Man-induced regime shifts in small estuaries - II: a comparison of rivers

This is Part II of two papers on man-induced regime shifts in small, narrow, and converging estuaries, with focus on the interaction between effective hydraulic drag, fine sediment import, and tidal amplification, induced by river engineering works, e.g., narrowing and deepening. Paper I describes a simple linear analytical model for the tidal movement in narrow, converging estuaries and a conceptual model on the response of tidal rivers to river engineering works. It is argued that such engineering works may set in motion a snowball effect bringing the river into an alternative steady state. Part II analyses the historic development in tidal range in four rivers, e.g., the Elbe, Ems, Loire, and Scheldt, all in northwest Europe; data are available for many decades, up to a century. We use the analytical model derived in Part I, showing that the effective hydraulic drag in the Ems and Loire has decreased considerably over time, as anticipated in Part I. We did not find evidence that the Upper Sea Scheldt is close to its tipping point towards hyperturbid conditions, but risks have been identified. In the Elbe, tidal reflections against the profound step in bed level around Hamburg seem to have affected the tidal evolution in the last decades. It is emphasized that the conceptual picture sketched in these papers is still hypothetical and needs to be validated, for instance through hind-cast modeling of the evolution of these rivers. This will not be an easy task, as historical data for a proper calibration of the models required are scarce

How progressive vegetation die-off in a tidal marsh would affect flow and sedimentation patterns : a field demonstration

Coastal marshes provide valuable ecosystem functions, but some are facing increasing risks of vegetation loss due to sea level rise and other stressors. A key question is how tidal flow and sedimentation patterns are affected by the spatiotemporal patterns of vegetation loss, as sediment accretion with sea level rise largely affects the potential for marsh recovery. Here, we performed a field study in a macrotidal reed marsh and simulated typical spatiotemporal patterns of vegetation loss by consecutive mowing. For each mowing pattern, the spatial patterns of flow velocities and sedimentation rates were recorded. Our results indicate that initial vegetation loss in inner marsh portions, with an intact vegetation belt alongside channel edges, has limited effect on tidal flows over the marsh. However, subsequent creation of unvegetated corridors connecting the bare inner marsh and the channels increases flow velocities in these corridors but not in remaining vegetation patches. Finally, when all vegetation is removed, sheet flow occurs over the whole marsh instead of concentrated channel flow. Effects on spatial sedimentation patterns are complex and not significant on all measuring locations. Nevertheless, our study indicates that complete vegetation removal results in redistributed sedimentation patterns, with a general tendency of locally reduced sedimentation rates close (< 15 m) to channels and increased sediment supply to inner marshes 15–50 m from channels. Our results highlight that feedbacks between spatial patterns of vegetation loss, tidal sediment transport, and deposition are key to understanding and mitigating risks of marsh loss in face of sea level rise

How progressive vegetation die-off in a tidal marsh would affect flow and sedimentation patterns : A field demonstration

Coastal marshes provide valuable ecosystem functions, but some are facing increasing risks of vegetation loss due to sea level rise and other stressors. A key question is how tidal flow and sedimentation patterns are affected by the spatiotemporal patterns of vegetation loss, as sediment accretion with sea level rise largely affects the potential for marsh recovery. Here, we performed a field study in a macrotidal reed marsh and simulated typical spatiotemporal patterns of vegetation loss by consecutive mowing. For each mowing pattern, the spatial patterns of flow velocities and sedimentation rates were recorded. Our results indicate that initial vegetation loss in inner marsh portions, with an intact vegetation belt alongside channel edges, has limited effect on tidal flows over the marsh. However, subsequent creation of unvegetated corridors connecting the bare inner marsh and the channels increases flow velocities in these corridors but not in remaining vegetation patches. Finally, when all vegetation is removed, sheet flow occurs over the whole marsh instead of concentrated channel flow. Effects on spatial sedimentation patterns are complex and not significant on all measuring locations. Nevertheless, our study indicates that complete vegetation removal results in redistributed sedimentation patterns, with a general tendency of locally reduced sedimentation rates close (< 15 m) to channels and increased sediment supply to inner marshes 15–50 m from channels. Our results highlight that feedbacks between spatial patterns of vegetation loss, tidal sediment transport, and deposition are key to understanding and mitigating risks of marsh loss in face of sea level rise

How progressive vegetation die-off in a tidal marsh would affect flow and sedimentation patterns: A field demonstration

Coastal marshes provide valuable ecosystem functions, but some are facing increasing risks of vegetation loss due to sea level rise and other stressors. A key question is how tidal flow and sedimentation patterns are affected by the spatiotemporal patterns of vegetation loss, as sediment accretion with sea level rise largely affects the potential for marsh recovery. Here, we performed a field study in a macrotidal reed marsh and simulated typical spatiotemporal patterns of vegetation loss by consecutive mowing. For each mowing pattern, the spatial patterns of flow velocities and sedimentation rates were recorded. Our results indicate that initial vegetation loss in inner marsh portions, with an intact vegetation belt alongside channel edges, has limited effect on tidal flows over the marsh. However, subsequent creation of unvegetated corridors connecting the bare inner marsh and the channels increases flow velocities in these corridors but not in remaining vegetation patches. Finally, when all vegetation is removed, sheet flow occurs over the whole marsh instead of concentrated channel flow. Effects on spatial sedimentation patterns are complex and not significant on all measuring locations. Nevertheless, our study indicates that complete vegetation removal results in redistributed sedimentation patterns, with a general tendency of locally reduced sedimentation rates close (< 15 m) to channels and increased sediment supply to inner marshes 15–50 m from channels. Our results highlight that feedbacks between spatial patterns of vegetation loss, tidal sediment transport, and deposition are key to understanding and mitigating risks of marsh loss in face of sea level rise

Baseline chest computed tomography for diagnosis of invasive aspergillosis in patients with acute myeloid leukaemia treated with intensive chemotherapy:A retrospective single-centre cohort study

Background: Invasive pulmonary aspergillosis (IPA) is a relatively common infection in patients with acute myeloid leukaemia (AML), and is associated with high mortality rates. Optimising early detection is key to reduce the burden of IPA in this population. In this retrospective cohort study, we evaluated the added value of baseline chest CT before start of classical induction chemotherapy. Methods: Adult patients receiving first-line intensive chemotherapy for AML were included if a baseline chest CT scan was available (±7 days). Data were collected from the electronic health record. IPA was classified using the EORTC/MSGERC 2020 consensus definitions. Results:Between 2015 and 2019, 99 patients were included. During first-line treatment, 29/99 (30%) patients developed a probable IPA. Baseline chest CT was abnormal in 61/99 (62%) and 14/61 (23%) patients had typical radiological signs for IPA. An abnormal scan showed a trend towards higher risk for IPA (hazard ratio (HR): 2.12; 95% CI 0.95–4.84). Ground glass opacities were a strong predictor for developing IPA (HR 3.35: 95% CI 1.61–7.00). No probable/proven IPA was diagnosed at baseline; however, a bronchoalveolar lavage (BAL) at baseline was only performed in seven patients. Twelve-week mortality was higher in patients with IPA (7/26, 27% vs. 5/59, 8%; p =.024). Conclusion: Baseline chest CT scan could be an asset in the early diagnosis of IPA and contribute to risk estimation for IPA. In patients with an abnormal baseline CT, performing a BAL should be considered more frequently, and not only in patients with radiological findings typical for IPA.</p

Historische evolutie van zeescheldehabitats:kwantitatieve en kwalitatieve analyse van invloedsfactoren

In een eerste historische analyse van de ingreep-effectrelaties op de Zeeschelde (Van Braeckel et al., 2006) werden data verzameld, ge&euml;valueerd naar kwaliteit en continu&iuml;teit en ecologisch ge&iuml;nterpreteerd. De eerste analyse van ingrepen, getij en evolutie van Zeescheldehabitats was vooral kwalitatief. Eenduidige oorzaak-gevolg relaties waren moeilijk te leggen omdat verschillende ingrepen gelijktijdig plaatsvonden en het relatief belang van autonome processen moeilijk in te schatten was. Kennishiaten werden ge&iuml;dentificeerd en een aantal onderzoeksvragen werden als stelling geformuleerd.&quot;br/&quot;In dit vervolgproject werd de systeemkennis verder uitgediept met nieuw verworven informatie: de tij-analyse werd meer in detail doorgevoerd en het historisch beeld van de ecotopen werd vervolledigd. Het doel van deze vervolgstudie was vooral om ingreep-effect relaties beter te kwantificeren.&quot;br/&quot;Met betrekking tot arealen werd vastgesteld dat slik- en schoroppervlakten langs de Zeeschelde en haar getijgebonden zijrivieren respectievelijk met 66% en 82% verminderden tussen 1850 en 2003. Sinds 1930 verdween ook 40% van het ondiep subtidaal. Zowel rechtstreekse habitatvernietiging als hydrodynamische en morfologische ontwikkelingen die voortschrijdende erosie veroorzaken, liggen hier aan de basis. Meer bepaald rechttrekkingen, inpolderingen, dijkwerken, zeespiegelstijging, wijziging en manipulatie van het debiet van de bovenafvoer, verruiming en verdieping van de vaargeul, vaargeulonderhoud en&quot;br/&quot;baggerstortwerken, zandwinning en morfologische aanpassingen worden steeds in die context vermeld.&quot;br/&quot;In deze studie wordt met behulp van numerieke modelleringen (1D en 2D modellen) getracht het effect van de verschillende ingrepen te kwantificeren. Het doel is om de relatieve bijdrage van rechttrekkingen, debietwijzigingen van de bovenafvoer, inpolderingen, verruimingen, zandwinning en de algemene zeespiegelstijging op de getij-indringing in het Schelde-estuarium te bepalen. Omdat de morfologische veranderingen van het Middelgat en het Gat van Ossenisse eveneens het getij be&iuml;nvloedden werden ook deze gemodelleerd in enkele bagger/stort-scenario‟s. Vertrekkend van modellen die de huidige topo-bathymetrie&quot;br/&quot;weergeven worden de ingrepen uit het verleden schematisch ongedaan gemaakt. Het resultaat is dus niet echt een reconstructie van het verleden maar eerder een &bdquo;hindcast‟ vanuit het heden. Door de resultaten van verschillende simulaties te vergelijken kunnen richting en grootteorde van het aandeel van de ingrepen op wijzigingen in de getij-variabelen begroot worden. Via deze getijkenmerken werd dan de potenti&euml;le impact op areaal en kwaliteit van habitats ingeschat.&quot;br/&quot;Een laagwater daling heeft de potentie in zich de slikoppervlakte te vergroten, althans indien de hoogtegradi&euml;nt niet te steil is. Op dezelfde wijze kan stijging van hoogwater potentieel schoruitbreiding met zich meebrengen op voorwaarde dat de zijdelingse ruimte voorhanden is. De werkelijke ontwikkeling van slikken en schorren ten gevolge van toenemend getijverschil is echter ook nog afhankelijk van sedimentvracht, hydro- en morfodynamiek en topografie. In de Zeeschelde is de beschikbaarheid van sediment geen beperkende factor. Door gebrek aan zijdelingse ruimte en luwten echter zal toename in het getijverschil zelden in duurzame slik- en schorwinst resulteren. Document type: Boo

High-resolution bed level changes in relation to tidal and wave forcing on a narrow fringing macrotidal flat : bridging intra-tidal, daily and seasonal sediment dynamics

Sediment dynamics in tidal flats, ranging from daily to seasonal timescales, are particularly relevant as they control key ecological and geomorphic processes that ultimately contribute to the long-term evolution of coastal and estuarine landscapes. Yet, insights into bed level changes, including the full range of relevant timescales from intra-tidal to daily and seasonal scales, are currently limited due to a lack of efficient methods to record high-resolution (1 year). Accordingly, this contribution intends to improve our understanding on spatio-temporal patterns of long-term (>1 year) high-resolution (daily) bed level dynamics in tidal flats in relation to the dominant hydrodynamic driving forces, namely tides and waves. Specifically, this study was conducted along two 200 m long cross-shore transects on an intertidal flat located in the macrotidal (>5 m tidal range) Scheldt estuary, Belgium. Results showed that daily bed level changes at the low tidal flat (i.e. 4.10 m below mean high tide level) were dominated by tidal currents, with a strong fortnightly (or spring-neap) signature, whereas wave activity was of secondary importance. Conversely, bed level changes in the high tidal flat (i.e. 0.65 m below mean high tide level) were almost exclusively dominated by wave activity. Additionally, seasonal deposition–erosion cycles that superimposed on the daily bed level changes were associated with the seasonality of wind wave activity and benthic biology. Analysis of wave and current-induced bed shear stresses at the respective locations confirmed this spatial variability of tidal-dominated sediment dynamics at the low tidal flat versus wave-dominance at the high tidal flat, and comparison with local critical bed shear stresses for sediment motion also revealed differences in morphological impacts of the hydrodynamics between the two transects. These distinctive responses of bed level dynamics across the tidal flat can be partly explained by the spatially varying sediment properties across the tidal flat and may be further mediated, on a seasonal time scale, by the growth of the algal mat with its effect on stabilizing the sediment bed. In view of the large spatial and temporal variability of internal and external forcing revealed in this study, comprehensive and detailed field measurements are even more necessary to understand and predict long-term bed level dynamics and related ecological implications in tidal flats

Estuaries as filters: the role of tidal marshes in trace metal removal.

Flux calculations demonstrate that many estuaries are natural filters for trace metals. Yet, the underlying processes are poorly investigated. In the present study, it was hypothesized that intertidal marshes contribute significantly to the contaminant filter function of estuaries. Trace metal concentrations and sediment characteristics were measured along a transect from the subtidal, over an intertidal flat and marsh to a restored marsh with controlled reduced tide. Metal concentrations in the intertidal and restored marsh were found to be a factor two to five higher than values in the subtidal and intertidal flat sediments. High metal concentrations and high accretion rates indicate a high metal accumulation capacity of the intertidal marshes. Overbank sedimentation in the tidal marshes of the entire estuary was calculated to remove 25% to 50% of the riverine metal influx, even though marshes comprise less than 8% of the total surface of the estuary. In addition, the large-scale implementation of planned tidal marsh restoration projects was estimated to almost double the trace metal storage capacity of the present natural tidal marshes in the estuary