Patch behaviour and predictability properties of modelled finite-amplitude sand ridges on the inner shelf

The long-term evolution of shoreface-connected sand ridges is investigated with a nonlinear spectral model which governs the dynamics of waves, currents, sediment transport and the bed level on the inner shelf. Wave variables are calculated with a shoaling-refraction model instead of using a parameterisation. The spectral model describes the time evolution of amplitudes of known eigenmodes of the linearised system. Bottom pattern formation occurs if the transverse bottom slope of the inner shelf, β, exceeds a critical value β<sub>c</sub>. For fixed model parameters the sensitivity of the properties of modelled sand ridges to changes in the number (<i>N</i>−1) of resolved subharmonics (of the initially fastest growing mode) is investigated. For any <i>N</i> the model shows the growth and subsequent saturation of the height of the sand ridges. The saturation time scale is several thousands of years, which suggests that observed sand ridges have not reached their saturated stage yet. The migration speed of the ridges and the average longshore spacing between successive crests in the saturated state differ from those in the initial state. Analysis of the potential energy balance of the ridges reveals that bed slope-induced sediment transport is crucial for the saturation process. In the transient stage the shoreface-connected ridges occur in patches. The overall characteristics of the bedforms (saturation time, final maximum height, average longshore spacing, migration speed) hardly vary with <i>N</i>. However, individual time series of modal amplitudes and bottom patterns strongly depend on <i>N</i>, thereby implying that the detailed evolution of sand ridges can only be predicted over a limited time interval. Additional experiments show that the critical bed slope β<sub>c</sub> increases with larger offshore angles of wave incidence, larger offshore wave heights and longer wave periods, and that the corresponding maximum height of the ridges decreases whilst the saturation time increases

Patch behaviour and predictability propierties of modelled finite-amplitude sand ridges on the inner shelf

The long-term evolution of shoreface-connected sand ridges is investigated with a nonlinear spectral model which governs the dynamics of waves, currents, sediment transport and the bed level on the inner shelf. Wave variables are calculated with a shoaling-refraction model instead of using a parameterisation. The spectral model describes the time evolution of amplitudes of known eigenmodes of the linearised system. Bottom pattern formation occurs if the transverse bottom slope of the inner shelf, β, exceeds a critical value βc. For fixed model parameters the sensitivity of the properties of modelled sand ridges to changes in the number (N−1) of resolved subharmonics (of the initially fastest growing mode) is investigated. For any N the model shows the growth and subsequent saturation of the height of the sand ridges. The saturation time scale is several thousands of years, which suggests that observed sand ridges have not reached their saturated stage yet. The migration speed of the ridges and the average longshore spacing between successive crests in the saturated state differ from those in the initial state. Analysis of the potential energy balance of the ridges reveals that bed slope-induced sediment transport is crucial for the saturation process. In the transient stage the shoreface-connected ridges occur in patches. The overall characteristics of the bedforms (saturation time, final maximum height, average longshore spacing, migration speed) hardly vary with N. However, individual time series of modal amplitudes and bottom patterns strongly depend on N, thereby implying that the detailed evolution of sand ridges can only be predicted over a limited time interval. Additional experiments show that the critical bed slope βc increases with larger offshore angles of wave incidence, larger offshore wave heights and longer wave periods, and that the corresponding maximum height of the ridges decreases whilst the saturation time increases.Postprint (published version

Mechanisms controlling crescentic bar amplitude

The formation of crescentic bars from self-organization of an initially straight shore-parallel bar for shore-normal incident waves is simulated with a two-dimensional horizontal morphodynamical model. The aim is to investigate the mechanisms behind the saturation process defined as the transition between the linear regime (maximum and constant growth of the crescentic pattern) and the saturated state (negligible growth). The global properties of the morphodynamical patterns over the whole computational domain are studied (“global analysis”). In particular, consideration of the balance of the potential energy of the emerging bar gives its growth rate from the difference between a production term (related to the positive feedback leading to the instability) and a damping term (from the gravity-driven downslope transport). The production is approximately proportional to the average over the domain of the cross-shore flow velocity times the bed level perturbation. The damping is essential for the onset of the saturation, but it remains constant while the production decreases. Thus, it is notable that the saturation occurs because of a weakening of the instability mechanism rather than an increase of the damping. A reason for the saturation of the crescentic bar growth is the change in bar shape from its initial stage rather than the growth in amplitude itself. This change is mainly characterized by the narrowing of the rip channels, the onshore migration of the crests, and the change in the mean beach profile due to alongshore variability. These properties agree with observations of mature rip channel systems in nature.Postprint (published version

Understanding coastal morphodynamic patterns from depth-averaged sediment concentration

This review highlights the important role of the depth-averaged sediment concentration (DASC) to understand the formation of a number of coastal morphodynamic features that have an alongshore rhythmic pattern: beach cusps, surf zone transverse and crescentic bars, and shoreface-connected sand ridges. We present a formulation and methodology, based on the knowledge of the DASC (which equals the sediment load divided by the water depth), that has been successfully used to understand the characteristics of these features. These sand bodies, relevant for coastal engineering and other disciplines, are located in different parts of the coastal zone and are characterized by different spatial and temporal scales, but the same technique can be used to understand them. Since the sand bodies occur in the presence of depth-averaged currents, the sediment transport approximately equals a sediment load times the current. Moreover, it is assumed that waves essentially mobilize the sediment, and the current increases this mobilization and advects the sediment. In such conditions, knowing the spatial distribution of the DASC and the depth-averaged currents induced by the forcing (waves, wind, and pressure gradients) over the patterns allows inferring the convergence/divergence of sediment transport. Deposition (erosion) occurs where the current flows from areas of high to low (low to high) values of DASC. The formulation and methodology are especially useful to understand the positive feedback mechanisms between flow and morphology leading to the formation of those morphological features, but the physical mechanisms for their migration, their finite-amplitude behavior and their decay can also be explored

Children’s Participation in the Justice System

The rights of children in youth justice and civil court proceedings, and in particular the right of children to be heard or to “participate” in such systems, is an area in which there has been much interest in recent years, particularly sparked by Article 12 of the UN Convention on the Rights of the Child. There are a wide variety of proceedings in which children’s interests are decided, for example, where they are accused of a crime, where their parents are in dispute on family breakdown and where there are child protection concerns. This chapter examines recent developments in standards at international level concerning children’s participation in proceedings, such as the drafting of General Comment No. 12 of the UN Committee on the Rights of the Child on the right to be heard and the Guidelines on Child-friendly Justice of the Council of Europe. It draws on recent international research in order to provide analysis of the extent to which such standards have affected practice and made a difference for children. It concludes that although the development of such standards is to be welcomed, and although these standards have achieved some improvements at domestic level, the more extensive modifications required for genuine participation of children in the justice system has not occurred.Effective Protection of Fundamental Rights in a pluralist worl

Modeling the morphodynamics of shoreface-connected sand ridges

The focus of this thesis is on the morphodynamics of shoreface-connected sand ridges, which are large-scale bedforms observed on the inner shelf of coastal seas where storms occur frequently. The main aim was to explore which physical processes control the formation, long-term evolution and main characteristics of these ridges. Existing idealized morphodynamic models were used and extended with potentially important physical processes. The main idea behind these models is that bedforms can evolve due to interactions between waves (which stir sediment from the bottom), storm-driven currents (which transport sediment) and the sandy seabed. The model setting resembles the storm-dominated Long Island inner shelf. The first extension with respect to previous work, as presented in chapter 2 and used in all chapters, is that wave variables are calculated with a shoaling-refraction module. Results (chapter 2) reveal that a necessary condition for growth of ridges is that the transverse bottom slope of the inner shelf exceeds a critical value. Exceeding the critical bottom slope, results in the evolution of ridges with a preferred longshore spacing, growth time and migration speed of about 7 km, 1800 yr and 1 m/yr, respectively. Variations in the latter for a change in offshore wave characteristics are the consequence of a different wave transformation across the inner shelf. In chapter 3 the shoaling-refraction wave module was added to a nonlinear version of the morphodynamic model, meant to study the long-term evolution of shoreface-connected ridges. Simulations show that, in time, the shape of the ridges becomes asymmetrical (steeper seaward flanks), their height saturates and their migration speed decreases. Analysis of the energy balance of ridges reveals that bed slope-induced sediment transports are crucial for the saturation process. Adding subharmonic modes (i.e., eigenmodes with wavelengths larger than that of the initially fastest growing mode), shows a 20% increase in the spacing of ridges in time. The evolution of the individual modal amplitudes is sensitive to the number of subharmonics included and thus is only predictable for a finite amount of time. Interestingly, ridges form patches in the transient stage, which is also observed. In chapter 4 refraction and shoaling processes due to the presence of bedforms (called wave-bedform feedbacks) are also accounted for. Including wave-bedform feedbacks results in a new mechanism for ridge growth, which also works for a flat bed. The spacing and offshore extent of ridges decrease, which is in accordance with observations. The influence of wave-bedform feedbacks on the initial formation of ridges for a bimodal sediment mixture was investigated in chapter 5. Results indicate that ridge growth and migration decrease in case of a mixture. In case that the entrainment of sediment is dependent on bottom roughness the coarsest sediment is found in the troughs, whereas in all other cases fine sediment occurs in the troughs. Some shoreface-connected ridges exhibit grain size patterns, which are in agreement with those obtained by the model, however not the Long Island ones. The modeled maximum variation in the mean grain size over the topography has improved with respect to previous work

Effect of wave–bedform feedbacks on the formation of, and grain sorting over shoreface-connected sand ridges

The influence of wave–bedform feedbacks on both the initial formation of shoreface-connected sand ridges (sfcr) and on grain size sorting over these ridges on micro-tidal inner shelves is studied. Also, the effect of sediment sorting on the growth and the migration of sfcr is investigated. This is done by applying a linear stability analysis to an idealized process-based morphodynamic model, which simulates the initial growth of sfcr due to the positive coupling between waves, currents, and an erodible bed. The sediment consists of sand grains with two different sizes. New elements with respect to earlier studies on grain sorting over sfcr are that wave-topography interactions are explicitly accounted for, entrainment of sediment depends on bottom roughness, and transport of suspended sediment involves settling lag effects. The results of the model indicate that sediment sorting causes a reduction of the growth rate and migration speed of sfcr, whereas the wavelength is only slightly affected. In the case where the entrainment of suspended sediment depends on bottom roughness, the coarsest sediment is found in the troughs; otherwise, the finest sediment occurs in the troughs. Compared to previous work, modeled maximum variations in the mean grain size over the topography are in better agreement with field observations. Settling lag effects are important for the damping of high-wavenumber mode instabilities such that a preferred wavelength of the bedforms is obtaine

Patch behaviour and predictability propierties of modelled finite-amplitude sand ridges on the inner shelf

The long-term evolution of shoreface-connected sand ridges is investigated with a nonlinear spectral model which governs the dynamics of waves, currents, sediment transport and the bed level on the inner shelf. Wave variables are calculated with a shoaling-refraction model instead of using a parameterisation. The spectral model describes the time evolution of amplitudes of known eigenmodes of the linearised system. Bottom pattern formation occurs if the transverse bottom slope of the inner shelf, β, exceeds a critical value βc. For fixed model parameters the sensitivity of the properties of modelled sand ridges to changes in the number (N−1) of resolved subharmonics (of the initially fastest growing mode) is investigated. For any N the model shows the growth and subsequent saturation of the height of the sand ridges. The saturation time scale is several thousands of years, which suggests that observed sand ridges have not reached their saturated stage yet. The migration speed of the ridges and the average longshore spacing between successive crests in the saturated state differ from those in the initial state. Analysis of the potential energy balance of the ridges reveals that bed slope-induced sediment transport is crucial for the saturation process. In the transient stage the shoreface-connected ridges occur in patches. The overall characteristics of the bedforms (saturation time, final maximum height, average longshore spacing, migration speed) hardly vary with N. However, individual time series of modal amplitudes and bottom patterns strongly depend on N, thereby implying that the detailed evolution of sand ridges can only be predicted over a limited time interval. Additional experiments show that the critical bed slope βc increases with larger offshore angles of wave incidence, larger offshore wave heights and longer wave periods, and that the corresponding maximum height of the ridges decreases whilst the saturation time increases

Effect of wave-topography interactions on the formation of sand ridges on the shelf

The role of wave-topography interactions in the formation of sand ridges on microtidal inner shelves is investigated with an idealized morphodynamic model. The latter uses the two-dimensional shallow water equations to describe a storm-driven flow on an inner shelf with an erodible bottom and a transverse slope. Both bed load and suspended load sediment transport are included. New are the incorporation of a wave module based on physical principles and a critical shear-stress for erosion. A linear stability analysis is used to study the initial growth of bed forms, by analyzing the initial growth of small perturbations evolving on an alongshore uniform basic state, which describes a storm-driven flow on a microtidal inner shelf. Model simulations show that wave-topography interactions cause the ridges to become more trapped to the coast. Both growth and migration of the ridges are controlled by suspended load transport. The physical mechanism responsible for ridge growth is related to transport by the storm-driven current of sediment that is entrained due to wave orbital motions induced by bed forms. This new mechanism even acts in absence of a transverse bottom slope. The orientation, spacing and shape of the modeled ridges agree well with field observations from different shelves