Progressive Development of the Perched Water Zone in Highway Slopes Made of Highly Plastic Clay

Based on National Oceanic and Atmospheric Administration data, after Hawaii and Louisiana, Mississippi is the rainiest state in the United States, having the most peak precipitation that occurs mainly in late winter. Development of perched water (DPW) has had a remarkable effect on the service life of highway slopes constructed on expansive clay. The objective of the current study is to map the DPW condition at highway slopes made of highly plastic clay (HPC) in Mississippi. Several highway slopes that are made of HPC in Jackson, MS, were instrumented using moisture sensors, water potential probes, and rain gauges. Based on the field investigations, it has been observed that a perched water condition exists in all the slopes constructed of Yazoo clay. To investigate the DPW condition and map the accumulation of the water within the slopes, a series of flow analyses have been conducted using the finite element method in Plaxis. The flow analysis is conducted considering the shrink/swell behavior of the Yazoo clay with the real-time rainfall events, as observed in the rain gauges. The numerical analysis was in good agreement with field monitoring results. Based on the analysis, it is observed that rainwater accumulated during the summer to fall season because of a high infiltration rate with the presence of desiccation cracks. On the other hand, the low permeability situation during the spring held the percolated water within the slopes. Repeated events of infiltration and water hold-up condition progressively develop the perched water zone in the slopes made of Yazoo clay.

Groundwater characterization and monitoring at a complex industrial waste site using electrical resistivity imaging

A contaminated industrial waste site in Washington State (USA) containing buried, metallic-waste storage tanks, pipes, and wells, was evaluated to determine the feasibility of monitoring groundwater remediation activities associated with an underlying perched aquifer system using electrical resistivity tomography. The perched aquifer, located ~65 m below ground surface and ~10 m above the regional water table, contains high concentrations of nitrate, uranium, and other contaminants of concern from past tank leaks and intentional releases of wastes to surface disposal sites. The extent of the perched water aquifer is not well known, and the effectiveness of groundwater extraction for contaminant removal is uncertain, so supplemental characterization and monitoring technologies are being evaluated. Numerical simulations of subsurface flow and contaminant transport were performed with a highly resolved model of the hydrogeologic system and waste site infrastructure, and these simulations were used as the physical basis for electrical resistivity tomography modeling. The modeling explicitly accounted for metallic infrastructure at the site. The effectiveness of using surface electrodes versus surface and horizontal subsurface electrodes, for imaging groundwater extraction from the perched water aquifer, was investigated. Although directional drilling is a mature technology, its use for electrode emplacement in the deep subsurface under a complex industrial waste site via horizontal wells has not yet been demonstrated. Results from this study indicate that using horizontal subsurface electrode arrays could significantly improve the ability of electrical resistivity tomography to image deep subsurface features and monitor remediation activities under complex industrial waste sites.

Modelling Groundwater Returns to Streams From Irrigation Areas with Perched Water Tables

Quantifying the magnitude and timing of groundwater returns to streams from irrigation is important for the management of natural resources in irrigation districts where the quantity or quality of surface water can be affected. Deep vadose zones and perched water tables can complicate the modelling of these fluxes, and model outputs may be biased if these factors are misrepresented or ignored. This study was undertaken in the Murray Basin in southern Australia to develop and test an integrated modelling method that links irrigation activity to surface water impacts by accounting for all key hydrological processes, including perching and vadose zone transmission. The method incorporates an agronomic water balance to simulate root zone processes, semi-analytical transfer functions to simulate the deeper vadose zone, and an existing numerical groundwater model to simulate irrigation returns to the Murray River and inform the management of river salinity. The integrated modelling can be calibrated by various means, depending on context, and has been shown to be beneficial for management purposes without introducing an unnecessary level of complexity to traditional modelling workflows. Its applicability to other irrigation settings is discussed.

Modelling Recharge from Irrigation Developments with a Perched Water Table and Deep Unsaturated Zone

Modelling of recharge under irrigation zones for input to groundwater modelling is important for assessment and management of environmental risks. Deep vadose zones, when coupled with perched water tables, affect the timing and magnitude of recharge. Despite the temporal and spatial complexities of irrigation areas; recharge in response to new developments can be modelled semi-analytically, with most outputs comparing well with numerical models. For parameter ranges relevant to the western Murray Basin in southern Australia, perching can reduce the magnitude of recharge relative to irrigation accessions and will cause significant time lags for changes to move through vadose zone. Recharge in the vicinity of existing developments was found to be similar to that far from existing developments. This allows superposition to be implemented spatially for new developments, thus simplifying estimation of recharge. Simplification is further aided by the use of exponential approximants for recharge responses from individual developments.

The Role of Soil Pipes and Pipeflow in Headcut Migration Processes

<p>Headcut formation and migration is sometimes mistaken as the result of overland flow without realizing that the headcut was formed by or significantly influenced by flow through soil pipes into the headcut. To determine the effects of a soil pipe and flow through a soil pipe on headcut migration, laboratory experiments were conducted under free-drainage conditions and conditions of a shallow water table. Soil beds with a 3-cm deep initial headcut were formed in a flume with a 1.5-cm diameter soil pipe 15 cm below the bed surface. Overland flow and flow into the soil pipe was applied at a constant rate of 68 L/min and 1 L/min, respectively, at the upper end of the flume. The headcut migration rate and sediment concentrations in both surface (channel) and subsurface (soil pipe) flows were measured with time. The typical response without a soil pipe was the formation of a headcut that extended in depth until an equilibrium scour hole was established at which time the headcut migrated upslope. The presence of a soil pipe below the channel, and particularly the phenomena of flow through a soil pipe and into the headcut, whether by seepage from a shallow water table or upslope inflow, significantly impacted the headcut migration. Pipeflow caused erosion inside of the soil pipe at the same time that runoff was causing a scour hole to deepen and migrate. When the headcut extended to the depth of the soil pipe, surface runoff entering the scour hole interacted with flow from the soil pipe also entering the scour hole. This interaction dramatically altered the headcut processes, greatly accelerated the headcut migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe in addition to pipeflow increased the sediment concentration by 42% and the headcut migration rate by 47% compared with pipeflow under free-drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 minutes compared with free-drainage condition. This study confirmed that pipeflow dramatically accelerates headcut migration especially under conditions of shallow perched water tables and highlights the importance of understanding these processes in headcut migration processes.</p>

A Wet Layered Sloping Sponge? The Role of Volcanic Ash Soils in Water Transport and Tracer Mixing at a Tropical Hillslope

<p class="Text"><span lang="EN-US">Hillslope soils developed on volcanic ash (Andosols) provide key hydrological services such as water storage and streamflow regulation in montane environments. Yet, little is known about how they influence subsurface water flow paths and flow transport and mixing dynamics. To fill this knowledge gap, we analyzed a unique 3-year dataset of hourly precipitation, soil moisture, and groundwater level and weekly precipitation and soil water stable isotope data collected along a steep hillslope transect underlain by Andosols. In combination with a detailed characterization of soil properties, we investigated how these soils influence water transport and tracer mixing in the subsurface. Our results indicate that the high organic matter (33-42%) and clay (29-31%) content of the soils’ organic horizon and an abrupt change in hydraulic conductivity between the highly conductive rooted soil layer and a low conductive underlying layer results in a perched water layer that remains near saturated year-round. Despite the formation of the latter, our isotope-based water age estimations depict that water resides within the organic horizon of the soils for short periods (2-4 weeks). The dynamics of soil moisture suggest a fast transfer of hydraulic potentials (few hours) along the entire soil profile in response to rainfall events. This hydraulic response is explained by the exponential shape of the soils’ water retention curves that facilitate a rapid vertical mobilization of water through the porous soil matrix. These findings indicate that the hydrological behavior of volcanic ash soils resemble that of a “layered sponge” in which vertical flow paths are dominant despite the formation of a perched water layer. </span></p>

Modelling rock walls destabilization caused by hydrostatic pressure in frozen/unfrozen bedrock (Hochvogel & Zugspitze, Germany)

<p>One of the most important but still unknown destabilizing factors of rock faces in periglacial environments is the contribution of water in terms of hydrostatic pressure (e.g. Piz Cengalo in 2017). Its presence has often been registered in major rock failures, but it has never been quantified. Perched water table >>20m above virtually impermeable permafrost bedrock can cause excessive hydrostatic stress on affected rockwalls. Climate change related intensification of rainstorms as well as permafrost degradation promote water accumulation. An increase in rockfall activity due to higher water pressure peaks is therefore expected, thus intensifying the risk for humans and infrastructures.</p><p>Here we conduct a hydromechanical stability analysis at two study sites in the Northern Calcareous Alps where this effect has been observed. We use the distinct element method developed in the software UDEC (Itasca); the required geometric and mechanical model input parameters were obtained from previous studies with direct investigations and laboratory tests in frozen/unfrozen conditions. Infiltration from rainfall or snow/ice melting is expected to create extreme pressure peaks, especially when permafrost seals fractured rock.</p><p>Here we present results from:</p><ol><li>the permafrost affected Zugspitze summit (Wetterstein Range), where sealing permafrost allows the meltwater to accumulate in the active layer. This causes high hydrostatic pressure, evaluated by relative gravimetry methods and with the help of a fracture mapping.</li> <li>a preparing high-magnitude rock fall at the Hochvogel (Allgäu Alps), where perched water could destabilize up to 260’000 m³. Displacement measurements on the summit showed acceleration following intense precipitation.</li> </ol><p>Our model proves that a column of water can bring the Zugspitze north face to instable equilibrium. This happens with different intensities according to frozen/unfrozen conditions and various depth of the active layer, if the hydrostatic pressure is adequate (0.2-0.4 MPa = 20-40 m water column).</p><p>Water could also increase the destabilization rates of the south-east face of Hochvogel by adding hydrostatic pressure. A Factor of Safety < 1 is reached when other water-related factors are considered, like: (i) reduction of cohesion in saturated joints, (ii) decrease of the interface friction angle in fractures and (iii) accelerates weathering along the shear plane</p>

The loess landslide on 15 march 2019 in Shanxi Province, China

On 15 March 2019, a fatal deep-seated landslide occurred at the village of Zaoling in Xiangning County of Shanxi Province, China. Extending to an area of about 120 m by 85 m, with an estimated displaced mass volume of 72,000 m3, the landslide left 20 people dead, 13 injured, and 8 buildings destroyed. There were no precursory signals prior to the event, and usual triggering mechanisms for a landslide were absent. Investigation conducted immediately after the incident revealed that the landslide was initiated in a 1.0 to 1.5-m thick-softened layer located at 40 m depth along the contact between the loess and interbedded paleosol layer. This softened layer was highly saturated due to the perched water on top of the relatively impervious paleosol layer and became a critical weak zone since the shear strength of loess is very sensitive to water content. We suggest that the perched water originated from extensive long-term unsaturated seepage of rainwater and local rapid percolation along preferential channels such as sinkholes and root network. The Zaoling landslide confirms that unlike most landslides in non-loess areas, loess landslides can occur without identifiable triggering events. They can result from gradual build up of instability due to slow (in the span of hundred years) accumulation of deep soil water. Based on the lessons learned from this landslide event, suggestions are given for the planning of urban and rural development in loess areas. Due to the fact that the process leading to the development of such a landslide is largely concealed, further research should be aimed at gaining a more thorough understanding of the mechanism of this landslide type.