Impact of backwater on water surface profile in curved channels

Owing to extensive construction of dams, the impact of backwater on flow may lead to navigation or flood control issues in curved channels. To date, the impact of backwater on the water surface profile in curved channels remains unknown and requires investigation. In this study, experiments were conducted in a glass-walled recirculating flume with a length of 19.4 m, a width of 0.6 m, and a depth of 0.8 m, and the impact of backwater on the water surface profile in a 90° channel bend was investigated. The experimental results showed that the backwater degree had a significant impact on the transverse and longitudinal flow depth distributions in the bend. The transverse slope of the flow (Jr) increased linearly with an increase in the Froude number of the approach flow upstream of the bend. Jr increased with the longitudinal location parameter ξ when −0.2 < ξ < 0.5, and decreased with ξ when 0.5 < ξ < 1.2. Furthermore, the results showed that Jr asymptotically decreased to zero with an increase in the degree of backwater. An equation was formulated to estimate the transverse slope of the flow in a 90° bend in backwater zones.

Rapid testing and prediction of soil–water characteristic curve of subgrade soils considering stress state and degree of compaction

The subgrade soil is generally in saturated or unsaturated condition. To analyze complex thermo-hydro-mechanical-chemical (THMC) behaviors of subgrade, it is essential to determine the soil–water characteristic curve (SWCC) that represents the relationship between matric suction and moisture content. In this study, a full-automatic rapid stress-dependent SWCC pressure-plate extractor was developed. Then, the influences of overburden stress and degree of compaction on the SWCC of subgrade soil such as high liquid limit silt (MH) and low liquid limit clay (CL) were analyzed. Accordingly, a new model taking into account the influences of overburden stress and degree of compaction based on the well-known Van Genuchten (VG) SWCC fitting model was presented and validated. The results show that with the increase of the degree of compaction and overburden stress, the saturated moisture content of subgrade soil decreases, while the air-entry value increases and the transition section curve becomes flat. The influences of the degree of compaction and overburden stress on the SWCC of MH is greater than that of CL. Meanwhile, there was a satisfactory agreement between the prediction and measurement, indicating a good performance of the new model for predicting the SWCC.

Rating Curve Estimation of Surface Water Quality Data Using LOADEST

Measurement of the nutrient concentrations in the stream is usually done on weekly, biweekly or monthly basis due to limited resources. There is need to estimate concentration and loads during the period when no data is available. The objectives of this study were to test the performance of a suite of regression models in predicting continuous water quality loading data and to determine systematic biases in the prediction. This study used the LOADEST model which includes several predefined regression models that specify the model form and complexity. Water quality data primarily nitrogen and phosphorus from five monitoring stations in the Neuse River Basin in North Carolina, USA were used in the development and analyses of rating curves. We found that LOADEST performed generally well in predicting loads and observation trends with general tendency/bias towards overestimation. Estimated Total Nitrogen (TN) varied from observation (“true” load) by -1% to 9%, but for the Total Phosphorus (TP) it ranged from -2% to 27%. Statistical evaluation using R2, Nash-Sutcliff Efficiency (NSE) and Partial Load Factor (PLF) showed a strong correlation in prediction.

Prediction of curved oil–water interface in horizontal pipes using modified model with dynamic contact angle

In this study,interface shapes of horizontal oil–water two-phase flow are predicted by using Young-Laplace equation model and minimum energy model.Meanwhile,the interface shapes of horizontal oil–water twophase flow in a 20 mm inner diameter pipe are measured by a novel conductance parallel-wire array probe(CPAP).It is found that,for flow conditions with low water holdup,there is a large deviation between the model-predicted interface shape and the experimentally measured one.Since the variation of pipe wetting characteristics in the process of fluid flow can lead to the changes of the contact angle between the fluid and the pipe wall,the models mentioned above are modified by considering dynamic contact angle.The results indicate that the interface shapes predicted by the modified models present a good consistence with the ones measured by CPAP.

Application of New Water Flooding Characteristic Curve in the High Water-Cut Stage of an Oilfield

The oil production predicted by means of the conventional water-drive characteristic curve is typically affected by large deviations with respect to the actual value when the so-called high water-cut stage is entered.In order to solve this problem,a new characteristic relationship between the relative permeability ratio and the average water saturation is proposed.By comparing the outcomes of different matching methods,it is verified that it can well reflect the variation characteristics of the relative permeability ratio curve.Combining the new formula with a reservoir engineering method,two new formulas are derived for the water flooding characteristic curve in the high water-cut stage.Their practicability is verified by using the production data of Mawangmiao and Xijiakou blocks.The results show that the error between the predicted cumulative oil production and production data of the two new water drive characteristic curves is less than the error between the B-type water drive characteristic curve and the other two water drive characteristic curves.It is concluded that the two new characteristic curves can be used to estimate more accurately the recoverable reserves,the final recovery and to estimate the effects of water flooding.

Modeling Soil Water Retention Curve with a Fractal Method

Many empirical models have been developed to describe the soil water retention curve (SWRC). In this study, a fractal model for SWRC was derived with a specially constructed Menger sponge to describe the fractal scaling behavior of soil; relationships were established among the fractal dimension of SWRC, the fractal dimension of soil mass, and soil texture; and the model was used to estimate SWRC with the estimated results being compared to experimental data for verification. The derived fractal model was in a power-law form, similar to the Brooks-Corey and Campbell empirical functions. Experimental data of particle size distribution (PSD), texture, and soil water retention for 10 soils collected at different places in China were used to estimate the fractal dimension of SWRC and the mass fractal dimension. The fractal dimension of SWRC and the mass fractal dimension were linearly related. Also, both of the fractal dimensions were dependent on soil texture, i.e., clay and sand contents. Expressions were proposed to quantify the relationships. Based on the relationships, four methods were used to determine the fractal dimension of SWRC and the model was applied to estimate soil water content at a wide range of tension values. The estimated results compared well with the measured data having relative errors less than 10% for over 60% of the measurements. Thus, this model, estimating the fractal dimension using soil textural data, offered an alternative for predicting SWRC.

Correct understanding and application of waterflooding characteristic curves

Through reviewing the generation process and essential characteristics of waterflooding curves, the essence and characteristics of Zhang Jinqing waterflooding curve and Yu Qitai waterflooding curve recommended in Chinese Petroleum Industry Standard 'Calculation methods for Recoverable Oil Reserves(SY/T5367—1998)' were discussed, and some technical issues related to the curves were examined in-depth. We found that:(1) All the waterflooding curves are based on empirical formulas derived from oilfield production experience and statistics methods, and can characterize oil displacement features by water quite well.(2) A new waterflooding curve can be derived by combining waterflooding parameters and using different mathematical calculations as long as the parameter combinations and mathematical operation meet a linear relationship, so proposing new waterflooding curves by changing the combination mode has no practical significance anymore.(3) The upwarp of waterflooding curve in the extremely high water cut stage is because the mobility ratio curve has an inflection point with the rapid rise of water cut after reaching a certain value, and the later rapid rise of mobility ratio changes the original two-phase flow dynamics.(4) After entering into water cut stage, all the waterflooding curves with linear relationship can be used to make prediction, even curves with inflection points, as long as they have a straight section above the inflection point.(5) Actual data of waterflooding oilfields has proved that Type A, Zhang Jinqing and Yu Qitai waterflooding curves all can predict accurately oil recoverable reserves in extremely high water cut stage and can be promoted.

Influences affecting the soil-water characteristic curve

The soil-water characteristic curve (SWCC) is the primary partially saturated soil information as its behavior and properties can be derived from it. Although there have been many studies of unsaturated soils and the SWCC, there is still no combined constitutive model that can simulate soil characteristics accurately. In cases when hydraulic hysteresis is dominant (e.g. under cyclic loading) it is particularly important to use the SWCC. In the past decades, several mathematical expressions have been proposed to model the curve. There are various influences on the SWCC as a source of information, so the curves obtained from conventional tests often cannot be directly applied; and the mathematical expressions from one scenario cannot be used to simulate another situation. The effects of void ratio, initial water content, stress state and high suction were studied in this work revealing that water content and stress state are more important than the other effects; but that the influences tend to decrease when suction increases. The van Genuchten model was modified to simulate better the changes in the degree of saturation at low values of suction. Predictions were compared with experimental results to determine the simulation capability of the model.

Predicting the entire soil-water characteristic curve using measurements within low suction range

The soil-water characteristic curve(SWCC) is widely used in the design and evaluation in the practice of geotechnical and geoenvironmental engineering such as the slope stability under the influence of environmental factors. The SWCC has distinct features in the capillary and adsorption zones due to different physical mechanisms. Measurements of the SWCC are typically limited within the capillary zone(i.e., low suction range). It is cumbersome and time-consuming to measure the SWCC in the adsorption zone(i.e., high suction range). This study presents a simple method to predict the entire SWCC within both the capillary and adsorption zones, using measured data only from low suction range(e.g., from 0 to 500 kPa). Experimental studies were performed on a completely weathered granite residual soil to determine its entire SWCC from saturated to dry conditions. The resultant SWCC, along with the SWCC measurements of 14 soils reported in the literature, were used to validate the proposed method. The results indicate that the proposed method has good consistency with a wide array of measured data used in this study. The proposed method is easy to use as it only requires a simple parameter calibration for a commonly used SWCC model. It can be used to improve the reliability in the prediction of the SWCC over the entire suction range when measurements are limited within the low suction range.

Effects of sample dimensions and shapes on measuring soil-water characteristic curves using pressure plate

It is well known that soilewater characteristic curve （SWCC） plays an important role in unsaturated soil mechanics, but the measurement of SWCC is inconvenient. In laboratory it requires days of testing time. For fine-grained clays, it may last for a couple of months using pressure plate tests. In this study, the effects of sample dimensions and shapes on the balance time of measuring SWCCs using pressure plate tests and the shape of SWCCs are investigated. It can be found that the sample dimensions and shapes have apparent influence on the balance time. The testing durations for circular samples with smaller diameters and annular samples with larger contact area are significantly shortened. However, there is little effect of sample dimensions and shapes on the shape of SWCCs. Its mechanism is explored and discussed in details through analysing the principle of pressure plate tests and microstructure of the sample. Based on the above findings, it is found that the circular samples with smaller dimensions can accelerate the testing duration of SWCC using the pressure plate.

Determination of strain-dependent soil water retention characteristics from gradation curve

The importance of soil water retention characteristics in modelling the hydro-mechanical response of unsaturated soils has been well recognised by many investigators in recent years.Determination of strain-dependent soil water retention curve(SWRC)is likely to be extraordinarily difficult.The first two authors have recently shown that SWRC can be computed from the gradation curve and the calculation result is consistent with the experimental results obtained from pressure plate tests.In this paper,based on a hypothesis related to change in the pore size distribution(POSD)due to volumetric strain of soil skeleton,a method to compute strain-dependent SWRC is presented.It is found that at initial degrees of saturation higher than 0.8,the influence of volumetric strain may be marginal whilst at initial degrees of saturation lower than 0.8,its influence is likely to be substantial.In all cases,the gradation curve of the soil affects the SWRC.

Soil Water Characteristic Curve of an Unsaturated Soil under Low Matric Suction Ranges and Different Stress Conditions

Accurate evaluation of unsaturated soil properties is critical for the design of geotechnical and geo-environmental structures such as road pavements, foundations, and earth dams. Water retention activity in soils which is used to predict the stability or seepage problems in the ground is one of the key features in unsaturated soil mechanics. Thus, many experimental works have reported on the unsaturated soil properties, and the soil-water characteristic curve (SWCC) test has contributed significantly to the interpretation of matric suction. Since traditional instruments cannot apply stress in SWCC tests, some researchers have developed suction controlled triaxial apparatus, by which SWCC tests are performed under different stress states. Determination of SWCCs under stress conditions similar to those in the field is key for interpretation of the hydro-mechanical behavior of unsaturated soils. This study conducted SWCC tests of unsaturated silt soil in low matric suction ranges under both drying and wetting conditions. The SWCCs were measured under one-dimensional and isotropic confining stresses ranging from 50 to 450 kPa. The micro porous membrane method was used instead of high air entry ceramic disk for controlling relatively low matric suction. The range of matric suction controlled was from 0 to 20 kPa. The study revealed that the measured SWCC in low matric suction ranges seems to be affected by the influence of stress conditions. Isotropic confining stress caused the void structure of the specimen to become dense and consequently, soil moisture flow movement also decreased. The water retention activity was obviously high, and the point regard to air entry value was larger. The study further suggests that the current methods adopted for estimating unsaturated soil properties require further development to take into account the effect of different stress conditions.

Attenuation-type and failure-type curve models on accumulated pore water pressure in saturated normal consolidated clay

Based on dynamic triaxial test results of saturated soft clay,similarities of variations between accumulated pore water pressure and accumulated deformation were analyzed.The Parr's equation on accumulated deformation was modified to create an attenuation-type curve model on accumulated pore water pressure in saturated normal consolidation clay.In this model,dynamic strength was introduced and a new parameter called equivalent dynamic stress level was added.Besides,based on comparative analysis on variations between failure-type and attenuation-type curves,a failure-type curve model was created on accumulated pore water pressure in saturated normal consolidation clay.Two models can take cycle number,coupling of static and dynamic deviator stress,and consolidation way into consideration.The models are verified by test results.The correlation coefficients are more than 0.98 for optimization of test results based on the two models,and there is good agreement between the optimized and test curves,which shows that the two models are suitable to predict variations of accumulated pore water pressure under different loading cases and consolidation ways.In order to improve prediction accuracy,it is suggested that loading cases and consolidation ways should be consistent with in-situ conditions when dynamic triaxial tests are used to determine the constants in the models.

A Method for Calculating Oil Field Relative Permeability Curve by Using Water Drive Characteristic Curve in High Water Cut Stage

With the production of strong bottom water reservoir, it will soon enter the ultra-high water cut stage. After entering the ultra-high water cut period, the main means of stable production is liquid extraction. Large liquid volume has a certain impact on the physical property distribution and fluid seepage law of the oilfield. The relative permeability curve measured according to the industry standard is not used for the prediction of development indicators and the understanding of the dynamic law of the oilfield. In order to understand the characteristics of water drive law in high water cut stage of water drive oilfield, starting from the water drive characteristic curve in high water cut stage, the method for calculating the relative permeability curve is deduced. Through numerical simulation verification and fitting the actual production data, it is confirmed that the obtained relative permeability curve is in line with the reality of the oilfield, It can provide some guiding significance for understanding the production law and water drive law of strong bottom water reservoir in ultra-high water cut stage.

Characteristics of In-Situ Soil Water Hysteresis Observed through Multiple-Years Monitoring

A soil water retention curve (SWRC) is an essential soil physical property for analyzing transport and retention of water in a soil layer. A SWRC is often described as a single-valued function that relates the soil water potential ψ to volumetric water content θ of the soil. However, an in-situ ψ − θ relation should show soil water hysteresis, though this fact is often neglected in analyses of field soil water regimes while long-term in-situ soil water hysteresis is not well characterized. This study aimed at probing and characterizing in-situ ψ − θ relations. The developments of large hysteresis in the in-situ ψ − θ relations were observed only a few times during the study period of 82 months. Any of the large hysteretic behaviors in the ψ − θ relations began with an unusually strong continual reduction in ψ. The completion of a hysteresis loop required a recorded maximum rainfall. Because the study field had very small chances to meet such strong rainfall events, it took multiple years to restore the fraction of soil water depleted by the unusually strong continual reduction in ψ. While wetting-drying cycles had occurred within a certain domain of ψ, hysteretic behaviors tended to be so small that the in-situ ψ − θ relation can be approximated as a single-valued function of θ(ψ). These observed patterns of the in-situ ψ − θ relations were characterized by kinds of difference in dθ/dψ between a drying process and a wetting process at a given ψ. Thus, more amounts of experimental facts about wetting SWRCs in parallel with drying SWRCs should be needed for correct modelling, analyzing, and predicting soil water regimes in fields. It is also necessary to increase our understandings about the long-term trends of occurrences of extreme weather conditions associated with possible change in climate.

Sequential Parameter Estimation Using Modal Dispersion Curves in Shallow Water

Existing sequential parameter estimation methods use the acoustic pressure of a line array as observations. The modal dispersion curves are employed to estimate the sound speed profile（SSP） and geoacoustic parameters based on the ensemble Kalman filter. The warping transform is implemented to the signals received by a single hydrophone to obtain the dispersion curves. The experimental data are collected at a range-independent shallow water site in the South China Sea. The results indicate that the SSPs are well estimated and the geoacoustic parameters are also well determined. Comparisons of the observed and estimated modal dispersion curves show good agreement.

Influence of Dry Density on Soil-Water Retention Curve of Unsaturated Soils and Its Mechanism Based on Mercury Intrusion Porosimetry

The soil-water retention curve(SWRC)can be used to evaluate the ability of unsaturated soils to attract water at various water contents and suctions. In this study, drying SWRCs for a kind of sandy soil were obtained in the laboratory by using self-modified SWRC apparatus. In addition, the porosity and the pore size distribution of the samples were investigated by a mercury porosimetry test in order to analyze the effect of dry density. Results showed that the soil-water retention of the soil specimens was strongly dependent on the dry density. Under zero suction, soil specimens with a higher dry density exhibited lower initial volumetric water content. The higher the dry density of soil, the more slowly the volumetric water content decreased with the increase of suction. There was a general and consistent trend for a soil specimen to possess a larger air-entry value and residual suction, while smaller slope of SWRC when it had a higher density. This was probably attributed to the presence of smaller interconnected pores in the soil specimen with a higher dry density. The proportion of large diameter pores decreased in comparison to pores with small diameters in the soil tested. The measured total pore volume of the soil specimen, which had a larger dry density, was lower than that of the relatively loose specimens.

Soil freezing process and different expressions for the soil-freezing characteristic curve

The soil-freezing characteristic curve(SFCC),which represents the relationship between unfrozen water content and subfreezing temperature(or suction at ice-water interface)in a freezing soil,can be used for understanding the transportation of heat,water,and solute in frozen soils.In this paper,the soil freezing process and the similarity between the SFCC of saturated frozen soil and soil-water characteristic curve(SWCC)of unfrozen unsaturated soil are reviewed.Based on similar characteristics between SWCC and SFCC,a conceptual SFCC is drawn for illustrating the main features of soil freezing and thawing processes.Various SFCC expressions from the literature are summarized.Four widely used expressions(i.e.,power relationship,exponential relationship,van Genuchten 1980 equation and Fredlund and Xing 1994 equation)are evaluated using published experimental data on four different soils(i.e.,sandy loam,silt,clay,and saline silt).Results show that the exponential relationship and van Genuchten(1980)equation are more suitable for sandy soils.The simple power relationship can be used to reasonably best-fit the SFCC for soils with different particle sizes;however,it exhibits limitations when fitting the saline silt data.The Fredlund and Xing(1994)equation is suitable for fitting the SFCCs for all soils studied in this paper.