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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    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.
    Data underlying “Multi-Scale monitoring and modelling of the Kapuas River Delta”
    Kästner, K. ; Vermeulen, B. ; Hoitink, A.J.F. ; Geertsema, T.J. - \ 2019
    hydrodynamics - morphodynamics - sand-bedded rivers - sediment transport - tropics
    unprocessed raw data: - water level (pressure gauge) - velocity (boat mounted VADCP and HADCP) - turbulence (bottom mounted coupled VADCP) - bathymetry (single beam) - side scan sonar - bed material grain size - water samples (suspended sediment concentration) - particle size (lisst casts) - salinity (gauged)
    Scale model of a training dam using lightweight granulates
    Vermeulen, Bart ; Boersema, Matthijs ; Hoitink, Ton ; Sieben, Arjan ; Sloff, Kees ; Wal, Maarten van der - \ 2018
    In: River Flow 2018 - Ninth International Conference on Fluvial Hydraulics. - EDP Sciences (E3S Web of Conferences ) - 7 p.

    Longitudinal training dams (LTDs) are a promising alternative for river groynes. Here we summarize findings of a recent study focused on the along river transition from a series of river groynes to an LTD, where the flow divides between the fairway and the side channel between the LTD and the river bank. A scale model is setup using lightweight granulates made of polystyrene to create conditions that are dynamically similar to a prototype situation in the River Waal. The key advantage of using lightweight granulates is that both the Shields number and the Froude number are similar in the model and the prototype. A high flow and a low flow experiment were carried out. The bedforms in the physical model have dimensions that correspond to theoretical dune height predictions, and also the channel incision due to width reduction is in accordance with expectations. The scour holes that develop near the tip of the groynes, however, are too deep, which may relate to improper scaling of the local turbulent vortices, initiated at the groynes. The morphodynamic developments in the flow divergence zone are subtle, and are overwhelmed by the mobile bed response to the presence of groynes. Considering that the physical model over-predicts the erosion caused by groynes, this suggests that the LTD configuration subject to study results in a comparatively stable bed morphology.

    Application of a Line Laser Scanner for Bed Form Tracking in a Laboratory Flume
    Ruijsscher, T.V. de; Hoitink, A.J.F. ; Dinnissen, S. ; Vermeulen, B. ; Hazenberg, P. - \ 2018
    Water Resources Research 54 (2018)3. - ISSN 0043-1397 - p. 2078 - 2094.
    Bed level monitoring - Dune tracking - Line laser scanner - LOESS - Physical scale model
    A new measurement method for continuous detection of bed forms in movable bed laboratory experiments is presented and tested. The device consists of a line laser coupled to a 3-D camera, which makes use of triangulation. This allows to measure bed forms during morphodynamic experiments, without removing the water from the flume. A correction is applied for the effect of laser refraction at the air-water interface. We conclude that the absolute measurement error increases with increasing flow velocity, its standard deviation increases with water depth and flow velocity, and the percentage of missing values increases with water depth. Although 71% of the data is lost in a pilot moving bed experiment with sand, still high agreement between flowing water and dry bed measurements is found when a robust LOcally weighted regrESSion (LOESS) procedure is applied. This is promising for bed form tracking applications in laboratory experiments, especially when lightweight sediments like polystyrene are used, which require smaller flow velocities to achieve dynamic similarity to the prototype. This is confirmed in a moving bed experiment with polystyrene.
    Prerequisites for Accurate Monitoring of River Discharge Based on Fixed-Location Velocity Measurements
    Kästner, K. ; Hoitink, A.J.F. ; Torfs, P.J.J.F. ; Vermeulen, B. ; Ningsih, N.S. ; Pramulya, M. - \ 2018
    Water Resources Research 54 (2018)2. - ISSN 0043-1397 - p. 1058 - 1076.
    discharge - index velocity - Indonesia - Kapuas River - rating curve - velocity profile
    River discharge has to be monitored reliably for effective water management. As river discharge cannot be measured directly, it is usually inferred from the water level. This practice is unreliable at places where the relation between water level and flow velocity is ambiguous. In such a case, the continuous measurement of the flow velocity can improve the discharge prediction. The emergence of horizontal acoustic Doppler current profilers (HADCPs) has made it possible to continuously measure the flow velocity. However, the profiling range of HADCPs is limited, so that a single instrument can only partially cover a wide cross section. The total discharge still has to be determined with a model. While the limitations of rating curves are well understood, there is not yet a comprehensive theory to assess the accuracy of discharge predicted from velocity measurements. Such a theory is necessary to discriminate which factors influence the measurements, and to improve instrument deployment as well as discharge prediction. This paper presents a generic method to assess the uncertainty of discharge predicted from range-limited velocity profiles. The theory shows that a major source of error is the variation of the ratio between the local and cross-section-averaged velocity. This variation is large near the banks, where HADCPs are usually deployed and can limit the advantage gained from the velocity measurement. We apply our theory at two gauging stations situated in the Kapuas River, Indonesia. We find that at one of the two stations the index velocity does not outperform a simple rating curve.
    Tidal controls on river delta morphology
    Hoitink, A.J.F. ; Wang, Z.B. ; Vermeulen, B. ; Huismans, Y. ; Kästner, K. - \ 2017
    Nature Geoscience 10 (2017)9. - ISSN 1752-0894 - p. 637 - 645.
    River delta degradation has been caused by extraction of natural resources, sediment retention by reservoirs, and sea-level rise. Despite global concerns about these issues, human activity in the world's largest deltas intensifies. Harbour development, construction of flood defences, sand mining and land reclamation emerge as key contemporary factors that exert an impact on delta morphology. Tides interacting with river discharge can play a crucial role in the morphodynamic development of deltas under pressure. Emerging insights into tidal controls on river delta morphology suggest that-despite the active morphodynamics in tidal channels and mouth bar regions-tidal motion acts to stabilize delta morphology at the landscape scale under the condition that sediment import during low flows largely balances sediment export during high flows. Distributary channels subject to tides show lower migration rates and are less easily flooded by the river because of opposing non-linear interactions between river discharge and the tide. These interactions lead to flow changes within channels, and a more uniform distribution of discharge across channels. Sediment depletion and rigorous human interventions in deltas, including storm surge defence works, disrupt the dynamic morphological equilibrium and can lead to erosion and severe scour at the channel bed, even decades after an intervention.
    Hydrology of inland tropical lowlands : The Kapuas and Mahakam wetlands
    Hidayat, Hidayat ; Teuling, Ryan ; Vermeulen, Bart ; Muh, Taufik ; Kastner, Karl ; Geertsema, Tjitske J. ; Bol, Dinja C.C. ; Hoekman, Dirk H. ; Sri Haryani, Gadis ; Lanen, Henny A.J. van; Delinom, Robert M. ; Dijksma, Roel ; Anshari, Gusti Z. ; Ningsih, Nining S. ; Uijlenhoet, Remko ; Hoitink, Ton - \ 2017
    Hydrology and Earth System Sciences 21 (2017)5. - ISSN 1027-5606 - p. 2579 - 2594.

    Wetlands are important reservoirs of water, carbon and biodiversity. They are typical landscapes of lowland regions that have high potential for water retention. However, the hydrology of these wetlands in tropical regions is often studied in isolation from the processes taking place at the catchment scale. Our main objective is to study the hydrological dynamics of one of the largest tropical rainforest regions on an island using a combination of satellite remote sensing and novel observations from dedicated field campaigns. This contribution offers a comprehensive analysis of the hydrological dynamics of two neighbouring poorly gauged tropical basins; the Kapuas basin (98g700gkm2) in West Kalimantan and the Mahakam basin (77g100gkm2) in East Kalimantan, Indonesia. Both basins are characterised by vast areas of inland lowlands. Hereby, we put specific emphasis on key hydrological variables and indicators such as discharge and flood extent. The hydroclimatological data described herein were obtained during fieldwork campaigns carried out in the Kapuas over the period 2013-2015 and in the Mahakam over the period 2008-2010. Additionally, we used the Tropical Rainfall Measuring Mission (TRMM) rainfall estimates over the period 1998-2015 to analyse the distribution of rainfall and the influence of El-Niño - Southern Oscillation. Flood occurrence maps were obtained from the analysis of the Phase Array type L-band Synthetic Aperture Radar (PALSAR) images from 2007 to 2010. Drought events were derived from time series of simulated groundwater recharge using time series of TRMM rainfall estimates, potential evapotranspiration estimates and the threshold level approach. The Kapuas and the Mahakam lake regions are vast reservoirs of water of about 1000 and 1500gkm2 that can store as much as 3 and 6.5 billiongm3 of water, respectively. These storage capacity values can be doubled considering the area of flooding under vegetation cover. Discharge time series show that backwater effects are highly influential in the wetland regions, which can be partly explained by inundation dynamics shown by flood occurrence maps obtained from PALSAR images. In contrast to their nature as wetlands, both lowland areas have frequent periods with low soil moisture conditions and low groundwater recharge. The Mahakam wetland area regularly exhibits low groundwater recharge, which may lead to prolonged drought events that can last up to 13 months. It appears that the Mahakam lowland is more vulnerable to hydrological drought, leading to more frequent fire occurrences than in the Kapuas basin.

    Distributary channels in the fluvial to tidal transition zone
    Kästner, K. ; Hoitink, A.J.F. ; Vermeulen, B. ; Geertsema, T.J. ; Ningsih, N.S. - \ 2017
    Journal of Geophysical Research: Earth Surface 122 (2017)3. - ISSN 2169-9003 - p. 696 - 710.
    Delta - Distributary - Grain size - Hydraulic geometry - Kapuas River - River
    Coastal lowland plains under mixed fluvial-tidal influence may form complex, composite channel networks, where distributaries blend the characteristics of mouth bar channels, avulsion channels, and tidal creeks. The Kapuas coastal plain exemplifies such a coastal plain, where several narrow distributaries branch off the Kapuas River at highly asymmetric bifurcations. A comprehensive geomorphological analysis shows that trends in the channel geometry of all Kapuas distributaries are similar. They consist of a short, converging reach near the sea and a nonconverging reach upstream. The two parts are separated by a clear break in scaling of geometrical properties. Such a break in scaling was previously established in the Mahakam Delta, which suggests that this may be a general characteristic in the fluvial to tidal transition zone. In contrast to the geometrical trend similarities, a clear difference in bed material between the main and side distributaries is found. In the main distributary, a continuous trend of downstream fining is established, similar to what is often found in lowland rivers. In the side distributaries, bed material coarsens in the downstream direction. This indicates an undersupply of sediment to the side distributaries, which may contribute to their long-term stability as established from historical maps. Tides may be the main agent preventing fine sediment to settle, promoting residual transport of fine material to the coastal ocean.
    Dune and alternate bar detection in a laboratory flume using a 3D laser scanner
    Ruijsscher, T.V. de; Dinnissen, S. ; Vermeulen, B. ; Hazenberg, Pieter ; Hoitink, A.J.F. - \ 2016
    In: River Flow - Proceedings of the International Conference on Fluvial Hydraulics, RIVER FLOW 2016. - CRC Press/Balkema - ISBN 9781138029132 - p. 1086 - 1091.

    Collecting high-precision data of migrating bed forms in flume experiments is often limited by the measurement equipment. With a 3D laser scanner a relatively large area of bed profiling can be monitored with high precision during the experiment. In the present study, the accuracy of such device is studied, applying corrections for internal calibration, path length and refraction. The total measurement error increases with increasing water depth and for water depths larger than 135 mm a filter should be applied to recognize a bed profile. For a sandy bed, application of a LOESS filter with different span widths leads to promising results for both dune and alternate bar patterns.

    Migration of banks along the Kapuas River, West Kalimantan
    Vermeulen, B. ; Huisman, A.K. ; Hoitink, A.J.F. ; Pramulya, M. - \ 2016
    In: River Flow - Proceedings of the International Conference on Fluvial Hydraulics, RIVER FLOW 2016. - CRC Press/Balkema - ISBN 9781138029132 - p. 1249 - 1253.

    In this study we analyse the migration rates along the Kapuas River, West Kalimantan. The migration rates are analysed by digitizing sets of Landsat images. Cloud detection and cloud shadow detection is used to mask the images, then they are combined to obtain water bodies. The difference between images from different years between 1973 and 2013 is used to obtain migration rates. A stream reconaissance was performed along the River to validate the results. The results of the stream reconaissance show good agreement with the Landsat based erosion rates.

    Multiscale structure of meanders
    Vermeulen, B. ; Hoitink, A.J.F. ; Zolezzi, G. ; Abad, J.D. ; Aalto, R. - \ 2016
    Geophysical Research Letters 43 (2016)7. - ISSN 0094-8276 - p. 3288 - 3297.
    bends - meanders - multiple loops - scale space tree - valley cuvature - wavelets

    River meander planforms can be described based on wavelet analysis, but an objective method to identify the main characteristics of a meander planform over all spatial scales is yet to be found. Here we show how a set of simple metrics representing meander shape can be retrieved from a continuous wavelet transform of a planform geometry. We construct a synoptic multiple looping tree to establish the meander structure, revealing the embedding of dominant meander scales in larger-scale loops. The method can be applied beyond the case of rivers to unravel the meandering structure of lava flows, turbidity currents, tidal channels, rivulets, supraglacial streams, and extraterrestrial flows.

    Flow structure caused by a local cross-sectional area increase and curvature in a sharp river bend
    Vermeulen, B. ; Hoitink, A.J.F. ; Labeur, R.J. - \ 2015
    Journal of Geophysical Research: Earth Surface 120 (2015)9. - ISSN 2169-9003 - p. 1771 - 1783.
    channel width - large-eddy simulation - nonhydrostatic pressure - reverse flow - scour - sharp bends

    Horizontal flow recirculation is often observed in sharp river bends, causing a complex three-dimensional flow structure with large implications for the morphological and planimetric development of meanders. Several field observations in small-scale systems show that sharp bends are often found in association with a strong increase in cross-sectional area, the deposition of outer bank benches, and reattachment bars near the inner bank. Recent studies show that these bends can also occur in large-scale systems. In this study, we present field measurements of a sharp bend in the Mahakam River, East Kalimantan, Indonesia. The cross-sectional area increases by a factor of 3 compared with the reach-averaged cross-sectional area. Along a river reach of about 150 km, cross-sectional area correlates strongly with curvature. The field measurements are analyzed together with the results from numerical simulation with a three-dimensional finite element model, which yields a comprehensive view of the intricate flow structure. In turn, the model is used to validate a new equation that captures the water surface topography dependence on cross-sectional area variation and curvature. The results show the importance of the increase in cross-sectional area in the development of horizontal recirculation. Vertical acceleration of the flow into the scour causes the pressure to deviate from a hydrostatic pressure distribution. Strong downflow (up to 12 cm s-1) advects longitudinal momentum toward the bed, causing the flow to concentrate in the lower part of the cross section. This increases the velocity magnitude throughout the cross section, which is expected to maintain the large scour depth found in several bends along the Mahakam River.

    Sharp bends associated with deep scours in a tropical river: The river Mahakam (East Kalimantan, Indonesia)
    Vermeulen, B. ; Hoitink, A.J.F. ; Berkum, S.W. van; Hidayat, H. - \ 2014
    Journal of Geophysical Research: Earth Surface 119 (2014)7. - ISSN 2169-9003 - p. 1441 - 1454.
    circular meander pools - flow separation - mackenzie delta - channel bends - concave-bank - evolution - discharge - division - width - hole
    Autogenic scouring in sharp river bends has received ample attention in laboratory and modeling studies. These studies have significantly advanced our understanding of how flow processes are influenced by strong curvature and how they affect the bathymetry. Here we present a 300 km reach of the Mahakam River in Indonesia, which features several sharp bends (W/R > 0.5), providing a unique field data set to validate existing knowledge on sharp bends. Scour depths were found to strongly exceed what can be expected based on existing understanding of sharp bends and are highly correlated with curvature. A comprehensive stream reconnaissance was carried out to compare the occurrence of sharp bends and deep scours with lateral bank migration. Histograms of the occurrence of erosive, stable, advancing, and bar-type banks as a function of curvature quantify the switch from a mildly curved bend regime to a sharp bend regime. In mild bends, outer banks erode and inner banks advance. In sharp bends the erosion pattern inverts. Outer banks stabilize or advance, while inner banks erode. In sharply curved river bends, bars occur near the outer banks that become less erosive for higher curvatures. Inner banks become more erosive for higher curvatures but nevertheless accommodate the larger portion of exposed bars. No relation was found between the land cover adjacent to the river and the occurrence of sharp bends. Soil processes may play a crucial role in the formation of sharp bends, which is inferred from iron and manganese concretions observed in the riverbanks, indicating ferric horizons and early stages of the formation of plinthic horizons. Historical topographic maps show the planform activity of the river is low, which may relate to the scour holes slowing down planimetric development.
    Improved flow velocity estmates from oving-boat ADCO measurements
    Vermeulen, B. ; Sassi, M.G. ; Hoitink, A.J.F. - \ 2014
    Water Resources Research 50 (2014)5. - ISSN 0043-1397 - p. 4186 - 4196.
    doppler current profiler - suspended sediment - turbulence measurements - acoustic measurement - river - discharge - transport - division - channel - vessel
    Acoustic Doppler current profilers (ADCPs) are the current standard for flow measurements in large-scale open water systems. Existing techniques to process vessel-mounted ADCP data assume homogeneous or linearly changing flow between the acoustic beams. This assumption is likely to fail but is nevertheless widely applied. We introduce a new methodology that abandons the standard assumption of uniform flow in the area between the beams and evaluate the drawbacks of the standard approach. The proposed method strongly reduces the extent over which homogeneity is assumed. The method is applied to two field sites: a mildly curved bend near a junction featuring a typical bend flow and a sharply curved bend that features a more complex sheared flow. In both cases, differences are found between the proposed method and the conventional method. The proposed technique yields different results for secondary flow patterns compared with the conventional method. The velocity components estimated with the conventional method can differ over 0.2 m/s in regions of strong shear. We investigate the number of repeat transects necessary to isolate the mean flow velocity vector from the raw ADCP signal, discarding the influences of noise, positioning and projection errors, and turbulence. Results show that several repeat transects are necessary. The minimum number of repeat measurements needed for robust mean velocity estimates is reduced when applying the proposed method
    River scale model of an training dam using lightweight granulates
    Vermeulen, B. ; Boersema, M.P. ; Hoitink, A.J.F. ; Sieben, J. ; Sloff, C.J. ; Wal, M.F. van der - \ 2014
    Journal of Hydro-environment Research 8 (2014). - ISSN 1570-6443 - p. 88 - 94.
    sediment transport - spur dikes - bed
    Replacing existing river groynes with longitudinal training dams is considered as a promising flood mitigation measure in the main Dutch rivers, which can also serve to guarantee navigability during low flows and to create conditions favourable for ecological development. Whereas the bed response in the streamwise uniform part of a river trained by a longitudinal dam can be readily predicted, the bed response at the transition zones is unclear. In the present study, we investigate the local morphological effects resulting at the intake section of a longitudinal training dam, where the flow is distributed over the main channel and a side channel in between the dam and the river shore. A sediment recirculating model with a nearly undistorted geometry with respect to the prototype was setup. Lightweight polystyrene granulates were used as a surrogate for sediment, to properly scale the Shields parameter without compromising Froude scaling, and reach dynamical similarity. A laser scanner allowed collecting high-resolution bed elevation data. Results obtained under typical low flow and high flow conditions show a general deepening of the bed in the area adjacent to the training dam, in response to narrowing of the main channel. Scour at an upstream river groyne embedded in the model showed a scour hole which was deeper than realistic. Throughout the entire domain, bedforms developed featuring geometrical properties that reproduced the prototype conditions appropriately. Based on a comparison with characteristics from the River Waal, regarded as the prototype without a longitudinal dam, lightweight sediments were considered to be a proper choice for this study, in which bedload is the main sediment transport mode. The main conclusion regards the absence of significant morphodynamic developments at the intake section, both during the high flow experiment and during the low flow experiment, which can be attributed to the alignment of the dam with the local streamlines.
    Rivers running deep : complex flow and morphology in the Mahakam River, Indonesia
    Vermeulen, B. - \ 2014
    Wageningen University. Promotor(en): Remko Uijlenhoet, co-promotor(en): Ton Hoitink. - Wageningen : Wageningen University - ISBN 9789462572065 - 150
    rivieren - morfologie - waterstroming - hydrologie - indonesië - rivers - morphology - water flow - hydrology - indonesia

    Rivers in tropical regions often challenge our geomorphological understanding of fluvial systems. Hairpin bends, natural scours, bifurcate meander bends, tie channels and embayments in the river bank are a few examples of features ubiquitous in tropical rivers. Existing observation techniques fall short to grasp the complex governing processes of flow and morphology. In this thesis new observational techniques are introduced and applied to study the Mahakam River, East Kalimantan, Indonesia. The observations reveal a new type of morphological regime, characterized by non-harmonic meanders, scour and strong variation of the cross-sectional area. The anomalous geometry induces a complex three-dimensional flow pattern causing longitudinal flow to be concentrated near the bed of the river.

    In Chapter 2 a wavelet based technique is introduced to characterize meander shape in a quantitative, objective manner. A scale space forest composed of a set of rooted trees represents the meandering planform. Based on the rooted trees, the locally dominant meander wavelengths are defined along the river. Sub-meander scale spectral density in the wavelet transform is used to determine a set of metrics quantifying bend skewness and fattening. Negative fattening parameterizes the so-called non-harmonic or hairpin bend character of meanders. The super-meander scale tree represents the embedding of meanders into larger-scale fluctuations, spanning from double-headed meander scales until the scale of the valley thalweg. The new approach is used to quantify the anomalous planform geometry of the Mahakam River in a comparison with the Red River and the Purus River.

    The geometry of the Mahakam River is analyzed into more detail in Chapter 3, where the highly curved non-harmonic meanders are related to deep scours in the river bed. A total of 35 scours is identified which exceed three times the average river depth, and four scours exceed the river depth over four times. The maximum scour depth strongly correlates with channel curvature and systematically occurs half a river width upstream of the bend apex. Most scours occur in a freely meandering zone of the river. A systematic reconnaissance of the river banks reveals a switch of erosion-deposition patterns at high curvature. Advancing banks normally observed at the inner side of a bend are mostly found at the outer side of high-curvature reaches, while eroding banks switch from the outer side for mildly curved bends to the inner side for bends with high curvature. The overall lateral migration rate of the river is low. These results indicate a switch of morphological regime at high curvatures, which requires detailed flow measurements to unravel the underlying physical processes.

    Taking flow measurements in the deep scours of the Mahakam River presents a challenge to contemporary methods in hydrography. Acoustic Doppler Current Profilers (ADCPs) are capable of profiling flow velocity over large distances from a research vessel, but the existent data processing techniques assume homogeneity of the flow between the divergent acoustic beams. This assumption fails for complex three dimensional flows as found in the scours. In Chapter 4 a new ADCP data processing technique is developed that strongly reduces the extent over which the flow needs to be assumed homogeneous. The new method is applied to flow measurements collected in a river bend with a scour exceeding 40 m depth. Results based on the new approach reveal secondary flow patterns which remain invisible adopting the conventional method.

    Chapter 5 aims to better understand flow in sharp bends, by combining analyses of the flow measurements from a deep scour with Large Eddy Simulations of the flow. The three-dimensional flow field is strongly dominated by horizontal circulations at both sides of the scour. The dramatic increase in cross-sectional area (from 2200 m2 to 7000 m2 ) plays a crucial role in the generation of the two horizontal recirculation cells. An existing formulation to predict water surface gradients in bends is extended to include the effect of cross-sectional area variations, next to the effect of curvature changes. Variation in the cross-sectional area develops adverse water surface gradients explaining the flow recirculation. The depth increase toward the scour causes a strong downward flow (up to 12 cm s − 1 ) creating a non-hydrostatic pressure distribution, steering the core of the flow toward the bed. The latter aspect is poorly reproduced by the Large Eddy Simulations, which may relate to the representation of turbulent shear stresses.

    In Chapter 6 a novel technique is introduced to better monitor turbulence properties in complex river flows from ADCP measurements, exploiting what is discarded in observations of the mean flow. It extends the so-called variance method, using two ADCPs instead of one. The availability of eight acoustic beams, four from each ADCP, changes an otherwise unsolvable set of equations with six unknowns into an overdetermined system of eight equations with six unknowns. This allows to solve for the complete Reynolds stress tensor, yielding profiles of Reynolds stresses over almost the entire water column. Widely applied assumptions on turbulence anisotropy ratios are shown to be incorrect, which reveals a knowledge gap in open channel turbulence.

    Chapter 7 uses the technique developed in Chapter 6 to investigate the degree in which bed shear stress can be monitored continuously from an ADCP mounted horizontally at the river bank (HADCP). A calibrated boundary layer model is applied to estimate time-series of cross-river bed-shear stress profiles from HADCP velocity measurements. It is concluded the HADCP measurement can represent the regional bed shear stresses, as inferred from a logarithmic velocity profile, reasonably well. These regional bed-shear stresses, in turn, poorly represent the local estimates obtained from coupled ADCP measurements, which are more directly related to processes of sediment transport and complex river morphology. Detailed observations of turbulence properties may be the key to improve our understanding of complex river flow and morphology.

    Improved processing of vessel mounted ADCP data in highly sheared turbulent flows
    Vermeulen, B. ; Sassi, M.G. ; Hoitink, A.J.F. - \ 2013
    Quantified turbulent diffusion of suspended sediment using acoustic Doppler current profilers
    Sassi, M.G. ; Hoitink, A.J.F. ; Vermeulen, B. - \ 2013
    Geophysical Research Letters 40 (2013)21. - ISSN 0094-8276 - p. 5692 - 5697.
    reynolds stress - boundary-layer - open channels - flow - suspension - transport - fluxes - adcp - sand
    Collocated profiles of the Reynolds stress tensor and eddy covariance fluxes are obtained to derive vertical profiles of turbulent momentum and sediment diffusivity in a tidal river, using coupled acoustic Doppler current profilers (ADCPs). Shear and normal stresses are obtained by combining the variances in radial velocities measured by the ADCP beams. The covariances between radial velocities and calibrated acoustic backscatter allow the determination of the three Cartesian components of the turbulent flux of suspended sediment. The main advantage of this new approach is that flow velocity and sediment concentration measurements are exactly collocated, and allowing for profiling over longer ranges, in comparison to existing techniques. Results show that vertical profiles of the inverse turbulent Prandtl-Schmidt number are coherent with corresponding profiles of the sediment diffusivity, rather than with profiles of the eddy viscosity.
    On the use of horizontal acoustic doppler profilers for continuous bed shear stress monitoring
    Vermeulen, B. ; Hoitink, A.J.F. ; Sassi, M.G. - \ 2013
    International Journal of Sediment Research 28 (2013)2. - ISSN 1001-6279 - p. 260 - 268.
    broad-band adcp - tidal channel - turbulence - velocity - flow - layer
    Continuous monitoring of bed shear stress in large river systems may serve to better estimate alluvial sediment transport to the coastal ocean. Here we explore the possibility of using a horizontally deployed acoustic Doppler current profiler (ADCP) to monitor bed shear stress, applying a prescribed boundary layer model, previously used for discharge estimation. The model parameters include the local roughness length and a dip correction factor to account for sidewall effects. Both these parameters depend on river stage and on the position in the cross-section, and were estimated from shipborne ADCP data. We applied the calibrated boundary layer model to obtain bed shear stress estimates over the measuring range of the HADCP. To validate the results, co-located coupled ADCPs were used to infer bed shear stress, both from Reynolds stress profiles and from mean velocity profiles. From HADCP data collected over a period of 1.5 years, a time series of width profiles of bed shear stress was obtained for a tidal reach of the Mahakam River, East Kalimantan, Indonesia. A smaller dataset covering 25 hours was used for comparison with results from the coupled ADCPs. The bed shear stress estimates derived from Reynolds stress profiles appeared to be strongly affected by local effects causing upflow and downflow, which are not included in the boundary layer model used to derive bed shear stress with the horizontal ADCP. Bed shear stresses from the coupled ADCP are representative of a much more localized flow, while those derived with the horizontal ADCP resemble the net effect of the flow over larger scales. Bed shear stresses obtained from mean velocity profiles from the coupled ADCPs show a good agreement between the two methods, and highlight the robustness of the method to uncertainty in the estimates of the roughness length.
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