Turbulence modulation in non-uniform and unsteady clay suspension flows

Cohesive sediment is common within natural environments, such as rivers, estuaries, shallow seas and deep oceans. High-magnitude, low-frequency events, such as storms, floods, and post-wildfire erosion, which occur more often due to climate change, can enhance the cohesive sediment supply to rivers. The higher sediment transport rates can have major impacts on water quality and aquatic ecosystems, including fish habitats, and also on channel morphology. Further, subaqueous sediment gravity currents are among the volumetrically most important sediment transport processes on Earth.In contrast to non-cohesive sediments, suspended cohesive sediment particles may form larger particles, or flocs, when the distance between the particles is sufficiently small. Networks of flocs in the flow, i.e., clay gels, enhance viscosity and yield stress, and thus are a key control on flow turbulence. Research into steady, uniform clay flows indicates a close interaction between turbulent and cohesive forces, controlling the dynamic structure of clay flows.Subaqueous sediment gravity currents can be classified into different flow types, similar to a clay flow classification scheme of open-channel flows. On one side of the spectrum, turbidity currents are relatively dilute flows, in which the particles are supported by the upward component of fluid turbulence; correlated to turbulent flows with characteristics of a turbulent, wall-bounded shear flow for open-channel flows. On the other side of the spectrum, debris or mud flows have limited internal turbulence and cohesive sediment provides grain support through yield strength; correlated to an almost fully suppressed turbulence in quasi-laminar plug flows for open-channel flows. Transitional flows bridge the gap between turbidity currents, i.e. turbulent flows, and debris flows, i.e. quasi-laminar plug flow. These transitional flow types contain transient turbulent behaviour.The transitional flow properties of clay-laden flow are based on steady, uniform open-channel flow. However, open-channel flows and gravity currents are naturally non-uniform, i.e. varying in space, or unsteady, i.e. varying in time. The formation of bonds between cohesive sediment particles is a time-dependent (thixotropic) process and, therefore, clay-laden flows need time to adjust to spatial or temporal variations in flow velocity. Despite clay being the most abundant sediment type on earth, the present knowledge on turbulent dynamics of non-uniform or unsteady clay-laden flows remains limited, whereas it directly influences the sediment transport capacity.To research the influence of suspended cohesive clay on changing flow dynamics under non-uniform flow conditions, new experiments were conducted using decelerating and accelerating clay suspension open-channel flows in a recirculating flume. The flows transition between clay flow types, with different degrees of turbulence enhancement and attenuation as the flow adapts to the change in velocity. The experimental results show that decelerating clay suspension flows have a longer adaptation time than accelerating clay suspension flows, which is potentially correlated to the formation of clay bonds in decelerating flows requiring more time than the breakage of clay bonds in accelerating flows. This hysteresis is more pronounced for higher concentration decelerating flows that pass through a larger variety of flow phases of turbulence enhancement and attenuation.To research the adaptation time of clay suspension flows to unsteady flow conditions, new experiments were conducted using unsteady clay suspension open-channel flows. The flow velocity was adjusted with increments of 0.1 m/s after which the adaptation time to reach the equilibrium conditions was quantified. The experimental results show that accelerating, strongly turbulence attenuated, clay flows require more time than weakly or non-turbulence attenuated clay flows due to the time required for turbulence to penetrate the plug flow in order to break the clay bonds. Relative to turbulent flows, the adaptation time of decelerating flows is reduced as the flow evolves through clay flow types as the increase in clay concentration allows for a higher frequency of inter-particle collision. Adaptation time is then increased again with stronger turbulence attenuated flows as the increasingly dominant cohesive forces reduce inter-particle collision and consequently the formation of clay bonds takes longer.To research the evolvement of non-uniform clay-laden gravity currents, new experiments with constant-flux flows in a submerged flume were conducted. Based on velocity measurements three different flow types are identified with increasing turbulence attenuation: a) turbidity current, b) turbulent plug flow and c) transitional plug flow. The combination of balance between turbulent and cohesive forces and the formation of clay bonds determines the evolvement of a gravity current. For low clay concentrations, entrainment of ambient water and the additional turbulence developed at the upper interface penetrates into the gravity current preventing the formation of clay bonds. Consequently, the gravity current evolves towards a more turbulent flow condition. For high clay concentrations, entrainment and turbulence generation remains in the outer region allowing the formation of clay bonds in the inner region. Consequently, the gravity current evolved towards a less turbulent flow condition as a plug flow develops in the inner region towards the bed.Turbulent flow dynamics have a direct influence on sediment erosion, transport and deposition patterns. Research into non-uniform and unsteady clay suspension flows indicates a complex interplay in cohesive and turbulent forces as the flow adapts to changes in velocity, which likely affects erosional and depositional processes in a variety of fluvial and submarine settings

Inadequacy of fluvial energetics for describing gravity current autosuspension

Gravity currents, such as sediment-laden turbidity currents, are ubiquitous natural flows that are driven by a density difference. Turbidity currents have provided vital motivation to advance understanding of this class of flows because their enigmatic long run-out and driving mechanisms are not properly understood. Extant models assume that material transport by gravity currents is dynamically similar to fluvial flows. Here, empirical research from different types of particle-driven gravity currents is integrated with our experimental data, to show that material transport is fundamentally different from fluvial systems. Contrary to current theory, buoyancy production is shown to have a non-linear dependence on available flow power, indicating an underestimation of the total kinetic energy lost from the mean flow. A revised energy budget directly implies that the mixing efficiency of gravity currents is enhanced

The Prognostic Accuracy of Clinical Judgment Versus a Validated Frailty Screening Instrument in Older Patients at the Emergency Department: Findings of the AmsterGEM Study

STUDY OBJECTIVE: To compare the prognostic accuracy of clinical judgment for frailty in older patients at the emergency department with a validated screening instrument and patient-perceived frailty. METHODS: A prospective cohort study in patients 70 years of age and older in 2 Dutch EDs with a follow-up of 3 months. A dichotomous question was asked to the physician and patient: "Do you consider the patient / yourself to be frail?" The Identification of Seniors At Risk-Hospitalized Patients (ISAR-HP) was used as a validated screening instrument. The primary composite outcome consisted of either functional decline, institutionalization, or mortality. RESULTS: A total of 736 patients were included. The physician identified 59% as frail, compared with 49% by ISAR-HP and 43% by patients themselves. The level of agreement was fair (Fleiss Kappa, 0.31). After 3 months, 31% of the patients experienced at least 1 adverse health outcome. The sensitivity was 79% for the physician, 72% for ISAR-HP, 61% for the patient, and 48% for all 3 combined. The specificity was 50% for the physician, 63% for ISAR-HP, 66% for the patient, and 85% for all 3 positive. The highest positive likelihood ratio was 3.03 (physician, ISAR-HP, patient combined), and the lowest negative likelihood ratio was 0.42 (physician). The areas under the receiver operating curves were all poor: 0.68 at best for ISAR-HP. CONCLUSION: Clinical judgment for frailty showed fair agreement with a validated screening instrument and patient-perceived frailty. All 3 instruments have poor prognostic accuracy, which does not improve when combined. These findings illustrate the limited prognostic value of clinical judgment as a frailty screener in older patients at the ED

The Accuracy of Four Frequently Used Frailty Instruments for the Prediction of Adverse Health Outcomes Among Older Adults at Two Dutch Emergency Departments: Findings of the AmsterGEM Study: Findings of the AmsterGEM Study

Study objective: Older adults presenting to the emergency department (ED) are at high risk of adverse health outcomes. This study aimed to evaluate the accuracy of 4 frequently used screening instruments for the prediction of adverse health outcomes among older adults in the ED. Methods: This was a prospective cohort study in patients ≥70 years of age presenting to the ED in 2 hospitals in the Netherlands. Screening instruments included the acutely presenting older patient screening program (APOP) (providing 2 risk scores—functional decline [APOP1] and mortality [APOP2]), the International Resident Assessment Instrument Emergendy Department screener (InterRAI ED), the Identification of Seniors At Risk-Hospitalized Patients (ISAR-HP), and the safety management system (VMS). The primary outcome measure was a composite outcome encompassing functional decline, institutionalization, and mortality at 3 months after ED presentation. Other follow-up time points were 1 and 6 months. Analyses were performed to assess prognostic accuracy. Results: In total, 889 patients were included. After 3 months, 267 (31%) patients experienced at least 1 adverse outcome. The positive likelihood ratio ranged from 1.67 (VMS) to 3.33 (APOP1), and the negative likelihood ratio ranged from 0.41 (ISAR-HP) to 0.88 (APOP2). Sensitivity ranged from 17% (APOP2) to 74% (ISAR-HP), and specificity ranged from 63% (ISAR-HP) to 94% (APOP2). The area under the curve ranged from 0.62 (APOP2) to 0.72 (APOP1 and ISAR-HP). Calibration was reasonable for APOP1 and VMS. The prognostic accuracy was comparable across all outcomes and at all follow-up time points. Conclusion: The frailty screening instruments assessed in this study showed poor to moderate prognostic accuracy, which brings into question their usability in the prediction of adverse health outcomes among older adults who present to the ED

Inadequacy of fluvial energetics for describing gravity current autosuspension

Gravity currents, such as sediment-laden turbidity currents, are ubiquitous natural flows that are driven by a density difference. Turbidity currents have provided vital motivation to advance understanding of this class of flows because their enigmatic long run-out and driving mechanisms are not properly understood. Extant models assume that material transport by gravity currents is dynamically similar to fluvial flows. Here, empirical research from different types of particle-driven gravity currents is integrated with our experimental data, to show that material transport is fundamentally different from fluvial systems. Contrary to current theory, buoyancy production is shown to have a non-linear dependence on available flow power, indicating an underestimation of the total kinetic energy lost from the mean flow. A revised energy budget directly implies that the mixing efficiency of gravity currents is enhanced

A narrative review of frailty assessment in older patients at the emergency department

Optimizing emergency care for the aging population is an important future challenge, as the proportion of older patients at the emergency department (ED) rapidly increases. Older patients, particularly those who are frail, have a high risk of adverse outcomes after an ED visit, such as functional decline, institutionalization, and death. The ED can have a key position in identifying frail older patients who benefit most from comprehensive geriatric care [including delirium preventive measures, early evaluation of after-discharge care, and a comprehensive geriatric assessment (CGA)]. However, performing extensive frailty assessment is not suitable at the ED. Therefore, quick and easy-to-use instruments are needed to identify older patients at risk for adverse outcomes. This narrative review outlines the importance and complexity of frailty assessment at the ED. It aligns the available screening instruments, including clinical judgment as frailty assessment, and summarizes arguments for and against frailty assessment at the ED

A narrative review of frailty assessment in older patients at the emergency department

Optimizing emergency care for the aging population is an important future challenge, as the proportion of older patients at the emergency department (ED) rapidly increases. Older patients, particularly those who are frail, have a high risk of adverse outcomes after an ED visit, such as functional decline, institutionalization, and death. The ED can have a key position in identifying frail older patients who benefit most from comprehensive geriatric care [including delirium preventive measures, early evaluation of after-discharge care, and a comprehensive geriatric assessment (CGA)]. However, performing extensive frailty assessment is not suitable at the ED. Therefore, quick and easy-to-use instruments are needed to identify older patients at risk for adverse outcomes. This narrative review outlines the importance and complexity of frailty assessment at the ED. It aligns the available screening instruments, including clinical judgment as frailty assessment, and summarizes arguments for and against frailty assessment at the ED