Downward seepage effects on dynamics of scour depth and migrating dune-like bedforms at tandem piers

The present study focuses on multi-scale statistical characterization of scour depth at tandem piers and migrating dune-like bedforms forming behind the piers in downward seepage affected alluvial channel, using an experimental approach. Experiments were conducted using two circular piers arranged in a tandem manner for no seepage, 10% seepage, and 15% seepage condition. The erosive capacity of reversal flow is hindered with downward seepage, which results in reducing velocity and Reynolds stress near the bed at upstream of piers in case of seepage runs. The bed elevation of the scoured region is decreasing with the increasing seepage percentage and the growth in scour depth is diminishing with time. The celerity of scour depth is decreasing with time as well as decreasing with increasing downward seepage. Probability density function of bedform elevation has positive tail slightly thicker than the Gaussian.

Using the Magnetotelluric Method for Detecting Aquifer Failure Characteristics under High-Intensity Mining of Thick Coal Seams

In the ecologically fragile mining area of northwest China, high-intensity mining has seriously affected the aquifer and surface eco-environment. In order to better implement water-preserved mining in ecologically fragile areas, the aquifer failure characteristics should be first detected accurately; therefore, it is necessary to find a convenient and fast detection method. Based on the analysis of the basic principles and influencing factors of the magnetotelluric (MT) method, the feasibility of using the MT method to detect aquifer failure is verified by testing the mined area with MT detection and field borehole measurement. Subsequently, the failure characteristics of overburden and unconsolidated aquifers under high-intensity mining are studied by MT detection and physical simulation. By comparing the physical simulation with the field measurement from the aspects of the maximum surface subsidence, interval of periodic weighting and step cracks, the reliability of the height of the water flowing fracture zone and caving zone obtained from physical simulation is verified. The analysis from MT detection and physical simulation shows that the results of the two methods are in accord with each other, which further confirms that the MT method can be used to detect the failure of overburdened structures and aquifers. The penetrating fractures are the main channel for the downward seepage of water resources, which is caused by the “two-zone” of overburden model and located in the “dimple” shape in the apparent resistivity (AR) isogram. It can provide a reference and technical support for the corresponding new water-preserved mining technology and the construction of digital mines.

Turbulent Flow Structures and Scour Hole Characteristics around Circular Bridge Piers over Non-Uniform Sand Bed Channels with Downward Seepage

In alluvial rivers bridge piers often cause local scour, a complex phenomenon as a result of the interaction between turbulent flow and bed material. In this paper, the results of an experimental study on the scour hole characteristics around single vertical pier sets on a non-uniform sand bed, under no seepage, and with downward seepage conditions, are described. In case of downward seepage, turbulent statistics, such as Reynolds stress, higher order moments, TKE-flux, and consequently sediment transport, decrease upstream of the pier, while increasing on both sides of it, where the enhanced erosive capacity of the flow results in an increase in the scour hole width. Moreover, the scour hole length shifts downstream. Empirical equations for the evaluation of scour hole characteristics, such as the length, width, area, and volume, including the downward seepage parameter, are proposed and experimentally tested. Model predictions give reasonably good agreement with the experimental data.

Double averaged turbulence characteristics of alluvial channel with downward seepage

Present work evaluates the double averaged turbulence characteristics of the sand bed channel subjected to the downward seepage through permeable bed. Measures of turbulent statistics are observed to increase with the application of downward seepage. The form induced stress in near bed has a reducing effect with no seepage and an increasing effect with seepage. The seepage increases the turbulent kinetic energy and turbulent intensities causing the bed particles to move rapidly. The quadrant analysis suggests that at near bed, the sweep events in flows with seepage are the main bursting events towards the Reynolds stress production, while ejection and sweep events in no seepage flow have almost equal contribution. The increase in sediment transport with seepage is caused by an increase in flow turbulence production and an associated decrease in turbulent kinetic energy dissipation and turbulent diffusion.

Statistical description of morphological characteristics of bedforms in seepage affected alluvial channels

In this study, experiments were performed in a curvilinear cross-sectional threshold alluvial channel with no seepage and with seepage conditions to understand the influence of downward seepage in an alluvial channel. We observed that a stable channel during the no seepage condition started to approach a stable channel in the transporting stage with downward seepage. Increased value of Shields stress was observed after the application of seepage. In addition, this study deals with the effect of downward seepage on the evolution of alluvial bedforms. In this regard, multi-temporal bed elevation profiles were collected along the test section of channel, which are used to characterize migrating bedforms. Results reveal greater fluctuations and variability on the channel bed under the influence of increased seepage discharge. Slope of the power spectral density with wave number was significantly increased with an increment in seepage percentage, showing more inhomogeneous arrangement of bedforms and larger roughness over the channel boundary.