Alluvial Streams Research Articles

Cyclic steps: A phenomenon of supercritical shallow flow from the high mountains to the bottom of the ocean

Cyclic steps constitute a characteristic bedform of Froude-supercritical shallow flow over an erodible bed. They are long-wave features that are bounded by hydraulic jumps and migrate upstream. They can be seen in alluvial streams, stream in cohesive sediment, bedrock streams, and on the seafloor in response to turbidity currents. Recent progress in the modeling of cyclic steps is summarized.

On the stable geometry of self-formed alluvial channels: theory and practical applicationThis article is one of a selection of papers in this Special Issue in honour of Professor M. Selim Yalin (1925–2007).

On the basis of previous work by the late Professor M. Selim Yalin and the author, the process of self-formation of alluvial streams and the final (equilibrium or regime) geometry of the self-formed stream are considered in the light of thermodynamic principles, including the first and second laws, and the Gibb’s equation; the stream is treated as an isolated and irreversible system. The present analysis suggests that stream self-formation is guided by the need of the stream to progressively decrease its average flow velocity to accommodate the increase in the entropy of the system with the passage of time. The reduction in flow velocity is achieved by an appropriate alteration of stream slope, cross-sectional geometry, and effective roughness, the regime development being the process of this appropriate alteration. A method is presented for the computation of regime width, depth, and slope. The method rests on the channel formation criterion derived from thermodynamic principles and the expression of regime flow width determined on the basis of zero net cross sediment transport rate at the regime state. The regime channels computed from this method are compared with field and laboratory data from various sources.

From flumes to rivers: Can sediment transport in natural alluvial channels be predicted from observations at the laboratory scale?

Doubt regarding the applicability of laboratory results to alluvial streams has led some to develop sediment transport predictors based solely on field data, and most current sediment transport formulae have typically been calibrated at least partially on field data. This paper examines the transferability of flume results to the field by exploring the extent to which a unified approach to the prediction of (1) flow regime, (2) depth, and (3) total sediment transport can be developed entirely with laboratory data. Relevance vector machine (RVM)‐based probabilistic models were constructed with only laboratory data, and their performances were tested against field data and found to be comparable with or better than currently available methods. Comparison of a laboratory‐trained RVM with a field‐trained RVM suggests that the prediction performances of the two models for unseen field data are not statistically different given the prediction uncertainty. For transferability, the choice of predictor variables is important with successful predictors being characterized by similar probability distribution in the laboratory and field data, e.g., as quantified by the Kullback‐Leibler divergence.

Sediment Entrainment Potential in Modified Alluvial Streams: Implications for Re-Mobilization of Stored In-Channel Sediment

Alteration of sediment dynamics is a common effect of human disturbance in fluvial systems. The longevity of human-induced changes and the implications of altered sediment dynamics for system-wide responses to disturbance are not entirely understood. In this study, I calculated sediment entrainment potential (τbf/τcr) for the D50 particle measured for 34 reaches located in three alluvial streams currently adjusting to historic disturbance, including channelization and land clearance, that have large in-channel sediment storage features. Analysis of the spatial variability of τbf/τcr values within the three study streams suggests that most study reaches are located in active transport zones, with the exception of three reaches in two of the study streams that appear to be located within sediment storage zones. The results of this research suggest that there is great potential for in-channel sediment storage features to be reworked and to serve as secondary sources of sediment well into the future.

Modeling the Evolution of Incised Streams. III: Model Application

Incision and the ensuing widening of alluvial stream channels represent important forms of channel adjustment. Two accom- panying papers have presented a robust computational model for simulating the long-term evolution of incised and restored or rehabili- tated stream corridors. This work reports on applications of the model to two incised streams in northern Mississippi, James Creek, and the Yalobusha River, to assess: 1 its capability to simulate the temporal progression of incised streams through the different stages of channel evolution; and 2 model performance when available input data regarding channel geometry and physical properties of channel boundary materials are limited in the case of James Creek. Model results show that temporal changes in channel geometry are satisfactorily simulated. The mean absolute deviation MAD between observed and simulated changes in thalweg elevations is 0.16 m for the Yalobusha River and 0.57 m for James Creek, which is approximately 8.1 and 23% of the average degradation of the respective streams. The MAD between observed and simulated changes in channel top width is 5.7% of the channel top width along the Yalobusha River and 31% of the channel top width along James Creek. The larger discrepancies for James Creek are mainly due to unknown initial channel geometry along its upper part. The model applications also emphasize the importance of accurate characterization of channel boundary materials and geometry.

Physical integrity: the missing link in biological monitoring and TMDLs

The Clean Water Act mandates that the chemical, physical, and biological integrity of our nation's waters be maintained and restored. Physical integrity has often been defined as physical habitat integrity, and as such, data collected during biological monitoring programs focus primarily on habitat quality. However, we argue that channel stability is a more appropriate measure of physical integrity and that channel stability is a foundational element of physical habitat integrity in low-gradient alluvial streams. We highlight assessment tools that could supplement stream assessments and the Total Maximum Daily Load stressor identification process: field surveys of bankfull cross-sections; longitudinal thalweg profiles; particle size distribution; and regionally calibrated, visual, stream stability assessments. Benefits of measuring channel stability include a more informed selection of reference or best attainable stream condition for an Index of Biotic Integrity, establishment of a baseline for monitoring changes in present and future condition, and indication of channel stability for investigations of chemical and biological impairments associated with sediment discontinuity and loss of habitat quality.

Assessing the effect of vegetation‐related bank strength on channel morphology and stability in gravel‐bed streams using numerical models

AbstractBank strength due to vegetation dominates the geometry of small stream channels, but has virtually no effect on the geometry of larger ones. The dependence of bank strength on channel scale affects the form of downstream hydraulic geometry relations and the meandering‐braiding threshold. It is also associated with a lateral migration threshold discharge, below which channels do not migrate appreciably across their floodplains. A rational regime model is used to explore these scale effects: it parameterizes vegetation‐related bank strength using a dimensionless effective cohesion, Cr*. The scale effects are explored primarily using an alluvial state space defined by the dimensionless formative discharge, Q*, and channel slope, S, which is analogous to the Q–S diagrams originally used to explore meandering‐braiding thresholds. The analyses show that the effect of vegetation on both downstream hydraulic geometry and the meandering‐braiding threshold is strongest for the smallest streams in a watershed, but that the effect disappears for Q* > 106. The analysis of the migration threshold suggests that the critical discharge ranges from about 5 m3/s to 50 m3/s, depending on the characteristic rooting depth for the vegetation. The analysis also suggests that, where fires frequently affect riparian forests, channels may alternate between laterally stable gravel plane‐bed channels and laterally active riffle‐pool channels. These channels likely do not exhibit the classic dynamic equilibrium associated with alluvial streams, but instead exhibit a cyclical morphologic evolution, oscillating between laterally stable and laterally unstable end‐members with a frequency determined by the forest fire recurrence interval. Copyright © 2008 John Wiley & Sons, Ltd.

An empirical approach to river bed degradation

Degradation in a homogeneous alluvial channel due to complete stoppage of sediment downstream of a high capacity reservoir has been studied empirically. Based on dimensional analysis, a method has been established for predicting transient bed profiles in degrading alluvial stream. In addition, prediction procedures for other degradation elements like maximum degradation at any time at upstream section and degradation extent has also been evolved. The prediction procedures when tested against known data gave satisfactory results.

Modeling the Evolution of Incised Streams. II: Streambank Erosion

Incision and ensuing widening of alluvial stream channels is widespread in the midsouth and midwestern United States and represents an important form of channel adjustment. Streambanks have been found to contribute as much as 80% of the total suspended load. The location, timing, and magnitude of streambank erosion are difficult to predict. Results from field studies to characterize the resistance of fine-grained materials to hydraulic and geotechnical erosion, the impact of pore-water pressures on failure dimensions and shearing resistance, and the role of riparian vegetation on matric suction, streambank permeability, and shearing resistance are used to enhance the channel evolution model CONCEPTS (conservational channel evolution and pollutant transport system). This paper discusses the conceptualization of the above-mentioned physical processes, and demonstrates the ability of the derived model to simulate streambank-failure processes. The model is tested against observed streambank erosion of a bendway on Goodwin Creek, Miss. between March 1996 and March 2001, where it accurately predicts the rate of retreat of the outside bank of the bendway. The observed change in average channel width within the central section of the bendway is 2.96m over the simulation period, whereas a retreat of 3.18m (7.4% larger) is simulated. The observed top-bank retreat within the central section of the bendway is 3.54m over the simulation period, whereas a retreat of 3.01m (15% smaller) is simulated.

Effects of variation in streamflow and channel structure on smallmouth bass habitat in an alluvial stream

AbstractWe evaluated the effects of streamflow‐related changes in channel shape and morphology on the quality, quantity, availability and spatial distribution of young‐of‐year and adult smallmouth bass Micropterus dolomieu habitat in an alluvial stream, the Baron Fork of the Illinois River, Oklahoma. We developed Habitat Suitability Criteria (HSC) for young‐of‐year and adult smallmouth bass to assess changes in available smallmouth bass habitat between years, and compare predicted smallmouth bass Weighted Usable Area (WUA) with observed WUA measured the following year. Following flood events between 1999 and 2000, including a record flood, changes in transect cross‐sectional area ranged from 62.5% to 93.5% and channel mesohabitat overlap ranged from 29.5% to 67.0% in study three study reaches. Using Physical HABitat SIMulation (PHABSIM) system analysis, we found that both young‐of‐year and adult smallmouth bass habitat were differentially affected by intra‐ and inter‐annual streamflow fluctuations. Maximum WUA for young‐of‐year and adults occurred at streamflows of 1.8 and 2.3 m3 s−1, respectively, and WUA declined sharply for both groups at lower streamflows. For most microhabitat variables, habitat availability was similar between years. Habitat suitability criteria developed in 1999 corresponded well with observed fish locations in 2000 for adult smallmouth bass but not for young‐of‐year fish. Our findings suggest that annual variation in habitat availability affects the predictive ability of habitat models for young‐of‐year smallmouth bass more than for adult smallmouth bass. Furthermore, our results showed that despite the dynamic nature of the gravel‐dominated, alluvial Baron Fork, HSC for smallmouth bass were consistent and transferable between years. Published in 2008 by John Wiley & Sons, Ltd.

Profile convexities in bedrock and alluvial streams

Longitudinal profiles of bedrock streams in central Kentucky, and of coastal plain streams in southeast Texas, were analyzed to determine the extent to which they exhibit smoothly concave profiles and to relate profile convexities to environmental controls. None of the Kentucky streams have smoothly concave profiles. Because all observed knickpoints are associated with vertical joints, if they are migrating it either occurs rapidly between vertical joints, or migrating knickpoints become stalled at structural features. These streams have been adjusting to downcutting of the Kentucky River for at least 1.3 Ma, suggesting that the time required to produce a concave profile is long compared to the typical timescale of environmental change. A graded concave longitudinal profile is not a reasonable prediction or benchmark condition for these streams. The characteristic profile forms of the Kentucky River gorge area are contingent on a particular combination of lithology, structure, hydrologic regime, and geomorphic history, and therefore do not represent any general type of equilibrium state. Few stream profiles in SE Texas conform to the ideal of the smoothly, strongly concave profile. Major convexities are caused by inherited topography, geologic controls, recent and contemporary geomorphic processes, and anthropic effects. Both the legacy of Quaternary environmental change and ongoing changes make it unlikely that consistent boundary conditions will exist for long. Further, the few exceptions within the study area–i.e., strongly and smoothly concave longitudinal profiles–suggest that ample time has occurred for strongly concave profiles to develop and that such profiles do not necessarily represent any mutual adjustments between slope, transport capacity, and sediment supply. The simplest explanation of any tendency toward concavity is related to basic constraints on channel steepness associated with geomechanical stability and minimum slopes necessary to convey flow. This constrained gradient concept (CGC) can explain the general tendency toward concavity in channels of sufficient size, with minimal lithological constraints and with sufficient time for adjustment. Unlike grade- or equilibrium-based theories, the CGC results in interpretations of convex or low-concavity profiles or reaches in terms of local environmental constraints and geomorphic histories rather than as “disequilibrium” features.

Hyporheic exchange and water chemistry of two arctic tundra streams of contrasting geomorphology

The North Slope of Alaska's Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the tundra surface and beneath streambeds during the summer, facilitating hyporheic exchange. Our goal was to understand how active layer extent and stream geomorphology influence hyporheic exchange and nutrient chemistry. We studied two arctic tundra streams of contrasting geomorphology: a high‐gradient, alluvial stream with riffle‐pool sequences and a low‐gradient, peat‐bottomed stream with large deep pools connected by deep runs. Hyporheic exchange occurred to ∼50 cm beneath the alluvial streambed and to only ∼15 cm beneath the peat streambed. The thaw bulb was deeper than the hyporheic exchange zone in both stream types. The hyporheic zone was a net source of ammonium and soluble reactive phosphorus in both stream types. The hyporheic zone was a net source of nitrate in the alluvial stream, but a net nitrate sink in the peat stream. The mass flux of nutrients regenerated from the hyporheic zones in these two streams was a small portion of the surface water mass flux. Although small, hyporheic sources of regenerated nutrients help maintain the in‐stream nutrient balance. If future warming in the arctic increases the depth of the thaw bulb, it may not increase the vertical extent of hyporheic exchange. The greater impacts on annual contributions of hyporheic regeneration are likely to be due to longer thawed seasons, increased sediment temperatures or changes in geomorphology.

How fast can a river flow over alluvium?

A new “vortex–drag” approach is proposed to compute the rate of dissipation of mean–motion energy in alluvial streams flowing over mobile bed forms. 1D–routing exercises strongly suggest that, for a given depth and energy–slope, there exists a certain flow velocity limit an alluvial stream is unlikely to exceed. This remarkable flow condition would actually correspond to a fundamental mode in terms of vortex Strouhal frequency (to which only higher harmonics can eventually be associated). Interestingly, the maximum possible velocity is not attained over a plane bed, but occurs when the wavy shape of the water–surface remains in phase with the sand wave. The model treats flow over sandy alluvium as a combination of boundary–attached and –detached flow features, both necessary to explain how well–marked protrusions can develop spontaneously in the bed profile and be maintained in response to extreme water–sediment discharge and stream power conditions. Through the vortex–drag concept and its mathematical development, a lot is learned about the nature of the mutual interactions existing between the various alluvial processes: bedform development, alluvial channel resistance, sediment transport capacity and turbulence–damping. Bedform types occurring in the upper alluvial regime are at the center of the analysis. Based on the model and on confrontation with available experimental evidence, it is suggested that the inphase wave [IPW] flow configuration is associated with minimum possible drag, creating low–turbulence high–velocity conditions ideal to evacuate extreme river discharges. Occurrence of IPW conditions is therefore important in terms of flooding alleviation, but will develop only if the right type of sediment is maintained available on the stream bed. The boundary–attached component of the model introduces a generalized Froude number formulation in alluvial hydraulics, to represent topographically–forced gravity waves occurring in non–shallow environments such as ripple fields. For the boundary–detached component of the model, we introduce a “control factor m” (m ≥ 1) which reflects a complex feedback–control loop process active in the separation cell immediately downstream of the bedform crest. Lift–up of bed particles (under the action of the corresponding vortices shedding out of the reattachment region) implies that control factor m should also play an important role in the further development of bursting–based conceptual models of non–cohesive sediment transport.

Conditions for a better agreement between sediment transport formulas and surveys in alluvial streams

This contribution proposes additional sediment sampling beyond what is customary done in practice, with the goal of enhancing the estimation of sediment transport rates from data collected in the field. The analysis is restricted to large, low-land, alluvial rivers, with bed and banks formed by non-cohesive sediment, which are free to change their depth, width and slope. In particular, the analysis concentrates on dune-covered rivers under steady or quasi-steady, lower-regime flow conditions. Pujol et al. (2004) have shown by means of the Engelund–Hansen sediment transport formula that when a relation is recalibrated using laboratory and field data complying with the above restrictions, the agreement between computed results and observations greatly improves. Motivated by such ideas, the approach proposed herein consists of taking several suspended sediment samples and in surveying bed longitudinal profiles, along streamlines covering several dunes. It is expected that this additional work will improve the quality of field observations, and will facilitate the application of sediment transport relations developed with laboratory observations to field conditions

Sediment threshold under stream flow: A state-of-the-art review

The doctoral research of Albert Frank Shields on sediment movement conducted in Technischen Hochshcule Berlin becomes a legend that is most often referred by many authors and has initiated a sizable number of researches over last seven decades. The Shields diagram is famous due to its application in ascertaining the threshold of sediment motion that is an essential prerequisite for the estimation of sediment transport in an alluvial stream. Since his pioneering work, numerous attempts have so far been made to study the sediment threshold both experimentally and theoretically. This paper presents a comprehensive state-of-the-art review of the important laboratory experimental and theoretical investigations on sediment threshold under steady stream flow highlighting the empirical concepts, hydrodynamic background and the mathematical treatment of the problem. The role of the turbulent bursting on sediment threshold is also discussed.

Influence of morphology and permafrost dynamics on hyporheic exchange in arctic headwater streams under warming climate conditions

We investigated surface‐subsurface (hyporheic) exchange in two morphologically distinct arctic headwater streams experiencing warming (thawing) sub‐channel conditions. Empirically parameterized and calibrated groundwater flow models were used to assess the influence of sub‐channel thaw on hyporheic exchange. Average thaw depths were at least two‐fold greater under the higher‐energy, alluvial stream than under the low‐energy, peat‐lined stream. Alluvial hyporheic exchange had shorter residence times and longer flowpaths that occurred across greater portions of the thawed sediments. For both reaches, the morphologic (longitudinal bed topography) and hydraulic conditions (surface and groundwater flow properties) set the potential for hyporheic flow. Simulations of deeper thaw, as predicted under a warming arctic climate, only influence hyporheic exchange until a threshold depth. This depth is primarily determined by the hydraulic head gradients imposed by the stream morphology. Therefore, arctic hyporheic exchange extent is likely to be independent of greater sub‐stream thaw depths.

Critical Evaluation of How the Rosgen Classification and Associated “Natural Channel Design” Methods Fail to Integrate and Quantify Fluvial Processes and Channel Response1

Abstract: Over the past 10 years the Rosgen classification system and its associated methods of “natural channel design” have become synonymous to some with the term “stream restoration” and the science of fluvial geomorphology. Since the mid 1990s, this classification approach has become widely adopted by governmental agencies, particularly those funding restoration projects. The purposes of this article are to present a critical review, highlight inconsistencies and identify technical problems of Rosgen’s “natural channel design” approach to stream restoration. This paper’s primary thesis is that alluvial streams are open systems that adjust to altered inputs of energy and materials, and that a form‐based system largely ignores this critical component. Problems with the use of the classification are encountered with identifying bankfull dimensions, particularly in incising channels and with the mixing of bed and bank sediment into a single population. Its use for engineering design and restoration may be flawed by ignoring some processes governed by force and resistance, and the imbalance between sediment supply and transporting power in unstable systems. An example of how C5 channels composed of different bank sediments adjust differently and to different equilibrium morphologies in response to an identical disturbance is shown. This contradicts the fundamental underpinning of “natural channel design” and the “reference‐reach approach.” The Rosgen classification is probably best applied as a communication tool to describe channel form but, in combination with “natural channel design” techniques, are not diagnostic of how to mitigate channel instability or predict equilibrium morphologies. For this, physically based, mechanistic approaches that rely on quantifying the driving and resisting forces that control active processes and ultimate channel morphology are better suited as the physics of erosion, transport, and deposition are the same regardless of the hydro‐physiographic province or stream type because of the uniformity of physical laws.

A method for improving predictions of bed‐load discharges to reservoirs

AbstractEffective management options for mitigating the loss of reservoir water storage capacity to sedimentation depend on improved predictions of bed‐load discharges into the reservoirs. Most predictions of bed‐load discharges, however, are based on the assumption that the rates of bed‐load sediment availability equal the transport capacity of the flow, ignoring the spatio‐temporal variability of the sediment supply. This paper develops a semiquantitative method to characterize bed‐load sediment transport in alluvial channels, assuming a channel reach is non‐supply limited when the bed‐load discharge of a given sediment particle‐size class is functionally related to the energy that is available to transport that fraction of the total bed‐load. The method was applied to 22 alluvial stream channels in the USA to determine whether a channel reach had a supply‐limited or non‐supply‐limited bed‐load transport regime. The non‐supply‐limited transport regime was further subdivided into two groups on the basis of statistical tests. The results indicated the pattern of bed‐load sediment transport in alluvial channels depends on the complete spectrum of sediment particle sizes available for transport rather than individual particle‐size fractions represented by one characteristic particle size. The application of the method developed in this paper should assist reservoir managers in selecting bed‐load sediment transport equations to improve predictions of bed‐load discharge in alluvial streams, thereby significantly increasing the efficiency of management options for maintaining the storage capacity of waterbodies.

Asynchronous forest–stream coupling in a fire‐prone boreal landscape: insights from woody debris

Summary We used dendrochronology to reconstruct the transfer of coarse woody debris across a forest–stream interface in a fire‐prone boreal landscape. A sequence of regulating factors was considered from source to sink of in‐stream woody debris (SWD), including fire history at the landscape scale, patterns of post‐fire recovery of riparian forest and inputs of SWD at the scale of a stream reach and its associated floodplain, and burial of SWD at an excavated site. Fires occurred repeatedly in the studied landscape (at least in 1708, 1733, 1791, 1811, c. 1838, c. 1850, 1882, 1941 and 1998), and were generally patchy on the floodplain because of the firebreak effect of the riparian corridor. Unburned forest remnants were regularly generated at the stream margin, thus permitting temporally continuous but spatially localized transfer of woody material across the forest–stream interface. These remaining forest patches also increased forest resilience by dispersing seeds and promoting conifer re‐establishment in burned areas. Because of higher severity compared with previous fires, the 1941 fire burned almost everywhere on the floodplain, creating only a few widely isolated unburned forest remnants. Consequently, following an abrupt post‐fire increase, SWD inputs almost completely ceased. In addition, post‐fire recovery of the riparian wood source is slow because of the spatially restricted seed source. In this alluvial stream, wood burial is faster than decay and largely determines the residence time of SWD. Because the residence time is about 150 years, the current density of SWD is high and contrasts sharply with the very low tree density at the stream margin. Although this long residence time helps maintain stream integrity while the forest is recovering from the 1941 fire, it is unlikely that SWD inputs would resume extensively before burial of the current SWD pool. Our research exemplifies the potentially complex impacts of disturbances on material transfer between a source and a sink ecosystem. We conclude that when certain components of ecosystems are coupled by unidirectional flow, those components will behave asynchronously if a disturbance impact at the source ecosystem does not propagate rapidly to the sink and the source and the sink recover at different rates.

Modelling of wind wave‐induced bottom processes during the slack water periods in Tampa Bay, Florida

AbstractEstuarine process‐oriented modelling studies were undertaken to understand bottom boundary layer processes in Tampa Bay, Florida. A stationary shallow water wave model, SWAN, was applied to predict wind wave‐induced rms bottom orbital currents in Tampa Bay on a 70 × 100 curvilinear grid. Simulations were performed by using two idealized wind forcing (i.e. northeasterly winds of 10 and 20 m s−1) and high‐resolution bathymetry. Calculations of bed load and total load of fine sand were made by using the transport formulas of Van Rijn (J. Hydraulic Eng. 1984; 110:1431–1456) and Engelund–Hansen (A Monograph on Sediment Transport in Alluvial Streams. Copenhagen Technical Press: 1972), respectively. Simulations of wind wave induced currents reveal that they are important for fine sand transport along the shallow margins of the Tampa Bay. Modelled bottom orbital currents ranged from 0.05 to 0.39 m s−1. Total loads of fine sand ranged from 2.26×10−10 to 1.05×10−5kg m−1 s−1 for northeasterly winds of 10 m s−1 and from 2.46×10−5 to 3.21×10−5 kg m−1 s−1 for northeasterly winds of 20 m s−1. Wind wave‐induced bottom resuspension is an important process affecting water quality in Tampa Bay. Copyright © 2006 John Wiley & Sons, Ltd.