Showing papers on "Stream power published in 2013"

River Longitudinal Profiles and Bedrock Incision Models: Stream Power and the Influence of Sediment Supply

Abstract: The simplicity and apparent mechanistic basis of the stream power river incision law have led to its wide use in empirical and theoretical studies. Here we identify constraints on its calibration and application, and present a mechanistic theory for the effects of sediment supply on incision rates which spotlights additional limitations on the applicability of the stream power law. On channels steeper than about 20%, incision is probably dominated by episodic debris flows, and on sufficiently gentle slopes, sediment may bury the bedrock and prevent erosion. These two limits bound the application of the stream power law and strongly constrain the possible combination of parameters in the law. In order to avoid infinite slopes at the drainage divide in numerical models of river profiles using the stream power law it is commonly assumed that the first grid cell is unchanneled. We show, however, that the size of the grid may strongly influence the calculated equilibrium relief. Analysis of slope-drainage area relationships for a river network in a Northern California watershed using digital elevation data and review of data previously reported by Hack reveal that non-equilibrium profiles may produce well defined slope-area relationships (as expected in equilibrium channels), but large differences between tributaries may point to disequilibrium conditions. To explore the role of variations in sediment supply and transport capacity in bedrock incision we introduce a mechanistic model for abrasion of bedrock by saltating bedload. The model predicts that incision rates reach a maximum at intermediate levels of sediment supply and transport capacity. Incision rates decline away from the maximum with either decreasing supply (due to a shortage of tools) or increasing supply (due to gradual bed alluviation), and with either decreasing transport capacity (due to less energetic particle movement) or increasing transport capacity (due less frequent particle impacts per unit bed area). We use this model to predict longitudinal profiles under varying boundary conditions and sediment supply rates and find that even in actively downcutting rivers, the river slope needed to maintain incision may be only slightly greater than the slope required to transport the imposed load. Hence, the channel slope-drainage area relationships of rivers actively cutting through bedrock may predominately reflect the grain size and supply rate of sediment and only secondarily the influence of bedrock resistance to erosion.

An integral approach to bedrock river profile analysis

Abstract: National Science Foundation (U.S.) (Geomorphology and Land-use Dynamics Program Award EAR-0951672)

Solutions of the stream power equation and application to the evolution of river longitudinal profiles

Abstract: [1] Erosion by bedrock river channels is commonly modeled with the stream power equation. We present a two-part approach to solving this nonlinear equation analytically and explore the implications for evolving river profiles. First, a method for non-dimensionalizing the stream power equation transforms river profiles in steady state with respect to uniform uplift into a straight line in dimensionless distance-elevation space. Second, a method that tracks the upstream migration of slope patches, which are mathematical entities that carry information about downstream river states, provides a basis for constructing analytical solutions. Slope patch analysis explains why the transient morphology of dimensionless river profiles differs fundamentally if the exponent on channel slope, n, is less than or greater than one and why only concave-up migrating knickpoints persist when n 1. At migrating knickpoints, slope patches and the information they carry are lost, a phenomenon that fundamentally limits the potential for reconstructing tectonic histories from bedrock river profiles. Stationary knickpoints, which can arise from spatially varying uplift rates, differ from migrating knickpoints in that slope patches and the information they carry are not lost. Counterparts to migrating knickpoints, called “stretch zones,” are created when closely spaced slope patches spread to form smooth curves in distance-elevation space. These theoretical results are illustrated with examples from the California King Range and the Central Apennines.

Bedrock Fluvial Incision and Longitudinal Profile Development Over Geologic Time Scales Determined by Fluvial Terraces

Abstract: Fluvial terraces preserve the history of river incision into bedrock over geologic time scales. In this paper we use terraces and a comparison of terrace longitudinal profiles to stream longitudinal profiles to develop a conceptual model of bedrock fluvial incision in diverse geologic, tectonic, and climatic settings. The conceptual model highlights a distinction between bedrock stream behavior in settings of relatively high versus relatively low tectonic activity. This distinction arises from the fundamentally different ways in which runoff is generated in these respective tectonic settings and the positive feedbacks that exist between topography and climate. The model allows for qualitative predictions of long profile shape that can be directly compared to the longitudinal profiles predicted by the stream power law. Our approach has the advantage of helping understand the geologic (and climatic) constraints on the wide variations in k, m, and n revealed in recent applications of the stream power law. We reconcile diverse longitudinal profile shapes and long-term rates of bedrock fluvial incision by considering how a drainage basin generates fluvial discharge and whether that discharge can produce the necessary stream power distributed across a valley bottom such that the long profile can rapidly accommodate changes in base level, climate, and/or rates of rock uplift. We propose that in tectonically active settings (Type I basins), the entire drainage basin experiences uplift which, in turn, builds steep slopes and concomitant increases in orographic precipitation that effectively generate the high peak discharges and fluvial-system wide stream power necessary to create and maintain concave-up long profiles and rates of incision equal and opposite to rates of rock uplift. Measured stream power for one of these basins is highly correlated to the width of the channel and valley bottom which argues for a conservation of energy along the profile and the apportionment of stream power to vertical incision, lateral incision, and bedload transportation. The stream power used for lateral incision processes periodically widen the channel bottom during transient, hydrologically-driven changes in discharge and sediment load, producing fluvial terraces. In contrast, drainage basins in tectonically inactive settings (Type II) may not have a hydrology characterized by high peak discharges, particularly for those drainage basins which do not receive large, highly seasonal and/or highly-variable precipitation (Type IIa). Streams in the tectonically-inactive setting are more dependent on local changes in stream power, spatially restricted to knickpoints, that require long periods of time to propagate through the system. A change in down stream base level in these settings has a particularly profound impact on long profile shape, especially where the river crosses resistant rock-types. Type II basins located where climate favors highly seasonal and/or variable precipitation (Type IIb) retain minor rock-type controlled convexities on otherwise concave profiles.

Climatic control of bedrock river incision

Abstract: Bedrock river incision drives the development of much of Earth's surface topography, and thereby shapes the structure of mountain belts and modulates Earth's habitability through its effects on soil erosion, nutrient fluxes and global climate. Although it has long been expected that river incision rates should depend strongly on precipitation rates, quantifying the effects of precipitation rates on bedrock river incision rates has proved difficult, partly because river incision rates are difficult to measure and partly because non-climatic factors can obscure climatic effects at sites where river incision rates have been measured. Here we present measurements of river incision rates across one of Earth's steepest rainfall gradients, which show that precipitation rates do indeed influence long-term bedrock river incision rates. We apply a widely used empirical law for bedrock river incision to a series of rivers on the Hawaiian island of Kaua'i, where mean annual precipitation ranges from 0.5 metres to 9.5 metres (ref. 12)-over 70 per cent of the global range-and river incision rates averaged over millions of years can be inferred from the depth of river canyons and the age of the volcanic bedrock. Both a time-averaged analysis and numerical modelling of transient river incision reveal that the long-term efficiency of bedrock river incision across Kaua'i is positively correlated with upstream-averaged mean annual precipitation rates. We provide theoretical context for this result by demonstrating that our measurements are consistent with a linear dependence of river incision rates on stream power, the rate of energy expenditure by the flow on the riverbed. These observations provide rare empirical evidence for the long-proposed coupling between climate and river incision, suggesting that previously proposed feedbacks among topography, climate and tectonics may occur.

Inland propagation of erosional escarpments and River profile evolution across the Southeast Australian passive continental margin

Abstract: Denudation at passive continental margins occurs over time as erosional escarpments propagate inland, cutting through regions of elevated topography flanking ocean basins. Understanding the actual processes and time variability in propagation rates associated with the advance of escarpments across passive margins remains a largely unsolved problem for tectonic geomorphology. Here we report results from new analyses of detailed river longitudinal profiles and surface exposure age measurements using cosmogenic radionuclides from the New England Tableland portion of the southeast Australian passive margin. In that area, many plateau-draining tributaries of the Macleay River cascade into narrow gorges across large-scale river knickpoints that represent the tips of the leading edge of the inland-advancing escarpment. Previous river profile analyses showed most knickpoints to be the same distance, about 200 km, upstream from the mouth of the river, despite order-of-magnitude variations in the areas drained at the gorge heads. The implication is that all knickpoints have migrated upstream at a speed of about 2 km/Myr averaged over the ∼100 Myr history of the margin. The new profile analyses confirm that, in general, distance upstream from the Macleay River mouth of the knickpoints is not related to area drained at the gorge head. We conclude that if sufficient fluvial transport power is available, the rate of upstream knickpoint migration is governed by slope failure mechanisms and the frequency of resulting mass wasting events on the steep rock slopes in the gorge head vicinity. In terms of the stream power model for channel incision, Z t A m' S n (where A is drainage area and S is channel gradient), the river profile analyses imply that the drainage area exponent m' = 0 (i.e., no dependence on drainage area so long as a threshold is met), and the slope exponent n > 1. Average erosion rates calculated from Be and Al radionuclide concentrations in samples collected across the knickpoint on Baker's Creek indicate that the plateau surface and the channel upstream of the gorge head - knickpoint are eroding slowly, ∼5 m/Myr, whereas the channel at and downstream of the knickpoint is eroding much more rapidly, >100 m/Myr. The pattern of increasing erosion rates across the knickpoint in the downstream direction is consistent with the paradigm of inland escarpment retreat across passive continental margins.

The available power from tidal stream turbines in the Pentland Firth

Abstract: This paper assesses an upper bound for the tidal stream power resource of the Pentland Firth. A depth-averaged numerical model of the tidal dynamics in the region is set-up and validated against fi...

Bedrock Channel Morphology in Relation to Erosional Processes

Abstract: Bedrock channel morphology reflects the interactions between erosive processes and the resistance of the channel substrate. The controls on these interactions change with spatial scale. Mineralogy, exposure age of the substrate, and local heterogeneities are particularly important in controlling substrate resistance at the micro scale (mm to cm). Substrate discontinuities created by bedding, joints, and lithologic contacts become progressively more important at the meso scale (cm to m), whereas regional structure and baselevel history may dominate substrate resistance at the macro scale (m to km). In a similar manner, turbulent fluctuations that create localized abrasion and cavitation are more important at the micro and meso scales, whereas longitudinal patterns of unit and total stream power exert a stronger influence on channel morphology at the macro scale. Most studies of bedrock channel morphology have described meso-scale erosional features. In the absence of direct measurements, investigators have inferred both the erosive processes that produced the observed features, and the controls on the location of the features. Fluvial erosion of bedrock may occur via; (1) corrosion, or chemical weathering and solution, (2) corrasion, or abrasion by sediment in transport along the channel, or (3) cavitation and other hydrodynamic forces associated with flow turbulence. Very few direct measurements of rate exist for any of these erosive processes. Bedrock channel morphologies may be divided into multiple or single flowpath channels, and subdivided on the basis of sinuosity, uniformity of bed gradient, and uniformity of erosion across a cross section. These categories may be used to infer dominant erosional processes and relative rates of erosion, but we cannot yet predict the occurrence of specific channel morphologies as a function of driving and resisting forces. In this context, the traditional assumption that substrate dominates bedrock channel morphology may be too restrictive.

Morphodynamic diversity of the world’s largest rivers

Abstract: Large alluvial rivers transport globally significant quantities of water, sediment, and nutrients to the oceans, temporarily storing and cycling this material within the bars, islands, and floodplains that define their morphology. The world’s largest rivers display a remarkable variety of morphologies. However, existing theory and numerical modeling fail to explain this diversity, which remains poorly understood. This study applies a new numerical model of water flow and sediment transport to show how the morphology of large sand-bed rivers is influenced by bed sediment mobility, bank erodibility, and rate of floodplain development. Simulations demonstrate that a wide range of river styles, including meandering, anabranching, and braiding, can occur over a relatively narrow range of environmental conditions. Results highlight the suspension of bed material, which limits the gravitational deflection of sediment in the direction of the local bed slope, as a key control on sediment transport direction and hence river morphology. Moreover, high mobility of bed and bank sediments are hypothesized to favor contrasting river styles, although both may be promoted by increasing stream power. These results explain the inability of existing stream power theory to predict the morphology of the world’s largest rivers, and highlight the potential for investigating river-floodplain co-evolution using physics-based simulation models.

Thermodynamics, maximum power, and the dynamics of preferential river flow structures at the continental scale

Abstract: The organization of drainage basins shows some reproducible phenomena, as exemplified by self-similar frac- tal river network structures and typical scaling laws, and these have been related to energetic optimization principles, such as minimization of stream power, minimum energy ex- penditure or maximum "access". Here we describe the or- ganization and dynamics of drainage systems using thermo- dynamics, focusing on the generation, dissipation and trans- fer of free energy associated with river flow and sediment transport. We argue that the organization of drainage basins reflects the fundamental tendency of natural systems to de- plete driving gradients as fast as possible through the maxi- mization of free energy generation, thereby accelerating the dynamics of the system. This effectively results in the maxi- mization of sediment export to deplete topographic gradients as fast as possible and potentially involves large-scale feed- backs to continental uplift. We illustrate this thermodynamic description with a set of three highly simplified models re- lated to water and sediment flow and describe the mecha- nisms and feedbacks involved in the evolution and dynam- ics of the associated structures. We close by discussing how this thermodynamic perspective is consistent with previous approaches and the implications that such a thermodynamic description has for the understanding and prediction of sub- grid scale organization of drainage systems and preferential flow structures in general.

Setting Goals in River Restoration: When and Where Can the River “Heal Itself”?

Abstract: Stream Restorat Approaches, Anal Geophysical Mon Copyright 2011 b 10.1029/2010GM Ecological research demonstrates that the most diverse, ecologically valuable river habitats are those associated with dynamically migrating, flooding river channels. Thus, allowing the river channel to “heal itself” through setting aside a channel migration zone, or erodible corridor, is the most sustainable strategy for ecological restoration. The width and extent of channel can be set from historical channel migration and model predictions of future migration. However, the approach is not universally applicable because not all rivers have sufficient stream power and sediment load to reestablish channel complexity on a time scale of decades to years, and many are restricted by levees and infrastructure on floodplains that preclude allowing the river a wide corridor. A bivariate plot of stream power/sediment load (y axis) and degree of encroachment (urban, agricultural, etc.) (x axis) is proposed as a framework for evaluating the suitability of various restoration approaches. Erodible corridors are most appropriate where both the potential for channel dynamics and available space are high. In highly modified, urban channels, runoff patterns are altered, and bottomlands are usually encroached by development, making a wide corridor infeasible. There, restoration projects can still feature deliberately installed components such as riparian trees and trails with the social benefits of public education and providing recreation to underserved families. Intermediate approaches include partial restoration of flow and sediment load below dams and “anticipatory management”: sites of bank erosion are anticipated, and infrastructure is set back in advance of floods, to prevent “emergency” dumping of concrete rubble down eroding banks during high water.

Improvement of streams hydro-geomorphological assessment using LiDAR DEMs

Abstract: Hydro-geomorphological assessments are an essential component for riverine management plans. They usually require costly and time-consuming field surveys to characterize the spatial variability of key variables such as flow depth, width, discharge, water surface slope, grain size and unit stream power throughout the river corridor. The objective of this research is to develop automated tools for hydro-geomorphological assessments using high-resolution LiDAR digital elevation models (DEMs). More specifically, this paper aims at developing geographic information system (GIS) tools to extract channel slope, width and discharge from 1 m-resolution LiDAR DEMs to estimate the spatial distribution of unit stream power in two contrasted watersheds in Quebec: a small agricultural stream (Des Feves River) and a large gravel-bed river (Matane River). For slope, the centreline extracted from the raw LiDAR DEM was resampled at a coarser resolution using the minimum elevation value. The channel width extraction algorithm progressively increased the centerline from the raw DEM until thresholds of elevation differences and slopes were reached. Based on the comparison with over 4000 differential global positioning system (GPS) measurements of the water surface collected in a 50 km reach of the Matane River, the longitudinal profile and slope estimates extracted from the raw and resampled LiDAR DEMs were in very good agreement with the field measurements (correlation coefficients ranging from 0 · 83 to 0 · 87) and can thus be used to compute stream power. The extracted width also corresponded very well to the channel as seen from ortho-photos, although the presence of bars in the Matane River increased the level of error in width estimates. The estimated maximum unit stream power spatial patterns corresponded well with field evidence of bank erosion, indicating that LiDAR DEMs can be used with confidence for initial hydro-geomorphological assessments. Copyright © 2013 John Wiley & Sons, Ltd.

A unit stream power based sediment transport function for overland flow.

Abstract: Soil erosion is a serious global problem requiring effective modeling for accurate assessment of sensitive areas and related erosion rates. The outcome of soil erosion models depends strongly on the estimation of sediment transport capacity. In most of the existing spatially distributed soil erosion models sediment transport capacity of overland flow is often estimated using stream flow transport capacity functions. The applicability of stream flow functions to overland flow conditions is questionable because hydraulic conditions like flow depth, slope steepness and surface roughness under overland flow are substantially different from stream flow conditions. Hence, the main objectives of this study were i) to check the suitability of five existing well known and widely used transport capacity functions (Yalin 1963; Low, 1989; Govers, 1990; modified Engelund and Hansen (Smith et al., 1995); and Abrahams et al., 2001) for use under overland flow conditions, and ii) to derive a new function based on unit stream power by dimensional analysis to quantify transport capacity for overland flow. To accomplish the objectives, experiments in a 3.0 m long and 0.5 m wide flume were carried out using four different sands (0.230, 0.536, 0.719, and 1.022 mm). The unit discharges used for experimentation ranged from 0.07 to 2.07 x 10(-3) m(2) s(-1) and slopes ranged from 5.2 to 17.6%. In this study, none of the predictions with the existing functions was in good agreement with measured results over the whole range of experimental conditions, especially at low flow intensities. The percentages of observations in which the discrepancy ratio ranged between 0.5 and 2.0 were: 65% (Yalin 1963), 74% (Low, 1989), 57% (Govers, 1990), 54% (modified Engelund and Hansen (Smith et al., 1995)), and 25% (Abrahams et al., 2001). The results showed that the selected functions reasonably estimate transport capacities only under those ranges of conditions for which they were formulated. Although the excess.shear stress concept based function (i.e. Low's function) produced excellent results, the degree of accuracy of the results varied substantially with grain size (P.O.(0.5-2.0): 53-100%). In contrast, the performance of the Govers' function, which is based on the unit stream power concept, was quite similar for all the selected sands (P.O.(0.5-2.0): 50-63%). Based on the unit stream power concept, a new function for low flow intensities was derived by dimensional analysis using the data gained from the flume experiments. (c) 2012 Elsevier B.V. All rights reserved.

A comparative evaluation of flood mitigation alternatives using GIS-based river hydraulics modelling and multicriteria decision analysis

Abstract: A multicriteria framework is developed for the selection of optimal flood mitigation and river training measures in a selected reach of Zaremroud River in Northern Iran. A river model, Hydrologic Engineering Center River Analysis System, combined with geographic information system analysis is used to simulate water levels for steady, gradually varied flow and mapping inundated flood extents. The modelling is performed for four different alternatives, considering various channel modifications with different dimensions and levee construction. Flood inundation area, flood level, flow velocity and stream power on the downstream and outside of the river bend are used as decision criteria for each alternative. Economic analysis is conducted to evaluate the cost-effectiveness of each alternative. The decision analysis method, technique for order of preference by similarity to ideal solution, is used to compare different flood hazard mitigation measures based on risk, and environmental and economic impacts criteria. The findings of the analysis are that a levee construction at the right side of the river bank adjacent to the residential area is superior to the other three alternatives, which is confirmed using a scenario analysis of different flood mitigation measures.

Field measurements of the energy delivered to the channel bed by moving bed load and links to bedrock erosion

Abstract: [1] Impact-driven fluvial erosion is directly related to the energy delivered to the channel bed by moving bed load. Using a novel protocol, we have measured this energy in four mountain streams in Austria and Switzerland. Similar to bed load transport rates, the energy delivered to the bed displays large scatter over several orders of magnitude even for a constant discharge. We found that only a small fraction (<1%) of the total energy available to the stream is delivered to the bed and can be used for erosive work. Empirical predictive equations can be defined for specific sites, but there is large site-to-site variability. Prediction of energy delivered to the bed using the saltation-abrasion model of bedrock erosion only provides the observed trends when measured bed load transport rates are used as input. Using an empirical transport law calibrated to the conditions at one of the study streams leads to overprediction of delivered energies by more than 2 orders of magnitude. This overprediction decreases with increasing discharge, and thus, at high discharges, better predictive results are obtained. We find a correlation between the channels' bed slope or characteristic grain sizes of the channel bed and the fraction of energy delivered to the bed of the total energy available to the stream. This observation provides a tentative link between fundamental fluvial incision processes to the stream power model family that has widely been used to model fluvial bedrock incision in landscape evolution simulations.

Flow hydraulic characteristic effect on sediment and solute transport on slope erosion

Abstract: Different hydraulic parameters, including the hydraulic shear stress, unit length shear force, steam power, unit steam power, and effective stream power were used to quantify flow detachment. Most former studies were conducted for flow detachment under uniform slope surface conditions, while a few studies compared different slope surface conditions. The uniform bare loess was prepared in laboratory experiments. Natural fallowed soil loess with stone covers was prepared in field experiments. The objective of this study was to assess the differences in hydraulic parameters and sediment detachment under these different soil surface conditions. Our results show that the unit sediment load (Rs) has a good linear relationship with the unit runoff rate (Rr) for the flume and field experiments, and the relationship can be expressed as the function: Rs = 0.262Rr − 0.802 (R2 = 0.947). The rate of Manning roughness coefficient to mean flow depth (n/h) is a good hydraulic indicator like as the stream power and Reynolds number for predicting the sediment load. Hydraulic parameters n/h, Re, and ω are good indicators for the unit area sediment load for both the flume and field experiments, while Fr, f, and τ are good indicators for the unit area sediment load only when the flume experiments and field experiments are individually analyzed. Among the three good indicators (ω, Re, and n/h), n/h is better than the other two for predicting sediment load in rill erosion for both flume and field experiments, as well as for the unit solute transport rate (MBr). The parameter of n/h probably is not only a good hydraulic parameter as an indicator for both sediment and solute transport, but also a good hydraulic parameter which link with runoff energies. The parameter n/h represents the flow wave of runoff and is an important factor to represent the energy for water and sediment transport, and the flow wave celerity (vw) is related to n/h by: vw = 1.585(n/h)− 0.527 (R2 = 0.978).

Predicting grain size in gravel-bedded rivers using digital elevation models: Application to three Maine watersheds

Abstract: Riverbed grain size controls suitability of spawning habitat for threatened fish species. Motivated by this relationship, we developed a model that uses digital elevation models (DEMs) to predict bed grain size. We tested the accuracy of our model and two existing models with channel measurements from high-resolution airborne light detection and ranging (LiDAR) DEMs. All three models assume that bed grain size is a function of reach-average high-flow channel hydraulics (measured by shear stress or stream power). Our test data are field measurements of median grain size ( D 50 ) at 276 stations along four rivers in Maine. Pleistocene continental glaciation strongly influences the longitudinal profiles, which have alternating steep and gradual segments. We exploit the resulting variations in sediment supply to understand the controls on model success or failure in predicting bed grain size. Results show that all three models have ∼70% success in predicting D 50 within a factor of two overall, and better where the rivers are coarse gravel bedded (∼80% success where D 50 ≥ 16 mm). This similarity is unsurprising given that the models primarily rely on channel gradient (S) and drainage area as inputs. Measurements of S from LiDAR DEMs yield only a modest improvement in model success over those from topographic maps. We find that our model works best in sediment-starved steep reaches. Model failures fall into two broad categories: (1) relatively fine-grained ( D 50 D 50 . We argue that models based on airborne infrared LiDAR DEMs may reach a maximum around 80%–85% accuracy due to these sub-reach-scale factors, which cannot be easily measured from DEMs. The overall success of the models in predicting grain size indicates that the morphology of these channels has adjusted to the imposed S and sediment load during the ∼15 k.y. since deglaciation and through the period of anthropogenic channel change over the past three centuries.

The role of hydrologic alteration and riparian vegetation dynamics in channel evolution along the lower Minnesota River

Abstract: . The Minnesota River carries the largest load of sediment to the Mississippi River in Minnesota, most of which comes from channel sources. This study investigates bank retreat in the lower Minnesota River since 1938. Specifically we asked, How have changes to river form influenced sediment transport and deposition in the lower Minnesota River and how did hydrological and ecological processes affect channel change? It was hypothesized that channel straightening, reduction in floodplain access, and streamflow increases contribute to increased channel-derived sediment load and decreased point bar deposition. Secondly, it was hypothesized that hydrologic changes have reduced woody riparian vegetation on sandbars, further promoting channel widening. To quantify channel sediment and phosphorus loading rates in the lower Minnesota River, we analyzed historic aerial photos for evidence of channel change, we performed long-term monitoring of erosion and deposition rates within the river corridor, and we calculated channel sediment transport rates. Results from this study showed that the Minnesota River has widened by 52% between Mankato and St. Paul since 1938, on average contributing 280,000 Mg of sediment per year and 153 Mg total phosphorus. The river also shortened by 7% since 1938, increasing bankfull shear stress and stream power. Sediment deposition rates in the floodplain have increased since European settlement by an order of magnitude. Ecohydrological studies showed that establishment of woody riparian plants has been inhibited on sandbars by prolonged summer flow duration and scour at high flow, reducing potential point bar growth. Findings from this study will be useful in prioritizing sediment and vegetation management actions.

9.29 Incised Channels: Disturbance, Evolution and the Roles of Excess Transport Capacity and Boundary Materials in Controlling Channel Response

Abstract: Channel incision is part of denudation, drainage-network development, and landscape evolution. Rejuvenation of fluvial networks by channel incision generally leads to further network development and an increase in drainage density as gullies migrate into previously nonincised surfaces. Large anthropogenic disturbances, similar to large or catastrophic natural events, greatly compress timescales for incision and related processes by creating enormous imbalances between upstream sediment delivery and available transporting power. Field examples of channel responses to anthropogenic and natural disturbances are presented for fluvial systems in the mid-continent, Pacific Northwest, USA, and central Italy. Responses to different types of disturbances are shown to result in similar spatial and temporal trends of incision for vastly different fluvial systems. Similar disturbances are shown to result in varying relative magnitudes of vertical and lateral (widening) processes, and different channel morphologies as a function of the type of boundary sediments comprising the bed and banks. This apparent contradiction is explained through an analysis of temporal adjustments to flow energy, shear stress, and stream power with time. Numerical simulations of sand-bed channels of varying bank resistance disturbed by reducing the upstream sediment supply by half, show identical adjustments in flow energy and the rate of energy dissipation. The processes that dominate adjustment and the ultimate stable geometries, however, are vastly different, depending on the cohesion of the channel banks and the supply of hydraulically controlled sediment (sand) provided by bank erosion.

Applicability of Different Hydraulic Parameters to Describe Soil Detachment in Eroding Rills

Abstract: This study presents the comparison of experimental results with assumptions used in numerical models. The aim of the field experiments is to test the linear relationship between different hydraulic parameters and soil detachment. For example correlations between shear stress, unit length shear force, stream power, unit stream power and effective stream power and the detachment rate does not reveal a single parameter which consistently displays the best correlation. More importantly, the best fit does not only vary from one experiment to another, but even between distinct measurement points. Different processes in rill erosion are responsible for the changing correlations. However, not all these procedures are considered in soil erosion models. Hence, hydraulic parameters alone are not sufficient to predict detachment rates. They predict the fluvial incising in the rill's bottom, but the main sediment sources are not considered sufficiently in its equations. The results of this study show that there is still a lack of understanding of the physical processes underlying soil erosion. Exerted forces, soil stability and its expression, the abstraction of the detachment and transport processes in shallow flowing water remain still subject of unclear description and dependence.

Relationship between bedload and suspended sediment in the sand-bed Exu River, in the semi-arid region of Brazil

Abstract: Suspended sediment and bedload discharges in sand-bed rivers shape semi-arid landscapes and impact sediment delivery from these landscapes, but are still incompletely understood. Suspended sediment and bedload fluxes of the intermittent Exu River, Brazil, were sampled by direct measurements. The highest suspended sediment concentration observed was 4847.4 mg L-1 and this value was possibly associated with the entrainment of sediment that was deposited in the preceding year. The bedload flux was well related to the stream power and the river efficiently transported all available bedload with a mean rate of 0.0047 kg m-1 s-1, and the percentage of bedload to suspended sediment varied between 4 and 12.72. The bed sediment of Exu River was prone to entrainment and showed a proclivity for transport. Thus, sand-bed and gravel-bed rivers of arid environments seem to exhibit the same mobility in the absence of armour layer.Editor D. Koutsoyiannis; Associate editor B. TouaibiaCitation Cantalice, J.R.B., Cu...