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To date no study has been done to analyse the effect on water level for the Dutch coast from the passage of storms over the North Sea. This research examines the feasibility of developing a joint probability method to determine the extreme water level for the Dutch coast, resulting from these storms. This has been done by means of a parametric model, which determines the hydraulic boundary conditions from a set of significant storm parameters. The objective of this research is to obtain physical knowledge in predicting the water level for the Dutch coast. This provides a better understanding of the contribution of storm characteristics to high water levels, and can therefore be very useful in the forecasting of extreme surges from the passage of these storms. Secondly, possible changes in meteorology can be taken into account in this model, in contrast to the current method. Another advantage is that the simulated wind field based on storm parameters may also be a tool for simulating and understanding the behaviour of waves for the North Sea and the joint probability with the water level.

The Lagos coast has been suffering high rates of erosion since the construction of three harbour moles, i.e. the West Mole, East Mole and the Training Mole, at the tidal inlet connecting the Lagos Lagoon to the South Atlantic Ocean. To provide for a permanent erosion mitigation measure and to create residential and commercial area for circa 400,000 people, the Eko Atlantic City project has been initiated in 2008. In front of the eroded coast, approximately 9 km² of land will be reclaimed and protected by a revetment. In this study the long-term and large-scale morphological behaviour of the Lagos coast is investigated and subsequently the long-term morphological impact of the Eko Atlantic City project is assessed. First, a conceptual model is created, in which the historical development of the coast is discussed. The long-term morphological behaviour of the coast downstream of the inlet is determined by two main factors: sediment accumulation at the West Mole and sediment import into the tidal inlet and the lagoon, induced by disturbance of the morphological equilibrium by sea level rise and dredging activities. Using the numerical simulation model Unibest, the long-term impact of Eko Atlantic City is assessed. It is concluded that the construction of Eko Atlantic City will not change the total erosion volumes occurring downstream of the inlet. However, as the revetment of the project retains the coast, the erosion will be shifted towards downstream. Downdrift of the project, the erosion rates are locally relatively high. The shape of the sea defence has been designed to minimize the local erosion effect. A monitoring and mitigation strategy has been recommended to monitor this effect and to instruct coastal protection management actions to be implemented if required.

In 2008 Hurricane Ike flooded large parts of the barrier islands in front of the Galveston Bay near Houston, Texas. The storm surge also entered the bay through the inlets causing great damage along the bay and the port of Houston. Because of the high probability that a hurricane would strike again the “Ike Dike” was developed. The concept consists of heightening and extending the floodwall on Galveston Island and Bolivar Peninsula. Barriers will be placed at the San Luis Pass and Bolivar Roads inlet whereas the Rollover Pass might be closed completely. The Bolivar Roads barrier will be a combination between a floodgate and a lifting gate structure. During non-storm conditions, when the gates are open, the flow area through the inlets are expected to reduce up to 40-60% due to the barriers. The reduction of flow area can cause a decrease of the tidal prism and tidal range. Also current velocities are expected to increase near the barriers and decrease in the main bay. The residence time of fresh water in the bay is expected to increase and the salinity to decrease. The reduction of the tidal prism, tidal range and current speeds can cause a redistribution of the sediment from marshes and flats to the channels within the bay. The blocking of sediment from the Mexican Gulf by the barriers can further enhance this. Changes of the hydrodynamics, water quality and morphology in the bay could potentially result in loss of habitat and disturb the ecology. A 2D hydrodynamic model has been created in order to investigate the impact of the barriers on the tidal prism, tidal range and circulation in the bay. The morphology and water quality of the bay are investigated qualitatively using literature, reference projects and the outcomes of the 2D model. It is concluded that a 40-60% decrease of the flow area at the Bolivar Roads entrance affects the hydrodynamics of the Galveston Bay. To mitigate or prevent these effects the design could be altered such that the flow area is reduced by less 20%, because the impacts of such a barrier are relatively small. Another mitigating solution could be to build compartment dams to preserve the tidal range.

The wetlands in the Mississippi delta (USA) are drastically subsiding and eroding. Many projects and researches are ongoing to determine how this “drowning effect” of the delta can be stopped. One of the solutions that could be feasible is implementing sediment diversions in the levees of the Lower Mississippi River in order to divert sediment into the delta. This thesis addresses the morphological effects of river diversions on the Lower Mississippi River. The main objective is to analyze and optimize trade-offs between delta building and river navigability. For this purpose a 2DH numerical model with Delf3D has been created; the model simulates the hydro- and morphodynamic behavior. The river reach which has been studied is the final 110 km of the river from Point a la Hache at River Kilometer 78 (RK 78) down to the mouth of the river (RK -30), below Head of Passes. The hydrodynamic model has been calibrated and verified with flow and stage data from daily observations on the river. With the available sediment data a calibration has been carried out of the morphological behavior in the river. The model has been used to simulate several scenarios to get insight in the problems in the delta. From analysis of the model results the river bed in the study area can be divided into three categories. Upstream of RK 4 the bed is subject to erosion, around RK 4 the bed is practically in equilibrium and downstream of RK 4 the bed is subject to sedimentation. The reach downstream of RK 4 is the dredging reach; after analyzing the long-term simulation of 20 years it is not expected that the dredging quantities will decrease in the future. Closing off West Bay diversion has a positive effect on the dredging quantities. The best diversion site for this study area is found in the inner bend of the river upstream of Empire (RK 47) at RK 53. This site is most efficient and diverts the largest quantities of sand through the diversion.

This study focused on the possible relationship between seasonal variations of suspended sediment concentration (SSC) and diatom biomass. In order to determine whether a relationship exists, the effect of other parameters on these seasonal variations was also studied. This was accomplished to prevent the risk of finding a spurious relationship between SSC and diatom biomass. This approach resulted in a literature study and the analysis of data. The literature study was necessary to provide the theoretical background, the data analysis was used for the quantification of the identified relationships. The literature survey showed that SSC variations do not affect diatom biomass in the Western Scheldt. Turbidity, caused by both SSC and organic matter, is too high throughout the year for diatoms to be active during inundation of the flats. Diatom, which are in fact algae, depend on the availability of light for their growth. The role of diatoms on SSC variations is an indirect one. Diatoms do not influence deposition of sediment, but they do play a role in stabilising fresh deposits. When this occurs on a large scale during a period of time, then suspended particles are constantly removed from the water column. A relation scheme was proposed in which all identified parameters and their relationships were summarised. Among the parameters that play a role in the seasonal variation of SSC and diatom biomass are wind, rain, discharge and biological activity. The flats play a role through seasonal changes in accretion and erosion of fine grained sediment. The interaction between SSC and diatoms takes place at the flats. Data acquisition and analysis was used to investigate the contribution of the various parameters to SSC and diatom biomass variations. It showed that wind is an important parameter, probably through the influence on hydrodynamic conditions. This was a result of the analysis of both short term, high frequency measurements and long term, low frequency measurements. The performed regression analyses explained only part of the SSC and diatom biomass variations. This indicates that the parameters on which no information was selected play a role. Also the fact that the analyses were performed on the scale of the entire estuary might contribute. Some indications for this assumption were provided in the analysis of spatial variations of SSC, discharge and chlorophyll a. Future research should aim at combining spatial and temporal scales and the use of other types of data and analysis techniques.

Dit rapport beschrijft de resultaten van de studie "Modellering van zand-slibsegregatie in de Nieuwe Merwede", uitgevoerd voor het Rijksinstituut voor Integraal Zoetwaterbeheer en Afvalwaterbehandeling (RIZA) van het Ministerie van Verkeer en Waterstaat. Als studiegebied heeft het RIZA gekozen voor de Nieuwe Merwede, een riviertak in het Noordelijk Deltabekken. De bovenstroomse rand ligt bij Werkendarn (km 961) en de benedenstroomse rand bij Anna Jacominaplaat (km 979). Op dit traject heeft na de sluiting van het Haringvliet (1970) veel sedimentatie van zand en slib plaatsgevonden. Uit bodernrnonsters en boringen blijkt dat er sprake is van zowel horizontale als vertikale zandslibstructuren. Zo blijkt dat in 1992 de toplaag in de Nieuwe Merwede erg zandig was. In de geul wordt sporadisch slib teruggevonden tussen 5 en 15%. In de kribvakken is het slibpercentage over het algemeen hoger dan in de geul (30 - 70%). Op sommige lokaties in de kribvakken worden ook zeer lage slibpercentages waargenomen. Ook in de vertikaal komen variaties in zand- en slibafzettingen voor. Uit de boringen blijkt dat in het benedenstroomse gedeelte van de Nieuwe Merwede na de sluiting van het Haringvliet (1970) flinke sliblagen zijn afgezet. In deze sliblagen komen dunne zandlagen voor. Deze sliblaag is afgedekt met een zandlaag die de toplaag vorrnde in 1992. In dit onderzoek wordt geprobeerd de horizontale en vertikale structuren middels een modelstudie te reproduceren. Hiervoor is gebruik gemaakt van een twee-dimensionaal horizontaal (2DH-) suspensietransport model voor zand en slib, uitgaande van respectievelijk een volledige zandbodem en een volledige slibbodem op de Nieuwe Merwede. Er is onder vier verschillende bovenafvoeren gerekend met een bodemschematisatie van 1970 en 1993. De vier bovenafvoeren worden representatief geacht voor de beschrijving van de totale kansverdeling. De simulaties zijn uitgevoerd met het waterbewegingsmodel WAQUA en het sedimenttransportmodel DELWAQ.Vit het zandmodel voor 1970 blijkt dat het zandtransport in het benedenstroomse gedeelte en het middendeel alleen onder hoge afvoer (Qbr = 6000 m3/s) plaatsvindt. Op het bovenstroomse gedeelte en vindt onder bovenafvoeren van 2200 en 3000 m3/s zandtransport plaats. Op jaarbasis is voor 1970 sprake van zandsedimentatie over het gehele traject. Ongeveer 94% van het zandaanbod sedimenteert in de Nieuwe Merwede. Het slibmodel geeft flinke slibsedimentatie onder lage en gemiddelde bovenafvoer op het benedenstroomse gedeelte (10 - 15 cm/jaar). Op basis van deze modelresultaten kan geconcludeerd worden dat de gevonden vertikale zand-slibstructuren (slibafzettingen met dunne zandlagen) in het benedenstroomse gedeelte het gevolg zijn geweest van afvoervariatie. Grote slibafzettingen zijn opgetreden gedurende lage en gemiddelde Bovenrijnafvoer, terwijl tijdens hoge afvoer (6000 m3/s) sprake dunne zandlagen zijn afgezet. In dit rapport is aangetoond dat het goed mogelijk is de grootschalige zand-slibstructuren in de Nieuwe Merwede, een rivier met een dominante bovenafvoer en schijngetij, te verklaren. De resultaten van het model komen kwalitatief en soms kwantitatief goed overeen met de resultaten verkregen uit boringen, bodemmonsters en lodingen. Het voorspellen van zandslibstructuren is van belang om de gedragskennis van gebieden in rivieren en estuaria, waar zowel zand als slib een rol spelen, uit te breiden.

After the catastrophic flooding of New Orleans due to hurricane Katrina in the year 2005, the city’s hurricane protection system has been improved to provide protection against storms with at least a 100 year return period. This thesis investigates the risk to life in the post-Katrina situation for the New Orleans metro bowl. The risk to life is evaluated through a ‘scenario’ based approach simulating defined flood events. Several scenarios are identified that could lead to flooding within the Metro Bowl and including the failure of hurricane protection or the river levees. The probability for each scenario is determined based on existing design guidelines and expert judgment. A two-dimensional hydrodynamic model is used to simulate flood characteristics and these outcomes are used as input for the loss of life estimate. In addition, the evacuation effectiveness has been considered. Results indicate that the estimated loss of life in case of flooding ranges from about 100 to nearly 500. The highest life loss value is found for breaching of the river levees. The probability and consequence estimates are combined to evaluate the individual risk and societal risk for New Orleans. The individual risk (IR) refers the probability of an individual residing in a given area perishing as a consequence of flooding. The IR for large parts of New Orleans metro is larger than 1/100,000 per year. The evaluated risk is compared to risk levels that have been found to the risks of other large scale engineering systems (e.g. other flood prone areas in the US and other regions, dams and the nuclear sector) as well as existing criteria for evaluating acceptable levels of individual and societal risk. The evaluated risk determined in this study exceeds tolerable or acceptable risk criterion as discussed in literature and other industries. Despite the major improvements to the flood protection system, the societal flood risk of New Orleans is still expected to be significant at a national (US) scale. Finally, the effect of various risk reduction measures on the risk level is investigated. Strategies for which costs and risk reduction effects are discussed, including: elevated homes, improved evacuation and increased protection ( relocation of population to higher areas). While decisions regarding flood risk are complex and involve many factors such as perception and economic considerations, the results indicate the necessity of further discussion regarding the management and reduction of the city’s risk to flooding.

De bodemsamenstelling van estuaria varieert vaak sterk in zowel horizontale als verticale richting. Met name de ruimtelijke verdeling van zand en slib (zgn. zand-slibsegregatie) is belangrijk, omdat voedingsstoffen en vervuilende stoffen zich voornamelijk hechten aan slib en niet aan zand. Hierdoor is het slibpercentage in de bodem een belangrijke parameter voor ecosystemen en een indicator voor de mogelijke aanwezigheid van bodemvervuiling. Voor een goed beheer van estuaria is kennis en voorspelling van dit fenomeen noodzakelijk. De ruimtelijke verdeling van zand en slib is nog altijd niet goed te verklaren en dus te voorspellen. Dit wordt met name veroorzaakt door de complexe interactie tussen de waterbeweging, sedimenttransport en bodemligging en -samenstelling. De waterbeweging ten gevolge van getij en golven maken het stroombeeld in een estuarium zeer complex. Daarnaast worden erosie- en sedimentatieprocessen van zand en slib nog maar beperkt begrepen. Een veel gebruikte hypothese voor het verklaren van zand-slibsegregatie is "hoe kalmer het water, hoe meer slib". De achtergrond van deze hypothese is dat slib in het algemeen bij lagere stroomsnelheden nog getransporteerd kan worden, terwijl zand inmiddels gesedimenteerd is. Doelstelling van dit onderzoek is het bekijken of er een lokale relatie bestaat tussen waterbeweging enerzijds en de bodemsamenstelling anderzijds. Deze relatie is onderzocht aan de hand van een stromingsmodel en bodemgegevens van de Molenplaat, een intergetijdegebied nabij Hansweert in de Westerschelde. De bodemsamenstellingsgegevens zijn verkregen uit een in 1990 uitgevoerde meetcampagne in de hele Westerschelde. Tijdens deze meetcampagne is de bovenste tien centimeter van de bodem bemonsterd en hiervan is de korrelgrootteverdeling bepaald. In totaal zijn in dit onderzoek 344 monsters gebruikt. Verder zijn secundaire parameters als valsnelheden en dieptegemiddelde kritische snelheden voor het begin van beweging voor zand berekend. Het gebruikte waterbewegingsmodel is het Molenplaatmodel uit 1995. Met dit model zijn dieptegemiddelde (2DH) berekeningen uitgevoerd voor drie verschillende getijsituaties: een springtij, een gemiddeld getij en een doodtij situatie. Tevens is een 3D berekening voor de springtij situatie gemaakt om te bekijken of dit betere waarden voor de waterbeweging oplevert. De monsterpunten zijn toegevoegd aan dit model als observatiepunten, zodat per monsterpunt de waterbeweging gedurende de getijperiode berekend wordt. Door een nabewerking van de modelresultaten zijn verschillende hydrodynamische parameters per monsterpunt berekend, zoals de maximale snelheid, de gemiddelde snelheid, de energiedissipatie etc. Met betrekking tot de relatie tussen sedimenteigenschappen en diepte blijkt dat er tussen 1m +NAP en 2m -NAP hoge concentraties slib in de bodem voorkomen. Op de hoogste delen van de platen (boven 1m +NAP) wordt echter vrijwel geen slib aangetroffen en dieper dan 2m -NAP is de trend ook niet meer waarneembaar. Ook de dso van de zandfractie lijkt samen te hangen met de diepte. Uit de correlatie blijkt: hoe dieper, hoe grover het zand. Combinaties tussen bodemsamenstellingsparameters en hydrodynamische parameters zijn gevisualiseerd in grafieken. Hoewel de waterbeweging van de getijberekeningen in absolute grootte natuurlijk verschilt, laat een vergelijking tussen de verschillende getijomstandigheden zien dat er weinig concrete verschillen zijn. De vormen van de puntenwolken wijken niet erg af wanneer de getijberekeningen worden vergeleken. Wanneer zand en slibpercentages worden uitgezet tegen maximale snelheden, bodemschuifspanningen en energiedissipaties, blijkt er steeds een bepaalde grens te zijn waarboven vrijwel geen slib meer voorkomt. Onder deze grens is echter geen duidelijk verband aanwezig. Zowel grof als fijn materiaal komen voor. Het lijkt erop dat onder deze grens wel de mogelijkheid is voor slib om neer te slaan, maar dat het van meerdere processen afhangt of dit daadwerkelijk gebeurt. Wanneer korreldiameters en aanverwante parameters als dso worden uitgezet tegen de waterbeweging, is steeds een redelijk lineair verband aanwezig. Hoe groter de optredende snelheden en schuifspanningen, hoe grover het sediment.

The area of New Orleans was hit by Hurricane Katrina in August 2005. A large part of the city got flooded due to bad design, construction and maintenance of the levee system. In order to increase the level of protection of the city, the levees are heightened and strengthened in the framework of the Hurricane and Storm Damage and Risk Reduction System (HSDRRS). On the long term the restoration of coastal wetlands is also part of the program. Since the 1930’s wetland erosion in coastal Louisiana has been recorded. The main cause of the erosion is the canalization of the Mississippi River, land subsidence and sea level rise, and salt water intrusion by alteration of drainage patterns. One way of initiating wetland restoration is the construction of diversions. The Violet Diversion is the largest diversion planned in the Pontchartrain Basin. Water from the Mississippi River is diverted into Lake Borgne and the Biloxi marsh in order to decrease salinities in those target areas. In order to get more insight in the impact of the diversion on salinity (gradients), hydrodynamic and salinity modeling of the Pontchartrain Basin is desired. Due to lack of data and time, model calibration on salinity was not accomplished. The goal of this study is to model a dynamic equilibrium of yearly averaged salinity in the Pontchartrain Basin. The lessons learned from this study can be a start for subsequent modeling efforts of the Violet Diversion. In Delft3D-FLOW a grid was set-up to model tidal propagation in Lake Borgne. The grid consists of a little less than 53,000 nodes. The initial bathymetry and roughness are taken from the ADCIRC SL15 model. The model is forced with the amplitudes and phases of the ten most important tidal constituents. In order to calibrate the model, the tidal channels are enlarged and the bottom friction is decreased. The necessity of these changes was already proven by the application of the harmonic method on the Pontchartrain Basin, as well as the moderate results of previous model studies. The model is calibrated on tidal amplitudes (accuracy within 10%) and fluxes through the tidal passes (accuracy within 1%). Phases were considered less important. Salinity was implemented by simulating initial salinities and river discharges on top of the tide. Comparing 2D with 3D simulations, gravitational circulation occurs in the 3D modeling. This causes an increased salt water intrusion from the Gulf of Mexico towards Lake Borgne and the Biloxi Marsh. However, the salinities in this target area are too low in the dynamic equilibrium situation. This is explained by the underestimation of transport by tides and Mississippi River discharge towards Lake Borgne. Previous model studies proved that circulation around the continental shelf cannot be neglected for tidal transport. Also, the Mississippi River discharge can flow around the Birdfoot. Due to the choice of the model domain, that flow cannot occur in this study. Using the tide-calibrated model for salinity studies, it is recommended to model in 3D to simulate the gravitational circulation. Nontidal water level elevations and currents should be included in the boundary conditions. This can be achieved by enhancing the model domain to capture a larger part of the Gulf of Mexico and the Mississippi Birdfoot. Then the flow around the Birdfoot can also be simulated. Wind should also be added to the hydrodynamic simulations. The measured salinities and the target salinities show seasonal variation. Therefore future modeling should strive for real-time simulation by forcing the model with time-series. The diversion flow can then be varied per month or season.

In this study, a process-based, depth-averaged Delft3D hydrodynamic and morphologic model of the Wax Lake Delta in Atchafalaya Bay, Louisiana was developed to simulate a five year period of delta development. The purpose of this modeling effort was to test the ability of process-based modeling tools to successfully simulate typical delta-building processes and the resulting morphologic and stratigraphic characteristics of the delta. Recent developments in conceptual modeling of mouth bar formation and full delta development have confirmed the capability of process-based models to simulate the processes necessary for delta growth and the resulting long-term, geologic scale morphologic and stratigraphic features. In this attempt to model the actual development of a prototype delta using similar techniques to those employed in the conceptual delta models, the applicability of physics-based modeling to delta evolution simulation will be further validated. Morphology qualitatively reproduced typical river-dominant delta growth through the establishment of new depositional lobes while maintaining approximate radial symmetry. More specific stratigraphic features were also reproduced. The successive stacking of coarsening upwards sequences observed in Wax Lake Delta mouth bar deposits was evident in the stratigraphy of modeled incipient jet deposits, a result of the varying discharge regime. Though incipient jet deposits developed in the model at the distal ends of distributary channels, the prograding bars did not aggrade sufficiently to induce flow bifurcation and the development of a mature mouth bar depositional lobe. The overall coarsening-upwards, though sand dominant stratigraphic sequence of typical friction-dominant river mouth deposits is reproduced. Significant mud-dominant prodelta deposition is observed basinward of the original delta front location. In the prototype Wax Lake Delta, deposition of fine sediments in this area is hindered by waves and wind-induced transport, so the modeled mud depositional bodies do not reflect prototype development. Distributary channels are significantly incised and narrowed over the course of the simulation. The narrowing proceeds from significant sand-dominant subaqueous levee deposition on the channel banks. Though distributary channels in the real Wax Lake Delta do frequently incise through the full deltaic sedimentary sequence with channel extension, the modeled incision is persistent throughout the simulation. Upstream accretion of established delta lobes through sand-deposition, similar to the observed primary process of subaerial development in the neighboring Atchafalaya Delta, was also present in the modeled development. The Wax Lake Delta is clearly river-dominant according to traditional classification schemes; however, the deposition of fine sediments is influenced by basin processes that resuspend and export significant quantities from Atchafalaya Bay. The processes contributing to the coarse sediment depositional features that dominate the Wax Lake Delta are qualitatively simulated under purely riverine forcings, but the fine sediment dynamics cannot be accurately simulated in the present, process-limited model. Recommendations for improving morphological simulation include model redevelopment with an alternative, total load transport formulation and the inclusion of limited marine forcings that inhibit fine-sediment deposition.

Tidal inlet systems, as found in the Dutch Wadden Sea, often feature both sand and mud. Due to differences in sediment properties, sand and mud particles respond different to identical forcing conditions, like short waves and tidal currents. Because of this, sand and mud can get transported to different locations. This process is referred to by sand-mud segregation. Sand-mud segregation can have considerable influences on bathymetry, potential pollution and flora & fauna. It is because of these aspects that predictions on sand-mud segregation are needed. To predict sand-mud segregation, commonly for a practical scenario, a morphodynamic model can be used. Though, modelling a practical scenario often comprises a complex bathymetry and various non-linear processes, that contribute to sand-mud segregation, occur. Because of this, practical sand-mud segregation models are often hard to understand, give little insight in the overall processes and discrepancies with reality are often found within the results. These problems can be overcome by considering a schematized scenario, where only the overall forcing conditions (tidal currents and short waves) are considered, along with a schematized bathymetry and geometry. By considering a schematized scenario of the Amelander tidal inlet system, the large-scale sand-mud segregation patterns can be reproduced. It is found, e.g. in observations from the field and theoretical descriptions, that mud is commonly found in less hydrodynamic active areas (as long as mud is available). Within the Amelander tidal inlet system, mud is therefore found in the shallow intertidal areas, far from the deeper hydrodynamically active areas, like the tidal inlet and tidal channels. The schematized process-based model (a newly developed sand-mud version of Delft3D, which accounts for (non) cohesive regimes, a layered stratigraphy and consolidation lag) also reproduces these large-scale spatial sand-mud segregation patterns, with the schematized process-based approach. By combining the schematized model approach with variations in relative forcing domination (by tidal currents or short waves), various scenarios can be considered. From observations in the field and associated sand-mud segregation theory, one always expects mud deposition in less hydrodynamically active areas. Given the properties of tidal currents and short waves, it is hypothesized that mud is commonly found in deeper/shallow areas, respectively for a relative dominance of tidal currents/short waves. Results from the schematized process-based model support this hypothesis. Mud is transported to deeper areas when short wave domination is imposed, while mud is commonly found in shallow areas for a tidally dominated system. A schematized process-based sand-mud segregation model is able to reproduce large-scale spatial sand-mud segregation patterns for a practical case (the Amelander tidal inlet system). Furthermore, a classification can be set up, relating these large-scale spatial sand-mud segregation patterns to relative forcing dominance (from tidal currents or short waves), which is based on observations, theory and the schematized process-based model.

In the tidal inlets in the Dutch Wadden Sea, the most important sediment types are sand, silt and clay. A distinction between the sediment types is made, because clay particles have cohesive properties. Consequently, the clay particles do not behave as individual particles but tend to stick together. Floes are formed whose size and settling velocity are larger than those of the individual particles. The bed is mostly composed of a mixture of different sediment types. The bed has cohesive properties (and is called a mud bed) if the clay percentage is higher than 5-10%. At present, computer-models are not able to simulate the sand-mud distribution in tidal inlets. The bed composition variation cannot be taken into account in the computations. This is caused by a lack of knowledge about the processes and mechanisms that take place during mud transport and sand-mud interaction. Therefore, the goal of this study is to gain more knowledge about sand-mud distribution in tidal inlets. In this study the tidal basin of the Amelander Inlet has been chosen as area of interest. The first part of the study consists of a data analysis. The data is taken from the Sediment Atlas (Rijkswaterstaat, 1998) and consists of the grain size distribution at sample points in the tidal basin of the Amelander Inlet. To obtain these grain size distributions, samples were taken every kilometre or every five hundred meters. The samples were taken in the period April-July 1995. The grain size distribution was determined with the Malvern method. It is know that the Malvern method underestimates the finer fractions in a sample. The second part of the study consists of using numerical models to try to simulate the observed sand-mud distribution in the Amelander Inlet. In the data analysis, research questions were raised which can be answered with the models in this part. These questions concern the wave penetration, the tidal penetration, the water depth at high water and the level of the sand and mud flats. The computations are made within the Delft3D modelling system. First, hydrodynamic computations are made in Delft3D-FLOW. After that, sediment transport computations are made separately for the sand bed and the mud bed. The main conclusions from this study are: 1) The areas where deposition takes place are mainly determined by the occurring tidal action. The influence of waves is large at small water depths, and determines how much and how fast the mud particles are suspended. Whether the mud particles are transported after being stirred-up, is determined by the actual tidal action. 2) Waves also are of influence for the deposition of mud layers. Waves in the tidal basin spread the mud particles from areas with high wave attack (high concentration) towards areas with low wave attack (low concentration). Due to this redistribution of mud, the mud particles, on a large scale, are transported from west to east in the tidal basin. 3) Qualitatively, the computed mud deposition agrees with the measured mud contents on a large scale. With the help of the second deposition computation and the erosion computation, part of the differences can be explained and the measured mud contents are understood better. Further experimenting with the wind speed, computing different phases in a certain sequence and the computation times for the different phases can improve the results and help understand and predict the measured mud contents on a more accurate level.

In het najaar van 2011 is Thailand zwaar getroffen door overstromingen, met meer dan 800 dodelijke slachtoffers en ongeveer 45 miljard dollar directe schade. Nederlandse expertise werd ingeroepen voor het crisismanagement om de schade te beperken.

Het stormseizoen is weer begonnen in Nederland. De duinen en dijken langs de Nederlandse kust kunnen in deze periode zwaar belast worden door een combinatie van hoge stormvloed en zware golven. Op dit moment worden deze waterkeringen getoetst en ontworpen met waterstandsgegevens die vooral gebaseerd zijn op statistische extrapolatie van langjarige tijdreeksen. Een parametrisch model, ontwikkeld bij de Technische Universiteit Delft samen met Royal HaskoningDHV, geeft meer inzicht in de achterliggende stormkarakteristieken die horen bij deze maatgevende condities. Ook biedt deze methode perspectief om op termijn de extreme combinaties van waterstand en golven beter te voorspellen.

Eind oktober 2012 gebeurde wat al jaren was voorspeld: een orkaan treft een groot gebied in het noordoosten van de VS, met uitzonderlijk veel schade en meer dan 130 dodelijke slachtoffers. De vraag is of de Amerikanen meer aandacht gaan krijgen voor preventieve maatregelen.

Afgelopen voorjaar was sprake van een bijzonder hoogwater op de Mississippi. De rivier voerde op haar hoogtepunt in totaal meer dan 65.000 kubieke meter per seconde af naar de Golf van Mexico, ongeveer vier keer zoveel als de 1/1.250 jaar ontwerpafvoer van de Rijn in Nederland, en overtrof op sommige plaatsen het historische hoogwater van 1927. Bovenstrooms in de Mississippi vonden op diverse plaatsen overstromingen plaats. Dijken werden opgeblazen om het water weg te leiden om overstromingen in steden te voorkomen. Benedenstrooms werd onder andere de Morganza Spillway, een grote overlaat die sinds 1973 niet meer gebruikt was, opengezet om New Orleans te beschermen tegen (nieuwe) overstromingen. De totale schade liep in de honderden miljoenen dollars. Dit artikel geeft een overzicht van dit unieke hoogwater en een doorkijk naar mogelijke maatregelen om het hoogwaterrisico te reduceren.