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Stream Power (stream + power)
Selected AbstractsLithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto RicoEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2010Andrew S. Pike Abstract An extensive survey and topographic analysis of five watersheds draining the Luquillo Mountains in north-eastern Puerto Rico was conducted to decouple the relative influences of lithologic and hydraulic forces in shaping the morphology of tropical montane stream channels. The Luquillo Mountains are a steep landscape composed of volcaniclastic and igneous rocks that exert a localized lithologic influence on the stream channels. However, the stream channels also experience strong hydraulic forcing due to high unit discharge in the humid rainforest environment. GIS-based topographic analysis was used to examine channel profiles, and survey data were used to analyze downstream changes in channel geometry, grain sizes, stream power, and shear stresses. Results indicate that the longitudinal profiles are generally well graded but have concavities that reflect the influence of multiple rock types and colluvial-alluvial transitions. Non-fluvial processes, such as landslides, deliver coarse boulder-sized sediment to the channels and may locally determine channel gradient and geometry. Median grain size is strongly related to drainage area and slope, and coarsens in the headwaters before fining in the downstream reaches; a pattern associated with a mid-basin transition between colluvial and fluvial processes. Downstream hydraulic geometry relationships between discharge, width and velocity (although not depth) are well developed for all watersheds. Stream power displays a mid-basin maximum in all basins, although the ratio of stream power to coarse grain size (indicative of hydraulic forcing) increases downstream. Excess dimensionless shear stress at bankfull flow wavers around the threshold for sediment mobility of the median grain size, and does not vary systematically with bankfull discharge; a common characteristic in self-forming ,threshold' alluvial channels. The results suggest that although there is apparent bedrock and lithologic control on local reach-scale channel morphology, strong fluvial forces acting over time have been sufficient to override boundary resistance and give rise to systematic basin-scale patterns. Copyright © 2010 John Wiley and Sons, Ltd. [source] Wood distribution in neotropical forested headwater streams of La Selva, Costa RicaEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2009Daniel Cadol Abstract Surveys of wood along 30 forested headwater stream reaches in La Selva Biological Station in north-eastern Costa Rica represent the first systematic data reported on wood loads in neotropical streams. For streams with drainage areas of 0·1,8·5 km2 and gradients of 0·2,8%, wood load ranged from 3 to 34·7 m3 wood/100 m channel and 41,612 m3 wood/ha channel. These values are within the range reported for temperate streams. The variables wood diameter/flow depth, stream power, the presence of backflooding, and channel width/depth are consistently selected as significant predictors by statistical models for wood load. These variables explain half to two-thirds of the variability in wood load. These results, along with the spatial distribution of wood with respect to the thalweg, suggest that transport processes exert a greater influence on wood loads than recruitment processes. Wood appears to be more geomorphically effective in altering bed elevations in gravel-bed reaches than in reaches with coarser or finer substrate. Copyright © 2009 John Wiley & Sons, Ltd. [source] Scale-dependent controls upon the fluvial export of large wood from river catchmentsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 6 2009Jung Il Seo Abstract The annual fluvial export of large wood (LW) was monitored by local reservoir management offices in Japan. LW export per unit watershed area was relatively high in small watersheds, peaked in intermediate watersheds, and decreased in large watersheds. To explain these variations, we surveyed the amount of LW with respect to channel morphology in 78 segments (26 segments in each size class) in the Nukabira River, northern Japan. We examined the differences in LW dynamics, including its recruitment, transport, storage, and fragmentation and decay along the spectrum of watershed sizes. We found that a large proportion of LW produced by forest dynamics and hillslope processes was retained because of the narrower valley floors and lower stream power in small watersheds. The retained LW pieces may eventually be exported during debris flows. In intermediate watersheds, the volume of LW derived from hillslopes decreased substantially with reductions in the proportion of channel length bordered by hillslope margins, which potentially deliver large quantities of LW. Because these channels have lower wood piece length to channel width ratios and higher stream power, LW pieces can be transported downstream. During transport, LW pieces are further fragmented and can be more easily transported. Therefore, the fluvial export of LW is maximized in intermediate watersheds. Rivers in large watersheds, where the recruitment of LW is limited by the decreasing hillslope margins, cannot transport LW pieces because of their low stream power, and thus LW pieces accumulate at various storage sites. Although these stored LW pieces can be refloated and transported by subsequent flood events, they may also become trapped by obstacles such as logjams and standing trees on floodplains and in secondary channels, remaining there for decades and eventually decaying into fine organic particles. Thus, the fluvial export of LW pieces is low in large watersheds. Copyright © 2009 John Wiley & Sons, Ltd. [source] Reduced-complexity flow routing models for sinuous single-thread channels: intercomparison with a physically-based shallow-water equation modelEARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2009A. P. Nicholas Abstract Reduced-complexity models of fluvial processes use simple rules that neglect much of the underlying governing physics. This approach is justified by the potential to use these models to investigate long-term and/or fundamental river behaviour. However, little attention has been given to the validity or realism of reduced-complexity process parameterizations, despite the fact that the assumptions inherent in these approaches may limit the potential for elucidating the behaviour of natural rivers. This study presents two new reduced-complexity flow routing schemes developed specifically for application in single-thread rivers. Output from both schemes is compared with that from a more sophisticated model that solves the depth-averaged shallow water equations. This comparison provides the first demonstration of the potential for deriving realistic predictions of in-channel flow depth, unit discharge, energy slope and unit stream power using simple flow routing schemes. It also highlights the inadequacy of modelling unit stream power, shear stress or sediment transport capacity as a function of local bed slope, as has been common practice in a number of previous reduced-complexity models. Copyright © 2009 John Wiley & Sons, Ltd. [source] A conceptual model for the longitudinal distribution of wood in mountain streamsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 3 2009Ellen Wohl Abstract Wood load, channel parameters and valley parameters were surveyed in 50 contiguous stream segments each 25 m in length along 12 streams in the Colorado Front Range. Length and diameter of each piece of wood were measured, and the orientation of each piece was tallied as a ramp, buried, bridge or unattached. These data were then used to evaluate longitudinal patterns of wood distribution in forested headwater streams of the Colorado Front Range, and potential channel-, valley- and watershed-scale controls on these patterns. We hypothesized that (i) wood load decreases downstream, (ii) wood is non-randomly distributed at channel lengths of tens to hundreds of meters as a result of the presence of wood jams and (iii) the proportion of wood clustered into jams increases with drainage area as a result of downstream increases in relative capacity of a stream to transport wood introduced from the adjacent riparian zone and valley bottom. Results indicate a progressive downstream decrease in wood load within channels, and correlations between wood load and drainage area, elevation, channel width, bed gradient and total stream power. Results support the first and second hypotheses, but are inconclusive with respect to the third hypothesis. Wood is non-randomly distributed at lengths of tens to hundreds of meters, but the proportion of pieces in jams reaches a maximum at intermediate downstream distances within the study area. We use these results to propose a conceptual model illustrating downstream trends in wood within streams of the Colorado Front Range. Copyright © 2009 John Wiley & Sons, Ltd. [source] Flow energy and channel adjustments in rills developed in loamy sand and sandy loam soilsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2009Jovan R. Stefanovic Abstract The storms usually associated with rill development in nature are seldom prolonged, so development is often interrupted by interstorm disturbances, e.g. weathering or tillage. In laboratory simulated rainfall experiments, active rill development can be prolonged, and under these conditions typically passes through a period of intense incision, channel extension and bifurcation before reaching quasi-stable conditions in which little form change occurs. This paper presents laboratory experiments with coarse textured soils under simulated rainfall which show how channel adjustment processes contribute to the evolution of quasi-stability. Newly incised rills were stabilized for detailed study of links between rill configuration and flow energy. On a loamy sand, adjustment towards equilibrium occurred due to channel widening and meandering, whereas on a sandy loam, mobile knickpoints and chutes, pulsations in flow width and flow depth and changes in stream power and sediment discharge occurred as the channel adjusted towards equilibrium. The tendency of rill systems towards quasi-stability is shown by changes in stream power values which show short-lived minima. Differences in energy dissipation in stabilized rills indicate that minimization of energy dissipation was reached locally between knickpoints and at the downstream ends of rills. In the absence of energy gradients in knickpoints and chutes, stabilized rill sections tended toward equilibrium by establishing uniform energy expenditure. The study confirmed that energy dissipation increased with flow aspect ratio. In stabilized rills, flow acceleration reduced energy dissipation on the loamy sand but not on the sandy loam. On both soils flow deceleration tended to increase energy dissipation. Understanding how rill systems evolve towards stability is essential in order to predict how interruptions between storms may affect long-term rill dynamics. This is essential if event-based physical models are to become effective in predicting sediment transport on rilled hillslopes under changing weather and climatic conditions. Copyright © 2008 John Wiley and Sons, Ltd. [source] Bed morphology and generation of step,pool channelsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 11 2008Roman B. Weichert Abstract Flume experiments have been carried out to study the formation processes and the bed morphology of step,pool channels. From the experiments different step types and step configurations could be distinguished depending on the stream power. These step types can be seen as an image of the generation mechanisms of step,pool systems. These results suggest that the bed roughness geometry develops towards a condition that provides the maximum possible bed stability for a given grain size distribution. In contrast to a variety of other studies, antidunes did not contribute to the generation of the step structures. However, the data of the presented study fits well into the region of antidune formation proposed by Kennedy for sand-bed rivers. This observation points out that step,pool field-data located in the Kennedy region do not inevitably prove that antidunes played a role in step development. It is rather proposed that in Kennedy's region of antidune formation there exist hydraulic conditions where the flow resistance is maximized. It is suggested that such maximum flow resistance is associated with an optimal distance between the bedforms and their height, independently of whether these are antidunes in sand- and gravel-bed rivers or step,pool units in boulder-bed streams. The considerations of the Kennedy region of antidune formation and the analysis of planform step types depending on stream power both suggest that steep channels have a potential for self-stabilization by modifying the step,pool structure towards a geometry that provides maximum flow resistance and maximum bed stability. Copyright © 2008 John Wiley & Sons, Ltd. [source] Optical remote mapping of rivers at sub-meter resolutions and watershed extentsEARTH SURFACE PROCESSES AND LANDFORMS, Issue 1 2008W. Andrew Marcus Abstract At watershed extents, our understanding of river form, process and function is largely based on locally intensive mapping of river reaches, or on spatially extensive but low density data scattered throughout a watershed (e.g. cross sections). The net effect has been to characterize streams as discontinuous systems. Recent advances in optical remote sensing of rivers indicate that it should now be possible to generate accurate and continuous maps of in-stream habitats, depths, algae, wood, stream power and other features at sub-meter resolutions across entire watersheds so long as the water is clear and the aerial view is unobstructed. Such maps would transform river science and management by providing improved data, better models and explanation, and enhanced applications. Obstacles to achieving this vision include variations in optics associated with shadows, water clarity, variable substrates and target,sun angle geometry. Logistical obstacles are primarily due to the reliance of existing ground validation procedures on time-of-flight field measurements, which are impossible to accomplish at watershed extents, particularly in large and difficult to access river basins. Philosophical issues must also be addressed that relate to the expectations around accuracy assessment, the need for and utility of physically based models to evaluate remote sensing results and the ethics of revealing information about river resources at fine spatial resolutions. Despite these obstacles and issues, catchment extent remote river mapping is now feasible, as is demonstrated by a proof-of-concept example for the Nueces River, Texas, and examples of how different image types (radar, lidar, thermal) could be merged with optical imagery. The greatest obstacle to development and implementation of more remote sensing, catchment scale ,river observatories' is the absence of broadly based funding initiatives to support collaborative research by multiple investigators in different river settings. Copyright © 2007 John Wiley & Sons, Ltd. [source] Regional analysis of bedrock stream long profiles: evaluation of Hack's SL form, and formulation and assessment of an alternative (the DS form)EARTH SURFACE PROCESSES AND LANDFORMS, Issue 5 2007Geoff Goldrick Abstract The equilibrium form of the fluvial long profile has been used to elucidate a wide range of aspects of landscape history including tectonic activity in tectonic collision zones, and in continental margin and other intraplate settings, as well as other base-level changes such as due to sealevel fluctuations. The Hack SL form of the long profile, which describes a straight line on a log,normal plot of elevation (normal) versus distance (logarithmic), is the equilibrium long profile form that has been most widely used in such studies; slope,area analysis has also been used in recent years. We show that the SL form is a special case of a more general form of the equilibrium long profile (here called the DS form) that can be derived from the power relationship between stream discharge and downstream distance, and the dependence of stream incision on stream power. The DS form provides a better fit than the SL form to river long profiles in an intraplate setting in southeastern Australia experiencing low rates of denudation and mild surface uplift. We conclude that, if an a priori form of the long profile is to be used for investigations of regional landscape history, the DS form is preferable. In particular, the DS form in principle enables equilibrium steepening due to an increase in channel substrate lithological resistance (parallel shift in the DS plot) to be distinguished from disequilibrium steepening due to long profile rejuvenation (disordered outliers on the DS plot). Slope,area analysis and the slope,distance (DS) approach outlined here are complementary approaches, reflecting the close relationship between downstream distance and downstream catchment area. Copyright © 2006 John Wiley & Sons, Ltd. [source] Long-term landscape evolution: linking tectonics and surface processesEARTH SURFACE PROCESSES AND LANDFORMS, Issue 3 2007Paul Bishop Abstract Research in landscape evolution over millions to tens of millions of years slowed considerably in the mid-20th century, when Davisian and other approaches to geomorphology were replaced by functional, morphometric and ultimately process-based approaches. Hack's scheme of dynamic equilibrium in landscape evolution was perhaps the major theoretical contribution to long-term landscape evolution between the 1950s and about 1990, but it essentially ,looked back' to Davis for its springboard to a viewpoint contrary to that of Davis, as did less widely known schemes, such as Crickmay's hypothesis of unequal activity. Since about 1990, the field of long-term landscape evolution has blossomed again, stimulated by the plate tectonics revolution and its re-forging of the link between tectonics and topography, and by the development of numerical models that explore the links between tectonic processes and surface processes. This numerical modelling of landscape evolution has been built around formulation of bedrock river processes and slope processes, and has mostly focused on high-elevation passive continental margins and convergent zones; these models now routinely include flexural and denudational isostasy. Major breakthroughs in analytical and geochronological techniques have been of profound relevance to all of the above. Low-temperature thermochronology, and in particular apatite fission track analysis and (U,Th)/He analysis in apatite, have enabled rates of rock uplift and denudational exhumation from relatively shallow crustal depths (up to about 4 km) to be determined directly from, in effect, rock hand specimens. In a few situations, (U,Th)/He analysis has been used to determine the antiquity of major, long-wavelength topography. Cosmogenic isotope analysis has enabled the determination of the ,ages' of bedrock and sedimentary surfaces, and/or the rates of denudation of these surfaces. These latter advances represent in some ways a ,holy grail' in geomorphology in that they enable determination of ,dates and rates' of geomorphological processes directly from rock surfaces. The increasing availability of analytical techniques such as cosmogenic isotope analysis should mean that much larger data sets become possible and lead to more sophisticated analyses, such as probability density functions (PDFs) of cosmogenic ages and even of cosmogenic isotope concentrations (CICs). PDFs of isotope concentrations must be a function of catchment area geomorphology (including tectonics) and it is at least theoretically possible to infer aspects of source area geomorphology and geomorphological processes from PDFs of CICs in sediments (,detrital CICs'). Thus it may be possible to use PDFs of detrital CICs in basin sediments as a tool to infer aspects of the sediments' source area geomorphology and tectonics, complementing the standard sedimentological textural and compositional approaches to such issues. One of the most stimulating of recent conceptual advances has followed the considerations of the relationships between tectonics, climate and surface processes and especially the recognition of the importance of denudational isostasy in driving rock uplift (i.e. in driving tectonics and crustal processes). Attention has been focused very directly on surface processes and on the ways in which they may ,drive' rock uplift and thus even influence sub-surface crustal conditions, such as pressure and temperature. Consequently, the broader geoscience communities are looking to geomorphologists to provide more detailed information on rates and processes of bedrock channel incision, as well as on catchment responses to such bedrock channel processes. More sophisticated numerical models of processes in bedrock channels and on their flanking hillslopes are required. In current numerical models of long-term evolution of hillslopes and interfluves, for example, the simple dependency on slope of both the fluvial and hillslope components of these models means that a Davisian-type of landscape evolution characterized by slope lowering is inevitably ,confirmed' by the models. In numerical modelling, the next advances will require better parameterized algorithms for hillslope processes, and more sophisticated formulations of bedrock channel incision processes, incorporating, for example, the effects of sediment shielding of the bed. Such increasing sophistication must be matched by careful assessment and testing of model outputs using pre-established criteria and tests. Confirmation by these more sophisticated Davisian-type numerical models of slope lowering under conditions of tectonic stability (no active rock uplift), and of constant slope angle and steady-state landscape under conditions of ongoing rock uplift, will indicate that the Davis and Hack models are not mutually exclusive. A Hack-type model (or a variant of it, incorporating slope adjustment to rock strength rather than to regolith strength) will apply to active settings where there is sufficient stream power and/or sediment flux for channels to incise at the rate of rock uplift. Post-orogenic settings of decreased (or zero) active rock uplift would be characterized by a Davisian scheme of declining slope angles and non-steady-state (or transient) landscapes. Such post-orogenic landscapes deserve much more attention than they have received of late, not least because the intriguing questions they pose about the preservation of ancient landscapes were hinted at in passing in the 1960s and have recently re-surfaced. As we begin to ask again some of the grand questions that lay at the heart of geomorphology in its earliest days, large-scale geomorphology is on the threshold of another ,golden' era to match that of the first half of the 20th century, when cyclical approaches underpinned virtually all geomorphological work. Copyright © 2007 John Wiley & Sons, Ltd. [source] Evolution of channel morphology and hydrologic response in an urbanizing drainage basinEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2006Peter A. Nelson Abstract The Dead Run catchment in Baltimore County, Maryland, has undergone intense urbanization since the late 1950s. Reconstruction of the channel planform from topographic maps dating back to the 1890s and aerial photographs dating back to the 1930s indicates that the channel has remained stable in planform since at least the 1930s. The relative stability of Dead Run contrasts with the alterations in channel morphology reported for other urbanizing streams in the Piedmont physiographic province of the eastern United States. Trend analyses of discharge records in Dead Run show that urban development and stormwater control measures have had significant impacts on the hydrologic response of the catchment. The flood hydraulics of the Dead Run catchment are examined for the event that occurred on 22 June 1972 in association with Hurricane Agnes. A two-dimensional hydraulic model, TELEMAC-2D, was used with a finite-element mesh constructed from a combination of high-resolution LiDAR topographic data and detailed field survey data to analyse the distribution of boundary shear stress and unit stream power along the channel and floodplain during flooding from Hurricane Agnes. The spatial and temporal distributions of these parameters, relative to channel gradient and channel/valley bottom geometry, provide valuable insights on the stability of the Dean Run channel. The stability of Dead Run's channel planform, in spite of extreme flooding and decades of urban development, is most likely linked to geological controls of channel and floodplain morphology. Copyright © 2006 John Wiley & Sons, Ltd. [source] Wood storage in a wide mountain river: case study of the Czarny Dunajec, Polish CarpathiansEARTH SURFACE PROCESSES AND LANDFORMS, Issue 12 2005omiej Wy Abstract Storage of large woody debris in the wide, mountain, Czarny Dunajec River, southern Poland, was investigated following two floods of June and July 2001 with a seven-year frequency. Within a reach, to which wood was delivered only by bank erosion and transport from upstream, wood quantities were estimated for eighty-nine, 100 m long, channel segments grouped into nine sections of similar morphology. Results from regression analysis indicated the quantity of stored wood to be directly related to the length of eroded, wooded banks and river width, and inversely related to unit stream power at the flood peak. The largest quantities of wood (up to 33 t ha,1) were stored in wide, multi-thread river sections. Here, the relatively low transporting ability of the river facilitated deposition of transported wood while a considerable length of eroded channel and island banks resulted in a large number of trees delivered from the local riparian forest. In these sections, a few morphological and ecological situations led to the accumulation of especially large quantities of wood within a small river area. Very low amounts of wood were stored in narrow, single-thread sections of regulated or bedrock channel. High stream power facilitated transport of wood through these sections while the high strength of the banks and low channel sinuosity prevented bank retreat and delivery of trees to the channel. Considerable differences in the character of deposited wood existed between wide, multi-thread channel sections located at different distances below a narrow, 7 km long, channellized reach of the river. Wood deposited close to the downstream end of the channellized reach was highly disintegrated and structured into jams, whereas further downstream well preserved shrubs and trees prevailed. This apparently reflects differences in the distance of wood transport and shows that in a mountain river wider than the height of trees growing on its banks, wood can be transported long distances along relatively narrow, single-thread reaches but is preferentially deposited in wide, multi-thread reaches. Copyright © 2005 John Wiley & Sons, Ltd. [source] A model of equilibrium bed topography for meander bends with erodible banksEARTH SURFACE PROCESSES AND LANDFORMS, Issue 10 2002Stephen E. Darby Abstract Channel curvature produces secondary currents and a transverse sloping channel bed, along which the depth increases towards the outer bank. As a result deep pools tend to form adjacent to the outer bank, promoting bank collapse. The interaction of sediment grains with the primary and secondary flow and the transverse sloping bed also causes meanders to move different grain sizes in different proportions and directions, resulting in a consistent sorting pattern. Several models have been developed to describe this process, but they all have the potential to over-predict pool depth because they cannot account for the influence of erodible banks. In reality, bank collapse might lead to the development of a wider, shallower cross-section and any resulting flow depth discrepancy can bias associated predictions of flow, sediment transport, and grain-size sorting. While bed topography, sediment transport and grain sorting in bends will partly be controlled by the sedimentary characteristics of the bank materials, the magnitude of this effect has not previously been explored. This paper reports the development of a model of flow, sediment transport, grain-size sorting, and bed topography for river bends with erodible banks. The model is tested via intercomparison of predicted and observed bed topography in one low-energy (5·3 W m,2 specific stream power) and one high-energy (43·4 W m,2) study reach, namely the River South Esk in Scotland and Goodwin Creek in Mississippi, respectively. Model predictions of bed topography are found to be satisfactory, at least close to the apices of bends. Finally, the model is used in sensitivity analyses that provide insight into the influence of bank erodibility on equilibrium meander morphology and associated patterns of grain-size sorting. The sensitivity of meander response to bank cohesion is found to increase as a function of the available stream power within the two study bends. Copyright © 2002 John Wiley & Sons, Ltd. [source] A stability criterion inherent in laws governing alluvial channel flowEARTH SURFACE PROCESSES AND LANDFORMS, Issue 9 2002He Qing Huang Abstract The stability criterion of maximum flow efficiency (MFE) has previously been found inherent in typical alluvial channel flow relationships, and this study investigates the general nature of this criterion using a wider range of flow resistance and bedload transport formulae. For straight alluvial channels, in which the effect of sediment sorting is insignificant, our detailed mathematical analysis demonstrates that a flow efficiency factor , occurs generally as the ratio of sediment (bedload) discharge Qs to stream power , (,QS) in the form of . When , is maximized (i.e. Qs is maximized or , is minimized), maximally efficient straight channel geometries derived from most flow resistance and bedload transport formulae are found compatible with observed bankfull hydraulic geometry relations. This study provides support for the use of the criteria of MFE, maximum sediment transporting capacity and minimum stream power for understanding the operation of alluvial rivers, and also addresses limitations to the direct application of its findings. Copyright © 2002 John Wiley & Sons, Ltd. [source] Hydrodynamics and geomorphic work of jökulhlaups (glacial outburst floods) from Kverkfjöll volcano, IcelandHYDROLOGICAL PROCESSES, Issue 6 2007Jonathan L. Carrivick Abstract Jökulhlaups (glacial outburst floods) occur frequently within most glaciated regions of the world and cause rapid landscape change, infrastructure damage, and human disturbance. The largest jökulhlaups known to have occurred during the Holocene within Iceland drained from the northern margin of Vatnajökull and along the Jökulsá á Fjöllum. Some of these jökulhlaups originated from Kverkfjöll volcano and were routed through anastomosing, high gradient and hydraulically rough channels. Landforms and sediments preserved within these channels permit palaeoflow reconstructions. Kverkfjöll jökulhlaups were reconstructed using palaeocompetence (point measurements), slope,area (one-dimensional), and depth-averaged two-dimensional (2D) hydrodynamic modelling techniques. The increasing complexity of 2D modelling required a range of assumptions, but produced information on both spatial and temporal variations in jökulhlaup characteristics. The jökulhlaups were volcanically triggered, had a linear-rise hydrograph and a peak discharge of 50 000,100 000 m3 s,1, which attenuated by 50,75% within 25 km. Frontal flow velocities were ,2 m s,1; but, as stage increased, velocities reached 5,15m s,1. Peak instantaneous shear stress and stream power reached 1 × 104 N m,2 and 1 × 105 W m,2 respectively. Hydraulic parameters can be related to landform groups. A hierarchy of landforms is proposed, ranging from the highest energy zones (erosional gorges, scoured bedrock, cataracts, and spillways) to the lowest energy zones (of valley fills, bars, and slackwater deposits). Fluvial erosion of bedrock occurred in Kverkfjallarani above ,3 m flow depth, ,7m s,1 flow velocity, ,1 × 102 N m,2 shear stress, and 3 × 102 W m,2 stream power. Fluvial deposition occurred in Kverkfjallarani below ,8 m flow depth, 11 m s,1 flow velocity, 5 × 102 N m,2 shear stress, and 3 × 103 W m,2 stream power. Hence, erosional and depositional ,envelopes' have considerable overlap, probably due to transitional flow phenomena and the influence of upstream effects, such as hydraulic ponding and topographic constrictions, for example. Holocene Kverkfjöll jökulhlaups achieved geomorphic work comparable to that of other late Pleistocene ,megafloods'. This work was a result of steep channel gradients, topographic channel constrictions, and high hydraulic roughness, rather than to extreme peak discharges. The Kverkfjöll jökulhlaups have implications for landscape evolution in north-central Iceland, for water-sediment inputs into the North Atlantic, and for recognizing jökulhlaups in the rock record. 2D hydrodynamic modelling is likely to be important for hazard mitigation in similar landscapes and upon other glaciated volcanoes, because it only requires an input hydrograph and a digital elevation model to run a model, rather than suites of geomorphological evidence and field-surveyed valley cross-sections, for example. Copyright © 2006 John Wiley & Sons, Ltd. [source] Modern and ancient fluvial megafans in the foreland basin system of the central Andes, southern Bolivia: implications for drainage network evolution in fold-thrust beltsBASIN RESEARCH, Issue 1 2001B. K. Horton ABSTRACT Fluvial megafans chronicle the evolution of large mountainous drainage networks, providing a record of erosional denudation in adjacent mountain belts. An actualistic investigation of the development of fluvial megafans is presented here by comparing active fluvial megafans in the proximal foreland basin of the central Andes to Tertiary foreland-basin deposits exposed in the interior of the mountain belt. Modern fluvial megafans of the Chaco Plain of southern Bolivia are large (5800,22 600 km2), fan-shaped masses of dominantly sand and mud deposited by major transverse rivers (Rio Grande, Rio Parapeti, and Rio Pilcomayo) emanating from the central Andes. The rivers exit the mountain belt and debouch onto the low-relief Chaco Plain at fixed points along the mountain front. On each fluvial megafan, the presently active channel is straight in plan view and dominated by deposition of mid-channel and bank-attached sand bars. Overbank areas are characterized by crevasse-splay and paludal deposition with minor soil development. However, overbank areas also contain numerous relicts of recently abandoned divergent channels, suggesting a long-term distributary drainage pattern and frequent channel avulsions. The position of the primary channel on each megafan is highly unstable over short time scales. Fluvial megafans of the Chaco Plain provide a modern analogue for a coarsening-upward, > 2-km-thick succession of Tertiary strata exposed along the Camargo syncline in the Eastern Cordillera of the central Andean fold-thrust belt, about 200 km west of the modern megafans. Lithofacies of the mid-Tertiary Camargo Formation include: (1) large channel and small channel deposits interpreted, respectively, as the main river stem on the proximal megafan and distributary channels on the distal megafan; and (2) crevasse-splay, paludal and palaeosol deposits attributed to sedimentation in overbank areas. A reversal in palaeocurrents in the lowermost Camargo succession and an overall upward coarsening and thickening trend are best explained by progradation of a fluvial megafan during eastward advance of the fold-thrust belt. In addition, the present-day drainage network in this area of the Eastern Cordillera is focused into a single outlet point that coincides with the location of the coarsest and thickest strata of the Camargo succession. Thus, the modern drainage network may be inherited from an ancestral mid-Tertiary drainage network. Persistence and expansion of Andean drainage networks provides the basis for a geometric model of the evolution of drainage networks in advancing fold-thrust belts and the origin and development of fluvial megafans. The model suggests that fluvial megafans may only develop once a drainage network has reached a particular size, roughly 104 km2, a value based on a review of active fluvial megafans that would be affected by the tectonic, climatic and geomorphologic processes operating in a given mountain belt. Furthermore, once a drainage network has achieved this critical size, the river may have sufficient stream power to prove relatively insensitive to possible geometric changes imparted by growing frontal structures in the fold-thrust belt. [source] | ||||||||||