Drawdown Conditions Research Articles

Native macrophyte community response to water-level manipulation for an invasive species

Disturbance regimes related to water level fluctuation play an important role in the structuring of aquatic macrophyte communities. In managed systems, winter water level drawdowns have shown promise in controlling populations of nuisance macrophytes, however, drawdowns may have unintended negative consequences on erosion, fauna, and non-target plant species. The Chippewa Flowage, located in Sawyer County, WI, has implemented targeted winter water level drawdowns with the goal of reducing nuisance levels of Myriophyllum spicatum. To evaluate the effects of water level drawdowns on Myriophyllum spicatum and the native macrophyte community, we conducted repeated full lake surveys of aquatic macrophytes at 2018 points under drawdown and normal water level management in 2005/06, 2015, and 2017. We supplemented these efforts with downscaled surveys in 2007, 2010, and 2016. Surveys revealed a statistically significant decline in both Myriophyllum spicatum occurrence and abundance, and average macrophyte abundance during active drawdown management. Kernel distributions of change in point-specific c-value between the two time periods showed a significant increase under drawdown conditions, and a decrease under normal management (D = 0.41, P ≤0.0001). Drawdown management appeared to successfully control Myriophyllum spicatum abundance and allowed for improvements in the floristic quality of the macrophyte community but resulted in reduced macrophyte abundance overall.

Study on the stable behavior of the river embankment system considering climate change impacts and pressure wave

Understanding the behavior of working conditions of the embankment structural system on reinforced concrete pile foundations is very important in design, construction, operation exploitation, and use. This study assessed the adaptive performance of the Hau River landslide protection embankment in regard to climate change scenarios, specifically the impacts of more extraordinary groundwater table drawdown conditions including pressure waves. The study uses Plaxis 2D V.22 software to simulate and calculate the safety factor of the stability of landslide protection embankment on the right bank of Hau River in the area of Binh Duc Ward, Long Xuyen City, An Giang Province. Simulation. The research results on the effectiveness of shore protection works by the seawall system show that the impact of rising water levels as forecasted by climate change projections (+2.8m elevation) is still within the safe limits. Besides, the dangerous scenario of lowering water levels from the design elevation of -1.0 m (Msf = 1.114) down to the climate change water level of -1.5m elevation (Msf = 1.113)) affecting the protected shoreline also has a minor influence on the stability of the structure. Moreover, the impact of waves on the protection works, which is negligible, is also reduced by the shore protection seawall.

Stability analysis of Hub dam under rapid drawdown

Abstract The purpose of this study was to calculate the Hub dam’s slope stability analysis in a rapid drawdown scenario. Using the slide software, we compared four distinct analytic techniques – the Bishop, Janbu, ordinary way of slicing, and Morgenstern-Price approach – to determine whether or not a given slope would remain stable at the highest possible water level. According to the simulation outcomes, the portion of the dam upstream directly impacts the safety factor. The upstream safety factors, calculated using the Bishop approach, are 2.122 for steady-state seepage and 2.023 for the fast drawdown scenario. This ensures the Hub dam’s stability even under rapid drawdown conditions.

Investigating the role of geo-fluvial parameters on model river banks: A flume study

This experimental investigation aimed to analyze the failure profile of a model river bank composed of sand and silty clay. A model river bank with a length of 446 cm, a depth of 32 cm, and a slope of 1V:1.5H was prepared in a flume that was 550 cm long, 150 cm wide, and 60cm deep. Froude-number-based reduced-scale distorted river modeling was considered in this study. For modeling different parameters of flume, the linear scale 1:250 and depth scale 1:20 have been taken. The study was divided into two distinct series: Series-A and Series-B, each designed to explore specific soil types and moisture content variations. In Series-A, physical model tests were conducted using cohesionless soil (sand) with an initial moisture content set at 6% and 13%. Conversely, Series-B involved model banks constructed with silty clay soil, with initial moisture contents of 10% and 15%. Both series utilized a consistent model bank slope of 1V:1.5H. The drawdown conditions, set at 50% and 80% of the model riverbank’s height, were integral to the experiments in both Series-A and Series-B. The impact of moisture content, drawdown, and bank material was observed in the failure of the model bank.

Determining the Thickness of the Filter Layers to Protect the Upstream Sideslope of the Homogeneous Earth Dam Due to Rapid Drawdown Condition with Experimental and Numerical Models

Determining the Thickness of the Filter Layers to Protect the Upstream Sideslope of the Homogeneous Earth Dam Due to Rapid Drawdown Condition with Experimental and Numerical Models

Effect of Core Geometry on Earth Dam Slope Stability

Embankment dams are widely constructed due to their suitability with different types of foundation, and it is constructed from the available material at the site. A zoned earth dam consists of a clay core at the center to control seepage supported by a shell. This paper investigated the impacts of side slopes, top width, and shape on upstream and downstream slope stability during different cases. Slide 6.0 software was used to evaluate the factor of safety of side slopes of an earth dam, and its result was validated. Different side slopes range from 0H:1V to 2 H:1V, and crest widths from 3m to 10 m were examined. In addition, for the slanting core case, several cases with varying angles of inclination were provided. The results indicated that by increasing the side slopes of the core, the factor of safety was reduced, especially in the steady state and rapid drawdown conditions. Increasing the top width also reduced the safety factor in the steady state condition. This reduction was because the core material had lower shear strength than the shell material. In addition, in a steady state and rapid drawdown conditions, the cohesion of core material sharply reduced. Compared with the vertical case, increasing the slanting core slopes influenced the slope stability insignificantly during the steady state. The slanting core had benefits in the steady state when the reservoir was full since, in this case, increasing core side slopes affected the slope stability insignificantly. The maximum core side slope in the earth dam and maximum top width, which insignificantly affected the dam’s slope stability, were 0.8:1 and 5 m, respectively.

Three-Dimensional Stability of Slopes under Water Drawdown Conditions

In practical engineering, slopes near reservoirs are susceptible to collapse during the rising and falling of water levels, resulting in huge financial losses. Three-dimensional (3D) upper-bound analysis is an effective vehicle to assess slope stability under water drawdown. However, in previously published 3D upper-bound analysis, the pore water pressure distribution caused by drawdown is often approximately determined by employing a pore-pressure coefficient, which is not theoretically sound and fails to give rigorous upper-bound estimations of slope stability. To overcome this shortcoming, the hydraulic head distribution of a slope subjected to drawdown was determined numerically using seepage flow calculations. The obtained hydraulic head distribution was subsequently incorporated into the 3D rotational failure mechanism of the kinematic approach of limit analysis, so as to deliver an upper-bound solution to slope safety factors. To validate the proposed approach, a case study on the Chenjiawan slope at the Three Gorges Reservoir, China, and comparisons with numerical calculations and previous studies are performed. Four different drawdown conditions are considered in this study and corresponding stability charts are provided for directly assessing the safety factors of slopes subjected to different drawdown conditions. The effects of different drawdown processes on the slope’s stability are studied, showing the unfavorable effect of the external drawdown process and the beneficial effect of the internal drawdown process.

Stability analysis of an old earth Samarkand Dam in Kazakhstan under rapid drawdown conditions

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.

Stability Assessment of the Slopes of an Oceanside Coral Limestone Quarry under Drawdown Condition of Semidiurnal Ocean Tides

Vipingo quarry in Kilifi county, Kenya, is one of the quarries supplying coral limestone for limestone manufacturing in the coastal region. Due to its close proximity to the Indian ocean, the semidiurnal ocean tides tend to have an influence on the stability of the quarry slopes adjacent to the shoreline. Finite element numerical analysis using the generalized Hoek–Brown criterion is conducted to assess the stability condition of the slopes followed by slope-angle optimization to determine the safest overall slope angle as well as analyzing the stability of the slopes due to action of varying ocean tides. The optimum overall slope angles for various excavation depths are found to be 52° for 20 m, 46° for 30 m, 42° for 40 m, and 39° for 50 m, which are the same even with varying distance of the slope face from the shoreline. A parametric analysis shows that there is no significant effect of the tides on slope stability for excavations above the water table, but as the quarry gets deeper, the slope stability is affected. A sensitivity factor (ζ) is introduced, being a measure of how much the slope safety factor is reduced as a result of the semidiurnal tidal action.

Transparent soil model test of a landslide with umbrella-shaped anchors and different slope angles in response to rapid drawdown

Umbrella-shaped anchors, as new stabilizing structures, have been increasingly adopted in engineering practice due to their simple structure, ease of installation, and high load capacity. Reservoir operation, especially during rapid drawdown, affects the long-term stability of reservoir bank slopes as this process affects the stress, seepage and deformation characteristics of the water-level fluctuation (WLF) zone. Thus, the characteristics of the deformation and failure mode of WLF zones reinforced by umbrella-shaped anchors under rapid drawdown conditions are important for the design and construction of anchoring measures. Five slope-anchor scale models based on the Yingpan landslide were established as transparent soil models considering different slope angles and drawdown rates. Displacement, velocity and macroscopic deformation data were obtained using particle image velocimetry (PIV) technology. The results indicated that the deformation process of both reinforced and unreinforced slopes under rapid drawdown conditions could be divided into four phases: initial, shallow sliding, multisliding and stable phases. In the anchor-reinforced models, loss of soil particles occurred when the water level dropped to reach the anchor heads, causing the anchor plates to loosen and slope deformation to accelerate, especially near the lower row of cables. The anchor heads failed first, and progressive slope sliding ensued. Under rapid drawdown conditions, soil liquefaction and subsequent loosening and sliding occurred, generating a far larger zone of accumulation than the zone of collapse. This indicates that soil liquefaction is another important factor of the link between rapid water level drawdown and slope instability causing local collapses. Combining the experimental results and 47 sets of field monitoring data, it could be concluded that the slope stability decreases with increasing slope angle of the WLF zone. This occurs because a higher water level is needed for deformation initiation in the WLF zone in the case of a higher slope angle.

Coupled Numerical and Analytical Stability Analysis Charts for an Earth-Fill Dam under Rapid Drawdown Conditions

The presented article provides a comprehensive study on the stability analysis of earth-fill dams under rapid drawdown and transient flow conditions used to prepare stability analysis charts by conducting coupled finite-element numerical and analytical limit equilibrium procedures. In this regard, the impacts of different rapid drawdown conditions on the safety factor of the Alavian earth-fill dam are determined. The slope stability charts present for both shallow and deep slip surfaces with various permeabilities are verified by ground information obtained with extensive instrumentation on the dam’s site. The results showed that by decreasing the permeability of the core’s material, despite preventing seepage, the instability risk of the upstream slope as a result of rapid drawdown intensifies. Also, as stability charts can be stated, with increasing the slip surface’s depth and decreasing the hydraulic hydration, the reliability decreases, and the sliding surfaces’ sensitivity increases based on the drawdown rates, which have been revealed to be from 0.2 to 0.6, the most critical state for safety factors, showing significant declines.

Dynamics of Embankment Slope Stability under Combination of Operating Water Levels and Drawdown Conditions

This study investigated the potential influence of operating water levels and loading conditions on the slope stability of an embankment dam. Four different operating reservoir levels (normal, reduced, embankment height, and overflow) were considered in the study. Numerical modeling was used to investigate the problem in the case of the Chardara dam within the Syrdarya catchment in Kazakhstan. Based on the drawdown rates and operating conditions, minimum factor of safety values ranging from 0.56 (total failure) to 2.5 were retrieved. Furthermore, a very high correlation was observed between drawdown days, the minimum factor of safety values, the maximum factor of safety values, and pore-water pressures, with correlation coefficients ranging from 0.561 to 0.997 (strong to very strong correlation). On the other hand, the highest negative correlation of 0.997 was observed between the minimum factor of safety values and pore-water pressures. Additionally, based on the results from the analysis of variance, three reservoir operating levels (normal, embankment height, and overflow) resulted in p-values less than 0.05, indicating that the variations in the factor of safety values from the drawdown rates were statistically significant. The findings of this study demonstrated that, not only may the drawdown rate be detrimental to the embankments, but that different operating levels can also affect slope stability in different ways.

The Perfomance of Skewers Foundation Combined with Revetment under Rapid Drawdown Conditions on Balikpapan Coastal Area in Soft Soil using Numerical Analysis Undrained B

This research analyses the skewer foundation with Plaxis 2D considering the stability before and after drawdown conditions of the sea level with variations in the point load on the foundation. The study is carried out using the back-analysis method with the assumption that the protective building foundation is in a balanced state with the safety factor (SF) = 1.5. As a result, the stability of the rapid drawdown gave a different value which is the skewer foundation before rapid drawdown showed a better result of the performance of foundation during point load than after drawdown. Before drawdown conditions, the value of the point load (P) is 12 Tons. Meanwhile, after drawdown conditions, the skewer foundation is only able to accept a load of 5 tons. The value of the movement of the skewer foundation before and after the drawdown of sea-level conditions occurred at the same settlement value of about 0.0179 m = 1.18 cm. In this condition, the skewer foundation is considered capable of resisting the settlement issue where the maximum settlement value for shallow foundations for clay soils is not greater than 65 mm.

Stability assessment of coastal clay slopes considering strength nonlinearity and sea level drawdown

Sea level drawdown may lead to the stability decrease and even the failure of coastal clay slopes. Since few attempts have been made to illustrate the impact of sea level drawdown on coastal slope stability considering three-dimensional (3 D) geometry and nonlinear strength, this paper extended 3 D limit analysis approach with nonlinear criterion for slope stability under different sea level drawdown conditions. The least upper bound on stability number was derived by using an optimization procedure. Stability charts were presented and the influences of sea level drawdown on slope stability were illustrated. The drops of sea levels in three cases were found to have different effects on the decreases in slope stability caused by nonlinear parameters. 3 D slope tends to be unstable in the case of rapid drawdown, but it will become stable when the inside water drops after sea level has rapidly decreased. In the case of sea level slow drawdown, slope stability will reduce to minimum and then increase. Effects of sea level drawdowns on slope stability become less significant for slope soils with obvious nonlinear strengths. Certain reinforcement measures can be taken to improve the stability of coastal slopes undergoing sea level rapid drawdown or slow drawdown.

Seismic analysis of Souk Tlata dam behaviour using finite element simulation

In this study, earthquake resistance of Souk Tlata earth dam was examined. It corresponds to a preliminary investigation of the behaviour of the dam under earthquake loading with a dominant frequency close to the fundamental frequency of the dam. The dam was modeled with plane-strain finite elements using Plaxis-2D program. The behaviour of both the fill and core of the dam is described using the simple non associated Mohr-Coulomb criterion. The development of the safety factors as a function of the displacement of the dam crest dam reveals that the static stability of the dam under conditions of drawdown is not assured. Dynamic analyses are conducted under a real earthquake motion, which corresponds to the main shock of May 21, 2003 in Algeria. The results show that the seismic loading, with peak acceleration exceeding 0.10g, induces significant settlement at the crest of the dam and substantial displacement in the upper part of the rip-rap in the upstream side.

The influence of material characteristics on dam stability under rapid drawdown conditions

The influence of material characteristics on dam stability under rapid drawdown conditions

The Influence of Shell Permeability on Stability of Upstream Slope during Rapid Drawdown – Khassa Chai Earth Dam as a Case Study

Several factors affect the stability of earth dam during sudden drawdowns such as permeability and mechanical properties of soil, upstream side slope, drawdown ratio, and drawdown rate. This paper investigates the influence of shell permeability on earth dam upstream slope stability and its role in the change of pore water pressure at different locations of the embankment during the sudden drawdown, using different limit equilibrium methods. To accomplish the objective of this study, (Geo Studio 2012 Software) as one of the powerful geotechnical programs was used for the modeling and numerical analysis. The study shows that decreasing in the shell permeability resulted in the reduction of pore water pressure dissipation and variation of shell hydraulic conductivity plays a vital role in the overall stability of the upstream slope under rapid drawdown conditions.

Analysis of Earth Dam Body Behavior under Rapid Drawdown Conditions in Reservoir Water Level by Laboratory and Numerical Methods

Analysis of Earth Dam Body Behavior under Rapid Drawdown Conditions in Reservoir Water Level by Laboratory and Numerical Methods

A Two-Phase Flow Model for Reserves Estimation in Tight-Oil and Gas-Condensate Reservoirs Using Scaling Principles

Summary A common approach to forecast production from unconventional reservoirs is to extrapolate single-phase flow solutions. This approach ignores the effects of multiphase flow, which exist once the reservoir pressure falls below the bubble/dewpoint. This work introduces a new two-phase (oil and gas) flow solution suitable to extrapolating oil and gas production using scaling principles. In addition, this study compares the application of the two-phase and the single-phase solutions to estimates of production from tight-oil wells in the Wolfcamp Formation of west Texas. First, we combine the oil and the gas flow equations into a single two-phase flow equation. Second, we introduce a two-phase pseudopressure to help linearize the pressure diffusivity equation. Third, we cast the two-phase diffusion equation into a dimensionless form using inspectional analysis. The output of the model is a predicted dimensionless flow rate that can be easily scaled using two parameters: a hydrocarbon pore volume and a characteristic time. This study validates the solution against results of a commercial simulator. We also compare the results of both the two-phase and the single-phase solutions to forecast wells. The results of this research are the following: First, we show that single-phase flow solutions will consistently underestimate the oil ultimate recovery factors (URFs) for solution gas drives. The degree of underestimation will depend on the reservoir and flowing conditions as well as the fluid properties. Second, this work presents a sensitivity analysis of the pressure/volume/temperature (PVT) properties, which shows that lighter oils (more volatile) will yield larger recovery factors for the same drawdown conditions. Third, we compare the estimated ultimate recovery (EUR) predictions for two-phase and single-phase solutions under boundary-dominated flow (BDF) conditions. The results show that single-phase flow solutions will underestimate the ultimate cumulative oil production of wells because they do not account for liberation of dissolved gas and its subsequent expansion (pressure support) as the reservoir pressure falls below the bubblepoint. Finally, the application of the two-phase model provides a better fit when compared with the single-phase solution. The present model requires very little computation time to forecast production because it only uses two fitting parameters. It provides more realistic estimates of URFs and EURs, when compared with single-phase flow solutions, because it considers the expansion of the oil and gas phases for saturated flow. Finally, the solution is flexible and can be applied to forecast both tight-oil and gas condensate wells.

DYNAMIC RESPONSE AND STABILITY ANALYSES OF AKOSOMBO DAM USING NUMERICAL ANALYSIS

Dams are very important in Ghanas economic development and environmental improvement. Although Ghana dams are seismically far from the active zone, accurately analysed dams should be evaluated since failure could severely impact the people in the flood environment and the regions economy on a large scale. This paper proposes a numerical procedure for the static, slope stability, and dynamic analysis of the Akosombo embankment dam. Nineteen horizontal acceleration time histories recorded data was used based on Maximum Design Earthquake (MDE), Maximum Credible Earthquake (MCE), Design Basis Earthquake (DBE) and Operating Basis Earthquake (OBE) data. The numerical results estimated showed that the Akosombo embankment dam is likely to experience moderate deformations during the different design earthquakes. The result also indicated that non-linear analysis capable of capturing dominant non-linear mechanisms could be used to assess the stability of embankment dams. The factor of safety (FS) calculated was greater than 1.5 for high reservoir, rapid drawdown condition and low reservoir condition whereas, the FS values were found to be 1.42 for slow drawdown condition.