Cold disc case: system-integrated SED (using the GRaTer package for a system at a distance of 30 pc), at the peak of the avalanche luminosity, for the reference GAT07 τ_⊥ ~ 2 × 10^-3 disc and for a denser τ_⊥ ~ 10^-2 disc. The dashed lines give the luminosity excess due to the avalanche (computed by subtracting the SED of a reference avalanche-free case). The cutoff of the dashed-blue line beyond 50 μm reflects the fact that, for this dense disc case, the avalanche-system is slightly less luminous, at these long wavelengths, than an avalanche-free counterpart. This is because, beyond 50 μm, the flux is mostly dominated by large grains, which are slightly eroded by the passage of the avalanche (see discussion in Sect. 4.4.1).