FIELD OBSERVATIONS OF INFRAGRAVITY WAVE RESPONSE TO VARIABLE SEA-SWELL WAVE FORCING

Full version: Access restricted permanently due to 3rd party copyright restrictions. Restriction set on 16.11.2017 by SE, Doctoral CollegeInfragravity waves are low frequency (0.005-0.04 Hz) waves that can dominate the spectrum of water motions and sediment transport processes within the inner surf zone. Despite the established importance of infragravity waves in shaping our coasts and numerous studies dating back to the 1950s, several aspects of infragravity wave analysis, generation and dissipation remain poorly understood. As much of the recent infragravity research has focussed on fetch-limited coasts, less is known about the climatology of these waves on energetic coastlines subject to both swell and fetch-limited waves. It has been postulated that bed friction only plays a significant role in the dissipation of infragravity waves where the bed is exceptionally rough, but the precise impact of bed roughness is not fully understood, particularly on extremely rough rock platforms. Finally, although there have been many methodologies proposed for the decomposition of reflective wave fields (an essential tool for studying infragravity wave dynamics), very little attention has been given to evaluating their accuracy, particularly the impact of uncorrelated noise. This study aims, primarily through the collection of an extensive field dataset and the establishment of accurate analysis tools, to provide new insight into the propagation, dissipation and reflection of infragravity waves on energetic coastlines of varied roughness, subject to both swell and fetch-limited waves. To ensure the accurate decomposition of infragravity wave signals into their incident and reflected components, a sensitivity analysis into the effect of uncorrelated noise on an array separation method is performed. Results show that signal noise, often prevalent in field data, introduces a significant bias to estimates of incident and reflected wave spectra, and corresponding reflection coefficients. This bias can exceed 100% for signal-to-noise ratios of 50% if signal noise is unaccounted for. Consequently, noise reduction should form an integral part of future infragravity wave studies. New research from a dissipative, fetch-unlimited sandy beach (Perranporth, Cornwall, UK) and a macrotidal, rocky shore platform (Freshwater West, Pembrokeshire, UK) uniquely demonstrates that the level of infragravity wave energy close to shore is linearly dependent on the offshore short wave energy flux H_o^2 T_p (r^2 = 0.93and 0.79, respectively). Infragravity waves approach the coast as bound waves lagging slightly (~4 s) behind the wave group envelope and are released in the surf zone where their heights can exceed 1 m. Considerable infragravity dissipation is observed in the surf zone and is a function of both frequency and H_o^2 T_p. Complex Empirical Orthogonal Function (EOF) analysis reveals (quasi-)standing waves at low infragravity frequencies 0.017 Hz), infragravity waves demonstrated progressively more dissipation (up to 90%) and progressive wave characteristics, with increasing frequency. Much of the observed dissipation occurs very close to shore (h <0.8 m) and the dependence of the reflection coefficient on a normalised bed slope parameter implies a mild sloping bed regime at these high infragravity frequencies, suggesting that the observed dissipation is dominated by wave breaking processes. This is supported by the results of bispectral analysis which show predominantly infragravity-infragravity interactions in shallow water and the development of infragravity harmonics indicative of steepening and eventual breaking of the infragravity waves. This study presents the first simultaneous field observations of infragravity waves on a macrotidal, rocky shore platform and adjacent sandy beach. Infragravity wave dissipation is observed on both the platform and beach and occurs at statistically similar rates, demonstrating that frictional dissipation due to bed roughness is not the dominant dissipation mechanism, even in this extreme case. Sea-swell waves are also unaffected by the extreme roughness of the platform, with relative wave heights on the beach and platform (γ = 0.38 and 0.43, respectively) scaling well with their respective gradients and are in very close agreement with formulations derived from sandy beaches. Overall, bed roughness is shown to have no significant impact on infragravity or sea-swell wave transformation, with offshore forcing and bed slope being the main controlling factors, particularly under moderate to high energy offshore forcing

Correcting wave reflection estimates in the coastal zone

publisher: Elsevier articletitle: Correcting wave reflection estimates in the coastal zone journaltitle: Coastal Engineering articlelink: http://dx.doi.org/10.1016/j.coastaleng.2016.09.004 content_type: article copyright: © 2016 Elsevier B.V. All rights reserved

Infragravity waves: From driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)

Day hospital versus admission for acute psychiatric disorders

Backgroun

Day hospital versus out-patient care for psychiatric disorders

Obstacle, adventure and endurance competitions in challenging or remote settings are increasing in popularity. A literature search indicates a dearth of evidence-based research on the organisation of medical care for wilderness competitions. The organisation of medical care for each event is best tailored to specific race components, participant characteristics, geography, risk assessments, legal requirements, and the availability of both local and outside resources. Considering the health risks and logistical complexities inherent in these events, there is a compelling need for guiding principles that bridge the fields of wilderness medicine and sports medicine in providing a framework for the organisation of medical care delivery during wilderness and remote obstacle, adventure and endurance competitions. This narrative review, authored by experts in wilderness and operational medicine, provides such a framework. The primary goal is to assist organisers and medical providers in planning for sporting events in which participants are in situations or locations that exceed the capacity of local emergency medical services resources

Day hospital versus admission for acute psychiatric disorders

BACKGROUND: Inpatient treatment is an expensive way of caring for people with acute psychiatric disorders. It has been proposed that many of those currently treated as inpatients could be cared for in acute psychiatric day hospitals. OBJECTIVES: To assess the effects of day hospital versus inpatient care for people with acute psychiatric disorders. SEARCH STRATEGY: We searched the Cochrane Controlled Trials Register (Cochrane Library, issue 4, 2000), MEDLINE (January 1966 to December 2000), EMBASE (1980 to December 2000), CINAHL (1982 to December 2000), PsycLIT (1966 to December 2000), and the reference lists of articles. We approached trialists to identify unpublished studies. SELECTION CRITERIA: Randomised controlled trials of day hospital versus inpatient care, for people with acute psychiatric disorders. Studies were ineligible if a majority of participants were under 18 or over 65, or had a primary diagnosis of substance abuse or organic brain disorder. DATA COLLECTION AND ANALYSIS: Data were extracted independently by two reviewers and cross-checked. Relative risks and 95% confidence intervals (CI) were calculated for dichotomous data. Weighted or standardised means were calculated for continuous data. Day hospital trials tend to present similar outcomes in slightly different formats, making it difficult to synthesise data. Individual patient data were therefore sought so that outcomes could be reanalysed in a common format. MAIN RESULTS: Nine trials (involving 1568 people) met the inclusion criteria. Individual patient data were obtained for four trials (involving 594 people). Combined data suggested that, at the most pessimistic estimate, day hospital treatment was feasible for 23% (n=2268, CI 21 to 25) of those currently admitted to inpatient care. Individual patient data from three trials showed no difference in number of days in hospital between day hospital patients and controls (n=465, 3 RCTs, WMD -0.38 days/month CI -1.32 to 0.55). However, compared to controls, people randomised to day hospital care spent significantly more days in day hospital care (n=265, 3 RCTs, WMD 2.34 days/month CI 1.97 to 2.70) and significantly fewer days in inpatient care (n=265, 3 RCTs, WMD -2.75 days/month CI -3.63 to -1.87). There was no significant difference in readmission rates between day hospital patients and controls (n=667, 5 RCTs, RR 0.91 CI 0.72 to 1.15). For patients judged suitable for day hospital care, individual patient data from three trials showed a significant time-treatment interaction, indicating a more rapid improvement in mental state (n=407, Chi-squared 9.66, p=0.002), but not social functioning (n=295, Chi-squared 0.006, p=0.941) amongst patients treated in the day hospital. Four of five trials found that day hospital care was cheaper than inpatient care (with cost reductions ranging from 20.9 to 36.9%). REVIEWER'S CONCLUSIONS: Caring for people in acute day hospitals can achieve substantial reductions in the numbers of people needing inpatient care, whilst improving patient outcome

An Investigation of Cross-Shore Velocity Profiles, Bed Shear Stress and Friction in the Swash Zone of a High Energy, Macrotidal Beach

Cross-shore velocity profiles are measured at 0.001 m vertical resolution and at 100 Hz over the lower 0.07 m of the water column on a high energy, macrotidal beach in Cornwall, United Kingdom. Data are used to quantify the vertical structure of cross-shore flow velocities and estimate corresponding bed shear stresses and friction coefficients. Analysis is performed on a raw swash event to an elevation of 0.07 m and an ensemble event to an elevation of 0.02 m. Cross-shore velocities exceed 2 m s-1 and are of a similar magnitude during both the uprush and the backwash. The observed swash zone boundary layer is at least 0.07 m thick during the strongest flows and is well-represented by the logarithmic model applied to this elevation, except near flow reversal. However, applying the logarithmic model to various sections of the velocity profile suggests that the boundary layer is only logarithmic to a mean elevation of 0.01-0.015 m. Maximum bed shear stresses estimated using the logarithmic model are 24.79 N m-2 and 15.25 N m-2 for the raw event and ensemble event respectively. Bed shear stresses are generally larger during the backwash than the uprush. Mean friction coefficients estimated from equating the logarithmic model and the quadratic drag law are 0.018 and 0.019 for the raw event and ensemble event respectively. Bed shear stresses and friction coefficients are both larger if the logarithmic model is fit only to the part of the profile that is known to be logarithmic, emphasizing the need for high resolution velocity profiles close to the bed for accurate sediment transport predictions on the foreshore

Infragravity waves: from driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)