  The dynamics of Bose-Einstein condensates in the lowest energy band of a
one-dimensional optical lattice is generally disturbed by the presence of
transversally excited resonant states. We propose an effective one-dimensional
theory which takes these resonant modes into account and derive variational
equations for large-scale dynamics. Several applications of the theory are
discussed and a novel type of "triple soliton" is proposed, which consists of a
superposition of a wavepacket at the upper band edge and two transversally
excited wavepackets which are displaced in quasi-momentum space.
