A meta‐analysis of reef island response to environmental change on the Great Barrier Reef

Reef islands on the Great Barrier Reef are influenced by a range of environmental factors. A meta‐analysis of 103 islands is presented to express variation in island size (area and volume) as a function of latitudinal and cross shelf gradients in regional oceanographic factors (exposure to incident waves, tidal range and tropical cyclone frequency) and local physical factors (position on the shelf, area, length and depth of supporting reef platform, vegetative cover). Models performed well for unvegetated sandcays (R² = 0.89), vegetated sandcays (R² = 0·72) and low wooded islands (R² = 0.78), with a moderate level of variation explained when all islands were simultaneously regressed (R² = 0.58). Future island dynamics were simulated for anticipated changes in cyclone regime, wave activity and sea level. For 38 islands mapped on the 1973 Royal Society and Universities of Queensland Expedition to the Northern Great Barrier Reef, change over the same 22 year period (1973–1995) was determined and the relative magnitude of observed and modelled changes was compared and found to be consistent through rank correlation analysis (Γ = 0.84 for unvegetated sandcays, Γ = 0.81 for vegetated sandcays). Simulations of island area or volume change from 2000 to 2100 indicated that under a 30% decrease in tropical cyclone activity, unvegetated sandcays continue to accrete at a lower rate, whereas all island types erode under a 38% increase in tropical cyclone activity. Vegetated sandcays initially accrete at higher levels of cyclone activity, entering an erosive state with a 60% increase in activity. Low wooded islands are unresponsive to environmental changes modelled. A sensitivity analysis of vegetated and unvegetated sandcays indicated that the presence of vegetation increases the tropical cyclone activity threshold at which islands begin to erode. Greatest sedimentary losses occur within the central band of high cyclone activity between Cooktown and Mackay. Copyright © 2015 John Wiley & Sons, Ltd.     

A method to dynamically subdivide parcels in land use change models

Spatial simulation models have become a popular tool in studying land use/land cover (LULC) change. An important, yet largely overlooked process in such models is the land subdivision, which is known to govern LULC change and landscape restructuring to a large extent. To fill this gap, we propose an efficient and straightforward method to simulate dynamic land subdivision in LULC change models. Key features in the proposed method are implementing a hierarchical landscape where adjacent cells of the same LULC type form patches, patches form properties, and properties form the landscape and incorporating real subdivision layouts. Furthermore, we use a queue-based modified flood-fill algorithm to dynamically reset LULC patches following a subdivision. The proposed subdivision method is demonstrated in action using a prototype agent-based LULC model developed for an amenity landscape in Australia. Results show that it is computationally feasible to run the subdivision method even as spatial resolution is increased, thus providing a proven means for spatial simulation models to dynamically split parcel land.     

A Comprehensive Numerical Study of Hydraulic Fracturing Process and Its Affecting Parameters

Production of hydrocarbons from shale formations has been made possible mainly due to hydraulic fracture (HF) technology. It increases the permeability of reservoir rocks around a well by connecting fractures and improving conductivity. HF behavior especially in presence of natural fractures have recently given much attention in studies. This paper investigates HF propagation and its associated parameters in various conditions. A higher order displacement discontinuity method is used to achieve higher accuracy in the results. First, behavior of crack opening displacement (COD) of an HF i.e. HF width in various conditions is studied. COD is a key parameter in determination of an HF operation success. It is proportional to production rate of oil and gas wells and provides a path for proppant entrance into the fractures. An equation considering many important parameters, based on numerous numerical modellings of various mechanical and geometrical effects on COD is proposed with coefficient of determination and standard error of 94.35% and 4.37 × 10^?4 respectively. The next part of the paper studies the HF propagation in a naturally fractured reservoir. These natural fractures alter the stress fields and hence affect the propagation of a hydraulic fracture. In fact, it is shown that in certain orientations of hydraulic fractures and natural fractures, the effect of natural fractures disappear or completely changes propagation path. Using a combination of several interaction criteria, a new modeling of HFs and NFs interaction is presented. The modellings showed that spacing and angle of intersection can significantly affect HFs propagation. The results of COD and HF propagation in presence of natural fractures may be considered in HG design and primary orientation of perforated fractures.     

A hybridized numerical and regression method for estimating the minimum rock pillar width of twin circular tunnels

Twin tunnels can be used for many applications. Interaction between two tunnels is an important problem in tunnel engineering that should be studied specially. Numerical investigations are well adapted to field data and numerical methods can be used in design of rock pillar of twin circular tunnels. So far, no relationship has been provided to estimate the minimum stable rock pillar width. In this paper, the interaction between twin circular tunnels has been studied using 2D finite element analysis. To do this, a great number of twin tunnels were modeled in Phase2 software with different conditions of rock mass (RMR value) and depth of tunnel. Models were analyzed and minimum stable rock pillar width was determined. This process was repeated for three different ratios of K (ratio of horizontal stress to vertical stress, 0.5, 1, and 1.5). Finally, according to the linear and nonlinear regression methods, the best merit function was fitted to result of numerical analysis. Then, new approximate formula was proposed to estimate the minimum rock pillar width according to RMR value and depth of twin circular tunnels with different K values. The formulae are very accurate (coefficient of correlation equals to minimum 0.96) that can be used for estimating the minimum rock pillar width of twin circular tunnels.     

Analyses of Inclined Cracks Neighboring Two Iso-Path Cracks in Rock-Like Specimens Under Compression

Crack propagation process in pre-cracked rock like specimens has been studied experimentally and numerically considering three cracks in the middle part of each specimen. The rock-like specimens are specially prepared from Portland pozzolana cement, fine sands and water. These pre-cracked cylindrical specimens (each containing a single inclined crack in the neighborhood of two iso-path cracks) are experimentally tested under compressive loading. The same problems are numerically simulated by a modified displacement discontinuity method using higher order displacement discontinuity elements and higher order special crack tip elements for crack tip treatment to increase the accuracy of the Mode I and Mode II stress intensity factors obtained based on linear elastic fracture mechanics theory. The crack propagation and coalescence paths of the inclined crack are estimated by implementing a suitable iteration algorithm of incremental crack length extension in a direction predicted by using the maximum tangential stress criterion. The numerical and analytical crack extension analyses are compared which are in good agreement and show the validity, applicability and accuracy of the present work.     

Assessing the recent (1834 – 2002) morphological evolution of a rapidly prograding delta within a GIS framework: Macquarie Rivulet delta,Lake Illawarra,New South Wales

The recent morphological evolution of Macquarie Rivulet delta, Lake Illawarra, New South Wales, has been assessed from historical parish maps from 1834, 1892, 1916 and 1927 along with aerial photographs from 1938 to 2002. These images were digitised, georeferenced and analysed in a GIS framework. The 1834 sketch map could not be georeferenced and it was excluded from the calculation of delta progradation areas. Between 1892 and 1927 morphological changes were minimal, probably because the areas were not resurveyed between these map editions. A significant change in morphology between 1927 and 1938 reflects a change in source material from maps to aerial photographs. The research highlights that great care must be taken when utilising historical data. Major morphological changes observed between 1938 and 2002 included the development of a large crevasse splay in the 1960s to 1970s and the abandonment and infilling of the delta's previously active distributary channels. The research shows that the morphological changes observed on the delta can be related to both natural processes and anthropogenic modifications to the catchment and thus could be used in the development of catchment management plans.     

Numerical simulation of crack propagation in layered formations

In the present study, a boundary element method based on the higher order displacement discontinuity formulation is presented to solve the general problem of hydraulic fracture propagation in layered formations. Displacement collocation technique is employed to model the higher order displacement variation along the crack and the special crack tip element near its ends. The hydraulic fracture propagation and its interaction with the layer interface in non-homogenous rock materials are studied by the proposed semi-analytical (hybridized boundary element-boundary collocation) method. The maximum tangential stress criterion (or σ-criterion) of fracture mechanics considering different elastic constants (Young modulus and Poisson’s ratio) is used to obtain the fracture path. The fracture propagation from stiff to soft and soft to stiff media for cracks having different inclination angles is modeled, and the effects of elastic constants on the hydraulic fracture propagation is studied. The results show that if the hydraulic fracture originates in the stiffer layer, its capability to cross the layer increases and is vice versa for the softer material. The comparison of the results gained from the numerical method with those in the literature show a good performance of the method in the case of propagation of hydraulic fracture in layered formations.