Enhancement of River Bank Stability by <em>Hibiscus tiliaceus</em> Root Reinforcement: A Nature-based Solution

The rapid drawdown of reservoir may have a significant impact on the stability of adjacent slopes. In order to avoid potential risks, it is important to calculate the change of slope safety factor prior to the reservoir operation. The finite element method is used to analyze the transient seepage during drawdown, and then the pore water pressures are introduced into the stability computation based on limit equilibrium to obtain the transient safety factor. Computations show that for slopes with a specific geometry, the safety factor ratio depends on the parameters K/(Syv) (where K is the permeability coefficient, v is the drawdown speed, and Sy is the specific yield) and c′/(γHtan ϕ′) (where c′ and ϕ′ are the effective cohesion and friction angle, γ is the soil unit weight, and H is the slope height). By considering a wide range of K/(Syv) and c′/(γHtan ϕ′) values and different slope geometries, the percent reduction in critical safety factor of slope during drawdown relative to that during steady-state seepage is obtained. The drawdown condition that causes a large percent reduction in safety factor is judged as a rapid drawdown, and the opposite is a slow drawdown, which does not affect the slope design. This paper presents a series of charts for engineers and designers to judge rapid and slow drawdown conditions. Before the reservoir operation, the appropriate drawdown speed is selected according to the charts to ensure a slow drawdown for adjacent slope, while in the slope stabilization design, only the rapid drawdown stability analysis needs to be performed.

A rapid draw of water from a reservoir can cause a temporary increase in the hydraulic gradient that may not be tolerated by the slope of an earth dam. The increased seepage forces may lead to slope instability, causing the collapse of the structure. The kinematic approach of limit analysis is used to examine stability of slopes subjected to a rapid or slow drawdown. Combinations of slope inclination, soil properties, and hydraulic conditions are found for which the slope becomes unstable. The results are presented in the form of charts for convenient practical use, and the safety factors can be obtained from the charts without the need for iteration. For granular slopes, particularly if shallow, subjected to drawdown, a simple translational mechanism with a shallow failure surface is not the most adverse mechanism for all water-draw regimes.Key words: slopes, stability, rapid drawdown, limit state analysis.

Aims Variabilities of vegetation and soil cause uncertainty to the factor of safety (FoS) of unsaturated vegetated slopes, yet the significance of these variabilities on the uncertainty of FoS is unclear. This study aims to quantify the effect of the uncertainties of root reinforcement and soil hydromechanical properties to the uncertainty of the FoS. Methods The variance‐based global sensitivity analysis was adopted to evaluate how the variance of FoS of vegetated slopes can be apportioned by the variabilities of soil and root parameters. A copula theory was applied to model the correlation amongst the parameters. Results For slip depths shallower than 0.30 m, the major source of the variance of the FoS included the parameters that define root reinforcement, followed by the parameters of soil shear strength. The variation of transpiration‐induced soil suction had limited effect on the FoS variance under heavy rainfall. Taking into account the correlations amongst the parameters had minor influence on their contribution to the variance of the FoS. Conclusions We observed threshold slip depths, where the relative contribution of uncertainties in root and soil parameters on the FoS uncertainty underwent a transition. Root reinforcement for slips as deep as 0.60 m can provide reliable slope stabilisation effects.

Testing the importance of native plants in facilitation the restoration of coastal plant communities dominated by exotics

Stability of Earth and Rockfill Dams under steady-state seepage and rapid draw-down conditions largely depends upon the flownet generated in the core of dam. Direction of flow lines in flow direction or in reverse direction of flow in core may destabilize the factor of safety of dam. An analytical study has been undertaken to evaluate and compare the variation in the flownet under Steady-State Seepage and Rapid Draw-Down in the core of a dam on account of variation in the thickness of core. A 180 m high earth and rockfill dam section, founded on strong base, was used as a base section for analysis. The flow lines were obtained for Steady-State Seepage under full reservoir condition and Rapid Draw-Down (Hdd = H/2) using software (which employs FEM technique). It is observed that for thin and thick core, the direction of flow lines in core are always from upstream slope to downstream slope under Steady-State Seepage, while for all cases of Rapid Draw-Down for Hdd = H/2 condition, when the vertical core is very thin, bulk of the flow lines are towards downstream slope and some are towards upstream slope. As the thickness of core increases, more flow lines are observed towards upstream face including below draw-down level. Stability analysis shows that the factor of safety is constant for core thickness in the range of 1.0 V: 0.25 H to 1.0 V: 0.50 H for steady-state seepage and from 1.0 V: 0.25 H to 1.0 V: 0.75 H under rapid draw-down condition. However, after this range, there is a decrease in factor of safety. The rate of decrease in the factor of safety is also observed to be similar in both conditions.KeywordsEarth and rockfill damVertical coreFlownet

The present work intends to demonstrate the advantages of considering transient flow regime in the stability analysis of the upstream slope for the rapid drawdown situation of a homogeneous earth dam. Upstream slope stability evaluations were carried out, considering pore pressure and suction from transient flow analysis while simulating rapid drawdown of the reservoir. The evaluations comprised different geometries of the upstream slope (from 1V:1.1H to 1V:2.5H) and heights varying from 10 m to 50 m, as well as several low permeability materials (SM, SM-SC, SC, ML, ML-CL, CL, MH and CH). In addition, equations relating the safety factor to such slopes or dam height were adjusted to the analysis data, in order to define the minimum slope for a certain dam height or the maximum height for a given upstream slope. The results have shown that, considering the transient flow condition, including suction, within the slope stability analysis of the rapid drawdown situation, increases the safety factor in relation to the simplified analysis that is usually adopted. This also results in much steeper slopes (for a safety factor of 1.1) than the ones recommended by the U.S. Bureau of Reclamation (USBR), suggesting the importance of performing transient flow analysis for rapid drawdown situations and considering its results instability analysis.

The risks of erosion to road embankments due to increased river water volume, particularly during flood events, often disrupt economic activities such as food and clothing distribution between locations. Therefore, this research investigated the critical impact of rapid drawdown on slope stability for road infrastructure adjacent to the Laeya River in Southeast Sulawesi, Indonesia. To achieve this aim, a comprehensive two-stage method was adopted, and the first included hydrological analysis to simulate flood water levels and predict rapid drawdown scenarios. In the second stage, a detailed geotechnical analysis through finite element method (FEM) was used to assess the stability of the slopes. Consequently, the research findings showed essential safety factor (SF) values under various conditions. The initial SF for the existing condition was 1.20, but after implementing treatment measures, such as slope geometry modification and soil compaction, the value improved to 1.54. However, during flood water level conditions, SF decreased to 1.50 due to the submergence of the slope base. Observation showed that rapid drawdown conditions led to a critical reduction in SF to 1.22, signifying the need for further research on the implications of drawdown. This research provided valuable perceptions and engineering solutions for improving slope stability in river-adjacent road infrastructure.

<p>SlideforMAP and SOSlope are part of a suite of software available through ecorisQ (www.ecorisq.org), an international, non-profit association promoting solutions for risk reduction of natural hazards. SlideforMap is a probabilistic model that quantifies the stabilizing effect of vegetation at the regional scale and localizes potential areas where forest protection could be improved. SOSlope is a hydro-mechanical model that computes the factor of safety at the slope scale, using a strain-step discrete element method, which includes the effects of vegetation root structure and composition. The research aims at investigating the landslide susceptibility at two different spatial scales, using both models. </p><p>The study area is located on a vegetated slope near an interregional connecting road (Tuscany, Emilia-Romagna, central Italy), which crosses the Foreste Casentinesi National Park (Tuscany) an important natural area for both touristic and recreational activities. </p><p>A shallow landslide susceptibility analysis was performed at two different spatial scales, combining the use of the two previously mentioned models. In particular, SlideforMap was applied to identify the main susceptible areas to landslides at regional scale. Next, the identified unstable areas were investigated at detailed scale using SOSlope which simulated an intense rainfall event. Specifically, both distributions of root and soil forces along the slope were analyzed; for the sake of comparison, beech (<em>Fagus sylvatica</em> L.) and spruce (<em>Picea abies</em> L.) parameters were used. Finally, a back-analysis was performed on real landslides. </p><p>The results showed the activation of root reinforcement spatially distributed in the studied slope. The basal root reinforcement map highlights significant differences, with beech showing higher reinforcement values compared to spruce. According to the factor of safety map, landslides may occur along the investigated unstable area. </p><p>SlideforMap and SOSlope may be useful tools to support land and forestry planning, allowing the localization and quantification of the protective effects of forests, root reinforcement included. Results demonstrated that the factor of safety can be used as benchmarks for silvicultural interventions, thus improving the whole planning activities in both forest and surrounding natural and man-made systems.</p>

It can be a great expense to examine individually the stability of earth and rockfill dams on rapid drawdown in civil engineering practice. The aim of this present work is to clarify the safe type on the rapid drawdown among the most common types of earth and rockfill dams and to introduce cheaply the types in dam design. First, a transient analysis of saturated‐unsaturated seepage coupled with stress is carried out in the cross sections of typical earth and rockfill dams the during rapid drawdown, and the safety factors of the upstream slopes are determined by the shear strength reduction method. Then, the typical dams are compared for the stability characteristics so that designers can select the safe type of earth and rockfill dams on rapid drawdown. The obtained results show that the decreasing rate of safety factor in a central core dam is 0.72–0.85 times than one of the homogeneous dams and 0.17–0.40 times than one of the sloping upstream core dams so that it is more stable than other earth and rockfill dams during rapid drawdown.

The rapid drawdown condition to control floods and irrigation is one of the things that may occur over the lifetime of the dam. Also, the stability of the dam at the rapid drawdown will be more important due to the faster reduction of the water level of the dam reservoir than the pore water pressure. In this study, the finite element method and GeoStudio software used to study the seepage from the body earth dam. Also, the complete elastic-plastic model of Mohr-Coulomb is considered in the analysis. In this study, the stability analysis of the Eyvshvan earth dam after rapid drawdown due water to release of the dam reservoir to downstream agricultural lands during drought crisis, is investigated. For the validation, first, the results of the pore water pressure instrument were compared with the results of numerical analysis. The results of multivariate regression analysis (coefficient of determination) showed very good agreement (R2=0.98). The results showed that the phreatic line remains after 29 days from the start of the rapid drawdown of the reservoir, while half of the volume of the drained reservoir remains at 1842 masl (1/3 of the crest). The analysis of dam stability during rapid drawdown using both Morgenstern-Price and Bishop Methods showed that the most critical situation would occur after 42 days of discharge with a factor of safety (FoS) of 1.71, with no stability hazard and the upstream slope would be safe. 2021 University of Tehran. All Rights Reserved.

The upstream or waterside slopes of water-retaining structures, such as earth dams and levees, must be designed for rapid drawdown (RDD). While multiple instances of RDD failure have occurred, only two case histories, Pilarcitos Dam and Walter Bouldin Dam, have sufficient shear strength data to be useful for validation of analysis procedures. This paper documents a RDD failure at Sparmos Dam in Northern Greece in 2016, including the results of index, compaction, and triaxial compression tests on soil from the dam. Based on the collected data, the factor of safety following RDD is calculated using two effective stress and two undrained methods. The B¯ effective stress method and both undrained methods are able to predict the Sparmos Dam failure, if a conservative interpretation of shear strength is employed. In contrast, the effective stress evaluation of RDD using uncoupled transient seepage analysis produces invalid results when appropriate values of the coefficient of consolidation are used. The Sparmos Dam case history highlights the need for ongoing study of RDD analysis procedures and shows that pore pressures from uncoupled transient seepage are unreliable for RDD.

Nature Based Solutions (NBS) for water resources management have potential to mitigate climate change impacts, including more frequent flooding and droughts. Successful uptake requires more knowledge on the effects of NBS type and design on high and low flows. The cost-benefits of NBS impacts on these and water yield are also essential. Here, we used a modelling framework to explore the impacts of two common NBS types (Runoff Attenuation Features [RAFs] and tree planting), both varying in design, specifically location and scale. Data from an upland Scottish catchment (∼1km2) informed a coupled physically-based hydrological (MIKE SHE) and hydraulic (MIKE 11) modelling approach. NBS scenario effects on high and low flows, as well as groundwater recharge were compared and hydrological indices specific to the whisky industry informed the study site's 25-year cost-benefit analysis. Overall, tree planting reduced low flows and recharge of groundwater, whereas RAFs had a positive but smaller effect. Both NBS types reduced high and medium flows, although tree planting reduced high flows less than RAFs. RAF design, particularly increases in storage volume spread over greater areas, increased effects on all aspects of flows and recharge. Greater areas of planting increased effects on all but the highest flows. NBS type and design affected timing of water storage availability, retention and transfer, but this also depended on antecedent wetness, so these should all be considered for optimal performance or avoiding negative effects. The cost-benefit analysis revealed that RAFs would be a financially feasible NBS approach for enhancing low flows, whereas tree planting would not. This study highlighted that implementing a modelling framework alongside cost-benefit analysis could help optimise type and design of NBS for cost-effective management of specific local water availability issues. Critically this could inform NBS implementation for management of flood and drought impacts, likely to become more frequent in future with climate change.

This study was carried out for safety evaluation, the practical application and improvement of design method of the agricultural reservoir embankment according to backside extension. Seepage analysis, slope stability analysis and finite element analysis were performed for steady state and transient conditions. Also, the pore water pressure, seepage quantity, safety factor and stress-strain behavior according to high water level and rapid drawdown were compared and analyzed. The pore water pressure at contact region between backside extension and old embankment was kept high after rapid drawdown. Therefore, backside extension is recommended that design method is required to be improved and reinforced more than the others raising embankment. The hydraulic gradients before and after backside extension showed high value at the base of the core, but they showed stable state at the upstream slope and downstream slope. The seepage quantity per 1 day and the leakage per 100 m for the steady state and transient conditions appeared to be safe against the piping. The safety factor of slope stability showed high at the steady state, and transient conditions did not show differences depending on the rapid drawdown. The safety factor was appeared high at the upstream slope before backside extension and downstream slope after extension. The excess pore water pressure for steady state and transient conditions showed negative(-) at the upstream slope, it was small at the downstream slope. The mean effective stress (p’) showed high at the base of the core and to be wild distribution after the extension. The displacement after extension showed 0.02-0.06 m in the upstream slope, the maximum shear strain after extension was smaller than that before extension.

Rapid drawdown has been identified as one of the most frequent causes of slope failures due to the effects associated with drought and operational changes when incorporating hydroelectric plants, which influence the filling level of earth dams. The main goal of this research is to obtain predictive models based on Artificial Neural Networks that return the factor of safety of the upstream slope in homogeneous earth dams in the face of the effect of rapid drawdown. Three geometries and 40 soils were defined to form the embankment, from which hybrid numerical models of transient water flow with unsaturated soils were built, considering three discharge speeds. From these results, a database was built to develop the predictive models, by means of the KNIME program and an algorithm based on Artificial Neural Networks. The behavior of the factor of safety as a function of time is also analyzed to establish its recovery intervals. Main results show that the minimum factor of safety is obtained between 52 % and 88 % of the total drawdown time. Regarding the predictive models, the adjusted R2 determination coefficients were greater than 95 % in all cases and the errors remained below 10 %. This demonstrates a high effectiveness of this algorithm and the validity of its application to geotechnical problems.

Despite being potential historical sites, old embankment dams are subjected to many stability challenges due to many factors, including a lack of sufficient stability assessment tools by the time the dam was built and changes in embankment material properties induced by natural and human activities. Therefore, with the current advancement in technology is of great importance to investigate the state of old embankment dams under different potential loading conditions. The stability challenges become of more significant concern when the embankment is subjected to a rapid drawdown loading scenario. In this study, the Samarkand dam located in Karaganda province in Kazakhstan which was put into operation in 1941 is investigated in terms of seepage and slope stability with the help of numerical modelling. Both steady and transient (rapid drawdown) flow conditions are taken into consideration. e finite element method-based modelling is achieved using SEEP/W and SLOPE/W of the GeoStudio software. From the analysis results, it was observed that the old dam can be subjected to a potential failure under rapid drawdown conditions as the minimum factor of safety values were decreasing with the increase in the drawdown rates. For instance, the minimum factor of safety from the instantaneous drawdown rate was equivalent to 32.85% less than the factor of safety retrieved from the long-term steady-state conditions. Also, from Analysis of Variance (ANOVA), a p-value of 9.97× 10-29 was obtained after subjecting the factor of safety values from instantaneous, 5 days, 10 days, and 1 m per day drawdown rates to ANOVA, indicating that the factor of safety differences among the analyzed drawdown rates were statistically significant.