Bank Stability And Toe Erosion Model Research Articles

Effects of vegetation on debris flow mitigation: A case study from Gansu province, China

Debris flows are traditionally controlled using civil engineering structures such as check dams. However, the misuse of such strategies may sometimes trigger environmental hazards such as the catastrophic landslide in 2010 in Zouqu county, China, and therefore other methods such as the use of vegetation as an eco-engineering tool are increasingly being adopted. The aim of the present research was to investigate the bioengineering effects of vegetation over time in an area prone to debris flows in Gansu province, China. We collected detailed data from 2012 to 2014 on vegetation type, density, and root system morphology, and measured profiles across the valley. In addition, we assessed the increased soil cohesion provided by the root development of three monospecific stands of Robinia pseudoacacia of different ages growing within the debris valley, and on a larger scale, their effects on channel morphology. These data were incorporated into a modified form of BSTEM (Bank Stability and Toe Erosion Model) and a cellular braided-stream model. The results indicate that with increasing age, the FOS (factor of safety) of the bank would be significantly increased, and that the flooded area in the valley caused by simulated flood events would be decreased by 18–24%, on average. Subsequently, field data were incorporated into a cellular model to simulate sediment movement and the effects of vegetation on the channel dynamics. The results demonstrate that the stability provided by vegetation could result in a less active valley system and that overall the development of debris-controlling vegetation could make a major contribution to ecosystem restoration. However, careful management is essential for making optimum use of the vegetation.

Bank stability and toe erosion model of the Kodku Khola bank, southeast Kathmandu valley, central Nepal

The Kodku Khola is a potential river from the southeast part of Kathmandu valley as it has been used for irrigation and household purposes from prehistoric time. The river is suffering from streambank instability causing great threat to the infrastructure, land and settlement areas. In this context, assessment of Bank Stability and Toe Erosion Model (BSTEM) of the Kodku Khola was undertaken for eight different sites using the BSTEM version 5.4 that calculates a Factor of Safety (Fs) for multilayer streambank, based on limit equilibrium-method. Streambank of the uppermost reach around the transects BK1 (Lower Badikhel) and BK2 (Upper Taukhel) area is stable, where Fs exceeds 1.3 and maximum lateral retreat of channel is 21.86-30.59 cm with 0.025-0.290 m2 of the total eroded area of the bank-toe resulting in less bank toe erosion. Canopy and understorey cover with consolidated bank materials are the causes of stable banks. Streambank of transects BK3 (Arubot) and BK4 (Thaiba) are unstable as Fs ranges from 0.75 to 0.92, and the maximum lateral retreat of channel ranges 70.83-208.81 cm with total eroded bank toe area of 0.117–1.695 m2 resulting in excessive bank toe erosion problems. Major causes of instability are the presence of unconsolidated bank material, high scouring, and sparse riparian vegetation. Within the transects BK6 and BK7 around Harisiddhi, streambanks are stable with less bank toe erosion hazard because of channelization. Where the Fs are low and banks are disturbed by encroachment, suitable bioengineering measures can be implemented to mitigate excessive bank toe erosion and failure.

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

AbstractStreambank 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 the Hydrologic Engineering Center's River Analysis System (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. Copyright © 2016 John Wiley & Sons, Ltd.

Uncertainty and sensitivity in a bank stability model: implications for estimating phosphorus loading

AbstractEutrophication of aquatic ecosystems is one of the most pressing water quality concerns in the United States and around the world. Bank erosion has been largely overlooked as a source of nutrient loading, despite field studies demonstrating that this source can account for the majority of the total phosphorus load in a watershed. Substantial effort has been made to develop mechanistic models to predict bank erosion and instability in stream systems; however, these models do not account for inherent natural variability in input values. To quantify the impacts of this omission, uncertainty and sensitivity analyses were performed on the Bank Stability and Toe Erosion Model (BSTEM), a mechanistic model developed by the US Department of Agriculture – Agricultural Research Service (USDA‐ARS) that simulates both mass wasting and fluvial erosion of streambanks. Generally, bank height, soil cohesion, and plant species were found to be most influential in determining stability of clay (cohesive) banks. In addition to these three inputs, groundwater elevation, stream stage, and bank angle were also identified as important in sand (non‐cohesive) banks. Slope and bank height are the dominant variables in fluvial erosion modeling, while erodibility and critical shear stress had low sensitivity indices; however, these indices do not reflect the importance of critical shear stress in determining the timing of erosion events. These results identify important variables that should be the focus of data collection efforts while also indicating which less influential variables may be set to assumed values. In addition, a probabilistic Monte‐Carlo modeling approach was applied to data from a watershed‐scale sediment and phosphorus loading study on the Missisquoi River, Vermont to quantify uncertainty associated with these published results. While our estimates aligned well with previous deterministic modeling results, the uncertainty associated with these predictions suggests that they should be considered order of magnitude estimates only. Copyright © 2016 John Wiley & Sons, Ltd.

Co‐evolution of riparian vegetation and channel dynamics in an aggrading braided river system, Mount Pinatubo, Philippines

AbstractIncreased bank stability by riparian vegetation can have profound impacts on channel morphology and dynamics in low‐energy systems, but the effects are less clear in high‐energy environments. Here we investigate the role of vegetation in active, aggrading braided systems at Mount Pinatubo, Philippines, and compare results with numerical modeling results. Gradual reductions in post‐eruption sediment loads have reduced bed reworking rates, allowing vegetation to finally persist year‐round on the Pasig‐Potrero and Sacobia Rivers. From 2009–2011 we collected data detailing vegetation extent, type, density, and root strength. Incorporating these data into the RipRoot model and BSTEM (Bank Stability and Toe Erosion Model) shows cohesion due to roots increases from zero in unvegetated conditions to > 10·2 kPa in densely‐growing grasses. Field‐based parameters were incorporated into a cellular model comparing vegetation strength and sediment mobility effects on braided channel dynamics. The model shows both low sediment mobility and high vegetation strength lead to less active systems, reflecting trends observed in the field. The competing influence of vegetation strength versus channel dynamics is a concept encapsulated in a dimensionless ratio between timescales for vegetation growth and channel reworking known as T*. An estimated T* between 1·5 and 2·3 for the Pasig‐Potrero River suggests channels are still very mobile and likely to remain braided until aggradation rates decline further. Vegetation does have an important effect on channel dynamics, however, by focusing flow and thus aggradation into the unvegetated fraction of braidplain, leading to an aggradational imbalance and transition to a more avulsive state. The future trajectory of channel–vegetation interactions as sedimentation rates decline is complicated by strong seasonal variability in precipitation and sediment loads, driving incision and armoring in the dry season. By 2011, incision during the dry season was substantial enough to lower the water‐table, weaken existing vegetation, and allow for vegetation removal in future avulsions. Copyright © 2015 John Wiley & Sons, Ltd.

Modeling streambank erosion and failure along protected and unprotected composite streambanks

Streambank retreat can be a significant contributor to total sediment and nutrient loading to streams. Process-based bank stability models, such as the Bank Stability and Toe Erosion Model (BSTEM), have been used to determine critical factors affecting streambank erosion and failure such as riparian vegetation and to estimate retreat rates over time. BSTEM has been successfully applied on a number of cohesive streambanks, but less so on composite banks consisting of both cohesive and noncohesive soils in highly sinuous streams. Composite streambanks can exhibit rapid and episodic bank retreat. The objectives of this research were twofold: (i) develop and apply simplified procedures for estimating root cohesion based on above- and below-ground biomass estimates and (ii) systematically apply BSTEM to a series of 10 composite streambanks distributed along the Barren Fork Creek in eastern Oklahoma to assess model sensitivity to root cohesion and model performance in predicting retreat. This research aimed to document the influence of riparian conservation practices on bank retreat rates and evaluated simplistic methods for incorporating such practices into such process-based models. Sites modeled included historically unprotected sites with no riparian vegetation and historically protected sites with riparian vegetation present during all or part of the 2003 to 2010 study period. The lateral retreat ranged from 4.1 to 74.8m across the 10 sites and was largest at the historically unprotected sites in which retreat averaged 49.2m. Protected sites had less bank retreat but with more variability in retreat rates per year. With calibration focused on the erodibility parameters, the model was able to match both the observed total amount of retreat as well as the timing of retreat at both the protected and unprotected sites as derived from aerial imagery. During calibration BSTEM was not sensitive to the specific value of the soil cohesion or the additional soil cohesion added due to roots for the cohesive topsoil layer, suggesting that the proposed simplified techniques could be used to estimate root cohesion values. The BSTEM modeling also provided an advantageous assessment tool for evaluating retreat rates compared to in situ bank retreat measurements due to the magnitude and episodic nature of streambank erosion and failures. Process-based models, such as BSTEM, may be necessary to incrementally model bank retreat in order to quantify actual streambank retreat rates and understand mechanisms of failure for the design of stabilization projects.

Evaluation of the bank stability and toe erosion model (BSTEM) for predicting lateral retreat on composite streambanks

Streambank erosion is known to be a major source of sediment in streams and rivers. The Bank Stability and Toe Erosion Model (BSTEM) was developed in order to predict streambank retreat due to both fluvial erosion and geotechnical failure. However, few, if any, model evaluations using long-term streambank retreat data have been performed. The objectives of this research were to (1) monitor long-term composite streambank retreat during a hydraulically active period on a rapidly migrating stream, (2) evaluate BSTEM's ability to predict the measured streambank retreat, and (3) assess the importance of accurate geotechnical, fluvial erosion, and near-bank pore-water pressure properties. The Barren Fork Creek in northeastern Oklahoma laterally eroded 7.8 to 20.9m along a 100-m length of stream between April and October 2009 based on regular bank location surveys. The most significant lateral retreat occurred in mid- to late-May and September due to a series of storm events, and not necessarily the most extreme events observed during the monitoring period. BSTEM (version 5.2) was not originally programmed to run multiple hydrographs iteratively, so a subroutine was written that automatically input the temporal sequence of stream stage and to lag the water table in the near-bank ground water depending on user settings. Eight BSTEM simulations of the Barren Fork Creek streambank were performed using combinations of the following input data: with and without a water table lag; default BSTEM geotechnical parameters (moderate silt loam) versus laboratory measured geotechnical parameters based on direct shear tests on saturated soil samples; and default BSTEM fluvial erosion parameters versus field measured fluvial erosion parameters from submerged jet tests. Using default BSTEM input values underestimated the actual erosion that occurred. Lagging the water table predicted more geotechnical failures resulting in greater streambank retreat. Using measured fluvial and geotechnical parameters and a water table lag also under predicted retreat (approximately 3.3m), but did predict the appropriate timing of streambank collapses. The under prediction of retreat was hypothesized to be due to over predicting the critical shear stress of the non-cohesive gravel, under predicting the erodibility of the non-cohesive gravel, and/or under predicting the imposed shear stress acting on the streambank. Current research improving our understanding of shear stress distributions, streambank pore-water pressure dynamics, and methods for estimating excess shear stress parameters for noncohesive soils will be critical for improving BSTEM and other streambank stability models.

Quantifying Reductions of Mass‐Failure Frequency and Sediment Loadings From Streambanks Using Toe Protection and Other Means: Lake Tahoe, United States1

Abstract: Streambank erosion by mass‐failure processes represents an important form of channel adjustment and a significant source of sediment in disturbed streams. Mass failures regularly occur by a combination of hydraulic processes that undercut bank toes and geotechnical processes that cause bank collapse by gravity. Little if any quantitative information is available on the effectiveness of bank treatments on reducing erosion. To evaluate potential reduction in sediment loadings emanating from streambanks, the hydraulic and geotechnical processes responsible for mass failure were simulated under existing and mitigated conditions using a Bank‐Stability and Toe‐Erosion Model (BSTEM). Two critical erosion sites were selected from each of the three watersheds known to contribute the greatest amounts of fine sediment by streambank processes in the Lake Tahoe Basin. A typical high‐flow annual hydrograph was selected for analysis. Bank‐material strength data were collected for each layer as were species‐specific root‐reinforcement values. The effects of the first flow event on bank‐toe erosion were simulated using an excess shear‐stress approach. The resulting geometry was then exported into the bank‐stability submodel to test for the relative stability of the bank under peak flow and drawdown conditions. In this way, BSTEM was used iteratively for all flow events for both existing and mitigated conditions. On average, 13.6% of the material was eroded by hydraulic shear, the remainder by mass failures, which occurred about five times over the simulation period. Simulations with 1.0 m‐high rock‐toe protection showed a dramatic reduction in streambank erosion (69‐100%). Failure frequency for the simulation period was reduced in most cases to a single episode. Thus, an almost 90% reduction in streambank loadings was achieved by virtually eliminating the erosion of only 14% of the material that was entrained by hydraulic forces. Consequently, simulations show average load reductions of about an order of magnitude. Results stress the critical importance of protecting the bank toe‐region from steepening by hydraulic forces that would otherwise entrain previously failed and in situ bank materials, thereby allowing the upper bank to flatten (by failure) to a stable slope.

Enhanced application of root‐reinforcement algorithms for bank‐stability modeling

AbstractRiparian vegetation is known to exert a number of mechanical and hydrologic controls on bank stability. In particular, plant roots provide mechanical reinforcement to a soil matrix due to the different responses of soils and roots to stress. Root reinforcement is largely a function of the strength of the roots crossing potential shear planes, and the number and diameter of such roots. However, previous bank stability models have been constrained by limited field data pertaining to the spatial and temporal variability of root networks within stream banks. In this paper, a method is developed to use root‐architecture data to derive parameters required for modeling temporal and spatial changes in root reinforcement. Changes in root numbers over time were assumed to follow a sigmoidal curve, which commonly represents the growth rates of organisms. Regressions for numbers of roots crossing potential shear planes over time showed small variations between species during the juvenile growth phase, but extrapolation led to large variations in root numbers by the time the senescent phase of the sigmoidal growth curve had been reached. In light of potential variability in the field data, the mean number of roots crossing a potential shear plane at each year of tree growth was also calculated using data from all species and an additional sigmoidal regression was run. After 30 years the mean number of roots predicted to cross a 1 m shear plane was 484, compared with species‐specific curves whose values ranged from 240 roots for black willow trees to 890 roots for western cottonwood trees. In addition, the effect of spatial variations in rooting density with depth on stream‐bank stability was modeled using the bank stability and toe erosion model (BSTEM). Three root distributions, all approximating the same average root reinforcement (5 kPa) over the top 1 m of the bank profile, were modeled, but with differing vertical distributions (concentrated near surface, non‐linear decline with depth, uniform over top meter). It was found that stream‐bank FS varied the most when the proportion of the failure plane length to the depth of the rooting zone was greatest. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of seepage undercutting on the stability of root‐reinforced streambanks

AbstractSeveral mechanisms contribute to streambank failure including fluvial toe undercutting, reduced soil shear strength by increased soil pore‐water pressure, and seepage erosion. Recent research has suggested that seepage erosion of noncohesive soil layers undercutting the banks may play an equivalent role in streambank failure to increased soil pore‐water pressure. However, this past research has primarily been limited to laboratory studies of non‐vegetated banks. The objective of this research was to utilize the Bank Stability and Toe Erosion Model (BSTEM) in order to determine the importance of seepage undercutting relative to bank shear strength, bank angle, soil pore‐water pressure, and root reinforcement. The BSTEM simulated two streambanks: Little Topashaw Creek and Goodwin Creek in northern Mississippi. Simulations included three bank angles (70° to 90°), four pore‐water pressure distributions (unsaturated, two partially saturated cases, and fully saturated), six distances of undercutting (0 to 40 cm), and 13 different vegetation conditions (root cohesions from 0·0 to 15·0 kPa). A relative sensitivity analysis suggested that BSTEM was approximately three to four times more sensitive to water table position than root cohesion or depth of seepage undercutting. Seepage undercutting becomes a prominent bank failure mechanism on unsaturated to partially saturated streambanks with root reinforcement, even with undercutting distances as small as 20 cm. Consideration of seepage undercutting is less important under conditions of partially to fully saturated soil pore‐water conditions. The distance at which instability by undercutting became equivalent to instability by increased soil pore‐water pressure decreased as root reinforcement increased, with values typically ranging between 20 and 40 cm at Little Topashaw Creek and between 20 and 55 cm at Goodwin Creek. This research depicts the baseline conditions at which seepage undercutting of vegetated streambanks needs to be considered for bank stability analyses. Copyright © 2008 John Wiley & Sons, Ltd.

The effects of variability in bank material properties on riverbank stability: Goodwin Creek, Mississippi

Bank retreat is an important area of research within fluvial geomorphology and is a land management problem of global significance. The Yazoo River Basin in Mississippi is one example of a system which is experiencing excessive erosion and bank instability. The properties of bank materials are important in controlling the stability of stream banks and past studies have found that these properties are often variable spatially. Through an investigation of bank material properties on a stretch of Goodwin Creek in the Yazoo Basin, Mississippi, this study focuses on: i) how and why effective bank material properties vary through different scales; ii) how this variation impacts on the outputs from a bank stability model; and iii) how best to appropriately represent this variability within a bank stability model. The study demonstrates the importance that the variability of effective bank material properties has on bank stability: at both the micro-scale within a site, and at the meso-scale between sites in a reach. This variability was shown to have important implications for the usage of the Bank Stability and Toe Erosion Model (BSTEM), a deterministic bank stability model that currently uses a single value to describe each bank material property. As a result, a probabilistic representation of effective bank material strength parameters is recommended as a potential solution for any bank stability model that wishes to account for the important influence of the inherent variability of soil properties.