Staff Publications

Staff Publications

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    'Staff publications' is the digital repository of Wageningen University & Research

    'Staff publications' contains references to publications authored by Wageningen University staff from 1976 onward.

    Publications authored by the staff of the Research Institutes are available from 1995 onwards.

    Full text documents are added when available. The database is updated daily and currently holds about 240,000 items, of which 72,000 in open access.

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    (En)gage the flow! : striking the balance in deltas under pressure
    Hoitink, A.J.F. - \ 2020
    Wageningen : Wageningen University & Research - ISBN 9789463955041 - 20
    Idealized Model for the Deflection of Sediment Into Lateral Branches of Lowland Rivers
    Kästner, K. ; Hoitink, A.J.F. - \ 2020
    Water Resources Research 56 (2020)6. - ISSN 0043-1397
    bifurcation - idealized model - potential flow - river - secondary flow - sediment diversion

    The division of sediment at river bifurcations is crucial for the morphodynamics of anastamosing rivers and distributary delta channel networks. Many river bifurcations are strongly asymmetric and have a planform where a small channel branches off to the side. Such a configuration is also typical for man-made diversions of water and sediment into canals. At asymmetric bifurcations, the division of sediment is influenced by the secondary current, which is caused by the turning of the flow toward the side. The secondary currents cause especially water from the lower parts of the water column to be diverted into the side branch. As the sediment concentration close to the bottom is high, side branches can receive a disproportionately large fraction of the incoming sediment load, relative to the water discharge. Lateral diversions have been extensively studied with physical and numerical experiments, with the goal to either mitigate or exploit this effect. However, a systematic mathematical analysis of the parameter space has not yet appeared in the literature. Here, we present a comprehensive analysis by way of an idealized model, revealing how the division of sediment is influenced by the width and depth of the branches. We show that the excess of sediment that is diverted into the side branch is lower when the inlet to the side branch is wider and shallower. This may have larger implications for the stability of delta channel networks, as inlets to side branches tend to be locally wider, which likely contributes to their morphological stability.

    Numerical simulation of water renewal timescales in the Mahakam delta, Indonesia
    Van, Chien Pham ; Brye, Benjamin De; Brauwere, Anouk De; Hoitink, A.J.F. ; Soares-Frazao, Sandra ; Deleersnijder, Eric - \ 2020
    Water 12 (2020)4. - ISSN 2073-4441
    Age - CART - Exposure time - Mahakam delta - Residence time - Return coefficient

    Water renewal timescales, namely age, residence time, and exposure time, which are defined in accordance with the Constituent-oriented Age and Residence time Theory (CART), are computed by means of the unstructured-mesh, finite element model Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM) in the Mahakam Delta (Borneo Island, Indonesia). Two renewing water types, i.e., water from the upstream boundary of the delta and water from both the upstream and the downstream boundaries, are considered, and their age is calculated as the time elapsed since entering the delta. The residence time of the water originally in the domain (i.e., the time needed to hit an open boundary for the first time) and the exposure time (i.e., the total time spent in the domain of interest) are then computed. Simulations are performed for both low and high flow conditions, revealing that (i) age, residence time, and exposure time are clearly related to the river volumetric flow rate, and (ii) those timescales are of the order of one spring-neap tidal cycle. In the main deltaic channels, the variation of the diagnostic timescales caused by the tide is about 35% of their averaged value. The age of renewing water from the upstream boundary of the delta monotonically increases from the river mouth to the delta front, while the age of renewing water from both the upstream and the downstream boundaries monotonically increases from the river mouth and the delta front to the middle delta. Variations of the residence and the exposure times coincide with the changes of the flow velocity, and these timescales are more sensitive to the change of flow dynamics than the age. The return coefficient, which measures the propensity of water to re-enter the domain of interest after leaving it for the first time, is of about 0.3 in the middle region of the delta.

    Diversion of flow and sediment towards a side channel separated from a river by a longitudinal training dam
    Ruijsscher, T.V. de; Vermeulen, B. ; Hoitink, A.J.F. - \ 2020
    Water Resources Research 56 (2020)6. - ISSN 0043-1397
    Side weir - Bifurcation - Sediment transport - ADCP - Longitudinal training dam
    A human‐made entrance to a side channel separated from the river by a longitudinal training dam can be considered a new, emergent type of river bifurcation. To understand the processes controlling the diversion of flow and sediment towards the side channel at such bifurcations, a comprehensive field‐monitoring programme was performed in the Waal River, which is the main branch of the Rhine River in the Netherlands. Local processes govern the flow field in the bifurcation region. The angle between the main river flow and the flow into the side channel increases with decreasing lateral and longitudinal distance to the bifurcation point, which corresponds to the head of the training dam. The general flow pattern can be well reproduced with a uniform depth, potential flow model consisting of a superposition of main channel flow and lateral outflow. For submerged flow conditions over the sill, the side channel hydraulic conditions influence the exchange processes, yet free flow side weir theory describes the flow field at this bifurcation type qualitatively well. The vertical flow structure in the side channel, which governs the sediment exchange between the main channel and the side channel, is steered by the geometrical details of the sill. The presence of the sill structure is key to controlling the morphological stability of this type of bifurcation given its primary influence on bed load sediment import and exerts an indirect impact on suspended sediment exchange.
    Replication Data for: Diversion of flow and sediment towards a side channel separated from a river by a longitudinal training dam
    Ruijsscher, Timo de; Vermeulen, Bart ; Hoitink, Ton - \ 2020
    Wageningen University & Research
    ADCP - Bifurcation - Longitudinal training dam - Lowland river - Sediment transport - Side weir
    A human-made entrance to a side channel separated from the river by a longitudinal training dam can be considered a new, emergent type of river bifurcation. To understand the processes controlling the diversion of flow and sediment towards the side channel at such bifurcations, a comprehensive field-monitoring programme was performed in the Waal River, which is the main branch of the Rhine River in the Netherlands. Local processes govern the flow field in the bifurcation region. The angle between the main river flow and the flow into the side channel increases with decreasing lateral and longitudinal distance to the bifurcation point, which corresponds to the head of the training dam. The general flow pattern can be well reproduced with a uniform depth, potential flow model consisting of a superposition of main channel flow and lateral outflow. For submerged flow conditions over the sill, the side channel hydraulic conditions influence the exchange processes, yet free flow side weir theory describes the flow field at this bifurcation type qualitatively well. The vertical flow structure in the side channel, which governs the sediment exchange between the main channel and the side channel, is steered by the geometrical details of the sill. The presence of the sill structure is key to controlling the morphological stability of this type of bifurcation given its primary influence on bed load sediment import and exerts an indirect impact on suspended sediment exchange.
    Impact of river discharge seasonality change on tidal duration asymmetry in the Yangtze River Estuary
    Yu, Xiayan ; Zhang, Wei ; Hoitink, A.J.F. - \ 2020
    Scientific Reports 10 (2020)1. - ISSN 2045-2322

    The Yangtze River Estuary (YRE) is one of the world’s largest river-tidal systems with rapidly changing hydrology and morphology following the construction of multiple dams. The effects of dam construction may extend to the region close to the coast, where channel stability depends on the asymmetry of the tide. Here, we focus on the possible effects of changing discharge regimes on tidal asymmetry in the YRE. Specifically, we focus on the difference in duration between ebb and flood, quantified as tidal duration asymmetry, because it has strong implications for residual sediment transport and can be derived from available water level data. To cope with nonstationary tides under the influence of a time-varying river discharge, a nonstationary harmonic analysis tool (NS_TIDE) is applied to explore the spatiotemporal variations in tidal duration asymmetry, under the influence of different combinations of tidal constituents. Tidal duration asymmetry initially increases, then slightly decreases, in an upstream direction. It experiences significant seasonal variations in response to rapidly varying discharge: tides are more asymmetric upstream of Zhenjiang in the dry season and more asymmetric downstream in the wet season. The combined effects of discharge regulation and morphological changes cause seasonal alterations in tidal duration asymmetry. In the wet season, reduced river discharge caused by water storage and climate change enhance the asymmetry upstream (+11.74% at Wuhu, +7.19 at Nanjing) while the asymmetry is weakened downstream (-2.90% at Zhenjiang, -7.19 at Jiangyin) following the TGD’s operation. Downstream channel erosion caused by post-TGD lower sediment loads has become the dominant factor weakening tidal asymmetry in most parts of the YRE in the dry season. Understanding these evolutions of tidal duration asymmetry under the hydrological and morphological effects has important implications for the management of estuarine ecosystem and navigation.

    Wood-induced backwater effects in lowland streams
    Geertsema, Tjitske J. ; Torfs, Paul J.J.F. ; Eekhout, Joris P.C. ; Teuling, Adriaan J. ; Hoitink, Antonius J.F. - \ 2020
    River Research and Applications (2020). - ISSN 1535-1459
    backwater effects - conceptual backwater effect model - stream hydraulics - woody debris in streams

    Placement of wood in streams has become a common method to increase ecological value in river and stream restoration and is widely used in natural environments. Water managers, however, are often hesitant to introduce wood in channels that drain agricultural and urban areas because of backwater effect concerns. This study aims to better understand the dependence of wood-induced backwater effects on cross-sectional area reduction and on discharge variation. A newly developed, one-dimensional stationary model demonstrates how a reduction in water level over the wood patch significantly increases directly after wood insertion. The water level drop is found to increase with discharge, up to a maximum level. If the discharge increases beyond this maximum, the water level drop reduces to a value that may represent the situation without wood. This reduction predominately depends on the obstruction ratio, calculated as the area covered by wood in the channel cross section divided by the total cross-sectional area. The model was calibrated with data from a field study in four lowland streams in the Netherlands. The field study showed that morphologic adjustments in the stream and reorientation of the woody material reduced the water level reduction over the patches in time. The backwater effects can thus be reduced by optimizing the location where wood patches are placed and by manipulating the obstruction ratio. The model can function as a generic tool to achieve a stream design with wood that optimizes the hydrological and ecological potential of streams.

    Resilience of River Deltas in the Anthropocene
    Hoitink, A.J.F. ; Nittrouer, J.A. ; Passalacqua, P. ; Shaw, J.B. ; Langendoen, E.J. ; Huismans, Y. ; Maren, D.S. van - \ 2020
    Journal of Geophysical Research: Earth Surface 125 (2020)3. - ISSN 2169-9003
    deltas - morphodynamics - resilience - rivers - sea level rise - sediment transport

    At a global scale, delta morphologies are subject to rapid change as a result of direct and indirect effects of human activity. This jeopardizes the ecosystem services of deltas, including protection against flood hazards, facilitation of navigation, and biodiversity. Direct manifestations of delta morphological instability include river bank failure, which may lead to avulsion, persistent channel incision or aggregation, and a change of the sedimentary regime to hyperturbid conditions. Notwithstanding the in-depth knowledge developed over the past decades about those topics, existing understanding is fragmented, and the predictive capacity of morphodynamic models is limited. The advancement of potential resilience analysis tools may proceed from improved models, continuous observations, and the application of novel analysis techniques. Progress will benefit from synergy between approaches. Empirical and numerical models are built using field observations, and, in turn, model simulations can inform observationists about where to measure. Information theory offers a systematic approach to test the realism of alternative model concepts. Once the key mechanism responsible for a morphodynamic instability phenomenon is understood, concepts from dynamic system theory can be employed to develop early warning indicators. In the development of reliable tools to design resilient deltas, one of the first challenges is to close the sediment balance at multiple scales, such that morphodynamic model predictions match with fully independent measurements. Such a high ambition level is rarely adopted and is urgently needed to address the ongoing global changes causing sea level rise and reduced sediment input by reservoir building.

    Effect of non‐migrating bars on dune dynamics in a lowland river
    Ruijsscher, T.V. ; Naqshband, S. ; Hoitink, A.J.F. - \ 2020
    Earth Surface Processes and Landforms 45 (2020)6. - ISSN 0197-9337 - p. 1361 - 1375.
    As dunes and larger scale bed forms such as bars coexist in rivers, the question arises whether dune dynamics are influenced by interaction with the underlying bed topography. The present study aims to establish the degree in which dune characteristics in two and three dimensions are influenced by an underlying topography dominated by non‐migrating bars. As a case study, a 20km stretch in the Waal River in the Netherlands is selected, which represents a sand‐bed lowland river. At this location, longitudinal training dams (LTDs) have recently been constructed to ensure sufficient navigation depth during periods with low water levels, and to reduce flood risk. By using data covering two‐year‐long periods before and after LTD construction, the robustness of the results is investigated. Before LTD construction, dune characteristics show large variability both spatially and temporally, with dunes being longer, lower, less steep and having a lower lee side angle when they are located on bar tops. The correlation between dune characteristics and the underlying bed topography is disrupted by unsteady conditions for which the dunes are in a state of transition. The bar pattern causes tilting of dune crest lines, which may result from a transverse gradient in bed load sediment transport. As a result of LTD construction, the hydraulic and morphological conditions have changed significantly. Despite this, the main conclusions still hold, which strengthens the validity of the results.
    Scale‐dependent evanescence of river dunes during discharge extremes
    Naqshband, S. ; Hoitink, A.J.F. - \ 2020
    Geophysical Research Letters 47 (2020)6. - ISSN 0094-8276
    During high river discharge extremes, the growth of dunes can reach a maximum beyond which a transition to upper stage plane bed may occur, enhancing the river's conveyance capacity and reducing flood risk. Our predictive ability of this bedform regime shift in rivers is exclusively built upon high Froude number flows dominated by asymmetric dunes with steep downstream‐facing slipfaces that are rare in natural rivers. By using light‐weight polystyrene particles as a substrate in an experimental flume setting, we present striking dune morphodynamic similarity between shallow laboratory flow conditions and deep rivers, preconditioned that both flow and sediment transport parameters are accurately scaled. Our experimental results reveal the first observation of upper stage plane bed in a shallow laboratory flume that is reached for a Froude number well below unity. This work highlights the need to rethink widely used dune scaling‐relationships, bedform stability diagrams, predictions of flow resistance, and flood risk.
    Replication Data for: Effect of non-migrating bars on dune dynamics in a lowland river
    Ruijsscher, Timo de; Naqshband, Suleyman ; Hoitink, Ton - \ 2020
    Wageningen University & Research
    bars - bed level - discharge - dunes - longitudinal training dams - multibeam echosounding - roughness - Waal River - water level
    As dunes and larger scale bed forms such as bars coexist in rivers, the question arises whether dune dynamics are influenced by interaction with the underlying bed topography. The present study aims to establish the degree in which dune characteristics in two and three dimensions are influenced by an underlying topography dominated by non-migrating bars. As a case study, a 20 km stretch in the Waal River in the Netherlands is selected, which represents a sand-bed lowland river. At this location, longitudinal training dams (LTDs) have recently been constructed to ensure sufficient navigation depth during periods with low water levels, and to reduce flood risk. By using data covering two-year-long periods before and after LTD construction, the robustness of the results is investigated. Before LTD construction, dune characteristics show large variability both spatially and temporally, with dunes being longer, lower, less steep and having a lower lee side angle when they are located on bar tops. The correlation between dune characteristics and the underlying bed topography is disrupted by unsteady conditions for which the dunes are in a state of transition. The bar pattern causes tilting of dune crest lines, which may result from a transverse gradient in bed load sediment transport. As a result of LTD construction, the hydraulic and morphological conditions have changed significantly. Despite this, the main conclusions still hold, which strengthens the validity of the results.
    Impact of trends in river discharge and ocean tides on water level dynamics in the Pearl River Delta
    Cao, Yu ; Zhang, Wei ; Zhu, Yuliang ; Ji, Xiaomei ; Xu, Yanwen ; Wu, Yao ; Hoitink, A.J.F. - \ 2020
    Coastal Engineering 157 (2020). - ISSN 0378-3839
    Nonstationary harmonic analysis - River discharge - Tidal amplitudes - Tidal forcing - Water levels

    The spectrum of tidal and subtidal water level variations in river deltas responds to river discharge variation, ocean tides, and human activities of many kinds. It remains a contemporary challenge to identify the main sources of changes in tidal dynamics in deltas, because of nonlinear interactions between tides and the river discharge in a changing setting. Understanding the main forcing factors controlling the evolution of mean water levels and the associated amplitudes and phases of tidal constituents can help to understand the causes of floods and the occurrence of low flows hindering navigation. Here, a nonstationary harmonic analysis tool (NS_TIDE) is applied to hydrological data from 14 stations in the Pearl River Delta (PRD) spanning the period 1961–2012. The water levels and main tidal constituent properties are decomposed into contributions of external forcing by river discharges and ocean tides, providing insight into the nonstationary tidal-fluvial processes. Significant temporal trends in mean water levels and tidal properties are observed in the PRD. Results indicate that there is spatial variability in the response of mean water levels and tidal properties to river discharge variation in the delta. The abrupt changes in bathymetry in the delta due to intensive sand excavation are likely responsible for the observed spatial variations in tidal response, which reduce the flood-dominant tidal asymmetry in this area.

    Global-scale human impact on delta morphology has led to net land area gain
    Nienhuis, J.H. ; Ashton, A.D. ; Edmonds, D.A. ; Hoitink, A.J.F. ; Kettner, A.J. ; Rowland, J.C. ; Törnqvist, T.E. - \ 2020
    Nature 577 (2020)7791. - ISSN 0028-0836 - p. 514 - 518.

    River deltas rank among the most economically and ecologically valuable environments on Earth. Even in the absence of sea-level rise, deltas are increasingly vulnerable to coastal hazards as declining sediment supply and climate change alter their sediment budget, affecting delta morphology and possibly leading to erosion1–3. However, the relationship between deltaic sediment budgets, oceanographic forces of waves and tides, and delta morphology has remained poorly quantified. Here we show how the morphology of about 11,000 coastal deltas worldwide, ranging from small bayhead deltas to mega-deltas, has been affected by river damming and deforestation. We introduce a model that shows that present-day delta morphology varies across a continuum between wave (about 80 per cent), tide (around 10 per cent) and river (about 10 per cent) dominance, but that most large deltas are tide- and river-dominated. Over the past 30 years, despite sea-level rise, deltas globally have experienced a net land gain of 54 ± 12 square kilometres per year (2 standard deviations), with the largest 1 per cent of deltas being responsible for 30 per cent of all net land area gains. Humans are a considerable driver of these net land gains—25 per cent of delta growth can be attributed to deforestation-induced increases in fluvial sediment supply. Yet for nearly 1,000 deltas, river damming4 has resulted in a severe (more than 50 per cent) reduction in anthropogenic sediment flux, forcing a collective loss of 12 ± 3.5 square kilometres per year (2 standard deviations) of deltaic land. Not all deltas lose land in response to river damming: deltas transitioning towards tide dominance are currently gaining land, probably through channel infilling. With expected accelerated sea-level rise5, however, recent land gains are unlikely to be sustained throughout the twenty-first century. Understanding the redistribution of sediments by waves and tides will be critical for successfully predicting human-driven change to deltas, both locally and globally.

    Het Haringvliet en de Permanent Oostelijke Aanvoer – Haringvliet Flows debat
    Rijcken, T. ; Veraart, J.A. ; Roncken, P.A. ; Hoitink, A.J.F. ; Herman, P. ; Heesen, M. - \ 2019
    Delft : FLOWS
    Subtidal Flow Reversal Associated With Sediment Accretion in a Delta Channel
    Zhang, Wei ; Feng, Haochuan ; Zhu, Yuliang ; Zheng, Jinhai ; Hoitink, A.J.F. - \ 2019
    Water Resources Research 55 (2019)12. - ISSN 0043-1397 - p. 10781 - 10795.
    deltas tidal networks salt intrusion bifurcation Yangtze Delta Stokes transport

    In branching delta channel networks, tides intrude into parallel river outlets causing complex tidal behavior. The tidal motion can impact the division of river discharge over distributary channels. Under some circumstances, the discharge averaged over a tidal cycle may even reverse, drawing ocean water intox the delta, such as observed in the Yangtze Delta, the Fly Delta and the Colorado Delta. Here, we study the flow reversal associated with sediment accumulation of a distributary channel in terms of tidal propagation and subtidal discharge dynamics, by focusing on the Yangtze Delta. The Yangtze Delta channel configuration represents a deep and wide main channel and a smaller side channel that has rapidly accreted over the past decades. A new mechanism is presented, which results in seawater transport across the shallow side channel, posing a risk to freshwater availability. The shallowest section in the side channel nearly acts as a tidal divide. In this section, the tide has the character of a standing wave, which implies that Stokes transport converges from opposite sides of the channel. This leads to significant variation of the total water storage in the side channel and subtidal water levels in the side channel being elevated above that in the main channel. The bulge of water that is stored during spring tide leaves the side channel toward neap tide, explaining reversal of the tide-averaged discharge and seawater intrusion when the river discharge is low.

    Flow and Suspended Sediment Division at Two Highly Asymmetric Bifurcations in a River Delta: Implications for Channel Stability
    Kästner, K. ; Hoitink, A.J.F. - \ 2019
    Journal of Geophysical Research: Earth Surface 124 (2019)10. - ISSN 2169-9003 - p. 2358 - 2380.
    ADCP - Indonesia - river bifurcation - river delta - sediment transport - tidal hydrodynamics

    The division of sediment at river bifurcations results from the complex interaction between three-dimensional flow, planform, and channel bed morphology, as well as the heterogeneity of the bed material. Sediment division processes cannot be incorporated in their full complexity in scale experiments and are difficult to reproduce with numerical models. Field measurements are thus necessary to advance our understanding of those processes in river deltas. However, such measurements are rare. We present measurements of the flow and sediment division at two tidally influenced bifurcations of the Kapuas River, a large sand-bedded suspended load-dominated river in West Kalimantan, Indonesia. At both bifurcations, a smaller channel branches off from the side of the main river, which makes the planform strongly asymmetric. The planform of both bifurcations has been stable at least since the end of the nineteenth century when the region was mapped for the first time. Based on our measurements, we explore possible factors that stabilize the bifurcations. We measure the flow velocities with a boat-mounted acoustic velocity profiler and determine the sediment concentration from acoustic backscatter, calibrated against water samples. The side branch of the first bifurcation receives a proportionally lower fraction of sediment than water. In contrast, the side branch at the second bifurcation receives a proportionally higher fraction of sediment than water. A comparison of flow velocity and suspended sand concentration indicates that the bed material sorting strongly influences the division of sediment, in particular at one of the bifurcations, which is situated in a meander bend.

    Analytical model captures intratidal variation in salinity in a convergent, well-mixed estuary
    Xu, Yanwen ; Hoitink, Antonius J.F. ; Zheng, Jinhai ; Kästner, Karl ; Zhang, Wei - \ 2019
    Hydrology and Earth System Sciences 23 (2019)10. - ISSN 1027-5606 - p. 4309 - 4322.

    Knowledge of the processes governing salt intrusion in estuaries is important, since it influences the eco-environment of estuaries as well as its water resource potential in many ways. Analytical models of salinity variation offer a simple and efficient method for studying salt intrusion in estuaries. In this paper, an unsteady analytical solution is presented to predict the spatio-temporal variation in salinity in convergent estuaries. It is derived from a one-dimensional advection-diffusion equation for salinity, adopting a constant mixing coefficient and a single-frequency tidal wave, which can directly reflect the influence of the tidal motion and the interaction between the tide and runoff. The deduced analytical solution is illustrated with an application to the Humen estuary of the Pearl River Delta (PRD) and proves to be an efficient and accurate approach for predicting the salt intrusion in convergent estuaries. The unsteady analytical solution is tested against observations from six study sites to validate its capability to predict intratidal variation in salt intrusion. The results show that the proposed unsteady analytical solution can be successfully used to reproduce the spatial distribution and temporal processes governing salinity dynamics in convergent, well-mixed estuaries. The proposed method provides a quick and convenient approach for deciding on water-fetching methods to make good use of water resources.

    Bed morphodynamics at the intake of a side channel controlled by sill geometry
    Ruijsscher, T.V. de; Hoitink, A.J.F. ; Naqshband, S. ; Paarlberg, A.J. - \ 2019
    Advances in Water Resources 134 (2019). - ISSN 0309-1708
    Bifurcation - Longitudinal training dam - Physical scale model - River morphology - Side channel

    As part of a general trend towards river management solutions that provide more room for the river, longitudinal training dams (LTDs) have recently been constructed in the inner bend of the Dutch Waal River, replacing groynes. LTDs split the river in a main channel and a bank-connected side channel with a sill at the entrance. In the present study, a physical scale model with mobile bed was used to study morphological patterns and discharge division in the entrance region of such a side channel. Alternative geometric designs of the sill are tested to investigate the controls on the diversion of water and sediment into the side channel. After reaching a morphodynamic equilibrium, two bar features were observed in the side channel under low flow conditions. An inner-bend depositional bar emerged against the LTD, resembling depositional bars observed in sharp river bends. A second bar occurred in the most upstream part of the side channel, next to the sill, induced by divergence of the flow by widening of the channel and an increasing flow depth after the sill, hence defined as a divergence bar. The morphologically most active system in the side channel emerges for the configuration in which the sill height decreases in downstream direction. For such a geometry, the sediment that settles during low flow is largely eroded during high flow, reducing maintenance needs. A qualitative comparison based on a lab experiment mimicking field conditions demonstrates the realism of the experiments.

    Supporting Information for: Scale-dependent evanescence of river dunes during discharge extremes
    Naqshband, Suleyman ; Hoitink, Ton - \ 2019
    Wageningen University & Research
    river dunes - sediment transport - upper stage plane bed
    Data contains dune slipface angles (steepest part of the dune lee-face) for different experimental conditions as shown in figure 4 of the submitted manuscript. The slipface angles are derived using a widely used bedform tracking tool.
    Propagation of tides along a river with a sloping bed
    Kästner, K. ; Hoitink, A.J.F. ; Torfs, P.J.J.F. ; Deleersnijder, E. ; Ningsih, N.S. - \ 2019
    Journal of Fluid Mechanics 872 (2019). - ISSN 0022-1120 - p. 39 - 73.
    river dynamics - shallow water flows - topographic effects

    Conceptually, tidal rivers are seen as narrow channels along which the cross-section geometry remains constant and the bed is horizontal. As tidal waves propagate along such a channel, they decrease exponentially in height. The more rapid the decrease, the stronger the river flow. Near the coast, the tidally averaged width and depth change little throughout the year, even if the river discharge varies strongly between the seasons. However, further upstream, the water depth varies considerably with the river discharge. Recent observations from the Kapuas River, Indonesia, show that the water surface forms a backwater profile when the river flow is low. In this case, the depth converges, i.e. it gradually decreases between the river mouth and the point where the bed reaches sea level. This effect distinctly influences how tidal waves propagate up river so that their wave height does not decrease exponentially any more. We present a theoretical analysis of this phenomenon, which reveals several so far overlooked aspects of river tides. These aspects are particularly relevant to low river flow. Along the downstream part of the tidal river, depth convergence counteracts frictional damping so that the tidal range is higher than expected. Along the upstream parts of the tidal river, the low depth increases the damping so that the tide more rapidly attenuates. The point where the bed reaches sea level effectively limits the tidal intrusion, which carries over to the overtide and the subtidal water level set-up.

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