In this work, we propose a numerical approach based on forward coupled mode modeling to simultaneously evaluate second- and third-order nonlinearities inside a dispersion-engineered waveguide towards the goal of generating broadband terahertz (THz) radiation by optical rectification (OR). While the model can be applied to various material platforms, we showcase it for the case of lithium niobate rib waveguides. By adequately choosing the waveguide parameters, and thereby its group velocity dispersion, we demonstrate numerically that it possible to generate the THz radiation in the 0.1-30 THz frequency range even at input pulse lengths of 160 fs, benefitting from intra-waveguide spectral broadening. We analyze the magnitude, spectral content, and phase of the THz emission in both farfield (in a silicon substrate) and nearfield.