Time evolution of the normalised water vapour abundance, C/O, O/H, and C/H abundance ratios at r = 0.5 AU as a function of gap depth for a disc with a gap at 10 AU. We plot the results for disc viscosity α = 3 × 10⁻⁴ and pebble fragmentation velocity u_frag = 5 ms⁻¹. Regimes I, II, and III correspond respectively to the scenario of shallow gap, traffic jam, and deep gap. Circles and triangles are selected examples from the three regimes at different time snapshots where we further investigate their observability (see Sect. 3.2).