It is found that the admixture of weak higher short-wavelength modes (several percent of the coupled power) in the spatial power spectrum of the lower-hybrid waves radiated from the waveguide grill into an inhomogeneous scrape-off-layer plasma in tokamaks has a profound influence on the ponderomotive effects of these waves. Using the barometric formula with the ponderomotive force potential for the electron density, we solve the coupling problem self-consistently for four modes using the shooting and matching method to fulfil surface impedance boundary conditions determined by infinite-grill theory. For the symmetric spectrum, below threshold, the total reflection coefficient is independent of the coupled power. Also, at these power levels, isolated density depressions (cavitons) are formed in the spatial locations where there is constructive superposition of short- and long-wavelength modes. Intense mutual mode conversion of these waves occurs at these sites. For power levels above threshold, the reflection coefficient increases until it saturates at higher powers. Numerical results are given for the 24-waveguide grill mounted on the ASDEX tokamak.