Eddy J. Langendoen

Bank and near-bank processes in an incised channel

Abstract Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through: 1. increase in soil bulk unit (specific) weight, 2. decrease or complete loss of matric suction, and, therefore, apparent cohesion, 3. generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength, 4. entrainment of in situ and failed material at the bank toe, and 5. loss of confining pressure during recession of stormflow hydrographs. Relatively small frequent flows during the winter have the ability to erode failed bank materials, maintain oversteepened, unstable bank surfaces and promote prolonged periods of bank retreat, channel migration and high yields of fine-grained sediment. Confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe, which steepens the bank. For example, more than 2 m of bank retreat occurred during the study period at the research site on Goodwin Creek, northern Mississippi. The loss of matric suction (negative pore pressures) due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability. Apparent cohesion, friction angle, soil bulk unit weight and moisture content were measured in situ. Matric suction was measured continuously, in situ with a series of five pressure-transducer tensiometers. A bank-failure algorithm, which combines the Mohr–Coulomb approach, for saturated conditions and the Fredlund modification for unsaturated conditions was developed for layered cohesive streambanks. The resulting equation has been used successfully to investigate the role of matric suction, positive pore-water pressures and confining pressure for layered streambanks composed of cohesive materials.

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.

Test of a Method to Calculate Near-Bank Velocity and Boundary Shear Stress

Detailed knowledge of the flow and boundary shear stress fields near the banks of natural channels is essential for making accurate calculations of rates of near-bank sediment transport and geomorphic adjustment. This paper presents a high-resolution laboratory data set of velocity and boundary shear stress measurements and uses it to test a relatively simple, fully predictive, numerical method for determining these distributions across the cross-section of a straight channel. The measurements are made in a flume with a fairly complex cross-section that includes a simulated, cobble-roughened floodplain. The method tested is that reported by Kean and Smith in Riparian Vegetation and Fluvial Geomorphology in 2004, which is modified here to include the effects of drag on clasts located in the channel. The calculated patterns of velocity and boundary shear stress are shown to be in reasonable agreement with the measurements. The principal differences between the measured and calculated fields are the result of secondary circulations, which are not included in the calculation. Better agreement with the structure of the measured streamwise velocity field is obtained by distorting the calculated flow field with the measured secondary flow. Calculations for a variety of narrower and wider configurations of the original flume geometry are used to show how the width-to-depth ratio affects the distribution of velocity and boundary shear stress across the channel.

Stability Analysis of Semicohesive Streambanks with CONCEPTS: Coupling Field and Laboratory Investigations to Quantify the Onset of Fluvial Erosion and Mass Failure

AbstractThe overarching goal of this study is to perform a comprehensive bank stability analysis that is phenomenologically sound by considering both mass failure and fluvial erosion. The nature of this study is twofold. First, field and experimental analyses are conducted to generate data for channel cross-section properties, soil index properties, and mechanical and erosional strengths at two sites in a representative, midsize, midwestern stream in southeastern Iowa that is subjected to frequent flash floods and characterized by active fluvial erosion and cantilever failure. Second, the channel surveys and data obtained from the field and laboratory analyses are used as input parameters for an established one-dimensional, channel evolution model, namely, the conservational channel evolution and pollutant transport system (CONCEPTS, version 2.0, Langendoen and Alonso 2008), to estimate the factor of safety for mass failure (FSm) and fluvial erosion (FSf) and simulate the bank retreat as a result of eithe...

EROSION PROCESSES IN GULLIES MODIFIED BY ESTABLISHING GRASS HEDGES

Concentrated flow can cause gully formation on sloping lands and in riparian zones of floodplains adjacent to incising stream channels. Current practice for riparian gully control involves blocking the gully with an earthen embankment and installing a pipe outlet. Measures involving native vegetation would be more attractive for habitat recovery and economic reasons. To test the hypothesis that switchgrass (Panicum virgatum L.) hedges planted at 0.5 m vertical intervals within a gully would control erosion, we established a series of hedges in several concentrated flow channels. Two of the channels were previously eroded trapezoidal channels cut into compacted fill in an outdoor laboratory. The other channels were located at the margin of floodplain fields adjacent to an incised stream channel (Little Topashaw Creek) in Chickasaw County, Mississippi. While vegetation was dormant following two growing seasons, we created artificial runoff events in our test gullies using synthetic trapezoidal-shaped hydrographs with peak discharge rates of approximately 0.03, 0.07, and 0.16 m3 s-1, flow rates similar to those observed during natural runoff events in gullies at Topashaw. During these tests, we monitored flow depth, velocity, turbidity, and soil pore water pressures. Flow depths were generally <0.3 m, and flow velocities varied spatially and exceeded 2.0 m s-1 at the steepest points in some tests. Erosion rates remained modest for the conditions tested, as long as slopes were less than 3 horizontal to 1 vertical (33%) and step height between hedges was less than 0.5 m. Stability modeling of soil steps reinforced with switchgrass roots showed that cohesive forces were 3 times greater than shearing forces for 0.5 m step heights, and that therefore mass failure was unlikely even with the surcharge weight of a 0.2 m depth of ponded water. For step heights greater than 1 m, however, mass failure was observed and predicted to be the dominant erosion mechanism.

Modification of meander migration by bank failures

Meander migration and planform evolution depend on the resistance to erosion of the floodplain materials. To date, research to quantify meandering river adjustment has largely focused on resistance to erosion properties that vary horizontally. This paper evaluates the combined effect of horizontal and vertical floodplain material heterogeneity on meander migration by simulating fluvial erosion and cantilever and planar bank mass failure processes responsible for bank retreat. The impact of stream bank failures on meander migration is conceptualized in our RVR Meander model through a bank armoring factor associated with the dynamics of slump blocks produced by cantilever and planar failures. Simulation periods smaller than the time to cutoff are considered, such that all planform complexity is caused by bank erosion processes and floodplain heterogeneity and not by cutoff dynamics. Cantilever failure continuously affects meander migration, because it is primarily controlled by the fluvial erosion at the bank toe. Hence, it impacts migration rates and meander shapes through the horizontal and vertical distribution of erodibility of floodplain materials. Planar failures are more episodic. However, in floodplain areas characterized by less cohesive materials, they can affect meander evolution in a sustained way and produce preferential migration patterns. Model results show that besides the hydrodynamics, bed morphology and horizontal floodplain heterogeneity, floodplain stratigraphy can significantly affect meander evolution, both in terms of migration rates and planform shapes. Specifically, downstream meander migration can either increase or decrease with respect to the case of a homogeneous floodplain; lateral migration generally decreases as result of bank protection due to slump blocks; and the effect on bend skewness depends on the location and volumes of failed bank material caused by cantilever and planar failures along the bends, with possible achievement of downstream bend skewness under certain conditions.

Validity of Uniform Flow Hypothesis in One-Dimensional Morphodynamic Models

Local-uniform-flow (LUF) hypothesis is a simplification of the governing equations describing river morphodynamics, which is needed to determine the evolution of the bed profile and bed-material composition in the case of large time and space scales. This paper presents a rigorous analysis of the full one-dimensional river hydrodynamic and morphodynamic mathematical model compared to its LUF approximation. The analysis establishes two criteria to assess the validity of the LUF hypothesis: (1) a criterion for rivers in equilibrium and (2) a criterion for evolving rivers (i.e., in nonequilibrium). The first criterion is based on the concept of the morphological box. Variations of the river bed longer than the box length are adequately reproduced by the LUF hypothesis, whereas only spatially averaged values are resolved within the box. The second criterion is based on the concept of an evolution window. Temporal variations represented by wave periods larger than the evolution window can be adequately reproduced by the LUF hypothesis, whereas variations with shorter periods are averaged within this window. The minimum size of morphological box and evolution window that limit the error introduced by the LUF hypothesis increases when the Froude number decreases. Further, the minimum size of the evolution window increases for decreasing sediment concentration and increasing mixing layer thickness (i.e., for larger bed forms). The LUF hypothesis is therefore best applied to small mountain rivers for which both the minimum size of the morphological box and the evolution window is relatively small, so that spatial and temporal variations can be resolved in more detail. Applications using the LUF hypothesis for large watersheds (including the lowland portion of the fluvial network) are possible, but are limited to simulations over larger spatial and temporal intervals.

Evaluating a process‐based model for use in streambank stabilization: insights on the Bank Stability and Toe Erosion Model (BSTEM)

Streambank retreat is a complex cyclical process involving subaerial processes, fluvial erosion, seepage erosion, and geotechnical failures and is driven by several soil properties that themselves are temporally and spatially variable. Therefore, it can be extremely challenging to predict and model the erosion and consequent retreat of streambanks. However, modeling streambank retreat has many important applications, including the design and assessment of mitigation strategies for stream revitalization and stabilization. In order to highlight the current complexities of modeling streambank retreat and to suggest future research areas, this paper reviewed one of the most comprehensive streambank retreat models available, the Bank Stability and Toe Erosion Model (BSTEM), which has recently been integrated with several popular hydrodynamic and sediment transport models including HEC-RAS. The objectives of this paper were to: (i) comprehensively review studies that have utilized BSTEM and report their findings, (ii) address the limitations of the model so that it can be applied appropriately in its current form, and (iii) suggest directions of research that will help make the model a more useful tool in future applications. The paper includes an extensive overview of peer reviewed studies to guide future users of BSTEM. The review demonstrated that the model needs further testing and evaluation outside of the central United States. Also, further development is needed in terms of accounting for spatial and temporal variability in geotechnical and fluvial erodibility parameters, incorporating subaerial processes, and accounting for the influence of riparian vegetation on streambank pore-water pressure dynamics, applied shear stress, and erodibility parameters. This article is protected by copyright. All rights reserved.

Turbulent Flow and Sand Transport over a Cobble Bed in a Laboratory Flume

AbstractImproving the prediction of sand transport downstream of dams requires characterization of the interaction between turbulent flow and near-surface interstitial sands. The advanced age and impending decommissioning of many dams have brought increased attention to the fate of sediments stored in reservoirs. Sands can be reintroduced to coarse substrates that have available pore space resulting from periods of sediment-starved flow. The roughness and porosity of the coarse substrate are both affected by sand elevation relative to the coarse substrate; therefore, the turbulence characteristics and sediment transport over and through these beds are significantly altered after sediment is reintroduced. Past work by the writers on flow over sand-filled gravel beds revealed that the fine-sediment level controls the volume of material available for transport and the area of sediment exposed to the flow. The present work expands on the gravel-bed experiments by conducting similar measurements of turbulent f...

CONCEPTS - A Process-Based Modeling Tool to Evaluate Stream-Corridor Restoration Designs

The success of proposed stream-corridor restoration measures to effectively restore stream structure and function is greatly aided by a thorough analysis of the potential geomorphic responses of the stream to the measures. Complex hydrologic and geomorphic processes determine channel form. Simple analysis tools are available for restoration practitioners, however few are process-based and dynamic. As a response to this, scientists at the US Department of Agriculture, Agricultural Research Service, National Sedimentation Laboratory (NSL) have developed the CONCEPTS model. CONCEPTS (Conservational Channel Evolution and Pollutant Transport System) is a process-based, dynamic computer model that simulates open-channel hydraulics, sediment transport, and channel morphology. CONCEPTS includes research conducted during the past five years by NSL scientists to improve our understanding of the processes controlling channel width adjustment and streambank mechanics. The streambank erosion component of CONCEPTS has been enhanced to include bank-material stratigraphy. Tests of the new component are underway with data obtained over a five-year period from a reach (a bendway) of Goodwin Creek, Mississippi. Initial results show that the enhanced component is able to predict undercutting of streambanks. It underpredicts the retreat of the top of the outside bank at the apex of the bendway by approximately 2 meters because critical shear stresses to initiate lateral erosion at the toe of the bank were too small.