| Summary: | A common and potentially dangerous phenomenon associated with flooding is seepage under levees and the formation of sand boils. Seepage flow due to hydrostatic head gradients of floods may cause deformation of pervious layers leading to heave, piping and sand boils. Underseepage may also cause irreversible changes in the characteristics of the porous medium. A series of independent flood events may have cumulative effects on pervious layers causing sand boils to grow. Current underseepage analyses for levees are based on steady-state flow. Transient seepage flow due to rapid changes in river head may contribute to cumulative effects and cause critical hydraulic head development under levees and subsequent sand boil formation.
This research examined transient effects on hydraulic head development under levees during a flood event. While the research is focused on levees, this study is applicable to any hydraulic structures (e.g., flood walls, dams, and retaining structures) subject to underseepage. An analytical model was developed for one-dimensional transient flow in a confined aquifer under a levee in response to river stage fluctuations. This analytical model was revised by considering leakage out of confined aquifers to simulate the occurrence of sand boils on the landside of levees. Transient flow nets were also constructed using complex variables. The performance of these analytical models was evaluated by comparing with the limited field studies, current U.S. Army Corps of Engineers underseepage analysis methodology for levees, and a finite element program. The effects of possible cumulative deformations on development of exit hydraulic gradients were also evaluated and discussed.
Transient flow models performed reasonably well compared with the limited field studies, the Army Corps seepage analysis method and SEEP2D finite element program. Cumulative analysis of underseepage by the transient flow model simulating sand boil formations showed significant increases in exit hydraulic gradients in response to possible cumulative changes in aquifer characteristics.
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