"uuid","repository link","title","author","contributor","publication year","abstract","subject topic","language","publication type","publisher","isbn","issn","patent","patent status","bibliographic note","access restriction","embargo date","faculty","department","research group","programme","project","coordinates"
"uuid:f392d303-dc5c-49ee-ae6c-ae9c40bd92d3","http://resolver.tudelft.nl/uuid:f392d303-dc5c-49ee-ae6c-ae9c40bd92d3","Dune Erosion on the Falsterbo Peninsula: Assessing the Dune System for Coastal Safety in Regions with Complex Interactions Between Waves and Water levels","Sukchaiwan, Emmy (TU Delft Civil Engineering & Geosciences)","Antonini, A. (mentor); Ragno, E. (graduation committee); Hallin, E.C. (graduation committee); Almström, B. (graduation committee); Österlund, G. (graduation committee); Delft University of Technology (degree granting institution)","2023","The Falsterbo Peninsula is a low-lying area that provides a home to 7,000 residents, as well as various bird and vegetation species. To protect this densely populated area, the municipality was granted a permit to build flood protections. As part of the strategy, the dune system in the study area will be used as natural barriers against storm surges. Despite being part of the protection strategy, the strength of the dune system in safeguarding the hinterland has not been assessed. This had led to the objective of this thesis, which aims to evaluate the strength of the current dune system. To achieve this objective, the following research question were formulated:
To what extent does the dune system on the Falsterbo Peninsula contribute to safeguarding the hinterland against the impact of historical storm conditions?
To seek answers to this question, the research was didvided into three parts. The first part of the methodology involves collecting the environmental data such as the wind, water level and wave data. Additionally, the data on the dune's morphology was collected during the field work. The second step of the methodology involves identification of extreme conditions within the time series spanning from 1959 to 2022. Considering the complex interaction between the waves and water levels, the extreme conditions were identified based on the combined effect of the two variables, which was represented in the total water level (TWL). The sampling method was based on the peak over threshold method applied to the time series of TWL. The choice of the threshold value was based on the scenarios of potential coastal flooding in the study area. The largest storm surge, the 1872 storm, was included in the analysis to evaluate its impact on the present dune system.
The dune erosion due to the selected extreme conditions was determined in the last part of the methodology. Two morphological models, the XBeach model and the storm impact model were employed to estimate the dune erosion in four transects withing the dune system.
The obtained dune erosion was expressed as a fraction of the available dune volume. The maximum dune erosion was found in the transect situated at the far-right end of the dune system when facing north. The maximum dune erosion under extreme conditions in the period 1959 to 2022, estimated by the XBeach and the storm impact model are 7.67% and 32.89%, respectively. Based on these results, it can be concluded that the present dune system is strong enough to provide protection to the hinterland against the impact of extreme condtions.
For the 1872 storm, the XBeach model estimated erosion percentage of 67.89%, whereas the storm impact model estimated more than 100%. This indicates that in the event of recurrence of the 1872 storm, a dune breach could be expected. While the 1872 storm may not be the design storm condition for the dune system, the storm impact model result highlights the need of reevaluation of the formulation of potential plans to reinforce the dune system.
For future studies, it is strongly recommended to establish a long-term monitoring program for the dune system on the Falsterbo Peninsula. The obtained dune erosion data can be used to calibrate the morphological models to enhance its accuracy in the predictions. Additionally, dune recovery data can aid in the understanding of the dune system as a whole.","XBeach; Storm impact model; Waves; Water level; dune erosion; Coastal protection; Dune system","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering | Coastal Engineering","",""
"uuid:3730f825-7934-4c97-a4c2-473ca99fc39c","http://resolver.tudelft.nl/uuid:3730f825-7934-4c97-a4c2-473ca99fc39c","Modelling the effect of twin storms on dune erosion: Applying XBeach to model the dune erosion for single and twin storms constructed using simulated weather data","Nieuwhuis, Thomas (TU Delft Civil Engineering & Geosciences)","de Vries, S. (mentor); Reniers, A.J.H.M. (graduation committee); Wegman, C. (graduation committee); Caspers, J.J. (graduation committee); Delft University of Technology (degree granting institution)","2023","When two storms quickly succeed one another, it can be called a twin storm. Storm conditions at the coast can lead to higher water levels and waves that can give rise to dune erosion. Therefore, after a storm the dunes are smaller than before the storm. Dune accretion is a much slower process than dune erosion; it can take weeks to months of time for a dune to grow the volume of sand it lost during a storm. If the interval time between two storms is smaller than the time it takes for a dune to gain the same volume of sand it loses during the storm, the second storm can lead to additional impact. So when storms quickly succeed one another, there is not enough time for the dune to significantly grow in between the storms. Due to additive impact, a twin storm might lead to more damage than a single storm with the same return period. In this research it is investigated whether twin storms give more dune erosion than a single storm with the same return period.
To answer this question, the research is divided into two parts. The first part consists of creating representative storm schematisations for single storms and for twin storms using simulated weather data from the European Centre for Medium Range Weather Forecasts (ECMWF). To start with, the storms from the dataset are divided into several clusters. For each cluster, one schematisation is made that represents the storm within that cluster.
In part two, the storm schematisations are used to calculate the dune erosion for each defined storm cluster. The dune erosion is modelled using XBeach, which is a model that is developed to simulate the evolution of the dune profile during extreme storms. The dune erosion will be modelled for two locations along the Dutch coast: Hoek van Holland and Nieuwvliet-Groede.
In this research, it turns out that at Hoek van Holland 25-30% of all storms can be considered a twin storm. At Nieuwvliet-Groede, 15-20% of all storms are twin storms. For Hoek van Holland very few of the constructed twin storms (approximately 10%) give more dune erosion than the constructed single storms. At location Nieuwvliet-Groede approximately 30% of the twin storms cause a larger dune erosion volume than the single storms. In most cases where twin storms result in more erosion than a single storm, this effect can be attributed to deviations in the storm schematisations.
For further research it’s recommended to investigate how the prediction of the time evolution of the water level can be improved. This is relevant because the dune erosion volume is strongly correlated with the water level. Furthermore, it is advised to do more research into the time scales and modelling of dune growth. Storm groups with interval periods of multiple weeks can be investigated with better predictions of dune growth.","dune erosion; twin storms; storm groups; ECMWF; XBeach","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering","",""
"uuid:96076fda-d0c7-450e-951d-b62d0f3675bf","http://resolver.tudelft.nl/uuid:96076fda-d0c7-450e-951d-b62d0f3675bf","Predicting erosion of vegetated dunes during hurricanes: Assessing the representation of vegetation effects in XBeach","Cuevas Salgado, Sebastian (TU Delft Civil Engineering & Geosciences; TU Delft Hydraulic Engineering)","de Vries, S. (mentor); Tissier, M.F.S. (graduation committee); van Dongeren, Ap (graduation committee); Quataert, Ellen (graduation committee); Delft University of Technology (degree granting institution)","2023","Coastal dunes serve as the primary defence mechanism against coastal storms for many coastal communities around the world. Vegetation plays a role in increasing dune resiliency as it enables dune growth, however not enough is known about its effects during storms. Research has shown that the current climate crisis will increase the intensity of coastal storms as the sea level rises and global temperatures increase. It is therefore of utmost importance to understand the effects of vegetation on coastal dunes. Particularly with current trends of nature-based solutions to plant vegetation in dune restoration projects.
In this master thesis, the impact of Hurricane Ian on two barrier islands is analyzed and modelled. The primary focus of the study was to investigate how dune vegetation influenced the erosional effects of the storm, with a particular emphasis on enhancing the existing methodologies for incorporating vegetation into morphological models such as XBeach. The research findings derived from the data analysis revealed that the most resilient dunes are high, broad and have a dense vegetation coverage. Moreover, the model outcomes highlight the substantial improvement in predictive accuracy achieved by integrating vegetation as a bed roughness coefficient within the XBeach model. Adding vegetation to the model directly influences current velocities, but does not affect water levels, wave heights, or infragravity waves. The primary influence of vegetation becomes pronounced when an island is inundated or breached, significantly reducing the currents and sediment transport caused by water level gradients. An additional effect of vegetation was observed in a comparative analysis between vegetated and unvegetated models. This analysis highlighted a delay in dune crest lowering when vegetation was present, showcasing the importance of vegetation in shaping the response of barrier islands during storm events.
Numerical modelling can help understand the complex processes that shape barrier islands during storms. This research emphasized the necessity of including vegetation in XBeach models to enhance their predictive capabilities. The best predictions occur when high-resolution bathymetric data is combined with land use land cover (LULC) data to include vegetation as a constant bed roughness parameter. Furthermore, reducing the land classes to four different ones based on dune vegetation zones improves the results of the model and facilitates the calibration of bed friction coefficients.
The most effective models applied in this study demonstrated impressive skill, ranging from good to excellent, accurately predicting breaches in the precise areas they occurred for both islands under investigation. This research contributes to the continued improvements of modelling with XBeach and provides a detailed method of analyzing the effects of dune vegetation on dune erosion to determine the impact of Hurricanes in coastal dunes.","Coastal dune development; Coastal erosion; Coastal modeling; XBeach; dune erosion; dune vegetation; land use land cover (LULC); XBeach vegetation; Coastal defence; barrier islands; Hurricane Ian; Morphodynamic modelling","en","master thesis","","","","","","","","","","","","Civil Engineering","",""
"uuid:c7a27842-2d77-4f63-ae9e-806844a89087","http://resolver.tudelft.nl/uuid:c7a27842-2d77-4f63-ae9e-806844a89087","Machine learning for post-storm profile predictions: Using XBeach and convolutional neural network structure U-Net to predict 1D dune erosion profile shapes at the Holland Coast","van Asselt, Koen (TU Delft Civil Engineering & Geosciences)","Antolínez, José A. Á. (mentor); Reniers, A.J.H.M. (mentor); Heinlein, A. (mentor); Athanasiou, Panagiotis (graduation committee); McCall, Robert (graduation committee); Delft University of Technology (degree granting institution)","2023","To reduce computational efforts, surrogate models have been developed for dune erosion prediction. Current surrogate models can describe the relationship between the XBeach input and output (Athanasiou, 2022) and provides a prediction of a morphological indicator based on a parameterized input (profile shape parameters and hydrodynamics). In this research, first steps are taken to set-up a surrogate model that is able to deal with spatial input and the prediction of actual profile shapes along the Holland Coast. Using a convolutional neural network structure (U-Net), several model set-ups are explored and scaled-up to a realistic storm scenario along the Holland Coast.","Dune erosion,; Neural network; XBeach; U-Net; CNN; Machine learning; Coast","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering","",""
"uuid:3636cb71-ef36-405c-b8f8-254e02a68605","http://resolver.tudelft.nl/uuid:3636cb71-ef36-405c-b8f8-254e02a68605","Numerical modeling of wave transmission over a living breakwater","Kadoglou, Kimon (TU Delft Civil Engineering & Geosciences)","van Gent, M.R.A. (graduation committee); Tissier, M.F.S. (graduation committee); Pearson, S.G. (graduation committee); de Ridder, Menno (graduation committee); Houtzager, Daan (mentor); Delft University of Technology (degree granting institution)","2023","Living Breakwaters have evolved from traditional breakwaters to create multi-purpose structures that provide environmental and social benefits. These breakwaters promote a healthy habitat for flora and fauna, while achieving the same structural capabilities as conventional structures. Reefy has developed a modular living breakwater that can serve both as an artificial reef and a stable breakwater. This study aims to model the impact of a Reefy breakwater on the transmitted wave height, using a process-based numerical model, namely XBeach.
This study builds upon a previous one conducted by van den Brekel [2021], where scaled experiments were performed in a physical wave flume, to investigate the hydrodynamic and ecological functionalities of the Reefy breakwater. In the wave flume experiments a total of 15 structure configurations together with 35 wave conditions, both regular and irregular, were tested. The first 7 of the configurations were 2D structures with a relatively simple shape and a porosity of 20%, and the rest were complex 3D structures with a porosity above 20%. The wave flume was recreated within the numerical model. The influence of the breakwaters on the wave heights are examined through the comparison between the calculated transmission coefficient of the wave flume and the numerical model. However, XBeach, and more specifically XBeach non hydrostatic+ mode which was used, does not resolve the complex interaction between waves, friction and porosity of the structures. As a result, a simple method had to be found to compensate for the loss of these phenomena, while successfully calculating the transmission coefficient.
The findings of this analysis demonstrate that treating the breakwater as an impermeable change in bathymetry, coupled with a 15% reduction in structure height, produces the desired outcomes. From the 35 wave conditions only the 8 irregular wave conditions were chosen to be of interest due to time constraints. Among these 8 wave conditions that were used as an input in XBeach, only 5 of them were satisfying the criterion that describes the range for which the numerical model should produce accurate results (kd<2). The transmission coefficient was defined as the ratio of the transmitted wave height behind the breakwater, and incident wave height in front of the structure. The wave decomposition method used was a modified Guza split method. In pursuit of finding a valid solution, the breakwaters were modeled firstly as impermeable structures with a decreased height, secondly as impermeable structures with a decreased width, thirdly as vegetation with the help of the vegetation module, and lastly the maximum wave steepness criterion as applied in the model was increased (maxbrsteep up to 1.4), without modifying the structure height.
Among the suggested solutions and after being validated for 13 experiments with both simple and complex structures, decreasing the width did not achieve the desired results while the vegetation module failed to produce consistent outcomes. As far as transmission coefficient is of interest, a 15% decrease in the structure height had the smallest mean absolute percentage error of almost 10% and a root mean square error of 8.2%. On the other hand, choosing to increase the maximum wave steepness (to a value of 1.4) is not a valid option for the complex 3D structures, but illustrates even better behavior than a change in the structure height when simple structures with 20% porosity are concerned. To summarize, this thesis provides alternatives to replicate in a simplified way the impact of porous structures, such as Reefy breakwaters, without the need of an extensive and time expensive numerical model.","Living Breakwaters; Submerged Breakwater; Porous structures; XBeach Non-hydrostatic; Nature – based solution; Numerical Modelling; Artificial Reefs","en","master thesis","","","","","","","","2025-05-03","","","","Civil Engineering | Hydraulic Engineering | Coastal Engineering","",""
"uuid:9873098c-0fcf-466e-ae84-6884df9b80ff","http://resolver.tudelft.nl/uuid:9873098c-0fcf-466e-ae84-6884df9b80ff","Influence of Sandy Foreshores on Overtopping in Non-Tidal Low-Energy Shallow Lake Environments","Le Grand, Oscar (TU Delft Civil Engineering & Geosciences; TU Delft Hydraulic Structures and Flood Risk)","Kok, M. (graduation committee); Lanzafame, R.C. (graduation committee); de Vries, S. (graduation committee); Vuik, V. (mentor); de Hoop, Jan-Bert (mentor); Kruyt, Claus (graduation committee); Delft University of Technology (degree granting institution)","2022","Sea level rise will increase the risk of flooding in coastal areas. This poses a risk to the coastal protection as well as rivers and lakes close to the coast. Solutions are needed to cope with this threat. The past decade, nature based solutions have gained significant interest. One of these solutions could be sandy foreshores. Due to the use of natural materials, sandy foreshores are a ‘nature-based solution’ opposed to a more traditional approach of dike reinforcement. For sandy foreshores to be a viable alternative to regular dike reinforcements, the order of magnitude of construction and maintenance costs need to be known. For this reason, it is necessary to be able to calculate a failure probability of a dike with sandy foreshore, to predict the required maintenance and to optimize the design based on life-cycle costs.
To improve the calculation method, a step-wise calculation of the failure probability for wave overtopping of a hybrid structure was developed. The calculation included an iterative process. The calculation methods consists out of calculating the failure probability for wave overtopping in Riskeer and a dune erosion calculation in Xbeach. In this research, overtopping was considered as the dominant failuremechanism. For the assessment, the dike was seen as an impermeable hard layer and the foreshore as a beach. Therefore, dune erosion could not erode the main structure and is considered a sub-mechanism of overtopping.
Next, a calculation was performed to predict the longshore transport along the beach in Almere Duin. The occurrence of different wind directions, together with a Delft3D model of the Markermeer, was combined to find the longshore transport. The LST was calculated with transport formula calibrated for coastal areas. The life cycle costs (LCC), including design and maintenance costs, were calculated for different strategies. The calculated alongshore transport together with cost estimates were used to calculate the LCC. Subsequently, the net present value of the maintenance costs was calculated to determine the LCC for each maintenance strategy. At the end the uncertainty and sensitivity of each strategy were analysed.
The time-varying protection level can be optimized by calculating the failure probability due to wave overtopping, with Riskeer and calculating the erosion of the foreshore, with Xbeach. To use this method, Riskeer and Xbeach should have the same design point and if the design point shifts, an extra iteration is necessary. For a 1/10 profile, only a 0.25 m lowering of the foreshore height was found and one extra iterative step was required to carry out the calculation. A maintenance strategy with a 100 year maintenance interval period was found to be optimal in this thesis and a Monte Carlo simulation, which included uncertainties, led to similar results. However, the total LCC were 21% to 34% higher, when uncertainties were included in the calculation. These findings suggest that it is not necessary, to carry out a probabilistic calculation, to find the optimal maintenance strategy. However, the models used, aswell as the local boundary conditions, such as the longshore transport and the cost of sand, were found to influence the life cycle costs significantly. For this reason, it is concluded that a location specific analysis is important to optimize the maintenance strategy.
During normal conditions, waves come predominantly form the East (90% of the wave climate), have significant wave height of 2 m and period of 8 s. Besides, San Andrés is situated on the Caribbean hurricane route, which can cause an enormous damage to the island. The storm season at San Andrés is between October and December, which is also when major erosion events take place.
The economy of San Andrés is mostly built upon tourism, specially related to its biodiverse ecosystems and Caribbean beaches. The island’s ecological environment is composed by mangroves, seagrasses, and coral reefs, attracting a wide spectrum of fauna and flora to its ecosystems.
During the Masterplan for Coastal Erosion (PMEC), San Andrés was pointed out as a location in which coastal erosion is problematic. In a follow-up of this Masterplan, the island was elected to be part of a program in which solutions against coastal erosion would be presented. This research is part of this project, as a parallel trajectory to get a more profound understanding of the system and the possible mitigation measures that could be applied on the island.
With increasing urbanization and frequency of extreme weather events, erosion is becoming a problem with which San Andrés and its residents are repeatedly having to deal. Erosion is specially problematic for the Northern part of the island, called Spratt Bight. This region is not only the most densely populated area of the island, but also economically and touristically very important. Its beach presents periodically eroding patterns during storm seasons, when wave action drives the sediment towards the East, decreasing its beach width almost to none. A decreasing beach width has a direct negative impact on tourism, making coastal erosion in Spratt Bight not only a coastal safety problem, but also an economic issue.
This study aims to look into the main hydro- and morphological processes driving coastal erosion in Spratt Bight and, using the Building with Nature philosophy, propose a set of solutions to mitigate this problem. To reach this objective data analysis and literature research has been carried out, after which different environmental conditions were modelled using the numerical model Delft3D.
During these activities it was found that independently of its direction, waves approaching San Andrés break upon the coral reef and induce a water level set up inside the coral lagoon. The difference in water level in- and outside the lagoon generates a current and sediment transport, which is directed towards the western opening in the coral reef.
When the Northern waves approach the island (1.5% of the wave climate), the same water level set-up phenomenon is observed. However, as waves are approaching form the North, they not only break upon the reef, but are also able to enter the sheltered lagoon through the western opening in the coral reef. These waves are able to bend around the reef reaching the shore and the headland on the Northern part of the island, inducing a longshore current and a sediment transport that is southeastward directed. The result is that Northern waves are mostly responsible for a strong westward and erosive sediment transport pattern. These waves are mostly observed between October and March, which coincides with the storm season in San Andrés.
Besides, it was found that the Eastern waves are responsible for restoring the (dynamic) equilibrium profile of Spratt Bight Beach. However, this restoring force has a less strong intensity, taking more time to restore the beach than to disrupt its equilibrium.
The solutions proposed include seagrass restoration to enhance ecology, restrain sediment transport and attenuate wave heights; the beneficial reuse of dredged material, to nourish Spratt Bight Beach; and finally, the implementation of artificial coral reefs as breakwaters to prevent the newly nourished sediment to be lost from the system. Besides, artificial coral reefs enhance the ecosystem by attracting fauna and flora increasing biodiversity. All proposed solutions have a positive impact on the beaches and therefore on tourism and the economy of the island. This makes them multifunctional solutions, serving the main goal of protecting the beach while at the same time creating benefits for other functions and values in the area. Following in this way the prescriptions of the Building with Nature design approach by van Eekelen and Bouw (2020).
However, the impacts of dune vegetation during collision regime storm conditions are not taken into consideration in morphological numerical modeling. Consequently, it is unclear how to evaluate vegetation impacts during design assessments. Therefore, the goal of this research is to investigate the potential effect of dune vegetation on dune erosion during collision regime storm conditions and subsequently link this with dune rehabilitation projects. This research is divided into two parts.
First, the capability of the numerical model XBeach to simulate the potential effects of dune vegetation during the collision regime is investigated. Four vegetation approaches are identified in the model, which could possibly represent different dune vegetation effects. Using beach-dune profiles and erosion volumes obtained from two wave flume experiments, the performance and sensitivity of the vegetation approaches are tested in XBeach. Thereby, a distinction is made between the effect of aboveground vegetation: hydrodynamic altering and belowground vegetation: soil stabilization. The model results show that XBeach is capable of simulating dune erosion with vegetation during collision regime storm conditions. This can be primarily attributed to the increase of the critical slope in the avalanching module. This approach represents the soil stabilization effects of belowground vegetation. The application of the root model was added for even better simulations. This approach accounts for the additional root cohesion provided by belowground vegetation by increasing the critical velocity for sediment pickup. The values to be used for both vegetation approaches could not be defined systematically yet due to the small number of experiments assessed and little research found in the literature. It is recommended to obtain more information about the effect of dune vegetation on the stabilizing effects and the critical slope for avalanching. The application of a higher roughness value and the vegetation module, which both represent aboveground vegetation by altering the hydrodynamics, have shown to contribute insignificantly to erosion reduction in the examined cases.
The belowground soil stabilization approaches are applied in a case study to give a first indication of the applicability of XBeach at large-scale and in dune rehabilitation projects. An XBeach model of Beira, Mozambique was set up. A higher critical slope demonstrated profile evolution and erosion reduction in line with observations in the literature. During wave impact, the avalanching module is the most suitable approach to account for the erosion-reducing effect of belowground vegetation. The root model appears to be a more appropriate strategy for accounting for belowground vegetation during milder conditions and shorter storm duration. It was proven that XBeach has very good potential to evaluate the effectiveness of dune vegetation in the design phase. The case study illustrates that vegetation will significantly increase the erosion resistance of the flood dunes. The dunes with vegetation can handle an additional 20-30 centimeters design water level or 0.5-0.8 meter storm wave height without breaching compared to the same dune without vegetation. The design with vegetation has shown to be more robust. However, the approaches are based on different assumptions and limitations and therefore the results should be considered carefully. It is strongly advised to conduct more research in this relatively new study area.
This study investigates and proposes a method for quantifying the advantages of vegetation on dune erosion reduction. The relevance and added value of mature and robust dune vegetation for a resilient coastal dune system and protection against erosion and floods are confirmed. As a result, the incorporation of vegetation in dune rehabilitation projects is promoted.
The intertidal zone is subject to shoaling, surf and swash zone processes. The grain size influences the beach slope, the initiation of motion and settling to the bed. The cross-shore sediment transport is the combination of sediment that is stirred up from the bed and subsequently transported. Breaking induced turbulence enhances stirring of sediment from the bed and keeps sediment in suspension. The amount of stirring and the transport direction depends on the wave conditions.
Input and control data for the model study was provided by the Scanex 2020 fieldwork campaign at Noordwijk, the Netherlands. The ADV velocity data combined with a pressure signal has been used for the tidal and incoming wave signal. Cross-shore profiles have been determined in Matlab based on terrestrial laser scans. Soil samples of the intertidal zone were taken with a sand scraper and analyzed with a sieve tower. For the initial grain size distribution is the average distribution of 14 samples on a transect was used. Based on wave, wind and soil sampling data a model period from 29-2-2020 02:00 to 10-3-2020 13:00 was selected.
The XBeach model used is as described by Reniers et al. (2013), but with a time-averaged turbulent kinetic energy and a different implementation of the Riemann boundary. The model consisted of a 176 x 3 grid with a grid size of dx=1 m and dy=5 m. For the initial bathymetry the laser scan of 29-2-2020 02:00 was used. The initial grain size was imposed on all the model grid cells. Additional to the standard run, runs have been performed to research the effect of a storm, the model sensitivity and the effect of aeolian transport.
The model shows a pattern of cross-shore grain size variations with coarser sediment from x=20 to x=56 m, finer sediment from x=57 to x=105 m and fluctuating grain size from x=106 to x=136 m compared to the initial grainsize. After 24 h a grain size pattern establishes with a clear deposition of fine sediment on the upper beach. The pattern remained stable for nearly the full model period. After 200 hours the fines become less prominent and move onshore. On the intratidal scale sediment becomes coarser when submerged and finer when emerged, except near the high water line where fine sediment is deposited.
The model reproduced the same pattern of grain size variations over the cross-shore as was found in the soil samples of 10-3-2020. As the cross-shore grain size pattern remained stable during the model period, processes on the spring-neap time scale or storm time scale seem to govern the cross-shore variations of the grain size. For the aeolian transport this would imply that for this model period the fine sediment supply is controlled on the same time scales. Nevertheless, considering that aeolian transport could have resulted in coarsening of the fines in the upper intertidal zone, processes over a single tide, could be more important than was visible in the model result.
This research investigates the hydrodynamic and morphological effects of different nourishment strategies at the Domburg coast. The study focusses on the application of the shoreface nourishment, as this strategy has not been previously applied at a coast without shore-parallel bars. The study aims at gaining more insight and knowledge on important mechanisms responsible for the spreading of nourished sediment, because this is not fully understood. Additionally, the performance of a numerical model to hindcast erosion and sedimentation in coastal zones is evaluated.
To this end, a morphological analysis based on measurements and a model analysis are conducted for three nourishment scenarios. The morphological analysis uses an extensive dataset of bathymetric surveys to compute volume changes, sediment fluxes, bar migration rates, momentary coastline (MKL) positions and nourishment longevities (defined as the period that the volume in between the dune foot and mean low water level is greater than before nourishment). The numerical model analysis is based on the output of a morphostatic XBeach model with simplified boundary conditions which computes the hydrodynamics and sediment transports.
The longevity of beach nourishments was found to be on average 3.3 years and the MKL regressed 3.9m/yr on average two years after construction. The eroded sediment did not accrete on the shoreface but was likely transported in the direction of the net sediment transport. The model output indicates that a beach nourishment only has a significant local effect on the hydrodynamics and transport rates.
The shoreface nourishment transforms from a landward skewed triangular shape into a more rounded body without the formation of a trough. The bar migrates onshore but not alongshore and the bar volume remains constant. The model shows a contraction of the tidal flow due to the shoreface bar, increasing the seaward velocity and causing a sheltered zone at the leeside and downdrift of the bar. Consequently, the alongshore transport gradients are increased. This causes extra erosion on the shoreface seaward of the bar while accretion is seen at the downdrift shoreface. The shoreface bar contributes little to the offshore dissipation of wave energy. No evidence for a wave-driven salient effect was found at Domburg from the model output.
The longevity of the 2019 beach nourishment is not prolonged by the presence of a shoreface bar, as this was found to be 3.1 years. Likewise, the MKL measured a regression of 4.3m/yr, similar to previous beach nourishments. Positively, the shoreface bar captures eroding beach sediment because accretion was found in the surf zone, as opposed to the 2014 beach nourishment. Therefore, a shoreface nourishment is moderately beneficial on maintaining the MKL but contributes to the sediment balance of the coastal cell.
Additionally, an alternative nourishment strategy was evaluated through the numerical model. A mega nourishment to the west of Domburg is a viable option as it is likely to feed the updrift and downdrift coastlines which have a sediment demand. It is recommended to further evaluate this nourishment strategy with different numerical models.
Modelling the response of a coastline during a storm event requires an offshore boundary condition that represents the incident short and long waves. The boundary condition for the incident infragravity waves is often derived by assuming a local equilibrium between the directionally spread sea-swell waves and bound infragravity waves following the K. Hasselmann (1962) method. This approach has proven to be problematic for two main reasons. First, the method tends to overestimate the incident infragravity wave height by assuming a local equilibrium is achieved at the model boundary; however, observations have shown that the transfer of energy from the short-wave groups to the underlying bound wave is gradual on a sloping bed. Second, by applying the equilibrium K. Hasselmann (1962) method it is implied that only bound infragravity waves are present at the boundary underestimating the total incident infragravity wave energy.
An analysis of XBeach in surfbeat mode revealed that the model can confidently predict infragravity wave behaviour on a natural beach slope with two well-developed bars during a storm event by calibrating the roller breaker slope coefficient and wave breaker coefficient. Infragravity wave heights were reasonably accurately predicted in the surfzone, but the model generally overpredicted the most energetic infragravity waves.
Further, the study investigated the impact of free infragravity waves on dune erosion predictions by simulating the behaviour of two 1D planar beach slopes to the inclusion of free long waves at the boundary. The study revealed that on a beach with a 1:35 slope the relative increase in dune erosion volume was 44.1% when the bound infragravity energy is equal to the free infragravity energy relative to the case excluding free waves. For the same conditions, a 20.6% increase was observed in the maximum runup. For a milder 1:70 slope a 36.4% and 6.1% increase in dune erosion and maximum runup was observed.
The results demonstrate that the dune response of a coastline is sensitive to the inclusion of incident free infragravity waves at the boundary. Moreover, neglecting the presence of free infragravity waves at the boundary may underpredict the dune response during a storm event.
High runup events and flooding on coral reef-lined coasts have been subject of recent studies, and some valuable insights are gained. It was found that the recent increases in wave attack, high runup events, and coastal flooding are primarily due to high offshore water levels coinciding with high energy swell events, circumstances which will become more frequent with sea-level rise. Increasing water depth over the reef changes the hydrodynamics across the reef in such a way that larger incident-band and infra gravity-band waves reach the shoreline, causing high runup levels and flooding of the land behind the shoreline.
However, little is known about the influence of longshore variations on runup and flooding of reef-lined coasts, while most show significant longshore variations, of which shore-normal paleo-stream channels are a prevalent one. This study aims to fill in that knowledge gap by examining the influence of paleo-stream channels on runup along reef-fronted shorelines, specifically during extreme wave conditions.
With a system analysis we determine the range of naturally occurring topographies of reef-channel systems and determine a representative reef. Results of this analysis are a useful starting point for future studies on this subject.
With a parametric study using the numeric model XBeach, we show that the presence of a channel results in a strong circulation on the reef flat and significant longshore variation of runup. Depending on the geometry and forcing, runup levels are increased next to the channel or inside the channel. This impact of the channel increases for higher incident waves, lower incident wave steepness, wider channels, a narrower reef and shorter channel spacing. Longshore variation of infragravity wave height is responsible for large scale variations in runup, while setup, short waves and very low frequency wave heights cause a local increase of runup inside the channel.
Results of the parametric study are valuable as they provide insight in which locations on a coast are most vulnerable to high runup events, using only widely available data such as reef geometry and offshore wave conditions. This is relevant for prediction of coastal hazards and to guide coastal management policies. Furthermore, this study provides insights for future studies on flood risk of reef-lined coasts, as it illustrates the importance of accounting for longshore variations while schematizing a coastline to predict high runup and coastal flooding, for instance to assess when a 1D schematization is sufficient and when a 2D model is required.
The findings are compared with two XBeach models (surf beat model and hydrostatic swash model) which are used to reproduce the observed morphological behavior of the upper intertidal bar. Both models partly reproduce the onshore migration but show deviating results regarding the final growth of the intertidal bar. In contrast to the surf beat model, the morphological changes in the hydrostatic swash model are primarily induced by swash zone processes, which is comparable to the processes in the TLS measurements. Finally, a conceptual model is developed in which four intertidal bar regimes are classified based on the tidal water level. The distinction determines the dominant cross-shore processes for the for-mation, migration, growth and destruction of intertidal bars. The model shows that the swash zone processes are dominant for the onshore migration and growth of intertidal bars in the overwash regime, while the surf zone processes are dominant in the submersion regime. The findings presented in this study provide a better understanding of the intertidal bar behavior. Although the XBeach models did not reproduce the observed behavior completely, there are some pronounced similarities. Further research is required to increase the knowledge of intertidal bar behavior at a variety of sandy coasts and to improve the performance of rocesses-based models like XBeach.
Failure was assessed on two failure mechanisms: 'Grensprofiel', which consists of two aspects. A minimum sand dike volume above storm surge level must be available in every transect alongshore in the sand dike. Also, each post-storm sand dike transect must be large enough to contain a legally defined 'limit profile'. The second mechanism is 'Initiation of flooding', which means that failure occurs when at any point landward of the 'Waterstaatswerk' boundary (the outer border of the primary coastal defence) a depth of at least 20 cm is observed. In all present scenarios no failure occurs for a normative 1/3000 year storm. De Slufter is therefore considered 'safe'. Maximum wave heights did not increase significantly for different bathymetry configurations due to the large amount of dissipation occurring in the slufter valley. No overwash or overtopping occurred in any of the modelled scenarios and morphological impact on the sand dike itself was relatively low. Failure does occur for the 1/3000 year storm with a sea level rise of 1.95 m and 3.17 m. Failure does not occur in the middle of the sand dike where the majority of the wave attack happens but in the southwestern and northeastern parts due to inundation over the ridge there (‘Initiation of flooding’). It is expected that the 'tipping point' of De Slufter, which is the sea level rise magnitude beyond which De Slufter does not adhere to safety standards, is at a sea level rise magnitude of 1.70 m.","De Slufter; Texel; coastal morphology; dune erosion; sea level rise; coastal safety assessment; XBeach Surfbeat; gully migration","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering","","53.139800, 4.798915"
"uuid:53037d4d-d7a9-4f71-9b22-8c329f2b384c","http://resolver.tudelft.nl/uuid:53037d4d-d7a9-4f71-9b22-8c329f2b384c","Computationally Efficient Modelling of Compound Flooding due to Tropical Cyclones with the Explicit Inclusion of Wave-Driven Processes: Research into the required processes and the implementation within the SFINCS model","Leijnse, Tim (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Reniers, Ad (mentor); Bricker, Jeremy (mentor); Labeur, Robert Jan (mentor); Nederhoff, Kees (mentor); van Ormondt, Maarten (mentor); Delft University of Technology (degree granting institution)","2018","Tropical cyclones (TCs) have tremendous impact on coastal communities in terms of damage due to flooding and high wind velocities, as shown by the recent hurricane season of 2017. Coastal flooding due to TCs can be contributed to different types of forcing (e.g. high offshore water levels, rainfall, etc.), or multiple at the same time (i.e. compound flooding). While the impact of TCs increases, there is the need for better predictions and early warning systems (EWSs). Probabilistic forecasting by modelling different ensembles is wanted to account for uncertainties, which requires fast models. Current modelling options are static models (bathtub approach), which are fast but too simple. Furthermore there are advanced process-based models (like Delft3D, XBeach) which are accurate but too computationally expensive. The solution is the use of a semi-advanced model which solves all relevant processes, but does that in a computationally efficient way. The semi-advanced SFINCS model is developed to solve all necessary processes with computationally efficiency in mind, but is still in its development phase.
This research assesses how compound flooding due to TCs can be modelled in an accurate and computationally efficient way. Besides assessing the relevant physical processes, the implementation in a semi-advanced model is tested. In multiple tests it is shown that for conceptual situations a semi-advanced model can give accurate results within certain limits. Also it is shown in what conditions the advection term of the momentum balance needs to be solved and that a swash zone model approach with an indirect random forcing can give realistic results in 1D runup tests.
A case study of the compound flooding at Jacksonville, Florida, during Hurricane Irma (2017) shows that using a semi-advanced model, a similar accuracy compared to Delft3D can be achieved while being two orders of magnitude faster. Furthermore, a first test with wave-driven flooding in a real case study at Hernani, the Philippines, during Typhoon Haiyan (2013) shows that wave-driven processes have to be explicitly included to model all types of compound flooding. The approach to include these processes with the semi-advanced SFINCS model gives reasonable results compared to XBeach, although they can be further improved with more research. The resulting computational efficiency seems to get in the right range as needed for EWSs.
Therefore this work explores to what extent the morphological developments of these artificial beaches can be modelled with a numerical model. The numerical models that were used in this work are XBeach and XBeach-G. The aim is to reproduce the short term (3 to 6 hours) cross-shore morphological developments of the DynaRev experiments (a series of physical model experiments to investigate the stability of an artificial composite beach under sea-level rise), with a focus on the beach section on which the gravel revetment is placed. The first step was to find the relevant processes that are missing in the currently available numerical models. The missing processes were identified by combining knowledge acquired through the modelling of composite beaches with the currently available models, substantiated by analysis of the DynaRev experiments and literature research.
It was concluded that the numerical models were lacking two important processes: namely the absence of a gravel transport formulation in XBeach and transport of sand in the gravel revetment. The missing processes were implemented into XBeach and the updatedmodel’s performances were verified with the DynaRev experiments as benchmark. The implementation of the XBeach-G gravel transport formulation in coherence with the existing XBeach code for sand transport required under-the-hood adaptations to improve the switches that are already in place for multiple sediment fractions. It was found that a combination of sand transport and gravel transport is possible, but this combination presents difficulties regarding suspended sediment transport in combination with the hydrodynamics and the groundwater dynamics.
The second process was transport of sand in the gravel revetment. This was first analysed by looking at the initial transport rates of sand inside the revetment with a newly introduced equation for transport of sandinside a filter layer (Jacobsen et al., 2017). It was shown that transport fluxes of sand inside the revetment are likely to occur as results of the groundwater dynamics. To see bed level changes due to these transport fluxes this transport equation needs to be implemented into the XBeach code. This was not possible with the current architecture of XBeach and the way it accounts for multiple fractions. Therefore a new accounting system for multiple fractions was introduced in this work and implemented in XBeach, named the two-line model. In this experimental model sand transport gradients in the revetment are translated into visible sinking of the revetment (erosion) and settling of sand within the gravel revetment (deposition). In the model’s current state the erosive locations appear to match with the DynaRev observations, whereas locations where deposition occurs are no match. The two-line model was sensitive and showed instabilities, mainly due to high groundwater velocities that were caused by wave breaking in themodel. The results of the experimental model can possibly be improved by including vertical groundwater velocities to model sand transports, possibly also in combination with the addition of suspended sediment transport of sand inside the revetment.","XBeach; XBeach-G; Morphodynamics; Composite beach; Modelling; Multiple fractions; Gravel","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering | Coastal Engineering","",""
"uuid:d01c0eca-50f3-4339-b7bf-dce035dd4ebb","http://resolver.tudelft.nl/uuid:d01c0eca-50f3-4339-b7bf-dce035dd4ebb","The determination of the feasibility of a 'Sand Breakwater' on the Nigerian coastline at Badagry","Peters, Jochem (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Aarninkhof, S.G.J. (graduation committee); Verhagen, H.J. (mentor); van den Bos, J.P. (graduation committee); Scholl, Olaf (mentor); Delft University of Technology (degree granting institution)","2018","The coastal waters of Nigeria are a difficult environment to build structures to control the local morphology and hydrodynamic conditions to ones desire. This difficulty is a consequence of the persistent high-energy swell wave climate and Longshore Sediment Transport (LST) in the order of one million cubic metres per
year. Besides these tough conditions, the costs of conventional coastal mitigation solutions are high because of the lack of suitable material (rock) close by. These characteristics of the Nigerian coast led to the interest, whether it is possible to design and construct a ’Sand Breakwater’ for a planned port on the Nigerian coastline at Badagry. A ’Sand Breakwater’ is the complete construction which functions as the protection of the port and creates shelter from incoming waves for ships inside, consisting out of components (partly) made out of sand and hard structures. Especially the very uni-directional and consistent character of the wave climate creates an opportunity to embrace natural processes in the design. In addition, much sand needs to be dredged for the creation of the approach channel along with the port’s basin. This volume of sand might possibly be re-used for a sand breakwater.
This report presents the feasibility of a sand breakwater on the Nigerian coastline at Badagry. This feasibility is determined both morphologically and economically. Long term coastline development was modelled with the one-line coastline evolution model LITPACK. Results show that the persistent uni-directional character of the wave climate prevailing at the coast of Badagry forces an equilibrium coastline orientation of 277.5 degrees with respect to True North (TN). Because of this, all other orientations of coast rather than the equilibrium orientation are vulnerable and will develop to the equilibrium orientation over time. The implementation of this equilibrium orientation in the design along with some hard structures led to multiple long term morphological
stable conceptual variants for a sand breakwater. The coastline impacts of these variants west of the sand breakwater were examined on large scale. In 38 years after construction a maximum of 20 meters coastline retreat occurs, which is considered acceptable. After this period, only shoreline accretion takes place.
To establish complete conceptual designs, the cross-shore profiles are first determined. The submerged part of the profile up to the top of the intertidal profile is constructed by integration of measured data in the design. The emerged part of the profile concerns the cross-section of a dune and was the critical part for the designing objective. To determine these profiles, first the design crest height is determined by applying maxmimum acceptable overtopping volumes (Tilmans, 1983). Knowing the design crest height, the dune width is determined by modelling storm impact. This width is established by examining storm impact and determine what minimal required width needs to be present for the sand breakwater to live up to its design criteria.
Storm impact for the found conceptual variants is examined with the numerical software XBeach to determine cross-shore sediment transport during storm impact. The impact of storms with shorter return periods results to be relatively high in comparison to the storms with longer return periods. This is taken into account by adapting the design conditions. The design storm conditions are based on the impact of a storms with a 1/100 year return period along with an expansion for a consecutive storm with a return period of 1/1 year. With this
storm impact known, the required crest width is determined and with that the minimal required cross-shore profiles for the multiple variants are established. Consequently, besides long term morphologically stable, the conceptual designs are capable to design storm impact, confirming the morphological feasibility.
In order to define the economical feasibility of a sand breakwater, a comparison between the different conceptual variants and a conventional design is made. This comparison is conducted via a rough cost comparison along with examining future prospects. In order to be able to make this cost comparison, rough volume estimations are done for all conceptual variants and conventional design. These volume estimations combined with unit prices lead to a rough cost comparison. From this rough cost comparison appears that a sand breakwater
showed to be in the same price range as a conventional design.
Three future prospects of a sand breakwater are determined. First of all the by-pass of sand is compared for the conceptual designs to the conventional design. The characteristic LST is problematic due to the occurrence of sand by-passing causing sedimentation in the approach channel and port. One of the conceptual variants shows that a sand breakwater is proven to be able to provide a period without by-passing of sand in the same order of magnitude as a conventional design.
Another future prospect is the required maintenance necessary to execute for the sand breakwater. The XBeach results of lower storm conditions show that maintenance costs are high. However, these results are assumed to be heavily overestimated. Results with more specific ’Nigerian conditions’ create reason to believe that maintenance for lower storm conditions will be much lower and in acceptable range.
The last future prospect concerns the accreted land which arises due to the blockage of LST. A sand breakwater creates the possibility of acquiring land for new port space which is not possible with a conventional breakwater. In addition, the value of a Building with Nature component is present in this project and could enhance its economical interest.
Not only the morphological but also the economical feasibility of a sand breakwater is confirmed. The ’Sand Breakwater’ succeeded to convert the drivers of the problem to its solution, leading to an innovative and in all likeliness even more cost-effective solution compared to the traditional approach.","Sand; Breakwater; Morphology; Coast; Beach; Ocean; Sediment; Longshore sediment transport; Building with Nature; feasibility study; wave climate; uni-directional; Litpack; XBeach; Design; Coastal development; coastal engineering; Coastal Engineering; Coastal defense","en","master thesis","","","","","","""The palest ink is stronger than the best memory.""","","","","","","Civil Engineering | Hydraulic Engineering","","6.392870, 2.863491"
"uuid:4f738968-be52-4484-af1d-60a0e74fdbab","http://resolver.tudelft.nl/uuid:4f738968-be52-4484-af1d-60a0e74fdbab","Non-hydrostatic wave modelling of coral reefs with the addition of a porous in-canopy model","de Ridder, Menno (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Reniers, A.J.H.M. (graduation committee); van den Bos, J.P. (graduation committee); Nederhoff, Kees (mentor); Babovic, Vladan (graduation committee); McCall, R.T. (graduation committee); van Rooijen, Arnold (graduation committee); Delft University of Technology (degree granting institution); National University of Singapore (degree granting institution)","2018","One sixth of the world's coastline consist of coral reefs and provide natural flood defence for the people who live in the coastal region behind the reef. However, a rising sea level, changing wave conditions and degradation of corals threaten the coastal safety of these reefs.Numerical models can be applied to study the reef-hydrodynamics and the effects of coral degradation on the reef-hydrodynamics. When non-linear processes are important or the individual waves need to be determined, a phase resolving model is preferred. Within this thesis two issues regarding the application of non-hydrostatic models to coral reefs were studied.
Due to the large bottom gradient in front of a reef, the offshore boundary has to be located in deep water, which means that frequency dispersion becomes important. The accuracy of frequency dispersion within non-hydrostatic models depends on the number of vertical layers. However, the addition of a vertical layer increases the computational time extremely. Therefore, a reduced two layer non-hydrostatic model (XBeach-nh+) was developed with the assumption of a constant non-hydrostatic pressure in the lower layer. In theory, XBeach-nh+ is capable of modelling the wave transformation from deep to shallow water, but the applied boundary conditions cannot force deep water waves. On top of that XBeach-nh+ has never been properly validated for reef environments.
Furthermore, the corals (growing on the reef flat) have a large effect on the reef-hydrodynamics by dissipating a large part of the wave energy. There exist different formulations to include vegetation into a non-hydrostatic wave model, but these formulations are mainly applicable for cylinder shaped geometries, whereas corals are more complex in shape. Apart from the shape, the in-canopy velocity can be significantly different from the free stream velocity. Therefore, a porous in-canopy model was implemented to model the in-canopy velocity, which was used to determine the canopy-induced force on the depth-averaged flow computation.
Firstly, the inclusion of the second reduced layer improves the dispersion relation up to a relative depth ($kh$) of 5 for linear waves. A simulation of biochromatic waves over a plane beach showed that XBeach-nh+ is capable of modelling the energy transfer between the major wave components. Both steeping and reflection of the sub-harmonic were modelled according to the measurements. Furthermore, the validation of random waves over a fringing reef showed the capability of XBeach-nh+ to model the reef-hydrodynamics for different wave conditions (rel. bias of -0.003 for total wave height, -0.081 for LF-waves and -0.103 for the setup). Moreover, the addition of the second reduced layer gives a more robust prediction for all modelled wave conditions, whereas the one-layer model contains more scatter.
Secondly, the in-canopy model captures the canopy-induced force when the canopy parameters were known. Both the in-canopy flow of unidirectional and oscillating flow fields was accurately modelled when the results were compared to the measured velocity though cylinders and corals. Although, the canopy parameters were not always known, it was shown that an un-calibrated in-canopy model, based on porosity and canopy height, gives a competitive result compared to a fully calibrated shear stress formulation. The applicability of XBeach-nh+ in 2-dimensional domain with a coral covered reef flat was shown by modelling a 5 day Swell event at Ningaloo Reef. Reasonably accurate results were achieved when using the in-canopy model, based on the canopy properties.","XBeach; Non-hydrostatic; Coral reefs; Hydrodynamics; in-canopy flow","en","master thesis","","","","","","","","","","","","","",""
"uuid:17620c42-8c1b-491c-b862-f4c1f6c92a48","http://resolver.tudelft.nl/uuid:17620c42-8c1b-491c-b862-f4c1f6c92a48","Modelling the effects of excavation pits on fringing reefs","Klaver, Sebastiaan (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering; TU Delft Water Management)","Aarninkhof, S.G.J. (mentor); Tissier, M.F.S. (mentor); van der Spek, A.J.F. (mentor); Nederhoff, Kees (mentor); Giardino, A (mentor); Jing, Yuan (mentor); Delft University of Technology (degree granting institution); National University of Singapore (degree granting institution)","2018","Many small island developing states (SIDS) are among the most vulnerable to climate change (e.g. sea level rise) and seasonal to inter-annual climate variability, and subsequently experience flooding due to swell waves and wind waves, coastal erosion and salinisation of freshwater lenses. To counteract this, reef flat mining for sand and aggregate offers a possible solution to the material demand for coastal protection infrastructure and other engineering projects. However, the knowledge on the effects of these pits is limited to only two studies at a single reef: near-shore wave transformation based on measurements (Ford et al., 2013) and a numerical modelling study (Yao et al., 2016). This MSc thesis provides insights on the effects of pits, related to hydrodynamics and wave runup, on a large variety of fringing reefs, by using a 1D and 2DH process-based wave-resolving hydrodynamic model (XBeach non-hydrostatic+, “XBnh+”). Model results indicate that excavation pits cause a decrease in amplitude for waves in the infra-gravity (IG) frequency band. For the majority of the modelled reefs, this was mainly due to a modification of the longest natural frequency of the reef caused by the pit, resulting in a decrease in resonant amplification. Both pit width and cross-shore location have a strong influence on this mechanism. The observed changes in variance in the high frequency (HF) band can be partly explained by a decrease in wave dissipation, as well as a decrease in wave-wave (triad) interaction, both associated with (locally) increased water depth, which causes an increase of the HF peak and a decrease of the HF tail. Of all modelled reefs, there is a 15% chance of an increase in wave runup due to the presence of a pit. This probability is lowest for pits with narrow width and/or located close to the reef crest. The change in runup ranges from +10% to -20%. An increase in runup is caused by a combination of increased wave energy at peak frequencies, as well as smaller reductions in resonant amplification. Moreover, the effects of pits on mean water level near-shore of the excavation can cause circulation patterns that potentially result in coastal erosion and entrapment of sediments.","fringing reef; hydrodynamics; XBeach; wave runup; atoll; excavation pits","en","master thesis","","","","","","NUS-TUD Double MSc Degree Programme","","","","","","Civil Engineering | Hydraulic Engineering","","7.121645, 171.186053"
"uuid:400f0dbe-c89b-4b5f-a2a4-37274cd9f976","http://resolver.tudelft.nl/uuid:400f0dbe-c89b-4b5f-a2a4-37274cd9f976","Stability of stones on mild slopes","Wendt, Emiel (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Uijttewaal, Wim (mentor); Hofland, Bas (graduation committee); Kuiper, Coen (graduation committee); Jumelet, H.D. (graduation committee); Delft University of Technology (degree granting institution)","2017","The static stability of stones on mild slopes under wave attack is investigated in this research. The first part of the research is focused on reproducing the physical scale model tests regarding profile change of Kramer (2016) numerically with the model XBeach-G. The erosion profiles modelled with the bed-load transport formulas of Nielsen (2006) and Van Rijn (2007) in XBeach-G do not match the erosion profiles of the profile change experiments of Kramer (2016). The bed-load transport formulas of Nielsen (2006) and Van Rijn (2007) are not able to model the sediment transport in XBeach-G accurately. Furthermore, XBeach-G cannot determine the velocity and acceleration near the bed, because the model solves the flow due to currents and waves for a single layer. Therefore, it can be concluded that XBeach-G should not be used to describe static stability of stones on mild slopes under wave attack. For the application of dynamically stable structures (which is not investigated in this research), XBeach-G functions satisfactorily (Postma, 2016). For further research, a model that solves the hydrodynamics for multiple layers should be applied. In this way, the hydrodynamics near the bed can be used to describe the static stability of stones.
The aim of the second part of the research is to develop a design method that describes the static stability of stones on mild slopes under wave attack. The basis of this design method is the initiation of motion of a stone and the hydrodynamic forces that initiate this movement. The hydrodynamic forces and corresponding mobility parameters are determined with the velocity and the acceleration near the bottom. Using Bubble Image Velocimetry (BIV), the velocity and the acceleration are derived from the videos of the BIV experiments of Kramer (2016) with regular waves breaking on a slope. It is found from the results of the BIV analysis that the effective, adapted Shields parameter θ’McCall can be used to describe movements of stones on mild slopes under wave attack. This mobility parameter has been determined with the bed shear stress of McCall (2015), which added an inertia term to include the influence of accelerations. For initiation of motion of stones, it appears that the stability parameter θcr could be a value of 0.024 (in case no slope correction factor has been applied). To substantiate a design method that describes the static stability of stones on mild slopes under wave attack, the value of 0.024 could be used to define a threshold for initiation of motion of stones. More experiments need to be executed to optimize this value of the stability parameter. Moreover, a statistical value for the stability parameter could be used (like θcr,1%) to describe the static stability of stones by means of a certain number of stones that are allowed to move for a certain number of waves.","stability of stones; initiation of motion; Shields parameter; mild slopes; design method; erosion profiles; BIV analysis; XBeach-G; McCall (2015); Van der Meer (1988); Nielsen (2006); Van Rijn (2007)","en","master thesis","","","","","","","","2018-12-20","","","","","",""
"uuid:dd8c8a2b-5905-4b51-96c5-1243af5ec507","http://resolver.tudelft.nl/uuid:dd8c8a2b-5905-4b51-96c5-1243af5ec507","The influence of incident waves on runup: A comparison between a phase-averaged and a phase-resolving XBeach model","de Beer, Anne (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Reniers, A.J.H.M. (mentor); Tissier, M.F.S. (graduation committee); de Schipper, M.A. (graduation committee); McCall, R.T. (graduation committee); Long, J.W. (graduation committee); Delft University of Technology (degree granting institution)","2017","Wave runup is generated by energy which remains after wave breaking and travels farther to the coast in the form of a bore. It can be seen as a thin wedge of water running up the beach face (Brocchini and Baldock, 2008). Under storm conditions runup is responsible for beach and dune erosion and accurate runup predictions are therefore required (Ruggiero et al., 2001; Stockdon et al., 2005). For runup and its components, the time-mean setup component and the time-varying swash component, empirical parameterizations have been developed in the past, but they cannot be validated for storm conditions due to a lack of data (Stockdon et al., 2005). The data gap can be filled by numerically simulated runup, for example with the process-based XBeach model. XBeach is a depth-averaged model which predicts nearshore hydrodynamics and can be used in a phase-averaged or a phase-resolving mode. However, both the significant incident and infragravity swash is underpredicted by the phase-averaged XBeach Surfbeat model (Palmsten and Splinter, 2016; Stockdon et al., 2014), which does not resolve incident wave motions. In order to predict runup under storm conditions with confidence the performance of XBeach under mild conditions should be assessed. Here runup simulated with XBeach Surfbeat and the phase-resolving XBeach Non-hydrostatic for the intermediate reflective beach of Duck was compared to measurements of the SandyDuck'97 experiment, where mild offshore conditions were present. A 2DH model was set up using measured bathymetry and forced with measured frequency-directional spectra. The hydrodynamics responsible for a difference in runup prediction were investigated and their origin in the cross shore was identified. It was shown that the prediction of significant incident and infragravity swash can be improved by using the phase-resolving XBeach Non-hydrostatic model instead of the XBeach Surfbeat model, while performance for setup remains similar. Incident swash predictions are improved by resolving the incident wave motions. The major part of the improvement in infragravity swash predictions is driven by differences in infragravity wave transformation between the two XBeach models. A small part also originates within the swash zone, for which incident bore merging can be a possible explanation. The difference in infragravity wave height predictions between the two XBeach models mainly develops in the surf zone where a different response to directional spreading and different degrees of shoaling most likely can explain the difference in infragravity wave height. Against expectations no correlation with the groupiness of the incident waves or with the phase difference between wave group and infragravity wave was found. A small part of the difference in infragravity wave height predictions is already present near the offshore boundary and probably results from interaction processes between high and low frequency wave boundary conditions. It can thus be said that on intermediate reflective beaches, where both incident and infragravity waves play a role, the resolving of incident wave motions is a necessity to predict runup accurately. In these situations the phase-resolving XBeach Non-hydrostatic model should therefore be used, instead of the phase-averaged XBeach Surfbeat model. Here only an intermediate reflective beach and a small range of energetic conditions were included. More types of beaches and storm condition forcing should be investigated to be able to validate the empirical parameterizations but also to further indicate applicability ranges of the two XBeach models. Also, more attention should be paid to hydrodynamic differences at the boundary and in the surf zone in order to find more conclusive reasons for differences between the two XBeach models.","XBeach; runup; numerical modelling; SandyDuck'97; infragravity waves; hydrodynamics; swash","en","master thesis","","","","","","","","","","","","Civil Engineering | Hydraulic Engineering | Coastal Engineering","",""
"uuid:822dc768-b9a5-485d-bfd6-d102c5af1924","http://resolver.tudelft.nl/uuid:822dc768-b9a5-485d-bfd6-d102c5af1924","Improving XBeach non-hydrostatic model predictions of the swash morphodynamics of intermediate-reflective beaches","Jongedijk, Cleo (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","McCall, Robert (mentor); Reniers, Ad (mentor); de Schipper, Matthieu (graduation committee); van der Werf, JJ (graduation committee); Delft University of Technology (degree granting institution)","2017","A very common observation is the episodic erosion of beaches during storms and the slow recovery (accretion) afterwards (Yates et al. 2009). Morphodynamic models parameterize physical processes in order to relate the fluid motions (hydrodynamics) to the bed level changes (morphodynamics) over a wide range of spatial and temporal scales. Despite recovery of the beach profile being a slow process, accretion mechanisms in the swash zone are complex to represent by a numerical model due to the shallow, rapidly varying flows and high concentration gradients (Brocchini & Baldock 2008). The swash zone on most beaches is readily accessible, but this accessibility does not translate into a broad knowledge of the underlying physical processes (Chardón-Maldonado et al. 2015). This research focused on assessing the representation of physical processes in the swash zone of intermediate-reflective beaches during erosive and accretive conditions in the non-hydrostatic version of the XBeach model (XBeach). This is a depth-averaged phase resolving model, mainly used for calculations of storm impact (hours-days) on sandy and gravel beaches. Based on conclusions in literature, relevant physical processes that contribute to accretion were determined. Using a simple planar beach bathymetry, the sediment transport formulations in XBeach and the individual influence of groundwater effects, bed slope effects, sediment response time and wave breaking induced turbulence were assessed. The results showed that for both accretive and erosive wave conditions, XBeach predicts erosion in the swash zone. Groundwater infiltration and wave breaking induced turbulence are likely to enhance onshore transport significantly Reniers et al. (2013), Turner &Masselink (1998). To verify the findings of the planar beach modeling approach, in the second part of this thesis the morphodynamical predictions of XBeach were compared to the dataset collected during the Bardex II experiment. Bardex II was performed in 2012 in the Delta Flume in the Netherlands and the dataset contains observations of a series of experiments focusing on the effect of varying wave, sea level and beach groundwater conditions on a sandy beach (D50=0.42 mm)(Masselink et al. 2013). Stored data and published resultswere used for comparison with the morphodynamical prediction performance of XBeach. Two timescales have been analyzed, the total morphological response of the beach on a timescale of 100 minutes and the intra-swash sediment transport processes on a timescale of 10 seconds. Two experiments from the series have been reproduced. The first, experiment A4, has an almost stable, slightly erosivemorphological response throughout the swash zone. The second, experiment A8, shows accretion in the upper swash and a stable profile in the rest of the swash zone. XBeach erroneously (over)predicted erosion above the mean sea level (MSL) for both A4 and A8 conditions. This conclusion was related to the results of the intra-swash sediment transport assessment. The modeled velocity in both uprush and backwash were higher than the Bardex II measured velocity and XBeach extremely underpredicted uprush sediment concentrations suggesting that turbulence induced by the bore is not enough taken into account. Over-predicted backwash sediment concentrations for both accretive and erosive conditions suggested that groundwater infiltration was not strong enough. However, enhancing wave breaking induced turbulence and groundwater infiltration did not lead to an improvement of the predictions of sediment concentrations in the swash. The sediment transport formulations of XBeachwere developed using a long wave resolving (short wave averaging) model, andwidely validated and calibrated on field and experimental data (Soulsby 1997, van Rijn et al. 2007, Van Thiel De Vries 2009). Therefore, in the last part of this thesis two possible improvements of the two sediment formulations of XBeach (van Thiel-van Rijn and Soulsby-van Rijn) when applying them in a short wave resolving model are assessed using a 1D sediment transport model. The decomposition of the velocity signal in amean and a fluctuating part improved mainly the predictions using Soulsby-van Rijn where the separate calibration of turbulent kinetic energy resulted in better predictions for both transport formulations. This analysis was performed only at one point on the cross-shore domain (slightly above MSL). Although the results for this position were promising, comparison of modeled sediment transport with Bardex II observations at different positions throughout the upper and lower swash zone is needed to give a full validation of the proposed adaptations to the transport formulations.","Swash; XBeach; Morphodynamics; numerical modeling; Bardex II; Delta Flume; sediment transport modelling; sand; Nearshore morphology; Morphology","en","master thesis","","","","","","","","","","","","","Comidas",""
"uuid:e684f420-9183-439e-857b-60a6a52fdc84","http://resolver.tudelft.nl/uuid:e684f420-9183-439e-857b-60a6a52fdc84","A hybrid solution for the Galveston Seawall: A study on the reduction of the hydraulic loads by a sand cover at the Galveston Seawall with the use of XBeach","Muller, Jos (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Figlus, Jens (mentor); de Vries, S. (mentor); Delft University of Technology (degree granting institution)","2017","The City of Galveston is protected from extreme storm impact by a 17-km concrete seawall facing the Gulf of Mexico. Recent investigations have shown that the seawall may not be sufficient any more to protect against a 1 in 100 year design storm. Since raising the seawall disconnects the city from the beach and may be very costly, a hybrid approach is being discussed in which the existing hard structure is covered by a dune. This numerical model study investigates the hydro- and morphodynamic effects of adding a sand cover to the Galveston Seawall under extreme storm conditions.","Galveston; Houston; Texas; hybrid; coastal; defence; seawall; XBeach; Ike; hurricane; dike; dune","en","student report","","","","","","Additional thesis","","","","","","Civil Engineering | Hydraulic Engineering | Coastal Engineering","","29.284696,-94.795859"
"uuid:8452f24d-afb7-4d08-b9d9-7ad384821f4e","http://resolver.tudelft.nl/uuid:8452f24d-afb7-4d08-b9d9-7ad384821f4e","Beach representation in morphodynamic predictions: Coupling models to improve beach behavior applied to Anmok beach","Caichac Avilés, Daniel (TU Delft Civil Engineering and Geosciences; TU Delft Hydraulic Engineering)","Aarninkhof, Stefan (mentor); Luijendijk, Arjen (graduation committee); McCall, Robert (graduation committee); de Boer, Wiebe (graduation committee); Reyns, J (graduation committee); Delft University of Technology (degree granting institution)","2017","Numerical process-based morphodynamic models are widespread in coastal engineering practice and have become the new standard when it comes to assessing the impact of natural or man-made structures on coastal environments. The most common practice among engineers is to focus on a single spatial and time scale, which means either neglecting certain processes under the assumption that they will average out, or performing detailed simulations for short time-spans in order to optimize the normally limited computational resources. Despite the efforts from several authors, at the moment there is a lack of a clear methodology which would allow incorporating the relevant physical phenomena only when required, hence optimizing the computational effort.
The above leads to the main research objective of this thesis: to gain insight in what is the added value of coupling process based morphodynamic models, regarding the morphological impacts near the beach. For this purpose two models that were originally conceived to resolve different timescales are selected; XBeach as a storm model, and the new suite from Deltares, Delft3D-Flexible Mesh (D3D-FM) as a long-term morphodynamic model. The area selected as study site is Anmok beach, located at the east coast of South Korea. The coastal erosion at this location is not yet well understood (mainly due to human interventions and storms) plus the micro-tidal wave-dominated environment makes this location ideal for this study. Recent researches on this site have found that there is a delicate balance between the stormy and calm periods, where the high energy wave events are the main drivers of local morphology.
One of the main findings in this thesis is that the coupling of independently calibrated models does not necessarily provide better morphodynamic results than the results obtained by running each model separately. Including different processes such as infragravity waves or Eulerian mass transport (which enhances the offshore sediment transport in the surf zone) during highly energetic events tend to generate large supratidal beach erosion. However, the post-storm recovery mechanisms present in long-term morphodynamic models are not sufficient to bring the sediment back to the beach. Therefore, it is recommended to include all the relevant physical processes (storm erosion and post storm recovery mechanisms) when following a coupling approach in order to have a coherent morphodynamic balance. Furthermore, the coupling of models can play an important role in identifying which processes are missing or are not fully represented by the different modelling packages.
The erosive effect of cumulative storms was shown to be relevant in the short to medium term and might become a key parameter when defining, for instance, the worst case scenario regarding shoreline retreat. Despite the fact that uncoupled long-term morphodynamic models produce better average results in the case of Anmok beach, the implementation of a coupled scheme was proven to be important when the erosion due to cumulative storm effects cannot be neglected.
Among the advantages of using D3D-FM for this work is the implementation of the Basic Model Interface, which meant an important reduction in bookkeeping efforts and the possibility to seamlessly couple D3D-FM with XBeach. This procedure allowed for the incorporation of more complex phenomena (such as infragravity waves) with an acceptable increase in computational time.
A research version of Delft3D (D3D) with specialized sediment transport equations in the swash zone was tested in an attempt to enhance the post-storm recovery mechanisms. The results obtained are promising in the sense that the accretion of the shoreline and lower dry beach was reasonably enhanced, especially when considering all the limitations involved in modelling the morphodynamics of the swash zone with a stationary wave model. Another important conclusion is that these models were capable of depositing the sediments at the lower backshore at best. Hence, there is still the need of a mechanism/process capable of transporting the sediments farther upslope into the dry beach or dunes, such as aeolian transport.
The decision to undergo with a coupled or uncoupled approach depends on case-by-case basis. For current practice, it is recommended to develop a coupled simulation in the medium-term, where both storms and calm periods have significant effects and where the intra seasonal variation could be a parameter of interest. For short-term simulations, an uncoupled storm model (e.g. XBeach) is recommended as is the most accurate for such a time span. For long-term simulations, the general recommendation is to run an uncoupled long-term model such as D3D. In this case the storm erosion and post-storm recovery processes are expected to average out, being the long-term model the most suitable package to obtain average morphological results.
For future work it is recommended to add an aeolian process-based model and incorporate the swash zone sediment transport module into D3D-FM as this would move us one step closer towards the development of a fully coupled model where all the relevant processes (storm erosion, post storm recovery and aeolian transport) are included.
Many previous studies have been done to identify the protection offered by coastal vegetation. This thesis aims to identify mangrove vegetation interactions under hazardous wave conditions. Moreover, the study includes a comprehensive parameter space by accommodating the variation in wave climate, vegetation and bathymetry observed in the field.
The hydrodynamic-mangrove interactions are analyzed from the perspective of coastal hazard mitigation by vegetation. The study focuses on wave attenuation, setup variation and runup reduction by mangroves.
The investigations are carried out using a numerical modeling scheme, XBeach-Surfbeat.
Mangrove vegetation can substantially mitigate the effects of coastal hazards by waves (wave energy, wave induced flooding) faced in the hinterland. However, the level of mitigation depends on several factors. The most important factors are, vegetation density, mangrove forest width, wave height and water level.
Denser mangrove vegetation and wider forests increases the wave attenuation while deeper water depths in mangrove forests reduces the attenuation capacity.
The improved understanding of the hydrodynamic-vegetation interactions gained in this study can be used as a foundation for a Bayesian network. A better understanding of the effect of the different parameters on flood mitigation can then be attained.
30 year) of measurement data of erosion events at Narrabeen beach (NWS, Australia) provides insight in erosion volumes and their return periods in this area. The aim of this study was to replicate these data using XBeach in order to assess the validity of both the Joint Probability Model (JPM) and XBeach on beaches such as Narrabeen. In this study, a large number of different storms were simulated using XBeach. The probability and thus return period of the resulting erosion volumes were determined using the JPM. XBeach was calibrated against two individual erosion events, one at Narrabeen beach and one at Hasaki beach (Japan). The best fit for the Narrabeen beach, obtained using a stationary mode, led to an overestimation of erosion volumes at lower return periods (< 3 year) but fell within the boundaries implied by a 95% confidence interval of the measurement data for higher return periods. When calibrated against the erosion volumes with low return periods (<2 year), XBeach slightly underpredicted the erosion volumes at higher return periods. Depending on the method of determining confidence levels, the results were outside or well within the confidence interval of the measurements. This could suggests that this method is a valid way to predict erosion volumes and their return periods, in cases where long term erosion volumes measurements are absent.","XBeach; JPM; Narrabeen; Extreme; Erosion","en","master thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:5e790ec1-9a1d-4a8e-b5b2-e11217f43a97","http://resolver.tudelft.nl/uuid:5e790ec1-9a1d-4a8e-b5b2-e11217f43a97","Dune erosion near sea walls; XBeach validation","De Vries, B.B.","Stive, M.J.F. (mentor); Van Dongeren, A.R. (mentor); Van Geer, P.F.C. (mentor); Van Thiel de Vries, J.S.M. (mentor); Smit, P.B. (mentor)","2011","During a storm dunes erode and provide sediment to the beach. The foreshore rises and the wave height decreases. Subsequently the wave-induced water level setup increases. Seawalls do not erode. The waves in front of the seawall remain high throughout the storm and the wave-induced water level setup hardly changes. Over a dune-dike connection a water level gradient drives a current which transports sediment from the dunes to the seabed in front of the seawall. The loss of sediment to the dike causes the foreshore of dunes near the connection to rise slower resulting in more erosion. The amount of additional erosion near structures depends on the angle of wave incidence w.r.t. the shore. Dunes situated downstream of the connection experience a significant increases in erosion. Near the upstream connection sediment is deposited on the seabed in front of the structure. Sediment will pile up against the structure resulting in less erosion near the connection. The influence of structures on dune erosion was investigated in a series of experiments and were aimed at 4 different connections between a dune and a structure. Two configurations of dunes and structures were investigated with 2 different wave periods. In XBeach the erosion in a dike breach is a function of the breach width and the wave period. The erosion increases for a smaller breach width and an increasing wave period. XBeach underestimates the erosion above the dune revetment. The erosion in the revetment breach is predicted well. For experiments V1 & V3 the predictive capabilities of XBeach in dune sections are very good. The performance of XBeach for experiments V2 & V4 is relatively good. The effect of the wave period for 2DH models with a large depth scale (nd = 60) is not properly simulated by XBeach.","dune; dike; connection; XBeach; validation","en","master thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","Coastal Morphology","",""
"uuid:632cd1ad-ca43-4289-a7d3-79b134459ed6","http://resolver.tudelft.nl/uuid:632cd1ad-ca43-4289-a7d3-79b134459ed6","Modelling Sediment Transport in the Swash Zone","Van Rooijen, A.A.","Stive, M.J.F. (mentor); Reniers, A.J.H.M. (mentor); Van Thiel de Vries, J.S.M. (mentor); McCall, R.T. (mentor); Henriquez, M. (mentor); Smit, P.B. (mentor)","2011","The swash zone is the part of the beach that reaches from the limit of wave run-up until the limit of wave run-down. It is recognized as being a dynamic area of the nearshore region, characterized by strong and unsteady flows, high turbulence levels, large sediment transport rates and morphological changes on a small timescale. Due to the complexity of the processes taking place in the swash zone, there are still great uncertainties about the driving forces for sediment transport. Morphodynamic process-based numerical models tend to overestimate the seaward directed sediment transport in the swash zone, especially for mild wave conditions. The main objective of this thesis is to obtain insight in the hydrodynamic processes responsible for sediment transport in the swash zone, and to use this knowledge to optimize a morphodynamic numerical model (XBeach) for simulating swash zone physics. First, an extensive literature review is carried out to provide the physical base. Second, a number of (theoretical) linear profile simulations are conducted to provide insight into the simulated swash characteristics for different beach states, and to assess the effect of including a number of swash processes (e.g. turbulence or groundwater flow) in the simulations. Third, measurements obtained during a field experiment in Le Truc Vert (France) are used to verify three hydrodynamic modelling approaches and two sediment transport models.","numerical modelling; XBeach; high frequency swash; low frequency swash; Le Truc Vert","en","master thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","Coastal Engineering","",""
"uuid:aa89b3d8-e599-4161-9914-61447efa36a0","http://resolver.tudelft.nl/uuid:aa89b3d8-e599-4161-9914-61447efa36a0","Low frequency wave resonance on fringing reefs","Pomeroy, A.W.M.","Stive, M.J.F. (mentor); van Dongeren, A.R. (mentor); van Thiel de Vries, J.S.M. (mentor); Ranasinghe, R.W.M.R.J.B. (mentor); Zijlema, M. (mentor); Lowe, R. (mentor)","2011","Reef systems have been estimated to exist along approximately 80% of the world’s coastlines with living coral reefs, relic limestone platforms and submerged rock formations being the most common types observed. The processes of wave breaking on a reef crest, setup on a reef and flow over and within a lagoon, have been the primary focus of research to date, while wave transformation shoreward of the reef crest and surf zone have also been studied. The propagation of low frequency waves has been shown to have a large influence on flow, sediment transport and morphology. Furthermore, it has been demonstrated that these waves may possess periods that, if closely correlated with the reef width and depth, may enter a standing wave type form and possibly resonate. Aim: The aim of this study was to determine the indicators of low frequency resonance in field, laboratory or numerical model data, and to identify the influence of different geometric parameters on the generation of low frequency wave resonance on a fringing reef. Methods: The indicators were tested by the use of the numerical model XBeach, which was demonstrated to consist of a numerical basis suitable for the analysis of reef systems. The model was calibrated with high-resolution field data obtained at the Ningaloo Reef (Western Australia). The tested indicators were then applied to the Ningaloo Reef field data to determine if a resonance signal could be identified at the site. Finally, a geometric parameter sensitivity analysis was conducted with an idealised reef profile based upon the Ningaloo Reef. The wave boundary of the model was forced with a JONSWAP-type spectrum that characterised the peak of a storm at the site. The influence of different geometric parameters (in both non-frictional and frictional cases) was investigated and compared to an analytical model. Results: For two time-series that are spatially lagged across a reef, three indicators need to be satisfied to demonstrate the presence of resonance. They are: the surface elevation variance across the basin must be coherent, a phase relationship associated with the mode of resonance considered must exist, and an amplification of the wave between two points considered at the frequency of resonance must occur. The results of the indicator tests showed strong agreement with a simple basin analytical model that was adapted to include the effect of a lagoon. Strong amplification (resonant) peaks were observed for the first two standing waveforms. The frequency of these peaks was affected by the setup on a reef while the amplitude was affected by the influence of friction. It was shown that for frictional values consistent with Ningaloo Reef, the amplification peaks ‘flatten’ to magnitudes similar to the progressive waves in the spectrum. The geometric sensitivity analysis indicated that the resonant frequency was more sensitive to the reef and lagoon length than the reef and lagoon depth. The amplification was greatest for the zero and first-mode of resonance. However this amplification was dampened with the introduction of friction. It was determined that resonance is not likely to occur on reef systems with the geometry, frictional characteristics and wave forcing similar to the studied section of Ningaloo Reef. Resonance may occur for reef systems with shorter reef and lagoon widths, lower frequency forcing and/or less frictional dissipation. The latter may occur for reefs that have a different roughness to Ningaloo Reef as well as for reef systems that are damaged or dying in which coral assemblages degrade into coral rubble.","resonance; fringing reef; low-frequency; XBeach; coral reef; standing wave; infragravity","en","master thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","CoMEM - Coastal and Marine Engineering and Management","",""
"uuid:27aa2fdc-6f32-4f29-b423-7fbaabed5fb9","http://resolver.tudelft.nl/uuid:27aa2fdc-6f32-4f29-b423-7fbaabed5fb9","Coastal response during the 1953 and 1976 storm surges in the Netherlands. Field data validation of the XBeach model","Voukouvalas, E.","Stive, M.J.F. (mentor); Van Dongeren, A.R. (mentor); Walstra, D.J.R. (mentor); Labeur, R.J. (mentor)","2010","The storm surge early-warning system that is going to be established in the Netherlands, combines the accurate weather forecast with the hydrodynamic and morphodynamic models in an operational mode, in order to estimate the potential impact on the coasts. This study focuses on the morphodynamic validity of the operational model system, by studying two historical storm surge events on prototype scale. Due to the lack of warning, the 1953 storm surge left behind thousands of casualties and extensive wreckage of the Dutch coastline. In order to reduce the probability of experiencing again such a devastating storm surge, the coastal defense policy in the Netherlands had been reorganized on national level and more effective countermeasures had been received. When the 1976 storm surge attacked the country, the civil awareness and the reinforced coastal defense abate the impact and the fatalities. The degradation of the coastline has been recorded as part of the Jarkus coastline monitoring programme, and after the conducted analysis the non-uniform impact along the North Holland province became evident. Two numerical models form the operational model schematization; the deformation and the propagation of the storm surge are simulated with the Delft3D model and the nearshore hydrodynamic and morphodynamic processes by the XBeach model. The available storm surge level records attest that the model underestimates the storm surge level for both of the events, up to 0.70m and the phase lag up to 45 minutes. In the studied site of Bergen, the performed sensitivity analysis of the XBeach model proves that the water level underestimation is an important parameter for the deviations between the coastal profile records as a result of the 1976 storm surge and the computed post storm profile, as higher computational skill is obtained when imposing the measured storm surge level record instead of the computed one. Furthermore, the XBeach model reacts as expected at the changes concerning the waves' asymmetry and the slumping of the water area through its avalanching mechanism, reflecting a higher computational skill. In contrast, the model is insensitive to the long waves' sediment stirring and to the additional imposed wind setup. Concerning the model performance in the area of Castricum, the model skill is excellent and very good convergence is obtained on estimating the volume change and the estimated dune retreat. In Julianadorp, while the influence of the groins is not accounted, due to the significant scouring that is observed in the backshore zone, the model performance is bad. The geological features and the bed profile of the studied areas present a different pattern of the energy distribution, which may indicate a possible reason for the longshore erosion variability during the 1976 storm surge. The operational model needs to be further verified with the 2DH simulations in order to account for the longshore profile development and the effects of the hard non-erodible structures, while it is also recommended to further investigate the extensive scour development in the backshore zone.","storm surge; 1953; 1976; operational model; XBeach","en","master thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:5ade9ef3-b263-4da9-98a4-3355f8ac34e3","http://resolver.tudelft.nl/uuid:5ade9ef3-b263-4da9-98a4-3355f8ac34e3","Modelling the 1775 storm surge deposits at the Heemskerk dunes","Pool, A.D.","Van Dongeren, A.P. (mentor); Van Heteren, S. (mentor); Van Gelder, P.H.A.J.M. (mentor); Storms, J.E.A. (mentor); Stive, M.J.F. (mentor)","2009","After a storm surge in November 2007, older storm surge deposits were discovered in the eroded dunes near Heemskerk, the Netherlands. These deposits undulate in height with a maximum elevation of over 6 m above mean sea level. Luminescence dating suggests that the layers were deposited by either the 1775 or the 1776 storm surge. The aim of this thesis is to model the 1775 storm surge and its capability to reach the height of NAP + 6.5 m at which the deposits have been discovered. Secondary objectives are (a) to give an estimation of the probability of exceedance of the 1775 storm surge and (b) to compare the effects of this storm surge on a open dune front (historical situation) and a closed dune front (present situation). A modelling framework has been set up to transform the available historical data into boundary conditions for the process-based model XBeach. A 1D probabilistic approach resulted in distributions for the 2% exceedance height for the storm surge level including set-up and wave run-up for six characteristic profiles. The distributions do not give reason to reject the hypothesis that, during the 1775 storm surge, the water level has reached the level of NAP + 6.5 m. Based on the existing exceedance line for IJmuiden, the probability of exceedance of the 1775 storm surge is estimated to be 3/10000. This is close to the existing Dutch design criteria for primary sea defences. A comparison of several 2DH simulations with a historical dune topography containing an open dune front and the same boundary conditions with a present dune topography containing a closed dune front, give the following results. With the open dune front, storm surges can easily enter the dune valleys behind the first dune row, but the water is almost always stopped by the second dune row. Wave energy dissipates quickly in the dune area. The present situation with a closed dune front gives high erosion rates along the entire dune front, while the historical situation with an open dune front gives more variation in erosion and deposition rates. Erosion rates are generally higher in the present situation with the closed dune front.","storm surge; deposits; historical storm; 1775; dunes; dune overwash; dune breaching; dune erosion; process-based modelling; xbeach; statistics; probabilistic modelling","en","master thesis","","","","","","","","","Civil Engineering and Geosciences","Hydraulic Engineering","","","",""
"uuid:416ae56c-8fa6-42b1-b532-acc457b28604","http://resolver.tudelft.nl/uuid:416ae56c-8fa6-42b1-b532-acc457b28604","Non-hydrostatic modelling of large scale tsunamis","Smit, P.B.","Stelling, G.S. (mentor); Labeur, R.J. (mentor); van Thiel de Vries, J. (mentor); Vatvani, D. (mentor)","2008","The Indian Ocean Tsunami has once again revived the discussion in the tsunami modelling community if the non-linear shallow water equations are a valid model for the propagation of tsunamis. It is suggested that the mechanism of frequency dispersion which is absent in these equations might be important in the correct modelling of large scale tsunamis. In this master Thesis a non-hydrostatic numerical model based upon the scheme proposed by Stelling and Zijlema (2003) is constructed and it is investigated if it can be an effective and efficient way to include the effect of frequency dispersion in the modelling of tsunamis in their propagation and run-up. The non-hydrostatic algorithm is incorporated into the existing explicit shallow water solver of XBeach. In this way the model is extended to allow for shorter wave propagation. The main reason for doing this was to show that the employed non-hydrostatic scheme can be easily incorporated as a simple add-on. The depth averaged formulation of the XBeach model prevented an easy extension towards multiple layers but, for a single layer, the addition of the non-hydrostatic pressures was indeed straightforward. No large modifications to the existing code where required. The numerical model is based on the application of mehrstellen verfahren for the pressure gradients in the vertical. This makes it possible to exactly set the surface pressure to zero which is important for the correct modelling of surface waves. The advective terms have been included in a momentum conservative way based on Stelling and Duinmeijer (2002). This allows for the correct modelling of braking waves. The resulting 2DV model is validated with analytical solutions available for: (i) an oscillating basin (ii) the propagation of a solitary waves (iii) the run-up of long waves on a beach and (iv) the dambreak solution. Furthermore the model is verified using experimental data by Synolakis (1987) on the run-up of solitary waves on a plane beach. In all cases it is concluded that the results are satisfactory. The 2DV model is subsequently expanded into a 3D model which is validated with a 3D version of the oscillating basin and verified with the Berkhoff shoal which includes shoaling, refraction and diffraction of waves. A surprising result is that the model using only a single layer is able to satisfactorily reproduce the measurements. The numerical model is applied to two tsunami benchmark tests conducted by Briggs (1995). The first test consists of the run-up of solitary waves on a vertical wall while the second deals with the run-up of solitary waves on a conical island. From the first test it is concluded that the model can correctly model these types of waves using only a single layer. Furthermore, when compared to hydrostatic solutions, the model is a dramatic improvement. The over steepening, typical of the non-linear shallow water equations, does not occur. From the results of the second test it is concluded that the model can accurately predict the inundation heights. However, very fine grids where needed due to the excessive numerical diffusion introduced by the upwind approximations. It can be concluded that the non-hydrostatic model by Stelling and Zijlema can indeed be an attractive way to include frequency dispersion into large scale tsunami propagation models. It is anticipated that the non-hydrostatic terms add about fifty percent to the duration of a simulation.","non-hydrostatic; xbeach; tsunami; large scale","en","master thesis","TU Delft, Civil Engineering and Geosciences, Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","","","","",""
"uuid:5a8919f1-65c6-4b81-8652-dd4f5d7d142b","http://resolver.tudelft.nl/uuid:5a8919f1-65c6-4b81-8652-dd4f5d7d142b","The longshore dimension in dune overwash modelling: Development, verification and validation of XBeach","McCall, R.T.","Stive, M. (mentor); Roelvink, J.A. (mentor); van Thiel de Vries, J.S.M. (mentor); van Dongeren, A.P. (mentor); Walstra, D.J.R. (mentor); Visser, P.J. (mentor); Zijlema, M. (mentor)","2008","The primary objective of this thesis is to generate a 2DH-numerical model to simulate dune overwash. The first stage in this is achieved by modifying the program code of an existing overwash model in development, XBeach, to enable 2DH calculations. In the second stage the hydrodynamics of the model are verified using theoretical and laboratory and field tests. In the third stage the model is validated by simulating overwash and washover on Santa Rosa Island, Florida, during Hurricane Ivan. The secondary objective of this thesis is to evaluate the effect of longshore bathymetry variations on the patterns and amount of overwash using the newly developed XBeach model. A numerical area model of part of Santa Rosa Island, Florida, is developed. The model is forced using Hurricane Ivan wave and surge conditions. The XBeach model shows five phases of morphology on the barrier island, leading from foredune erosion to breaching of the island. The model results are compared to high resolution post-storm altimetry data. It is shown that although the XBeach model produces morphological features common to overwash conditions, the amount of erosion is an order greater than the measured erosion. Sensitivity studies are carried out to determine the influence of the hydraulic boundary conditions on the final erosion profile. It is found that the model is sensitive to total surge levels and surge level gradients across the island, but insensitive to wave heights. It is shown that under inundation overwash conditions the amount of erosion and patterns of deposition are almost entirely determined by longshore bathymetry features with length scales in the order of kilometres. The primary recommendation given in this thesis is to develop and implement better sediment transport relations in XBeach and to account for effects of vegetation on the hydrodynamics and morphodynamics of the subaerial barrier island.","xbeach; overwash; dune; modelling; hurrican","en","master thesis","TU Delft, Civil Engineering and Geosciences, Hydraulic Engineering","","","","","","","","Civil Engineering and Geosciences","","","","",""