sub merged sills on the physical habitat of Yangtze finless porpoise is necessary. This requires a good understanding of the hydrodynamics changes caused by two submerged sills as well as the requirements from Yangtze finless porpoise on its physical habitat.  In this thesis, a study is performed to find a reliable evaluating system of Yangtze finless porpoise's physical habitat quality under the impact of hydraulic engineering. Habitat suitability index model, developed by the United States Fish and Wildlife Service (USFWS or FWS),is applied to the physical habitat of Yangtze finless porpoise. By literature reviewing and inquiring biologists, the vital ecological behaviors of Yangtze finless porpoise are identified as movement and feeding, and vital habitat variables are identified as water depth, flow velocity and substrate type.   A previous developed hydro-morphodynamic numerical model was made available and is further developed to gain a better understanding of the changes to local hydrodynamic variables such as water depth, flow velocity. Substrate type, as one of the identified habitat variable from the results of habitat suitability index model, is determined by taking as implified method of comparison between soil critical shear stress and local bed shear stress. Three scenarios are designed to simulate the situation in the dry, normal-water and monsoon season.   This thesis forecasts the changes to the physical habitat quality of Yangtze fineless porpoise through understanding the changes to local hydrodynamic condition and combining the results from habitat suitability index model. Overall,the submerged sills offer limited potential habitat area, slightly extend the high-quality habitat area and improve the existing habitat.     

Along river banks, on the transition zone between the river and see, tidal marshes can develop. The tidal marshes accommodate multiple plant and fish species. As a result of the stressful conditions in tidal marshes, unique intertidal ecology develops. However, due to the densifying of the river banks, tidal marshes are disappearing.
Restoration of tidal marshes can reintroduce the unique intertidal ecology on several locations in the estuary. To obtain restoration of tidal marshes, constructed tidal marshes come into play, which can be built at designated places along the river.
However, constructed tidal marshes are not necessarily built to restore the unique tidal nature, but can also have other functions as recreation and contribution in green city area.

Nevertheless, is it still unknown how constructed tidal marshes behave and what the optimal design is. Namely, a mismatch exists between the policymakers and designers on the one side and academic knowledge on the other side. To fill up this gap, accessible knowledge from the experts should be available for designers.
Therefore, this research provides guidelines for designers and gains more insight into the behaviour of constructed tidal marshes. This thesis focusses on extracting general knowledge from the results of a numerical model, applied on a case study.

Simulations of a numerical 1D Sobek model applied on the case study achieves the influence of model configurations on accretion. Therefore, tidal forcing, marsh design and system adjustments are divided into multiple components. The model simulations of the separate components give the influence on bed shear stress and potential sedimentation.
Furthermore, executed fieldwork calibrates the model on measured flow velocities. Besides, the Manning coefficient is estimated, and cross-sections are obtained by gps measurements. The parameters are subsequently used as model input.

From the simulations, it can be concluded that tidal asymmetry determines the duration of stagnant water and thus the settling of fine particles. A considerable increase in sedimentation is reached when flats are participating. However, an increasing flat area encourages ebb-dominance and can even lead to erosion.
Next, when the width of the cross-section is large compared to the depth, more sedimentation is predicted. With flood-dominant bed shear stresses, a broad cross-section leads to accretion.
In the case study, the presence of sand particles is not expected, as sand is deposited close to the inlet, where the bed shear stress is ebb-dominant. In contrast, silt settles throughout the system. Especially at low energetic conditions, such as bends, silt settles. As the occurring bed shear stress is mostly flood-directed, it is likely that sediment entering the system, does not leave the system anymore, and accretion of the bends is presumed.
The placement of gate culverts in flood direction leads to higher accretion rates due to the longer slack duration. However, at the exact locations of structures, higher bed shear stresses can develop, and erosion is expected.

This thesis research has been conducted to analyse the sediment dynamics in the Yangon Estuary, in order to define the cause for the sedimentation in Monkey Point Channel. For years, Yangon Port has been suffering from limited accessibility due to the sedimentation in the channel and currently daily dredging is required in order to let ships enter the port.
The main characteristics of Yangon Estuary are; tidal range of 6 m, high suspended sediment concentrations (up to 6 g/l), a monsoon climate and the presence of a confluence within the tidal range. Research to the cause of sedimentation is done by making a conceptual model of the estuary based on a study area analysis and a literature study to estuarine, riverine and sediment processes. In addition, numerical models are used to improve the conceptual model in a quantitative manner. The conceptual model resulted in the following findings.
The fluctuations in SSC are dominantly determined by the tide. During the monsoon season suspended sediment is brought in the system mainly by the Yangon River discharge, causing for averaged SSC values of 1 g/l during the season. In dry season, the SSC shows a fortnightly rhythm caused by the spring/neap cycle. Strong spring tidal currents cause for high bed shear stresses on the shoals, exceeding 1 N/m2, which erodes the stationary fluid mud layers. Due to the dispersion of the mud, the SSC rises rapidly up to 5 g/l. During neap tides the bed shear stress is much less, enabling the fine sediments to settle on the shoals and restore the fluid mud layers. Hence, the SSC values reduce to less than 0.5 g/l. During the monsoon, the spring flood current is reduced due to the seaward directed river discharge. Therefore, the spring flood currents are insufficient to erode the stationary fluid mud layers, hence the SSC are less than during spring tide in the dry season.
In contrast to the fluctuations in erosion and deposition of mud on the shoals, almost continuously deposition and consolidation of mud is possible in Monkey Point Channel. This is possible because the channel is located in the stagnation-zone where both during flood (bifurcation) and ebb (confluence) the flow velocities and bed shear stresses sufficiently low to let mud settle. After deposition the mud is not resuspended, and consolidation causes the mud layer to become thicker and become problematic for the ship traffic.
To conclude; the combination of high SSC with the possibility of consolidation of deposited mud in the stagnation-zone results in too hard layers to sail through. This potential cause corresponds with the dredged material from the channel bed and the sedimentation rate in the channel.
It is recommended to relocate the navigation channel outside the stagnation-zone. The combination of land reclamation at the western estuary bank downstream of the confluence and a dredged channel shows the most promising results. However, more research is required to determine the long-term sedimentation rates in the new channel.

Like many rivers around the world, the Loire river in France has a history of human interventions in order to facilitate navigation and port development. Next to affecting bed levels directly, the heavy modification of the river-estuary has induced significant changes in the hydrodynamic and morphodynamic behaviour of the Loire. As a result, the Loire river is experiencing an ongoing bed degradation. In the estuary, the interventions have led to an amplification of the tidal amplitude, a shift of the tidal and salt intrusion limit in upstream direction and a larger tidal asymmetry. According to Winterwerp and Wang (2013), this tidal deformation leads to an increased import of fine sediment in the estuary and a reduced hydraulic drag. This in turn enhances tidal deformation further. Due to this positive feedback mechanism the Loire estuary has evolved into a hyper-turbid state, associated with large suspended sediment concentrations and the formation of fluid mud.

The behaviour of the river-estuary on large temporal and spatial scales and its response to historical and possible future interventions is not yet sufficiently understood. Therefore, an idealized, computationally efficient, three-dimensional morphodynamic model of the river-estuary is developed with the FLOW module of the Delft3D software suite, in which the main processes influencing the morphology of the Loire river-estuary are included.

The current estuarine geometry, lacking the presence of large intertidal areas, induces a flood-dominant tidal signal. In the mouth of the estuary, the baroclinic pressure gradient introduces a mean landward-directed velocity near the bed and amplifies the mean seaward-directed velocity near the surface. This gravitational circulation causes the formation of an Estuarine Turbidity Maximum at the tip of the salt wedge, which is strengthened further by the flood-dominance of the tide. Model results confirm the feedback mechanism between tidal deformation, the import of fine sediment and the effective hydraulic drag as described above. However, next to tidal deformation, strengthening of the gravitational circulation also plays a large role in this mechanism for the Loire.

To mitigate bed degradation, several measures are simulated that decrease the transport capacity of the flow, increase the sediment supply or do both. Over the first 10 years, sediment nourishments are the most effective according to the model. However, after 100 years of simulation the removal of all groynes present within the domain leads to the most sedimentation relative to the reference scenario. To decrease the import of mud into the estuary, two measures have been investigated. Increasing bed levels in the main channel of the estuary mainly leads to a decrease of the tidal range, whereas restoring tidal flats turns the tide from flood- into ebb-dominant. In both cases, a decrease of the salt intrusion length and a significant weakening of the gravitational circulation occurs, leading to a tidally averaged export of fine sediment and very low mud concentrations.

Bed roughness is an important parameter for the prediction of sediment transport as well as calculating flow conditions near the bed. Numerical models are extensively used to make these predictions. In these models it is fundamental to have proper values for bed roughness. The bed roughness is being estimated using a dataset from an ongoing pilot project in the Dutch western Wadden Sea. This dataset consists of 40 days of velocity measurements using ADV (8 Hz) and ADCP (1 Hz) instruments and concentration of SPM measurements using OBS instruments.
To estimate the bed roughness, the dataset is analyzed using four methods for calculating the bed shear stress. The logarithmic profile, turbulent kinetic energy, vertical turbulent kinetic energy and the Reynolds stress method.

The data has been processed and averaged per tidal phase, so statistical analysis can be applied to it. From this analysis, it is found that the concentration of SPM increases at 6 cm with increasing wind speed. As a result of this increasing of SPM the bed roughness also increases. This leads to the hypothesis that suspended sediment makes the bed rougher, and is not primarily governed by horizontal advection but also local resuspension.
A 1DV numerical model is used in which horizontal advection is excluded to test this hypothesis.

Simulations are performed with stationary boundary conditions, using combinations of water depth (0.2 to 2.8 m) and velocities (5 cm/s to 70 cm/s). These simulations are imposed with an initial homogeneous concentration. For every combination, the concentration is increased gradually until the concentration profile becomes L-shaped.

Besides simulations with stationary conditions, timeseries of water depth and velocity are used to simulate one tidal cycle.
All simulations performed with this numerical model do not take wind and waves into account and water-bed exchange is excluded.
From the simulations with stationary boundary conditions the roughness is calculated using the LP method. It is found that bed roughness increases with increasing initial homogeneous concentration.

From the simulation of one tidal cycle, it is found that the roughness increases towards the turn of the tide. After the turning of the tide, the concentration profile becomes L-shaped, and the roughness is decreased. The concentration profile becomes homegeneous again after a certain threshold of velocity and waterdepth and at the same time the roughness increases again. During a tidal cycle, it is possible to have a collapsed concentration profile, which indicates a lower bed roughness and thus a smoother bed.

The typically mild slopes of the muddy coastline in the south of Myanmar, enable the tide to propagate deep into the mainland, carrying muddy and saline water. The Nga Moe Yeik Creek (a tributary of the Yangon River) is closed off at the upstream end during the dry season and is subject to high siltation rates. Extensive dredging is necessary to ensure navigability. However, dredging activities stopped in 2015 because dredging appeared to be insufficient.

Flushing the creek, by opening the Nga Moe Yeik Creek Sluice Gates more often could contribute to a solution to minimise the siltation rates, leading to lowered dredging volumes and an improved navigability of the creek. However, it may decrease the availability of irrigation and drinking water behind the sluice in the dry season. The responsible authorities are looking for an optimal solution. The current insight in the siltation problem is limited due to a lack of measured data collected in the Nga Moe Yeik Creek. The general consensus is that siltation rates can be decreased by an improved operating scheme of the Nga Moe Yeik Sluice Gates.

Since data resources are scarce or not available at all, data on bathymetry, water levels and suspended sediment concentration is collected from the field between May and July 2017. The observed water levels show a flood-dominant tidal wave with a tidal range in the order of 4 - 6 meters. Depths are measured in a zigzag pattern in the creek stretch between mouth and sluice. Suspended sediment concentrations are measured between 0.3 and 1 gram/L. Tidally and depth averaged concentrations in the order of 0.5 gram/L and 0.4 gram/L are observed at respectively 2 and 17 km from the mouth of the creek.

Part of this data is used to set up a Delft3D-flow model of the system that simulates the main sediment transport processes in the Nga Moe Yeik Creek. The initial hydrodynamic model is calibrated against observed water levels. In the next phase, the sediment transport model is used in a qualitative analysis to determine best suitable sediment parameters. Using a combination of model and collected data it is aimed to increase the insight in the siltation problem.

Both model and data showed similar hydrodynamic and sediment transport processes. The water level observations showed that the tidal wave deforms asymmetrically and becomes more flood-dominant as it propagates into the creek. A similar phenomenon is observed in the model results. Forced with a constant 2 gram/L the model generated tidally and depth averaged sediment concentrations in the same order of magnitude as the observations. From field observations and personal communication with local authorities, it is known that the siltation is most severe in the upstream end of the creek. This is also confirmed by the model results. In the scope of this work it is assumed that the model gives reliable results on the assessment of water levels and siltation.

The final model was used to assess several flushing scenarios (continuously or pulsatile flushing). Although all scenarios reduced thalweg siltation in the upper domain, pulsatile flushing reduced siltation slightly better. The scenario with 3 pulses gives best results followed by the scenario with 6 pulses. Less effective is the current flushing scheme with a continuous discharge. However to be able to give an quantitative answer on the extend of mitigation,more specific values of river discharge and critical bed shear stress are necessary.

Over the last decades, the People’s Republic of China experience large economic development, which have led to severe shortages of usable land. To counteract this, the Chinese central government promoted the development of coastal provinces by reclaiming large sections of coastal zones. One of these regions is Nantong Binhai Park or Tongzhou Bay on the Jiangsu coast. This region is characterized with large intertidal mudflats and tidal channels and has a high potential for the development of agri-aquaculture as well as the construction of a deep-sea port. However, these intertidal mudflats are a vital part of ecosystems and support numerous species, such as several endangered migratory shorebirds within the East Asian–Australasian Flyway. Acknowledging the importance of both port development and existing endangered ecosystems, a more integrated coastal design is needed. Possible solution could be found through the Building with Nature approach, in which natural processes and ecological needs are included in the design. This requires a good understanding of the governing processes as well as the requirements from society, ecology and engineering perspectives.

In this thesis, a preliminary study was performed into a more sustainable alternative for the planned Tongzhou Bay Port, in which a basic connection was made between engineering and ecological point of view. A hydro-morphodynamic numerical model was made available and further developed for the modelling of the governing processes in the development of the intertidal mudflats at Tongzhou Bay. The model showed satisfactory results with respect to observations and net flow and transport features were closer analyzed. The different ecosystem types at Tongzhou Bay were quantified with ecotope classification and further validated with satellite-tracked data of two endangered shorebird species. Based on this validation, high-value ecotopes were determined based on these species and was used as an indication for the performance of alternative port configurations with respect to ecology.

With a better understanding of the governing processes for the development of the intertidal mudflats and ecotopes, a series of alternative port designs were conceived through several design cycle iterations. Guiding principles were the preservation of high-value ecotopes, the enhancement of these area’s development and the potential area created for future port development by each variant. Results showed that by understanding natural transport and siltation patterns, not only could alternative port configurations lead to the preservation of existing high-value ecotopes, but a stronger development of these areas with respect to the natural development could be realized as well. The preliminary results of this thesis show the potential of more sustainable alternative port designs through the Building-with-Nature principle, in creating benefits for both economic and ecological development of Tongzhou Bay.

This thesis investigates the origins of large sedimentation volumes in the Port of Santos, São Paulo - Brazil. The port is located in the Santos - São Vicente estuarine system, delimited by scarps and subject to intense rainfall. The terminals are placed inside the estuary and their access is provided by a 24 km long channel with mean depths of 15 m DHN, subject to annual maintenance dredging with volumes in the order of 4 Mm³ [CODESP, 2016a].
A literature-based sediment mass balance is formulated for the system. Sediment sources and sinks are assessed based on data of the period 2005 to 2012 and expert judgment. The largest sediment input is given by winter storms (65%) followed by river sediment loads (25%). Up-estuary import of fine sediment is found as a third source, computed from the deficit between sources and sinks (trapping in the mudflats and maintenance dredging).
An explorative process based model is set to evaluate the hypothetical sediment balance and expand the understanding of the system behavior. A period of six months is simulated to capture the expected seasonal variability in salinity gradients. Conditions for fluid mud formation are found in the entire channel. A rerun with salinity switched off promotes an expressive reduction (70%) in the cumulative fine sediment import. Up-estuary sediment transport is found as a result of salinity-induced residual currents, which increase in strength with deeper channels [van Maren et al., 2015].
Fine sediment deposits in the final stretches of the channel that probably have been accounted in previous studies as from river discharges, according to this study are provided from marine sources. Model results suggest an annual import of 1.1 Mton of sediment, six times larger than the literature-based estimate. Additionally, results indicate that little quantity of riverine sediment reaches the channel. The literature mass-balance is therefore modified to incorporate these findings: river contribution is set to 15%, up-estuary transport to 30%, and winter storms to 55%.