During extreme high-water events, the phreatic water level in levees will rise over time due to infiltration of water. This can promote slope instability or internal erosion, and eventually lead to structural failure. A potential solution is the application of an impermeable seal, such as a geotextile, to the levee’s outer slope to locally reduce the inflow of water. In this study, the spatiotemporal effect of a seal on the phreatic surface level is investigated experimentally, both at laboratory scale for a homogeneous sand levee, and at full-scale for a more realistic levee design. On the two-dimensional laboratory scale, it was found that application of a seal does not significantly change the steady-state phreatic level, as expected from a theoretical perspective. However, the time for the phreatic surface level to reach steady state after a sudden external water rise was found to increase 25% to 50% in the cases with a seal. Similar results were found for the full-scale three-dimensional experiments, which showed that details of the soil-structure interface significantly influenced the effectiveness of the impermeable seal, increasing the time to steady state between 12% and 25%. A simple numerical transient groundwater flow model confirms that the quality of the seal governs the response of the phreatic level. This model required the inclusion of an interface layer to properly model the imperfect soil-seal conditions. It is concluded that application of an impermeable seal to a levee before sudden water rise does not influence the new steady-state phreatic level. However, the seal slows down the infiltration process, especially for a case where the outer slope is damaged.