Bank Profile Research Articles

The Role of Collapsed Bank Soil on Tidal Channel Evolution: A Process‐Based Model Involving Bank Collapse and Sediment Dynamics

AbstractWe develop a process‐based model to simulate the geomorphodynamic evolution of tidal channels, considering hydrodynamics, flow‐induced bank erosion, gravity‐induced bank collapse, and sediment dynamics. A stress‐deformation analysis and the Mohr‐Coulomb criterion, calibrated through previous laboratory experiments, are included in a model simulating bank collapse. Results show that collapsed bank soil plays a primary role in the dynamics of bank retreat. For bank collapse with small bank height, tensile failure in the middle of the bank (Stage I), tensile failure on the bank top (Stage II), and sectional cracking from bank top to the toe (Stage III) are present sequentially before bank collapse occurs. A significant linear relation is observed between bank height and the contribution of bank collapse to bank retreat. Contrary to flow‐induced bank erosion, bank collapse prevents further widening since the collapsed bank soil protects the bank from direct bank erosion. The bank profile is linear or slightly convex, and the planimetric shape of tidal channels (gradually decreasing in width landward) is similar when approaching equilibrium, regardless of the consideration of bank erosion and collapse. Moreover, the simulated width‐to‐depth ratio in all runs is comparable with observations from the Venice Lagoon. This indicates that the equilibrium configuration of tidal channels depends on hydrodynamic conditions and sediment properties, while bank erosion and collapse greatly affect the transient behavior (before equilibrium) of the tidal channels. Overall, this contribution highlights the importance of collapsed bank soil in investigating tidal channel morphodynamics using a combined perspective of geotechnics and soil mechanics.

Numerical modelling of the erosion of marsh boundaries due to wave impact

The study describes an improvement of the 1D version of the hydro-morphodynamic model XBeach to simulate the erosion of saltmarsh edges under wave attack. The effects of sand-mud mixture and characteristics of vegetation on the erosive processes are implemented, along with a nonlinear version of the Exner equation to better simulate the horizontal migration of a vertical bank profile due to wave impact. Numerical experiments reveal that the effect of soil composition is crucial for the development of bank morphology where steep scarps are associated with muddier soils while gently smoothed cross-shore profiles are associated with sandier soils. The effect of vegetation in controlling the morphodynamic evolution under wave attack is greater when the sand fraction increases, thus contributing to the development of small steep scarps. The model is further tested against field data of a saltmarsh bank profile and wind conditions collected before and after the passage of Hurricane Isaac in Lake Borgne, Louisiana. After validation of the hydrodynamics, a tuning procedure to match the final profile is carried out on the main model parameters. The results illustrate the potential utility of the model for application to geomorphic settings subjected to wave impact and characterized by high mud content and halophytic vegetation; yet, the model still has limitations and further improvements are needed. Finally, the results demonstrate the ability of saltmarshes to withstand erosion during the peak of large storms when the marsh is submerged, and highlight transient conditions as important drivers of geomorphic change during storms.

P‐189: Distinguished Poster: 3D Pixel Configurations for Optical Out‐coupling of OLED Displays‐ Part I: Optical Simulation

Due to considerations on image quality and fabrication/integration compatibility, current active‐matrix OLED displays (AMOLEDs) hardly adopt any effective light out‐coupling techniques/structures and it remains an unsolved critical technical issue. Recently, through optical simulation, we proposed a 3D pixel configuration as a potential and promising way to boost light out‐coupling of OLED display pixels. With extending the bottom reflective electrode to bank slope of the pixel definition layer (PDL) and selectively filling the bank opening (concave area) with high‐index filler to form the optically reflective concave structure, it is possible to give several‐fold light extraction enhancement. In this paper, we further extend the simulation work by evaluating effects of more realistic graded bank slopes vs. straight bank slope, less challenging non‐patterned (blanket) high‐index filler vs. patterned filler, and real OLED layer stacks and properties. The optical simulation clearly reveals that light extraction enhancement can be mostly kept with more realistic graded bank profiles and even less ideal (but also less challenging) non‐patterned filler can provide substantial extraction enhancement. The graded bank structure also provides beneficial effects on viewing characteristics.

Assessment of geomorphological bank evolution of the alluvial threshold rivers based on entropy concept parameters

ABSTRACTThe complex stream bank profiles in alluvial channels and rivers that are formed after reaching equilibrium has been a popular topic of research for many geomorphologists and river engineers. The entropy theory has recently been successfully applied to this problem. However, the existing methods restrict the further application of the entropy parameter to determine the cross-section slope of the river banks. To solve this limitation, we introduce a novel approach in the extraction of the equation based on the calculation of the entropy parameter (λ) and the transverse slope of the bank profile at threshold channel conditions. The effects of different hydraulic and geometric parameters are evaluated on a variation of the entropy parameter. Sensitivity analysis on the parameters affecting the entropy parameter shows that the most effective parameter on the λ-slope multiplier is the maximum slope of the bank profile and the dimensionless lateral distance of the river banks.

Enhanced formulation of the probability principle based on maximum entropy to design the bank profile of channels in geomorphic threshold

It is essential to consider the geometric shape and dimensions of a stable channel bank profile in the design and implementation of open channels. A threshold channel refers to a state in which particles do not move along the channel (especially the banks) even after the channel has reached stability. This type of channel is a simple model of rivers and natural channels. The way sediment particles form at the channel banks is significant. The transverse bank slope (St) varies monotonically from zero at the channel bed to a maximum value at the channel centerline. Accordingly, studying the transverse bank slope based on maximizing its entropy value can offer acceptable results in predicting the shape and dimensions of stable channels. The mean and maximum channel transverse slope values at any discharge (Q) are useful for predicting channel dimensions. In different Q ranges, a channel adjusts its dimensions and geometric shape to attain a constant range of entropy parameter values. In the present study, the entropy concept is employed to study the bank profile shape after a threshold channel with a gravel bed reaches equilibrium. Moreover, the ratio (β) between the maximum (SX) and mean (SN) transverse slope values of a channel bank profile (β = SN/SX) and the relation with the entropy parameter (K) are assessed based on a wide range of observations and field data in various hydraulic conditions. According to the results, the ratio of SN to SX (β) at the banks of a stable channel at each discharge tends towards a constant value and does not change with changes in discharge and time. Subsequently, the mean value of the entropy parameter (K) per discharge is constant at approximately 1–3. Therefore, four common threshold channel types are defined in this study based on the variations in Q and mean sediment size (d50). New relationships between K, β and other parameters in open channels are developed to facilitate the calculation and design of stable channels. The proposed method produces the transverse bank slope values and vertical alignment of a stable channel bank’s cross sections with high accuracy. Thus, it is possible to calculate the shape and dimensions of a stable channel in different hydraulic conditions (varying Q and d50).

A method based on the Tsallis entropy for characterizing threshold channel bank profiles

As a stable channel forms, a point is reached when particles shift to motion threshold (threshold channel). As such, the most common challenges related to alluvial channels include the variation in the transverse bank slope (st) and channel geometric profile formation. The present study introduces a novel reliable method of estimating the transverse slope distribution of stable channel bank profiles in different hydraulic conditions by using the Tsallis entropy concept. Accordingly, an equation for calculating st and subsequently the vertical boundary level of bank profiles is presented based on the principle of maximum entropy. The threshold channel bank profile features are defined according to the probability density function and cumulative density function to adjust the general form of the Tsallis entropy for this specific engineering problem. An extensive, detailed study of Lagrange multipliers is carried out and a wide range of observational data are used to determine the Tsallis entropy model accuracy in measuring the st of stable channel banks. The results indicate that the Tsallis entropy model is capable of detecting the ascending trend governing the profiles of stable channel banks in different hydraulic conditions. This model exhibits excellent predictive performance with a mean absolute relative error (MARE) of 0.265 and a coefficient of determination (R2) of 0.975. The proposed Tsallis entropy-based equation can be used to predict river morphology changes for river management plans.

Three‐dimensional pixel configurations for optical outcoupling of OLED displays—optical simulation

AbstractBecause of considerations on image quality and fabrication/integration compatibility, current active‐matrix OLED displays (AMOLEDs) hardly adopt any effective light outcoupling techniques/structures, and it remains an unsolved critical technical issue. Recently, through optical simulation, we proposed a three‐dimensional (3D) pixel configuration as a potential and promising way to boost light outcoupling of OLED display pixels. By extending the bottom reflective electrode to bank slope of the pixel definition layer (PDL) and selectively filling the bank opening (concave area) with high‐index filler to form the optically reflective concave structure, it is possible to give several‐fold light extraction enhancement. In this paper, we further extend the simulation work by evaluating effects of more realistic graded bank slopes vs straight bank slope, less challenging nonpatterned (blanket) high‐index filler vs patterned filler, and real OLED layer stacks and properties. The optical simulation clearly reveals that light extraction enhancement can be mostly kept with more realistic graded bank profiles and even less ideal (but also less challenging) nonpatterned filler can provide substantial extraction enhancement. The graded bank structure also provides beneficial effects on viewing characteristics.

Analyzing bank profile shape of alluvial stable channels using robust optimization and evolutionary ANFIS methods

In natural rivers and artificial channels in addition to the channel dimensions (widening, reduction in slope and depth in the channel banks), formed shape profile in the case that the sediment on the banks with no movement (thresholds state) is of considerable importance for engineers. To determine the bank shape profiles, various theoretical, empirical and statistical relations have been provided based on physical and numerical models by different researchers. In this study, a simple model of adaptive neuro-fuzzy inference systems (ANFIS) is combined with two algorithms of differential evolution (DE) and singular value decomposition value (SVD) and the performance of these models to predict the stable shape profiles of the channels is evaluated and compared. In this paper, the main goal is to assess extensively the effect of hybrid models based on optimized algorithms (ANFIS-DE) and multi-objective evolutionary algorithm (ANFIS-DE/SVD) in improvement of performance of ANFIS and ANFIS-DE models, respectively. Accordingly, the results assessment show that all three ANFIS, ANFIS-DE and ANFIS-DE/SVD models are perfectly able to predict shape profiles in accordance with the observed profiles for the threshold channels. Using optimized and evolutionary algorithms has a positive impact on the performance of simple model of ANFIS. As compared to the simple ANFIS model, ANFIS-DE approximately 10.1% and ANFIS-DE/SVD model 7.2% is improved compared to the ANFIS-DE model. The accuracy of ANFIS-DE/SVD model showed better performance as well about 18.6% compared to the simple ANFIS model. Therefore, it can be said that not only DE optimization algorithms have a significant impact on increasing the performance of a simple ANFIS model but also using evolutionary algorithms (ANFIS-DE/SVD) reduce the ANFIS-DE model error accordingly. Polynomial equations of bank profiles proposed by hybrid ANFIS models in the present study can be used in design and implementation of cross section of stable channels.

Entry Trajectory Options for High Ballistic Coefficient Vehicles at Mars

Future large-scale Mars surface exploration missions require landed masses beyond the capability of current entry, descent, and landing technology. Hypersonic trajectory options for large ballistic coefficient vehicles are explored to assess the potential for improved landed mass capability in the absence of landed accuracy requirements. Hypersonic trajectories appropriate for use with supersonic parachute and supersonic retropropulsion descent systems are studied. Optimal control techniques are used to determine hypersonic bank-angle control profiles that achieve favorable conditions at terminal descent initiation. Terminal descent initiation altitude-maximizing bank strategies for parachute descent systems are explored across vehicle and mission design parameters of interest. A tradeoff between altitude and flight-path angle at terminal descent initiation is identified. Hypersonic trajectories that minimize required propellant for propulsive descent are identified and studied parametrically. A hypersonic ballistic coefficient and lift-to-drag ratio are shown to have the largest effects on required propellant mass fraction; changes to the vehicle state at entry interface have a smaller effect. The space of reachable supersonic retropropulsion ignition states is presented over a range of vehicle and trajectory parameters. Overall, results indicate execution of an appropriate hypersonic bank profile can significantly increase the parachute deploy altitude for parachute descent systems or reduce the amount of propellant required when compared to full lift-up entry for supersonic retropropulsion descent systems operating at Mars.

An experimental investigation on effect of drawdown rate and drawdown ratios on stability of cohesionless river bank and evaluation of factor of safety by total strength reduction method

ABSTRACTThis paper presents an experimental investigation on the stability of a model river bank composed of homogeneous cohesionless soil under rapid drawdown condition. The effects of major influencing parameters controlling Factor of Safety (FoS) of a river bank under depletion of water level have been considered in the present study. A series of laboratory model studies have been carried out to investigate the cases of drawdown rate and drawdown ratios rendering the bank to maximum damage. Moreover, the effect of water level drawdown on the response variables namely pore water pressure, deformation of the bank profile, and shear strength of bank material have been observed and analysed. Stability analysis of the experimental model banks under drawdown conditions was carried out by evaluating FoS using the principle of reduced shear strength methodology. The in-situ total shear strength after each drawdown was measured using a laboratory vane shear apparatus. The minimum shear strength among these values has been identified. Now the FoS against each drawdown ratio has been computed from the ratio of total shear strength obtained after drawdown for that particular drawdown ratio to the minimum shear strength as obtained. This experimental programme examined the optimum combination of drawdown rate and drawdown ratio causing mass failure of the bank. It was revealed from the variation of pore pressure after drawdown that changes of pressures at points close to the toe of the bank slope is strongly controlled by the stress-state induced by drawdown. From the failure observations it was found that the drawdown rate is the dominating cause of maximum deformation of the bank than that of drawdown ratio. The findings of the present research work may help in predicting the actual failure scenario and stability condition of prototype river under similar boundary conditions.

Coupled modeling of bank retreat processes in the Upper Jingjiang Reach, China

AbstractBank retreat processes in the Upper Jingjiang Reach (UJR) have attracted much attention in recent years due to a high occurrence frequency, and these processes can be influenced significantly by the near‐bank fluvial erosion and variations in river stage and groundwater level. A coupled model was thus proposed in this study, integrating the module of bank‐toe deformation with the modules of groundwater level change and bank stability analysis. The proposed model was validated through comparisons between the calculated and measured bank erosion volumes and bank profiles at four typical sections in the UJR, with relatively close agreement being obtained. Multiple tests were conducted to investigate the model performance. These results show that: (i) a more rapid drawdown of river stage resulted in a more significantly delayed response of groundwater level to river stage, indicating that the bank may suffer a higher pore water pressure at the same river stage during the rapid drawdown period; (ii) a larger bank erosion volume was predicted, with the delayed response of groundwater level being considered. In addition, initial groundwater level may take a great impact on bank erosion simulations, especially for the banks with relatively lower soil permeability and shear strength); (iii) based on the adopted method of bank stability analysis, the occurrence of the first failure may greatly influence the sequential failures, and thus the total number and volume of bank failure; (iv) the calculated bank erosion volume was most sensitive to a change in the critical shear stress, especially to its decrease, when the erodibility coefficient was determined by an empirical function of the critical shear stress. © 2018 John Wiley & Sons, Ltd.

Optimum cross section index (OCI): a new approach for identification of an optimum channel—a case study of the Ichamati River, India

The Ichamati River drains the east and south side of the North 24 Parganas district which belongs to the Ganga-Brahmaputra Delta (GBD), covered by deep Quaternary sediments produced under tropical monsoon climate in India. The district is densely populated. The river has huge impact on its population. In this paper, particular attention has been paid to a new method, named optimum cross section index (OCI), which is a numerical representation of a river cross section with respect to its optimum condition. To establish the validity and reliability of OCI, as circumstantial evidences, we have measured and analyzed the geometric parameters, such as width, depth, hydraulic radius, wetted perimeter, and bank profile at 22 points, and applied the areal difference asymmetric index (ADAI). The paper considers that the changing characters of the geometric form of the cross sections and ADAI stand for the OCI. All the techniques reveal that (1) the area of channel decreases abruptly upstream downward, (2) the upstream channel is more symmetrical than the lower reach, and (3) the area of optimum channel gradually increases upstream downward. Results of the study show that the river is going to be deteriorating from upstream downward. OCI has been constructed based on width and depth, which are relatively easily available data. So, this model may be well assessable to identify the optimum channel for a river management.

A methodological approach of predicting threshold channel bank profile by multi-objective evolutionary optimization of ANFIS

More accurate stable channel design methods are necessary for analyzing the complex bank profile cross sections of alluvial channels that achieve equilibrium state. This study introduces a new hybrid method that combines an adaptive neuro-fuzzy inference system (ANFIS), Differential Evolution (DE) algorithm and Singular Value Decomposition (SVD) to predict the bank profile of a threshold channel. SVD and DE serve to optimally determine the consequent linear parameters and antecedent nonlinear parameters of the TSK-type fuzzy rules in ANFIS. Moreover, by defining two objective functions and using the Pareto curve, the tradeoff of function is selected as the optimal modeling point. The authors carried out laboratory experiments at four discharge rates of 1.16, 2.18, 2.57 and 6.20 l/s to measure the coordinates of the points in a stable channel boundary profile. The ANFIS-DE/SVD results are compared with the results of a simple ANFIS model and 7 previous research works (based on numerical and experimental models and mathematical principles). The RMSE error index (0.019) of the ANFIS-DE/SVD model is lower than the ANFIS model (0.027), but both models outperform the best available model (CKM, Cao and Knight, 1998) (RMSE = 0.120). The ANFIS-DE/SVE model is more accurate for larger y (water surface level) values than the simple ANFIS model. Moreover, the superior performance of the hybrid ANFIS-DE/SVD over the simple ANFIS model is more pronounced at greater discharge rates. ANFIS-DE/SVD estimates the bank profile shape of a stable channel as a third-degree polynomial equation, which can be used to design and implement stable alluvial channels.

Urban wetland seed bank profiles in south-central New York State1,2

Wetlands are important habitats in urban landscapes that reduce flooding and improve water quality, yet urban wetland seed banks are rarely studied. Our main objective was to profile urban wetland seed banks in south-central New York. We exposed sediment cores from four wetlands in Broome County, New York, to flooded and drawdown treatments for 16 mo, and recorded community composition and seedling density. We found high spatial variation in species richness and seedling density among the four sites. Species richness ranged from 28 to 56 species, with sample-based extrapolation (Sest), Jackknife1, and Chao1 analyses estimating similar expected species richness values (Sest projected 37.9–77.0 species, Jackknife1 analysis estimated 40.5–77.8 species, and Chao1 projected 32.2–79.1 species). Mean seedling density ranged from 3,367 seedlings/m2 to 19,132 seedlings/m2. These seed banks were dominated by obligate wetland species (75.8–93.3%). Invasive species comprised a high percentage of seedlings for three wetlands (40.8–80.9%), but not for the fourth site (4.2%). Lythrum salicaria, Typha sp., and Ludwigia palustris were common species based on relative seedling density for three seed banks, while Leersia oryzoides, Schoenoplectus tabernaemontani, and Alisma triviale were common species in the fourth site. Similarity indices between the standing vegetation and their respective seed banks, based on presence/absence data, were low (13–34%). Species richness and seedling densities were within the ranges of natural wetland seed bank studies.

Reliable method of determining stable threshold channel shape using experimental and gene expression programming techniques

The geometric dimensions and bank profile shape of channels with boundaries containing particles on the verge of motion (threshold channels) are significant factors in channel design. In this study, extensive experimental work was done at different flow velocities to propose a reliable method capable of estimating stable channel bank profile. The proposed method is based on gene expression programming (GEP). Laboratorial datasets obtained from Mikhailova et al. (Hydro Tech Constr 14:714–722, 1980), Ikeda (J Hydraul Div ASCE 107:389–406 1981), Diplas (J Hydraul Eng ASCE 116:707–728, 1990) and Hassanzadeh et al. (J Civil Environ Eng 43(4):59–68, 2014) were used to train, test, validate and examine the GEP model in various geometric and hydraulic conditions. The obtained results demonstrate that the proposed model can estimate bank profile characteristics with great accuracy (determination coefficient of 0.973 and mean absolute relative error of 0.147). Moreover, for practical calculations of channel dimensions, the model provides a specific mathematical relationship to solve problems with different discharge rates (Q) and particles with various median grain sizes (D50). The proposed model’s performance is compared with 8 relationships suggested previously by researchers (based on empirical and theoretical analyses) and a relationship obtained using a nonlinear regression model with different experimental data. The polynomial VDM and two exponential functions, i.e. IKM and DIM, are introduced as the superior existing models. According to the present study results, the proposed GEP model can predict the bank profile shape trend well and similar to the experimental datasets. Sensitivity analysis was also applied to assess the impact of each input variable (x, Q and D50) on the presented relationship. According to the current study, the GEP model provides a suitable equation for predicting the bank profile shape of stable channels.

Uncertainty analysis of intelligent model of hybrid genetic algorithm and particle swarm optimization with ANFIS to predict threshold bank profile shape based on digital laser approach sensing

Measuring the bank profile shape when bank sediment particles are undergoing incipient motion (threshold) is notable for researchers. In the present study, an advanced laser gauge sensor instrument is utilized to measure the coordinates of points located in a stable channel bank cross section. A robust Particle Swarm Optimization (PSO) algorithm combined with a Genetic Algorithm (GA) is developed in an Adaptive Neuro-Fuzzy Inference System (ANFIS) model to predict and compare the bank profile with measured data. The results show that the proposed ANFIS-PSO/GA optimization method had a determination coefficient (R2) and Mean Absolute Relative Error (MARE) of 0.9951 and 0.1575, respectively, and was thus highly accurate in stable bank shape prediction. With increasing flow discharge, the ANFIS-PSO/GA model accuracy augmented, and the model could simulate the widening of the water surface width quite well with an appropriate bank profile function that can be used in the design and implementation of stable channel geometry. Moreover, an uncertainty analysis was carried out to calculate the reliability of the proposed model based on the Monte-Carlo method. The results show that the ANFIS-PSO/GA model has less uncertainty in different hydraulic conditions of channels for predicting the vertical level of threshold bank profile and can predict satisfactorily the channel bank profiles.

Modelling of the retreat process of composite riverbank in the Jingjiang Reach using the improved BSTEM

AbstractBank retreat in the Jingjiang Reach is closely related not only to the near‐bank intensity of fluvial erosion but also to the composition and mechanical properties of bank soils. Therefore, it is necessary to correctly simulate bank retreat to determine the characteristics of fluvial processes in the Jingjiang Reach. The current version of bank stability and toe erosion model (5.4) was improved to predict riverbank retreat, by inputting a dynamic water table, and calculating the approximation of the distribution of dynamic pore water pressure in the soil near the river bank face, and considering the depositional form of the failed blocks, which is assumedly based on a triangular distribution, with the slope approximately equalling the stable submerged bank slope and half of collapsed volume deposited in the bank‐toe region. The degrees of riverbank stability at Jing34 were calculated using the improved bank stability and toe erosion model. The results indicate the following trends: (a) the degrees of riverbank stability were high during the dry season and the rising stage, which led to minimal bank failure, and (b) the stability degrees were low during the flood season and the recession stage, with the events of bank collapse occurring frequently, which belonged to a stage of intensive bank erosion. Considering the effects of bank‐toe erosion, water table lag, and the depositional form of the collapsed bank soil, the bank‐retreat process was simulated at the right riverbank of Jing34. The model‐predicted results exhibit close agreement with the measured data, including the total bank‐retreat width and the collapsed bank profile. A sensitivity analysis was conducted to determine the quantitative effects of toe erosion and water table lag on the degree of bank stability. The calculated results for toe erosion indicate that the amount of toe erosion was largest during the flood season, which was a primary reason for bank failure. The influence of water table lag on the degree of stability demonstrates that water table lag was an important cause of bank failure during the recession stage.

Quantifying streambank movement and topography using unmanned aircraft system photogrammetry with comparison to terrestrial laser scanning

AbstractStreambank movement is an integral part of geomorphic changes along river corridors and affects a range of physical, ecological, and socio‐economic systems including aquatic habitat, water quality, and infrastructure. Various methods have been used to quantify streambank erosion, including bank pins, ground surveys, lidar, and analytical models, however, due to high‐cost or labour intensive fieldwork these are typically feasible or appropriate only for site‐specific studies. Advancements in unmanned aircraft systems (UAS) and photogrammetry provide opportunities for more rapid and economical quantification of streambank erosion and deposition at variable scales. This work assesses the performance of UAS‐based photogrammetry for capturing topography of streambank surfaces and quantifying bank movement. UAS data are compared to terrestrial laser scanner (TLS) and GPS surveying from streambank sites located in Vermont that featured a variety of bank conditions and vegetation. Cross‐sectional analysis of data from UAS and TLS revealed that the UAS reliably captured the bank surface within 0.2 m of TLS and GPS surveys across all sites during leaf‐off conditions. Mean error between UAS and TLS was only 0.11 m in early spring conditions. Dense summer vegetation resulted in decreased accuracy and was a limiting factor in the ability of the UAS to capture the ground surface. At areas with observed bank movement, the change in cross‐sectional area estimated using UAS data compared reliably to TLS survey for net cross‐sectional changes greater than 3.5 m2, given a 10% error tolerance. At locations with smaller changes, error increased due to the effect of vegetation, georeferencing, and overhanging bank profiles. UAS‐based photogrammetry shows significant promise for capturing bank topography and movement at fine resolutions in a flexible and efficient manner.

Understanding mass fluvial erosion along a bank profile: using PEEP technology for quantifying retreat lengths and identifying event timing

AbstractThis study provides fundamental examination of mass fluvial erosion along a stream bank by identifying event timing, quantifying retreat lengths, and providing ranges of incipient shear stress for hydraulically driven erosion. Mass fluvial erosion is defined here as the detachment of thin soil layers or conglomerates from the bank face under higher hydraulic shear stresses relative to surface fluvial erosion, or the entrainment of individual grains or aggregates under lower hydraulic shear stresses.We explore the relationship between the two regimes in a representative, US Midwestern stream with semi‐cohesive bank soils, namely Clear Creek, IA. Photo‐Electronic Erosion Pins (PEEPs) provide, for the first time, in situ measurements of mass fluvial erosion retreat lengths during a season. The PEEPs were installed at identical locations where surface fluvial erosion measurements exist for identifying the transition point between the two regimes. This transition is postulated to occur when the applied shear stress surpasses a second threshold, namely the critical shear stress for mass fluvial erosion.We hypothesize that the regimes are intricately related and surface fluvial erosion can facilitate mass fluvial erosion. Selective entrainment of unbound/exposed, mostly silt‐sized particles at low shear stresses over sand‐sized sediment can armor the bank surface, limiting the removal of the underlying soil. The armoring here is enhanced by cementation from the presence of optimal levels of sand and clay. Select studies show that fluvial erosion strength can increase several‐fold when appropriate amounts of sand and clay are mixed and cement together. Hence, soil layers or conglomerates are entrained with higher flows.The critical shear stress for mass fluvial erosion was found to be an order of magnitude higher than that of surface fluvial erosion, and proceeded with higher (approximately 2–4 times) erodibility. The results were well represented by a mechanistic detachment model that captures the two regimes. Copyright © 2017 John Wiley & Sons, Ltd.

Effect of downward seepage on the shape of an alluvial channel

Longitudinal and cross-sectional profiles of alluvial channels are altered by the presence of seepage. However, the available literature suggests that seepage has not been considered in analyses of bank profiles of threshold alluvial channels. In the experimental study reported here, downward seepage was applied to parabolic channels based on tractive force theory in order to determine its effect on the longitudinal and cross-sectional profiles. It was observed that sediment transport in the threshold channels increased when downward seepage was applied. The parabolic shape did not hold, but channels formed a new cross-sectional shape with a flat bed and two curved banks and achieved stability. The reason for the increased sediment transport in a threshold channel after the application of downward seepage was sought by analysing the time-averaged velocities. An empirically derived exponential expression was developed for the bank profiles of threshold alluvial channels affected by seepage. A relationship among hydraulic parameters for a stable channel with downward seepage is also suggested on the basis of the experimental observations.