Quantifying uncertainty in extreme values of design parameters with resampling techniques

In this paper, a methodology for the selection of statistical models for describing the extreme wave heights on the basis of resampling techniques is presented. Two such techniques are evaluated: the jackknife and the bootstrap. The methods are applied to two high-quality datasets of wave measurements in the Mediterranean and one from the East Coast of the USA. The robustness of the estimates of the extreme values of wave heights at return periods important for coastal engineering design is explored further. In particular, we demonstrate how an ensemble error norm can be used to select the most appropriate extreme probability model from a choice of cumulative distribution functions (CDFs). This error norm is based on the mean error norm of the optimised CDF for each resampled (replicate) data series. The resampling approach is also used to present confidence intervals of the CDF parameters. We provide a brief discussion of the sensitivity of these parameters and the suitability of each model in terms of uncertainty with resampling techniques. The advantages of resampling are outlined, and the superiority of the bootstrap over the jackknife in quantifying the uncertainty of extreme quantiles is demonstrated for these records.     

Parameterisation and transformation of wave asymmetries over a low-crested breakwater

This paper presents the results of an investigation of the transformation of wave skewness and asymmetry as waves propagate obliquely over low-crested breakwaters, (LCBs), based on an analysis of measurements collected in the DELOS project. Considering the effect of the local Ursell number on wave asymmetries, a set of practical empirical formulae were established using least squares regression for both smooth and rubble mound LCBs. Predictions are in good agreement with measurements. Wave skewness on both sides of LCBs is linearly correlated for rubble mound LCBs but weakly correlated for smooth LCBs. While wave asymmetry on both sides of LCBs has a weakly quadratic correlation. The effect of the relative freeboard on the relationships of wave asymmetries between both sides is significant for rubble mound LCBs, but the same does not hold for smooth LCBs. With the presence of LCBs, wave skewness retains a positive sign on both sides but asymmetry changes from negative on the incident side to positive on the transmission side. Bispectral analysis shows that positive skewness and negative asymmetry arises from self–self and sum interactions but positive asymmetry is due to difference interactions between frequencies. The findings provide improved understanding of changes in wave skewness and asymmetry in the vicinity of structures, which may help mitigate scour and improve the stability of breakwaters.     

Cation substitution in synthetic meridianiite (MgSO4·11H2O) II: variation in unit-cell parameters determined from X-ray powder diffraction data

We have prepared aqueous MgSO₄ solutions doped with various divalent metal cations (Ni²⁺, Zn²⁺, Mn²⁺, Cu²⁺, Fe²⁺, and Co²⁺) in proportions up to and including the pure end-members. These liquids have been solidified into fine-grained polycrystalline blocks of metal sulfate hydrate + ice by rapid quenching in liquid nitrogen. In a companion paper (Fortes et al., in Phys Chem Min 39) we reported the identification of various phases using X-ray powder diffraction, including meridianiite-structured undecahydrates, melanterite- and epsomite-structured heptahydrates, novel enneahydrates and a new octahydrate. In this work we report the changes in unit-cell parameters of these crystalline products where they exist over sufficient dopant concentrations. We find that there is a linear relationship between the rate of change in unit-cell volume as a function of dopant concentration and the ionic radius of the dopant cation; large ions such as Mn²⁺ produce a substantial inflation of the hydrates’ unit-cell volume, whereas smaller ions such as Ni²⁺ produce a modest reduction in unit-cell volume. Indeed, when the data for all hydrates are normalised (i.e., divided by the number of formula units per unit-cell, Z, and the hydration number, n), we find a quantitatively similar relationship for different values of n. Conversely, there is no relationship between the degree of unit-cell inflation or deflation and the limit to which a given cation will substitute into a certain hydrate structure; for example, Co²⁺ and Zn²⁺ affect the unit-cell volume of MgSO₄·11H₂O to a very similar degree, yet the solubility limits inferred in our companion paper are >60 mol. % Co²⁺ and <30 mol. % Zn²⁺.     

Modelling of possible mud volcanism on Titan

Possible sedimentary basins on Titan are potential sites for the formation of mud volcanoes. In order to constrain the appearance of such features in remotely sensed imagery being acquired by the Cassini spacecraft, we have modelled the formation of mud volcanoes on Titan for a series of plausible mud compositions, climatic conditions and geological settings, as well as addressing the full range of eruption variables; mud viscosity, conduit diameter and eruption duration. We find that for an acetylene mud source containing 20 wt% liquid methane in pore spaces, overlain by a sheet of water ice 500-m thick, a mud volcano can grow as high as 140 m. Assuming reasonable eruption parameters, such an edifice may develop into a pancake-like dome several kilometres in diameter. If observed and properly characterised, mud volcanoes would provide an important window on the subsurface distribution and dynamics of solids and liquids in sedimentary basins on Titan.     

Spatial–temporal patterns of cloud-to-ground lightning over the northwest Iberian Peninsula during the period 2010–2015

The spatial–temporal patterns of cloud-to-ground (CG) lightning covering the period 2010–2015 over the northwest Iberian Peninsula were investigated. The analysis conducted employed three main methods: the circulation weather types developed by Jenkinson and Collison, the fit of a generalized additive model (GAM) for geographic variables, and the use of a concentration index for the ratio of lightning strikes and thunderstorm days. The main activity in the summer months can be attributed to situations with eastern or anticyclonic flow due to convection by insolation. In winter, lightning proves to have a frontal origin and is mainly associated with western or cyclonic flow situations which occur with advections of air masses of maritime origin. The largest number of CG discharges occurs under eastern flow and their hybrids with anticyclonic situations. Thunderstorms with greater CG lightning activity, highlighted by a higher concentration index, are located in areas with a higher density of lightning strikes, above all in mountainous areas away from the sea. The modeling of lightning density with geographic variables shows the positive influence of altitude and, particularly, distance to the sea, with nonlinear relationships due to the complex orography of the region. Likewise, areas with convex topography receive more lightning strikes than concave ones, a relation which has been demonstrated for the first time from a GAM.     

Limitations of ROI Testing for Venting Design: Description of an Alternative Approach Based on Attainment of a Critical Pore-Gas Velocity in Contaminated Media

We utilize data from a Superfund site where radius of influence (ROI) testing was conducted in support of a venting design to describe limitations of ROI evaluation in more detail than has been done previously, and to propose an alternative method of design based on specification and attainment of a critical pore-gas velocity in contaminated subsurface media. Since accurate gas permeability estimation is critical to pore-gas velocity computation, we assess the usefulness of ROI testing data on estimation of radial permeability, vertical permeability, and leakance. We apply information from published studies on rate-limited vapor transport to provide the basis for selection of a critical design pore-gas velocity for soils at this site. Using single-well gas flow simulations, we evaluate whether this critical pore-gas velocity was achieved at measured ROIs. We then conduct a series of multi-well gas flow simulations to assess how variation in anisotropy and leakance affect three-dimensional vacuum and pore-gas velocity profiles and determination of an ROI. Finally, when attempting to achieve a critical design pore-gas velocity we evaluate whether it is more efficient to install additional wells or pump existing wells at a higher flow rate.     

Structures and profiles of novel sulfur-linked chlorophyll derivatives in an Antarctic lake sediment

High performance liquid chromatography-mass spectrometry has permitted the identification of a homologous series of novel alkylsulfide derivatives of chlorophyll a containing between one and five carbon atoms, in sediment from a coastal Antarctic lake. The sulfur-containing compounds are present in varying abundance in stratigraphic horizons representing a phase when the lake was a marine basin. Throughout this marine phase photic zone anoxia is recorded by the presence of bacteriochlorophyll c and d-derivatives. Distributional variations between sulfurised and non-sulfurised chlorophyll a-derivatives throughout the sediment section studied indicate that the extent of sulfurisation is not controlled by chlorophyll a abundance alone.     

Coupling Suspension Feeders to the Chesapeake Bay Eutrophication Model

A model predicting suspension-feeding bivalve biomass and its interactions with water quality has been developed and coupled with the Chesapeake Bay Eutrophication Model. This coupling included deposition of filtered particulate matter to the sediments and the recycling of inorganic nutrients back to the water column. Because individual size is a crucial determinant of bivalve filtration and respiration rates, an empirical function, was developed from data, relating computed areal biomass to size, which was then used to adjust these rates during the simulation. Biomass was strongly related to the eutrophication model's predictions of organic and total solids distributions, as well as to bottom water dissolved oxygen. The tight coupling between seasonal organic matter concentration and biomass suggested that food, or the ability of suspension feeders to ingest it given present total solids loadings, is a limiting factor baywide. Hypoxia and anoxia also reduced benthic biomass in affected locations. High site-specific temporal variability observed in the data may contain a large component of spatial patchiness, on scales below which the present estuarine eutrophication model could resolve. Further insights will be needed to incorporate the effects of patchiness, as well as other important spatial and temporal signals, such as predation and recruitment.