Studying mangroves in the field can be very difficult; therefore it is in some situations better to model in a flume. The question is, to what extend are the situation in the field and the situation in the flume comparable with each other? This study focusses on reflection in the wave flume. Wave energy in a flume can be damped by placing a wave absorber at the back of the flume, which absorbs part of the wave energy; part of the waves is reflected. In this study rock is used to build the wave absorber. Two aspects are studied: the influence of the lay-out of the wave absorber on the reflection in the flume, with as goal to find an optimal lay-out, fitting on a maximum horizontal length of 3.00 m, to minimise reflection; and the method of measuring the amount of reflection in a wave flume.

Shore-normal breakwaters are constructed in coastal zones both for beach protection (erosion reduction) and port development (wave sheltering). These breakwaters have an effect on the waves, the (wave-driven) currents and hence, the sediment transport along the coast. The waves and tide force an equilibrium sediment transport along the coast in a natural unaltered coast. When breakwaters are constructed this sediment transport in the alongshore direction is (partially) blocked. This results in accretion at the updrift side of the breakwater and coastline retreat at the lee-side. Coastal erosion behind a breakwater can result in floods, destroy property or simply narrow the beach. Not only the short term effects of these structures (what happens during and short after construction) is important to know, also the long term effects need to be known; because the breakwaters are build for decades coastal influence of these breakwaters needs to be known for these time-scales as well. Therefore it is relevant to investigate the scale (in time and space) of these adverse effects beforehand. In coastal engineering, numerical models are used to predict the impact of coastal constructions like breakwaters. High detailed models which take for all physical processes into account will result in accurate predictions but result in large computation times. Simplified models have smaller computation times and are more suitable for coastal impact predictions on larger spatial (10 - 100 km) and temporal (decades) scale. The objective of the thesis is to improve the coastline change predictions of models at decadal scale by reviewing the common practice and a new, very fast, module for lee-side wave computations and investigating different wave processes that can improve the wave modelling. This research will focus on (1) understanding of what wave processes are important for the coastline change and (2) advise on what models to use for which conditions and how to the improve model predictions. The approach of this thesis is triple. First five different modelling approaches for wave modelling are compared, varying in computation time and usability, to a very accurate model that will be used as ground truth. The wave conditions will vary in direction and both wind waves and swell waves will be modelled. From this step de accuracy of these models for different wave conditions can be analyzed. The second step is to get an better understanding of the processes that are involved with the wave modelling. This information can be used to improve the accuracy of model approaches with high computation times. The last step is to see how this relates to sediment transport and thus the coastline change at the lee-side of a breakwater. This can be concluded after the investigation of 3 wave model (SWASH, SWAN and the Kamphuis module) SWAN model approach is for cases with wind waves a good model approach: the wave height, wave direction and the sediment transport are well representative. Also the setup differences are very similar to the ground truth model. SWAN is not very accurate for cases with a small directional spreading. For these cases the diffraction is more important and there is not enough wave energy in the sheltered area nor is the wave direction well represented. Kamphuis with Snellius (refraction) is for the case with a wide directional spreading, given the computation speed, pretty good. For the cases with a small directional spreading the wave height is not accurate. The conclusions related to the influence of the wave processes are: The refraction is investigated by comparing the Kamphuis module (which does not incorporate refraction) with the ground truth model. When Snell's law was applied to the kamphuis module the model results did show good results. Therefore the refraction is (especially outside the sheltered zone) very important to the wave direction. There is a large difference for cases with small and large directional spreading. The SWAN model gives results can can be expected when no diffraction is present. Directional spreading both influences the wave height directly behind the breakwater as well as the influence lengt of the breakwater. Diffraction does not play an important role for wind waves. The large directional spreading results in much smaller energy differences between the sheltered and the non-sheltered zone. Even SWAN without diffraction gives a good representation for wind waves. For swell waves however this is very different.Then diffraction is very important. There is a much larger difference in wave energy between the shelterd and the non-shelterd zone and for a good representation a model that can coop with diffraction is a must. Current induced refraction of the waves has influences for waves of an incoming agle 0 to 30 degrees because this will result in rip currents along the breakwater that turn the waves up to an extra 10 to 15 degrees. The relative sediment transport of the five modelling aproaches was also analyzed. The sediment transport proxy for the five modelling approaches showed that SWAN computes the wave height and direction well for wind waves. For swell waves however the model did not show good results. The reason is that there is no diffraction in SWAN, the diffraction computations with SWAN did not show much improvements either. The sediment transportation proxy for the Kamphuis module with Snellius where expected to be good for wind waves, because the wave height and wave direction where well represented. However the length of the erosion pit was much smaller and also the shape of the erosion pit is very different from the other two models.

As coastal wetland ecosystems, thriving at the interface between land and water, mangroves can benefit us in lots of ways: providing food and timber, purifying water and sequestrating carbon, protecting coastal area through wave attenuation and coastal stabilization. Regarding the protection of coastal area, mangroves damp the flow in short term and mitigate coastal erosion in long term. In mangrove forests, the capability of mangroves to capture sediments leads to accumulation of sediments within the mangrove forests. Hence, the elevation of the floodplain, where the mangroves grow, is usually higher than the main river bed. The slope connecting the main river channel and floodplain has been found with different angle. The topography of the mangrove area is influenced by both sediment dynamics and hydrodynamics. The physical processes behind this is still not fully understood. This study aims to study the hydrodynamics in a partially vegetated compound channel by using the 2DH RANS model in Delft3D and to verify to what extend the flow characteristics can be reproduced. The reference numerical model is set up to mimic the flume experiments so that the model can be calibrated. The influence of different physical and environmental parameters on flow characteristics is studied. Also, the limitation of Delft3D in simulating the flow in a partially vegetated compound channel has been found.

This study explores the influence of the wave characteristics on the attenuation process of waves through coastal mangroves, which are threatened by the coastal mangrove squeeze phenomenon. Coastal mangrove squeeze is the phenomenon where coastal regions, even when sediment availability is sufficient, are eroding due to a lack of accommodation space caused by the land use on the landside and by sea level rise on the waterside. Along the Mekong Delta Coast, only a narrow strip of mangroves of less than 140 m is left at the locations where a strong erosion of up to 100myr¡1 is observed. Furthermore, observations at the south eastern and the eastern coasts of the Mekong Delta are, that a mangrove width ranging from approximately 30m to260m and 140m on average, appears to be stable. Therefore, a hypothesis regarding coastal mangrove squeeze is proposed based on the empirical relationship between mangrove forest width and coastline evolution. The hypothesis is proposed, that a minimum space of coastal mangroves is required for a sustainable development of the mangrove forest...@en

Large shallow lakes plays a significant role in the rapid urbanization process. A series of problems have occurred due to urbanization including water quality degradation, flood intensity increase, ecological and environmental issues etc. One of the most important threat comes from eutrophication, as it deteriorates water quality, introduces harmful algal blooms, harms lake ecosystems, affecting human health and hinders social economic development. This thesis presents a series of studies focusing on wind induced hydrodynamic circulation in large shallow lake, with the implication of Taihu Lake from lake scale hydrodynamic study, to lake scale water quality implication, and to basin scale implication. The proposed modelling approach could serve as a basis and provide information on lake scale wind effects on hydrodynamic circulation and catchment scale urbanization implication on water environment for management and planning of Taihu Lake and Taihu Basin. @en

Increasingly large sand nourishments are used for the maintenance of sandy coasts around the world. There is, however, still very limited understanding of their behaviour (i.e. redistribution) and effects on the marine environment. This PhD thesis explores the morphological reshaping of shoreface nourishments (i.e. long bunds of sand placed at the sub-tidal bar with volumes of 1 to 5 million m3) based on data at 19 field sites as well as the reshaping of mega nourishments (i.e. temporary land reclamations) using data of the 'Sand Motor' at the Dutch coast (with a volume of 21 million m3). Considerable cross-shore profile change takes place at shoreface nourishments, consisting of a landward movement of the nourishment crest and erosion of the seaward edge of the nourishment, erosion directly landward of the shoreface nourishment (in the first 100 to 150 m) and some accretion in the inner surfzone (at MSL -2m). Especially the water-level gradient driven currents and onshore transport due to wave skewness are responsible for the morphological change, which could be modelled with Xbeach using a lookup table with initial sedimentation-erosion rates for possible climate conditions. Mega nourishments, on the other hand, reshape predominantly in alongshore direction as a result of the alongshore wave-driven current. Design graphs showing the erosion rates, life span and maintenance volumes were made for the mega nourishments, which can be used for the planning phase of projects. Making a differentiation between the non-rotating foreshore and active surfzone proved to be essential for an accurate representation of the wave-driven alongshore transport in 1D coastline models. Furthermore, the lifetime of the nourishment is related to the sensitivity of the alongshore wave-driven transport to a shoreline rotation, which is affected especially by the wave energy. The recent upscaling of the sand nourishment volume in the last decades (i.e. to Sand Motor scale) and increasing anthropogenic pressure also comes with questions regarding the impact that is made on the natural environment. This thesis investigated the development of the bed sediment composition at the 'Sand Motor' using field measurements and numerical modelling, since bed composition is relevant for marine ecology. Considerable alongshore heterogeneity of the bed composition (D50) was observed as the Sand Motor evolved over time with (1) a coarsening of the lower shoreface of the exposed part of the Sand Motor (+90 to +150 µm) and (2) a deposition area with relatively fine material (50 µm finer) just North and South of the Sand Motor. The alongshore heterogeneity of the D50 is most evident outside the surfzone (i.e. seaward of MSL -4 m), while alongshore variation in D50 was relatively small in the surfzone itself (i.e. landward of MSL -4 m). Preferential erosion of the finer sand fractions takes place during mild to moderate wave conditions, while a reduction of the local armouring of the bed takes place during storms which mobilize all sediment fractions and mix the the top-layer of the bed with the relatively finer substrate. A 3D multi-fraction morphological model gave a good hindcast of 2.5 year of observed spatial and temporal changes in D50 at the Sand Motor, which showed that the coarsening of the bed after construction of the Sand Motor is mainly due to the tidal contraction at the Sand Motor. The currents transport especially the fine grains of the sand mixture, which are more easily suspended than the coarse grains. This difference in suspension behaviour is the main cause of the observed bed composition changes at the lower shoreface. Within the surfzone the difference in suspension behaviour of the size fractions will be smaller, as the energetic conditions can suspend all size fractions. The current findings imply that large-scale bed composition changes can take place at any coastal structure which has a considerable impact on the tidal currents. @en

Mangroves are an interesting species of vegetation, surviving and thriving at the interface of land and water, in the inter tidal brackish coastal waters between the mean sea level and mean high water. Mangroves are a highly productive and complex ecosystem, providing numerous services and goods to people and marine environment. Mangroves are home to a large variety of underwater animals. Mangrove wood is valuable because it is resistant to rot and insects and can be harvested for pulp or charcoal production. Most importantly, complex roots, stems and canopies of mangroves provide effective protective means for coastal and estuarine regions. Waves and tidal flows are significantly slowed down as they make their way into and through the roots, stems and canopies of the mangrove forest. Nutrients and sediments can be deposited, providing necessary conditions for a sustainable development of the mangrove ecological system in particular and a stable coastal area in general. However, despite the important role of mangroves along the Mekong delta estuary, a large part of the mangrove forests has been converted into fish farms. In many regions, only a narrow strip of mangroves remained and, as a consequence, mangrove forests degraded and banks and shorelines experience severe erosion. Although numerous attempts were implemented to restore the mangroves and to enhance the river bank stability, these were not really successful. A possible explanation may be that knowledge about the hydrodynamics and exchange processes in and around the mangrove forest vegetation area is still not yet rigorously researched. In order to understand the dynamics of mangroves, the remaining width of mangrove forests and erosion (accretion) rate were observed, collected and analysed in terms of a morphological perspective. It is found that the river bank erosion appears to relate to the width of mangrove forest. The larger the width of the mangroves, the less erosion of the river bank and vice versa. In this context, the concept “Squeeze Phenomenon,” explaining the degradation of mangroves together with the erosion of the river bank, is introduced. Based on a schematised numerical model, changes in hydrodynamics and exchange processes caused by the limited width of the forest are proposed to be the fundamental reason for the “Squeeze Phenomenon.”… @en

Beach erosion, the loss of sand from a beach due to longshore and/or cross-shore sediment transport mechanisms, is a challenging problem. In order to stabilize the beach and to slow down the rate of beach erosion, the construction of hard hydraulic structures is a traditional option. Groins are one of the oldest man-made hydraulic structures designed to intercept the longshore sediment transport and to stimulate sediment deposition within the groin compartments. However, erosion is likely to appear at the downdrift beach stretch of a groin system, due to lack of sufficient sand feeding from the updrift groined beach reaching the downdrift beach. To alleviate sand starvation at a downdrift beach of groins, groins are suggested to be gradually shorter and more permeable approaching the downdrift terminal groin. The primary advantage of permeable groins, compared to impermeable groins, is they do not entirely block longshore currents. The large openings of permeable groins allow littoral drift to flow through. The shoreline response to permeable groins is comparable to a straight line, other than a zig-zag shape response to impermeable groins. Nevertheless, even though the benefits of permeable groins seem obvious, the research on the subject of the hydrodynamics of permeable groins in coastal waters is limited. @en

Nearshore sandbars have a lifetime of many years, during which they exhibit cyclic, offshore directed behaviour with strong alongshore coherence. A bar is generated near the shoreline and grows in height and width while migrating offshore, before finally decaying at the seaward limit of the surf zone. It may take 10 to 15 years for a bar to exhibit this cycle. Four to five bars may occur simultaneously within a cross-shore bed profile. Alongshore variations in cross-shore bar position and bar amplitude are commonly observed. A strong or abrupt alongshore variability is referred to as a bar switch. At large spatial scales, the inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the bar cycle return period (Tr, i.e. the duration between two successive bar decay events) may differ by a factor of three to four. This type of change in Tr appears to be always present in time and is characterized as a persistent bar switch. At smaller (kilometer) scales, bar switches typically occur in areas with similar Tr-values on both sides of a bar switch and occasionally disappear when the bars re-attach. These are characterized as non-persistent bar switches. The assimilation of shoreface nourishments into the coastal system involves a strong interaction with the pre-existing sandbar system. Typically the placement of a shoreface nourishment just seaward of an outer bar reverses the bar cycle temporalily, inducing a landward migration of the bar system. The shoreface nourishment becomes absorbed in the coastal system as the new outer bar. At the distal ends of the shoreface nourishment bar switches often manifest, owing to a distinct difference in the bar migration cycle phase that is induced. Given the importance of the bar-nourishment interaction, an improved understanding of the nearshore bar dynamics is expected to improve the efficacy of shoreface nourishments. Furthermore, the long-term evolution of the nearshore barred profiles is generally considered indicative of the quality of the modelling for the response of the entire nearshore coastal system. Therefore, the ability to perform reliable and robust a-priori, long-term predictions has broad societal relevance in view of anticipated adverse impacts of climate change and sea level rise on the stability of coasts worldwide. Until now the anatomy of the nearshore sandbars has primarily been studied using field data. Although these studies have provided insight into how the geometric bar parameters respond to the external forcings, no comprehensive conceptual framework is available that explains the full life cycle of a sandbar and its associated characteristics. The overarching objective of this study is to elucidate the anatomy of the inter-annual bar morphology using a combined data and model approach. This overarching objective is in turn devolved into three objectives aiming to understand key features of bar morphology and a further objective to enable a comprehensive modelling approach based on the acquired insights. The latter objective involves the development of an input-reduction framework for advanced process-based forward modelling of the inter-annual bar morphology.   1) To elucidate the morphodynamic processes that result in cross-shore transient sandbar amplitude responses (i.e. the transition from bar growth in the intertidal and across surf zone to sandbar decay at the seaward edge of the surf zone). 2) To establish the role of cross-shore processes in non-persistent bar switches. 3) To identify the dominant environmental variables and the associated mechanisms that govern the bar cycle return period. 4) To develop an input-reduction framework to enable the application of state-of-the-art process based forward area models to simulate the multi-annual bar behaviour and nearshore morphology.   A comprehensive study approach is adopted in which observations of the nearshore morphology are combined with detailed forward modeling of the bar dynamics at Noordwijk (The Netherlands) utilizing wave and waterlevel observations as boundary conditions. The Noordwijk model acts as a reference for additional simulations at Egmond (The Netherland) and at Hasaki (Japan) to address the specific characteristics of the nearshore sandbar morphodynamics as outlined above.  The transient cross-shore bar amplitude response Based on a three-year hindcast of a bar cycle at Noordwijk (Netherlands) and on additional synthetic runs using a wave-averaged cross-shore process model, the dominant mechanisms that govern the bar amplitude growth and decay during net inter-annual offshore migration are identified. The bar amplitude response is particularly sensitive to the water depth above the bar crest, hXb, and the angle of wave incidence, θ. These variables largely control the amount of waves breaking on the bar and the strength and cross-shore distribution of the associated longshore current. The longshore current has its maximum landward of the bar crest, inducing additional stirring of sediment on the landward bar slope and trough. The enhanced sediment concentration in the trough region shifts the cross-shore transport peak landward of the bar crest, forcing bar amplitude growth during offshore migration. For increased hXb-values wave breaking becomes less frequent, reducing the influence of the longshore current on sediment stirring. Therefore, the resulting dominance of the cross-shore current results in a sediment transport peak at, or just seaward of, the bar crest causing bar amplitude decay. All four types of bar response (viz. all combinations of onshore/offshore migration and bar amplitude growth/decay) can occur for a single wave height and wave period combination, depending on hXb and θ. Additional hindcast runs in which the wave direction was assumed time-invariant confirmed that hXb and θ largely control the transient bar amplitude response.  The mechanics of non-persistent bar switches Intra-site alongshore variability is greatest when bars display km-scale disruptions, indicative of a distinct alongshore phase shift in the bar cycle. An outer bar is then, for example, attached to an inner bar, referred to as a non-persistent bar switch. This large-scale alongshore variability is investigated by applying the reference model at 24 transects along a 6 km section of the barred beach at Noordwijk (The Netherlands). When alongshore variability is limited, the model predicts that the bars migrate offshore at approximately the same rate (i.e. the bars remain in phase). Only under specific bar configurations with high wave-energy levels is an increase in the alongshore variability predicted. This suggests that cross-shore processes may trigger a switch in the case of specific antecedent morphological configurations combined with storm conditions. It is expected that three-dimensional (3D) flow patterns augment the alongshore variability in such instances. In contrast to the observed bar behaviour, predicted bar morphologies on either side of a switch remain in different phases, even though the bars are occasionally located at a similar cross-shore position. In short, the 1D profile model is not able to remove a bar switch. This data-model mismatch suggests that 3D flow patterns are key to the dissipation of bar switches.  The mechanics of persistent bar switches and the bar cycle return period To date, data-analytic studies have had only partial success in explaining differences in Tr, establishing at best weak correlations to local environmental characteristics. In the present approach the process-based profile reference model is utilized to investigate the non-linear interactions between the hydrodynamic forcing and the morphodynamic profile response for two sites. Despite strong similarity in environmental conditions, the sites at Noordwijk and Egmond on the Holland coast exhibit distinctly different Tr values. The detailed comparison of modelling results enables a consistent investigation of the role of specific parameters at a level of detail that could not have been achieved from observations alone, and provides insights into the mechanisms that govern Tr. The results reveal that the bed slope at the barred zone is the most important parameter governing Tr. As a bar migrates further offshore, a steeper slope results in a stronger relative increase in hXb which reduces wave breaking and in turn reduces the offshore migration rate. The deceleration of the offshore migration rate as the bar moves to deeper water - the morphodynamic feedback loop - contrasts with the initial enhanced offshore migration behaviour of the bar. The initial behaviour is determined by the intense wave breaking associated with the steeper profile slope. These mechanisms explain the counter-intuitive observations at Egmond where Tr is significantly longer than at Noordwijk despite Egmond having the more energetic wave climate which typically reduces Tr.  Input reduction for inter-annual advanced forward model applications In order to avoid excessively long computation times, input reduction is imperative for the application of advanced forward morphodynamic area models to consider long-term (>years) predictions. Here, an input reduction framework for wave-dominated coastal settings is introduced. The framework comprises 4 steps, viz. (1) the selection of the duration of the original (full) time series of wave forcing, (2) the selection of the representative wave conditions, (3) the sequencing of these conditions, and (4) the time span after which the sequence is repeated. In step (2), the chronology of the original series is retained, while that is no longer the case in steps (3) and (4). We apply the framework to two different sites (Noordwijk, The Netherlands and Hasaki, Japan) with multiple nearshore sandbars but contrasting long-term offshore-directed behaviour: at Noordwijk the offshore migration is gradual and not coupled to individual storms, while at Hasaki the offshore migration is more episodic, and wave chronology appears to control the long-term evolution. The performance of the model with reduced wave climates is compared with a simulation with the actual (full) wave-forcing series. It is demonstrated that input reduction can dramatically affect long-term predictions, to such an extent that the main characteristics of the offshore bar cycle are no longer reproduced. This was the case at Hasaki, in particular, where all synthetic series that no longer retain the initial chronology (steps 3 and 4) lead to rather unrealistic long-term simulations. At Noordwijk, synthetic series can result in realistic behaviour, provided that the time span after which the sequence is repeated is not too large; the reduction of this time span has the same positive effect on the simulation as increasing the number of selected conditions in step 2. It is further demonstrated that, although storms result in the largest morphological change, conditions with low to intermediate wave energy must be retained to obtain realistic long-term sandbar behaviour. The input-reduction framework must be applied in an iterative fashion to obtain a reduced wave climate that is able to simulate long-term sandbar behaviour sufficiently accurately within an acceptable computation time. These results imply that it is essential to consider input reduction as an intrinsic part of any model set-up, calibration and validation effort. The study outcomes indicate clearly that a relatively simple model can be utilized to study the highly non-linear interaction between the nearshore hydrodynamics and morphology in great detail. This was achieved through carefully designed numerical experiments in which the influence of a specific process or environmental variable was isolated and identified. Although the model only considers cross-shore processes, the numerical experiments generated new insights into the importance of 3D processes under particular morphological conditions of the nearshore barred profiles. Even though the model was successfully calibrated at Noordwijk, the application at Egmond showed a significantly reduced predictive capacity. The model was able to reproduce the main characteristics of the inter-annual bar morphodynamics, but the bar cycle return period was under-estimated by about 30%. This suggests that the model can capture trends fairly well, but is unable to produce accurate absolute predictions - a finding that has broader implications. As stated earlier, accurate predictions of the long-term evolution of the nearshore barred profiles are generally considered indicative of the quality of the modelling of the entire nearshore coastal system. Consequently, further improvement of morphodynamic process-based models, particularly for the nearshore zone, constitutes a major research priority. @en