242 research outputs found
Flood risk analysis for the river Scheldt estuary
This report gives an overview of the history of flood risk management along the Scheldt estuary. Thepurpose of this overview is twofold. It documents the results obtained in the first activity of the Task25 Pilot, being focused on flood risk analysis, both in historical perspective and with regard to currentpolicies. Secondly, it provides an introduction to the Scheldt estuary for other partners in theFLOODsite project which will use the Scheldt Pilot for their own tasks and activities.The Scheldt estuary is a clear example where integrated water management is required. Integratedmeans in this case: meeting the objectives for 3 major basic functions and values of the estuary: safety,accessibility (navigation) and ecology. Recently finalised and ongoing projects, such as Proses and theSIGMA plan show a highly multidisciplinary approach in order to fulfil these objectives.From the perspective of flood risk management we see a significant difference in approach betweenBelgium and the Netherlands. The Belgian New Sigma Plan has used an approach that is strikinglysimilar to the FLOODsite methodology, i.e. going through the three phases Risk Analysis, RiskAssessment and Flood Management Strategy formulation. In contrast, the Dutch flood riskmanagement was developed over a long historical period, with the Delta Plan as the final apotheosis.In the time the Delta Plan was formulated, there was hardly any attention and technology available todetermine flood probability and flood damage
Adaptation of river channels to a wetter or drier climate: Insights from the Lower Pilcomayo River, South America
Climate change has a direct influence on both hydrology and floodplain vegetation of water courses, which are key players in river morphodynamics. The river system response to climate change is complex and the effects of non-linear interactions between alterations in water, sediment and vegetation remain incompletely understood. Which of these components becomes dominant in shaping the river channel when climate becomes drier or wetter? To answer this question, we investigate the cross-sectional response of sand-bed rivers to climate change focusing on channel width and depth, which respond directly to changes in boundary stresses. Thanks to the exceptional availability of long time-series of daily discharge and cross-sectional profiles, the Pilcomayo River is an ideal living lab for this investigation. We constructed a two-dimensional model of the river using the open-source state-of-the-art, structured Delft3D code. The highly dynamic behaviour and quick morphological adaptation of the Pilcomayo allowed reducing the period of time covered by the simulations because the river cross-section adapts its morphology to a new value of the water discharge within hours or days, which is crucial for modelling investigations. Calibration and validation were successfully performed by comparison with historical data. We considered several scenarios representing current, dryer and wetter climates. The results show that a dryer climate reduces the river channel depth and enlarges the width. A wetter climate increases the channel depth but produces negligible widening. Vegetation, sparser with a drier climate and denser with a wetter climate, is found to control the channel width. This analysis is unique and shows which alterations can be expected in alluvial sand-bed rivers with natural vegetated banks due to climate change
Bank erosion processes measured with UAV-SfM along complex banklines of a straight mid-sized river reach
We apply structure from motion (SfM) photogrammetry with imagery from an
unmanned aerial vehicle (UAV) to measure bank erosion processes along a
mid-sized river reach. This technique offers a unique set of characteristics
compared to previously used methods to monitor banks, such as high resolution
and relatively fast deployment in the field. We analyse the retreat of a
1.2 km restored bank of the Meuse River which has complex vertical scarps
laying on a straight reach, features that present specific challenges to the
UAV-SfM application. We surveyed eight times within a year with a simple
approach, combining different photograph perspectives and overlaps to
identify an effective UAV flight. The accuracy of the digital surface models
(DSMs) was evaluated with real-time kinematic (RTK) GPS points and airborne
laser scanning of the whole reach. An oblique perspective with eight photo
overlaps and 20 m of cross-sectional ground-control point distribution was
sufficient to achieve the relative precision to observation distance of ââŒâ1 : 1400 and 3 cm root mean square error (RMSE), complying with the
required accuracy. A complementary nadiral view increased coverage behind
bank toe vegetation. Sequential DSMs captured signatures of the erosion cycle
such as mass failures, slump-block deposition, and bank undermining. Although
UAV-SfM requires low water levels and banks without dense vegetation as many
other techniques, it is a fast-in-the-field alternative to survey reach-scale
riverbanks in sufficient resolution and accuracy to quantify bank retreat and
identify morphological features of the bank failure and erosion processes.
Improvements to the adopted approach are recommended to achieve higher
accuracies.</p
Modeling meander morphodynamics over self-formed heterogeneous floodplains
This work addresses the signatures embedded in the planform geometry of meandering rivers consequent to the formation of floodplain heterogeneities as the river bends migrate. Two geomorphic features are specifically considered: scroll bars produced by lateral accretion of point bars at convex banks and oxbow lake fills consequent to neck cutoffs. The sedimentary architecture of these geomorphic units depends on the type and amount of sediment, and controls bank erodibility as the river impinges on them, favoring or contrasting the river migration. The geometry of numerically generated planforms obtained for different scenarios of floodplain heterogeneity is compared to that of natural meandering paths. Half meander metrics and spatial distribution of channel curvatures are used to disclose the complexity embedded in meandering geometry. Fourier Analysis, Principal Component Analysis, Singular Spectrum Analysis and Multivariate Singular Spectrum Analysis are used to emphasize the subtle but crucial differences which may emerge between apparently similar configurations. A closer similarity between observed and simulated planforms is attained when fully coupling flow and sediment dynamics (fully-coupled models) and when considering self-formed heterogeneities that are less erodible than the surrounding floodplain
Long-duration laboratory experiment of slow development of steady alternate bars
River morphodynamics and sediment transportRiver morphology and morphodynamic
Influence of bethanechol on salivary parameters in irradiated patients
Some studies have shown evidence that the prophylactic use of bethanechol chloride (BC) may be useful in preventing the incidence and/or severity of xerostomia (XT). However, the indication of BC in irradiated patients with XT needs to be better characterized. The study aimed to evaluate the influence of BC on XT, salivary flow rate, and salivary composition in patients previously submitted to head and neck radiotherapy. Forty five irradiated patients complaining of XT used 50 mg/day of BC for 3 months, and the salivary parameters were evaluated in 4 Phases (Before BC therapy, after one month of BC, 2 months of BC, and 3 months of BC). Biochemical analysis included buffering capacity; pH; total protein concentration (TP); amylase concentration (AM); catalase (CAT) and peroxidase (PX) activities. In addition, unstimulated and stimulated salivary flow rates were determined and XT was classified. According to the XT grading system used, patients showed improvement in XT between Phase 1, and Phases 2, 3 and 4. In addition, some changes were observed in TP concentration (decreased); AM concentration (increased); and PX and CAT activities (decreased and increased, respectively) after Phase 2, for stimulated saliva collection (p<0.05). Our results suggested that when BC was used to treat salivary gland dysfunction induced by head and neck radiotherapy, improvement in XT symptoms, and some changes in saliva composition were shown
Effects of sediment flushing operations versus natural floods on Chinook salmon survival
Flushing is a common measure to manage and reduce the amount of sediment stored in reservoirs. However, the sudden release of large volumes of sediment abruptly increases the suspended solids concentration and alters the riverbed composition. Similar effects can be produced also by natural flood events. Do flushing operations have more detrimental impacts than natural floods? To answer this question, we investigated the impact of flushing on the survival of the Chinook salmon () in the Sandy River (OR, USA), assuming that sediment is flushed from hypothetical bottom gates of the, now decommissioned, Marmot Dam. The effects of several flushing scenarios are analyzed with a 2D morphodynamic model, together with habitat suitability curves and stress indicators. The results show that attention has to be paid to duration: the shorter the flushing operation, the lesser the stresses on fish survival and spawning habitats. Flushing causes high stress to salmon eggs and larvae, due to unbearable levels of suspended sediment concentrations. It also decreases the areas usable for spawning due to fine-sediment deposition, with up to 95% loss at peak flow. Without the dam, the corresponding natural flood event would produce similar effects, with up to 93% loss. The study shows that well-planned flushing operations could mimic a natural impact, but only partly. In the long-term, larger losses of spawning grounds can be expected, since the removal of fine sediment with the release of clear water from the reservoir is a lengthy process that may be undesirable due to water storage reduction
Sediment Transport of Fine Sand to Fine Gravel on Transverse Bed Slopes in Rotating Annular Flume Experiments
Largeâscale morphology, in particular meander bend depth, bar dimensions, and bifurcation dynamics, are greatly affected by the deflection of sediment transport on transverse bed slopes due to gravity and by secondary flows. Overestimating the transverse bed slope effect in morphodynamic models leads to flattening of the morphology, while underestimating leads to unrealistically steep bars and banks and a higher braiding index downstream. However, existing transverse bed slope predictors are based on a small set of experiments with a minor range of flow conditions and sediment sizes, and in practice models are calibrated on measured morphology. The objective of this research is to experimentally quantify the transverse bed slope effect for a large range of nearâbed flow conditions with varying secondary flow intensity, sediment sizes (0.17â4 mm), sediment transport mode, and bed state to test existing predictors. We conducted over 200 experiments in a rotating annular flume with counterrotating floor, which allows control of the secondary flow intensity separate from the streamwise flow velocity. Flow velocity vectors were determined with a calibrated analytical model accounting for rough bed conditions. We isolated separate effects of all important parameters on the transverse slope. Resulting equilibrium transverse slopes show a clear trend with varying sediment mobilities and secondary flow intensities that deviate from known predictors depending on Shields number, and strongly depend on bed state and sediment transport mode. Fitted functions are provided for application in morphodynamic modelin
Limits to scale invariance in alluvial rivers
Assumptions about fluvial processes and processâform relations are made in general models and in many siteâspecific applications. Many standard assumptions about reachâscale flow resistance, bedâmaterial entrainment thresholds and transport rates, and downstream hydraulic geometry involve one or other of two types of scale invariance: a parameter (e.g. critical Shields number) has the same value in all rivers, or doubling one variable causes a fixed proportional change in another variable in all circumstances (e.g. powerâlaw hydraulic geometry). However, rivers vary greatly in size, gradient, and bed material, and many geomorphologists regard particular types of river as distinctive. This review examines the tension between universal scaling assumptions and perceived distinctions between different types of river. It identifies limits to scale invariance and departures from simple scaling, and illustrates them using large data sets spanning a wide range of conditions. Scaling considerations and data analysis support the commonly made distinction between coarseâbed and fineâbed reaches, whose different transport regimes can be traced to the different settlingâvelocity scalings for coarse and fine grains. They also help identify two endâmember subâtypes: steep shallow coarseâbed âtorrentsâ with distinctive flowâresistance scaling and increased entrainment threshold, and very large, lowâgradient âmega riversâ with predominantly suspended load, subdued secondary circulation, and extensive backwater conditions
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