Return Period Storm Research Articles

Finite Element Analysis for Structural Performance of Offshore Platforms under Environmental Loads

Offshore structures for oil and gas exploitation are subjected to various ocean environmental phenomena which can cause highly nonlinear action effects. Offshore structures should be designed for severe environmental loads and strict requirements should set for the optimum performance. The structural design requirements of an offshore platform subjected to wave induced forces and moments in the jacket can play a major role in the design of the offshore structures. For an economic and reliable design; good estimation of wave loadings are essential. The structure is discretized using the finite element method, wave force is determined according to linearized Morison equation. Hydrodynamic loading on horizontal and vertical tubular members and the dynamic response of fixed offshore structure together with the distribution of displacement, axial force and bending moment along the leg are investigated for regular and extreme conditions, where the structure should keep production capability in conditions of the one year return period wave and must be able to survive the 100 year return period storm conditions. The results show that the nonlinear response analysis is quite crucial for safe design and operation of offshore platform. Fixed Jacket type offshore platforms under extreme wave loading conditions may exhibit significant nonlinear behavior. The effect of current with different angles when hitting the offshore structure with the wave and wind forces, is very important for calculate the stress, the response displacement and deformation shapes. As the current increase or decrease the effect of wave force according to the hitting angle of current.

Long-term change of daily and multi-daily precipitation in southern Sweden

Daily rain series from southern Sweden with records dating back to the 1870s have been analysed to investigate the trends of daily and multi-day precipitation of different return periods with emphasis on the extremes. Probabilities of extreme storms were determined as continuously changing values based on 25 years of data. An extra set of data was used to investigate changes in Skane, the southernmost peninsula of Sweden. Another 30-year data set of more than 200 stations of a dense gauge network in Skane was used to investigate the relation between very large daily rainfall and annual precipitation. The annual precipitation has increased significantly all over southern Sweden due to increased winter precipitation. There is a trend of increasing maximum annual daily precipitation at only one station, where the annual maximum often occurs in winter. The number of events with a short return period is increasing, but the number of more extreme events has not increased. Daily and multi-daily design storms of long return periods determined from extreme value analysis with updating year by year are not higher today than during the last 100 years. The largest daily storms are not related to stations with annual rainfall but seem to occur randomly. Copyright (c) 2013 John Wiley & Sons, Ltd. (Less)

Nonlinear response of fixed jacket offshore platform under structural and wave loads

The structural design requirements of an offshore platform subjected to wave induced forces and moments in the jacket can play a major role in the design of the offshore structures. For an economic and reliable design; good estimation of wave loadings are essential. A nonlinear response analysis of a fixed offshore platform under structural and wave loading is presented, the structure is discretized using the finite element method, wave plus current kinematics (velocity and acceleration fields) are generated using 5th order Stokes wave theory, the wave force acting on the member is calculated using Morison's equation. Hydrodynamic loading on horizontal and vertical tubular members and the dynamic response of fixed offshore structure together with the distribution of displacement, axial force and bending moment along the leg are investigated for regular and extreme conditions, where the structure should keep production capability in conditions of the 1-yr return period wave and must be able to survive the 100-yr return period storm conditions. The result of the study shows that the nonlinear response investigation is quite crucial for safe design and operation of offshore platform.

Hydrological analysis for water level projections in Taihu Lake, China

AbstractChanges in precipitation can be expected to have a major influence on the frequency and severity of flooding in the Taihu Basin. As part of an integrated assessment of flood risk in the Taihu Basin, this paper addresses the hydrological conditions and the potential impacts of climate change on Taihu Lake water levels. The hydrological system is simulated by using the distributed hydrological model VIC (Variable Infiltration Capacity), with future climate scenarios generated by the regional climate model PRECIS (Providing Regional Climates for Impacts Studies). The results indicate that maximum and mean flood water levels under the A2 and B2 scenarios for 2021–2050 will be higher than the levels during the baseline period (1961–1990), with larger increases under the B2 scenario than for the A2 scenario. The storm return periods leading to extreme water levels in the Taihu Lake in the future is projected to decrease significantly in comparison to the baseline period, implying the potential for the more frequent occurrence of major floods. These results indicate the importance of planning for future flood control in the Taihu Basin.

Impacts of Climate Change on Extreme Precipitation Events Over Flamingo Tropicana Watershed1

Abstract: Climate change, particularly the projected changes to precipitation patterns, is likely to affect runoff both regionally and temporally. Extreme rainfall events are expected to become more intense in the future in arid urban areas and this will likely lead to higher streamflow. Through hydrological modeling, this article simulates an urban basin response to the most intense storm under anthropogenic climate change conditions. This study performs an event‐based simulation for shorter duration storms in the Flamingo Tropicana (FT) watershed in Las Vegas, Nevada. An extreme storm, defined as a 100‐year return period storm, is selected from historical records and perturbed to future climatic conditions with respect to multimodel multiscenario (A1B, A2, B1) bias corrected and spatially disaggregated data from the World Climate Research Programme's (WCRP's) database. The cumulative annual precipitation for each 30‐year period shows a continuous decrease from 2011 to 2099; however, the summer convective storms, which are considered as extreme storms for the study area, are expected to be more intense in future. Extreme storm events show larger changes in streamflow under different climate scenarios and time periods. The simulated peak streamflow and total runoff volume shows an increase from 40% to more than 150% (during 2041‐2099) for different climate scenarios. This type of analysis can help evaluate the vulnerability of existing flood control system and flood control policies.

Storm surge impact in the hydrodynamics of a tidal lagoon: the case of Ria de Aveiro

Picado, A., Lopes, C.L., Mendes, R., Vaz, N. and Dias, J.M., 2013. Storm surge impact in the hydrodynamics of a tidal lagoon: the case of Ria de Aveiro.Storm surges are a hazardous phenomenon, since they may flood large coastal areas, causing socio-economical and habitation losses. Thus, the study of their characteristics and effects in coastal regions is crucial to prevent their negative consequences. This work aims at assessing the storm surges impact in the hydrodynamics of a tidal lagoon located in the north-western Portuguese coast (Ria de Aveiro). Storm surge amplitudes of 0.58 m, 0.84 m and 1.17 m for 2, 10 and 100 return periods, respectively, were determined adjusting the annual maximum amplitudes to a Generalized Extreme Value (GEV) distribution. To assess the hydrodynamic changes in the Ria de Aveiro under storm surge conditions, numerical modeling simulations were carried out, considering four scenarios: a single astronomical tidal forcing (reference) and astronomical tide plus 2, 10 and 100 years return period surges. Maximum levels and velocities for the entire lagoon and the tidal prism for the main cross-sections were determined and compared with the reference scenario. Generally, the model results suggest that during storm surge events the maximum levels increase in whole domain, with the largest increase found for the 100 return period storm surge scenario (1.17 m). The most significant changes occur at the main channels head for all scenarios, revealing that these regions are the most vulnerable to marginal flooding. Also, storm surges induce higher velocities and tidal prisms in the lagoon, increasing the marginal risk of erosion, as well as the salinization of the lagoon marginal lands.

Return Period of a Sea Storm with at Least Two Waves Higher than a Fixed Threshold

Practical applications in ocean engineering require the long-term analysis for prediction of extreme waves, that identify design conditions. If extreme individual waves are investigated, we need to combine long-term statistical analysis of ocean waves with short-term statistics. The former considers the distribution of standard deviation of free surface displacement in the considered location in a long-time span, of order of 10 years or more. The latter analyzes the distribution of individual wave heights in a sea state, which is a Gaussian process in time domain. Recent advanced approaches enable the combination of the two analyses. In the paper the analytical solution is obtained for the return period of a sea storm with at least two individual waves higher than a fixed level. This solution is based on the application of the Equivalent Triangular Storm model for the representation of actual storms. One of the corollaries of the solution gives the exact expression for the probability that at least two waves higher than fixed level are produced during the lifetime of a structure. The previous solution of return period and the relative probability of exceedance may be effectively applied for the risk analysis of ocean structures.

Long-Term Modelling of Wave Run-Up and Overtopping during Sea Storms

ABSTRACT Arena, F.; Malara, G.; Barbaro, G.; Romolo, A., and Ghiretti, S., 2013. Long-term modelling of wave run-up and overtopping during sea storms. This paper is concerned with the determination of the return period of a sea storm in which response (run-up or overtopping) of a coastal structure exceeds a fixed threshold. The method is based on long-term statistical analysis of the sea states interacting with the structure and on practical formulas proposed in the past decades for determining response of coastal structures. The proposed methodology accounts for nonstationarity of sea states in time domain by the equivalent triangular storm model, which allows us to determine closed-form solutions of the return period of a storm where maximum significant wave height is larger than a fixed threshold. In this paper the analytical solution is supplemented by a practical application pertaining calculation of the response of a coastal structure. A rubble-mound breakwater is supposed to be placed at a known wa...

Effects of Land Use Change on Hydrologic Response at a Watershed Scale, Arkansas

The population has increased at a fast pace in the last two decades in Arkansas. One direct result from this process is urbanization, especially in northwest Arkansas. Land use and land cover (LULC) had been changed from forests, pastures, and farmland to urbanized areas over a short time span. Increased flood discharge and decreased water quality have been negative side effects of urbanization on the environment. In this study, five watersheds were selected as representative of the entire state. These watersheds were selected to examine the hydrological response to land use change over the last two decades in Arkansas. The curve number, a term used to describe the runoff depth, was calculated for the five watersheds. Out of the five watersheds, the Illinois River watershed experienced the greatest change in land use from 1993–2006, whereas the other four watersheds experienced slight land use change. The land transformation model (LTM) was used to forecast the land use of the Illinois River watershed in the year 2019. Simulated urbanized area in the Illinois River watershed from 1993–2019 increased from 3.7 to 23.6%. The curve number of the Illinois River watershed would increase from 68.9 in 1993 to 72.1 in 2019. The Spring Creek-Osage Creek watershed, a subwatershed in the Illinois River watershed, was selected for detailed evaluation of the hydrologic response attributable to land use change. The storm water management model (SWMM) was used to calculate the flood discharges resulting from different storm events. The simulation results showed that during 1993–2019, the peak flood discharge of the Spring Creek-Osage Creek watershed for the storm return periods 2, 5, 10, 25, 50, and 100 years increased 230, 180, 160, 140, 130, and 110%, respectively, whereas the total flood volume increased 270, 220, 200, 170, 160, and 140%. The results in this study might suggest to small cities like Bethel Heights that it is better to plan the drainage systems for the coming decades before such cities have already sprawled. This study also applies a combined methodology of a regional scale urbanization-flood model, which could be used to improve flood frequency analysis.

The Calibration on Nonlinear Single Return Period Storm Intensity Model Parameters by the New Blending Accelerating Genetic Algorithm

Through improving the real-coded genetic algorithms and embedding the algorithm of accelerating the local search in the Powell direction and the operation of accelerating cycle, thereby it is constructed the New blend Accelerating Genetic Algorithm (hereinafter referred to as NBAGA). The applied examples of the calibration on nonlinear single return period storm intensity model parameters show that this method takes into account of the advantages of both the advantages of the improved real-coded genetic algorithms and the Powell Algorithm, therefore this method is an excellent nonlinear optimization method which can search the global optimal exact solutions quickly and in greater probability, as well as doing subtle search locally.

Multivariate return periods of sea storms for coastal erosion risk assessment

Abstract. The erosion of a beach depends on various storm characteristics. Ideally, the risk associated with a storm would be described by a single multivariate return period that is also representative of the erosion risk, i.e. a 100 yr multivariate storm return period would cause a 100 yr erosion return period. Unfortunately, a specific probability level may be associated with numerous combinations of storm characteristics. These combinations, despite having the same multivariate probability, may cause very different erosion outcomes. This paper explores this ambiguity problem in the context of copula based multivariate return periods and using a case study at Durban on the east coast of South Africa. Simulations were used to correlate multivariate return periods of historical events to return periods of estimated storm induced erosion volumes. In addition, the relationship of the most-likely design event (Salvadori et al., 2011) to coastal erosion was investigated. It was found that the multivariate return periods for wave height and duration had the highest correlation to erosion return periods. The most-likely design event was found to be an inadequate design method in its current form. We explore the inclusion of conditions based on the physical realizability of wave events and the use of multivariate linear regression to relate storm parameters to erosion computed from a process based model. Establishing a link between storm statistics and erosion consequences can resolve the ambiguity between multivariate storm return periods and associated erosion return periods.

The Return Period Analysis of Natural Disasters with Statistical Modeling of Bivariate Joint Probability Distribution

New features of natural disasters have been observed over the last several years. The factors that influence the disasters' formation mechanisms, regularity of occurrence and main characteristics have been revealed to be more complicated and diverse in nature than previously thought. As the uncertainty involved increases, the variables need to be examined further. This article discusses the importance and the shortage of multivariate analysis of natural disasters and presents a method to estimate the joint probability of the return periods and perform a risk analysis. Severe dust storms from 1990 to 2008 in Inner Mongolia were used as a case study to test this new methodology, as they are normal and recurring climatic phenomena on Earth. Based on the 79 investigated events and according to the dust storm definition with bivariate, the joint probability distribution of severe dust storms was established using the observed data of maximum wind speed and duration. The joint return periods of severe dust storms were calculated, and the relevant risk was analyzed according to the joint probability. The copula function is able to simulate severe dust storm disasters accurately. The joint return periods generated are closer to those observed in reality than the univariate return periods and thus have more value in severe dust storm disaster mitigation, strategy making, program design, and improvement of risk management. This research may prove useful in risk-based decision making. The exploration of multivariate analysis methods can also lay the foundation for further applications in natural disaster risk analysis.

NONLINEAR ANALYSIS OF OFFSHORE STRUCTURES UNDER WAVE LOADINGS

The structural design requirements of an offshore platform subjected wave induced forces and moments in the jacket can play a major role in the design of the offshore structures. For an economic and reliable design; good estimation of wave loadings are essential. A nonlinear response analysis of a fixed offshore platform under wave loading is presented, the structure is discretized using the finite element method, wave force is determined according to linearized Morison equation. Hydrodynamic loading on horizontal and vertical tubular members and the dynamic response of fixed offshore structure together with the distribution of displacement, axial force and bending moment along the leg are investigated for regular and extreme conditions, where the structure should keep production capability in conditions of the one year return period wave and must be able to survive the 100 year return period storm conditions. The result of the study shows that the nonlinear response investigation is quite crucial for safe design and operation of offshore platform.

Is Xynthia an exceptional event? Reflections on evolution and return period of storms, storm tides and high surges associated on the French Atlantic coast

À partir de données constituées des vents maximum instantanés quotidiens sur neuf stations de la façade atlantique française, des observations et des traitements statistiques sont menés afin de tenter d’apprécier la fréquence des niveaux d’eau atteints lors de l’épisode Xynthia qui a sinistré les littoraux de Vendée et de Charente-Maritime en février 2010. À cet effet, les vents instantanés de plus de 89 km/h, 102 km/h, et 118 km/h sont analysés. Dans un premier temps, il a été possible d’évaluer l’évolution des tempêtes depuis un demi-siècle sur les neuf stations, dont se dégagent très peu de tendances significatives. Nous avons ensuite croisé ces séries avec les coefficients de marée supérieurs à deux valeurs seuil. Dans un second temps, sur des séries plus restreintes (1984-2010), nous avons procédé à un croisement supplémentaire consistant à ne retenir que les situations où les vitesses de vent maximum sont survenues à moins de trois heures de la pleine mer, afin d’estimer notamment la fréquence et le temps de retour des marées de tempête et des situations présentant un risque de submersion élevé. Il est ainsi montré que des événements comparables à Xynthia sont rares (seulement dix cas observés en 27 ans), voire exceptionnels dans certaines stations, dont La Rochelle fait partie. Les temps de retour des marées de tempête varient fortement selon les sites, de 177 ans à La Rochelle pour une tempête de 140 km/h sachant que la mer est haute, à seulement 5 ans à Belle-Île. Enfin, quelques données factuelles sont présentées afin de rappeler la difficulté de la prévision des valeurs de surcotes, ainsi que l’existence de moyens désormais à disposition pour progresser dans ce domaine.

Regional Intensity-Duration-Frequency Curves Derived from Ensemble Empirical Mode Decomposition and Scaling Property

This paper proposes deriving regional intensity-duration-frequency (IDF) curves for Edmonton, Canada, based on the scaling property of precipitation data using ensemble empirical mode decomposition (EEMD). Selected sets of annual maximum precipitation data were decomposed by the EEMD to intrinsic mode functions (IMFs), and the scaling property was investigated. Next, representative scale exponents were extracted. The results show that quantiles derived from general extreme value (GEV) probability distribution (PD) with parameters derived by the probability-weighted moment (PWM) are more accurate than those derived from the extreme value type I (EVI) PD with parameters derived by the method of moment (MOM), whose underestimation of rainfall intensity becomes obvious for high return period (greater than 25 years) and short duration (less than 1 h). The results also show that for Edmonton, generally three of four IMFs of the precipitation data showed a simple scaling property, and regional IDF curves derived from the scaling IDF and EEMD approach predict accurate storm intensities for rain-gauging sites at both the calibration and validation stages, but there could be errors associated with predicted storms of high return periods (100 year). DOI: 10.1061/(ASCE)HE.1943-5584.0000612. © 2013 American Society of Civil Engineers.

Overwash hazard assessment

Overwash is a natural storm-related process that occurs when wave runup overcomes the dune crest. Because coastlines are globally occupied, overwash is a hazardous process and there is a need to identify vulnerable areas. This study proposes a method to detect overwash-prone areas in the Ancao peninsula, Portugal, and eventually outlines a vulnerability map. Dune base (DLOW) and crest (DHIGH) topography were surveyed. Three different storm scenarios (5-, 10- and 25- year return period storms) and associated waves and sea level were determined. According to these data, extreme wave runup (RHIGH) was calculated by a parameterisation set for intermediate-reflective beaches. The maps for collision and overwash regimes were designed by comparing RHIGH values with DLOW and DHIGH. Almost the entire dune base along the peninsula is vulnerable to collision regime. The overwash process was identified mainly along the tidal inlet hazard area and within the human-occupied portion of the beach, where overwash is expected to occur within 5 years of return period. The developed method considers the main overwash driving forces and proved to identify hazardous areas previously observed in the area. Occasional differences between modelled and observed overwash areas can be attributable to equipment errors, morphology interpretation subjectivity and maladjustments in runup parameterisation. Nevertheless, the method proved to be effective in reproducing the overall Ancao peninsula vulnerability and can be widely applied. Therefore, it is a simple and potentially important tool for coastal management that enables mitigation strategies for occupied coasts and assessment of geological and ecological consequences in natural areas.

High-resolution refinement of a storm loss model and estimation of return periods of loss-intensive storms over Germany

Abstract. A refined model for the calculation of storm losses is presented, making use of high-resolution insurance loss records for Germany and allowing loss estimates on a spatial level of administrative districts and for single storm events. Storm losses are calculated on the basis of wind speeds from both ERA-Interim and NCEP reanalyses. The loss model reproduces the spatial distribution of observed losses well by taking specific regional loss characteristics into account. This also permits high-accuracy estimates of total cumulated losses, though slightly underestimating the country-wide loss sums for storm "Kyrill", the most severe event in the insurance loss records from 1997 to 2007. A larger deviation, which is assigned to the relatively coarse resolution of the NCEP reanalysis, is only found for one specific rather small-scale event, not adequately captured by this dataset. The loss model is subsequently applied to the complete reanalysis period to extend the storm event catalogue to cover years when no systematic insurance records are available. This allows the consideration of loss-intensive storm events back to 1948, enlarging the event catalogue to cover the recent 60+ years, and to investigate the statistical characteristics of severe storm loss events in Germany based on a larger sample than provided by the insurance records only. Extreme value analysis is applied to the loss data to estimate the return periods of loss-intensive storms, yielding a return period for storm "Kyrill", for example, of approximately 15 to 21 years.

Quantifying extreme behavior in geomagnetic activity

Understanding the extremes in geomagnetic activity is an important component in understanding just how severe conditions can become in the terrestrial space environment. Extreme activity also has consequences for technological systems. On the ground, extreme geomagnetic behavior has an impact on navigation and position accuracy and the operation of power grids and pipeline networks. We therefore use a number of decades of one‐minute mean magnetic data from magnetic observatories in Europe, together with the technique of extreme value statistics, to provide a preliminary exploration of the extremes in magnetic field variations and their one‐minute rates of change. These extremes are expressed in terms of the variations that might be observed every 100 and 200 years in the horizontal strength and in the declination of the field. We find that both measured and extrapolated extreme values generally increase with geomagnetic latitude (as might be expected), though there is a marked maximum in estimated extreme levels between about 53 and 62 degrees north. At typical midlatitude European observatories (55–60 degrees geomagnetic latitude), compass variations may reach approximately 3–8 degrees/minute, and horizontal field changes may reach 1000–4000 nT/minute, in one magnetic storm once every 100 years. For storm return periods of 200 years the equivalent figures are 4–11 degrees/minute and 1000–6000 nT/minute.

Critical storm thresholds for significant morphological changes and damage along the Emilia-Romagna coastline, Italy

The definition of storm morphological thresholds along the coast of the Emilia-Romagna Region strictly depends on its configuration and variability. The region is located in northern Italy, facing the Adriatic Sea. The coastline is characterised by very different levels of economic development, ranging from natural zones with dunes to highly developed stretches protected by breakwaters and groynes. The Integrated Coastal Zone Management effort is mainly concentrated on preserving urban areas that generate significant income for the regional economy. Natural areas, while small in comparison to the urbanised zone, are important for environment preservation. Because of such a multiplicity of issues at stake, it was decided to produce two different thresholds: one for the morphological impact on natural sectors and another for inundation and damage to structures along urbanised zones.The “forcing” component of the threshold definition for natural areas was calculated by summing the effects of surge+tide+waves (run-up elevation) to find the Maximum Water Level (MWL) reached by the sea during one, ten and one-hundred year storm return periods. For urbanised zones, historical storm information was collected starting from the 1960s in order to identify the forcing conditions causing real damages. Each storm was classified in terms of wave height, period, direction and surge level. Morphological information were obtained from Lidar flights performed in 2003 and 2004 and from direct surveys undertaken in September 2008 and February 2009 as part of the monitoring programme for the MICORE Project.The computed MWL for each return period was then compared to beach elevations along natural areas in order to calculate the Dune Stability Factor (DSF), an index that accounts for the eroded sediment volume above the MWL during a storm. Based on analysis along 41 profile lines at a 500m spacing, it was found that the 1-in-1year return period wave height+1-in-1year return period surge are able to erode and/or overwash 2/3 of the dunes.The historical storm hydrodynamic information was used to estimate which wave and surge conditions are able to inundate at least 2/3 of the beach profiles. The MWL was again compared to beach elevations, this time along 63 anthropogenic profiles spaced 500m apart (or 1/3 of the urbanised coastline). It was found that a wave heights >=2m and surge+tide levels >=0.7m are able to flood between 18% and 36% of the built-up coast.The defined thresholds are related to the present coastal characteristics and are not “static”, meaning that they are likely to change according to future evolution of the coastline. They are very important because they can be used as thresholds to issue warnings and alert the Civil Protection. Moreover they are the first thresholds defined for the Emilia-Romagna coastline and will be used as starting values to generate “dynamic” thresholds based on numerical model predictions of morphological change for a given wave and surge level.

Sediment yield response to large storm events and forest harvesting, Motueka River, New Zealand

Suspended sediment yields (SY) were measured at seven sites in the Motueka catchment to characterise major influences on sediment yield variation and estimate sediment delivery to Tasman Bay. Relationships between storm event SY and peak flow were used with long-term flow records to calculate SY. At large catchment scale, there were clear differences in SY related to geology and rainfall, but the major influence on SY was the impact of a >50-year return period storm that caused an increase in event SY ranging from 10–20 times in the headwaters to 2–3 times at the coast. Forest harvesting was an important influence at small catchment scale, producing a five-fold increase in event SY. The magnitude of the SY increase, the time to recover and the total area affected were all smaller than the impact of large storms. Annual SY at the coast was extremely variable, ranging from 49,000 t to 1.7 Mt.