Accumulation of gas inside a valley exposed to a light crosswind is experimented in this paper to evaluate the influence of gas in accelerating forest fire phenomena. Experimentations were done inside a wind tunnel using a 1/400 forest model configured as a valley with two different internal angles. The forest is modelled so that a parallel is possible with a real forest thanks to a similitude law. The emission of gas is ensured by 400 tubes introduced inside the mesh cylinders supplied with ethane. The 400 tubes are divided into four independent parts of 100 tubes to be able to study independently the influence of the different zones before and inside the valley on the accumulation of gases. We focused on the measurements of velocity by Laser-Doppler Velocimetry (LDV) and concentration with a Flame Ionization Detector (FID). Analyzing the velocity, turbulence and concentration results, we observed a stagnation point in the thalweg for the flattest valley and a recirculation zone for the deepest one where gas accumulation reached up to 4 times the concentration measured outside the valley. The study of the influence of the different emission zones showed that gas accumulation mainly comes from the zones inside the valley.