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Influence of vegetation induced moisture transfer on unsaturated soils.

Ali, Nazri 2007. Influence of vegetation induced moisture transfer on unsaturated soils. PhD Thesis, Cardiff University.

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Abstract

This study explores issues related to the numerical simulation of moisture migration patterns in the unsaturated zone and in the vicinity of established vegetation. A one- dimensional water-uptake model was first developed to simulate moisture migration beneath uniform vegetation (crop) cover. The main thrust of the research was then to develop a practicable water-uptake model that can be used to simulate moisture migration patterns beneath mature trees. The moisture flow model used is based on Richard's Equation extended to incorporate a sink term and integrated with appropriate water-uptake models (i.e. ID, 2D, and 2D axi-symmetric form). A numerical solution was achieved via the finite element method for spatial discretisation along with a finite difference time-marching scheme. A series of numerical simulations have been presented that demonstrate the newly introduced sink term functioned correctly and that the model is capable of representing typical water extraction processes from a variety of crop types. A new two-dimensional axi-symmetric model was then developed and applied to simulate moisture migration near established trees. In particular, the approach adopted utilizes radial symmetry and assumes a linear distribution of water extraction rate with both depth and radius. The new model has been validated by direct comparison to field measurements recorded (by others) for mature trees located on a clay sub soil. The model has been shown to be capable of representing water-uptake over a full-annual cycle. Time dependent boundary conditions, based on rainfall data, and hysteresis effects have also been explored. Overall, a good correlation between field data and simulated results has been achieved. Hysteresis effects, when the soil has already experienced repeated wetting and drying cycles, were not found to be of great significance for the class of problem considered. The research also provides a preliminary assessment of the significance of tree induced water content (and therefore suction) changes on the stability of unsaturated soil slopes. The 'typical' example considered, indicated that relatively small suction changes (in the order of 10 kPa) can influence the factor of safety against slope failure by approximately 7%. These findings should be considered in addition to the variety of other vegetation related strength effects. In conclusion, a relatively straight forward approach to modelling water-uptake by vegetation has been developed. The resulting model is thought to be valuable for a range of geoengineering problems.

Item Type: Thesis (PhD)
Status: Unpublished
Schools: Engineering
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
ISBN: 9781303209383
Funders: University Technology Malaysia, Higher Education of Malaysia
Date of First Compliant Deposit: 30 March 2016
Last Modified: 09 Jan 2018 18:17
URI: http://orca-mwe.cf.ac.uk/id/eprint/54597

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