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.
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This thesis examines the effects of the stratified tidal flow on the morphodynamics of the Dutch inner shelf. The south portion of the Dutch inner shelf is strongly influenced by the Rhine River ROFI (Region Of Freshwater Influence), which is generated by the discharge from the Rhine River through the Rotterdam waterways. Under stratified conditions, the three-dimensional structure of the tidal currents develops a strong cross-shore shear so that the bottom and surface currents become 180deg out of phase. The sheared flow created by stratification operates in the inner shelf and nearshore zones so that the flow asymmetries imparted by stratification are expected to impact the morphodynamics, however the role of the stratified tidal flow on the morphodynamics along the Dutch coast has been often neglected or oversimplified. In this context, this thesis aims to provide new insights on how the stratified tidal flow dictates the morphodynamics outside the surfzone.

In the south portion of the Dutch coast is located the Sand Engine, a 21.5 million m3 experimental mega-nourishment that was built in 2011. This intervention created a discontinuity in the previous straight sandy coastline, altering the local hydrodynamics in a region that is influenced by the Rhine River ROFI. Estimates of the centrifugal acceleration directly after construction of the Sand Engine showed that its curved shape impacted the cross-shore flow, suggesting that the Sand Engine might have played a role in controlling the cross-shore exchange currents during the first three years after the completion of the nourishment. Presently, the curvature effects are minute owned to the morphodynamic evolution of the Sand Engine. Observations document the development of strong baroclinic-induced cross-shore exchange currents dictated by the intrusion of the river plume fronts as well as the classic tidal straining which are found to extend further into the nearshore (from 12 to 6 m depth), otherwise believed to be a mixed zone.

In the inner shelf, shoaling waves are as effective in mobilizing sediment as the other co-existing flows. The influence of stratification on the hydrodynamics is translated into near-bed shear velocity in the layer immediately above the sea floor. The tide-induced bed shear stress is able to periodically agitate the bed near the peaks of flood and ebb cycles mostly during spring tides. Results from observations suggested that, under stratified conditions, relatively high values of bed shear stress are sustained for a prolonged period of time. The results also revealed that the non-tidal flow, such as the wind-induced flow, plays a role in controlling the bed mobility. However, wave-induced bed shear stress in general does not set sediment in motion during fair weather conditions and thus the stirring role of the waves is mostly important during storms.

The co-exiting near-bed flows in the inner shelf are responsible for moulding the seafloor so that the resulting types of bedforms can reveal important information on the hydrodynamic forcings that dictate the sediment mobility. Observations showed that 56% of the ripples in the Dutch inner shelf are classified as current ripples. Wave ripples occur only during storm conditions, comprising 3%. The frequency of occurrence of transitional bed types composes 23% and poorly developed ripples is found to develop mostly during neap tides making up 15% of the observed bed types. The feedback of the different types of bedforms on the overlying boundary layer plays a fundamental role in the dynamics of the sediment load.

The morphological response of the bed to the stratified and non-stratified tidal flow leads to differentiations of the ripple migration as well as the sediment transport modes (bedload and suspended load). The bedforms at the measurement site are strongly controlled by tides so that their behavior exhibits not only a spring-neap signature, but also a distinct semi-diurnal fluctuation. Under the influence of the Rhine ROFI, the bedform mean dimensions (ripple height and wavelength) are reduced, indicating that their development is affected by the stratified tidal flow. In the absence of (ambient) stratification, the tidal current ripples are more developed, attaining relatively larger dimensions. The net alongshore bedload transport is south-directed, whereas the net alongshore suspended load is north-directed regardless of stratification. Moreover, the net alongshore bedload transport is higher during stratified conditions but the net alongshore suspended transport is smaller. Regarding the cross-shore sediment transport, the findings show that ambient stratification promotes onshore-directed bed- and suspended load net transport. The gross suspended transport rates are 10 times greater than the gross bedload transport rates. ... Read more

The main objective of this study is to unravel the physical processes that control typical coastal systems in Central Vietnam while challenged by the fact that it is a data limited environment. Inlets and bays are the typical coastal system along the central coast of Vietnam. These coastal systems are strongly influenced by tropical monsoon conditions, which are characterised by variations in seasonal wave conditions and seasonal river flow. These systems are even more vulnerable to extreme weather conditions, such as floods and storms, because of the complex topography of a relatively narrow and steep mountain range which is directly connected to a dense river network in the low-lying coastal plains at the downstream end. Economic and ecological values in the coastal area are under pressure as a result of the intensification of natural disasters and human interventions. Notable examples of this are Cua Dai beach and Da Nang bay. Cua Dai beach lies adjacent to the Cua Dai inlet which is a typical seasonal varying tidal inlet connected to the catchment area of the Vu Gia-Thu Bon River. Cua Dai beach has suffered extreme erosion in the recent decade. Da Nang bay is a complex bay beach headland downstream of the Vu Gia River that discharges into this bay. Also, this system is affected by human interventions. Due to the common downstream basin of the Vu Gia-Thu Bon River system, both being typical coastal systems in Central Vietnam that experience data limitations, this study attempts to combine and understand the hydrodynamics and morphodynamics of Cua Dai inlet and the complex Da Nang embayment. In order to identify and quantify the main processes governing the evolution of Cua Dai beach to explain the morphological changes and extreme erosion in recent years, a new approach was developed. Historical shoreline positions and sediment budget changes are the two parameters of main importance in the approach to quantify the erosion processes in the Cua Dai coastal inlet. Historical shoreline changes were derived fromsatellite images and associated sediment budgets were estimated based on shoreline change rates using additional assumptions, such as defining a closure depth and a time invariant beach profile. To gain insight into the sediment transport along the Cua Dai beach, additional numerical models and empirical equations are used to investigate the variation in alongshore sediment transport induced by waves. Further analysis on how seasonal variation in both waves and river discharge impacts the morphodynamics of the ebb tidal delta and its adjacent coasts is performed based on process-based modelling. ... Read more

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. ... Read more

This thesis investigates the behaviour of the often used point-wise skill score, the MSESSini a.k.a. BSS, and develops new error metrics that, as opposed to point-wise metrics, take the spatial structure of morphological patterns into account. The MSESSini measures the relative accuracy of a morphological prediction over a prediction of zero morphological change, using the mean-squared error (MSE) as the accuracy measure. The main findings about the MSESSini are: 1) a generic ranking, based on values for MSESSini, has limited validity, since the zero change reference model fails to make model performance comparable across different prediction situations; 2) the combination of larger, persistent and smaller, intermittent scales of cumulative change may lead to an increase of skill with time, without the prediction on either of these scales becoming more skilful with time; 3) in the presence of inevitable location errors, the MSESSini favours predictions that underestimate the variance of cumulative bed changes and 4) existing methods to correct for measurement error are inconsistent in either their skill formulation or their suggested classification scheme. In order to overcome the inherent limitations of point-wise metrics, three novel diagnostic tools for the spatial validation of 2D morphological predictions are developed. First, a field deformation or warping method deforms the predictions towards the observations, minimizing the squared point-wise error. Error measures are formulated based on both the smooth displacement field between predictions and observations and the residual point-wise error field after the deformation. In contrast with the RMSE, the method captures the visual closeness of morphological patterns. Second, an optimal transport method defines the distance between predicted and observed morphological fields in terms of an optimal sediment transport field. The optimal corrective transport field moves the misplaced sediment from the predicted to the observed morphology at the lowest quadratic transportation cost. The root-mean-squared value of the optimal transport field, the RMSTE, is proposed as a new error metric. As opposed to the field deformation method, the optimal transport method is mass-conserving, parameter-free and symmetric. The RMSTE, unlike the RMSE, is able to discriminate between predictions that differ in the misplacement distance of predicted morphological features. It also avoids the consistent reward of the underestimation of morphological variability that the RMSE is prone to. Third, a scale-selective validation approach allows any metric to selectively address multiple spatial scales. It employs a smoothing filter in such a way that, in addition to the domain-averaged statistics, localized validation statistics and maps of prediction quality are obtained per scale (geographic extent or areal size of focus). The employed skill score weights how well the morphological structure and variability are simulated, while avoiding to reward the underestimation of variability. To fully describe prediction quality multiple metrics are required with a weighting determined by the goal of the simulation. Point-wise metrics should be supplemented with an error decomposition, as to avoid undesired underestimation of variability. Further, a set of performance metrics must include a metric, e.g. the RMSTE, that accounts for the spatial structure of the observed and predicted morphological fields. ... Read more

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. ... Read more

This thesis investigates how selected multivariate probabilistic methods can be adapted for risk analysis and decision making in coastal and offshore environments. In particular, the thesis makes a contribution to decision support tools for risk reduction efforts in coastal environments and to statistical simulation methods for wave conditions. Generally, very few observations on negative impacts in coastal or offshore environments are available for risk analysis or decision making due to the rare nature of extreme events. However, synthetic impact data can be generated by propagating relevant hydro meteorological conditions to the environment of interest through a chain of multiple models. Especially in coastal environments, this chain often includes computationally intensive models. ... Read more

This thesis explores the nature of aeolian sediment availability and its influence on aeolian sediment transport. The aim is to improve large scale and long term aeolian sediment transport estimates in (nourished) coastal environments. The generally poor performance of aeolian sediment transport models with respect to measurements in coastal environments is often accredited to limitations in sediment availability. Sediment availability can be limited by particular properties of the bed surface. For example, if the beach is moist or covered with non-erodible elements, like shells. If sediment availability is limited, the aeolian sediment transport rate is governed by the sediment availability rather than the wind transport capacity. Aeolian sediment availability is rather intangible as sediment availability is not only affected by aeolian processes, but also by marine and meteorological processes that act on a variety of spatial and temporal scales. The Sand Motor 21 Mm3 mega nourishment is used to quantify the spatiotemporal variations in aeolian sediment availability and its effect on aeolian sediment transport. The Sand Motor was constructed in 2011 along the Dutch coast. Aeolian sediment accumulation in the Sand Motor region is low compared to the wind transport capacity, while the Sand Motor itself is virtually permanently exposed to wind and accommodates large fetches. Aeolian sediment availability is therefore likely to dominate aeolian sediment accumulation. Multi-annual bi-monthly measurements of the Sand Motor's topography are used for a large scale aeolian sediment budget analysis. The analysis revealed that aeolian sediment supply from the dry beach area, that is almost permanently exposed to wind, diminished a half year after construction of the Sand Motor. The reduction in aeolian sediment supply is likely due to the development of a beach armor layer. In the subsequent years, two-third of the aeolian sediment deposits originate from the low-lying beaches that are frequently flooded and therefore often moist. The importance of the low-lying beaches in the Sand Motor region is tested during a six-week field campaign. Gradients in aeolian sediment transport are measured during the field campaign as to localize aeolian sediment source and sink areas. A consistent supply from the intertidal beach area was measured that was temporarily deposited at the higher dry beach. The temporary deposits were transported further during high water, when sediment supply from the intertidal beach ceased, resulting in a continuous sediment supply to the dunes. The temporary deposition of sediment at the dry beach was likely promoted by the presence of a berm that affects the local wind shear. Moreover, the berm edge coincided with the onset of the beach armor layer that might have further promoted deposition of sediment. The measurements on spatiotemporal variations in aeolian sediment availability and supply inspired an attempt to capture the characteristics of aeolian sediment availability in coastal environments in a comprehensive model approach. The resulting model simulates spatiotemporal variations in bed surface properties and their combined influence on aeolian sediment availability and transport. The implementation of multi-fraction aeolian sediment transport in the model introduces the recurrence relation between aeolian sediment availability and transport through self-grading of sediment. The model was applied in a four-year hindcast of the Sand Motor mega nourishment as first field validation. The model reproduces the multi-annual aeolian sediment erosion and deposition volumes, and the relative importance of the intertidal beach area as source of aeolian sediment well. Seasonal variations in aeolian sediment transport are incidentally missed by the model. The model accuracy is reflected in a R2 value of 0.93 when comparing time series of measured and modeled total aeolian sediment transport volumes in the four years since construction of the Sand Motor. The results suggest that indeed significant limitations in sediment availability, due to soil moisture content and beach armoring, govern aeolian sediment transport in the Sand Motor region. A comparison with a simulation without limitation in sediment availability suggests that aeolian sediment availability in the Sand Motor region is limited to about 25% of the wind transport capacity. Moreover, both spatial and temporal variations in aeolian sediment availability as well as the recurrence relation between aeolian sediment availability and transport are essential to accurate long term and large scale aeolian sediment transport estimates. ... Read more