查看更多>>摘要：Spontaneous imbibition is relevant to many processes in nature and industry. In this work, the effect of pore structure of porous media on counter-current imbibition is studied. The one-dimensional counter-current imbibition experiments were conducted in packed porous media with different pore structure. Experimental results showed that counter-current imbibition can only proceed for a short time in homogeneous porous media for all fluids. In heterogeneous porous media, counter-current imbibition can only proceed a short time for water-air experiments as well. However, imbibition can proceed continuously until the imbibition front reaches the end of packed porous media for water-oil experiments and the mass of water infiltration is proportional to the square root of time. The efficiency of water infiltration behind the imbibition front decreases with increase in viscosity of non-wetting phase fluid. In addition, the numerical simulation of counter-current imbibition in porous media with different pore structure were conducted using phase-field method. Simulation results showed that very little oil was produced in homogeneous porous media and more oil was produced in heterogeneous porous media. It is indicated that the difference between capillary driving pressure and capillary back pressure, which are closely related to pore structure and pore size distribution of porous media, has important influence on counter-current imbibition. Furthermore, the snap-off of oil or gas production channels in narrow pore throats may lead to termination of imbibition when the water saturation behind the imbibition front is higher.
Ji, HongyuChen, ShenliangPan, ShunqiXu, Congliang...
13页
查看更多>>摘要：Intensified human interventions in river basins and deltas lead to more complexities of environmental changes during the Anthropocene. Changes in river regime especially a dramatic reduction in sediment delivery increase challenges of the morphological and ecological sustainability of river deltas. In evaluating deltaic risks and sustainable solutions, researches are often limited to single geomorphic units of the deltaic system, and investigations of sediment source to sink transfer at river deltas under recent river regimes are often missing. The Yellow River Delta (YRD) presents as a typical megadelta under stressors induced by changing environments. This study utilizes a period of 20-yr high-resolution topography data of the deltaic channel and its subaqueous delta to investigate sediment transport and source to sink process by integrated methods of field measurements and numerical simulations. The results indicate that the deltaic channel has transitioned from net accretion to erosion after the implementation of the Water-Sediment Regulation Scheme (WSRS) in 2002. The active river mouth experienced a slow accretion phase since the river channel diverted to Qing 8 channel, with a reduced vertical deposition rate of 0.15 m/yr, whilst its adjacent Gudong littoral zone had a -0.11 m/yr erosion rate. Under the new fluvial regime, the river-borne suspended sediment tends to transport southwards to the Laizhou Bay, followed by the river-derived sediment transport eastward and northward to the offshore delta. It is clear that with the continued human activities in the region, the YRD is at the potential state of deltaic transition both in the deltaic channel and its subaqueous delta. This transition is believed to be beneficial to the deltaic channel stability, but it could significantly impact on the geomorphic and ecologic sustainability of the entire deltaic system.
Anh, Le NgocTran, Dung DucThong, NguyenVan, Can Thu...
27页
查看更多>>摘要：Recent riverbed sand mining activities have modified estuarine morphology and sedimentation patterns in many Southeast Asian deltas. The Soai Rap Estuary along the Dong Nai River (one of the largest rivers in Southern Vietnam) is one of the most intensively mined areas in the region, where large-scale sand mining activities have started since the 1990 '& nbsp;s. In this paper, we used hydrodynamic modelling (TELEMAC-2D) to investigate the estuarine morphological changes and sediment dynamics related to the intensive riverbed mining in the Soai Rap Estuary. After the hydraulic calibration and validation of the model, we simulate four scenarios: two with dredging, one with sand excavation, and a baseline condition. Our results show that dredging substantially changed the hydraulic regimes, morphological and sediment dynamics of the estuary. Compared to the baseline, dredging substantially influences flow velocity and tidal amplitude. These activities reduce the amount of sediment accumulating upstream by 18% to 36%, while increasing sediment deposition in the Soai Rap Estuary by 6% to 13%. Furthermore, sediment accumulation in the Ganh Rai Bay area decrease by 5% to 10% due to increased sediment transport from the sea to the estuary. Our modelled results also show that the mining pits (pockmarks) created by sand extractions efficiently trap sediment, causing discontinuities in the longitudinal transport of sediment and resulting morphological patterns. We conclude that the morphology and sediment transport regime of the Soai Rap estuary has been significantly altered by the river sand dredging. As such, we recommend that the local government put in place measures to ensure that sand extraction is carried out sustainably in the future, to minimise further alterations to the natural sedimentation patterns and geomorphology of this estuary.
查看更多>>摘要：The exact solution for the concentration breakthrough curves resulting from a solute injection in radially converging flow in an aquifer obtained using Laplace transforms has a complex form difficult to compute, and cannot be given in closed-form expression. This makes it difficult to use for tracer tests analysis in radial flow. To overcome this difficulty new simple approximate but accurate closed-form expressions are derived in the present paper for slug and continuous injections. The improvement of accuracy of these new closed-form expressions is demonstrated. The paper establishes also that the breakthrough curves resulting from slug injections into a radially diverging or radially converging flow are exactly identical at the sampling well in domains having the same spatial extension. Solutions were also derived for tracers or solutes subject to degradation.The new closed-form expressions were used for the analysis of 12 field tracer tests to highlight the difference in transport parameters determination compared to previous approximate expressions.
查看更多>>摘要：A semi-analytical solution is developed for drawdown of transient confined-unconfined flow towards a fully penetrating well in a confined aquifer. The proposed solution considers the aquifer anisotropy and the drainage from the unsaturated zone above the water table in the transient unconfined region. Based on the work by Moench (1995), the parameter of gamma related to vertical hydraulic conductivity, aquifer thickness, specific yield and a delayed coefficient is introduced to investigate the delayed responses of flow behaviours, i.e., drawdown and development of the transient unconfined region. The results indicate that the flow behaviours simulations by the proposed solution are identical to that by Visual-MODFLOW Flex and finite difference method. The drawdown delayed responses occur in the early and intermediate segments, and disappear at the late segment during the transient confined-unconfined conversion. The increasing of the vertical hydraulic conductivity and the delayed coefficient can decrease the drawdown and limit the development of transient unconfined region, thus, and postpone the time of transient confined-unconfined conversion in a radial distance. The release of water by gravity storage nearby the piezometer surface is quick, resulting in an obvious log-log S-shaped drawdown-time curve. As buried depth increases, the effect of artesian storage is more pronounced. This paper provides much-needed insights of drawdown delayed response during the confined-unconfined conversion.
查看更多>>摘要：Process-based models have been widely used for hydrologic modeling, and it is a common practice to use sensitivity analysis methods for excluding non-influential hydrologic processes from further investigation and/or model improvement. This study develops a new method called multi-model absolute difference-based sensitivity (MMADS) analysis method to screen non-influential system processes and parameters. MMADS is conceptually similar to the Morris method for addressing parametric uncertainty, but has a unique feature to address both process model uncertainty (i.e., a process may be represented by multiple process models) and process model parameter uncertainty (i.e., parameters associated with a process model are random). MMADS first evaluates absolute differences of a quantity of interest (i.e., a system model output) by varying process models and/or process model parameter values, and then calculates the mean and variance of the differences for investigating process influence. The mean measures overall influence of the process on the quantity of interest, and the variance estimates influence of nonlinear effects of the process and/or its interactions with other processes. MMADS is an extension of the Morris method from a parameter space to a joint parameter-model space for explicitly addressing both process model uncertainty and model parameter uncertainty. The performance of MMADS is evaluated by using two numerical experiments. One experiment is based on Sobol's G*-function with ten product elements, and has analytical solutions of the MMADS mean and variance of absolute differences. The other experiment is for groundwater flow modeling which considers three processes (i.e., recharge, geology, and snowmelt) that interact with each other. Results indicate that MMADS is computationally efficient and can identify non-influential processes of complex hydrological systems.
NSTL
Elsevier