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Wetting Front (wetting + front)

Distribution by Scientific Domains

Engineering	100%

Selected Abstracts

The Lisse Effect Revisited

GROUND WATER, Issue 6 2002
Edwin P. Weeks
The Lisse effect is a rarely noted phenomenon occurring when infiltration caused by intense rain seals the surface soil layer to airflow, trapping air in the unsaturated zone. Compression of air by the advancing front results in a pressure increase that produces a water-level rise in an observation well screened below the water table that is several times as large as the distance penetrated by the wetting front. The effect is triggered by intense rains and results in a very rapid water-level rise, followed by a recession lasting a few days. The Lisse effect was first noted and explained by Thal Larsen in 1932 from water-level observations obtained in a shallow well in the village of Lisse, Holland. The original explanation does not account for the increased air pressure pushing up on the bottom of the wetting front. Analysis of the effect of this upward pressure indicates that a negative pressure head at the base of the wetting front, ,f, analogous to that postulated by Green and Ampt (1911) to explain initially rapid infiltration rates into unsaturated soils, is involved in producing the Lisse effect. Analysis of recorded observations of the Lisse effect by Larsen and others indicates that the water-level rise, which typically ranges from 0.10 to 0.55 m, should be only slightly larger than |,f| and that the depth of penetration of the wetting front is no more than several millimeters. [source]

Quantifying the impact of soil water repellency on overland flow generation and erosion: a new approach using rainfall simulation and wetting agent on in situ soil

HYDROLOGICAL PROCESSES, Issue 17 2007
G. Leighton-Boyce
Abstract The conventional view of soil water repellency is that it promotes overland flow and soil erosion, but this is not always borne out by observations. This study aimed to isolate the effects of repellency on long-unburnt and recently burnt terrain on infiltration, overland flow and erosion at the small plot scale (0·36 m2). Rainfall simulations (30 min; intensity 100 mm h,1), using untreated water, and water treated with surfactants to eliminate repellency, were conducted on in situ repellent soils in fire-prone Eucalyptus globulus plantations, north-central Portugal at (i) a long-unburnt site with and without litter, and (ii) a recently burnt site. On long-unburnt terrain, the mean overland flow coefficient (33%) was 16 times higher and mean slopewash was 23 times higher under repellent compared with wettable conditions. On recently burnt terrain, no overland flow was recorded under wettable conditions, while under repellent conditions the mean coefficient was 70%. The water storage capacity of the litter layer under 10-year-old eucalyptus stands for dry antecedent conditions was at least 3 mm water depth per cm litter depth, implying at least a delay to the onset of overland flow. Severe repellency (36% ethanol) was found to persist through a 30-min storm (100 mm h,1) when a litter layer was present. A continuous wetting front was observed in the upper ,1 cm of exposed soil, indicating a breakdown in repellency at the time of observation. Below ,1 cm, repellent, dry soil conditions generally persisted through the simulated storm event. A major implication is that prediction of hydrological impacts of repellency must also take into account the infiltration characteristics of any litter layer and any non-repellent soils, if present. Copyright © 2007 John Wiley & Sons, Ltd. [source]

Directions of preferential flow in a hillslope soil, 1.

HYDROLOGICAL PROCESSES, Issue 4 2005
Quasi-steady flow
Abstract Preferred infiltration is mainly perceived as vertically down whereas subsurface storm flow is thought to occur parallel to slopes. The transition from vertical to lateral flow in a layered hillslope soil is the focus of the contribution. Transient flow is assumed to move as a wetting front. Three time-domain reflectometry (TDR) wave-guides, each 0·15 m long, were mounted in the shape of a truncated tetrahedron with its peak pointing down. Each wave-guide focuses the front velocity along its axis. The three front-velocity vectors are decomposed into their x, y and z components, which are then assembled to the resultant velocity vector. The volume density flux of preferred flow is the product of the front velocity and the mobile water content. The latter is the amplitude of transient soil moisture measured with each wave-guide. The resultant vector of the volume flux density is computed similarly to the velocity vector. The experimental approach allows for the rapid assessment of transient flows without relying on the variation of water potentials. The experiments indicate that the directions of the resultant vectors of velocity and volume flux density can be estimated if the moisture variations of the three TDR wave-guides are strongly correlated during the passing of the wetting front. Copyright © 2004 John Wiley & Sons, Ltd. [source]

Numerical simulation of a dam break for an actual river terrain environment

HYDROLOGICAL PROCESSES, Issue 4 2007
C. B. Liao
Abstract A two-dimensional (2D) finite-difference shallow water model based on a second-order hybrid type of total variation diminishing (TVD) approximate solver with a MUSCL limiter function was developed to model flooding and inundation problems where the evolution of the drying and wetting interface is numerically challenging. Both a minimum positive depth (MPD) scheme and a non-MPD scheme were employed to handle the advancement of drying and wetting fronts. We used several model problems to verify the model, including a dam break in a slope channel, a dam break flooding over a triangular obstacle, an idealized circular dam-break, and a tide flow over a mound. Computed results agreed well with the experiment data and other numerical results available. The model was then applied to simulate the dam breaking and flooding of Hsindien Creek, Taiwan, with the detailed river basin topography. Computed flooding scenarios show reasonable flow characteristics. Though the average speed of flooding is 6,7 m s,1, which corresponds to the subcritical flow condition (Fr < 1), the local maximum speed of flooding is 14·12 m s,1, which corresponds to the supercritical flow condition (Fr , 1·31). It is necessary to conduct some kind of comparison of the numerical results with measurements/experiments in further studies. Nevertheless, the model exhibits its capability to capture the essential features of dam-break flows with drying and wetting fronts. It also exhibits the potential to provide the basis for computationally efficient flood routing and warning information. Copyright © 2006 John Wiley & Sons, Ltd. [source]