On the swimming strategies to escape a rip current: a mathematical approach

Rip currents represent significant hazards to swimmers all around the world. The danger arises when a misinformed swimmer uses an inadequate strategy to escape the rip, such as fighting the current directly. This can lead to fatigue, panic, and in some cases drowning. There exists a range of strategies put forward by experts (both lifeguards and scientists) to escape rip currents. However, these recommendations are based on a limited number of scientific studies and there is still much discrepancy surrounding the best strategy to escape a rip. Thus, here we present a simple, physics-based theoretical model aimed at assessing different escape strategies in terms of their associated ‘energetic cost’ (in work and power) for any given rip current and swimmer’s proficiency level. Many combinations of swimmers and rips are considered, including both idealised and a realistic rip current. Our quantitative results back the common knowledge that swimming against the rip (which is strongly discouraged by lifeguards) is almost universally the worst possible strategy, especially when compared against strategies favoured by experts, such as floating with the current before attempting to swim back to the shore. For a realistic rip, our results suggest that swimming directly against the rip can require several times more power from the swimmer than other strategies advised by lifeguards, thus lending further scientific support to experts’ recommendations. This study may help promote education around the dangers posed by rip currents and how best to address them.

     

Evaluation of swimmer-based rip current escape strategies

Rip currents are the primary hazard on surf beaches, and early studies described them as fast, shore-normal flows that extended seaward of the surf zone. Based on this traditional view, commonly promoted safety advice was to escape a rip current by swimming parallel to the beach. However, recent studies have shown dominant rip current re-circulation within the surf zone and have endorsed floating as an appropriate escape strategy. Here, a first quantitative assessment of the efficacy of various rip current escape strategies, with a focus on the underlying physical processes, is presented. A field study was conducted at Shelly Beach, NSW, Australia, measuring three rip currents (two open beaches, one topographic) over 3 days in varying wave conditions. Floating was found to be a longer duration, more variable escape strategy ( $ \overline{t} $  = 3.8 min, σ = 2.4 min), than swimming parallel ( $ \overline{t} $  = 2.2 min, σ = 1.0 min). Neither of the scenarios is 100 % foolproof, and both fail in some scenarios, making simplified safety recommendations difficult. Swim parallel failures are related to swimming against the alongshore current of the rip circulation. Float failures related to surf zone exits, with the highest exit rate occurring in the topographic rip. Float failures also occurred due to multiple re-circulations without the person attaining safe footing on the bar. The variable spatial and temporal behaviour of rip currents suggests that a single escape strategy safety message is inappropriate. Instead, a combined approach and scenario-specific safety advice should be considered by beach safety practitioners to promote to the public.     

Perception of the rip current hazard on Galveston Island and North Padre Island,Texas, USA

The hazard posed by a rip current depends in part on the ability of beach users to identify a rip current and to associate surf conditions with the potential for rip currents. Understanding which visual features beach users associated with rip currents is an important step in the development of appropriate programs and educational materials aimed at improving the ability of beach users to identify a rip current. A face-to-face survey (n = 392) was conducted to assess the ability of beach users to identify a rip current using five near eye-level photographs that simulate the view of the beach and surf zone as the respondent approached the beach. The survey was conducted on three heavily used public beaches in Texas (Galveston, Port Aransas, and Corpus Christi) at the height of the summer beach season in 2012. Only 13 % of respondents correctly selected the photograph showing the most hazardous conditions and correctly identified the precise location of the rip current on the photograph. The majority of beach users (87 %) incorrectly indicated that the photograph with the heaviest surf represented the most hazardous surf conditions and greatest potential for the development of rip currents, or failed to identify rip currents in photographs. Respondents who were able to correctly identify the rip current tended to recognize the breaking wave pattern, areas of darker water, or the proximity to structures as key visual characteristics of rip currents and also had higher self-reported abilities to swim in rough water and escape a rip current.     

Active hydrothermal fluids circulation triggering small-scale collapse events: the case of the 2001–2002 fissure in the Lakki Plain (Nisyros Island,Aegean Sea,Greece)

Rip currents are fast moving, offshore flows that have the ability to move even the strongest swimmers into deeper waters. Miami Beach, Florida is one of the most visited beaches in the USA and a sought after destination for citizens and international tourists alike. It is also known to be a rip current “hot spot.” These factors greatly increase the risk of drowning; however, no previous research has focused on beachgoer perception of rip-related risks in South Florida. Over a 12-month period, 203 public surveys were collected to determine the rip current knowledge of beachgoers at Miami Beach based on factors such as swimming ability and frequency of beach visits. The responses were analyzed by constructing a normalized component factor to determine the respondent’s comprehensive knowledge of rips, and multiple regression models were used to assess the net influences of sociodemographic and behavioral characteristics on the responses. A significant proportion of the survey respondents showed insufficient knowledge, indicating they are at risk of drowning in a rip current. Frequent beachgoer’s exposure to the beach environment, maturation, and nativity is identified as the main contributors to knowledge net of other sociodemographic compositions. The most at-risk groups were determined to be young adults, foreign tourists, poor swimmers, and those who infrequently visited the beach. Miami Beach needs to initiate a rip current safety campaign to target these at-risk beachgoers, where interventions beyond familial and educational institutions should be introduced.     

Beach Comfort Index – a new approach to evaluate the thermal conditions of beach holiday resorts using a South African example

The climatic conditions of seven beach holiday resorts in South Africa (Alexander Bay, Cape Town, George, Port Elizabeth, East London, Durban and Richard's Bay) have been examined with regard to the thermal perception of beach holiday makers. The evaluation of the thermal conditions is based on the human energy balance calculations which have been adapted to the requirements of beach goers. A model has been developed to calculate the missing radiation input data and parameters such as ‘beach day’ and ‘beach hour’ are introduced and defined. The average monthly number of beach days is calculated for every location showing the highest amounts in Durban and Richard's Bay and the highest annual variations in Cape Town. The Beach Comfort Index Map of South Africa shows the maximum, minimum and average number of beach days per month and the probability of beach days and thus providing understandable and interpretable information on the thermal conditions of the sites from the point of view of beach holiday makers. This revised version was published online in July 2006 with corrections to the Cover Date.     

Deformation of an elastic layer coupling in different ways to a base due to a very long vertical strike-slip dislocation

The closed-form analytic expressions for the displacement and stresses at any point of an elastic layer lying over a base due to a very long vertical strike-slip dislocation are obtained. The interface between the layer and the base is assumed to be either ‘smooth-rigid’ or ‘rough-rigid’ or ‘welded’. The variations of displacement and stresses with the horizontal distance from the fault for different types of coupling of the layer with the base have been studied. It is found that the displacement for ‘welded interface’ lies between the displacements due to ‘smooth rigid’ and ‘rough-rigid’ interfaces for different positions of the observer and different values of the ratio of rigidities of the layer and half-space.     

‘How do the American people know…?’: embodying post-9/11 conspiracy discourse

Conspiratorial thought has been highly visible in post-September 11th America, manifest through the continued growth of a public ‘9/11 Truth Movement’ as well as at the state-level, through the Bush administration’s conspiracy rhetoric of Islamic terrorists intent on infiltrating the US homeland. In this paper, I demonstrate how conspiracy can be understood as a ‘knowledge-producing discourse’; dialectically engaged across multiple subject positions and through which geopolitical narratives are performatively produced and contested at interconnected scales of bodies, homes, city streets and national ‘homelands’. Through drawing on, and challenging, the conceptual and methodological approaches of a burgeoning feminist geopolitics, I ground my analysis in the embodied performances of ‘patriotic dissent’ by members of the 9/11 Truth Movement in New York City, as well as through my own situated and ethical engagement with positions of political difference.     

Ability of beach users to identify rip currents at Pensacola Beach, Florida

Quasi-permanent rip current hot spots at Pensacola Beach, Florida, pose a significant hazard to beach users, largely because the hot spots are located at or close to the primary access points. While an increase in the number of lifeguards has led to a decrease in the number of drownings since 2004, the number of rescues and contacts has increased to over a 30,000 year. Despite warning signs at access points along the beach, it is not clear whether beach users are able to identify a rip channel or an active rip current. To assess beach users’ knowledge of rip currents and their ability to identify rip channels and currents, 97 surveys were conducted between June and September of 2010 at Pensacola Beach. Beach users were asked to identify rip channels in oblique photographs taken on green, yellow and red flag days when the potential for rip currents is low, medium and high, respectively. A majority of participants suggested that they could identify a rip channel or current (if present), but less than 20 % of beach users were able to identify the rip channels and currents. The majority of participants identified heavy surf areas as the location of the rips versus the relatively flat water of the current or the darker color water of the channel. Results further suggest that most beach users, and particularly local participants, are overconfident in their ability to identify rip channels and currents. The focus of beach users on heavy surf as an indication of the rip current potential and the overconfidence in identifying a rip channel or current affects the spatial distribution of beach users and to some degree the location of rescues and drownings. While it can be quite difficult for the average beach user to identify rip channels and active rip currents, the results of the study suggest a need for further education efforts to reduce the rip hazard, particularly in areas where lifeguards are not permanently stationed.     

Contesting Chinatown: Place-making and the Emergence of ‘Ethnoburbia’ in Brisbane,Australia

This paper juxtaposes the actual areas of settlement and settlement activities of Chinese migrants in Brisbane’s southern suburbs since the mid-1980s, with the concomitant, ‘government planned’ construction of the city’s Chinatown as an ‘exotic,’ ‘ethnic,’ and ‘cosmopolitan’ landmark. It argues that while the latter, as with Chinatowns in other Australian and world cities, has continued to appropriate the symbols of so-called ‘Chineseness’ to sell the locale to non-Chinese, the former, in recalling the notion of ‘ethnoburbia,’ significantly evinces actual Chinese migrants’ agency and role in place-making. In the process, these migrants have not only established a local, ethnically meaningful environment, but also challenged the still current metonymic Western image of the Chinese as preferring life in ‘ethnic’ urban enclaves that had its origin from early Chinese settlement and fundamentally detached from the lives of other ethnic groups per se. Indeed greater recognition and comprehension of such locales may go a long way to illustrate that many Chinese living in Australia today do not fit the territorial, place-based identity often associated with them in the imagination of mainstream society.     

A model for multiphase flow and transport in porous media including a phenomenological approach to account for deformation—a model concept and its validation within a code intercomparison study

Multiphase flow processes in unsaturated cohesive soils are often affected by deformation due to swelling and shrinking as a result of varying water contents. This paper presents a model concept which is denoted ‘phenomenological’ in terms of the processes responsible for soil deformation, since the effects of deformation on flow and transport are only considered by constitutive relations that allow an adaptation of the hydraulic properties. This new model is validated in a detailed intercomparison study with two state-of-the-art models that are capable of explicitly describing the processes relevant for the deformation. A ‘numerical experiment’ with a state-of-the-art reference model is used to produce ‘measurement data’ for an inverse-modelling-based estimation of the model input parameters for the phenomenological concept.     

Low energy trajectories for the Moon-to-Earth space flight

The Moon-to-Earth low energy trajectories of ‘detour’ type are found and studied within the frame of the Moon-Earth-Sun-particle system. These trajectories use a passive flight to the Earth from an initial elliptic selenocentric orbit with a high aposelenium. The Earth perturbation increases the particle selenocentric energy from a negative value first to zero and then to a positive one and therefore leads to a passive escape of the particle motion from the Moon attraction near the translunar libration pointL ₂. This results in the particle flight to a distance of about 1.5 million km from the Earth where the Sun gravitation decreases the particle orbit perigee distance to a small value that leads to the particle approaching the Earth vicinity in about 100 days of the flight. A set of the Moon-to-Earth ‘detour’ trajectories is defined numerically. Characteristics of these trajectories are presented. The ‘detour’ trajectories give essential economy of energy (about 150 m/s in Delta V) relative to the usual ones.     

Facies characteristics of the basal part of the Talchir Formation,Talchir Basin,India — Depositional history revisited

The lowest unit of the Talchir Formation of Talchir Basin, Orissa, was described by pioneer workers as the ‘basal boulder bed’. In an attempt to explain the co-existence of gravel and clay, materials of contrasting hydraulic properties, a probable situation resembling the effects of the action of ground-ice enabled boulders to be carried down by sluggish currents resulting in an intermixture of large boulders and fine mud was conceived. Misinterpretation of this conclusion led to a general tendency to describe the ‘basal boulder bed’ as ‘glacial tillite’. However, the unit described as ‘basal boulder bed’ is actually represented by a matrix rich conglomerate with pockets of normally graded silty clay. The present study reveals that the depositional imprints preserved in this part of the sedimentary succession indicate emplacement of successive debris flows generated through remobilization of pre-existing unconsolidated sediments. Small pockets of fine-grained turbidites presumably deposited from the entrained turbidity currents associated with the debris flows suggest the composite character of the debris flow deposit.     

A liminal territory: Gaza,executive discretion,and sanctions turned humanitarian

In September 2007 Israel’s security cabinet approved a ‘hostile entity’ classification for the Gaza Strip and intensified its economic and diplomatic blockade of this Hamas-controlled region. Taking the ‘hostile entity’ classification as a point of entry, this paper examines the construction of Gaza as an insurgent zone, a liminal space within which Israel’s executive discretion has authorizing force. Central to this process, it argues, is a blurring of lines between the civilian and combatant—the elimination of a purely civilian space. This paper begins with an analysis of the discursive strategies employed to collapse the space between the civilian body and battlefield in Gaza. It then turns to an examination of socio-spatial practices mobilized around the ‘hostile entity’ classification, foremost Israel’s sanctions policy, and argues this counter-insurgency strategy entails regulation and management of the Palestinian body combined with the active subjugation of Palestinian life to the power of death. Centrally, this paper attends to the relationship between geopolitics and violence at the scale of the (Palestinian) body.     

Problems associated with tourism development in Southern Africa: The case of Transfrontier Conservation Areas

Tourism in Southern Africa is synonymous with the wildlife safari. In the post-colonial era the establishment of so-called ‘peaceparks’ that straddle the borders of states has come to be seen as a key not only to increasing tourism in the Southern African region, but also to the modernizing of conservation policies and the development of rural economies. This paper focuses on the global and continental presence of transfrontier conservation areas, the link between conservation and tourism development, and the current factors that constrain and influence the realization of an ‘African Dream’ — ‘establishment of the greatest animal kingdom’. The unstable political situation in Zimbabwe and how this negatively affects wildlife conservation and tourism in the Gonarezhou part of the Greater Limpopo Transfrontier Park, serves as a case study. This revised version was published online in August 2006 with corrections to the Cover Date.     

A comparative analysis of several vulnerability concepts

A comparative analysis of six vulnerability models aims to identify differences and similarities between several approaches towards understanding vulnerability. The analysis yields a set of characteristics for explaining the condition of vulnerability (multiple contexts, multiple dimensions, temporal variability, multiple scales and scale-interdependency). In addition, ‘adaptation’ and ‘adaptive capacity’ are identified as key elements of vulnerability. The results of the analysis are put into a wider context not only of vulnerability but also of resilience and risk research. It is demonstrated that ‘adaptation’ and ‘adaptive capacity’ serve as hinges not only for conceptualising vulnerability but between ‘vulnerability’ and ‘resilience’ alike. Based on the results of the comparative analysis, a model of vulnerability focussing on the household and community scale is developed, which displays the key findings of this work.     

Can the visualization of rip currents prevent drowning accidents? Consideration of the effect of optimism bias

Drowning accidents at beach in Japan are caused by rip currents. To reduce these accidents, a new technology that can detect rip currents and notify beachgoers by using the Internet of Things (IoT) and Artificial Intelligence (AI) was proposed. However, studies on the effect of visualizing rip currents or considering the effect of optimism bias have not been conducted. This study investigates if visualization of rip currents might help in preventing drowning accidents, while considering the effect of optimism bias. The participants were 90 Japanese beachgoers. They were asked to answer questions based on their knowledge of the beach and rip currents, their optimism bias regarding rip currents, and awareness with or without visualization. The results of the analyses suggest that despite optimism bias, the visualization of rip currents increases the tendency of beachgoers to perceive and avoid rip currents. As described above, it was found that by visualizing the rip current, beachgoers were able to perceive and avoid rip currents. In addition, an understanding of rip currents is positively related to the intent to avoid rip currents even when rip currents are visualized. Therefore, it is necessary not only to enhance the avoidance tendency by visualizing rip currents, but also to further enhance knowledge of beachgoers to deepen the understanding of rip currents including the danger associated and methods to avoid them.

     

Finite element modelling for evaluation of apparent resistivity over complex structures

The paper presents a computational algorithm designed for efficient modelling of apparent resistivity over complex geological structures, using finite element method. The algorithm can be used to study variations of apparent resistivities using any electrode configuration at any point on the earth’s surface, not necessarily regular. A Schlumberger apparent resistivity sounding curve over a buried anticline, is presented here as an example and compared with the corresponding analytical curve, to demonstrate the correctness of the FEM algorithm. The various potential derivatives required for the computation of apparent resistivities evaluated through different electrode configurations have been obtained by calculating the ‘influence coefficients’ using reciprocal theorems, an approach successfully applied in structural engineering. In essence, a set of self balancing nodal currents, obtained from the appropriate derivative(s) of the shape functions of the elements contributing to the point of observation, is applied as the load vector. The resulting quantities corresponding to the potential distribution in traditional finite element method, then, turn out to be the potential derivatives at the point of observation for different positions of the current electrodes. These are known as influence coefficients. The continuum nature of the domain beyond the region of interest has been modelled by using ‘infinite elements’ across which the potential is assumed to decay exponentially.     

Nagaur — tertiary interface under Punjab-Haryana semi-desertic plains in western India

The subsurface Nagaur Basin in northwest Haryana and southwest Punjab hosts evaporite sequence, conformably overlain by Nagaur red beds and grey beds. A polymictic conglomerate intervenes the Nagaur sequence and the overlying Tertiary sequence. The focus is on the new finds of (i) distinctive ‘grey beds’ overlying ubiquitous red beds, (ii) ‘Malout conglomerate and grit’ horizon (Palaeocene-Miocene) marking an unconformity over the Nagaur Group (early Cambrian) and (iii) ‘entrapped gas and dried-up leaves/carbonaceous flakes’ in the conglomerate unit and succeeding Tertiary rocks. Palaeoenvironmental significance has been discussed. The Nagaur ‘red beds’ has been likened to ‘continental red beds’ of Schreiber (1978) and ‘reworked red beds’ of Krynine (1949). The ‘grey beds’ are channel fillings, fresh material having been derived from fresh cuttings by streams/rivers following structural grains in the Aravalli landscape where the interfluves provided weathered (oxidised) materials for the red beds. Association of continental red beds and grey beds is known in the Newark Series (Triassic) of the Eastern United States and the Keweenawan ‘late Precambrian’ of the Lake Superior region.     

Re-viewing the Entrapment controversy: Megaprojection, (mis)representation and postcolonial performance

Becoming the tallest building in the world in the mid-1990s, the Petronas Towers was the centre piece of an image of national progress and development that Malaysian authorities sought to project internationally. The release of Fox Movies' Entrapment in Malaysia in May 1999 provoked political outrage and popular disappointment at the way in which the Petronas Towers in Kuala Lumpur had been spliced alongside riverside ‘slums’ filmed in the town of Malacca some 150 km away. This paper provides a critical reading of the spliced scene in the movie. At one level, the angry response of the Malaysian Prime Minister, Dr Mahathir Mohamad, to the scene diagnoses a geopolitics of asymmetrical representational power. However, I show how Mahathir's criticism of Entrapment in Malaysia was as much a defence of domestic political legitimacy (and national economic investibility) as it was ‘opposition’ or ‘resistance’ to hegemonic ‘Western’ (mis)representation. In addition, while the material and symbolic work of reimaging Kuala Lumpur had sought to negate (neo)orientalist imaginings of ‘Asian’ cities, the controversial scene rendered visible environmental ‘underdevelopment’ that has no place in a modern (vision of) Malaysia. Entrapment thus performed something in inducing Malaysian cities and citizens to ‘clean up’ their act, to practice ‘fully developed’ ways of seeing, being and being seen. This revised version was published online in August 2006 with corrections to the Cover Date.     

Capitalism, masculinity and whiteness in the dialectical landscape: The case of Tarzan and the Tycoon

The intent of this paper is to explore how ‘reel’ and ‘real’ landscapes can be rendered dialectic. The analysis focuses on the filming of Tarzan's Secret Treasure (1941) at the wildlife preserve of Wakulla Springs in Florida, which was administered by the financial tycoon Ed Ball. Using Mitchell's recent commentaries on ‘good’ and ‘bad’ dialectics as our entry point, we offer two readings of this cinematic process. The first renders the on and off screen Wakulla Springs as ideological fodder, serving to valorise capital, masculinity and whiteness. The second reading addresses how this place has been actively constructed from discourses pertaining to class, gender and race, such that Wakulla exists as a series of fully relational spaces within diverse contexts, both ‘real’ and ‘reel’. In structuring our analysis in this manner, we hope to make clear why it does indeed matter for film geographers to acknowledge the conceptual presumptions brought to bear in the framing of the real and the reel as objects of analysis. This revised version was published online in August 2006 with corrections to the Cover Date.