Grootschalige horizontale structuren in een rivier met uiterwaarden: Een experimentele studie

Dit afstudeerwerk bestaat uit het ontwerpen van een fysisch model, het uitvoeren van experimenten, het verwerken van de metingen en het interpreteren van de resultaten. Allereerst wordt in hoofdstuk 2 aan de hand van een probleemanalyse ingegaan op de literatuur betreffende de (ondiepe) menglaagstroming en wordt de doelstelling van het afstudeerwerk nader gepreciseerd. In hoofdstuk 3 wordt het ontwerp van het fysisch model toegelicht, waarbij niet alleen de afmetingen en de instellingen van het fYsisch model worden beschreven, maar waarin ook de keuze van de gebruikte meetapparatuur wordt gemotiveerd. In hoofdstuk 4 wordt het meetprogramma bepaald, wat opgezet is aan de hand van gedefinieerde verwachte karakteristieke eigenschappen van de grootschalige structuren. In hoofdstuk 5 worden de gebruikte analysemethoden beschreven. Naast standaard analysemethoden, zoals bijvoorbeeld het bepalen van spectra, worden speciaal op deze meetsituaties gerichte analysemethoden beschreven. Hoofdstuk 6 bevat vervolgens de resultaten van de bewerkingen welke zijn aangegeven in hoofdstuk 5. In dit hoofdstuk worden bovendien de resultaten gei"nterpreteerd en worden verklaringen gegeven voor de verschijnselen die gemeten zijn. Tenslotte worden in hoofdstuk 7 conclusies getrokken en aanbevelingen gedaan, waarbij ook een terugkoppeling naar de in rivieren optredende situatie wordt gemaakt.Hydraulic EngineeringCivil Engineering and Geoscience

Ondiepe menglagen

Civil Engineering and Geoscience

A stochastic formulation for erosion of cohesive sediments

The linear formulation for erosion E = M(?b??c), often applied in engineering applications, has two properties, which do not always comply with field and laboratory observations, they are as follows: (1) The erosion rate is zero below the critical bed shear stress ?c and increases linearly with bed shear stress ?b, when exceeding the critical bed shear stress; incipient motion (?b $\simeq$ ?c) is poorly represented. (2) The erosion rate is constant in time for constant values of M and ?c, whereas observations often suggest time dependency. In this paper we analyze the process of incipient motion and time dependency by using a stochastic forcing (bed shear stress) and a stochastic bed strength (critical bed shear stress). It is well known that the bed shear stress is not constant but varies due to turbulence. This stochastic nature of the turbulent motion is accounted for by a probability density distribution for the bed shear stress, which is based on the formulation of Hofland and Battjes (2006). This distribution is implemented in the linear erosion formulation. An analytical solution for the erosion rate is obtained, which only depends on the mean bed shear stress. A parametrization is made for efficient application in numerical models. The sediment in the bed is considered to be nonuniform. Therefore, it is subdivided into several classes, distinguished by the critical bed shear stress and not necessarily by the grain size. The variability of the critical bed shear stress is treated in a discritized way. Sediment balance equations are solved for each class. Considering different classes, the total erosion rate becomes time dependent, as the erosion depends on the availability of sediment. The model is applied to two annular flume data sets, Jacobs (2009) and Amos et al. (1992a). The results show that with a proper choice of the required parameters, the time dependence of the erosion rate and the concentration can be reproduced. We conclude that the occurrence of incipient motion can be explained from a stochastic forcing. Time?varying erosion rates can be explained from a stochastic bed strength distribution or from a vertical gradient in bed strength. The latter is, however, not likely and not measurable in the top layers of dense consolidated cohesive sediment beds.Hydraulic EngineeringCivil Engineering and Geoscience

Temporal bed level variations in the Yangtze tidal flats (abstract)

The Yangtze River is one of the largest rivers in the world and the longest one in Asia. Its estuary forms an important entrance for shipping, but is also a key ecological system. Especially the inter-tidal flats are valuable habitats. The health and integrity of the estuarine tidal flat are however under pressure due to anthropogenic influences upstream (e.g. the Three Gorges Dam) and in the estuary (navigation works). The processes responsible for the dynamics of tidal flats are still not fully understood. Although it is clear that tides, river run-off and waves steer the sediment transport, the full non-linear interaction needs further research. In this paper, the seasonal variation of the bed level is analysed, using a data set of tides, wind and river run-off.Hydraulic EngineeringCivil Engineering and Geoscience

Evolution of the Yangtze Tidal Flats: A One-Dimensional Approach (abstract)

The Yangtze river (China) is one of the biggest rivers in the world in terms of length, area, discharge and sediment load. The large sediment supply by the Yangtze river resulted in the development of large coastal wetlands in its estuary, which form rich and important ecosystems. Human activities are increasingly influencing the Yangtze estuary, for instance by the construction of plentiful dams in the river. The precise consequences of human interventions and the effects of climate change like global sea level rise, for the tidal flats are uncertain. In the last decades, the flats have been increasing in size, while measurements from recent years indicate a stop of this growth. Investigation of the development of the tidal flats is important for preserving nature, planning further land reclamation projects and coastal defence issues.Hydraulic EngineeringCivil Engineering and Geoscience

Simulating the large-scale spatial sand-mud distribution in a schematized process-based tidal inlet system model

Tidal basins, as found in the Dutch Wadden Sea, are characterized by strong spatial variations in bathymetry and sediment distribution. In this contribution, the aim is at simulating the spatial sand-mud distribution of a tidal basin. Predicting this spatial distribution is however complicated, due to the non-linear interactions between tides, waves, sediment transport, morphology and biology. To reduce complexity, while increasing physical understanding, an idealized schematization of the Amelander inlet system is considered. Delft3D is applied with a recently developed bed module, containing various sediment layers, combined with formulations for both cohesive and non-cohesive sediment mixtures. Starting with uniform mud content in the spatial domain, the development of the sediment distribution is simulated. Realistic sand-mud patterns are found, with accumulation of mud on the tidal flats. The schematization is further used to determine the sensitivity of the sand-mud patterns to changes in tide, while assessing the influence of tidal dominance on the large-scale sand-mud patterns. The patterns are enhanced/diminished under the influence of higher/lower tides.Hydraulic EngineeringCivil Engineering and Geoscience

Bed-level changes on intertidal wetland in response to waves and tides: A case study from the Yangtze River Delta

Short-term bed-level variability in tidal wetlands has important implication both for ecology and engineering. In this study, we combined in situ measurements with model simulations to quantify short-term bed-level changes on a meso-macrotidal wetland in the Yangtze River Delta. On the middle flat, we observed erosion during neap-to-mean tides under onshore moderate-to-strong winds, and bed recovery during subsequent spring tides, when winds were both offshore and weaker, suggesting that winds can overturn the neap–spring cyclicity of bed-level changes even on meso–macrotidal mudflats. The magnitude of bed-level changes was smaller on both sides of the middle flat, while the smallest changes occurred on the salt marsh. Observed bed-level changes were reconstructed using a single-point bed-level change model, which incorporates in situ measured parameters of hydrodynamics (waves and currents), suspended sediment concentrations, and bed sediment properties. We conclude that the relative importance of waves and tides in intertidal wetland erosion and accretion can vary temporally (due to changes in balance between wave and tidal energies) and spatially (because of changes in elevation and vegetation in the cross-shore profile). Our study also reflects the advantage of combination of in situ measurement with simulation in detecting short-term variability of tidal flats.Coastal EngineeringEnvironmental Fluid Mechanic

Do intertidal flats ever reach equilibrium?

Various studies have identified a strong relation between the hydrodynamic forces and the equilibrium profile for intertidal flats. A thorough understanding of the interplay between the hydrodynamic forces and the morphology, however, concerns more than the equilibrium state alone. We study the basic processes and feedback mechanisms underlying the long-term behavior of the intertidal system, restricting ourselves to unvegetated intertidal flats that are controlled by cross-shore tidal currents and wind waves and applying a 1-D cross-shore morphodynamic model. The results indicate that by an adjustment of the profile slope and shape, an initial imbalance between deposition and erosion is minimized within a few decades. What follows is a state of long-term seaward progradation or landward retreat of the intertidal flat, in which the cross-shore profile shape is largely maintained and the imbalance between deposition and erosion is not further reduced. These long-term trends can be explained by positive feedbacks from the morphology onto the hydrodynamic forces over the flat: initial accretion (erosion) decreases (increases) the shear stresses over the flat, which induces further accretion (erosion). This implies that a static equilibrium state cannot exist; the flat either builds out or retreats. The modeled behavior is in accordance with observations in the Yangtze Estuary. To treat these unbalanced systems with a one-dimensional numerical model, we propose a moving (Lagrangian) framework in which a stable cross-sectional shape and progradation speed can be derived for growing tidal flats, as a function of the wave climate and the sediment concentration in deeper water.Hydraulic EngineeringCivil Engineering and Geoscience

Feedbacks between Fine-Grained Sediment Deposits and Bedforms in a Predominantly Sandy Seabed: Field Observations from the Southern North Sea

Environmental Fluid MechanicsCoastal Engineerin

Rivierwaterbouwkunde

Deze syllabus beoogt een overzicht te geven van de basiskennis die nodig is om ingrepen in de rivier voor te bereiden en de gevolgen ervan te kunnen inschatten. Er wordt ingegaan op het gebruik dat de mens maakt van de rivier en op de kennis van de hydraulica, het sedimenttransport en de morfologie. Aan het einde van het dictaat worden praktijkvoorbeelden behandeld, waarvan een aantal is geënt op het Nederlandse deel van de Rijn.Civil Engineering and GeosciencesHydraulic Engineerin