· · · Institutional Repository

A research concerning the most important bottlenecks in the computational modelling of a free meandering river. With the software package Delft3D of WL|Delft Hydraulics a meandering part of the French river Allier is modelled. The Allier is an important tributary of the river Loire. The modelled part is located near Moulins in a beautiful nature reservation. In this reservation it is prohibited to protect the riverbanks. Because of this the river can freely migrate. In the modelled year (2003) a cliff erosion of up to 60 m was measured. The point-bars of the river Allier are densely vegetated in many areas. To simulate the impact of the vegetation on the morphodynamics, two different methods are used: the increased Nikuradse roughness height approach and a method by dr. ir. M.J. Baptist proposed in his PhD. study, 'Modelling floodplain biogeomorphology'. In his approach the vegetation is schematized as stiff vertical rods with a certain diameter, density and height. Concerning the next three aspects the model showed significant errors: the cliff erosion, the typical sedimentation processes in the inside bends due to secondary flow and the spreading of different sediment fractions over the area. Because of the model errors it was not possible to say which one of the tested vegetation models performed best. But the bed shear stresses simulated with the approach by Baptist are much more physically reliable than the bed shear stresses by the Nikuradse approach. Also the physical representation of vegetation by rods instead of huge grains speaks in favour of the method proposed by Baptist. The assumption, however, that the Nikuradse approach leads to too large transport capacities in comparison with the approach by Baptist showed to be untrue. The simulation results showed that, whether the Nikuradse or the Baptist approach simulates the largest sediment transport capacity, depends mostly on the water depth, flow velocity, sediment diameter and especially the used transport formula. Both of the vegetation methods showed that the impact of vegetation on the simulated behaviour is very strong. Even a single tree, simulated as a very rough grid cell, showed to have a strong influence on the local morphodynamics.

De afgelopen tien jaar is gebleken dat de rivieren in Nederland, en met name de Rijn, een afvoerregime vertonen dat verandert, en dat er regelmatig extreem lage en hoge afvoeren zijn. De meest recente extreem lage waterstanden werden geregistreerd in 2003. In 1993 en 1995 werd Nederland geconfronteerd met overstromingen wegens extreem hoge waterstanden. De oorzaak van de extreem hoge en lage waterstanden wordt gelegd bij klimaatverandering. Voor de binnenvaart leiden extreem lage waterstanden tot afname van het inzetbare laadvermogen en extreem hoge waterstanden leiden tot vaarbeperking en eventueel tot een vaarverbod. Met het vooruitzicht op meer extreem lage en hoge waterstanden en de voorspelde toename van het goederenvervoer, kan de prestatie van de binnenvaart onder druk komen te staan.

Deze scriptie behandelt het onderwerp herbestemming van kerken in het algemeen over de redenen van leegstand, de keuze tussen behoud, herbestemmen en sloop en over de haalbaarheid. Hierbij is extra aandacht uitgegaan naar hoe de Katholieke en Protestantse kerk hun kerkgebouw gebruiken en wat de verschillen ten opzichte van herbestemming zijn. Vervolgens zijn elf woonkerken onderzocht en vergeleken. Het rapport bevat een lijst met 142 herbestemde kerken.

Failure of coastal protection due to flooding is mainly caused by a combination of high water level and waves. If one considers these as independent variables, the probability of failure can be calculated easily, but this would be an incorrect assumption. These two variables are de-pendent, so a method suitable for multivariate statistics is to be applied, if one does not want to over- of underestimate the probability of failure. 'Method de Haan' is a method that is suit-able for extrapolation of dependent variables. In general this method shifts a cloud of observa-tions in exponential space into the area of failure or other relevant area. Because this method does not have a physical basis it is important that the relation between the variables that are used, holds in cases of extreme events if one wants to determine events with a very low fre-quency of occurrence. In this study the relation between (skewed) set-up and wind speed is examined. Most impor-tant reason to examine these variables is that at RIKZ (National Institute for Coastal and Ma-rine Management) these variables are used to determine the hydraulic variables in case of extreme events. A relation between water level and wave height is used to determine the cor-responding wave heights. Because the highest water levels are of importance skewed set-up is used as a variable instead of water level. By using this variable a more homogeneous data set is generated and the high-est water level can be constructed afterwards. Skewed set-up is the surge effect determined by the highest water level around during a tidal cycle, where this moment does not necessarily coincides with the moment the astronomical tide occurs. Data of four locations are used, covering a period of 23 years of simultaneous data. In order to use homogeneous data selections based on wind speed and set-up are carried out. Three dif-ferent thresholds are imposed for these selection procedures. The extrapolation path with method de Haan using these data sets is compared with a reference situation and results of hydrodynamic model runs where manipulated storm fields are used as input. In 'WAQUA', over 80 model runs are carried out, where a wind and atmospheric pressure field of the 1953 storm, is manipulated. The intensities, storm duration and phase of the storm relative to the astronomical tide are varied. Based on model runs with WAQUA the relation between skewed set-up and aver-age/maximum wind speed at a certain location appears to hold under extreme circumstances. However, only one wind drag relation (Charnock) is used in the model runs. The relation be-tween skewed set-up and average/maximum wind speed with method de Haan (extrapolation path) is physically more plausible if the asymptotic dependence is good, judged by the asymp-totic plots. The extrapolation path also is better if data are selected based on thresholds re-garding set-up. These paths lie closer to the reference situation, although strongly varying with different thresholds that were used.

ESA's space interferometry mission Darwin will make use of optical delay lines (ODL) to help control the optical path differences (OPD) between the satellites to the nanometer level. In order to determine the required ODL control bandwidth, this study investigates the order of magnitude of the high-frequency (> 1 Hz) disturbance forces, and their effects on the OPD. The internal disturbance forces are examined for the three subsystems which are believed to cause most mechanical vibrations. The frequency-dependence of the external disturbance forces is determined in LEO for a precursor interferometry demonstration mission, by Fourier-transforming the accelerometer data of the GRACE mission. These results, together with a literature survey on the space environment in L2, lead to an overall view on the order of magnitude of the high-frequency disturbance forces that can be expected on Darwin. In addition the micrometeoroid impacts are studied. The internal disturbance forces are found to be dominating. Their high-frequency component remains small but has still the same order of magnitude as the OPD-requirement of 5 nmRMS, for a 1 Hz control bandwidth. Also the micrometeoroid environment shows a possible threat for Darwin.

The expansion of human and economic activities together with the increasing pressure due to natural causes and the limited availability of land in the Dutch coastal provinces require examination of coastal extension. Therefore, five alternatives of land reclamation are designed to accommodate industrial activities and an offshore airport near the Port of Rotterdam. Three alternatives are located southwest of the Maasvlakte and two alternatives north of the approach channel of the Port of Rotterdam. Tidal elevation is simulated for each alternative with Delft3D, 2DH-mode (depth-averaged), to indicate the impact of each alternative on the tidal flow along the Dutch coast and hydrodynamics in the approach channel. Appraisal of these results indicates that the alternative of a Peninsula attached to the Maasvlakte pointing in southwest direction has the least impact on the tidal flow and is further examined to indicate the impact on the wave climate and morphology. The impact of the land reclamation on the wave climate is noticeable in the area enclosed by Scheveningen, Goeree Overflakkee and the peninsula. With normal wave conditions, the impact at the coastline of Goeree Overflakkee is < 5%. The impact of the peninsula on morphology is examined by simulation with sediment transport. The short-term simulations are done with Delft3D, sed-online (Van Rijn 2004). The long-term simulations are done with Delft3D-Ram (Rapid Assessment of Morphology, where flow and morphological calculations are decoupled). Both simulations determine significant erosion at the west side of the land reclamation (due to contraction of the tidal flow) and relatively less erosion will occur west of Schouwen Duiveland and Goeree Overflakkee. It is expected that less sedimentation will occur in the approach channel of the Port of Rotterdam. The relative longshore transport, for short- and long-term simulations, is determined at three cross-shore transects, i.e. Hook of Holland, Scheveningen and Noordwijk. Integration of the mean total longshore transport over the transect length indicates an increase in longshore direction, which could indicate erosion along this part of the coastline. The land reclamation will have an impact on the silt transport along the Dutch coast. A rough indication of influence on the silt transport is examined, leading to the following crude estimates: Siltation in the area enclosed by the peninsula and Goeree Overflakke will increase by considering tidal storage, which leads to an increase of this tidal area with a factor 2. Siltation in harbour basins of the Port of Rotterdam may decrease slightly (10 to 20%) as the water entering the system will originate from deeper water at the North Sea. The reduction of the siltation may be of the order of 1 2 [Mm3/year]. Siltation in Eastern-, Western Scheldt and Wadden Sea may change slightly. Finally, it can be determined that the large-scale land reclamation may be a good solution for the long-term spatial problems in the coastal zone when considering the aspects examined and the simplifications used in this study. Most side-effects are small or can be compensated and are only located in the direct vicinity of the land reclamation. Other side-effects contribute positively to the coastal safety of the Dutch coast. Further detailed studies should indicate the long-term and large-scale effects on sediment transport and the effects on the area between the land reclamation and Goeree Overflakkee.

Grote schepen worden steeds vaker uitgerust met grote boegschroeven om beter te kunnen manoeuvreren in havens. Wanneer een schip wegvaart wordt de boegschroef gebruikt om met de voorkant van het schip van de kade weg te draaien. De straal van de boegschroef uit een tunnelbuis wekt een turbulente stroming op die loodrecht tegen een verticale kademuur botst en alzijdig wegstroomt. Vervolgens ontstaan hoge stroomsnelheden boven de bodem. Als de bodembescherming met steenbestorting voor een kademuur niet berekend is op deze hoge stroomsnelheden ontstaat instabiliteit van de stenen. De afgelopen jaren zijn een reeks onderzoeken gedaan naar de belasting van boegschroefstralen op verticale kademuren. Deze zijn gebaseerd op proeven uit de praktijk en op fysische (laboratorium) modellen. De problemen hierbij waren dat de uitkomsten van de proeven moeilijk te verifin waren. In het fysische model heeft men te maken met beperkingen aan de afmetingen, waardoor verstoringen in het stroombeeld zouden kunnen ontstaan. Het doel van dit onderzoek is een numeriek (computer) model te ontwerpen, waarbij de mogelijkheid bestaat om verscheidene configuraties van dit probleem door te rekenen. Hierna wordt dit model gevalideerd in een fysisch model. In eerste instantie wordt uitgegaan van een grote domeinruimte waarin het hele schip met boegschroef wordt geplaatst. In deze domeinruimte wordt een vergelijking gemaakt tussen een open domein (aan- en afvoer van water van en naar het domein) en een gesloten domein (geen aan- en afvoer). Hieruit blijkt dat een gesloten fysisch model een redelijk goed beeld geeft van een open stroming. Het gesloten domein wordt afgebakend tot een optimaal domein (het kleine domein). Uit berekeningen in het model blijkt dat verstoringen van de stroomsnelheidwaarden, die optreden door circulatiestroming, minimaal zijn. Hieruit wordt geconcludeerd dat met het kleine domein berekeningen kunnen worden gemaakt voor deze specifieke situatie en vergeleken mogen worden met een situatie in een fysisch model. Het numerieke model komt in een 1:1 vergelijking met het fysische model niet goed overeen. In beide modellen moeten aanpassingen worden gedaan om een gevalideerde berekening te maken die aangeeft hoe betrouwbaar het numerieke rekenmodel is.

Mangrove forests cover large parts of the tropical and subtropical shorelines and grow in the inter-tidal zone. Mangrove forests act as a natural coastal protection. The most important features of this protection are the prevention of erosion by the collecting of sediment and the reduction of the wave climate. There is a need to quantify and model hydraulic and morphologic processes, wave attenuation in particular, in order to predict and simulate the natural hydraulic behaviour and evaluate the consequences of any artificial measure in mangrove forests. In this study it is found, that the best numerical approximation to model wave attenuation in vegetation with SWAN (spectral wave model), was a formulation described by Dalrymple (1984). The formulation expresses the amount of dissipated wave energy in terms of vegetation characteristics and wave parameters. It has been implemented in SWAN, resulting in an adapted SWAN model in which wave attenuation can be modelled due to homogenous, horizontally variable or vertically variable vegetation. Subsequently the implementation for homogenous vegetation was validated on the results of physical model tests from Mendez and Losada (2004). This validation was successful and gave evidence the implementation was done correctly. The drag coefficient (Cd) is the only calibration parameter in the Dalrymple (1984) formulation and depends on the vegetation characteristics and wave parameters. For the vegetation characteristics it turned out that the drag coefficient depends mainly on the relative spacing. The dependency of the drag coefficient on the wave parameters is determined by the horizontal orbital velocity and the peak period, and could be best described by the relation between the drag coefficient and the Keulegan-Carpenter number. Next a sensitivity analysis was performed which resulted in the expected relations between the wave attenuation and the vegetation characteristics, respectively the hydraulic conditions. In a test simulation with patchy vegetation it was found to be possible to model wave induced setup causes by the energy dissipation in the vegetation, however the potential flow itself can not be modelled by SWAN, because the model cannot simulate flow. Two measurement campaigns in Vietnam on wave attenuation in mangrove forests were modelled as test cases in the adapted SWAN model. In these cases hydraulic conditions and vegetation characteristics were known from measurements and thus the drag coefficient could be calibrated. Finally it is concluded that the implementation in SWAN was successful for different types of vegetation. Nevertheless the dependency of the drag coefficient on both wave parameters and vegetation characteristics implies that the model can only be used when vegetation characteristics and wave parameters have been measured and related to the drag coefficient.