On the behaviour of tidal current directions due to the presence of submarine sand waves

Side-scan sonar records reveal that the crestlines of megaripples in the troughs as well as on both sides of the slopes of sand waves form an angle with the crestline of the sand waves itself. The existence of such an angle between the crestline of megaripples and sand waves indicates that changes of the (tidal) current direction across sand waves can be expected. Direct evidence that such (tidal) current direction changes occur above sand waves is derived from measurements of the Air-Sea Interaction Drift Buoy (ASIB) system at the sea surface and from side-scan sonar records of the sea bed. Both systems were operated from on hoard research vessels during two C-STAR field experiments in the Hoek van Holland study area off the Dutch coast in April 1996 and in April 1997. Measurements performed by electrical resistance wires on board the ASIB system indicate that variations of the short and moderate wave direction and the wave directional spread are associated with changes of the (tidal) current direction at the sea surface. According to the results of the measurements derived from the ASIB system and the side-scan sonar records it can be summarised that the variation of the direction of short and moderate period water waves as well as changes of the (tidal) current speed and direction across large sand waves must be considered for correct modelling of the wave-current interaction mechanism

On the behaviour of hydrodynamic processes due to the presence of submarine sand waves

Radar signatures of the sea bed in coastal waters show that submarine sand waves superimposed on sandbanks or tidal current ridges change their orientation and character abruptly at the crest of the ridge. These observations were made when studying air- and spaceborne radar images of the southern North Sea (McLeish et al., 1981). Similar phenomena were already reported by analysing side-scan sonar records from the large sandbanks in the North Sea (Houbolt, 1968). Such observations could be evidence for changes of tidal current direction as the tidal flow approaches the crest of the sand ridge. Sand waves are flow-transverse bedforms which are oriented more or less perpendicular to the maximum tidal current velocity. Furthermore, several side-scan sonar records reveal megaripples on both sides of sand waves. The ripple height generally increases in the direction of the sand wave crest. Side-scan sonar records also reveal that the crestlines of megaripples in the troughs as well as on both sides of the slopes of the sand waves form a maximum angle of 450 with the crestline of the sand waves itself. The existence of such an angle between the crestline of megaripples and sand waves indicates that changes of the tidal current direction across sand waves can be expected. Direct evidence that such tidal current direction changes occur above sand waves is derived from measurements of the Air-Sea Interaction Drift Buoy (ASIB) system at the sea surface and from side-scan sonar records of the sea bed. Both systems were operated from on board research vessels during two C-STAR field experiments in the Hoek van Holland study area off the Dutch coast in April 1996 and in April 1997. Measurements performed by electrical resistance wires on board the ASIB system indicate that variations of the short and moderate wave direction and the wave directional spread are associated with changes of the tidal current direction at the sea surface. According to the results of the measurements derived from the ASIB system and the side-scan sonar records it can be summarized that the variation of the direction of short and moderate period water waves as well as changes of the tidal current direction across large sand waves must be considered for correct modelling of the wave-current interaction mechanism

Surveillance of COVID-19 mortality in Belgium, epidemiology and methodology during 1st and 2nd wave (March 2020 - 14 February 2021)

This report provides epidemiological figures about the characteristics of COVID-19 deaths during the first wave (1 March 2020 until 21 June 2020), the inter wave period (22 June until 30 August 2020) and the second wave (31 August 2020 until 14 February 2021) of the COVID-19 epidemic in Belgium. This is the period before the effects of the nationwide vaccination campaign that started early in 2021 could be assessed. In total 21,860 COVID-19 deaths occurred (43.9% in the first wave and 54.7% in the second wave). The COVID-19 mortality surveillance system was implemented at the start of the epidemic to acquire real-time COVID-19 mortality data on a daily basis. The surveillance combined information on COVID-19 related deaths from three surveillances (the hospital surveillance, the nursing home (NH) surveillance and notifications to regional health inspection authorities) through nine data sources. This information included the date of death, date of birth, sex, case classification, type of place of death, type of place of residence (e.g. living in a NH), postal code of the place of death and residence. Continuous improvements as regards the data collection resulted in retrograde adaptations of mortality&nbsp;numbers. The overall sex distribution was fairly even (49.1% in male and 50.8% in female). Almost all deaths occurred in the age group over 64 years and approximately half of the deaths occurred in the age group over 84 years.&nbsp;Data on hospitalized COVID-19 patients showed that higher age, male sex and several comorbidities such as cardiovascular disease and diabetes were risk factors for mortality. Additionally, the estimated COVID-19 case fatality in Belgium confirmed that it was higher for the elderly and male population. In the second wave, more deaths occurred in hospitals (61%) than in nursing homes for elderly (NHs) (38%). In contrast, during the first wave, this distribution was more equal (50% in hospitals and 49% in NHs). The test capacity increased and the testing strategy broadened over time, leading to an increase in the proportion of laboratory-confirmed COVID-19 cases among deaths (69% and 95% in the first and second waves respectively).&nbsp;COVID-19 age-standardized mortality rates (ASMR), which take into account the age distribution of the population, showed that Brussels presented the highest ASMR for the total period and the first wave, while Wallonia has the highest ASMR for the second wave (more precisely in the provinces of Hainaut and Liège). The crude COVID-19 mortality rates for residents of NHs were higher in Flanders than in the other regions, both for the total period and for the second&nbsp;wave. International comparison and ranking of COVID-19 crude mortality rates are misleading because of very heterogeneous methods used (e.g. case definition, testing and screening strategy, reporting method, availability of specific surveillance in NHs, etc.). Methods might also have changed during the course of the epidemic within the same country. A better comparison will probably be possible when countries have finished analyzing the official death certificates. The fast initiation of the COVID-19 surveillance in NHs and the inclusion of deaths of possible COVID-19 cases nevertheless allowed Belgium to provide accurate figures on COVID-19 deaths. This helped to assess the seriousness of the epidemiological situation in NHs. COVID-19 mortality was strongly correlated with excess all-cause mortality in Belgium. The excess mortality was a key indicator in the COVID-19 epidemic to validate that the epidemiological reporting of COVID-19-related mortality was correctly conducted during the&nbsp;epidemic</p

Excess mortality during the first and second waves of the COVID-19 epidemic in Belgium

COVID-19 mortality was highly correlated with excess all-cause mortality during the first two waves of the epidemic. Epidemiologic surveillance of COVID-19 deaths was accurately conducted during the epidemic and it is even likely that COVID-19 deaths were underreported during the ascending phases of excess mortality by 1,193 deaths. The first wave of the epidemic has a larger excess mortality than the second wave. People aged 85 and over were the most affected during the two periods of excess mortality. The year 2020 has a 17.5% excess mortality with 18,765 additional deaths, eight times the average excess mortality of the past five years. In the absence of the implementation of restrictive measures (e.g., social distancing, etc.) and non-pharmaceutical interventions (e.g., hand hygiene, personal protective equipment, etc.), it is possible that the excess mortality during this period would have been greater. Mortality analysis encompassing entire winter seasons is more accurate for flu, which often spreads over several winter months, spread over two calendar years. It provides very different results than a typical annual analysis from January to December. Even in the 21st century, epidemics of respiratory infectious diseases can be major lethal events of rapid onset in a susceptible and vulnerable&nbsp;population</p

Oversterfte tijdens de eerste en tweede golf van COVID-19-epidemie in België

De COVID-19-sterfte was sterk gecorreleerd met de oversterfte door alle oorzaken tijdens de eerste twee golven van de epidemie. Epidemiologische surveillance van COVID-19- sterfgevallen werd nauwkeurig uitgevoerd. Het is waarschijnlijk dat COVID-19-sterfgevallen werden ondergerapporteerd met 1193 sterfgevallen tijdens de toenemende fasen van de oversterfte . De eerste golf van de epidemie kende een grotere oversterfte dan de tweede golf. Personen van 85 jaar en ouder werden het zwaarst getroffen tijdens de twee perioden van oversterfte. Het jaar 2020 vertoont een oversterfte van 17,5% met 18 765 extra sterfgevallen, acht keer de gemiddelde oversterfte van de afgelopen vijf jaar. Zonder de toepassing van beperkende maatregelen (b.v. sociale distantie, enz.) en niet-farmaceutische interventies (b.v. handhygiëne, persoonlijke beschermingsmiddelen, enz.) is het mogelijk dat de oversterfte gedurende deze periode groter zou zijn geweest. Analyses van mortaliteit die hele winterseizoenen omvatten zijn nauwkeuriger voor griep, gezien de jaarlijkse griepepidemie zich vaak over meerdere wintermaanden en twee kalenderjaren verspreidt. Dit levert heel andere resultaten op dan de typische jaarlijkse analyses van januari tot december. Zelfs in de 21e eeuw kunnen epidemieën van luchtweginfecties grote dodelijke gebeurtenissen zijn met een snel begin bij een vatbare en kwetsbare&nbsp;bevolking.</p