A flow barrier system (FBS) that includes a Richards barrier acts in an unsaturated hydrogeologic system to prevent the advective flow of water down through the barrier. Thus, an FBS placed above any solid waste material buried in the unsaturated zone could greatly aid in isolating the waste by keeping the waste away from flowing water. The FBS, consisting of a layer of highly conductive, fine-grained material overlying a sloped gravel layer, is proposed to isolate high-level radioactive waste (HLW) at a candidate disposal facility located in the unsaturated zone at Yucca Mountain in Nevada. A series of laboratory experiments were conducted to (a) assure that the FBS of a specific design can divert the anticipated maximum advective flow (under ideal conditions as well as for the case of a disturbed interface between the two layers caused by, for example, improper initial emplacement or faulting due to seismic activity), (b) investigate water imbibition into the gravel, and (c) measure the diffusion coefficient of the tuff gravel under partially saturated conditions. The main results show that (a) the FBS used in the study can divert point- source flow rates as high as 2.6 × 105 l/yr; (b) this FBS will continue performing with offsets of the interface as great as 50 cm or more; (c) after 12 months of testing, moisture penetrates the gravel only several grain diameters; and (d) the gravel effective diffusion coefficient is <10-11 cm2/s under such low partial saturations. These results indicate that a properly designed FBS can be successful at isolating the HLW under the anticipated range of environmental conditions that exist both now and in the future at Yucca Mountain.