  We investigate the effect of galactic winds on the Lyalpha forest in
cosmological simulations of structure and galaxy formation. We combine high
resolution N-body simulations of the evolution of the dark matter with a
semi-analytic model for the formation and evolution of galaxies which includes
detailed prescriptions for the long-term evolution of galactic winds. This
model is the first to describe the evolution of outflows as a two-phase process
(an adiabatic bubble followed by a momentum--driven shell) and to include
metal--dependent cooling of the outflowing material. We find that the main
statistical properties of the Lyalpha forest, namely the flux power spectrum
P(k) and the flux probability distribution function (PDF), are not
significantly affected by winds and so do not significantly constrain wind
models. Winds around galaxies do, however, produce detectable signatures in the
forest, in particular, increased flux transmissivity inside hot bubbles, and
narrow, saturated absorption lines caused by dense cooled shells. We find that
the Lyalpha flux transmissivity is highly enhanced near strongly wind-blowing
galaxies, almost half of all high-redshift galaxies in our sample, in agreement
with the results of Adelberger et al. (2005). Finally, we propose a new method
to identify absorption lines potentially due to wind shells in the Lyalpha
forest: we calculate the abundance of saturated regions in spectra as a
function of region width and we find that the number with widths smaller than
about 1 Angstrom at z=3 and 0.6 Angstrom at z=2 may be more than doubled. This
should be detectable in real spectra.
