Numerical and experimental study of the vertical distribution of velocity and hydraulic gradient in the capillary fringe

<p>Capillary fringe plays an important role in the fate and transport of infiltrated solutes from agricultural lands. In this study, flow patterns and the vertical distribution of the velocity and hydraulic gradient inside the capillary fringe were investigated using FEFLOW calibrated by experimental data. An experimental box along with a real sample of capillary fringe from the study area (Sand and clay pit Brüggen, Germany) was used for the experiments. The dimension of the filled part of the box was 0.75 m long, 0.55 m high, and 0.150 m wide. To maintain a constant hydraulic gradient throughout the experiments the upstream and downstream groundwater levels were fixed to 7 cm and 3 cm, respectively. The horizontal velocity at different points inside the capillary fringe and the vadose zone was measured by injecting the fluorescent dye tracer (Uranin). At the end of the experiments, the soil samples are collected from different parts of the box for water content measurement. The results indicate that FEFLOW successfully estimates water content, overall flow pattern, and more importantly horizontal movement inside the capillary fringe. The streamlines are parallel to the groundwater table in the middle part.  Based on both experimental and numerical results, while there is a downward movement near the outflow, an upward movement was seen near the inflow. In previous studies, the velocity profile inside the capillary fringe was estimated using Darcy’s law, unsaturated hydraulic conductivity, and constant hydraulic gradient. The detailed comparison of measured water content and velocity with numerical modeling results showed that the constant hydraulic gradient assumption above the water table in previous studies is not valid. The vertical hydraulic gradient profile calculated by FEFLOW showed that the hydraulic gradient at the middle part of the box changes from 0.042 to 0.03. Moreover, the shape of the vertical hydraulic gradient profile is a function of the location in the box and soil type.</p><p><strong>Keywords: </strong>Solute transport, Unsaturated zone, Streamline, Pore velocity, Hydraulic conductivity, FEFLOW</p>

Vertical hydraulic gradient research in hyporheic zone of Beberibe river in Pernambuco State (Brazil)

ABSTRACT The interaction between groundwater and surface water occurs naturally and is dependent on the dynamics in the hyporheic zone. The hyporheic zone is the interface between the surface water source and the phreatic aquifer and it’s a system that also influences the water quality. An important feature is the ability to flux in this zone. This work aimed to evaluate the vertical hydraulic gradient in the hyporheic zone at two points in Beberibe river, Olinda-PE, to understand the hyporheic environment characteristics and water flow dynamics in experimental area, in addition to identify the existence of hydraulic connection between surface water and groundwater. The experimental phase consisted of infiltration tests in riverbed with cylinder infiltrometer and vertical hydraulic gradients readings with differential piezometer, complemented with grain size information, for an assessment of the water budget between the river and the aquifer. Analyzing the behavior of the interaction over the period of 10 months, it was observed that the Beberibe river (middle course) contributes to the groundwater recharge in most of the time. The average infiltration rate was 1.02 mm/min in point 1 and 0.85 mm/min in point 2. It was concluded that there is a variability in flow direction, which often is top-down, but may undergo change in the stream showing upstream after long periods of rainfall. Another conclusion is that grain size distribution of bed sediment, that is predominantly sandy, influences hydraulic conductivity of hyporheic zone and influences consequently the vertical flow rates.

Treating causes not symptoms: restoration of surface - groundwater interactions in rivers

Many river restoration projects seek to address issues associated with impaired hydrological and ecological connectivity in longitudinal (e.g. effects of dams, weirs) or lateral (e.g. alienated floodplain) dimensions. Efforts to restore the vertical dimension of impaired stream–groundwater exchange are rare, hampered by limited understanding of the factors controlling this linkage in natural alluvial rivers. We propose a simplified two-axis model of the ‘primary drivers’ (sediment structure and vertical hydraulic gradient) of stream–groundwater exchange that acknowledges their interaction and provides a practical template to help researchers and river managers pose hypothesis-driven solutions to restoration of damaged or lost vertical connectivity. Many human activities impact on one or both of these drivers, and we review some of the tools available for treating the causes (rather than symptoms) in impacted stream reaches. For example, creating riffle-pool sequences along stream reaches will enhance vertical hydraulic gradient, whereas flushing flows can remove clogging layers and sustain sediment permeability. Our model is a first step to specifying mechanisms for recovery of lost vertical connectivity. Assessing results of river restoration using this approach at reach to catchment scales will provide scientific insights into the interplay of hydrology, fluvial geomorphology and river ecosystem function at appropriately broad scales.