We report numerical analysis of planar and channel silica waveguide amplifiers with high erbium concentration. The numerical approach is based on the combined use of a full-vectorial finite-element method and the Runge-Kutta algorithm. It allows one to investigate real structures, accounting for both the active ions interactions by cross-relaxation and upconversion and the effects of waveguide geometry and refractive index profile. We show that, depending on input pump power and waveguide structure, ion-ion interactions affect the amplifier performance for high erbium concentrations. A three-dimensional waveguide designed for single-mode operation ensures higher pumping efficiency and lower noise characteristics than a two-dimensional one and seems to be the best candidate to overcome this drawback.< >