This paper presents results of an experimental investigation on detonation wave propagation in semi- confined geometries. Large scale experiments were performed in layers up to 0.6 m filled with uniform and non-uniform hydrogen-air mixtures in a rectangular channel (width 3 m; length 9 m) which is open from below. A semi confined driver section is used to accelerate hydrogen flames from weak ignition to detonation. The detonation propagation was observed in a 7 m long unobstructed part of the channel. Pressure measurements, ionization probes, soot-records and high speed imaging were used to observe the detonation propagation. Critical conditions for detonation propagation in different layer thicknesses are presented for uniform H2/air-mixtures, as well as experiments with uniform H2/O2 mixtures in a down scaled transparent channel. Finally detail investigations on the detonation wave propagation in H2/air-mixtures with concentration gradients are shown.