Showing papers on "Stream power published in 2010"

Evolution of vertical knickpoints (waterfalls) with resistant caprock: Insights from numerical modeling

Abstract: [1] Vertical knickpoints (waterfalls) mark a prominent process transition zone whose governing mechanics is not represented by conventional stream power incision models. We examine the evolution of vertical knickpoints with resistant caprock utilizing numerical simulations that explicitly represent (1) face failure mechanisms, (2) flow acceleration and amplified erosion above a knickpoint lip, (3) deposition and removal of coarse debris below the knickpoint, and (4) base level lowering or tectonic uplift rates. Our model demonstrates that knickpoint retreat rate, where the subcaprock is weak or vertically jointed and base level fall rates are steady, is likely to become tied to downstream conditions and equal to the downstream incision rate divided by channel gradient. Mechanically, this coupling occurs where the subcaprock reaches a threshold height for failure in shear or buckling or where the weathering rate of the subcaprock is higher than the downstream incision wave velocity (Vi_ds). The height of the subcaprock face can influence its gravitational stability and the knickpoint lateral erosion rate and lead to a feedback between downstream incision and retreat rate. Retreat rate can be lower than Vi_ds during transients, which could be long (>106 years) and set by the weathering rate of the subcaprock or influenced by lag debris evacuation. Key variables other than discharge can be important in setting retreat rates. These include base level lowering rate, the rock strength of stratigraphic units downstream of the knickpoint, and the size and flux of sediment contributed from above the knickpoint or from the canyon walls. Two types of oversteepened reaches can form in association with a vertical knickpoint: (1) an upstream, free fall-induced, oversteepened reach whose length is longer than the flow acceleration zone and (2) a downstream coarse debris-induced oversteepened reach. Although the model was constructed with caprock-type knickpoints in mind, some of its elements and insights are also relevant to homogenous substrates.

Effects of stream flow patterns on riparian vegetation of a semiarid river: Implications for a changing climate

Abstract: As global climate change affects recharge and runoff processes, stream flow regimes are being altered. In the American Southwest, increasing aridity is predicted to cause declines in stream base flows and water tables. Another potential outcome of climate change is increased flood intensity. Changes in these stream flow conditions may independently affect vegetation or may have synergistic effects. Our goal was to extrapolate vegetation response to climate-linked stream flow changes, by taking advantage of the spatial variation in flow conditions over a 200 km length of the San Pedro River (Arizona). Riparian vegetation traits were contrasted between sites differing in low-flow hydrology (degree of stream intermittency) and flood intensity (stream power of the 10-year recurrence flood). Field data indicate that increased stream intermittency would cause the floodplain plant community to shift from hydric pioneer trees and shrubs (Populus, Salix, Baccharis) towards mesic species (Tamarix). This shift in functional type would produce changes in vegetation structure, with reduced canopy cover and shorter canopies at drier sites. Among herbaceous species, annuals would increase while perennials would decrease. If flood intensities increased, there would be shifts towards younger tree age, expansion of xeric pioneer shrubs (in response to flood-linked edaphic changes), and replacement of herbaceous perennials by annuals. Woody stem density would increase and basal area would decrease, reflecting shifts towards younger forests. Some effects would be compounded: Annuals were most prevalent, and tree canopies shortest, at sites that were dry and intensely flooded. Vegetational changes would feedback onto hydrologic and geomorphic processes, of importance for modeling. Increased flood intensity would have positive feedback on disturbance processes, by shifting plant communities towards species with less ability to stabilize sediments. Feedbacks between riparian vegetation and stream low-flow changes would be homeostatic, with reduced evapotranspiration rates ameliorating declines in base flows arising from increased aridity. Copyright © 2009 John Wiley & Sons, Ltd.

Incision and channel morphology across active structures along the Peikang River, central Taiwan: Implications for the importance of channel width

Abstract: River morphology and dynamics are strongly influenced by active tectonics. We report channel dynamics for the Peikang River, which flows through the Hsuehshan Range in central Taiwan. Using a digital elevation model and field surveys, we constrain channel morphology for an ∼90 km stretch of river to calculate unit stream power and boundary shear stress along the river path. Incision rates are estimated with optically stimulated luminescence dating of sand deposited on strath terraces. We find a strong correlation between unit stream power/shear stress and incision rate, but only if variation in channel width is considered. A calibrated river incision rule implies river incision rates of ∼9–13.5 mm/yr upstream of the Meiyuan and Tili faults and suggests that one or both of these structures are presently active. Our results indicate that the Shuilikeng fault is also actively deforming, as incision rates increase to ∼6–10 mm/yr across it, compared to 1–4 mm/yr in adjacent reaches. Prominent narrowing across the Shuilikeng fault, and the absence of significant gradient variation indicate that channel width is a first-order morphological adjustment to differential incision. Only when the channel width-to-depth ratio reaches a minimum does the channel slope significantly adjust to local changes in base level, as is the case upstream of the Meiyuan and Tili faults.

Lithological and fluvial controls on the geomorphology of tropical montane stream channels in Puerto Rico

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.

The job of the river

Abstract: Earth scientists have traditionally conceptualized rivers and streams as geomorphic machines, whose role is to transfer sediment and to sculpt the landscape. Steady-state relationships between sediment supply and transport capacity have traditionally been considered normative in fluvial systems. Rivers are hydrological entities, however, whose function is to redistribute excess moisture on land. The geomorphic work of the river – erosion, transport, deposition, etc. – is a byproduct of the hydrological job of the river. There is therefore no reason to expect any particular relationship between sediment supply and transport capacity to develop as a normative condition in fluvial systems. The apparent steady-state equilibrium slope adjustments of rivers are a byproduct of four basic phenomena: (1) hydraulic selection, which favors channels and branching networks over other flux patterns; (2) water flows along the available path of least resistance; (3) energy dissipation; and (4) finite relaxation times. Recognizing converging trends of stream power or slope and sediment supply as common (but far from inevitable) side effects rather than self-regulation has important implications for interpreting and predicting fluvial systems, and for river management and restoration. Such trends are variable, transient, contingent, and far from universal. Where they occur, they are an emergent byproduct of fundamental physical mechanisms, not a goal function or attractor state. Copyright © 2009 John Wiley & Sons, Ltd.

Functional relationships between vegetation, channel morphology, and flow efficiency in an alluvial (anabranching) river

Abstract: [1] Water and sediment flux interactions are examined in Magela Creek, an alluvial (anabranching) sand bed river in the northern Australian tropics. Dense riparian vegetation stabilizes the channels and floodplains thereby preventing erosional instability at flow depths up to 6.2 times bankfull and discharges up to 15 times bankfull. Narrow anabranching channels characterize >92% of the alluvial reach and transport bed load more efficiently than short reaches of wide single-channels, yet overall 29 ± 12% of the bed load is sequestered and the average vertical accretion rate is 0.41 ± 0.17 mm yr−1 along the 12 km study reach. The most effective discharge for transporting sediment (40–45 m3 s−1) is consistent at all 5 stations (10 channels) examined and is equivalent to the channel-forming discharge. It has an average recurrence interval of 1.01 years, occurs for an exceptionally long portion (13–15%) of the annual flow duration, and averages a remarkable 2.1 times bankfull. The high flow efficiency (i.e., bed load transport rate to stream power ratio) of the anabranches is facilitated by low width/depth channels with banks reinforced by vegetation. Colonnades of bank top trees confine high-velocity flows overbed (i.e., over the channel bed) at stages well above bankfull. At even larger overbank flows, momentum exchange between the channels and forested floodplains restrains overbed velocities, in some cases causing them to decline, thereby limiting erosion. Magela Creek exhibits a complicated set of planform, cross-sectional and vegetative adjustments that boost overbed velocities and enhance bed load yield in multiple channels while restraining velocities and erosion at the largest discharges.

Rill erosion in natural and disturbed forests: 2. Modeling Approaches

Abstract: [1] As forest management scenarios become more complex, the ability to more accurately predict erosion from those scenarios becomes more important. In this second part of a two-part study we report model parameters based on 66 simulated runoff experiments in two disturbed forests in the northwestern U.S. The 5 disturbance classes were natural, 10-month old and 2-week old low soil burn severity, high soil burn severity, and logging skid trails. In these environments the erosion rates were clearly detachment limited, and the rill erodibility parameters calculated from four hydraulic variables increased by orders of magnitude. The soil shear stress based erodibility parameter, Kr, was 1.5 × 10−6 s m−1in the natural plots, 2.0 × 10−4 s m−1 in the high soil burn severity plots, and 1.7 × 10−3 s m−1 in the skid trail plots; Kr values for the low soil burn severity plots had negative sign. The erodibility value for the skid trail plots fell within ranges reported for tilled agricultural fields and also for forest roads. The Kr values decreased as erosion occurred in the plots and therefore should not be a constant parameter. The stream power produced the largest R2 value (0.41) when hydraulic predictors and the sediment flux were log-transformed, but none of the four hydraulic variables (soil shear stress, stream power, unit stream power, and unit length shear force) explained much of the variability in sediment flux rates across the five levels of disturbance when evaluated in the linear form of the erosion models under consideration.

Effects of sediment load on hydraulics of overland flow on steep slopes.

Abstract: Eroded sediment may have significant effects on the hydraulics of overland flow, but few studies have been performed to quantify these effects on steep slopes. This study investigated the potential effects of sediment load on Reynolds number, Froude number, flow depth, mean velocity, Darcy–Weisbach friction coefficient, shear stress, stream power, and unit stream power of overland flow in a sand-glued hydraulic flume under a wide range of hydraulic conditions and sediment loads. Slope gradients were varied from 8·7 to 34·2%, unit flow rates from 0·66 to 5·26×10−3 m2 s−1, and sediment loads from 0 to 6·95 kg m−1 s−1. Both Reynolds number (Re) and Froude number (Fr) decreased as sediment load increased, implying a decrease in flow turbulence. This inverse relationship should be considered in modeling soil erosion processes. Flow depth increased as sediment load increased with a mean value of 1·227 mm, caused by an increase in volume of sediment-laden flow (contribution 62·4%) and a decrease in mean flow velocity (contribution 37·6%). The mean flow velocity decreased by up to 0·071 m s−1 as sediment load increased. The Darcy–Weisbach friction coefficient (f) increased with sediment load, showing that the total energy consumption increased with sediment load. The effects of sediment load on f depended on flow discharge: as flow discharge increased, the influence of sediment load on f decreased due to increased flow depth and reduced relative roughness. Flow shear stress and stream power increased with sediment load, on average, by 80·5% and 60·2%, respectively; however, unit stream power decreased by an average of 11·1% as sediment load increased. Further studies are needed to extend and apply the insights obtained under these controlled conditions to real-world overland flow conditions. Copyright © 2010 John Wiley & Sons, Ltd.

Substrate controls on the longitudinal profile of bedrock channels: Implications for reach‐scale roughness

Abstract: [1] In this paper we examine the relationships among bedrock properties and hydraulics in shaping bedrock channel morphology at the reach scale. The Ocoee River and four other bedrock streams in the Blue Ridge province of the southeastern United States, which have reach-scale differences in bedrock erodibility controlled by lithologic and structural variation, are the focus of this study. We describe a simple conceptual model for concentrated erosion in bedrock channels and test three hypotheses in order to investigate the interactions among rock erodibility, characteristics of undulating rib-like bed forms, reach-scale gradient, and hydraulic roughness and energy dissipation. Substrate differences correlate with variation in reach morphology (i.e., gradient, bed form orientation, and amplitude), such that less erodible substrates are associated with steeper reach gradient and with transversely oriented ribs of greater amplitude. One-dimensional modeling in HEC-RAS indicated that in the reach with the least erodible substrate and greatest bed slope and rib amplitude, the reach-averaged hydraulic roughness was the greatest. Increased hydraulic roughness in steeper reaches points to the importance of positive and negative feedbacks in these systems: Greater substrate erosional resistance limits profile lowering, which likely creates steeper bed slopes and greater stream power, creating a self-enhancing feedback. This local increase in stream power is balanced by increased roughness resulting from the erosional processes that produce bedrock ribs, which represents a self-regulating feedback. The overall result reflects quantifiable adjustments between substrate resistance and hydraulic driving forces in bedrock channels.

Alluvial river response to active tectonics in the Dehradun region, Northwest India: A case study of the Ganga and Yamuna rivers.

Abstract: In environments characterised by active tectonics, it is widely accepted that river morphology will be affected by active fault displacement. For example, there is documented evidence of change in channel slope, channel width, channel braiding patterns, grain size distribution trends, and stream power in response to active faults. Therefore, river morphology can carry a measurable signature of tectonic activity. Furthermore, it can be hypothesised that fluvial systems are in fact more sensitive to local faulting than raw topographic expression. This would mean that young active faults will affect river morphology before they are expressed in the local topography. Therefore, detailed morphological measurements of rivers in tectonic settings could allow for an early detection of faulting which is not yet expressed in the landscape. The Dehradun region of the Northwest Himalayan foothills is an ideal test case for this hypothesis. In this area, the Ganges and Yamuna rivers flow across an active thrust fault system, which is not yet clearly visible in the landscape. Therefore, longitudinal river profiles of the Ganges and Yamuna rivers from the Lesser Himalaya mountain front to ~35 km into the Gangetic Plain were extracted from a 90 m Digital Elevation Model and hydraulic features, including width of channel belt, channel slope, and geomorphic characteristics, were collected from IRS 23.5 m satellite images. Stream power was calculated by using channel slope and monthly discharge data. These data were complemented by field measurements of bed material grain sizes at ~5 km intervals. All data were analysed for the ~80 km Ganges and Yamuna reaches flowing from the Main Boundary Thrust, through the alluvial Dehra Dun valley and across the suspected active Himalayan Frontal Thrust, and ~35 km out into the Indo-Gangetic foreland. The longitudinal profile, width of channel belt, channel slope, braiding relationship, pattern of stream power, and grain size distribution all indicate river response to active slip on the Himalayan Frontal Thrust. Most importantly, channel slope (0.063) increases in response to an uplifted bed, and elevation drop over the fault axis. The Yamuna channel slope shallows upstream of, and proximal to the HFT (~0.0025) and steepens across the fault axis to (~0.0035) in response to an uplifted bed and elevation drop. Width of channel belts narrow across the HFT in response to constriction by uplifted topography from ~1500 m upstream to ~500 m across the HFT. This is reflected in the braiding index as both rivers flow as one channel across the HFT and as many channels in the Dun and foreland. The grain size trend along the Ganges reflects downstream fining by selective entrainment throughout the entire study reach with no variation interpreted as having a high enough stream power to move all sediment. The Yamuna indicates downstream fining through the Dun valley, yet grain size increases immediately, upstream of the HFT. This is interpreted as being due to low stream power within the Mohand Anticline caused by a low discharge and shallowing channel slope. This study concludes that the Ganges and Yamuna rivers are responding to active tectonic uplift of the HFT in the Dehradun basin, Northwest India.

Hydrodynamic modeling of juvenile mussel dispersal in a large river: the potential effects of bed shear stress and other parameters

Abstract: Because unionid mussels have a parasitic larval stage, they are able to disperse upstream and downstream as larvae while attached to their host fish and with flow as juveniles after excystment from the host. Understanding unionid population ecology requires knowledge of the processes that affect juvenile dispersal prior to establishment. We examined presettlement (transport and dispersion with flow) and early postsettlement (bed shear stress) hydraulic processes as negative censoring mechanisms. Our approach was to model dispersal using particle tracking through a 3-dimensional flow field output from hydrodynamic models of a reach of the Upper Mississippi River. We tested the potential effects of bed shear stress (τb) at 5 flow rates on juvenile mussel dispersal and quantified the magnitude of these effects as a function of flow rate. We explored the reach-scale relationships of Froude number (Fr), water depth (H), local bed slope (S), and unit stream power (QS) with the likelihood of juvenile set...

Spatial and temporal variation in suspended sediment, organic matter, and turbidity in a Minnesota prairie river: implications for TMDLs

Abstract: The Minnesota River Basin (MRB), situated in the prairie pothole region of the Upper Midwest, contributes excessive sediment and nutrient loads to the Upper Mississippi River. Over 330 stream channels in the MRB are listed as impaired by the Minnesota Pollution Control Agency, with turbidity levels exceeding water quality standards in much of the basin. Addressing turbidity impairment requires an understanding of pollutant sources that drive turbidity, which was the focus of this study. Suspended volatile solids (SVS), total suspended solids (TSS), and turbidity were measured over two sampling seasons at ten monitoring stations in Elm Creek, a turbidity impaired tributary in the MRB. Turbidity levels exceeded the Minnesota standard of 25 nephelometric units in 73% of Elm Creek samples. Turbidity and TSS were correlated (r (2) = 0.76), yet they varied with discharge and season. High levels of turbidity occurred during periods of high stream flow (May-June) because of excessive suspended inorganic sediment from watershed runoff, stream bank, and channel contributions. Both turbidity and TSS increased exponentially downstream with increasing stream power, bank height, and bluff erosion. However, organic matter discharged from wetlands and eutrophic lakes elevated SVS levels and stream turbidity in late summer when flows were low. SVS concentrations reached maxima at lake outlets (50 mg/l) in August. Relying on turbidity measurements alone fails to identify the cause of water quality impairment whether from suspended inorganic sediment or organic matter. Therefore, developing mitigation measures requires monitoring of both TSS and SVS from upstream to downstream reaches.

Sediment response to catchment disturbances

Abstract: 1 IntroductionThe surface of the planet undergoes a constant battle forbalance between uplift and erosion. Plate tectonics causeland to be created and mountains to be born, whileweathering and erosion act to lower the land surface. Riverstransport the products of weathering and erosion to storagepoints, or sinks; the ultimate sink being the deep oceans. Fora given landscape, an equilibrium is reached whereby therates of sediment distribution between sources and sinks arefairly constant. In the case of a river channel, the amount ofwater and sediment moved through a landscape is balancedby the energy available to do this, such that a change ineither of the two generates adjustments in stream power. Achange in environmental conditions often results in aresponse, the magnitude of which is a function of the typeand intensity of “disturbance” to the system. In the case ofthe river channel, this might manifest itself in the form ofchannel aggradation or degradation, whereby sediments areeither deposited or eroded in response to changes in availableenergy or sediment supply.Changestoalandscapearebothnaturalandanthropogenic.In terms of landscapes, we tend to think of a “disturbance”eventasonethatisfairlyextremeandproducesameasureableresponse in the rate or type of processes occurring in thelandscape. Examples of natural disturbances include earth-quakes, volcanic activities and wildfires. Examples ofanthropogenic disturbances include deforestation, intensiveagriculture, urbanization, dam construction and gravel min-ing. Typically, in natural systems, the types of geomorphicresponses to these events include increased rates of sedimentmobilization (i.e., soil erosion, mass movements), transport(i.e., delivery to and within river channel systems) anddeposition (i.e., in floodplains, lakes, estuaries). A goodexample is the 1980 eruption of Mount St Helens volcano,USA, where annual suspended sediment yields after theeruption were as much as 500 times greater than typicalbackground levels. Even 20 years after the eruption, averagesuspendedsedimentyieldsfromthedebris-avalanchedepositsremained 100 times (10

Hydraulic parameter estimations of a 2D model validated with sedimentological findings in the point bar environment.

Abstract: There have been a number of flume tests of flow round bends with idealized geometry and recently hydraulic simulations of such experiment. However, studies of hydraulic models in natural river bend are rather limited because of greater complexity of the flow characteristics and lack of detailed data. In this article, we study how 2D hydraulic model and raster-based hydraulic parameter calculations predict flow characteristics on the natural point bar environment. We will compare calculations of various hydraulic parameters (velocity, bed shear stress and stream power) by the 2D model and the associated sedimentology of the point bars. As a result of comparison, the usability of the 2D model for flow-form-product relationship predictions will be evaluated in natural river bend environment. The study shows that the 2D model can be generally utilized to predict the flood-generated flow-form-product relationship in coarse-grained and structurally complex point bar environments with sand-dominated bedload. For example, point bar sections submerged in water depths greater than 50 cm showed a relatively good match laterally between the model and sedimentological estimations. Furthermore, this approach allows us to estimate flood processes on a local scale in similar point bar environments with width–length ratio. The flow direction estimates of the 2D model coincided relatively well with the sedimentological estimations on the bar head. However, flow directions on the downstream section could not be modelled because the 2D model cannot handle the helicoidal flow of the river bend. Copyright © 2010 John Wiley & Sons, Ltd.

Constraining the timescales of sediment sequestration associated with large woody debris using cosmogenic 7Be

Abstract: [1] The beneficial ecogeomorphic functions associated with large woody debris (LWD) in fluvial environments are well documented and include positive sediment impacts such as channel margin sequestration, increased substrate heterogeneity, and decreased channel embeddedness, as well as numerous secondary benefits such as nutrient retention and increased habitat heterogeneity. Despite an extensive literature documenting such positive sediment attributes of LWD in forested channels, a quantitative analysis of in-channel sediment storage times associated with channel obstructions has traditionally been difficult to assess. In this study along a 9 km stretch of the Ducktrap River in coastal Maine we present a novel application of fallout cosmogenic 7Be (t1/2 = 53 days) coupled with a constant initial activity (CIA) sediment aging model to quantitatively assess transitional bed load storage times in bars associated with in-channel obstructions (LWD and boulders). We find that reach-scale variability in unit stream power and LWD frequency affect sediment storage times, with transport-limited reaches providing longer-term sediment sequestration (generally > 100 days) associated with in-channel obstructions than supply limited ones (<100 days). Estimates of sediment baraccumulation rates also varied between reaches from 0.2 g cm−2 d−1 in the supply limited reach to 0.7 g cm−2 d−1 in the transport-limited reach. Last, greater frequency of sites, increased sediment volumes and storage times, and naturally viable recruitment mechanisms for LWD in forested channels document its superior ecogeomorphic function when compared to boulders in this study, even in the Ducktrap river, where twentieth century logging has greatly reduced the size, frequency, and geomorphic efficacy of in-channel wood. This study has implications for channel restoration efforts and documents a novel application of 7Be and CIA methodology to constraining transitional bed load storage times in the fluvial environment.

The influence of sediment size, relative grain size and channel slope on initiation of sediment motion in boulder bed rivers. A lichenometric study

Abstract: Sediment transport of four boulder bed rivers is studied using lichenometry. The presence of lichens on boulders in the river channel is used to date the last mobilization of the blocks. Using size frequency diagrams and regional growth curves calibrated with dated reference points it is possible to determine the flood event responsible for the last mobilization of each boulder with lichens present. The specific stream power of flood events over the last 60 years is then calculated, and thresholds of sediment transport based on the sediment size are calculated. The results from the four studied rivers are compared to similar relationships in the literature. Sediment motion thresholds appear to be very variable within the same type of river (mountainous boulder bed rivers). The critical specific stream power necessary to mobilize a particle of a given diameter may vary by up to 10 times from one river to the next. Bed sediment size and river slope may explain this large range of stream powers. Calculation of the relative size of the transported particles (Di/D50) also shows that both hiding and protrusion effects, as well as channels slope, are important factors in sediment transport. Copyright © 2010 John Wiley & Sons, Ltd.

Large wood storage in channelized and unmanaged sections of the czarny dunajec river, polish carpathians: implications for the restoration of mountain rivers

Abstract: The amounts and distribution pattern of large wood stored in a 17 km long reach of the wide, mountain Czarny Dunajec River, southern Poland, were investigated following a 7-year flood to determine the potential of naturally recruited woody debris for restoration of mountain rivers affected by channelization and channel incision. The quantity of stored wood as well as the number and mean mass of wood accumulations were found to be directly related to the length of eroded, forested banks and river width, and inversely related to unit stream power at the flood peak. A regression line for the relationship with river width testifies to the existence of minimum channel width, below which no wood was typically retained in the river. Numerous wood accumulations of relatively high mass were retained in wide, multi-thread river sections. In these sections, wood was deposited in a variety of depositional sites, considerably contributing to the diversity of physical and biotic processes in the river. In contrast, very small number of wood accumulations of relatively low mass were retained in narrow, single-thread sections of regulated or bedrock channel. Here, wood was mainly deposited along the channel margins and has exerted little influence on the river functioning. The recognised pattern of wood storage indicates that in a mountain river wider than the height of trees growing on its banks, environmentally-significant quantities of large wood are deposited in wide, multi-thread sections already remaining in a relatively healthy environmental state but cannot be retained in narrow, channelized or incised river sections. This questions the utility of naturally recruited large wood for restoration of mountain rivers affected by channelization and channel incision. Successful restoration of such rivers would require the previous reduction of their transporting power or should be undertaken with the use of artificially placed wood structures.

Impact of climate change on future discharges and flow characteristics of the Tana River, Sub-Arctic northern Fennoscandia.

Abstract: . Climate change is expected to have a substantial impact on hydrology on both a global and regional scale. Although the anticipated warming is expected to be greatest in the northern regions and cause alteration in the hydrological cycle, it has yet to be resolved, to what extent these hydrological changes will alter such flow characteristics as flow velocity, bed shear stress and stream power in Sub-Arctic rivers. Future changes in the fluvial erosion potential are studied in the Sub-Arctic Tana River, on the border of Finland and Norway. We modelled future discharge scenarios for the years 2070 to 2099 with a conceptual hydrological model incorporating three emission scenarios, with two global and one regional climate model. These simulated flood discharges were used as input hydrographs to model flow characteristics with a two-dimensional hydraulic model. Differences in the spatial distribution of flow characteristics between frequent (HQ1/2a) and infrequent floods (HQ1/250a) were examined. Compared to the present, in most simulations, both HQ1/250a and HQ1/2a flood discharges diminished, with spring floods occurring earlier also. Although the relative reduction in flow characteristics (velocities, bed shear stresses and stream powers per unit area) was more notable in 1/2a compared to 1/250a floods, the discharge peaks of the former would theoretically still be able to transport the fine sediments that form the river bed. Based on most of the climate scenarios, autumnal floods become more frequent in the future and hence, their role in sediment transport may become more significant compared to the present-day situation.

Most Influential Parameters for the Bed‐Load Sediment Flux Equations Used in Alluvial Rivers

Abstract: Khorram, Saeed and Mustafa Ergil, 2010 Most Influential Parameters for the Bed-Load Sediment Flux Equations Used in Alluvial Rivers Journal of the American Water Resources Association (JAWRA): 46(6):1065–1090 DOI: 101111/j1752-1688201000468x Abstract: Problems of bed-load sediment transport equations in alluvial rivers are addressed in this study where user-friendly parameters were developed To determine the influences of 300 parameters on the final result, 52 selected bed-load equations for noncohesive particles (sand and gravel separately) were gathered and individually investigated The influences of discrepancies among the computed and measured datasets were obtained by sensitivity analysis through multilinear regression method The most influential parameters for the bed-load sediment flux equations used to describe sand particles in alluvial rivers are: the gravitational power due to Shields’ parameter with an energy slope, the universal stream power due to critical Shields’ parameter with an energy slope, the Shields’ parameter ratio, the critical unit stream power, and the Shields’ parameter with energy slope For gravel particles, the most influential parameters are: the universal stream power due to critical Shields’ parameter with an energy slope, the Shields’ parameter ratio, the gravitational power due to Shields’ parameter with an energy slope, the Shields’ parameter with an energy slope, and the Froude number of the channel It is expected that researchers working in this field will be able to use these predicted parameters to generate new bed-load sediment flux equations that give results that more closely agree with the actual values measured in alluvial rivers

Reconciled bedload sediment transport rates in ephemeral and perennial rivers.

Abstract: It has been thought for some time that bedload sediment transport rates may differ markedly in ephemeral and perennial rivers and, supporting this thought, there has been observation of very high rates of bedload transport by flash floods in the ephemeral river Nahal Yatir. However, until now, there has been no quantitative model resolving the observation, nor a theory capable of explaining why bedload transport rates by unsteady flash floods can be reasonably well described by bedload transport capacity formulae initially derived for steady flows. Here a time scale analysis of bedload transport is presented as pertaining to Nahal Yatir, which demonstrates that bedload transport can adapt sufficiently rapidly to capacity determined exclusively by local flow regime, and accordingly the transport capacity formulations developed for steady flows can be applied even under unsteady flows such as flash floods. Complementing the time scale analysis, a series of computational exercises using a coupled shallow water hydrodynamic model are shown to adequately resolve the observation of the very high rates of bedload transport by flash floods in Nahal Yatir. While bedload transport rates in ephemeral and perennial rivers differ remarkably when evaluated against a pure flow parameter such as specific stream power, they are essentially reconciled if assessed with a physically sensible parameter incorporating not only the flow regime but also the sediment particle size. The present finding underpins the practice of fluvial geomorphologists relating measured bedload transport to local flow and sediment characteristics only, irrespective of whether the flow is unsteady or steady. Copyright © 2010 John Wiley & Sons, Ltd.

A Sensitivity Analysis of Total‐Load Prediction Parameters in Standard Sediment Transport Equations1

Abstract: Khorram, Saeed and Mustafa Ergil, 2010 A Sensitivity Analysis of Total-Load Prediction Parameters in Standard Sediment Transport Equations Journal of the American Water Resources Association (JAWRA) 46(6):1091–1115 DOI: 101111/j1752-1688201000469x Abstract: The lack of a well-defined, strong correlation between sediment transport load and the dominant variable selected for the development of a sediment transport equation is one of the fundamental reasons for discrepancies between computed and measured results under different flow and sediment conditions Although several scholars have suggested different parameters, they unfortunately could not yet solve the problem Twenty-three total-load equations for noncohesive particles were studied by providing insight into the relative strengths, weaknesses, and limitations Three hundred parameters were investigated individually by using sensitivity analysis to pinpoint the key physical properties that control the errors It is found that, the most influential parameter for the total-load sediment flux equations used in alluvial rivers for the sand particles is the gravitational power due to Shields’ parameter with an energy slope For the gravel particles, the most influential parameter is the universal stream power due to critical Shields’ parameter with an energy slope Several graphs are presented to emphasize the effect of these parameters that were either directly used or were embedded within those equations Recommendations and guides are also presented for the researchers working in this field

Tidal Stream Energy Assessment in Specific Channels of Zhoushan Sea Area

Abstract: In this paper,twenty-six hours' continuous measured data is analyzed,which includes data of five observation stations in Gaoting channel,and that of eight observation stations in Guanmen channel.The results show that the two channels appear irregular semidiurnal tidal stream characteristics,and the synthetic vertical average velocities are above 1 m/s.Two common methods-farm and flux,which are used for tidal stream energy assessment are also introduced in this paper.From the assessment results,it can be found that the tidal stream power can be developed in Gaoting channel exists between 4.67~5.31 MW,and that in Guanmen channel exists between 7.92~9.37 MW.If exchanging all tidal stream power to annual generated energy,it accounts for 6%～7% of electrical power consumption of Zhoushan in 2005.

Sediment records as archives of the Late Pleistocene–Holocene hydrological change in the alluvial Narmada River basin, western India

Abstract: The rivers of western India are monsoon dominated and have been so throughout the late Quaternary. Sediment accumulation in these river basins has been controlled by climatic and tectonic changes over a time span from the Late Pleistocene to the recent. The lithofacies assemblages associated with the various sediment archives in the Narmada basin range from the boulders of the alluvial fans to overbank fines on the alluvial plains. Estimates, based on clast size, of stream power and competence, bed shear stress and discharge reveal that hydrological conditions during the Late Pleistocene (∼90 ka) were comparable to the present day. The size of the transported clasts and the thickness of the accumulated sediment indicate the influence of basin subsidence rather than an increase in discharge. Discharge estimates based on sedimentary structures preserved in the alluvial-plain facies suggest that the channel had a persistent flow, with a low width–depth ratio and large meander wavelength. The hydrological changes during the Holocene are more pronounced where the early Holocene is marked by a high-intensity hydrological regime that induced erosion and incision of the earlier sediments. The mid-Holocene stream channel was less sinuous and had a higher width–depth ratio and a higher meander amplitude in comparison with the present-day channel. Palaeo-fluvial reconstructions based on the sediment archives in the alluvial reach of the river basin are important tools in understanding the long-term hydrological changes and the intricate fluvial architecture preserved in the Narmada River basin ensures scope for detailed studies to identify phases of weak and enhanced hydrological regimes.

Relationships between soil detachment rate and runoff hydrodynamic indexes of purple soil slope and red soil slope

Abstract: Soil detachment process is one of the very important processes of erosion,transportation and deposition.Quantitative study of the soil detachment process is the basis of building the significant physical implication of soil erosion and prediction model.Taken the red soil and purple soil slopes respectively as the research fields,which was 3.0 long and 1.0 wide,a series of scouring experiments at 4 slope gradients(5°、 10°、15°、20°) and 5 water discharges(1.3l /min、3.0l /min、5.5l /min、6.5l /min、8.5l /min) were conducted.The relationships between soil detachment rate and runoff shear stress,unit stream power,stream power and cross-section specific energy are studied in this paper.The results show that the soil detachment rate increased with the increase in water discharge at the same slope gradient.Soil detachment rate increased at the beginning and then decreased,at last,the soil detachment rate became stable at the same water discharge.The time of soil detachment rate reaching its peak value became shorter along with the increase in water discharge.There exited a good linear relationship between the soil detachment rate and runoff shear stress,unit stream power,stream power and cross-section specific energy.All of these four indexes could be used for reflecting the soil detachment rate,but the stream power was better than the others.For the red soil the critical shear stress was about 1.25N /m2 and for the purple soil it was about 2.16N /m2.For the red soil the critical unit stream power was about 0.00014m /s and for the purple soil it was about 0.0018m /s.For the red soil the critical stream power was about 0.0083N /(m.s) and for the purple soil it was about 0.062 N /(m.s).For the red soil the critical cross-section specific energy was about 0.24cm and for the purple soil it was about 0.23 cm.