| Author | Babel, Mukand Singh |
| Call Number | AIT Diss. no. WA-93-02 |
| Subject(s) | Hydraulics
|
| Note | A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Engineering, School of Engineering and Technology |
| Publisher | Asian Institute of Technology |
| Series Statement | Dissertation ; no. WA-93-02 |
| Abstract | Studies of unsaturated flow and transport have received considerable attention as a result of environmental concerns over the potential soil and groundwater pollution caused by human activities. Preferential flow in unsaturated zone has been recognized as an important mechanism of rapid migration of contaminants to the underlying aquifers. The present study attempts to analyze the water flow and solute transport in variably saturated and saturated soils under stable and unstable flow conditions in unsaturated zone. One-dimensional laboratory tracer studies were carried out to investigate whether the boundary condition of non-ponding infiltration produces unstable wetting in homogeneous sandy soils. Experiments were conducted in very coarse and coarse sands and for each porous medium two initial conditions of air dry and field capacity were considered. The effluent concentration data were analyzed to estimate the transport parameters, namely the pore water velocity and the dispersion coefficient. A two-dimensional flow and transport model in unsaturated-saturated porous media was developed based on the Richards' equation for water flow and the classical convective-dispersion equation for solute transport under stable flow conditions. The finite difference method with Iterative Alternating Direction Implicit scheme was employed to solve the flow equation. The transport equation was solved by the fo1ward particle tracking method in conjunction with the finite difference method. The unsaturated hydraulic prope1ties of the experimental sands were represented by the empirical equation of van Genuchten (1980). Experimental results indicated that the unstable flow phenomenon can occur in homogeneous coarse sands under unsaturated infiltration. This phenomenon was found to be very predominant in air dry compared to field capacity initial conditions. Results also showed that during the fingered preferential flow, the front moves in fingers rather than in planer form and hence only some fraction of soil matrix participates in the flow and transport processes. The relationship between the estimated dispersion coefficient and pore water velocity was found to be non-linear with exponent nearly equal to two. Numerical results obtained with the assumption of stable flow conditions demonstrated the occmTence of partial volume flow for both air dry and field capacity initial conditions. The fingering theory of Hillel and Baker (1988) was used to predict the fractional area required for the flow to move down. The steady state flux through the fingered area was determined from a mass balance equation. The calculated fingered flux was used to analyze the flow and transport behavior under unstable flow conditions. The theory was found to be applicable for the situation of unsaturated infiltration in homogeneous dry sands. When fingered flow was considered, the pore water velocity remained constant irrespective of the flux as long as the flux is smaller than the conductivity at water entry suction. The pore water velocity reduced drastically from air dry to field capacity suggesting that the fingering phenomenon is suppressed at higher initial wetness. With regard to the mass transport under unstable flow conditions, the model simulated reasonably well the observed arrival time of the solute at the bottom of the dry sand columns. Discrepancy in measured and simulated breakthrough curves is attributed to the lateral movement of solute between the preferential paths and the surrounding soil matrix. The model was successfully applied to simulate two-dimensional solute transport in unsaturated-saturated zones in a sub-surface tile drainage system under stable flow conditions. |
| Year | 1993 |
| Corresponding Series Added Entry | Asian Institute of Technology. Dissertation ; no. WA-93-02 |
| Type | Dissertation |
| School | School of Engineering and Technology |
| Department | Department of Civil and Infrastucture Engineering (DCIE) |
| Academic Program/FoS | Water Resources Research Engineering (WA) |
| Chairperson(s) | Gupta, Ashim Das; |
| Examination Committee(s) | Loof, Rainer;Murty, V. V. N.;Noppadol Phien-wej;Bouwer, Herman; |
| Scholarship Donor(s) | Government of Australia; |
| Degree | Thesis (Ph.D.) - Asian Institute of Technology, 1993 |