· · · Institutional Repository

The Eastern Scheldt tidal basin and its ebb tidal delta have changed drastically in the recent centuries under the influence of natural processes and human interventions. A Storm Surge Barrier, one of constructions within framework of Delta Project in the Eastern Scheldt, is considered as the largest impact to changes in hydrodynamics and morphology of the ebb tidal delta. The barrier was built in 1987 to maintain the tide and to protect the area from flooding. The barrier closes under severe storm surge conditions and remains open during normal conditions to preserve the tide-dominated character of the ecosystem. However, this storm surge barrier caused a reduction of tidal volume and tidal current in the Eastern Scheldt. Since 1987, there has been a remarkable reduction of the tidal prism by 30%, of the average tidal range by 12% and of the average tidal current velocities by 33%. As a result, sediment export from the basin towards its ebb tidal delta has ceased. This led and still to changes in morphology of the ebb tidal delta. In order to adapt to changes in tidal volume, the tidal channels become sinks of sediment and this sediment has been and still is taken from the tidal shoals nearby. Sediment volume of these shoals therefore has decreased drastically. The deposition in channels and erosion on the shoals caused a number of negative effects to ecology, safety, navigation, recreations and fishery in the Eastern Scheldt. To analyse the morphological developments of the ebb tidal delta before, during and after the implementation of the Storm Surge Barrier in the Eastern Scheldt, bathymetry maps which present the changes in deposition and erosion parts in the ebb tidal delta in every four years (from 1960 to 2004) were used. In summary, the morphology of the Eastern Scheldt ebb tidal delta experienced enormous changes due to the Delta Project. These changes comprise shoaling and reorientation of channels- from updrift to downdrift (northward) migration of channels, landward migration of shoals, development and diminishing of ebb and flood chutes and development of scour holes near the entrance of the storm surge barrier. As a result, it would take a long time (centuries) to establish a new dynamic equilibrium between hydraulic conditions and geomorphology in the Eastern Scheldt inlet.

The Lai Giang inlet located in Binh Dinh province incorporates various features of the estuaries in the Central part of Vietnam. Belonging to the micro-tidal and wave-dominant coast and influenced by the monsoon regime, the inlet has a seasonal character. During the dry season, as the river flow diminishes, the wave action causes high level of sedimentation and closes up the inlet eventually. In flood season, as the river discharge is high, the channel is scoured and the inlet begins to migrate. The high sedimentation level and migration of the Lai Giang inlet has been a serious problem of Binh Dinh province for a long time, because it is the only exit for the floodway. It is an anchorage and also the connection between the sea and the aquaculture area of Hoai Nhon district. The high level of sedimentation at the entrance of the inlet prevents river flood from flowing smoothly, thus leading to overflow in lowlands and navigation issues. In recent decades, the exploitation and protection of Lai Giang area have been studied in various forms of scientific researches and projects by different scientists and local professional agencies. However, the studies have only focused on hydraulics, hydrology and on adjusting the flow of Lai Giang river. There are only general and basic studies on the entrance of the inlet. The main objective of this research is to understand the morphological behaviour of Lai Giang inlet. The specific interest is focused on the main factors which are the tidal characteristic, the wave climate and the river flow during the flood season, and the interaction between all these factors that influence the morphological changes. The study starts with the collecting and analysing all the documentations to come up with a conceptual model of the Lai Giang inlet to explain how the sedimentation and the migration processes happen. Then, the Delft3D modelling software, which can model (tidal) flow, waves and sediment transport, has been applied to confirm the hypothesis and gain further knowledge. According to the data analysis, the conceptual model as well as the descriptive and quantitative result of the model, we can make the following main conclusions: 1. The wave climate in this area has seasonal characteristic and is dominated by two main directions; Northeast and Southeast in winter and summer monsoon, respectively. 2. During the summer monsoon, the longshore sediment transport moves towards the north, bypasses the entrance of the inlet and gradually builds up on the down-drift spit due to the Southeast Wave. 3. During winter monsoon, the Northeast wave intensifies the southward longshore sediment transport leading to the large amount of sedimentation in front of the inlet. At the same time, the significant river flow flushes away the sediment deposits at the main ebb channel located nearer to the up-drift spit. Thus the sedimentation could not take place at the up-drift spit. The sediment displacement at the up-drift and down-drift spit made the inlet migrate to the north gradually. Finally, the possibility to stabilize the inlet is discussed to give the optimum solution for this area.

The Colombian government aims to establish a connection that bridges the Atlantic Ocean and the Pacific Ocean. The new connection will largely take place on the current Atrato River, which has not yet been upgraded for a more extensive use of navigation in the near future. A bottleneck in the river is the delta area. Without structural dredging activities the river is completely inaccessible. This study investigates the morphodynamics of a river mouth of the Atrato delta, in order to improve its accessibility to provide navigation in the future and to minimize the dredging activities. In short, the main objectives of this study are (1) to gain insights into the morphodynamics of the delta, (2) to develop a hydrodynamic model on an unstructured grid, (3) to develop a morphodynamic model on a structured grid and consequently (4) apply these models to investigate measures that aim to improve the accessibility of a river mouth. In this study an approach is used to model the hydrodynamics first at a large scale and subsequently modelling the morphodynamics at a more detailed level. A hydrodynamic model is set up with an unstructured grid, which appears to be a powerful tool to model a topographical complex area such as the Atrato delta. The hydrodynamic model gives insights into the hydrodynamics of the delta and is capable to create specific time-dependent boundary conditions for the more detailed morphodynamic model. This approach solves the problem of enormous backwater adaptation lengths, which are initiated by the tidal movement and a large amount of bifurcations. Subsequently, a model with a structured grid has been set up to simulate the morphology around a river mouth. The morphodynamic model is able to identify the major processes that cause sediment deposition. The key process appears to be the sudden spreading and deceleration of the riverine water when the river flows into the Golfo de Urabá. As the result of the relative strength of the outflowing fresh water compared to the tidal power, the flow velocities are almost constantly located seawards. This explains the low sediment deposition rates in the river part of the delta. The enhanced flocculationin saline water results in a compact zone of sedimentation. This report discusses four possible measures that aim to improve the navigability in the delta and to minimize dredging activities. Modelsare set up to simulate their effects on the delta hydrodynamically as well as morphodynamically. The modelled measures are (1) a sand trap, (2) an artificial outlet channel, (3) sediment diversion channels and (4) the artificial extension of a river mouth. The model forecasts that none of the investigated measures will avoid dredging activities completely. A navigable artificial river mouth situated more upstream in the river even leads to closuresof the current river mouths. Of the examined measures, the implementations of a sand trap and sediment diversion channels raise the most potential to reduce the sediment supply to the current river mouths, without deteriorating the navigability in other parts of the river. The artificial extension of a river mouth with breakwaters does not decrease the sediment supply towards the river mouths. However, the sediment deposition process takes place in deeper water, which minimizes the dredging maintenance for at least several years. The findings of this study should be regarded as the first step in the extension of knowledge on the morphodynamics of the Atrato delta. At this stage the models will help to understand the morphodynamics at the delta in more detail and hopefully encourage the collection of more valuable data. For that purpose a measuring and monitoring program is presented in this report.

The Ecobeach project is a newly invented method to defend the coast against erosion. Ecobeach is a coastal defense system based on the principle of passively draining the beach. This Thesis looks into possible morphological changes induced by the Ecobeach test at Egmond aan Zee performed by Rijkswaterstaat and BAM.

Coastal lagoons are shallow coastal water bodies separated from the ocean by a barrier and they support a range of natural services that are highly valued by society, including but not limited to fisheries productivity, storm protection, and tourism. Songkhla lagoon is the largest lagoonal water resource in Thailand and Southeast Asia. The lagoon is a combined freshwater and estuarine complex of high productivity which represents an extraordinary combination of environmental resources believed to be unique in the region. Climate change, as a response to increased greenhouse gases in the Earth’s atmosphere, is now a widely accepted phenomenon. Sea level rise, temperature, precipitation, and storminess are expected to change significantly with global climate change and to impact coastal lagoons. The nature and magnitude of these impacts are still not very clear. The general objective of the research is to determine the climate change impacts on mixing and circulation at Songkhla lagoon, Thailand. To archive this objective the lagoon was be modeled with Delft 3D, a model developed by Deltares. After the verification with the available data for the region, different scenarios were created to represent the possible changes in mean sea level and riverflow due to global warming. Then, these results were compared to the current conditions to determine the main changes in mixing and circulation in this coastal lagoon. The results suggest an increase in water velocities of the inlet in future scenarios and a decrease of flushing time. Salinity and stratification showed more complex changes in futures scenarios.

A process-based numerical model was applied to investigate wave reworking in the deltaic environment. The two main objectives were (1) to develop this model and (2) consequently apply it to study the effects of wave reworking on the morphology and stratigraphy of a delta. A depth-average Delft3D model with two sediment fractions (fine silt and fine sand) was developed. The initial condition was the morphology and stratigraphy of a pre-defined fluvial-dominated delta. This initial condition was first subjected to gentle perpendicular waves for a period of 44 months, for a situation with no active river discharge, to resemble a degrading delta. Next, the reference model was subjected to waves for the same period and varying riverine water and sediment discharges were added to the model. The results of these simulations gave a realistic representation of sediment reworking by waves in the deltaic environment. The deltaic environment rapidly adjusted to changes in the forcing. The base case showed the effects of delta front erosion, channel infill and sediment sorting. Due to the difference in energy required for stirring up and transport of sediments, sand sediments remained in the deltaic environment while silt sediments were transported. This process of sediment sorting is dominant in wave reworking and is adequately represented by the model. Sand sediments are deposited on the edges of the delta front and thereby shield the underlying fine sediments. The results for the fluvial input case showed similar realistic behaviour and exhibited a switch towards wave-influenced delta morphology and behaviour, as defined by classical delta classifications. Also deposition of sand-ridges around the river mouth was observed. Sand deposits prevented further erosion of fine sediments of the delta front and sand-ridges shield the deltaic environment behind the ridge. This study also demonstrated that the influence of riverine sediment discharge is a steering factor for delta development. The model proved to be robust in the sensitivity analysis and provides greater insight in the coupling of morphology and stratigraphy of deltas and delta behaviour. The model therefore contributes to the understanding of the response to changes of processes in the deltaic environment, which is of increasing importance to help to sustain deltas for future generations.

Dune evolution on natural and nourished beaches on an engineering timescale is not well understood. Using a study of both literature, and data which has been gathered of the Dutch coast, a new hypothesis regarding the causes of variability in dune evolution is proposed. Following this hypothesis assumptions are made on the basis of which a conceptual model is developed that includes both the accretive and erosive processes that govern dune evolution.

In this study, a process-based, depth-averaged Delft3D hydrodynamic and morphologic model of the Wax Lake Delta in Atchafalaya Bay, Louisiana was developed to simulate a five year period of delta development. The purpose of this modeling effort was to test the ability of process-based modeling tools to successfully simulate typical delta-building processes and the resulting morphologic and stratigraphic characteristics of the delta. Recent developments in conceptual modeling of mouth bar formation and full delta development have confirmed the capability of process-based models to simulate the processes necessary for delta growth and the resulting long-term, geologic scale morphologic and stratigraphic features. In this attempt to model the actual development of a prototype delta using similar techniques to those employed in the conceptual delta models, the applicability of physics-based modeling to delta evolution simulation will be further validated. Morphology qualitatively reproduced typical river-dominant delta growth through the establishment of new depositional lobes while maintaining approximate radial symmetry. More specific stratigraphic features were also reproduced. The successive stacking of coarsening upwards sequences observed in Wax Lake Delta mouth bar deposits was evident in the stratigraphy of modeled incipient jet deposits, a result of the varying discharge regime. Though incipient jet deposits developed in the model at the distal ends of distributary channels, the prograding bars did not aggrade sufficiently to induce flow bifurcation and the development of a mature mouth bar depositional lobe. The overall coarsening-upwards, though sand dominant stratigraphic sequence of typical friction-dominant river mouth deposits is reproduced. Significant mud-dominant prodelta deposition is observed basinward of the original delta front location. In the prototype Wax Lake Delta, deposition of fine sediments in this area is hindered by waves and wind-induced transport, so the modeled mud depositional bodies do not reflect prototype development. Distributary channels are significantly incised and narrowed over the course of the simulation. The narrowing proceeds from significant sand-dominant subaqueous levee deposition on the channel banks. Though distributary channels in the real Wax Lake Delta do frequently incise through the full deltaic sedimentary sequence with channel extension, the modeled incision is persistent throughout the simulation. Upstream accretion of established delta lobes through sand-deposition, similar to the observed primary process of subaerial development in the neighboring Atchafalaya Delta, was also present in the modeled development. The Wax Lake Delta is clearly river-dominant according to traditional classification schemes; however, the deposition of fine sediments is influenced by basin processes that resuspend and export significant quantities from Atchafalaya Bay. The processes contributing to the coarse sediment depositional features that dominate the Wax Lake Delta are qualitatively simulated under purely riverine forcings, but the fine sediment dynamics cannot be accurately simulated in the present, process-limited model. Recommendations for improving morphological simulation include model redevelopment with an alternative, total load transport formulation and the inclusion of limited marine forcings that inhibit fine-sediment deposition.

For local protection of coastal stretches where buildings are situated in or form part of the coastal defense line the deep water reef concept has been developed. Two reef concepts were designed; a shore-parallel breakwater (of 3 km length) and a segmented breakwater (three blinds of 1 km length), both located in 10 m water depth at 1.5 km off the coast of Scheveningen. In the present research the hydrodynamics and long term morphodynamics of the two deep water reefs are analyzed with the physics-based numerical model Delft3D. The two reef concepts are modeled in 2DH mode with the Parallel Online morphological approach. The hydrodynamic processes at submerged breakwaters are different from those operating at emerged breakwaters. Wave transmission by the submerged breakwater crest creates two additional effects; 1) a water level set-up with a discharge over the reef and 2) reduced wave action at the landward area of the reef. The water level set-up drives two large-scale circulation cells off the reef tips, with velocities up to 1 m/s. In the surf zone the longshore currents are directed towards the shadow zone, where the second effect creates a sheltered area. The magnitude of these effects is for the parallel reef concept amongst other parameters dependent on the wave height and wave transmission, whereas for the blinds reef concept this is also dependent on the wave direction due to the orientation of the blinds. The longshore varying currents and reduced waves in the surf zone cause the coastline to development into a salient form. At the lee of the reef accretion occurs, whereas at the southward and northward sides of the sheltered area erosion occurs. The morphological development is similar for the two reef concepts and of the same order of magnitude. The blinds reef is the preferred alternative for construction, as less severe erosion is modeled to occur. Equilibrium will likely be attained in 15 to 25 years after construction.