Seepage analysis can be largely divided into two; the steady state analysis and transient analysis.

Steady state flow analysis is where the boundary conditions inside and outside of the ground does not change with time. Therefore, the inflow is always equal to the outflow within the analysis range. Transient analysis on the other hand, can display different inflow and outflow with time, even when the same boundary conditions as the steady state analysis are applied.

A permeable ground layer (aquifer) exists where the groundwater can seep through, and if the head difference or flux exists at the boundary,  the seepage phenomenon occurs.

Seepage flow occurs along the waterway that connects through the empty pores between soil particles. This flow complies with Darcy’s law. According to this law, the seepage quantity through the soil volume is equal to the multiplication of permeability coefficient, hydraulic gradient and cross sectional area. Darcy’s law originally started from the saturated domain, but can also be applied to the unsaturated domain.

The unsaturated domain includes all non-saturated domains, from the fully dried condition to the almost saturated condition. As the degree of saturation falls below 100% , air bubbles will also exist in the pores and if the saturation is very low, the water particles will attached between soil particles in a concave form.

Negative pore pressure is referred to as suction pressure. In most cases, suction pressure increases as the degree of saturation decreases.

Transient analysis is used when the boundary conditions inside or outside of the ground changes according to time.

The main differences between transient analysis and steady state analysis are that the boundary conditions change as time passes and the fact that the transient analysis requires volumetric water content. When the underground water level goes up or down, the influence factors such as the water content in the unsaturated domain and porosity are needed.

Comparing the water filling of a reservoir between the fully dried initial state and the partially saturated state, there is significant difference in the time it takes for the seepage in the reservoir body to reach a steady state. Hence, transient analysis can be used to estimate the time it takes to saturate the interior of the body, or deduce a more economic design variable by comparing it with the saturated case.