1,720,976 research outputs found
Discharge regimes, tides and morphometry in the Mahakam delta channel network
No full text The Mahakam delta in Indonesia constitutes a text book example of a mixed tide and fluvial dominated delta. Understanding the factors that control the division of water and sediment discharge over channels in the delta is relevant in the contexts of geology, ecology and river engineering. In the Mahakam river and its delta, the tide interacts with the river outflow. River-tide interaction exerts an influence on the discharge regimes and on the division of water and sediment at the bifurcations in the delta. Bifurcations control the dispersal of sediments that eventually govern the shape and evolution of the delta. In this thesis, spatial and temporal aspects of delta evolution are shown to be reflected in scaling relations between the geometric properties of delta channels and the discharge conveyed by the channels, which is known as downstream hydraulic geometry (HG). Downstream HG relations as established in this research, feature a transition from the landward part to the seaward part of the delta characterized by a clear break in scaling behavior. The variation of river discharge throughout the network is largely impacted by river-tide interaction, which is captured by downstream HG relations. </p
Rivers running deep : complex flow and morphology in the Mahakam River, Indonesia
Get PDFRivers 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. </p
Discharge regimes, tides and morphometry in the Mahakam delta channel network
No full textThe Mahakam delta in Indonesia constitutes a text book example of a mixed tide and fluvial dominated delta. Understanding the factors that control the division of water and sediment discharge over channels in the delta is relevant in the contexts of geology, ecology and river engineering. In the Mahakam river and its delta, the tide interacts with the river outflow. River-tide interaction exerts an influence on the discharge regimes and on the division of water and sediment at the bifurcations in the delta. Bifurcations control the dispersal of sediments that eventually govern the shape and evolution of the delta. In this thesis, spatial and temporal aspects of delta evolution are shown to be reflected in scaling relations between the geometric properties of delta channels and the discharge conveyed by the channels, which is known as downstream hydraulic geometry (HG). Downstream HG relations as established in this research, feature a transition from the landward part to the seaward part of the delta characterized by a clear break in scaling behavior. The variation of river discharge throughout the network is largely impacted by river-tide interaction, which is captured by downstream HG relations
Runoff, discharge and flood occurrence in a poorly gauged tropical basin : the Mahakam River, Kalimantan
No full text Tidal rivers and lowland wetlands present a transition region where the interests of hydrologists and physical oceanographers overlap. Physical oceanographers tend to simplify river hydrology, by often assuming a constant river discharge when studying estuarine dynamics. Hydrologists, in turn, generally ignore the direct or indirect effects of tides in water level and discharge records. This thesis aims to improve methods to monitor, model and predict discharge dynamics in tidal rivers and lowland wetlands, by focussing on the central and lower reaches of the River Mahakam (East Kalimantan, Indonesia), and the surrounding lakes area. The 980-km long river drains an area of about 77100 km2 between 2°N - 1°S and 113°E - 118°E. Due to its very mild bottom slope, a significant tidal influence occurs in this river. The middle reach of the river is located in a subsiding basin, parts of which are below mean sealevel, featuring peat swamps and about thirty lakes connected to the river via tie channels. Upstream of the lakes area, at about 300 km from the river mouth, an acoustic Doppler current profiler (H-ADCP) has been horizontally deployed at a station near the city of Melak (Chapter 2). The H-ADCP profiles of velocity are converted to discharge adopting a new calibration methodology. The obtained time-series of discharge show the tidal signal is clearly visible during low flow conditions. Besides tidal signatures, the discharge series show influences by variable backwater effects from the lakes, tributaries and floodplain ponds. The discharge rate at the station exceeds 3250 m3s-1 with a hysteretic behaviour. For a specific river stage, the discharge range can be as high as 2000 m3s-1. Analysis of alternative types of rating curves shows this is far beyond what can be explained from kinematic wave dynamics. Apart from backwater effects, the large variation of discharge for a specified river stage can be explained by river-tide interaction, impacting discharge variation especially in the fortnightly frequency band. A second H-ADCP station has been setup in the lower reach of the Mahakam, near the city of Samarinda, where the tidal discharge amplitude generally exceeds the discharge related to runoff (Chapter 3). Conventional rating curve techniques are inappropriate to model river discharge at this tidally influenced station. As an alternative, an artificial neural network (ANN) model is developed to investigate the degree to which tidal river discharge at Samarinda station can be predicted from an array of level gauge measurements along the tidal river, and from tidal level predictions at sea. The ANN-based model produces a good discharge estimation, as established from a consistent performance during both the training and the validation periods, showing the discharges can be predicted from water levels only, once that a trained model is available. The ANN models perform well in predicting discharges up to two days in advance. Chapter 4 addresses the role of backwater effects and tidal influences on discharge time-series used to calibrate a rainfall-runoff model. The HBV rainfall-runoff model is implemented for the Mahakam sub-catchment upstream of Melak (25700 km2). In a first approach, the model is calibrated using a discharge series derived from the H-ADCP measurements from Melak station. In a second approach, discharge estimates derived from a rating curve are used to calibrate the model. Adopting the first approach, a comparatively low model efficiency is obtained, which is attributed to the backwater and tidal effects that are not captured in the model. The second approach produces a relatively higher model efficiency, since the rating curve filters the backwater effects out of the discharge series. Seasonal variation of terms in the water balance is not affected by the choice for one of the two calibration strategies, which shows that backwaters do not have a systematic seasonal effect on the river discharge. To allow for investigation of the causes of backwater effects, satellite radar remote sensing is employed to monitor water levels in wetlands (Chapter 5). A series of Phased Array L-band Synthetic Aperture Radar (PALSAR) images is used to observe the dynamics of the Mahakam River floodplain. To analyze radar backscatter behavior for different land cover types, several regions of interest are selected, based on land cover classes. Medium shrub, high shrub, fern/grass, and degraded forest are found to be sensitive to flooding, whereas peat forest, riverine forest and tree plantation backscatter signatures only slightly change with flood inundation. An analysis of the relationship between radar backscatter and water levels is carried out. For lakes and shrub covered peatland, for which the range of water level variation is high, a good water level-backscatter correlation is obtained. In peat forest covered peatland, subject to a small range of water level variation, water level-backscatter correlations are poor, limiting the ability to obtain a floodplain-wide water surface topography from the radar images. Chapter 6 continues to investigate the degree in which satellite radar remote sensing can serve to distinguish between dry areas and wetlands, which is a difficult task in densely vegetated areas such as peat domes. Flood extent and flood occurrence information are successfully extracted from a series of radar images of the middle Mahakam lowland area. A fully inundated region is easily recognized from a dark signature on radar images. Open water flood occurrence is mapped using a threshold value taken from radar backscatter of the permanently inundated areas. Radar backscatter intensity analysis of the vegetated floodplain area reveals consistently higher backscatter values, indicating flood inundation under forest canopy. Those observations are used to establish thresholds for flood occurrence mapping in the vegetated area. An all-encompassing flood occurrence map is obtained by combining the flood occurrence maps for areas with and without vegetation. Chapter 7 synthesizes the findings from the previous chapters. It is concluded that the backwater effects and subtle tidal influences may prevent the option to predict river discharge using rating curves, which can best be interpreted as a stage-runoff relationship. H-ADCPs offer a promising alternative to monitor river discharge. For a tidal river, an ANN model can be used as a tool for data gap filling in an H-ADCP based discharge series, or even to derive discharge estimates solely from water levels and water level predictions. Discharge can be predicted several time-steps ahead, allowing water managers to take measures based on forecasts. The stage-runoff relationship derived from a continuous series of H-ADCP based discharge estimates may be expected to be much more accurate than a similar rating curve derived from a small number of boat surveys. The flood occurrence map derived from PALSAR images can offer a detailed insight into the hydroperiod, the period in which a soil area is waterlogged, and flood extent of the lowland area, illustrating the added value of radar remote sensing to wetland hydrological studies. In future work, radar-based floodplain observations may serve to calibrate hydrodynamic models simulating the processes of flooding and emptying of the lakes area.</p
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Morphological processes in lowland streams : implications for stream restoration
No full textWaterschappen in Nederland zijn constant op zoek naar kostenefficiënte alternatieven voor beekherstel. Eén van deze methoden is het verwijderen van oeverbeschoeiing, vervolgens zouden autogene morfologische processen een kronkelende loop moeten vormen. In een grootschalig veldexperiment (Hooge Raam) zijn deze morfologische processen onderzocht. De invloed van exogene processen op de initiatie van meandering zijn in detail bestudeerd in het Gelderns-Nierskanaal. In de Lunterse beek speelden stuweffecten een subtstantiële rol in de morfologische ontwikkelingen die uiteindelijk tot een bochtafsnijding hebben geleid. De term ’hermeandering´ is misleidend, vanwege de suggestie dat laaglandbeken de kenmerken zouden hebben van actief meanderende rivieren. Dit onderzoek heeft zich met name gericht op korte morfologische tijdschalen vanwege de relevantie voor het Nederlandse waterbeheer
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
- …
