1,721,022 research outputs found
Wave Damping due to Wooden Fences along Mangrove Coasts
No full textDao, T.; Stive, M.J.F.; Hofland, B., and Mai, T., 2018. Wave damping due to wooden fences along mangrove coasts. In the Mekong Delta, as in many other mangrove settings, wooden fences are considered beneficial coastal structures to provide sheltering for mangrove replantation efforts by reducing waves and currents and promoting sedimentation. One of the most quantitative previous studies on fence-induced wave reduction offered a first understanding of relevant process parameters. The present application of the advanced numerical time-domain wave model SWASH increases this understanding substantially and explains previously unexplained phenomena that were encountered in this earlier study. The findings reveal that wave damping increases with increasing fence thickness and with increasing density of the woody material in the fences. It further shows that the transmitted wave height (represented by the transmission coefficient) is inversely proportional to the Ursell number, implying that nonlinear waves are damped more effectively.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Coastal EngineeringHydraulic Structures and Flood Ris
Nederland veilig, nu en later
No full textDe Jonge Deltacommissie is een open en onafhankelijk netwerk dat bestaat uit alle jonge professionals die ideehebben ingezonden om Nederland waterveilig te houden. De Jonge Deltacommissie zal na het uitbrengen van het advies van de Deltacommissie een vervolg krijgen. De Jonge Deltacommissie wil dat Nederland de komende eeuwen goed beschermd is tegen overstromingen en wateroverlast en meent dat, dat ook kan. De Jonge Deltacommissie wil dat er voldoende zoet water in Nederland beschikbaar blijft. De Jonge Deltacommissie wil een bestuurder met lef en visie! Ieder bestuurlijk organisatorisch model kent zijn beperkingen een bestuurder met lef stapt over deze beperkingen heen. De Jonge Deltacommissie meent dat leven met water risicos met zich meebrengt en dat deze in zekere mate beheersbaar en voorspelbaar moeten blijven Helaas lukt dit bij water niet altijd. Wij menen dat de onvoorspelbaarheid van water en de risicos die dit met zich meebrengt geen inrichtingsvoorwaarden voor ons leven in Nederland moet zijn, maar juist een kans moet zijn voor de inrichting van water in Nederland. De Jonge Deltacommissie heeft in korte tijd 64 ideeverzameld. Deze ideezijn tijdens een druk bezocht symposium aangeboden aan de Deltacommissie. Op eigen initiatief hebben jonge professionals ideeuitgewerkt, ingestuurd en gepresenteerd aan elkaar, aan een raad van Oude Wijzen en aan de Deltacommissie. De Jonge Deltacommissie voorziet in een behoefte als een platform maar ook als een denktank. Het thema waterveiligheid leeft bij jonge professionals. Wat opvalt is dat de jonge professionals kijken naar meerdere oplossingsrichtingen. Ze kijken niet alleen naar technische oplossingen, maar juist ook ruimtelijke, economische, educatie en bestuurlijke oplossingsrichtingen. De ingediende ideegeven zo een goed beeld van de breedte van het tegenwoordige debat over water en waterveiligheid. Voor al deze oplossingsrichtingen wordt uitgegaan van een bepaalde mate van bescherming hoewel dat in de ideeniet altijd voorop staat. Zo staan ook randvoorwaarden als extreme neerslag en droogte centraal. Ook worden radicale plannen niet geschuwd, zoals gedeeltelijke terugtrekking. Deze plannen lijken vaak onhaalbaar, en zullen dat misschien ook zijn, maar verdienen wel aandacht te krijgen. Geen enkel idee bevat de absolute oplossing. Sterker nog, enige mate van restrisico zal er altijd zijn. Aandacht voor dit restrisico is noodzakelijk. Zo bevat een groot aantal ideeook aandacht voor gevolg beperkingen van een ramp als een overstroming in Nederland en het omgaan met de gevolgen, bijvoorbeeld door evacuatie en verzekeringsconstructies. Dat er meerdere problemen kunnen zijn in een gebied waarvoor er ook andere oplossingen nodig zijn. Onze ideezijn bouwstenen voor het maken van plannen. Per probleemgebied, en per probleem, kunnen combinaties van ideevan jonge professionals en andere ideesamen worden uitgewerkt en vergeleken met andere alternatieven. Tijdens het symposium is gebleken dat de inzenders van de ideehier aan kunnen werken, aandacht hebben voor de voor- en de nadelen en ideeleiden tot bijstellen en combineren. De inbreng van jonge professionals is belangrijk en wordt gewaardeerd. Zowel de Deltacommissie, overheid als de provincie Zuid-Holland, Innovatieprogrammas als Leven met Water, Netwerk Deltatechnologie, en het Innovatieplatform hebben zich uitgesproken voor een voortzetting van de Jonge Deltacommissie. De organisaties van de inzenders (universiteiten, bedrijfsleven, overheid, onderzoeksinstituten) hebben deze jonge professionals de ruimte geboden om zich te organiseren, en ideeuit te werken en te delen. Ook de bedrijven en universiteiten die de leden van het kernteam beschikbaar hebben gesteld hebben tijd en ruimte geboden aan deze mensen om alles te organiseren. Wat we zien is dat ook bij deze organisaties het belang van frisse ideeen ontwikkeling van young professionals wordt herkend. De ideevan jonge professionals zijn opgesteld om NU al te gebruiken. De ideegeven een impuls om verder na te denken over oplossingsrichtingen en innovaties. We kunnen hiermee invulling geven aan de aanbevelingen van de Deltacommissie. Voor de genoemde probleemgebieden kunnen we plannen maken en alternatieven eerlijk, helder en transparant beoordelen en afwegen. We hebben hiervoor een pamflet opgesteld met als motto: Aan de slag, we zijn er klaar voor.Civil Engineering and Geoscience
Cross-sectional stability of double inlet systems
No full textBarrier coasts and their associated tidal inlet systems are a common feature in many parts of the world. They constitute dynamic environments that are in a continuous stage of adapting to the prevailing tide and wave conditions. Commonly, these coastal areas are densely populated and (partly) as a result there often exists a strong conflict of interests between issues related to coastal safety, economic activities and ecology. To manage these different interests, it is important to gain more understanding of the long-term morphological evolution of tidal inlet systems and their adaptation to natural changes and human intervention. In this thesis the focus is on double inlet systems, where two tidal inlets connect a back-barrier basin to an ocean or a coastal sea. To investigate the morphological evolution of double inlet systems and their adaptation to internal or external change, the equilibrium configuration and stability properties of the cross-sectional areas of the two tidal inlets are studied in detail. To that extent, a widely used empirical relationship for cross-sectional inlet stability is combined with (i) a lumped-parameter (L-P) model (Chapters 2 and 3) and (ii) a two-dimensional, depth-averaged hydrodynamic (2DH) model for the water motion (Chapter 4). The Marsdiep-Vlie inlet system in the western Dutch Wadden Sea and the Faro-Armona inlet system in the Portuguese Ria Formosa serve as case studies throughout this thesis. With the assumptions of a cross-sectionally averaged, uniform inlet flow velocity and a uniformly fluctuating basin surface elevation, model results of the L-P model show that stable equilibrium configurations where both inlets are open exist. It is necessary, however, to account for the important processes either explicitly, e.g. including a topographic high in the back-barrier basin as observed in the Wadden Sea (Chapter 2), or parametrically, e.g. allowing for inlet entrance/exit losses for relatively short inlets such as in the Ria Formosa (Chapter 3). By solving the depth-averaged, linear shallow water equations on the f-plane with linearised bottom friction, the 2DH model explicitly accounts for spatial variations in surface elevation in the ocean, inlets and basin. Model results show that these spatial variations, induced by e.g. basin bottom friction, radiation damping, and Coriolis effects, are crucial to simulate and explain the long-term evolution of double inlet systems. This approach further allows the identification of a stabilising and destabilising mechanism associated with the persistence or closure of one (or both) of the inlets in a double inlet system and hence with its long-term evolution.Hydraulic EngineeringCivil Engineering and Geoscience
Modelling rhythmic morphology in the surf zone
No full textCoastal erosion is increasingly being prevented by the application of shoreface nourishments. Although they are applied successfully, their development and their impact on the coastal system is still difficult to predict. Especially the effects of shoreface nourishments on the already existing rhythmic bed forms in the surf zone are unknown. The importance of insight into the occurrence and behaviour of these rhythmic features in the surf zone lies in the fact that the rhythmic breaker bar topography might be mirrored on to the dry beach, thereby locally creating narrower and therefore weaker beaches. Besides, currents associated with these bed forms are hazardous for swimmers. The aim of this thesis is therefore to model these rhythmic morphological features in the surf zone with and without the presence of a shoreface nourishment in order to gain this insight. Using linear stability analyses and morphodynamic computations, it shows that, among others, the length of the shoreface nourishment is an important parameter for its impact on the length scale of the rhythmic bed forms in the surf zone.Civil Engineering and Geoscience
Coastal Dynamics of the Danube Delta
No full textLarge parts of the Danube Delta coast suffered severe erosion during last decades due to both natural trends and human interventions. To investigate and quantify the processes controlling the Danube Delta coast evolution a systematic approach was employed. First, a hindcast of the wave climate was simulated using the numerical model SWAN and wind data recorded locally during 11 consecutive years. Second, the sediment transport was calculated using the simulated wave climate, field data and an one line numerical model (UNIBEST CL+). A sediment budget and predictions for the future evolution of the study zone were derived further on. Third, the process based numerical model Delft-3D was used to investigate the episodic water level changes which produce locally large shoreline retreats. Fourth, an ideal spit was design and modelled with Delft-3D to understand the relationship between the two sediment transport components: cross and along shore. The results, along with historical data, were used to study the dynamics of the Sahalin spit, a particular geomorphologic feature formed at southernmost distributary mouth. Based on this particular case a conceptual model for formation and evolution of a spit at a river mouth was proposed. The thesis concludes with recommendations to mitigate the coastal erosion in a sustainable way by artificially supplying the retreating beach sectors.Hydraulic EngineeringCivil Engineering and Geoscience
Morphodynamics of seasonally closed coastal inlets at the central coast of Vietnam
No full textSituated in a monsoon-prone humid tropical region, Vietnam is affected by both oceanic and continental climates causing disasters to the country like riverine flooding and storm induced damage. The coastal districts of Vietnam have a population of about 18 million habitants, account for nearly one fourth of the total population of the country and locate about 50% of the major towns and cities of Vietnam. Most of the people currently living in the coastal zone have their livelihood mainly relying on marine resources and they are also the most vulnerable to sea-related natural disasters, such as storms and floods. The natural disasters occurring in the coastal strip in the central part of Vietnam, caused by meteorological and oceanographical factors, are intensified by human interventions, like the damming of rivers for various purposes or the extensive deforestation for the creation of agricultural lands. With more than 1,000 km of coastline, the central coast of Vietnam has more than sixty inlets and river mouths discharging into the South China Sea. These systems play a vital role in social-economic activities in the region. The steep rivers with abundant natural but temporally unevenly distributed flows make the low-lying coastal plains in the region prone to inundation by flooding, while the river is almost dry during the rest of the year. Specific topographical features and hydrological characteristics of the region produce a particularly high seasonal geomorphological variation of tidal inlets and river mouths, from narrowing, shoaling or entirely closing in the dry season to widening or breaching in the flood period. Frequent disasters set back development efforts in this poorest region of Vietnam and trap people in a cycle of poverty. Stabilising inlets at the central coast of Vietnam therefore is recognised as one of the priority tasks to mitigate potential risks caused by natural disasters, especially by floods and storms on low-lying coastal plains, and to promote a safe and stable condition for social-economic development in the region. To carry out this task, Vietnam needs both substantial financial and human resources, particularly knowledge and experience in coastal engineering, which is not trivial for a developing country. Additionally, strong seasonal variation of inlets and estuaries contribute to the complexity of problems and raise a necessity to implement a strategy for inlet and river mouth stabilisation under the constraints of a shortage of resources and knowledge. This thesis focuses on tidal inlets and estuaries in a wave-dominated, micro-tidal environment under the influence of episodic river flooding in the central coast of Vietnam. Natural behaviour and morphological stability of tidal inlets, which significantly interact with channel migration, entrance shoaling or closure have been identified and analysed based on field observations, historical satellite images, topographical maps and bathymetrical data. Based on the regional natural settings and hydrodynamic-morphological features, tidal inlets along the central coast of Vietnam can be divided into two main categories, namely, (1) barrier lagoon inlets and (2) wave dominated estuary inlets. A conceptual model for channel evolution and seasonal opening/closure of tidal inlets is proposed which describes the cyclic evolution of a typical tidal inlet at the central coast of Vietnam. In the conceptual model, the inlet entrance is forced both by the alongshore current which deposits sediment in the inlet channel and by the ebb tidal and river generated currents which erode sediment from the inlet channel. The interpretation of the Escoffier diagram is extended conceptually to explain the seasonal variation of both open equilibrium and closure. The variation is regulated by the seasonal variation of river flow and littoral drift. The conceptual model indicates the two major processes which dominate in the dry and the flood season leading to a deviation from the stable and unstable equilibrium points in the Escoffier diagram. This supports our understanding of seasonal variation of coastal inlets and estuaries in a region that experiences monsoons and storms causing a large fluctuation in littoral drift and ebb flow at the central coast of Vietnam. To get deeper insight into the underlying processes and cross-sectional stability of an schematised tidal inlet, regulated by tides only and regulated by both tides and waves, the process-based morphodynamic modelling system Delft-3D has been applied. In the model the tidal period, amplitude, basin area and initial inlet dimensions were changed systematically to create different hydrodynamic environments for inlet evolution. The model successfully reproduces the evolution of the channel flow area towards equilibrium for a tidal inlet and is able to describe the main behaviour of an inlet in response to a range of tide and wave conditions and geometries. The model results are in good agreement with empirical relationships (O'Brien, 1969; Jarrett, 1976) and the analytical solution (DiLorenzo, 1988) of Escoffier's diagram. To investigate location stability of inlet channels, seven experiments were designed to cover 3 different stability ranges (poor, fair to good stability). Reliable model results increase the understanding of the processes underlying the migration and closure of a tidal inlet. It is found that tidal inlet behaviour and location stability is linked to the number of channels on the ebb delta, the curvature if there is only one channel, type of bar on the ebb delta, the migration of the updrift barrier island, the distance between the inlet throat and the outer of margin of entrance bar. The model results demonstrate that the process-based model is able to reproduce the morphological evolution of a tidal inlet fairly consistent with the Bruun et al. (1978) empirical criteria for location stability. A typical example of a tidal inlet migrating due to oblique waves which includes features such as ebb channel migration, shifting and diminishing, and the bypassing of ebb shoals from the updrift to the downdrift barriers is investigated and discussed in detail. In another case inlet closure due to prolongation of the ebb channel and infilling with littoral-drift material in the foreshore is also observed. Furthermore, the model results indicate that Escoffier's closure curve is solely applicable to the stability of the channel gorge and thus insufficient to explain the closure of a tidal inlet due to littoral sand infilling into the main ebb channel. In this study solutions are developed for the stabilisation of tidal inlets at the central coast of Vietnam. The solutions are based on the natural behaviour and evolution of two different types of tidal inlets in the region, namely 1) barrier lagoon inlets and 2) inlets formed at the mouth of wave dominated estuaries. For each type of inlet, both short-term and long-term solutions as well as structural and non-structural solutions are taken into account. The solution for the stabilisation of inlets at the central coast of Vietnam is to restrict and/or response to problems. To verify proposed solutions for the stabilisation of inlets along the central coast of Vietnam, process-based modelling is employed to simulate the evolution of a schematised tidal inlet that is stabilised by two jetties and by using river flow to flush the inlet channel. The simulation results show that the inlet after stabilisation by jetties remains open but the inlet channel is highly variable due to the accumulation and erosion of sediment in between two jetties. A sedimentation and erosion pattern is found which is related to the distance in between the two jetties and the strength of the tidal power. An optimum distance between the two jetties that takes into account the effectiveness of the jetties and the structural safety during a major flood event need further study. For inlets that are stabilised by using river flow to flush the inlet, a set of simulation scenarios in which different flushing discharges and flushing durations was designed. For a limited number of simulation scenarios, the model results show that with the same amount of flushing volume, the scenario that has a longer flushing duration and a sufficient flushing discharge is more efficient than the scenario that uses a high flushing discharge over a short duration. This means that the flushing efficiency is closely related to the flushing duration rather than the flushing discharge. Moreover, the flushing moment (at the beginning of the ebb phase or at the beginning of the flood phase) will also contribute to the efficiency of the solution but needs more study.Department of Hydraulic EngineeringCivil Engineering and Geoscience
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