Confinement deterioration and rollover from the radiative improved (RI)mode to the low (L) confinement mode in plasmas with a strong gas puff inthe tokamak TEXTOR-94 are modelled by the one-dimensional transport codeRITM (radiation from impurities in the transport model). The anomaloustransport coefficients in the plasma core include contributions from theion temperature gradient and dissipative trapped electron instabilities.This model allows us to reproduce the L-RI bifurcation with impurityseeding in good agreement with experimental observations. Whereas thetransport at the plasma edge under the RI-mode conditions might bedescribed by the electrostatic turbulence caused by electric currents inthe scrape-off layer of the limiter, the present computations show thatthe level of the edge transport must be increased by at least a factor ofthree in order to reproduce the evolution of the plasma density profileand the effective ion charge during RI-to-L back transitions. An increaseof this order is in agreement with reflectometer measurements and couldtentatively be explained by the effect of neutrals on resistive driftmodes.