Verkenning effect van golfperiode op veiligheid duinenkust: Een probabilisitische beschouwing

Bij de beoordeling van de veiligheid van de Nederlandse duinenkust wordt gebruik gemaakt van een rekenmodel dat, voor vastgestelde ontwerpomstandigheden, de mate van duinafslag bepaalt. De kern van dit rekenmodel wordt gevormd door het zogenaamde DUROS-rekenmodel (TAW, 1984). Op basis van beschikbare informatie en het verwachte bereik van de golfperioden {Tp< 12 s) werd, tijdens de ontwikkeling van het DUROS-rekenmodel, de vorm van het afslagprofiel verondersteld onafhankelijk te zijn van de golfperiode. Echter, volgens de huidige inzichten moet er rekening mee gehouden worden dat tijdens extreme omstandigheden veel langere golfperioden voorkomen dan eerder verondersteld werd. Om een schatting te kunnen maken van de invloed van deze langere golfperioden op de mate van duinafslag zijn kleinschalige proeven uitgevoerd door WL | Delft Hydraulics in de Scheldegoot. Op basis van de analyse van die proeven is enig inzicht verkregen in de gevoeligheid van de mate van duinafslag voor de golfperiode. Het doel van dit onderzoek is een indicatie te geven van de toename in ontwerpafslag bij de recente inzichten, betreffende de golfperiode, t.o.v. de huidige beoordelingsmethode voor de veiligheid van de duinenkust. Om een uitspraak te kunnen doen over deze invloed is de probabilistische simulatie (numerieke integratie), die mede te grondslag ligt aan de huidige toetsingsmethode, uitgebreid met het effect van de golfperiode. Er is een fictieve situatie gecreëerd waarbij voor de golfperiode met een overschrijdingskans van 10-4 per jaar een piek-golfperiode van Tp = 18.5 s is gekozen. Voor deze situatie zijn een aantal simulaties uitgevoerd waarbij de relatieve toename van het afslagvolume bij Tp = 18.5 s t.o.v. Tp= 12 s (als de overige factoren gelijk blijven) is gevarieerd van 0 tot 50 %. Deze relatieve toename, welke de gevoeligheid van het afslagvolume voor de golfperiode weergeeft, wordt hier Scenario genoemd. Uit de uitgevoerde numerieke integraties, onder genoemde omstandigheden, kan geconcludeerd worden dat afslagvolume met een overschrijdingskans van 10-5 per jaar ongeveer met 0.8* Scenario zal toenemen. Dit betekent dat er voor een Scenario van 25 % toename bij Tp = 18.5 s t.o.v. = 12 s rekening gehouden moet worden met 0.8 * 25% = 20 % extra afslagvolume bij een overschrijdingskans van 10"^ per jaar.Hydraulic EngineeringCivil Engineering and Geoscience

The effect of wave period on dune erosion

Dune erosion is a relatively fast developing cross-shore transport process which occurs during severe wave attack combined with high water levels. Large quantities of sand coming from the dunes are transported seaward. If no seaward or lateral losses occur, the sand balance will be closed in cross-shore direction. The eroded sand will deposit on the beach and foreshore in such a way that a so-called erosion profile appears. For the judgement of the safety of the Dutch dune coast a method is used to compute, for given design conditions, the amount of dune erosion. The core of the method is the so-called DUROS-model. This model does not take into account the effect of the wave period on the amount of dune erosion and was developed for the at that time supposed reach of peak wave periods for the Dutch coast (Tp <= 12 s). According to present understanding one has to take longer wave periods into account (Tp up to 16 or 18 s). In addition, preliminary explorations supposed the dune erosion process to be dependent on the wave period. However, the used model was not validated for these high wave periods. Recently, a series of small scale physical model tests, with peak wave periods varying from about 10 to 19 s (prototype), have been carried out by WL | Delft Hydraulics in the Schelde flume. According to that research dune erosion is clearly dependent on the wave period. The aim of this study was to analyse the above mentioned test series and to verify the model DUROSTA (and briefly UNIBEST-TC) concerning the wave period influence on dune erosion (based on recent small scale tests). (These time-dependent cross-shore transport models take the wave period into account.) Most important conclusion about the physical model research was that assuming the applied scale relations and a relatively long storm duration an increase of dune erosion volume above storm surge level of 2535 % is found for a prototype peak wave period of Tp = 18.4 s with respect to Tp = 12 s. The conclusions about the model DUROSTA are more complicated. Since the wave heights were measured at several locations along the cross-shore profile, it became clear that the model (with default settings) did not completely reproduce the measured cross-shore wave height development. The wave height has been tuned for each test to the measured (incident) wave height near the wave board. This resulted in a good reproduction of the measured relative increase of dune erosion with increasing wave period. From several ways of tuning more near the dune appeared that DUROSTA only computes consistently more dune erosion for a longer wave period if the wave height near the dune is larger. This in contrast to a number of the tests where, although approximately equal measured wave heights near the dune (including reflection and long waves), the larger the wave period clearly results in more dune erosion.Civil Engineering and Geoscience

Reliability methods in OpenEarthTools

OpenEarthTools contains, apart from a lot of other tools in various programming languages, the probabilistic reliability methods FORM and Monte Carlo. This document aims at describing and providing background information and examples on the FORM and Monte Carlo implementation available in OpenEarthTools.Hydraulic EngineeringCivil Engineering and Geoscience

The role of bathymetry, wave obliquity and coastal curvature in dune erosion prediction

This study aims at reducing uncertainty in dune erosion predictions, in particular at complex dune coasts, in order to improve the assessment method for dune safety against flooding. To that end, state-of-the-art process-based dune erosion models are employed to further investigate issues insufficiently covered by the current Dutch safety assessment method. The influence of cross-shore bathymetry, coastal curvature and incident wave angle are the main focus. With a 2DH XBeach model, it is shown that the interaction between cross-shore and alongshore processes is of major importance for wave obliquity and coastline curvature leading to dune erosion increased rates up to 30 to 50%. In addition, a Bayesian Network approach is introduced to create a practical framework for the inclusion of additional aspects and uncertainties to the safety assessment method for complex coasts.Hydraulic EngineeringCivil Engineering and Geoscience

Sensitivity of dune erosion prediction during extreme conditions

Coastal dunes play an important role in protecting low-lying hinterland from flooding. In The Netherlands, dunes form the major part of the primary sea defence, that protects half of the population and over two-third of the economy from the sea. To design and maintain coastal dunes that properly protect the hinterland, a regular safety evaluation is performed every six years and measures are taken if the prescribed safety standard is not met. The empirical dune erosion model DUROS+ (Vellinga, 1986; van Gent et al., 2008) is the core of the currently used safety evaluation method for dunes. This model does only take the nearshore bathymetry explicitly into account. It is not clear beforehand whether the offshore bathymetry has a significant influence on the dune erosion rate under extreme storm conditions. This paper aims at investigating the sensitivity of the dune erosion rate for different parts of the cross-shore profile. Three clearly distinct cross-shore profiles along the Dutch coast are used to estimate the erosion under extreme storm conditions, using XBEACH (Roelvink et al., 2009), and distinguish between the sensitivity to the lower (seaward) part and the upper (landward) part of the profile. The division between the two profile parts has been made at the MSL-4 m contour. The profiles used for this investigation are considered to cover the range of different profiles as present along the Dutch coast. It can be concluded that the landward profile part is of main importance for an accurate dune erosion estimation. A different seaward profile part can influence the erosion, under extreme storm conditions, by about 10 %. On the other hand, a different landward profile part can lead to a few hundred percent difference in erosion.Hydraulic EngineeringCivil Engineering and Geoscience

Reliability of dune erosion assessment along curved coastlines

The dune assessment methods used to ensure the safety of the lower areas in The Netherlands are based on simple empirical relations that are, strictly speaking, only valid for infinitely long, uniform and straight coasts. The wide application of these relations is mainly justified due to intentional overestimation of the expected dune erosion. In context of climate change and expected sea level rise, it is worthwhile to investigate the needs for this overestimation before any physical measures are taken. This paper describes a research to the influence on the dune erosion process of two longshore phenomena that are neglected in these relations so far: coastal curvatures and oblique wave attack. It is shown that the normative retreat distance can increase over 100% for curvatures relevant for The Netherlands. Furthermore, it is shown that the sensitivity of dune erosion models changes when working with curved coastlines. It is also shown that the importance of several model parameters is influenced by coastal curvatures and special attention should be paid to the correlations between storm surge levels and wave parameters.Hydraulic EngineeringCivil Engineering and Geoscience

A Bayesian network approach to coastal storm impact modeling

In this paper we develop a Bayesian network (BN) that relates offshore storm conditions to their accompagnying flood characteristics and damages to residential buildings, following on the trend of integrated flood impact modeling. It is based on data from hydrodynamic storm simulations, information on land use and a depth-damage curve. The approach can easily be applied to any site. We have chosen the Belgian village Zeebrugge as a case study, although we use a simplified storm climate. The BN can predict spatially varying inundation depths and building damages for specific storm scenarios and diagnose under which storm conditions and where on the site the highest impacts occur.Hydraulic EngineeringCivil Engineering and Geoscience

Morphological modelling of strongly curved islands

Land reclamations and island coasts often involve strongly curved shorelines, which are challenging to be properly modeled by numerical morphological models. Evaluation of the long term development of these types of coasts as well as their response to storm conditions requires proper representation of the governing physical processes. Not all types of numerical models are equipped to represent an entire island and allow waves from any direction. In this paper we demonstrate XBeach’s capabilities of plying a curvilinear grid around a small-scale circular island and exchanging model variables between the lateral boundaries by the recently implemented cyclic boundaries. The small-scale physical model tests by Kamphuis and Nairn (1984) were modeled with XBeach using both the stationary and the nonhydrostatic wave model on a rectangular as well as a curvilinear grid. The wing-bars that typically develop in Kamphuis’ tests are represented in the model, albeit that the angle of the bars is different. In a different XBeach model series, we investigate the behaviour of a larger scale curved coastline model under extreme storm conditions, for varying coastal radius. The results show that maximum erosion occurs at the locations where the incident wave direction is under an angle of 45 degrees with the coastline, and the coastal radius is of secondary importance.Hydraulic EngineeringCivil Engineering and Geoscience

Probabilistic sensitivity analysis of dune erosion calculations

Coastal dunes protect low lying coastal areas against the sea. Extreme waves and water levels during severe storms may cause breaching of the dunes. Consequently, serious damage due to flooding and direct wave attack could occur, resulting in loss of life and property. Proper coastal management implies that reinforcement measures will be taken if the actual safety level does not meet the agreed standard. It is therefore essential to be able to assess the safety of a dune coast against breaching. This study concerns a probabilistic sensitivity analysis of various variables that are included in the current Dutch safety assessment method. The aim is to get more insight in the influence of the stochastic characteristics of the various variables which are taken into account in the current method. Although for the actual assessment a semi-deterministic method is used, the design values of the variables are based on a probabilistic investigation. Using the underlying probabilistic investigation as a reference, the various distribution functions have been varied in order to get more insight in the influence of each of these stochastic characteristics on the rate of dune erosion.Hydraulic EngineeringCivil Engineering and Geoscience

A regional application of bayesian modeling for coastal erosion and sand nourishment management

This paper presents an application of the Bayesian belief network for coastal erosion management at the regional scale. A "Bayesian erosion management network" (BERM-N) is developed and trained based on yearly cross-shore profile data available along the Holland coast. Profiles collected for over 50 years and at 604 locations were combined with information on different sand nourishment types (i.e., beach, dune, and shoreface) and volumes implemented during the analyzed time period. The network was used to assess the effectiveness of nourishments in mitigating coastal erosion. The effectiveness of nourishments was verified using two coastal state indicators, namely the momentary coastline position and the dune foot position. The network shows how the current nourishment policy is effective in mitigating the past erosive trends. While the effect of beach nourishment was immediately visible after implementation, the effect of shoreface nourishment reached its maximum only 5-10 years after implementation of the nourishments. The network can also be used as a predictive tool to estimate the required nourishment volume in order to achieve a predefined coastal erosion management objective. The network is interactive and flexible and can be trained with any data type derived from measurements as well as numerical models.Coastal Engineerin